Prosthetics with ceramic-metal bridge prosthesis. Bridge Design Principles
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Biomechanics of the bridge
\. Bridge prosthesis with bilateral support on molars and premolars. Rule: the force falling on the support will be the greater, the closer the abutment tooth is located to the chewed food (Fig. 17).
Rice. 17. Influence of vertical load on the biomechanics of the pons
prosthesis:
a - the load is applied to the middle of the short body of the bridge;
b - the load is applied to the middle of the long body of the bridge;
B - the load is applied to one of the supporting teeth.
2. Biomechanics of a bridge prosthesis with unilateral support (console-
The larger the size of the artificial tooth, the greater the load on the abutment tooth.
An overload occurs for the abutment, unusual in direction (Fig. 18).
Rice. 18. Biomechanics of a bridge prosthesis with unilateral support under the action of a vertical force P.
3. Biomechanics of the bridge prosthesis supported by the anterior teeth.
The design feature of these prostheses is that their intermediate part is located along the arc. At the same time, the same force acts on the intermediate part, which causes a fan-shaped divergence of the front teeth in people with periodontal diseases.
Principles of designing bridge prostheses in terms of
Biomechanics
1. The intermediate part of the prosthesis must be linear to avoid rotational loads.
2. Teeth with a not very high clinical crown should be used as a support to reduce the horizontal load.
3. The width of the chewing surface of the body of the prosthesis should be less than the width of the chewing surfaces of the replaced teeth to reduce the load on the supporting teeth.
4. To reduce the overload of supporting teeth, it is necessary to increase their number, avoid the use of bridges with one-sided support and reduce the width of the chewing surface of the body of the prosthesis.
5. It is necessary to restore the contact points between the abutment and natural teeth to evenly distribute the horizontal forces.
6. Competent design of bridges in terms of normal occlusion.
7. It is necessary to design such bridges that would meet the requirements of aesthetics to the maximum extent. To do this, the most aesthetically advantageous facing materials are used, as well as the supporting elements and the intermediate part of the prosthesis are designed to ensure reliable fastening of the lining made of plastic, porcelain or composite material.
Improving the aesthetic results of prosthetics is achieved by using combined prostheses, the base of which consists of a metal frame lined with porcelain or plastic facets or completely covered with a layer of porcelain (ceramic) mass. The most promising in aesthetic terms are cast prostheses lined with ceramics or light-cured composite plastics of a new generation.
To make it easier to carry out hygienic care for prostheses, details
Answers to exam questions
d part
The prosthesis lames are given a streamlined shape, devoid of undercuts. Not only mechanical, but also electrolytic polishing is used to create a smooth surface. The intermediate part, replacing the lateral defect, should not be adjacent to the gum, there should be a flushing space of 2-3 mm. It was believed that the best shape of the body of the prosthesis is one that has the shape of a triangle in cross section. But recently there have been supporters of the saddle shape of the body of the prosthesis, which resembles a natural tooth. It is especially widespread in metal-ceramic prostheses (only in the anterior section).
abstract
on the topic: Prosthetics with bridges:
features, biomechanics, design principles.
Artist: Zaitseva I.I.
intern doctor - dentist
Ozersk, 2000
1. Bridge prostheses, design features.
2. Biomechanics of bridge prostheses.
3. Basic principles of construction of bridge prostheses.
4. Indications for prosthetics with bridges. The value of periodontal reserve forces when using bridge structures.
Bridges are understood as structures that rely on teeth that limit the defect of the dentition. This is the most ancient type of prostheses, which is confirmed by findings during excavations of ancient monuments and tombs. The birthplace of modern bridges is considered to be the United States of America, where they received the greatest development and distribution in the second half of the last century. It is not known who exactly introduced the term “bridge”, but it is clear that it is borrowed from the technical vocabulary and reflects the engineering features of the design. However, the similarity of bridge prostheses with building structures - bridges - is purely formal, based on the fact that a bridge prosthesis, like any bridge, has supports. This is where the similarity ends.
The bridge prosthesis, based on natural teeth, transmits masticatory pressure to the periodontium. Most often, bridges rest on the teeth located on both sides of the defect, that is, they have a bilateral support. In addition, unilaterally supported bridges can be used. In this case, as a rule, the abutment tooth in relation to the defect is located distally. For example, in the absence of a maxillary lateral incisor, the canine should be used for support rather than the central incisor. Bridges with one-sided support are most often used for the loss of individual front teeth.
To support bridges, artificial crowns (stamped, cast, combined, semi-crowns, crowns on an artificial stump with a font) or inlays are used. In addition to the supporting elements, the design of bridge prostheses includes an intermediate part located in the area of the defect in the dentition.
According to the manufacturing method, bridges are divided into brazed, the parts of which are connected by soldering, and solid, having a solid frame. In addition, the bridge prosthesis can be entirely made of metal (all-metal), plastic, porcelain, or through a combination of these materials (combined - metal-plastic, metal-ceramic).
For the manufacture of bridges, chromium-nickel, cobalt-chromium, silver-palladium alloys, 900-carat gold, acrylic plastics and porcelain are used.
The disadvantage of brazed bridges is the presence of solder, which consists of metals that cause intolerance in some patients - zinc, copper, bismuth, cadmium. One-piece cast bridge prostheses are free from this drawback.
There are certain requirements for bridge prostheses, primarily related to the rigidity of the structure. Relying on teeth that are borderline with a defect, the bridge-like prosthesis performs the function of extracted teeth and, thus, transfers an increased functional load to the supporting teeth. Only a prosthesis with sufficient strength can resist it.
Equally important are the aesthetic qualities of bridges. Increasingly, there are patients who do not want to have metal parts of the prosthesis visible when smiling or talking. Metal-ceramic structures are considered the best in this respect.
In terms of hygiene, bridges are subject to special requirements. Here, the shape of the intermediate part of the prosthesis and its relation to the surrounding tissues of the prosthetic bed of the mucous membrane of the alveolar process, the gums of the supporting teeth, the mucous membrane of the lips, cheeks, and tongue are of great importance. In the anterior and lateral sections of the dental arch, the intermediate part is not the same. If in the anterior section it should touch the mucous membrane without pressure on it (tangential form), then in the lateral section between the body of the prosthesis and the mucous membrane covering the edentulous alveolar process, there should be free space that does not prevent the passage of chewed foods (wash space).
Forms of the intermediate part of the bridge prosthesis:
1 - tangent for front teeth
2 - hanging with high clinical crowns of teeth
3 - hanging with low clinical crowns of teeth
4 - saddle all-metal
5.6 - hanging with lining of the labial or labial-chewing surface
7 - saddle-shaped with lining of visible surfaces - chewing and partially lateral artificial teeth of the lower jaw.
With a tangent form, the absence of pressure on the mucous membrane is checked with a probe. If its tip is easily inserted under the body of the prosthesis, then there is no pressure on the gums, and at the same time there is no visible gap that does not look aesthetically pleasing when smiling or talking.
In the lateral part of the dentition, creating a flushing space, they try to avoid food retention under the intermediate part of the prosthesis, which can cause chronic inflammation of this area of the mucous membrane. That is why the wash space is made quite large, especially in the lower jaw. In the upper jaw, taking into account the degree of exposure of the lateral teeth when smiling, the flushing space is made slightly smaller than in the lower jaw, and in the area of premolars and canines that open when smiling, it can be minimized, up to touching the mucous membrane. In each case, this issue is resolved individually.
In cross section, the shape of the intermediate part of the prosthesis resembles a triangle. Regarding the saddle shape, opinions differ. B.N. Bynin in 1947 considered it possible to use a saddle-shaped intermediate part only in removable bridges because of the danger of pressure sores on the mucous membrane. In recent years, in connection with the introduction of highly aesthetic metal-ceramic structures, there has appeared a supporter of using a saddle-shaped prosthesis body in them.
Biomechanics of bridges
The nature of the distribution and the magnitude of the masticatory pressure falling on the body of the bridge and transmitted to the supporting teeth depend primarily on the place of application and direction of the load, the length and width of the body of the prosthesis. It is obvious that for living organs and human tissues the laws of mechanics are not absolute. For example, the state of periodontal tissues depends on the general state of the body, age, local state of the surrounding organs and tissues, the activity of the nervous system, and many other factors that determine the reactivity of the body as a whole. However, it is important for the clinician to know not only the reaction of the periodontium to the functional overload of the abutment teeth bearing bridges, but also the distribution of elastic stresses both in the bridge itself and in the periodontal tissues of the abutment teeth.
If the functional load falls on the middle of the intermediate part of the bridge, then the entire structure and periodontal tissues are loaded evenly and, therefore, find themselves in the most favorable conditions.
However, such conditions in the process of chewing food are extremely rare. At the same time, it should be borne in mind that with an increase in the length of the intermediate part or insufficiently pronounced elastic properties of the alloy, the body of the prosthesis can bend down and cause additional functional overload in the form of a counter or convergent inclination of the supporting teeth.
In this regard, functional overload is unevenly distributed in periodontal tissues, contributing to the development of a local dystrophic process. Thus, in order to prevent possible changes in the periodontium of supporting teeth under bridges, the body of the prosthesis must have sufficient thickness and not exceed the maximum length, which excludes metal deflection in the area of the dentition defect.
When a chewing load is applied to one of the abutment teeth, both supports are displaced along a circle, the center of which is the opposite, less loaded abutment tooth. This explains the tendency of the abutment teeth to diverge or diverge. Under these conditions, functional overload is also distributed unevenly in periodontal tissues.
If bridges are used with a pronounced samtal occlusal curve or with a significant deformation of the occlusal surface of the dentition, for example, against the background of partial loss of teeth, part of the vertical load is transformed into a horizontal one. The latter displaces the prosthesis samtally, causing the abutment teeth to tilt in the same direction.
Similar conditions arise when mobile teeth are used as one of the supports. However, in this case, the displacement of the prosthesis can reach critical values, aggravating the pathological condition of the periodontium.
Very dangerous for the periodontium are vertical loads falling on the body of a bridge with one-sided support. In this case, the functional load causes the abutment tooth to tilt towards the missing adjacent one. In periodontal tissues, there is also an uneven distribution of elastic stresses. In magnitude, these conditions are significantly superior to those that develop in bridges with bilateral support. Under the influence of a vertical load falling on the body of such a prosthesis, a bending moment occurs. The supporting tooth leans towards the defect, and the periodontium experiences a functional overload of an unusual direction and size. The result may be the formation of a pathological pocket on the side of tooth movement and resorption of the hole at the root apex on the opposite side.
The nature of the distribution and the amount of masticatory pressure that falls on the intermediate part of the bridge and is transmitted to the supporting teeth, depends primarily on the application and direction of the load, the length and width of the intermediate part of the prosthesis. It is important to know not only the reaction of the periodontium to the functional load of the supporting teeth, but also the distribution of elastic stresses both in the bridge itself and in the periodontal tissues of the supporting teeth.
It is necessary to consider various options for applying the load on the bridge:
1. load in the middle of the intermediate part of the bridge, when the entire structure, as well as the periodontium, is loaded evenly and, therefore, is in the most favorable conditions;
2. With an increase in the length of the intermediate part or insufficiently pronounced elastic properties of the alloy, the intermediate part of the prosthesis can bend and cause additional functional overload in the form of an oncoming, or convergent, inclination of the supporting teeth.
3. when a load is applied to one of the abutment teeth, both abutment teeth are displaced, both supports are displaced in a circle, the center of which is the opposite, less stressed abutment tooth;
4. with a pronounced sagittal occlusal curve or with a significant deformation of the occlusal surface of the dentition, when part of the vertical load is transformed into a horizontal one. Similar conditions also arise when mobile teeth are used as one of the supports;
5. Vertical loads falling on the intermediate part of the bridge prosthesis with unilateral support cause the abutment tooth to tilt towards the missing adjacent one. With lateral movements of the lower jaw during chewing, rotation of the abutment tooth occurs - a torque, which enhances the functional overload of the periodontium.
6. with a one-sided support, which consists of two supporting teeth, when there is an immersion in the alveolus of the supporting tooth adjacent to the artificial one. The second supporting tooth is under the action of retracting forces.
The distribution of horizontal forces has distinctive features:
1. with a horizontal load applied to the middle part of the body of the bridge, the abutment teeth experience uniform pressure and transfer the load to the periodontium from the side of the alveolar wall opposite to the application of force;
2. If pressure is applied to one of the supporting teeth, there is a displacement of this tooth along the circumference, the center of which is the other supporting tooth with intact periodontium.
