Effects of screw channel angulation on the fracture resistance of one-piece zirconia crown with titanium base.

Abstract

The design of the angulated screw channel in implant restorations allows the possibility to correct angulation discrepancies, especially in the anterior maxilla. However, the effects of varied screw channel angulations on fracture resistances and fracture patterns of the implant restorations are still uncertain, and thus the aim of this study. Angulated screw channel monolithic zirconia crowns (Nobel Biocare) with three different angulation groups-straight (ASC1), 15° (ASC15), and 25° (ASC25)-were digitally designed from a left central incisor prototype scan. Following fabrication, 10 samples of each group were individually mounted onto implant replicas embedded in standardized type V stone gypsum cylinder jigs (25mm × 25mm). All screws were manually torqued to 35 Ncm according to the manufacturer's recommendation, and screw access openings were subsequently sealed with resin composite. To mimic the off-axis loading of the central incisor, the specimens were then loaded at a cephalometric interincisal relationship of 135° between the long axis of the crown and the Instron force applicator, with crosshead speed set at 0.5mm/min. Fractured abutment surfaces were examined, and selected specimens were further evaluated by scanning electron microscopy. Screw torque values were also measured after the catastrophic loading. One-way ANOVA was used to compare load-to-fracture values between groups, with the statistical significance set at 0.05 (p values). The mean load-to-fracture values in descending order were 331.24N (±34.00N) in ASC15, 325.22N (±35.50N) in ASC25, and 302.04N (±45.10N) in ASC1, with no statistically significant differences between groups. Considerable screw torque losses were found in all groups after catastrophically loading. The average torque loss was 84% in ASC1, 86% in ASC15, and 94% in ASC25. 16 out of 30 specimens experienced screw loosening; one ASC1 screw underwent slight deformation. Crowns of all tested groups exhibited cohesive fracture patterns at the screw-metallic-zirconia interfaces. Within the limitations of this in vitro study, one-piece monolithic zirconia implant crowns with varied screw channel angulations shared similar fracture-strength and fracture-mode characteristics. The zirconia-titanium base junctions exhibited the weakest link of all restorations.