Recording relation of cast to surveyor

During various procedures such as mouth preparations, block-out on the master cast, placement of clasp arms in relation to undercut areas, and shaping of wax patterns, it is often necessary to transfer the cast to the surveyor multiple times. For these procedures, the relation between the cast and the vertical arm of the dental survoyor must be recorded to serve as a reference.

Since the base of each cast varies, recording the position of the surveyor’s platform has no significance. Furthermore, every cast needs to be assessed separately, and every positional record needs to be part of the cast rather than its basis.

This method is called tripoding the cast and will result in the original position of the cast and, thus, the original path of placement. Although many techniques have been proposed for this purpose, two methods are more accurate and practical than the others. The first method is based on marking three separate points on the tissue surface of the cast using a carbon marker while the vertical arm of the dental surveyor is fixed in position. These points should preferably be located away from the areas where the metal framework design will be drawn. To enhance visibility, circles may be drawn around these points. This position represents the initial tilt of the cast and therefore indicates the path of placement of the prosthesis. When the cast is re-mounted on the surveyor, the tilt is adjusted until the carbon  marker contacts all three points simultaneously in the same plane (Figure 7-14a).

The second method involves making vertical notches on both sides and the posterior aspect of the cast base using a sharp instrument held against the surveyor blade (Figure 7-14b). The original position of the cast can be determined by tilting it until all three lines become parallel to the surveyor blade. However, the notches created on the cast base must be duplicated for each cast and preserved during trimming of the base.