Basic principles for the design of bridge prostheses:
1. the supporting elements and the intermediate part of the bridge should be on the same line;
2. when constructing a bridge, abutment teeth with a not very high clinical crown of the abutment should be used;
3. The width of the chewing surface of the intermediate part of the bridge should be less than the width of the chewing surfaces of the teeth being replaced.
4. The value of chewing pressure is inversely proportional to the distance from the point of its application to the abutment tooth. Quite the opposite pattern is observed in the design of bridges with unilateral support. To reduce the functional overload of the supporting teeth, it is necessary to increase their number, avoiding the use of bridges with one-sided support and reducing the width of the chewing surface of the intermediate part of the prosthesis;
5. it is necessary to restore the contact points between the supporting elements of the bridge prosthesis and adjacent natural teeth;
6. competent design of bridges in terms of normal occlusion;
7. it is necessary to design such bridges that would meet the requirements of aesthetics to the greatest extent.
Indications for use in orthopedic treatment of patients with defects in the dentition with bridges and the choice of their design are determined mainly by the following factors: the size of the defect, its topography, the condition of the supporting teeth and antagonist teeth.
All patients who have defects in the dentition in the area of the anterior or visible lateral teeth (premolars) require their replacement with prostheses (usually bridges) not only for functional, but also for aesthetic reasons.
R&D: Biomechanics of non-removable bridge prostheses
INTRODUCTION
CHAPTER 1. TREATMENT OF PARTIAL EDENTIA WITH FIXED PROSTHESES
1 General characteristics of bridge prostheses
2 Biomechanics of bridges
3 Basic principles of bridge design
CHAPTER 2. INDICATIONS FOR PROSTHETIC BRIDGES
1 General features of manufacture and application
CONCLUSION
INTRODUCTION
Biomechanics is a branch of physiology that studies the mechanical properties of living tissues, organs and the body as a whole, as well as the physical phenomena that occur in them during their life.
The biomechanics of bridges are considered together with the biomechanics of the mandible. The movements of the lower jaw during eating occur in different directions and therefore, from the point of view of mechanics, forces act on the bridge prosthesis: pressure, traction, horizontal forces. Their action depends on the movement of the lower jaw, the consistency of food, the biometric structure of the bridge and the place of fixation.
Relevance of the topic. Bridges are the most common prosthetic construction used in orthopedic dentistry to restore defects in the dentition. This method, along with advantages, such as a fixed structure, complete restoration of chewing function, psychological comfort for the patient, has a significant drawback: if the design of the bridge is incorrectly chosen, functional overload and subsequent loss of abutment teeth, pathological changes in the periodontium and alveolar bone are noted. Under the action of masticatory pressure, elastic deformations occur in the walls of the alveoli, causing compressive or tensile stress, the nature and severity of which are directly dependent on the magnitude, direction and zone of force application, the thickness of the alveolar wall, the angle of the tooth, the presence of contact points. With a parallel arrangement of the longitudinal axes of the abutment teeth, the elastic deformation in the periodontal tissues is minimal, which is the best option when choosing a bridge design. In the same cases, when the action of the masticatory load is directed at an angle to the longitudinal axis of the tooth, the degree of deformation increases by 2–2.5 times. Repeated and prolonged angular loads lead to a change in transmural pressure, disruption of local blood circulation, resulting in dystrophic changes in periodontal tissues. Therefore, the correct choice of the design of the bridge prosthesis is so important, taking into account the qualitative picture of the distribution of forces, the points of their application and the quantitative assessment of the acting loads. Target.As mentioned above, the restoration of dentition defects is an urgent task, and bridges are the most common prosthetic structure used in orthopedic dentistry to solve this problem. Based on this, the purpose of this work is to describe both the undoubted advantages of biomechanical prosthetics with fixed bridge prostheses, and the disadvantages that occur with the wrong choice of bridge design. Tasks.Conduct a selection of available literature and Internet resources on a given topic. Conduct an analysis of the literature found and prepare a written work that will cover the following topics: indications and contraindications for use and features of the manufacture of fixed bridges; general characteristics and description of the basic principles of designing bridges, taking into account biomechanics. CHAPTER 1. TREATMENT OF PARTIAL EDENTIA WITH FIXED PROSTHESES
Bridges as a therapeutic agent are widely used in the treatment of partial adentia and the restoration of the function of chewing and speech. These types of prostheses, as well as other medical devices, are a preventive measure designed to prevent diseases of the dental system, gastrointestinal tract, and mental disorders. In this case, an important condition is the absence of side effects on the system and the body of both the design of the prosthesis and its elements, and the materials from which the prostheses are made. Corrosion resistance, biological compatibility of materials with tissues and the environment of the body determines their clinical suitability. Fixed dentures should not violate the hygienic state of the mouth. Bridges can replace small and medium (in the anterior - no more than four teeth, in the lateral - no more than three) included defects in the dentition and less often - end defects. Rice. 1 dental bridge - 4 units Conditions: dentition defects must be linear; abutment teeth must be stable, without pathological changes detected clinically and radiographically, with pronounced clinical crowns; it is desirable that the long axes of the supporting teeth are parallel (there is no convergence); it is desirable to have a physiological bite. Contraindications to the use of bridges: Large defects limited to teeth with different functional orientations. Defects limited distally to a tooth with pathological mobility. Defects limited to teeth, in which pathological changes are detected during clinical and radiological examination (chronic granulomatous periodontitis). Defects limited to teeth with low clinical crowns. Loss of all anterior teeth (321 1 123) is a curvilinear defect. With the help of bridge prostheses, especially meta-ceramic and metal-plastic ones, it is possible to restore the appearance of the patient. Bridge prostheses normalize the speech of the patient. Bridge prostheses allow to eliminate the functional overload of the periodontium, temporomandibular joints and masticatory muscles. Bridges are a preventive measure that prevents further destruction of the masticatory apparatus. Almost complete compliance of the prosthesis design with the natural dentition ensures rapid adaptation to it (3-7 days). .1 General characteristics of bridges
Bridges are understood as structures that rely on teeth that limit the defect of the dentition. This is the most ancient type of prostheses, which is confirmed by findings during excavations of ancient monuments and tombs. The birthplace of modern bridges is considered to be the United States of America, where they received the greatest development and distribution in the second half of the last century. The bridge prosthesis, based on natural teeth, transmits masticatory pressure to the periodontium. Most often, bridges rest on the teeth located on both sides of the defect, that is, they have a bilateral support. In addition, unilaterally supported bridges can be used. In this case, as a rule, the abutment tooth in relation to the defect is located distally. For example, in the absence of a maxillary lateral incisor, the canine should be used for support rather than the central incisor. Bridges with one-sided support are most often used for the loss of individual front teeth. To support bridges, artificial crowns (stamped, cast, combined, semi-crowns, crowns on an artificial stump with a font) or inlays are used. In addition to the supporting elements, the design of bridge prostheses includes an intermediate part located in the area of the defect in the dentition. According to the manufacturing method, bridges are divided into brazed, the parts of which are connected by soldering, and solid, having a solid frame. In addition, the bridge prosthesis can be entirely made of metal (all-metal), plastic, porcelain, or through a combination of these materials (combined - metal-plastic, metal-ceramic). For the manufacture of bridges, chromium-nickel, cobalt-chromium, silver-palladium alloys, 900-carat gold, acrylic plastics and porcelain are used. The disadvantage of brazed bridges is the presence of solder, which consists of metals that cause intolerance in some patients - zinc, copper, bismuth, cadmium. One-piece cast bridge prostheses are free from this drawback. There are certain requirements for bridge prostheses, primarily related to the rigidity of the structure. Relying on teeth that are borderline with a defect, the bridge-like prosthesis performs the function of extracted teeth and, thus, transfers an increased functional load to the supporting teeth. Only a prosthesis with sufficient strength can resist it. Equally important are the aesthetic qualities of bridges. Increasingly, there are patients who do not want to have metal parts of the prosthesis visible when smiling or talking. Metal-ceramic structures are considered the best in this respect. Rice. 2 Ceramic-metal structures In terms of hygiene, bridges are subject to special requirements. Here, the shape of the intermediate part of the prosthesis and its relation to the surrounding tissues of the prosthetic bed of the mucous membrane of the alveolar process, the gums of the supporting teeth, the mucous membrane of the lips, cheeks, and tongue are of great importance. In the anterior and lateral sections of the dental arch, the intermediate part is not the same. If in the anterior section it should touch the mucous membrane without pressure on it (tangential form), then in the lateral section between the body of the prosthesis and the mucous membrane covering the edentulous alveolar process, there should be free space that does not prevent the passage of chewed foods (wash space). Forms of the intermediate part of the bridge prosthesis: Tangent for anterior teeth Hanging with high clinical crowns of teeth Hanging with low clinical crowns of teeth Saddle all-metal 6 - hanging with lining of the labial or labio-chewing surface Saddle-shaped with lining of visible surfaces - chewing and partially lateral artificial teeth of the lower jaw. With a tangent form, the absence of pressure on the mucous membrane is checked with a probe. If its tip is easily inserted under the body of the prosthesis, then there is no pressure on the gums, and at the same time there is no visible gap that does not look aesthetically pleasing when smiling or talking. In the lateral part of the dentition, creating a flushing space, they try to avoid food retention under the intermediate part of the prosthesis, which can cause chronic inflammation of this area of the mucous membrane. That is why the wash space is made quite large, especially in the lower jaw. In the upper jaw, taking into account the degree of exposure of the lateral teeth when smiling, the flushing space is made slightly smaller than in the lower jaw, and in the area of premolars and canines that open when smiling, it can be minimized, up to touching the mucous membrane. In each case, this issue is resolved individually. In cross section, the shape of the intermediate part of the prosthesis resembles a triangle. In recent years, in connection with the introduction of highly aesthetic metal-ceramic structures, there has appeared a supporter of using a saddle-shaped prosthesis body in them. 1.2 Biomechanics of bridges
The nature of the distribution and the magnitude of the masticatory pressure falling on the body of the bridge and transmitted to the supporting teeth depend primarily on the place of application and direction of the load, the length and width of the body of the prosthesis. It is obvious that for living organs and human tissues the laws of mechanics are not absolute. For example, the state of periodontal tissues depends on the general state of the body, age, local state of the surrounding organs and tissues, the activity of the nervous system, and many other factors that determine the reactivity of the body as a whole. However, it is important for the clinician to know not only the reaction of the periodontium to the functional overload of the abutment teeth bearing bridges, but also the distribution of elastic stresses both in the bridge itself and in the periodontal tissues of the abutment teeth. If the functional load falls on the middle of the intermediate part of the bridge, then the entire structure and periodontal tissues are loaded evenly and, therefore, find themselves in the most favorable conditions. However, such conditions in the process of chewing food are extremely rare. At the same time, it should be borne in mind that with an increase in the length of the intermediate part or insufficiently pronounced elastic properties of the alloy, the body of the prosthesis can bend down and cause additional functional overload in the form of a counter or convergent inclination of the supporting teeth. In this regard, functional overload is unevenly distributed in periodontal tissues, contributing to the development of a local dystrophic process. Thus, in order to prevent possible changes in the periodontium of supporting teeth under bridges, the body of the prosthesis must have sufficient thickness and not exceed the maximum length, which excludes metal deflection in the area of the dentition defect. When a chewing load is applied to one of the abutment teeth, both supports are displaced along a circle, the center of which is the opposite, less loaded abutment tooth. This explains the tendency of the abutment teeth to diverge or diverge. Under these conditions, functional overload is also distributed unevenly in periodontal tissues. If bridges are used with a pronounced sagittal occlusal curve or with a significant deformation of the occlusal surface of the dentition, for example, against the background of partial loss of teeth, part of the vertical load is transformed into a horizontal one. The latter displaces the prosthesis sagittally, causing the abutment teeth to tilt in the same direction. Similar conditions arise when mobile teeth are used as one of the supports. However, in this case, the displacement of the prosthesis can reach critical values, aggravating the pathological condition of the periodontium. dental prosthesis Very dangerous for the periodontium are vertical loads falling on the body of a bridge with one-sided support. In this case, the functional load causes the abutment tooth to tilt towards the missing adjacent one. In periodontal tissues, there is also an uneven distribution of elastic stresses. In magnitude, these conditions are significantly superior to those that develop in bridges with bilateral support. Under the influence of a vertical load falling on the body of such a prosthesis, a bending moment occurs. The supporting tooth leans towards the defect, and the periodontium experiences a functional overload of an unusual direction and size. The result may be the formation of a pathological pocket on the side of tooth movement and resorption of the hole at the root apex on the opposite side. With lateral movements of the lower jaw during chewing, rotation of the abutment tooth occurs - a torque that aggravates the functional overload of the periodontium. The moments of torsion and bending are determined by the length of the body of the bridge, the height of the clinical crown of the abutment tooth, the length of the edge, the presence or absence of adjacent teeth, the magnitude of the applied force and the state of the periodontal reserve forces. The probability of developing functional overload in the stage of decompensation can be significantly reduced by increasing the number and using a bridge with a unilateral support in the case of included defects with a length of no more than one tooth. When using an artificial tooth with a one-sided support in the form of two supporting teeth, there is a predominant immersion in the alveolus of the supporting tooth adjacent to the artificial one. The other supporting tooth is under the influence of pulling forces. Thus, there is a kind of rotation of the prosthesis around the center located in the supporting tooth, which carries the artificial tooth. In this case, the difference in squeezing and stretching of periodontal tissues reaches quite large values and can also adversely affect the supporting tissues. If pressure is applied to one of the abutment teeth, especially with its pathological mobility, this tooth is displaced along a circle, the center of which is another abutment tooth with an unaffected periodontium. The latter is thus subjected to rotation around the longitudinal axis. 1.3 Basic principles of bridge design
