Distortion of gold partial denture castings

Abstract

In the technique survey it is significant that three of the seventy-nine castings (3.8 per cent) were unacceptable. Of these, one was the direct result of the displacement of the wax pattern by secondary investment; one failure was due to accidental mechanical distortion during the final polish; the third failure was due to the casting being warped during the soldering operation. For the entire technique survey, thirty-six castings (45.5 per cent) showed some visible evidence of warpage, but not sufficient to prohibit their use. The majority of unexplained discrepancies occurred either after the soldering or the annealing operation. Eleven of the seventy-nine castings failed to fit properly after the initial polish and sprue cutting. Of these eleven, six misfits were the result of an over-polishing of the rest surfaces, and one was the result of the displacement of the wax pattern. The laboratory studies show that eight castings developed slight discrepancies immediately following the soldering operation. These discrepancies increased with the successive stages of softening and hardening operations. Both the technique and laboratory castings showed similar changes, with the only significant changes being in the seventy of distortion. In nearly all castings the lingual bar tended to spread at its distal ends in such a manner as to force the section of the bar below the major abutment teeth into closer contact with the cast, sometimes to the point of serious impingement. At the same time a relief space tended to develop under the center section of the bar, lingual to the incisor teeth. The major occlusal rests showed a tendency to raise off the tooth surface, frequently more on the buccal surface, due to a twisting action of the major connector. This action tended to force the lingual portion of the resin retention lug into closer contact with the edentulous ridge, and the buccal portion of the lug farther from the ridge. Some castings which showed less of the twisting tendency and more of a straight spreading of the distal ends resulted in the major occlusal rests contacting only on the lingual slope of the buccal cusps with a resulting space under the lingual side of the rest. When surveying all of the results for both the technique and laboratory studies, one factor seems to stand out from the others. The more heat treatment the castings receive, the greater are the chances of producing distortion. By heat treatment is meant any operation which consists of heating the casting to a high temperature and then cooling it. This included the casting process, together with the method of cooling, and the soldering, annealing, and hardening operations.