Fracture resistance of implant-supported screw- vs cement-retained porcelain fused to metal single crowns

Objectives. The present in vitro study aimed at evaluating the fracture resistance of both implant-supported screw- and cement-retained porcelain fused to metal (PFM) single crowns. A scanning electron microscope (SEM) evaluation of the mode of failure of the spec- imens was also performed. Methods. Forty PFM premolar-shaped identical single crowns were realized. The restorations were divided into two groups: cement-retained (group 1) and screw-retained (group 2) pros- theses. Compressive loading tests and SEM fractographic analyses were performed. The data were statistically analysed by means of the Student’s t-test, with a confidence interval of 95%. Results. The mean fracture load value was 1657 (±725) N in group 1 and 1281 (±747) N in group 2; the statistical analysis pointed out no significant differences between the two groups (p = 0.115). The mean work at maximum load value was 0.775 (±0.619) J in group 1 and 0.605 (±0.526) J in group 2; the statistical analysis pointed out no significant differences between the two groups (p = 0.355). All the samples were affected by cohesive fractures of the porcelain. Screw-retained crowns showed microcracks at the level of the occlusal access to the screw and extensive fractures in the whole thickness of the ceramics. On the contrary, cement-retained restora- tions were affected by less wide paramarginal fractures of the porcelain. Significance. A stronger implant-prosthetic connection was noticed in cemented restora- tions group than in screw-retained single crowns. Even though negatively influenced by the presence of the occlusal access to the screw, the metal–ceramics bond can be considered predictable in both the implant-prosthetic connection systems analysed.