When designing bridges, certain principles should be followed. According to the first principle, the supporting elements of the bridge and its intermediate part must be on the same line. The curvilinear shape of the intermediate part of the bridge leads to the transformation of vertical and horizontal loads in rotation. Rice. 3 Features of the design of bridges: a - abutment tooth with a high clinical crown and a short root; b - an increase in the clinical crown with atrophy of the hole; c - reduction in the width of artificial teeth when constructing the body of the bridge The load is applied to the most protruding part of the body of the bridge. If we draw a perpendicular to the straight line, connecting the long axes of the abutment teeth, from the point of the body of the prosthesis that is farthest from it, then it will be the arm of the lever that rotates the prosthesis under the action of chewing load. The amount of rotational force is thus directly dependent on the curvature of the body of the bridge. Reducing the curvature of the intermediate part will help reduce the rotational action of the transformed masticatory load. The second principle is that when constructing a bridge, abutment teeth with a not very high clinical crown should be used. The magnitude of the horizontal load is directly proportional to the height of the clinical crown of the supporting tooth. The use of abutment teeth with high clinical crowns and shortened roots is especially harmful for the periodontium. In this case, there is a high probability of a rapid transition of the compensated form of functional overload into a decompensated one with the appearance of pathological mobility of the supporting teeth. Similar conditions also arise with atrophy of the alveolar process, when the height of the clinical crown of the tooth increases due to the reduction of the intra-alveolar part of the root. At the same time, it should be borne in mind that with excessively low clinical crowns, the design of a bridge prosthesis is also difficult due to a decrease in rigidity and a decrease in the area of adherence of the body to the supporting elements. Especially often the connection is destroyed in complete bridges. The third principle suggests that the width of the chewing surface of the bridge should be less than the width of the chewing surface of the replaced teeth. Since any bridge prosthesis functions due to the reserve forces of the periodontium of the abutment teeth, the narrowed chewing surfaces of the body reduce the load on the abutment teeth. Moreover, when designing the body of the prosthesis, it is advisable to take into account the presence of antagonistic teeth and their appearance - whether they are natural or artificial. If the pressure is concentrated closer to one of the supporting ones due to the loss of part of the antagonists, then the body of the prosthesis in this place may be narrower than in other areas. Thus, in order to avoid excessive functional overload, the chewing surface of the body of the bridge is made narrower, and the amount of narrowing in certain areas is determined individually in accordance with the characteristics of the clinical picture. An increase in the width of the chewing surfaces of the intermediate part of the bridge leads to an increase in the functional overload of the supporting teeth, not only due to an increase in the total area that receives chewing pressure, but also due to the appearance of rotational forces along the edge of the body of the prosthesis, which extends beyond the width of the supporting teeth. The fourth principle is based on the fact that the magnitude of chewing pressure is inversely proportional to the distance from the point of its application to the supporting tooth. Thus, the closer the load is applied to the abutment, the more pressure falls on this abutment and, conversely, as the distance from the place of application of the load to the abutment tooth increases, the pressure on this abutment decreases. A completely opposite pattern is found when constructing bridges with one-sided support. The larger the size of the suspended artificial tooth, the more the adjacent abutment tooth is loaded. To reduce the functional overload of the abutment teeth, it is necessary to increase their number, avoid the use of bridges with one-sided support, and reduce the width of the chewing surface of the prosthesis body. The fifth principle is associated with the need to restore the contact points between the supporting elements of the bridge and adjacent natural teeth. This allows you to restore the continuity of the dental arch and contributes to a more even distribution of chewing pressure, especially its horizontal component, among the remaining teeth in the oral cavity. It is especially important to observe this principle with a well-defined sagittal occlusal curve, when horizontal loads transformed from vertical ones tend to tilt the abutment teeth in the mesial direction. A properly restored contact point will transfer some of the horizontal force to adjacent natural teeth. This helps to keep the abutment teeth stable and prevents them from tilting mesially. The sixth principle provides for the competent design of bridges in terms of normal occlusion. There are two groups of patients. The first includes patients whose task of prosthetics is to restore the correct occlusal relationships in the defect area with careful modeling of the occlusal surface of the bridge prosthesis that fits into the patient's functional occlusion. Here, first of all, care should be taken to prevent premature contacts, reduce the interalveolar distance and functional overload of the periodontium after prosthetics. In the second group, we include patients who need not only the prosthesis of a defect in the dentition with a bridge, but also a simultaneous change in functional occlusion within the entire dentition. This may be necessary in case of partial loss of teeth, increased abrasion, periodontal diseases, anomalies of occlusion, complicated by partial loss of teeth, etc. Common to all these pathological conditions is a decrease in the interalveolar distance. Thus, for the second group of patients, more complex prosthetics are required, taking into account changes in the occlusion of dentures. Seventh principle: it is necessary to design such bridges that would meet the requirements of aesthetics to the maximum extent. To do this, the most aesthetically advantageous facing materials are used, as well as the supporting elements and the intermediate part of the prosthesis are designed to ensure reliable fastening of the lining made of plastic, porcelain or composite material. CHAPTER 2. INDICATIONS FOR PROSTHETIC BRIDGES
When determining the indications for prosthetics with bridges, it should be borne in mind, first of all, the extent of the defect in the dentition - these can be small and medium defects and less often end ones. A special role is played by the requirements for abutment teeth. The planning of a bridge prosthesis becomes only after a thorough clinical and paraclinical study: it is necessary to pay attention to the size and topography of the defect, the condition of the teeth limiting the defect, and the periodontium, the condition of the edentulous alveolar process, the type of bite, occlusal relationships, the condition and position of the teeth that have lost antagonists. The most important is the condition of the periodontium of the abutment teeth, limiting the defect of the dentition. The stability of the teeth, as a rule, indicates a healthy periodontium. Pathological mobility, on the contrary, is a reflection of deep changes in periodontal tissues, the condition of which requires a particularly careful assessment. At the same time, it should be remembered that stable teeth with signs of periodontal disease in the form of neck exposure, gingivitis, pathological gingival and bone pockets need additional x-ray examination. The same applies to teeth with fillings and carious defects, abrasion of crowns, artificial crowns, discoloration. Diagnostic models are a good tool for assessing occlusal relationships and the position of abutment teeth. Ideal for prosthetics with bridges are teeth with an average height of clinical crowns. With high clinical crowns, the risk of traumatic occlusion in the stage of decompensation increases significantly. With low clinical crowns, it is difficult to design a bridge. In addition, prosthetics with bridges are greatly facilitated with correct occlusal relationships and a healthy periodontium. Equally important is the correct position of the supporting teeth, when their long axes are parallel to each other. With deformations of the dentition, accompanied by an inclination of the supporting teeth that have lost antagonists, the use of bridges is significantly more difficult. As a support, the doctor often has to use teeth that have been treated for caries, pulpitis, chronic apical periodontitis. The latter can serve as a support after thorough filling of all root canals, provided that the clinical course is favorable and there is no history of exacerbation. Past periodontal diseases reduce its reserve forces and reduce the resistance of the periodontium to functional overload. When using bridges, it is large enough and can provoke an exacerbation of inflammation. That is why strict requirements are imposed on the quality of treatment of chronic apical periodontal diseases before prosthetics. When determining the indications for prosthetics with bridges, the question of the number of supporting teeth with a different size of the defect in the dentition is important. An objective assessment of the condition of the periodontium is one of the main prerequisites for orthopedic treatment. It is known that the ability of the periodontal teeth to perceive a particular load can be measured not only with the help of gnathodynamometry, which is characterized by large errors, but also by determining the size of the root surface. Clinical observations show that socket atrophy is not always a reliable indicator of periodontal endurance. It is also necessary to take into account the degree of tooth mobility. Thus, periodontal endurance can be most reliably assessed from three positions: the degree of atrophy of the tooth socket, tooth mobility and the area of their roots. Based on this premise, when deriving the conditional coefficients of periodontal endurance, we considered it expedient to take the area of the root of the lower central incisor as the smallest unit of endurance. Given the dependence of periodontal endurance on the degree of atrophy of the hole while maintaining the stability of the teeth, it is important to establish the magnitude of the decrease in the area of the root, approaching the shape of the cone. To carry out the corresponding calculations, the diameters of the necks and the lengths of the roots of permanent teeth according to V.A. Naumov were taken as the initial data. Comparison of these values with the total area of the roots made it possible to calculate the residual area of the roots of the teeth with atrophy of the hole by 1/4, 1/2, 3/4, as well as to derive the values of periodontal endurance for each degree of atrophy of the hole. Until now, it was believed that the reserve forces of the periodontium decrease in proportion to the atrophy of the hole. This did not take into account the anatomical feature of the roots of the teeth - an almost uniform narrowing from the neck to the tops of the roots. In addition, in accordance with the theory of the bilateral structure of the human body, it was conditionally believed that the periodontium of the teeth was able to withstand a double load, and the calculation of the remaining reserve forces was carried out on the basis that half of the periodontal strength was used when crushing food. This estimate of the reserve forces of the periodontium is inaccurate. Thus, the periodontium of the first permanent molars (37 kg) has the maximum endurance. At the same time, according to Schroeder, chewing boiled meat requires an effort of 39-40 kg. In addition, chewing pressure is expanded in the direction (vertical and lateral) and acts, as a rule, on several adjacent teeth. Its extreme value surpasses the effort required to chew food. When compiling a periodontogram, there is no need to calculate the efforts expended, for example, on biting or chewing food. It is important to assess the state of the periodontium and its reserve forces both in individual teeth and in the dentition as a whole. One of the most significant indicators of the condition of the periodontium is the stability of the teeth. With the appearance of pathological tooth mobility, the reserve forces of the periodontium disappear. Observations in the clinic show that in most patients, progressive atrophy of the sockets is accompanied by the appearance of pathological tooth mobility. But in some cases, for example, with developing primary traumatic occlusion, pathological mobility can occur without noticeable atrophy of the hole, and vice versa - despite the far advanced atrophy of the alveolar process in case of systemic and sluggish periodontal diseases of a dystrophic nature, teeth can remain stable for a long time and participate in chewing food. Thus, the assessment of the periodontal condition should be carried out taking into account the degree of socket atrophy and pathological tooth mobility. As the data of gnatodynamometry show, there is a fairly pronounced difference in the endurance of the periodontal teeth of the upper and lower jaws. Comparison of the area of the roots of the teeth confirms the existence of these differences in a healthy periodontium. Apparently, this can be explained by the peculiarities of the structure of the jaws: the upper jaw is more airy, less adapted to the perception of masticatory pressure, and the lower one is more compact, and more resistant to masticatory pressure. The difference in the areas of the root surfaces, as it were, compensates for these anatomical differences and contributes to a more uniform distribution of masticatory pressure on the jaws. The state of periodontal reserve forces depends on many factors: the shape and number of roots; location of teeth in the dentition; the