Steps in cavity preparation - Part 2 Resistence form
Resistance form may be defined as that shape and form of cavity walls that best enable both the restoration and the tooth to withstand occlusal forces without fracture.
Fundamental principles involved are:
1.Box shape or mortise shaped with flat floor, which helps the tooth to resist occlusal loading by virtue of being at right angles to the forces of mastication.
2.Slightly curved than acute line angles decrease the stress concentration of stresses and hence reduce the incidence of fracture.
3.Conservation of strong cusps and ridges with sufficient dentin support.
Weakened areas should be included in cavity preparation to prevent fractures (capping of the weakened cusps).
4.To provide enough thickness of restorative material to prevent fracture under load.
5.Slight roundening of the line angles to prevent stress concentration.
STRESS PATTERNS OF TEETH
According to Gabel application of mechanical principles to the design of restorations will help conceive favorable stress patterns for the teeth and the restorations. These principles vary according to the type of restoration and cavity.
TYPE OF RESTORATION:
The minimal thickness of amalgam and cast gold to resist fracture is approximately 1.5mm, though a little more depth is required for amalgam to achieve the requisite bulk. However in composite and glass ionomer, the depth is not the criteria for achieving resistance form. Porcelain also requires a depth of 2mm for inlays and 1.5mm for crowns.
TYPE OF CAVITY:
CLASS I:
A flat pulpal floor is appropriate. In case of deep caries where a rounded pulpal floor may result, the stress is doubled in the deepest portion of the cavity. Fractures in these rezsstorations are due to insufficient dentinal thickness in the center. Bending stresses are proportional to square of depth. Therefore for large restorations depth should be increased with increase in diameter.
CLASS II:
A proximo-occlusal inlay restoration acts like a curved beam of cantilever type. Due to differences in modulus of elasticity of dentin and the material there will be displacement of the restoration in the gingival seat area with the axio-pulpal line angle as axis of restoration. This is prevented by a lock in the form of groove pins, etc in the gingival floor. In M.O.D. cavity axio-pulpal line angle should be more rounded.
CLASS III:
Due to the thickness of incisal edge the cavity is extended lingually as close to the incisal edge as possible.
CLASS V:
The functional cusp and functional fossa relationship dictates the stress pattern.
EFFECT OF GROOVES:
Grooves provide resistance to a certain degree. Courdadee and Jimmerman have shown that localized areas of stress are produced in tooth tissue by provision of supplemental intracoronal retention in the form of pins.
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