Porcelain Fracture Resistance of Screw-Retained, Cement-Retained, and Screw-Cement-Retained Implant-Supported Metal Ceramic Posterior Crowns

Purpose: The purpose of this in vitro study was to compare the porcelain fracture resistance between screw-retained, cement-retained, and combined screw-and cement-retained metal-ceramic (MC) implant-supported posterior single crowns; and to investigate the effect of offsetting the occlusal screw-access opening on porcelain fracture resistance of screw-retained and cement-retained MC implant-supported posterior single crowns. Materials and Methods: Forty standardized MC molar-shaped restorations were fabricated. The 40 restorations were divided into four groups (SRC, SRO, CRP, and CSC) of 10 specimens each. Group SRC: screw-retained, screw-access hole placed in the center of the occlusal surface; Group SRO: screw-retained, screw-access hole placed 1 mm offset from the center of the occlusal surface toward the buccal cusp; Group CRP: cement-retained, zinc phosphate cement was used; Group CSC: cement-retained with a screw-access hole in the center of the occlusal surface. The screw-retained restorations and abutments were directly attached to 3i implant fixtures embedded in acrylic resin blocks. Subsequently, all test specimens were thermocycled and vertically loaded in a universal testing machine at a crosshead speed of 2 mm/min until fracture. Mean values of load at fracture (in N) were calculated in each group and compared with a one-way ANOVA and Tukey's Studentized test (alpha = 0.05). Results: Mean values of loads required to fracture the restorations were as follows (N): Group SRC: 1721 +/- 593; Group SRO: 1885 +/- 491; Group CRP: 3707 +/- 1086; Group CSC: 1700 +/- 526. Groups SRC, SRO, and CSC required a significantly lower force to fracture the porcelain than did the CRP group (p < 0.05). Conclusion: The cement-retained restorations showed significantly higher mean fracture loads than the restorations having screw-access openings in their occlusal surface. The position of the screw-access hole within the occlusal surface did not significantly affect the porcelain fracture resistance.