nature of the occlusion, age, past general and local diseases, etc. In addition, the functional structures of the periodontium are hereditary, so the influence of the hereditary factor on the ability of the periodontium to adapt to the changed functional load cannot be denied. So, periodontal teeth have very limited capabilities, therefore, the assessment of periodontal endurance and the calculation of the number of supporting teeth when planning the design of bridges should be carried out as follows. For example, in the absence of two (first and second) molars of the lower jaw, the sum of the endurance coefficients of a healthy periodontium of supporting teeth (35" and 38") is 4.0 units, and the sum of the coefficients of extracted teeth (36" and 37") is 5.1. Endurance periodontal 38 "conditionally accepted as equivalent to 37". Thus, the abutment teeth are in a state of functional overload, exceeding their endurance by 1.1 units. And this does not really contradict the idea, arising from the theory of traumatic occlusion, that any bridge prosthesis causes functional overload of the periodontium. However, its value may be different. In the above example, the endurance of the abutment teeth is exceeded by 1.1 units. In other cases, this difference can be much larger. So, when removing three teeth in the lateral part of the lower jaw (35,36,37), the sum of the periodontal endurance coefficients of the supporting teeth (34.38) will be 3.8 units, and the removed ones - 6.7. The difference is 2.9, that is, it is less (by 0.9) than the sum of the periodontal endurance coefficients of the abutment teeth. In this case, the functional overload of the periodontium is great, there is a risk of acute traumatic occlusion in the stage of decompensation. As clinical observations show, the difference in the sums of the periodontal endurance coefficients of supporting and extracted teeth should not exceed 1.5 - 2.0 units. As for mobile teeth, devoid of reserve forces, it should be considered that the endurance of their periodontium, regardless of the degree of mobility, is equal to zero. The use of such teeth as abutments without simultaneous splinting with other, stable teeth is contraindicated. A special place in determining the indications is occupied by bridges with one-sided support. The greatest danger to the periodontium of abutment teeth is the use of such structures to replace large molars. At the same time, it should always be borne in mind that when replacing end defects, such a bridge prosthesis can be used in case of contraindications to the use of removable structures or provided that its antagonists are artificial teeth of a removable prosthesis of the opposite jaw. Absolute contraindications for the use of bridges are large defects limited by teeth with different functional orientation of periodontal fibers, relative - defects limited by mobile teeth with low clinical crowns; defects with abutment teeth that have a small reserve of periodontal forces (with high clinical crowns and short roots). 2.1 General features of the manufacture and use of bridges
Porcelain coating can be used not only in the manufacture of single crowns, but also in bridges. Plastic as a facing material for cast prostheses has a number of disadvantages. These, first of all, include the possibility of developing allergic reactions when plastic comes into contact with both soft tissues of the marginal periodontium (gingiva), and adjacent areas of the mucous membrane of the lips, cheeks, tongue and edentulous alveolar process. In addition, the connection of plastic with a metal frame, based on the creation of mechanical retention points, is not very strong. A comparison of the aesthetic qualities of plastic and porcelain testifies to the undeniable advantage of the latter. Thus, the porcelain coating has a number of indisputable advantages that give the prosthesis a special value. When planning ceramic-metal bridges, special attention should be paid to the indications for their use. In doing so, the following circumstances must be borne in mind. Firstly, when planning such prostheses, it is necessary to carefully study the possibility of covering the abutment teeth with metal-ceramic crowns (this issue is discussed in detail in the corresponding chapter). Secondly, a separate problem is the determination of the possibility of porcelain lining the intermediate part of the bridge. To do this, it is necessary to assess the size of the interalveolar space in the area of the defect in the dentition. It should be sufficient to construct artificial metal-ceramic teeth with a beautiful anatomical shape and size. Thirdly, some authors consider medium defects, with a length of 2-3 teeth, when using alloys of noble metals, or medium and large, with a length of 2-4 teeth, when using stainless steel alloys, as an indication for the use of such prostheses. Other authors limit the use of metal-ceramic bridges to small and medium-sized defects with a length of 2-3 teeth. It is believed that an increase in the length of the intermediate part of the bridge can be the cause of minor deformations, leading to porcelain spalling. In addition, the length of the prosthesis is directly proportional to the height of the abutment teeth. Rice. 4 Dental bridge - 3 units However, in this case, one should also be aware of the possible deformation and its consequences. It is also useful to keep in mind the danger of excessive overloading of the periodontium of supporting teeth in case of applying large bridges by the method or using them not according to indications, for example, without increasing the number of supports in case of periodontal diseases. A thorough clinical and radiological assessment of the condition of the periodontium, supplemented by an assessment of its reserve forces, including using a periodontogram, makes it possible to more accurately determine the possibility of prosthetics with a metal-ceramic bridge prosthesis. In addition, it should be borne in mind that this bridge design can be used with equal success to replace defects in both the anterior and posterior dentition. Preparation of teeth is carried out according to known rules, taking into account the way of insertion of the prosthesis and the degree of deformation of the dentition, manifested in the inclination of the supporting teeth. The most accurate result will give a double print. The working model is prepared according to the method of preparing a collapsible gypsum model from high-strength gypsum. Abutment teeth must be covered with temporary crowns to prevent displacement of the prepared teeth towards the antagonists. With the help of temporary bridges, it is possible to protect the abutment teeth from the influence of the external environment and their displacement both in the vertical and in the mesio-distal direction. When planning ceramic lining of abutment crowns, one should take into account the type of bite, the depth of overlap of the anterior teeth, the height of clinical crowns and their vestibulo-oral size. When facing artificial crowns for lateral teeth, in addition, it is necessary to keep in mind the degree of their exposure when smiling or talking. A strip of metal in the form of a garland over the neck of the tooth is left only on the surfaces invisible for a simple examination of the oral cavity - palatine or lingual. However, in each specific case, a detailed plan is drawn up for facing all elements of the bridge prosthesis - the supporting parts and the body. The currently recommended drastic reduction in the area of the veneered surfaces should be carefully coordinated with the patient in order to avoid conflict after prosthetics. The attentive attitude of the doctor to the possible ethical and psychological incompatibility prevents the occurrence of such a situation. Modeling the intermediate part of the bridge is aimed at achieving the best aesthetic effect after prosthetics. As you know, there are two types of intermediate part: with or without flushing space. If in the anterior sections of the jaws the tangent form is most often used, then in the lateral sections the solution may be different. So, when replacing missing premolars and the first molar of the upper jaw and a wide smile, the body of the prosthesis can have a tangent shape. On the lower jaw in the lateral sections, an intermediate part with a flushing space is more often used. However, in some patients, this general scheme may be disturbed due to unusual clinical conditions: anomalies in the development of the jaws and alveolar processes, the height of the supporting or all remaining teeth in the oral cavity, the degree of exposure of the crowns of the teeth and alveolar processes when smiling, the length of the upper and lower lips, cross-sectional shape of the edentulous alveolar process, etc. At the same time, when designing the body of a ceramic-metal bridge, one should strive to maximize the reproduction of the anatomical shape of the lost teeth with occlusal relationships characteristic of each patient. An obstacle to this is often the deformation of the occlusal surface of the dentition. Correcting it before prosthetics allows you to improve the quality of prosthetics and get a high aesthetic effect. Failure to comply with this rule leads to thinning of the metal frame and weakening of the entire structure of the metal-ceramic prosthesis. The shortening of the interalveolar distance is also the reason for the decrease in the height of the artificial pontic teeth. In this case, the surface of the body of the prosthesis, facing the mucous membrane of the alveolar process, may not be covered with porcelain and remain metallic. This modeling allows you to make the frame of the intermediate part thicker, which provides it with the necessary rigidity. When modeling the intermediate part, each tooth should repeat the anatomical shape of the restored one, but be reduced in size by the thickness of a uniform porcelain coating. If a garland (collar) is modeled on the oral side, then it can be a continuation of a similar garland on the supporting crowns. Its dimensions and location are planned in advance when designing the entire prosthesis. Attention should be paid to the need to model the equator and hillocks. The absence of the latter in combination with the low height of the frame of artificial teeth of the body of the prosthesis can be the reason for the chipping of the porcelain coating. The transition of the garland to the rest of the framework, as well as the transition of the framework of the supporting crowns to the intermediate part of the bridge, should be smooth and not have sharp undercuts, sharp edges or protrusions. The successful development of periodontics and modern implantology has led to the development of new methods for preserving the alveolar ridge and surgical replacement of its defects. New methods of soft tissue plasty have influenced the shape of the gingival surface of the intermediate part of the bridge prosthesis (PBMP). Contrary to the traditional requirement to achieve minimal contact without pressure, at present, after plasty, the connection of PPCH is carried out with an oval gingival surface, maintaining direct contact and slight pressure on the underlying soft tissues throughout the entire length. With this design of the body of the bridge prosthesis, very high aesthetic results of treatment can be achieved. If surgical preparation is undesirable or contraindicated, the method of choice for replacing small ridge defects is the use of pink ceramics. The flushing form of the pontic contributes to the maintenance of soft tissues and periodontium in a healthy state with good hygiene of the abutment teeth. However, due to the distance from the alveolar ridge, a space is created where food debris accumulates. Functional, phonetic and aesthetic disadvantages of this design require its use exclusively in the area of the lower posterior teeth. In the absence of a defect in the alveolar ridge, a very good aesthetic result can be achieved with a saddle pontic. However, the extended area of contact with the alveolar ridge prevents the removal of soft plaque. As shown by clinical studies, in 85% of cases, such constructions caused severe inflammation up to ulceration of the mucous membrane. Reducing the contact surface by creating a semi-saddle shape also did not provide a noticeable improvement in hygienic conditions with a concave gingival surface of the bridge body. As already noted, the most common is the tangent form of the PCHMP. The convex gingival surface, in point contact with the alveolar ridge, provides conditions for good hygiene and does not irritate the underlying soft tissues. However, often the individual contour of the alveolar ridge requires compromise solutions in order to prevent aesthetic, functional and phonetic deficiencies. So, in the presence of vertical atrophy of the alveolar ridge, the intermediate part looks unnaturally long and has black triangles due to the absence of gingival papillae. In this case, in addition to aesthetic problems, functional disorders appear due to the ingress of saliva and exhaled air into the vestibule of the oral cavity, as well as the accumulation of food residues. With an oval gingival surface of the PCHMP, extensive contact with soft tissues is provided, imitating the natural transition of an artificial tooth into soft tissues. However, to achieve this effect, appropriate design of soft tissues is necessary. For this purpose, special methods have been developed that involve the design of the intermediate part, tooth extraction in the form of guided regeneration (immediate prosthesis technique), and plastic surgery in combination with orthopedic measures. The contact of the gingival surface of the PPCH with the mucous membrane suggests an increased readiness of the patient for oral hygiene, which should be assessed at the preparatory stage. Careful planning of PPCH is especially necessary for patients with a high smile line. Surgical restoration of limited defects of the alveolar part of the jaw is carried out by various methods. These include guided bone regeneration using membranes, the introduction of autogenous bone, xenogenic or alloplastic materials, and a combination of both. At the same time, the use of resorbable membranes avoids repeated surgical intervention. To restore the defects of the crest of the alveolar part with soft tissues, the following techniques are used: round stalked flap; overlay graft; subepithelial graft or connective tissue and its modifications. Thus, surgical repair of local defects of the alveolar process can be a good help in solving orthopedic problems of prosthetics of dentition defects with bridges. Moreover, these methods can also be combined with implantation if implant-supported bridges are planned. The surface cleanliness of the cast frame largely depends on the accuracy of the gating system. Wax models of sprues and feeders are made of special casting wax (voskolit-2) with a diameter of 2-2.5 mm (for sprues) and 3-3.5 mm (for feeders). Sprues are installed in the most thickened parts of the supporting crowns and artificial teeth of the intermediate part and connect them to a common feeder located along the dental arch. The feeder is connected to the gate cone with the help of additional branches. It is useful to additionally install sprues of a smaller diameter (0.5 I mm) in thin places of the supporting crowns, which remove air. The modeled wax reproduction of the prosthesis is carefully removed from the model and the casting mold is made and the framework is subsequently cast. The cast frame is processed in a sandblaster, freed from sprues and checked on a combined model. After this, the outer surface is treated with abrasive heads, bringing the thickness of the metal caps to 0.2-0.3 mm, and the intermediate part is separated from the antagonists by at least 1.5 mm and not more than 2 mm. Violation of this rule leads to chipping of the ceramic