Primary Resistance Form Definition:
Primary Resistance form is that shape and placement of the cavity walls to best enable both the tooth and restoration to withstand, without fracture the stresses of Masticatory forces delivered principally along the long axis of the tooth.
Resistance form is the design of a cavity in such a way that the remaining tooth substance and the restorative material can withstand masticatory stress.
To achieve this ,the prepared cavity should possess the following 6 attributes discussed below:
- Flat Floor
- Adequate bulk of the restorative material
- Absence of weak cusps or marginal ridges
- Occlusal cavity margins in areas not subjected to excessive occlusal trauma . In practice one-quarter (1/4) of the intercuspal width
- Flat floor at right angles to the line of stress
- Walls of the cavity parallel to the direction of the stress
1) Flat floor: A flat pulpal floor should be given while the cavity is being prepared to avoid unwanted stresses and forces on the pulpal floor.
2) Adequate bulk of the restorative material:
Bulk of the material should be 1.5-2mm the prepared cavity should be deep enough to take adequate bulk of the restorative material capable of withstanding masticatory stress. The bulk required will depend on the flexural strength of restorative material. In the case of amalgam it is estimated that a minimum of 1.5-2mm thickness of the restorative material is required to withstand masticatory stress.
3) Absence of weak cusps or marginal ridges:
After cavity preparation, the tooth should not be left with any weak cusps or marginal ridges. Any weak cusp must be removed and restored with a metallic restorative material, such as silver amalgam or dental gold. If a marginal ridge is found to be too weak in the cause of an occlusal cavity preparation, a Class II cavity may have to be prepared instead, so as to eliminate the weak marginal ridge. This is particularly indicated where the ridge is only of enamel thickness and unsupported by sound dentine
4) Occlusal cavity margins in areas not subjected to excessive occlusal trauma . In practice one-quarter (1/4) of the intercuspal width
The cavity should be designed that the occlusal margins of the cavity are in areas not subjected to excessive occlusal trauma, otherwise the enamel wall of the cavity and/or the margins of the restorative material may fracture. In practice, this may be achieved by placing an occlusal margins of a cavity about one-quarter (1/4) of the intercuspal distance. Note, that efforts should always be made to conserve sound tooth tissue.
5) Flat floor at right angles to the line of stress
The floor of prepared cavity should be flat and right angles to the line of occlusal stress, which is usually in the direction of long axis of the tooth. Sound tooth tissue should, however, not be removed simply to obtain a flat pulpal floor of prepared cavity
6) Walls of the cavity parallel to the direction of the stress
To achieve this , the walls of the cavity are prepared parallel to the corresponding tooth surfaces.
Fundamental principles involved to Obtain Primary Resistance Form are:
- 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.
- Slightly curved than acute line angles decrease the stress concentration of stresses and hence reduce the incidence of fracture.
- 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).
- To provide enough thickness of restorative material to prevent fracture under load.
- Slight roundening of the line angles to prevent stress concentration.