The physical properties of oral restorations must adequately withstand the stresses of mastication.
Several methods may be used to ensure proper per- formance of a restoration. With a constant force, the stress is inversely proportional to the contact area;
therefore stresses may be reduced by increasing the area over which the force is distributed. In areas of high stress, materials having high elastic moduli and strength properties should be used if possible.
If a weaker material has desirable properties, such as esthetic qualities, one may minimize the stress by increasing the bulk of the material when possible or ensuring proper occlusion on the restoration.
Restorations and appliances should be designed so the resulting forces of mastication are distributed as uniformly as possible. In addition, sharp line angles, nonuniform areas, and notched, scratched, or pitted surfaces should be avoided to minimize stress concentrations. For example, joints between abut- ments and pontics of fixed partial dental prostheses should be properly radiused to distribute stress dur- ing function. Implant screws should not be scratched or notched when inserted.
Restorative materials are generally weaker in ten- sion than in compression. Restorations should be designed to minimize areas of high tension. Material flaws can further contribute to areas prone to fail- ure. Fatigue is also an important consideration. For example, repeated flexure of an improperly loaded implant-supported restoration can concentrate stresses in the abutment screw or implant body, lead- ing to fatigue fracture.
The dentist is often concerned not so much with the fracture of an appliance as with the deflection that occurs when a force is applied. This is the case with a fixed partial dental prosthesis, which may be cast as a single unit or may consist of soldered units.
As discussed earlier in this chapter, the deflection of a beam, or in this case a fixed partial dental pros- thesis, supported on each end with a concentrated load in the center depends directly on the cube of the beam length and indirectly on the cube of the beam thickness. Doubling the length of the beam, there- fore, increases the deflection by eight times. This also indicates that decreasing the thickness of the beam by one-half increases the deflection by eight times. If too much bulk were required to develop the stiffness desired, changing to a material with a higher elastic modulus, or stiffness, would be beneficial. If repeated failures occur, consider increasing the occlusogingi- val dimension of the proximal connectors, balancing the occlusion over a larger surface area, and narrow- ing the occlusal table.
These isolated examples of applied knowledge of biting forces and stresses in dental structures indi- cate why an understanding of this subject is neces- sary to the practicing dentist.
In summary, three interrelated factors are impor- tant in the long-term function of dental restorative materials: (1) material choice, (2) component geom- etry (e.g., to minimize stress concentrations), and (3) component design (e.g., to distribute stresses as uni- formly as possible). It should be noted that failures can and do occur. In such instances, a failure analysis should be performed by answering several questions:
(1) Why did it fail? (2) How did it fail? (3) Did the material or design fail? and (4) How can this failure be prevented in the future? Lastly, remember that dental material behavior depends on interrelated physical, chemical, optical, mechanical, thermal, electrical, and
biological properties, and improvement of one spe- cific property often leads to a reduction in another property.
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69 In Chapter 4, we introduced fundamental concepts in biomechanics and physical properties of dental materials. The data presented were collected with a variety of test instruments. In this chapter we describe the individual tests in more detail.