coating. If casting defects are found, the frame must be reworked. An attempt to hide defects with ceramics also leads to the destruction of the latter during the use of the prosthesis. Fitted on the model and prepared for ceramic coating, the framework is transferred to the clinic to check the accuracy of manufacturing. When checking the framework in the oral cavity, one should first of all pay attention to the accuracy of the position of the support caps in relation to the marginal periodontium. The framework of the bridge should be easy to apply and accurately positioned in relation to the neck of the tooth. The criterion for this, as a rule, is the minimum immersion of the edge of the cap into the gingival pocket (no more than 0.5 mm) in areas prepared without a ledge. Where the tooth is prepared with a ledge, the edge of the cap should fit snugly against it. Difficulty applying the framework can be the result of many reasons, the main of which are defects in the working model, deformation of the wax reproduction of the framework, shrinkage of the alloy during casting of the framework, inaccurate coating of the wax framework with the formation of air bubbles (especially on the inner surface of the cutting edge or chewing part of the crown), inaccurate preparation of abutment teeth. Consistently, excluding each of the possible causes, they achieve an accurate establishment of the framework on the supporting teeth. After the framework is applied, the volume of the abutment teeth closed with metal caps and the artificial metal teeth of the intermediate part should be carefully assessed. If the frame occupies the entire volume, including that intended to accommodate the facing ceramic coating, it is necessary, first of all, to carefully evaluate the thickness of the frame in order to identify its possible increase. Another reason for such an error may be insufficient preparation of the abutment teeth. The manufacture of a bridge prosthesis without eliminating the mistakes made will lead to an increase in the volume of artificial teeth and abutment crowns of the prosthesis in comparison with adjacent natural teeth. The prosthesis will stand out among natural teeth and, instead of restoring aesthetics, will lead to its violation. The correction consists in reducing the thickness of the frame of the supporting caps and cast artificial teeth of the intermediate part to the required dimensions; if the thickness of the metal caps meets the requirements, it is necessary to carry out additional preparation of the supporting teeth and remake the framework of the bridge. Occlusal relationships should be evaluated especially carefully when checking the finished framework. General requirements involve the creation of a gap between the antagonists of 1.5-2 mm in the position of central occlusion. In case of lateral and anterior occlusions, one should keep in mind the possibility of premature contacts of the framework with opposing teeth. If found, they must be eliminated. After checking the metal framework, it is useful to re-determine the central relationship of the jaws, since the position of the framework on the abutment teeth is often somewhat different from its position on the working model. For the most accurate formation of the occlusal surface of the ceramic prosthesis, it is necessary to fix exactly the position of the frame that it occupies in the oral cavity. When creating a ceramic coating on a bridge prosthesis, first of all, the technology described earlier, adopted for single crowns, is used. The differences concern mainly the intermediate part. Of particular importance for the aesthetic qualities of the prosthesis are the interdental spaces and the shape of the contact surfaces of adjacent artificial teeth. For their formation, after applying the dentin and enamel layers, separation is carried out with a modeling needle to the opaque layer. For the same purpose, a special varnish separator is used, which is applied to every second tooth. During the subsequent firing, the varnish is applied in reverse order. Particularly carefully in the bridge prosthesis, the cervical part of the artificial teeth adjacent to the mucous membrane of the edentulous alveolar process is modeled. This part of the tooth is of great importance for the overall appearance of the entire prosthesis. We mean, first of all, the shape and size of the cervical part, its overlay in relation to the alveolar process, the depth and width of the interdental spaces, the inclination of the long axis of the artificial tooth. Thus, the occlusal surface of the paws must meet the most stringent requirements and, above all, correspond to the age characteristics of the microrelief in a given individual, provide a full-fledged chewing function and not have premature contacts with opposing teeth. Compliance with all these requirements is checked in the oral cavity. The finished prosthesis is carefully examined, the quality of the ceramic coating and the polishing of the metal garland are assessed. Before applying, it is necessary to carefully examine the inner surface of artificial crowns. When applying dyes or correcting the anatomical shape, ceramic mass can get into the crowns, especially along the inner edge. Parts of it, barely noticeable during examination, can cause inaccurate or difficult application of the prosthesis. With a shaped head of small diameter at low speeds of the drill, the particles of the ceramic mass are ground off. The same is done with the oxide film covering the inner surface of the combined crowns. Only after such preparation, the prosthesis is carefully placed on the supporting teeth. In this case, large efforts should be avoided, as they can cause chipping of the porcelain coating if the prosthesis is not correctly fitted. First of all, we are talking about a possible excess of ceramic mass on the proximal surfaces of abutment crowns, accused of adjacent natural teeth. To detect this deficiency, carbon paper is inserted into the interdental space with a coloring surface to the ceramic veneer, and then a prosthesis is applied. If an imprint is found, it is necessary to grind the ceramic in this place, preventing possible pressure on it when applying the entire prosthesis. The correction of the contact surfaces is repeated until the prosthesis is fully applied with visible contact of the crowns with adjacent teeth. The patient's lack of a feeling of pressure of the prosthesis on the adjacent teeth indicates the accuracy of the correction of the supporting crowns. The final check of the prosthesis consists in clarifying the occlusal relationships for various types of articulation, as well as the shape and color of the artificial teeth. The manufacture of the prosthesis is completed, if necessary, by tinting the ceramic coating and glazing. In the oral cavity, the prosthesis is reinforced with cement. The technique is simple and allows you to speed up the modeling process without condensing the ceramic mass and maintain a constant moisture content of the ceramics. Modeling starts from the vestibular surfaces, imitating the most striking features of the anatomical shape and color of the teeth. The palatal and lingual surfaces of the artificial teeth are then modeled, usually before the first firing. Layer-by-layer modeling should begin with the application of ceramic masses of a denser consistency (opaque masses). Subsequent layers should be less dense, not shifting the first layer. A more liquid consistency is used for incisal masses. The density of the ceramic mass before application can be ensured using a special "liquid N, Ivoclar". In the manufacture of large bridges, it is recommended to follow the following sequence. At the first stage, the anterior teeth are modeled (first firing), at the second stage, the chewing teeth are modeled and the anterior teeth are corrected (second firing), and at the third stage, the chewing teeth are corrected with possibly necessary correction of the anterior teeth (third firing). This sequence allows ceramic layering as the simplest way to speed up modeling, maintain constant ceramic moisture, and avoid ceramic mass condensation. When modeling a multilayer ceramic coating using intensely colored porcelain powders to create depth effects, the following should be taken into account: since the ceramic layer is applied taking into account its subsequent shrinkage during the firing process, a shift in individual color features laid down during the initial application may occur; correction of the anatomical shape by applying additional portions of porcelain can also cause a shift or loss of individual details of the color effect; Condensation of ceramic coating layers can lead to spreading of individual fine details of reproducible features. CONCLUSION
The bridge prosthesis as a therapeutic tool must meet the requirements of toxicology, technology, aesthetics, hygiene and function. The requirements of toxicology are reduced to the use of materials that, having anti-corrosion properties, are at the same time non-toxic, do not cause allergies, do not irritate the oral mucosa, do not combine with saliva and do not change its properties. There are certain requirements for bridge prostheses, primarily related to the rigidity of the structure. In the anterior and lateral sections of the dental arch, the intermediate part is not the same. If in the anterior section it should touch the mucous membrane without pressure on it (tangential form), then in the lateral section between the body of the prosthesis and the mucous membrane covering the edentulous alveolar process, there should be free space that does not prevent the passage of chewed foods (wash space). LIST OF USED LITERATURE
1. Abakarov S.I. /Modern designs of fixed dentures - St. Petersburg Folio - 2000. - 105p. Alabin I.V., Mitrofanenko V.P. / Anatomy, physiology and biomechanics of the dentition. - M., 2002. - 241s. Budylina S.M., Degtyareva V.P. /Physiology of the maxillofacial region. - 2000. - 352p. Voronov A.P., Lebedenko I.Yu. /Orthopedic dentistry. - M.: Medicine, 1997 - 210s. Mironova M.L. /Removable prostheses: a textbook for honey. colleges and schools. - GEOTAR-media, 2009. - 456s. Kopeikin V.N., Mirgazizov M.Z. /Orthopedic dentistry. - M.: Medicine, 2001. Kopeikin V.N., Dolbnev I.B., / Dental technology. - M.: Medicine, 1997. - 178s. Kurlyandsky V.Yu. /Ceramic and solid non-removable dentures. - M.: Medicine, 1998 - 100s. Pogodin V.S., Ponomareva V.A. / Guide for dental technicians - M.: Medicine, 2001. - 127p. Savchenkov Yu.I., Pats Yu.S. /Physiology for the dentist: a textbook. - 2000. - 90s. Handbook of dentistry / Ed. V.M. Bezrukov. - M.: Medicine, 1998. http://moodle.agmu.ru http://kbsu.ru http://lib.znate.ru http://dentaltechnic.info
Clinical rationale for the use of bridges:
. Prosthetics with bridges allows you to restore up to 85-100% of chewing efficiency. In this case, chewing pressure is transmitted to the supporting teeth and is regulated by the periodontal-muscular reflex (natural).
Relying on teeth that are borderline with a defect, the bridge-like prosthesis performs the function of extracted teeth and, thus, transfers an increased functional load to the supporting teeth. Only a prosthesis with sufficient strength can resist it. From the point of view of hygiene, special requirements are imposed on bridge prostheses.
Here, the shape of the intermediate part of the prosthesis and its relation to the surrounding tissues of the prosthetic bed of the mucous membrane of the alveolar process, the gums of the supporting teeth, the mucous membrane of the lips, cheeks, and tongue are of great importance. Similar works to - Biomechanics of fixed bridge prostheses
Ministry of Health of the Russian Federation
Federal State Budgetary Educational Institution of Higher Education
Department of Prosthetic Dentistry and Materials Science with a Course in Adult Orthodontics
abstract
On the topic:
"Prosthetics with ceramic-metal bridges"
St. Petersburg, 2017
Introduction…………………………………………………………………………….3
1. Theoretical foundations of prosthetics with ceramic-metal bridges:
1.1 General characteristics of bridges…………………………….5
1.2 Biomechanics of bridges……………………………………….
1.3 Basic principles for the design of bridges………….13
2. Practical features of prosthetics with ceramic-metal bridges:
2.1 Indications for prosthetics with bridges…………………
2.2 General features of manufacture and use…………………………..
Conclusion………………………………………………………………………..
INTRODUCTION
Porcelain-fused-to-metal restorations are based on the principle of combining the strength and precision of a cast metal framework with the aesthetics of porcelain, allowing them to come close to, and in some cases surpass, natural teeth.
A porcelain-fused-to-metal prosthesis consists of a cast metal part, or framework, that fits the prepared tooth exactly, and ceramics bonded to it. The frame may be slightly larger than a thin thimble, or it may distinctly resemble a cast crown with some of the metal removed. Broken contours are restored with porcelain, which will hide or mask the metal frame, reproduce the desired shape and color, and make the prosthesis very similar to natural teeth. The metal frame in the ceramic-metal prosthesis is covered with three main layers of porcelain.
In this essay, I will consider ceramic-metal bridges.
1. THEORETICAL FOUNDATIONS OF PROSTHETICS WITH CERAMIC BRIDGES.
1.1 GENERAL CHARACTERISTICS OF BRIDGES
Bridges are understood as structures that rely on teeth that limit the defect of the dentition. This is the most ancient type of prostheses, which is confirmed by findings during excavations of ancient monuments and tombs. The birthplace of modern bridges is considered to be the United States of America, where they received the greatest development and distribution in the second half of the last century.
The bridge prosthesis, based on natural teeth, transmits masticatory pressure to the periodontium. Most often, bridges rest on the teeth located on both sides of the defect, that is, they have a bilateral support. In addition, unilaterally supported bridges can be used. In this case, as a rule, the abutment tooth in relation to the defect is located distally. For example, in the absence of a maxillary lateral incisor, the canine should be used for support rather than the central incisor. Bridges with one-sided support are most often used for the loss of individual front teeth.
To support bridges, artificial crowns (stamped, cast, combined, semi-crowns, crowns on an artificial stump with a font) or inlays are used. In addition to the supporting elements, the design of bridge prostheses includes an intermediate part located in the area of the defect in the dentition.
According to the manufacturing method, bridges are divided into brazed, the parts of which are connected by soldering, and solid, having a solid frame. In addition, the bridge prosthesis can be entirely made of metal (all-metal), plastic, porcelain, or through a combination of these materials (combined - metal-plastic, metal-ceramic).
For the manufacture of bridges, chromium-nickel, cobalt-chromium, silver-palladium alloys, 900-carat gold, acrylic plastics and porcelain are used.
The disadvantage of brazed bridges is the presence of solder, which consists of metals that cause intolerance in some patients - zinc, copper, bismuth, cadmium. One-piece cast bridge prostheses are free from this drawback.
There are certain requirements for bridge prostheses, primarily related to the rigidity of the structure. Relying on teeth that are borderline with a defect, the bridge-like prosthesis performs the function of extracted teeth and, thus, transfers an increased functional load to the supporting teeth. Only a prosthesis with sufficient strength can resist it.
Equally important are the aesthetic qualities of bridges. Increasingly, there are patients who do not want to have metal parts of the prosthesis visible when smiling or talking. Metal-ceramic structures are considered the best in this respect.
