FORCES ACTING ON CAST RESTORATION INTRODUCTION The word biomechanics clearly denotes the application of mechanics to biologic systems. It is often compared with engineering principles. Stinners says that the engineers can definitely predict the amount of stresses to be induced where as dentist canâ€™t predict it. Because we deal with human tissues and do not always follow the rest of the dynamic nature of the actual biting (forces) stresses during mastication, it is difficult to measure Skinnerâ€™s mentions about the forces. -
Average of 77kg (170.16).
Cuspid 14.34 kg
Incisors 9.25kg Never the less the design of the tooth is somewhat of an engineering
marvel in that the tooth is generally able to absorb impact energies. The modulus of resilience of dentin is greater than that of enamel and thus is better able to absorb impact energy. Enamel is brittle with low proportional limit, low modulus of resilience, but when it is supported by dentin, it does withstand the forces, loss of support from and decreases its strength by 85%. A healthy uncut tooth is the strongest stress bearing structure, when this health is hampered by caries or trauma, the restorative dentistry comes in picture. Here the interaction of orofacial complex with forces should be clear.
Let us consider about a cast restoration e.g., inlay, it exhibits mechanical problems of resilience and retention. When the forces are applied at right angles to the flat bur of a filling, the filling by rolling it out of the cavity walls prevents the displacement. [The degree of resistance to displacement offered by each type of preparation. In case of a Class II elimination of one of the vertical 2 nd walls considerably reduces the resistance to displacement. Even though the gi extension is made the retentive qualities are not improved. Dropping the pulpal floor at some also slightly increases the resist to displacement. Also a groove placed in the gingival wall helps for same purpose Mcmath suggests that the gingival groove should not be wider than half of the width of the gingival wall. The depth of the groove end. The gingival groove prevents the lateral spreading of the casting. When considering the axioproximal walls. The compressive forces resulting from vertical pressure have an important bearing on two retention of the inlay the relationship of the vertical axial wall with buccal and lingual walls is critical. The question arises whether they end flare axioproximally or parallel to each other. Let use see the degree of resistance to displacement offered by each type of preparation. In a preparation with 2nd walls when force F is applied, there is a tendency of point X to rise occlusally on arc XY but this is resisted by the dentin lying within the area XYZ. In case of slight divergence slight resistance is found for displacement. Little available dentin in the area XYZ prevents rolling.
But in divergent walls, there is absolutely no dentin to prevent this displacement hence the casting rolls out of the cavity. Therefore, parallelism of the walls offers maxi rotational resistance. Taper of 2-5째 is also acceptable which is required for convinance. Same principles can be applied to the buccal and lingual walls of the occlusal dovedail. Role of dentin A well seated inlay contacts the walls and the base is in contact with the dentinal floor when the vertical forces are exerted there is a tendency to force through the custom because of the taper shape it gives a wedging effect. This tends to distend the lateral walls outward if the lateral walls are also placed in dentin like the floor, this elastic material exerts opposing stresses and grips the casting making the dislodgement difficult. Let us see the stresses and forces. The dentinal walls resist the tensile forces of displacement. Contact is assumed to have been made between the inlay and the walls of the cavity. If the inlay is further forced downward, it moves little distance, if the tooth structure is prevented from deformal it in the metal which deforms. But there are some opposing forces which are equal in magnitude i.e. the force tending the stop the inlay being pushed further into the cavity will exactly equal the force of friction tending to hold it there. When the force is removed, the gold of the inlay expands maintaining the contact with the tooth structure where as the tooth structure compresses, inducing stresses within it; while the gold expands an equal amount receiving stresses within itself it proceeds until a point has been reached where the induced unit compressive stress in the tooth structure is exactly equal to the residual compressive unit stress in the gold of the inaly. If these two stresses are not equal movement of the differential area would continue.
Resistance to horizontal displacing forces Application of a vertical force to the inclined plane will dislodge a filling horizontal even in a cavity with flat pulpal and gingival walls, the filling is rotated occlusoproximally with the rotation point being the gingival marginal wall. These forces are always effective marginal wall. These forces are always effective unless counteracted by an opposing movement. The counteracting movements are in 4 ways: 1) Occlusal dovetail â€“ properly prepared occlusal lock in between two strong cusps is the strongest means of resisting the displacement of the inlay. 2) Gingival wall gingival groove prevents the lateral displacement of the inlay because of the inherent weakness of this groove, the chances of # of gingival wall are more. Hence the gingival wall is prepared in two planes, cavosurface and axial when the horizontal displacing force is applied to the occlusal inclined plane the lateral displacement of the filling will be resisted by the angular wedge portion of the inlay; which extends into the acute angle formed by the axiogingival walls; until the inlay is raised vertically to clear the ridge of the gingival wall. Simple inward and downward slope of the gingival wall is not enough maxillary angulation of 45Â° i.e. the ideal depth of the inner bevel resists the displacement. 3) Pulpal wall offers no resistance to horizontal displacement other than friction pulpal wall prepared in two planes i.e. with inclined planes helps in preventing the lateral displacement lowering the grooves or pinhole serve the same purpose. 4) Properly contoured and placed contact areas definitely prevents this rotational movement of the proximal inlay.
