The Effect of Die Spacer on Retention and Fitting of Complete Cast Crowns

PURPOSE: This study evaluated the effect of die spacer on the fit and retention of complete cast crowns by using three different cements. MATERIALS AND METHODS: Standardized full crown restoration preparations were completed on 99 extracted molar teeth, impressions were made with poly(vinyl siloxane), and stone dies were made. Dies were covered with four layers of die spacer using three techniques: (1) covering the occlusal and 1/3 of the axial surfaces, (2) covering the occlusal and 2/3 of the axial surfaces, and (3) covering the entire preparation except the apical 0.5 mm of the preparation. Complete metal crowns were cast using Pors-on 4 alloy. Crowns were then assigned to one of three luting agent groups: resin modified glass ionomer cement, resin cement, or zinc phosphate cement. The castings were placed on their respective teeth and the marginal opening was recorded by two methods: 72 specimens were examined before and after cementation using optical microscopy with 0.001 mm resolution, and 27 specimens were examined after cementation with scanning electron microscopy. After cementation, the teeth were thermocycled for 700 cycles between 5 degrees C and 55 degrees C. The tensile retentive strength was measured on a universal testing machine with a crosshead speed of 0.5 mm/min. The data obtained for the fitting were recorded in millimeters and the data for the tensile retentive strength were recorded in KgF. The statistical analysis was performed by analysis of variance and post hoc Tukey's test (p< 0.05). RESULTS: Before cementation, better marginal fit was obtained when the die spacer covered all but the area 0.5 mm short of the margin of the preparation; however, after cementation, the resin modified glass ionomer cement group had the best fit with the same application of die spacer. Castings luted with resin cement required the greatest tensile force to produce cement failure. CONCLUSIONS: Increasing the area of the die surface covered with spacer improved the fit of the cast restoration. After cementation, the resin modified glass ionomer showed better adaptation; however, the optical microscopy and scanning microscopy correlate well. Resin cement had the highest resistance to tensile forces.