In terms of hygiene, bridges are subject to special requirements. Here, the shape of the intermediate part of the prosthesis and its relation to the surrounding tissues of the prosthetic bed of the mucous membrane of the alveolar process, the gums of the supporting teeth, the mucous membrane of the lips, cheeks, and tongue are of great importance. In the anterior and lateral sections of the dental arch, the intermediate part is not the same. If in the anterior section it should touch the mucous membrane without pressure on it (tangential form), then in the lateral section between the body of the prosthesis and the mucous membrane covering the edentulous alveolar process, there should be free space that does not prevent the passage of chewed foods (wash space).
Forms of the intermediate part of the bridge prosthesis:
1 - tangent for front teeth
2 - hanging with high clinical crowns of teeth
3 - hanging with low clinical crowns of teeth
4 - saddle all-metal
5.6 - hanging with lining of the labial or labial-chewing surface
7 - saddle-shaped with lining of visible surfaces - chewing and partially lateral artificial teeth of the lower jaw.
With a tangent form, the absence of pressure on the mucous membrane is checked with a probe. If its tip is easily inserted under the body of the prosthesis, then there is no pressure on the gums, and at the same time there is no visible gap that does not look aesthetically pleasing when smiling or talking.
In the lateral part of the dentition, creating a flushing space, they try to avoid food retention under the intermediate part of the prosthesis, which can cause chronic inflammation of this area of the mucous membrane. That is why the wash space is made quite large, especially in the lower jaw. In the upper jaw, taking into account the degree of exposure of the lateral teeth when smiling, the flushing space is made slightly smaller than in the lower jaw, and in the area of premolars and canines that open when smiling, it can be minimized, up to touching the mucous membrane. In each case, this issue is resolved individually.
In cross section, the shape of the intermediate part of the prosthesis resembles a triangle. In recent years, in connection with the introduction of highly aesthetic metal-ceramic structures, there has appeared a supporter of using a saddle-shaped prosthesis body in them.
1.2 BRIDGE BIOMECHANICS
The nature of the distribution and the magnitude of the masticatory pressure falling on the body of the bridge and transmitted to the supporting teeth depend primarily on the place of application and direction of the load, the length and width of the body of the prosthesis. It is obvious that for living organs and human tissues the laws of mechanics are not absolute. For example, the state of periodontal tissues depends on the general state of the body, age, local state of the surrounding organs and tissues, the activity of the nervous system, and many other factors that determine the reactivity of the body as a whole. However, it is important for the clinician to know not only the reaction of the periodontium to the functional overload of the abutment teeth bearing bridges, but also the distribution of elastic stresses both in the bridge itself and in the periodontal tissues of the abutment teeth.
If the functional load falls on the middle of the intermediate part of the bridge, then the entire structure and periodontal tissues are loaded evenly and, therefore, find themselves in the most favorable conditions.
However, such conditions in the process of chewing food are extremely rare. At the same time, it should be borne in mind that with an increase in the length of the intermediate part or insufficiently pronounced elastic properties of the alloy, the body of the prosthesis can bend down and cause additional functional overload in the form of a counter or convergent inclination of the supporting teeth.
In this regard, functional overload is unevenly distributed in periodontal tissues, contributing to the development of a local dystrophic process. Thus, in order to prevent possible changes in the periodontium of supporting teeth under bridges, the body of the prosthesis must have sufficient thickness and not exceed the maximum length, which excludes metal deflection in the area of the dentition defect.
When a chewing load is applied to one of the abutment teeth, both supports are displaced along a circle, the center of which is the opposite, less loaded abutment tooth. This explains the tendency of the abutment teeth to diverge or diverge. Under these conditions, functional overload is also distributed unevenly in periodontal tissues.
If bridges are used with a pronounced sagittal occlusal curve or with a significant deformation of the occlusal surface of the dentition, for example, against the background of partial loss of teeth, part of the vertical load is transformed into a horizontal one. The latter displaces the prosthesis sagittally, causing the abutment teeth to tilt in the same direction.
Similar conditions arise when mobile teeth are used as one of the supports. However, in this case, the displacement of the prosthesis can reach critical values, aggravating the pathological condition of the periodontium.
Very dangerous for the periodontium are vertical loads falling on the body of a bridge with one-sided support. In this case, the functional load causes the abutment tooth to tilt towards the missing adjacent one. In periodontal tissues, there is also an uneven distribution of elastic stresses. In magnitude, these conditions are significantly superior to those that develop in bridges with bilateral support. Under the influence of a vertical load falling on the body of such a prosthesis, a bending moment occurs. The supporting tooth leans towards the defect, and the periodontium experiences a functional overload of an unusual direction and size. The result may be the formation of a pathological pocket on the side of tooth movement and resorption of the hole at the root apex on the opposite side.
With lateral movements of the lower jaw during chewing, rotation of the abutment tooth occurs - a torque that aggravates the functional overload of the periodontium. The moments of torsion and bending are determined by the length of the body of the bridge, the height of the clinical crown of the abutment tooth, the length of the edge, the presence or absence of adjacent teeth, the magnitude of the applied force and the state of the periodontal reserve forces. The probability of developing functional overload in the stage of decompensation can be significantly reduced by increasing the number and using a bridge with a unilateral support in the case of included defects with a length of no more than one tooth.
When using an artificial tooth with a one-sided support in the form of two supporting teeth, there is a predominant immersion in the alveolus of the supporting tooth adjacent to the artificial one. The other supporting tooth is under the influence of pulling forces. Thus, there is a kind of rotation of the prosthesis around the center located in the supporting tooth, which carries the artificial tooth. In this case, the difference in squeezing and stretching of periodontal tissues reaches quite large values and can also adversely affect the supporting tissues.
The distribution of horizontal forces has distinctive features. Intact dentitions are the most resistant to horizontal loads. This is due to the anatomical structure of the teeth and their roots, the position of the teeth on the alveolar process, the relationship of the dentition with various types of articulation, as well as the structural features of the upper and lower jaws. With the loss of teeth, the conditions for the distribution of vertical loads change. So, with a horizontal load applied to the middle part of the body of the bridge, the abutment teeth experience uniform pressure and transfer the load to the periodontium from the side opposite to the application of the force of the alveolar wall.
If pressure is applied to one of the abutment teeth, especially with its pathological mobility, this tooth is displaced along a circle, the center of which is another abutment tooth with an unaffected periodontium. The latter is thus subjected to rotation around the longitudinal axis.
1.3 BASIC PRINCIPLES FOR THE DESIGN OF BRIDGES
When designing bridges, certain principles should be followed. According to the first principle, the supporting elements of the bridge and its intermediate part must be on the same line. The curvilinear shape of the intermediate part of the bridge leads to the transformation of vertical and horizontal loads in rotation.
The load is applied to the most protruding part of the body of the bridge. If we draw a perpendicular to the straight line, connecting the long axes of the abutment teeth, from the point of the body of the prosthesis that is farthest from it, then it will be the arm of the lever that rotates the prosthesis under the action of chewing load. The amount of rotational force is thus directly dependent on the curvature of the body of the bridge. Reducing the curvature of the intermediate part will help reduce the rotational action of the transformed masticatory load.
The second principle is that when constructing a bridge, abutment teeth with a not very high clinical crown should be used. The magnitude of the horizontal load is directly proportional to the height of the clinical crown of the supporting tooth. The use of abutment teeth with high clinical crowns and shortened roots is especially harmful for the periodontium.
In this case, there is a high probability of a rapid transition of the compensated form of functional overload into a decompensated one with the appearance of pathological mobility of the supporting teeth.
Similar conditions also arise with atrophy of the alveolar process, when the height of the clinical crown of the tooth increases due to the reduction of the intra-alveolar part of the root. At the same time, it should be borne in mind that with excessively low clinical crowns, the design of a bridge prosthesis is also difficult due to a decrease in rigidity and a decrease in the area of adherence of the body to the supporting elements. Especially often the connection is destroyed in complete bridges.
The third principle suggests that the width of the chewing surface of the bridge should be less than the width of the chewing surface of the replaced teeth. Since any bridge prosthesis functions due to the reserve forces of the periodontium of the abutment teeth, the narrowed chewing surfaces of the body reduce the load on the abutment teeth.
Moreover, when designing the body of the prosthesis, it is advisable to take into account the presence of antagonistic teeth and their appearance - whether they are natural or artificial. If the pressure is concentrated closer to one of the supporting ones due to the loss of part of the antagonists, then the body of the prosthesis in this place may be narrower than in other areas. Thus, in order to avoid excessive functional overload, the chewing surface of the body of the bridge is made narrower, and the amount of narrowing in certain areas is determined individually in accordance with the characteristics of the clinical picture. An increase in the width of the chewing surfaces of the intermediate part of the bridge leads to an increase in the functional overload of the supporting teeth, not only due to an increase in the total area that receives chewing pressure, but also due to the appearance of rotational forces along the edge of the body of the prosthesis, which extends beyond the width of the supporting teeth.
The fourth principle is based on the fact that the magnitude of chewing pressure is inversely proportional to the distance from the point of its application to the supporting tooth. Thus, the closer the load is applied to the abutment, the more pressure falls on this abutment and, conversely, as the distance from the place of application of the load to the abutment tooth increases, the pressure on this abutment decreases. A completely opposite pattern is found when constructing bridges with one-sided support. The larger the size of the suspended artificial tooth, the more the adjacent abutment tooth is loaded.
To reduce the functional overload of the abutment teeth, it is necessary to increase their number, avoid the use of bridges with one-sided support, and reduce the width of the chewing surface of the prosthesis body.
The fifth principle is associated with the need to restore the contact points between the supporting elements of the bridge and adjacent natural teeth. This allows you to restore the continuity of the dental arch and contributes to a more even distribution of chewing pressure, especially its horizontal component, among the remaining teeth in the oral cavity. It is especially important to observe this principle with a well-defined sagittal occlusal curve, when horizontal loads transformed from vertical ones tend to tilt the abutment teeth in the mesial direction. A properly restored contact point will transfer some of the horizontal force to adjacent natural teeth. This helps to keep the abutment teeth stable and prevents them from tilting mesially.
The sixth principle provides for the competent design of bridges in terms of normal occlusion. There are two groups of patients. The first includes patients whose task of prosthetics is to restore the correct occlusal relationships in the defect area with careful modeling of the occlusal surface of the bridge prosthesis that fits into the patient's functional occlusion. Here, first of all, care should be taken to prevent premature contacts, reduce the interalveolar distance and functional overload of the periodontium after prosthetics.
In the second group, we include patients who need not only the prosthesis of a defect in the dentition with a bridge, but also a simultaneous change in functional occlusion within the entire dentition. This may be necessary in case of partial loss of teeth, increased abrasion, periodontal diseases, anomalies of occlusion, complicated by partial loss of teeth, etc. Common to all these pathological conditions is a decrease in the interalveolar distance. Thus, for the second group of patients, more complex prosthetics are required, taking into account changes in the occlusion of dentures.
Seventh principle: it is necessary to design such bridges that would meet the requirements of aesthetics to the maximum extent. To do this, the most aesthetically advantageous facing materials are used, as well as the supporting elements and the intermediate part of the prosthesis are designed to ensure reliable fastening of the lining made of plastic, porcelain or composite material.
CHAPTER 2 PRACTICAL FEATURES OF PROSTHETICS WITH CERAMIC BRIDGES
2.1 INDICATIONS FOR BRIDGES
When determining the indications for prosthetics with bridges, it should be borne in mind, first of all, the extent of the defect in the dentition - these can be small and medium defects and less often end ones. A special role is played by the requirements for abutment teeth. The planning of a bridge prosthesis becomes only after a thorough clinical and paraclinical study: it is necessary to pay attention to the size and topography of the defect, the condition of the teeth limiting the defect, and the periodontium, the condition of the edentulous alveolar process, the type of bite, occlusal relationships, the condition and position of the teeth that have lost antagonists.
The most important is the condition of the periodontium of the abutment teeth, limiting the defect of the dentition. The stability of the teeth, as a rule, indicates a healthy periodontium. Pathological mobility, on the contrary, is a reflection of deep changes in periodontal tissues, the condition of which requires a particularly careful assessment. At the same time, it should be remembered that stable teeth with signs of periodontal disease in the form of neck exposure, gingivitis, pathological gingival and bone pockets need additional x-ray examination. The same applies to teeth with fillings and carious defects, abrasion of crowns, artificial crowns, discoloration.
Diagnostic models are a good tool for assessing occlusal relationships and the position of abutment teeth.
Ideal for prosthetics with bridges are teeth with an average height of clinical crowns. With high clinical crowns, the risk of traumatic occlusion in the stage of decompensation increases significantly. With low clinical crowns, it is difficult to design a bridge.
