FINISHING OF THE CAVITY WALLS:
It is the further development, when indicated, of a specific cavosurface design and degree of smoothness that produces the maximum effectiveness of the restorative material being used.
- To create the best marginal seal possible between the restorative material and the tooth structure.
- Afford a smooth marginal junction.
- Provide maximum strength of both tooth and the restorative material at and near the margin.
Factors to be considered:
- Direction of enamel rods.
- Support of enamel rods at DEJ and cavity margins.
- Type of restorative material to be used.
- Location of the margin.
- Degree of smoothness desired.
- The strongest enamel margin is that margin which is composed of full length of enamel rods that are supported on the cavity side by short enamel rods, all of which extend to sound dentin.
- Line angles formed by the junction of enamel rods should be rounded whether acute or obtuse.
There are two primary features related to the finishing of enamel walls:
1. The design of cavosurface angle:
For amalgam the cavosurface angle should be 90 degrees due to low edge strength of amalgam. However when extending the facial and lingual walls in treating extensive occlusal caries, tilting the bur is often indicated to conservatively extend the margin and provide a 90 – 100 degrees cavosurface angle. For cast gold/ metal/ composite restorations bevelling of external walls is done. Margins of gold foil restorations form a cavosurface angle much less obtuse than for gold / metal castings and composites. The bevel of the cavity margin in preparation for castings should produce a cavosurface angle of 30 – 40 degrees marginal metal. Providing a 30 degrees bevelled metal will provide with a sliding lapp type fit that definitely improves adaptation of metal to tooth at this margin.
2. The degree of smoothness of wall:
The advent of high-speed cutting procedures ahs produced two pertinent factors related to finishing enamel walls (i) lessening of tactile sense (ii) rapid removal of tooth structure.
Plain cut fissure burs produce the finest surface. The prepared wall of inlay/ onlay requires a very smooth surface to permit undistorted impressions and close adaptation of the casting to enamel margins. In amalgam, goldfoil, composite, a very smooth surface is not desired as it decreases the retention.