ALKUMRU H N, GEMALMAZ D, BERKSUN S, KASAPOGLU C (Marmara University Faculty of Dentistry, Istanbul, Ankara University Faculty of Dentistry, Ankara, Turkey)
The purpose of the present study was to evaluate the multidimensional thermal cycling distortion of 3-unit PFM frameworks at different firing stages. A master model was designed to represent the 2 abutments of a 3-unit fixed partial denture replacing a missing mandibular molar. Ten individual castings were made using standardized techniques. Half of the copings were cast in a Ni-Cr alloy and the other half in a PD-Cu alloy. The distortion of the frameworks were measured by means of inner fit changes, horizontal linear measurements of the framework length and vertical fit changes of each abutment. The measurements were made (1) initially, (2) after degassing firing and, (3) after glaze firing. The differences between the initial measurements and the subsequent measurements created the distortion values of the retainers in three dimensions. The mean values of inner fit change recorded for porcelain application firing were higher in magnitude than the values of metal conditioning firing.
The distortion seen in a greater magnitude after porcelain veneering can be attributed to the factors, contamination of porcelain in metal coping and reduction in resilience of metal. In both alloy groups, both retainers showed increase in vertical fit that implied poorer vertical fit after porcelain application. In addition, the results of both vertical fit measurements and internal fit change measurements revealed that the magnitude of change seen for premolar retainer was greater than that of molar retainer.
Supported in part by BEGO, Germany.