PRECISION OF FIT - THE PROCERA ALLCERAM CROWN

Statement of problem. Strength, color stability, and precision of fit are requirements for all-ceramic restorations. The Procera AllCeram cr own system, composed of a densely sintered high-purity alumina core co mbined with a low fusing surface porcelain, appears to satisfy most of these requirements. However, eval uation of marginal fit has not been reported. Purpose. This study measured the precision of fit of the Pr ocera AllCeram crown fabricated with Procera CAD/CAM technology for th e premolar and molar teeth fit to a die. Material and methods. Five iv orine maxillary first premolars and first molars were prepared for ful l-coverage crowns. Preparations were standardized with a convergence a ngle of 10 degrees, chamfer margins of 1.3 to 1.5 mm circumferentially , and occlusal reduction of 2.0 mm. AllCeram crowns were fabricated fo r the dies, and the fit of the crown to the die was determined by usin g a standardized procedure with a silicone impression material that se rved a dual role: (1) as a retrievable luting agent, and (2) to replic ate the internal aspects of the crown. Laser videography was used to m easure the gap dimension between the crowns and the dies at the margin al opening, the axial wall, the cusp tip, and the occlusal adaptation measurement locations. Mean gap dimensions and standard deviations (SD s) were calculated for marginal opening, internal adaptation, and prec ision of fit. Results. Mean gap dimensions and standard deviations at the marginal opening for the premolar and molar crowns were 56.0 mu m SD +/- 21 and 63.0 mu m SD +/- 13 mu m, respectively. The mean gap dim ensions and SDs of the internal adaptation were 69.0 mu m SD +/- 17 mu m for axial wall, 48.0 mu m SD +/- 12 mu m for cusp tip, and 36.0 mu m SD +/- 7 mu m for occlusal adaptation for the premolar crowns; and 4 9.0 mu m SD +/- 3 mu m axial wall, 67.0 mu m SD +/- 21 mu m cusp tip, and 74.0 mu m SD +/- 29 mu m occlusal adaptation for molar crowns. Pre cision of fit and SDs for premolar and molar crowns were 52.0 mu m SD +/- 19 mu m and 63.0 mu m SD +/- 20 mu m, respectively. Mean marginal openings and precision of fit gap dimensions for the crown groups were not significantly different at the .05 level. However, gap dimensions that defined the internal adaptation at the measurement locations wer e different (P less than or equal to .05). Conclusion. Mean gap dimens ions for marginal openings, internal adaptation, and precision of fit for the crown groups were below 70 mu m These findings show that the c rowns studied can be prescribed with confidence knowing that the preci sion of fit will consistently be less than 70 mu m.