During centric and excursive movements of the mandible, both the restoration and tooth structure are periodically loaded both separately and jointly. This brings about different stress patterns depending upon the actual morphology of the occlusal area of tooth and the contacting tooth elements. Let us classify the loading situation and their induced stress patterns in the following way: A cusp contacting the fossa away from the restored proximal surface in proximoocclusal reaction at a centric closure. Because of elasticity of dentin the restoration will bend at the axiopulpal line angle which creates tensile stresses at the isthmus portion and compressive stresses in the underlying dentin. A large cusp contacting the fossa adjacent to the restored proximal surface in a centric closure. This large cusp will tend to separate the proximal part of the restoration from the occlusal part this creates tensile stresses at the isthmus portion and compressive forces in the tooth structure apical to the restoration. Occluding cuspal elements contact facial and lingual tooth structure surrounding a proximoocclusal restoration during all movements, there will be a conclusion of the occluding cuspal elements contact facial or lingual parts of the restoration completely replacing the facial or lingual tooth structures during centric and excursive movements. Occluding cusp contacting a restoration MRs, there will be concentrated tensile stresses at the junction of MR and the restoration of the rest. Similarly there will be conclusion of tensile stresses underneath the corresponding areas if the cusp occludes the grooves or crossing ridges; axial protions.
Plus the remaining tooth structure does show some stress pattern. It is necessary to know the possible displacements that can happen to the restoration the forces that can cause them and the fulcrum of these movements. There are four such displacements for a Class II restoration. 1) Proximal displacement of the entire restoration. An obliquely applied force ‘A’ develops a compact ‘V’ in a vertical direction and a component ‘H’ in horizontal direction ‘V’ will try to seal the restoration where as ‘H’ will try to rotate it proximal around axis ‘X’ at the gingival walls. 2) Proximal displacement of proximal portion. If a restoration is considered to be L shaped with a long arm of L occlusally and short arm proximally when the long arm is loaded by vertical force ‘V’ it will seat the more into the tooth. Because of the elasticity of d, it changes its position from 1 and 2. But as the metallic restorations are more rigid than dentin, the short arm of L moves proximally and the fulcrum of the rotation is the axiopulpal line angle to prevent these displacements facial gingival grooves are given. But again there might be connection of stresses along with these grooves. 3) Lateral rotation of the restoration around hemispherical floors (pulpal and gingiva). These floors instead of hemispherical they should be made inverted tuncated cone shaped. 4) Occlusal displacement An inverted tuncated cone shape helps to seat the restoration. Although magnitude of these four displacements is minute, it has been repeated thousands of times per day. This can increased microleakage .
Also the axiopulpal line angle is rounded (avoids stress) connection improves visibility for the facial and lingual gingiva axial corners and increase mate outk). Flat pulpal and gingival floors at the isthmus should be perfectly flat resist the forces at the most advantageous angulation. All the parts of restoration should have individual retentive modes. E.g., dovetail fox O, grooves for proximal. There should not be any occlusal discontinuity. Material strength with low modulus of elasticity deforms if excess stress restoration where cast restoration behaves exactly transmitting forces to the both Ans might lead to tooth. Margins â€“ No unsupported enamel (enamel if not supported by denticles its strength by 85% flashes are advocated only in mate with high edge strength. So one has to look out for the design features for the protection of the mechanical integrity of the restoration and also of the remaining tooth structure, in short resistance and retention form. Design features for the protection of the mechanical integrity of the restoration. 1. Isthmus in between O part and proximal or facial or lingual parts. Mathematical, mechanical and photoelastic analysis of stresses at this particular part reveal four things: a) the fulcrum of bending occurs at the axiopulpal line angles, b) stresses are more to the surface of the restoration i.e. away from that fulcrum, c) tensile stresses predominate at the marginal ridge areas of a Class II restoration.
Materials tend to fail, starting from surface near MR and proceeding internally, towards axiopulpal line angle. To solve this problems 2 solutions tried i.e. i) by increasing mate bulk at the axiopulpal line angle and placing the surface stresses away from the fulcrum but this needs undue deepen of the cavity and also increased stresses in the material. ii) by origing axiopulpal line angle closer to the surface to decreases tensile stresses near MR, but tooth in of a material lacks resistant to O. forces. Ultimately combining these solutions i.e. the mate bulk can be increased and bringing the axiopulpal line angle closer to the surface by just slanting the axial wall towards pulpal floor. Design features for the protection of physiomechanical integrity of the tooth structure. 1. Isthmus should be as narrow as possible to minimize the tensile stresses in the tooth structure. 2. Occlusal surface preserving sound tooth structure O dovetail. Box or deepening of the O surface away from the lesion. 3. Proximal surface prepared in a stepped manner. Two planed gingival floor. The inner angulated dentinal plane (45째 ideally).
Published on Apr 30, 2014
Published on Apr 30, 2014
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