In addition, prosthetics with bridges are greatly facilitated with correct occlusal relationships and a healthy periodontium. Equally important is the correct position of the supporting teeth, when their long axes are parallel to each other. With deformations of the dentition, accompanied by an inclination of the supporting teeth that have lost antagonists, the use of bridges is significantly more difficult.
As a support, the doctor often has to use teeth that have been treated for caries, pulpitis, chronic apical periodontitis. The latter can serve as a support after thorough filling of all root canals, provided that the clinical course is favorable and there is no history of exacerbation. Past periodontal diseases reduce its reserve forces and reduce the resistance of the periodontium to functional overload. When using bridges, it is large enough and can provoke an exacerbation of inflammation. That is why strict requirements are imposed on the quality of treatment of chronic apical periodontal diseases before prosthetics.
When determining the indications for prosthetics with bridges, the question of the number of supporting teeth with a different size of the defect in the dentition is important. An objective assessment of the condition of the periodontium is one of the main prerequisites for orthopedic treatment.
It is known that the ability of the periodontal teeth to perceive a particular load can be measured not only with the help of gnathodynamometry, which is characterized by large errors, but also by determining the size of the root surface.
Clinical observations show that socket atrophy is not always a reliable indicator of periodontal endurance. It is also necessary to take into account the degree of tooth mobility. Thus, periodontal endurance can be most reliably assessed from three positions: the degree of atrophy of the tooth socket, tooth mobility and the area of their roots.
Based on this premise, when deriving the conditional coefficients of periodontal endurance, we considered it expedient to take the area of the root of the lower central incisor as the smallest unit of endurance.
Given the dependence of periodontal endurance on the degree of atrophy of the hole while maintaining the stability of the teeth, it is important to establish the magnitude of the decrease in the area of the root, approaching the shape of the cone. To carry out the corresponding calculations, the diameters of the necks and the lengths of the roots of permanent teeth according to V.A. Naumov were taken as the initial data. Comparison of these values with the total area of the roots made it possible to calculate the residual area of the roots of the teeth with atrophy of the hole by 1/4, 1/2, 3/4, as well as to derive the values of periodontal endurance for each degree of atrophy of the hole.
Until now, it was believed that the reserve forces of the periodontium decrease in proportion to the atrophy of the hole. This did not take into account the anatomical feature of the roots of the teeth - an almost uniform narrowing from the neck to the tops of the roots. In addition, in accordance with the theory of the bilateral structure of the human body, it was conditionally believed that the periodontium of the teeth was able to withstand a double load, and the calculation of the remaining reserve forces was carried out on the basis that half of the periodontal strength was used when crushing food. This estimate of the reserve forces of the periodontium is inaccurate. Thus, the periodontium of the first permanent molars (37 kg) has the maximum endurance. At the same time, according to Schroeder, chewing boiled meat requires an effort of 39-40 kg. In addition, chewing pressure is expanded in the direction (vertical and lateral) and acts, as a rule, on several adjacent teeth. Its extreme value surpasses the effort required to chew food. When compiling a periodontogram, there is no need to calculate the efforts expended, for example, on biting or chewing food. It is important to assess the state of the periodontium and its reserve forces both in individual teeth and in the dentition as a whole.
One of the most significant indicators of the condition of the periodontium is the stability of the teeth. With the appearance of pathological tooth mobility, the reserve forces of the periodontium disappear. Observations in the clinic show that in most patients, progressive atrophy of the sockets is accompanied by the appearance of pathological tooth mobility. But in some cases, for example, with developing primary traumatic occlusion, pathological mobility can occur without noticeable atrophy of the hole, and vice versa - despite the far advanced atrophy of the alveolar process in case of systemic and sluggish periodontal diseases of a dystrophic nature, teeth can remain stable for a long time and participate in chewing food. Thus, the assessment of the periodontal condition should be carried out taking into account the degree of socket atrophy and pathological tooth mobility.
As the data of gnatodynamometry show, there is a fairly pronounced difference in the endurance of the periodontal teeth of the upper and lower jaws. Comparison of the area of the roots of the teeth confirms the existence of these differences in a healthy periodontium. Apparently, this can be explained by the peculiarities of the structure of the jaws: the upper jaw is more airy, less adapted to the perception of masticatory pressure, and the lower one is more compact, and more resistant to masticatory pressure. The difference in the areas of the root surfaces, as it were, compensates for these anatomical differences and contributes to a more uniform distribution of masticatory pressure on the jaws.
The state of periodontal reserve forces depends on many factors: the shape and number of roots; location of teeth in the dentition; the nature of the occlusion, age, past general and local diseases, etc. In addition, the functional structures of the periodontium are hereditary, so the influence of the hereditary factor on the ability of the periodontium to adapt to the changed functional load cannot be denied.
So, periodontal teeth have very limited capabilities, therefore, the assessment of periodontal endurance and the calculation of the number of supporting teeth when planning the design of bridges should be carried out as follows.
For example, in the absence of two (first and second) molars of the lower jaw, the sum of the endurance coefficients of a healthy periodontium of supporting teeth (35" and 38") is 4.0 units, and the sum of the coefficients of extracted teeth (36" and 37") is 5.1. Endurance periodontal 38 "conditionally accepted as equivalent to 37". Thus, the abutment teeth are in a state of functional overload, exceeding their endurance by 1.1 units. And this does not really contradict the idea, arising from the theory of traumatic occlusion, that any bridge prosthesis causes functional overload of the periodontium. However, its value may be different. In the above example, the endurance of the abutment teeth is exceeded by 1.1 units. In other cases, this difference can be much larger. So, when removing three teeth in the lateral part of the lower jaw (35,36,37), the sum of the periodontal endurance coefficients of the supporting teeth (34.38) will be 3.8 units, and the removed ones - 6.7. The difference is 2.9, that is, it is less (by 0.9) than the sum of the periodontal endurance coefficients of the abutment teeth. In this case, the functional overload of the periodontium is great, there is a risk of acute traumatic occlusion in the stage of decompensation. As clinical observations show, the difference in the sums of the periodontal endurance coefficients of supporting and extracted teeth should not exceed 1.5 - 2.0 units. As for mobile teeth, devoid of reserve forces, it should be considered that the endurance of their periodontium, regardless of the degree of mobility, is equal to zero. The use of such teeth as abutments without simultaneous splinting with other, stable teeth is contraindicated.
A special place in determining the indications is occupied by bridges with one-sided support. The greatest danger to the periodontium of abutment teeth is the use of such structures to replace large molars. At the same time, it should always be borne in mind that when replacing end defects, such a bridge prosthesis can be used in case of contraindications to the use of removable structures or provided that its antagonists are artificial teeth of a removable prosthesis of the opposite jaw.
Absolute contraindications for the use of bridges are large defects limited by teeth with different functional orientation of periodontal fibers, relative - defects limited by mobile teeth with low clinical crowns; defects with abutment teeth that have a small reserve of periodontal forces (with high clinical crowns and short roots).
2.2 GENERAL FEATURES OF MANUFACTURING AND APPLICATION
Porcelain coating can be used not only in the manufacture of single crowns, but also in bridges. Plastic as a facing material for cast prostheses has a number of disadvantages. These, first of all, include the possibility of developing allergic reactions when plastic comes into contact with both soft tissues of the marginal periodontium (gingiva), and adjacent areas of the mucous membrane of the lips, cheeks, tongue and edentulous alveolar process. In addition, the connection of plastic with a metal frame, based on the creation of mechanical retention points, is not very strong. A comparison of the aesthetic qualities of plastic and porcelain testifies to the undeniable advantage of the latter. Thus, the porcelain coating has a number of indisputable advantages that give the prosthesis a special value.
When planning ceramic-metal bridges, special attention should be paid to the indications for their use. In doing so, the following circumstances must be borne in mind. Firstly, when planning such prostheses, it is necessary to carefully study the possibility of covering the abutment teeth with metal-ceramic crowns (this issue is discussed in detail in the corresponding chapter). Secondly, a separate problem is the determination of the possibility of porcelain lining the intermediate part of the bridge. To do this, it is necessary to assess the size of the interalveolar space in the area of the defect in the dentition. It should be sufficient to construct artificial metal-ceramic teeth with a beautiful anatomical shape and size. Thirdly, some authors consider medium defects, with a length of 2-3 teeth, when using alloys of noble metals, or medium and large, with a length of 2-4 teeth, when using stainless steel alloys, as an indication for the use of such prostheses.
Other authors limit the use of metal-ceramic bridges to small and medium-sized defects with a length of 2-3 teeth. It is believed that an increase in the length of the intermediate part of the bridge can be the cause of minor deformations, leading to porcelain spalling. In addition, the length of the prosthesis is directly proportional to the height of the abutment teeth.
However, in this case, one should also be aware of the possible deformation and its consequences. It is also useful to keep in mind the danger of excessive overloading of the periodontium of supporting teeth in case of applying large bridges by the method or using them not according to indications, for example, without increasing the number of supports in case of periodontal diseases. A thorough clinical and radiological assessment of the condition of the periodontium, supplemented by an assessment of its reserve forces, including using a periodontogram, makes it possible to more accurately determine the possibility of prosthetics with a metal-ceramic bridge prosthesis. In addition, it should be borne in mind that this bridge design can be used with equal success to replace defects in both the anterior and posterior dentition.
Preparation of teeth is carried out according to known rules, taking into account the way of insertion of the prosthesis and the degree of deformation of the dentition, manifested in the inclination of the supporting teeth. The most accurate result will give a double print. The working model is prepared according to the method of preparing a collapsible gypsum model from high-strength gypsum. Abutment teeth must be covered with temporary crowns to prevent displacement of the prepared teeth towards the antagonists. With the help of temporary bridges, it is possible to protect the abutment teeth from the influence of the external environment and their displacement both in the vertical and in the mesio-distal direction.
When planning ceramic lining of abutment crowns, one should take into account the type of bite, the depth of overlap of the anterior teeth, the height of clinical crowns and their vestibulo-oral size. When facing artificial crowns for lateral teeth, in addition, it is necessary to keep in mind the degree of their exposure when smiling or talking. A strip of metal in the form of a garland over the neck of the tooth is left only on the surfaces invisible for a simple examination of the oral cavity - palatine or lingual. However, in each specific case, a detailed plan is drawn up for facing all elements of the bridge prosthesis - the supporting parts and the body. The currently recommended drastic reduction in the area of the veneered surfaces should be carefully coordinated with the patient in order to avoid conflict after prosthetics. The attentive attitude of the doctor to the possible ethical and psychological incompatibility prevents the occurrence of such a situation.
Modeling the intermediate part of the bridge is aimed at achieving the best aesthetic effect after prosthetics. As you know, there are two types of intermediate part: with or without flushing space. If in the anterior sections of the jaws the tangent form is most often used, then in the lateral sections the solution may be different. So, when replacing missing premolars and the first molar of the upper jaw and a wide smile, the body of the prosthesis can have a tangent shape. On the lower jaw in the lateral sections, an intermediate part with a flushing space is more often used. However, in some patients, this general scheme may be disturbed due to unusual clinical conditions: anomalies in the development of the jaws and alveolar processes, the height of the supporting or all remaining teeth in the oral cavity, the degree of exposure of the crowns of the teeth and alveolar processes when smiling, the length of the upper and lower lips, cross-sectional shape of the edentulous alveolar process, etc. At the same time, when designing the body of a ceramic-metal bridge, one should strive to maximize the reproduction of the anatomical shape of the lost teeth with occlusal relationships characteristic of each patient.
An obstacle to this is often the deformation of the occlusal surface of the dentition. Correcting it before prosthetics allows you to improve the quality of prosthetics and get a high aesthetic effect. Failure to comply with this rule leads to thinning of the metal frame and weakening of the entire structure of the metal-ceramic prosthesis. The shortening of the interalveolar distance is also the reason for the decrease in the height of the artificial pontic teeth. In this case, the surface of the body of the prosthesis, facing the mucous membrane of the alveolar process, may not be covered with porcelain and remain metallic. This modeling allows you to make the frame of the intermediate part thicker, which provides it with the necessary rigidity.
When modeling the intermediate part, each tooth should repeat the anatomical shape of the restored one, but be reduced in size by the thickness of a uniform porcelain coating. If a garland (collar) is modeled on the oral side, then it can be a continuation of a similar garland on the supporting crowns. Its dimensions and location are planned in advance when designing the entire prosthesis. Attention should be paid to the need to model the equator and hillocks. The absence of the latter in combination with the low height of the frame of artificial teeth of the body of the prosthesis can be the reason for the chipping of the porcelain coating. The transition of the garland to the rest of the framework, as well as the transition of the framework of the supporting crowns to the intermediate part of the bridge, should be smooth and not have sharp undercuts, sharp edges or protrusions.
The successful development of periodontics and modern implantology has led to the development of new methods for preserving the alveolar ridge and surgical replacement of its defects. New methods of soft tissue plasty have influenced the shape of the gingival surface of the intermediate part of the bridge prosthesis (PBMP).
Contrary to the traditional requirement to achieve minimal contact without pressure, at present, after plasty, the connection of PPCH is carried out with an oval gingival surface, maintaining direct contact and slight pressure on the underlying soft tissues throughout the entire length. With this design of the body of the bridge prosthesis, very high aesthetic results of treatment can be achieved.
If surgical preparation is undesirable or contraindicated, the method of choice for replacing small ridge defects is the use of pink ceramics.
The flushing form of the pontic contributes to the maintenance of soft tissues and periodontium in a healthy state with good hygiene of the abutment teeth. However, due to the distance from the alveolar ridge, a space is created where food debris accumulates. Functional, phonetic and aesthetic disadvantages of this design require its use exclusively in the area of the lower posterior teeth.
In the absence of a defect in the alveolar ridge, a very good aesthetic result can be achieved with a saddle pontic. However, the extended area of contact with the alveolar ridge prevents the removal of soft plaque. As shown by clinical studies, in 85% of cases, such constructions caused severe inflammation up to ulceration of the mucous membrane. Reducing the contact surface by creating a semi-saddle shape also did not provide a noticeable improvement in hygienic conditions with a concave gingival surface of the bridge body.
As already noted, the most common is the tangent form of the PCHMP. The convex gingival surface, in point contact with the alveolar ridge, provides conditions for good hygiene and does not irritate the underlying soft tissues. However, often the individual contour of the alveolar ridge requires compromise solutions in order to prevent aesthetic, functional and phonetic deficiencies. So, in the presence of vertical atrophy of the alveolar ridge, the intermediate part looks unnaturally long and has black triangles due to the absence of gingival papillae. In this case, in addition to aesthetic problems, functional disorders appear due to the ingress of saliva and exhaled air into the vestibule of the oral cavity, as well as the accumulation of food residues.
With an oval gingival surface of the PCHMP, extensive contact with soft tissues is provided, imitating the natural transition of an artificial tooth into soft tissues. However, to achieve this effect, appropriate design of soft tissues is necessary. For this purpose, special methods have been developed that involve the design of the intermediate part, tooth extraction in the form of guided regeneration (immediate prosthesis technique), and plastic surgery in combination with orthopedic measures. The contact of the gingival surface of the PPCH with the mucous membrane suggests an increased readiness of the patient for oral hygiene, which should be assessed at the preparatory stage. Careful planning of PPCH is especially necessary for patients with a high smile line.
Surgical restoration of limited defects of the alveolar part of the jaw is carried out by various methods. These include guided bone regeneration using membranes, the introduction of autogenous bone, xenogenic or alloplastic materials, and a combination of both. At the same time, the use of resorbable membranes avoids repeated surgical intervention. To restore the defects of the crest of the alveolar part with soft tissues, the following techniques are used: round stalked flap; overlay graft; subepithelial graft or connective tissue and its modifications.
Thus, surgical repair of local defects of the alveolar process can be a good help in solving orthopedic problems of prosthetics of dentition defects with bridges. Moreover, these methods can also be combined with implantation if implant-supported bridges are planned.
The surface cleanliness of the cast frame largely depends on the accuracy of the gating system. Wax models of sprues and feeders are made of special casting wax (voskolit-2) with a diameter of 2-2.5 mm (for sprues) and 3-3.5 mm (for feeders). Sprues are installed in the most thickened parts of the supporting crowns and artificial teeth of the intermediate part and connect them to a common feeder located along the dental arch.
The feeder is connected to the gate cone with the help of additional branches. It is useful to additionally install sprues of a smaller diameter (0.5 I mm) in thin places of the supporting crowns, which remove air. The modeled wax reproduction of the prosthesis is carefully removed from the model and the casting mold is made and the framework is subsequently cast.
The cast frame is processed in a sandblaster, freed from sprues and checked on a combined model. After this, the outer surface is treated with abrasive heads, bringing the thickness of the metal caps to 0.2-0.3 mm, and the intermediate part is separated from the antagonists by at least 1.5 mm and not more than 2 mm. Violation of this rule leads to chipping of the ceramic coating. If casting defects are found, the frame must be reworked. An attempt to hide defects with ceramics also leads to the destruction of the latter during the use of the prosthesis. Fitted on the model and prepared for ceramic coating, the framework is transferred to the clinic to check the accuracy of manufacturing.
When checking the framework in the oral cavity, one should first of all pay attention to the accuracy of the position of the support caps in relation to the marginal periodontium. The framework of the bridge should be easy to apply and accurately positioned in relation to the neck of the tooth.
The criterion for this, as a rule, is the minimum immersion of the edge of the cap into the gingival pocket (no more than 0.5 mm) in areas prepared without a ledge. Where the tooth is prepared with a ledge, the edge of the cap should fit snugly against it. Difficulty applying the framework can be the result of many reasons, the main of which are defects in the working model, deformation of the wax reproduction of the framework, shrinkage of the alloy during casting of the framework, inaccurate coating of the wax framework with the formation of air bubbles (especially on the inner surface of the cutting edge or chewing part of the crown), inaccurate preparation of abutment teeth. Consistently, excluding each of the possible causes, they achieve an accurate establishment of the framework on the supporting teeth.
After the framework is applied, the volume of the abutment teeth closed with metal caps and the artificial metal teeth of the intermediate part should be carefully assessed. If the frame occupies the entire volume, including that intended to accommodate the facing ceramic coating, it is necessary, first of all, to carefully evaluate the thickness of the frame in order to identify its possible increase. Another reason for such an error may be insufficient preparation of the abutment teeth. The manufacture of a bridge prosthesis without eliminating the mistakes made will lead to an increase in the volume of artificial teeth and abutment crowns of the prosthesis in comparison with adjacent natural teeth. The prosthesis will stand out among natural teeth and, instead of restoring aesthetics, will lead to its violation. The correction consists in reducing the thickness of the frame of the supporting caps and cast artificial teeth of the intermediate part to the required dimensions; if the thickness of the metal caps meets the requirements, it is necessary to carry out additional preparation of the supporting teeth and remake the framework of the bridge.
Occlusal relationships should be evaluated especially carefully when checking the finished framework. General requirements involve the creation of a gap between the antagonists of 1.5-2 mm in the position of central occlusion. In case of lateral and anterior occlusions, one should keep in mind the possibility of premature contacts of the framework with opposing teeth. If found, they must be eliminated.
After checking the metal framework, it is useful to re-determine the central relationship of the jaws, since the position of the framework on the abutment teeth is often somewhat different from its position on the working model. For the most accurate formation of the occlusal surface of the ceramic prosthesis, it is necessary to fix exactly the position of the frame that it occupies in the oral cavity.
When creating a ceramic coating on a bridge prosthesis, first of all, the technology described by us earlier, adopted for single crowns, is used. The differences concern mainly the intermediate part. Of particular importance for the aesthetic qualities of the prosthesis are the interdental spaces and the shape of the contact surfaces of adjacent artificial teeth. For their formation, after applying the dentin and enamel layers, separation is carried out with a modeling needle to the opaque layer. For the same purpose, a special varnish separator is used, which is applied to every second tooth. During the subsequent firing, the varnish is applied in reverse order. Particularly carefully in the bridge prosthesis, the cervical part of the artificial teeth adjacent to the mucous membrane of the edentulous alveolar process is modeled. This part of the tooth is of great importance for the overall appearance of the entire prosthesis. We mean, first of all, the shape and size of the cervical part, its overlay in relation to the alveolar process, the depth and width of the interdental spaces, the inclination of the long axis of the artificial tooth.
Modeling of the chewing surface is carried out primarily from the point of view of restoring function, but the quality of restoring the anatomical shape is no less important. Thus, the occlusal surface of the paws must meet the most stringent requirements and, above all, correspond to the age characteristics of the microrelief in a given individual, provide a full-fledged chewing function and not have premature contacts with opposing teeth. Compliance with all these requirements is checked in the oral cavity. The finished prosthesis is carefully examined, the quality of the ceramic coating and the polishing of the metal garland are assessed. Before applying, it is necessary to carefully examine the inner surface of artificial crowns. When applying dyes or correcting the anatomical shape, ceramic mass can get into the crowns, especially along the inner edge. Parts of it, barely noticeable during examination, can cause inaccurate or difficult application of the prosthesis. With a shaped head of small diameter at low speeds of the drill, the particles of the ceramic mass are ground off. The same is done with the oxide film covering the inner surface of the combined crowns. Only after such preparation, the prosthesis is carefully placed on the supporting teeth. In this case, large efforts should be avoided, as they can cause chipping of the porcelain coating if the prosthesis is not correctly fitted. First of all, we are talking about a possible excess of ceramic mass on the proximal surfaces of abutment crowns, accused of adjacent natural teeth. To detect this deficiency, carbon paper is inserted into the interdental space with a coloring surface to the ceramic veneer, and then a prosthesis is applied. If an imprint is found, it is necessary to grind the ceramic in this place, preventing possible pressure on it when applying the entire prosthesis. The correction of the contact surfaces is repeated until the prosthesis is fully applied with visible contact of the crowns with adjacent teeth. The patient's lack of a feeling of pressure of the prosthesis on the adjacent teeth indicates the accuracy of the correction of the supporting crowns. The final check of the prosthesis consists in clarifying the occlusal relationships for various types of articulation, as well as the shape and color of the artificial teeth.
The manufacture of the prosthesis is completed, if necessary, by tinting the ceramic coating and glazing. In the oral cavity, the prosthesis is reinforced with cement. The technique is simple and allows you to speed up the modeling process without condensing the ceramic mass and maintain a constant moisture content of the ceramics. Modeling starts from the vestibular surfaces, imitating the most striking features of the anatomical shape and color of the teeth. The palatal and lingual surfaces of the artificial teeth are then modeled, usually before the first firing. Layer-by-layer modeling should begin with the application of ceramic masses of a denser consistency (opaque masses). Subsequent layers should be less dense, not shifting the first layer. A more liquid consistency is used for incisal masses. The density of the ceramic mass before application can be ensured using a special "liquid N, Ivoclar".
In the manufacture of large bridges, it is recommended to follow the following sequence. At the first stage, the anterior teeth are modeled (first firing), at the second stage, the chewing teeth are modeled and the anterior teeth are corrected (second firing), and at the third stage, the chewing teeth are corrected with possibly necessary correction of the anterior teeth (third firing). Such a sequence, according to the author, allows the use of layer-by-layer deposition of ceramics, as the simplest way to speed up modeling, maintain a constant humidity of the ceramics and not apply condensation of the ceramic mass.
When modeling a multilayer ceramic coating using intensely colored porcelain powders to create depth effects, the following should be taken into account: since the ceramic layer is applied taking into account its subsequent shrinkage during the firing process, a shift in individual color features laid down during the initial application may occur; correction of the anatomical shape by applying additional portions of porcelain can also cause a shift or loss of individual details of the color effect; Condensation of ceramic coating layers can lead to spreading of individual fine details of reproducible features.
FINDINGS
The bridge prosthesis as a therapeutic tool must meet the requirements of toxicology, technology, aesthetics, hygiene and function.
The requirements of toxicology are reduced to the use of materials that, having anti-corrosion properties, are at the same time non-toxic, do not cause allergies, do not irritate the oral mucosa, do not combine with saliva and do not change its properties.
There are certain requirements for bridge prostheses, primarily related to the rigidity of the structure.
Relying on teeth that are borderline with a defect, the bridge-like prosthesis performs the function of extracted teeth and, thus, transfers an increased functional load to the supporting teeth. Only a prosthesis with sufficient strength can resist it. From the point of view of hygiene, special requirements are imposed on bridge prostheses.
Here, the shape of the intermediate part of the prosthesis and its relation to the surrounding tissues of the prosthetic bed of the mucous membrane of the alveolar process, the gums of the supporting teeth, the mucous membrane of the lips, cheeks, and tongue are of great importance.
In the anterior and lateral sections of the dental arch, the intermediate part is not the same. If in the anterior section it should touch the mucous membrane without pressure on it (tangential form), then in the lateral section between the body of the prosthesis and the mucous membrane covering the edentulous alveolar process, there should be free space that does not prevent the passage of chewed foods (wash space).
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