The Effect of a Screw Access Channel on the Fracture Resistance of Cement-Retained Metal- Ceramic Implant Crowns

Abstract

Purpose: To determine if the presence of a screw-access channel results in a statistically significant difference in fracture strength of molar and premolar cemented metal-ceramic implant crowns. Materials and Methods: Twenty abutment-crown specimens were fabricated and tested in this study. Ten metal-ceramic premolar crowns and 10 metal-ceramic molar crowns were fabricated on their respective custom abutments. Of the 10 specimens in both the premolar and molar groups, five were fabricated as the experimental group with a screw access channel while the other five had their occlusal surface intact and served as the control group. Polytetrafluoroethylene tape was inserted in the screw access channel of the custom abutment for all specimens and the crowns were cemented using resin-modified glass ionomer cement. Following cementation, the screw access channels of the 10 specimens in the two experimental groups were filled with composite resin. Specimens were individually mounted onto a custom fixture with an implant analog and tested on a servohydraulic testing machine at a crosshead speed of 0.5 mm/min at least 24 hours after cementation. Specimens were tested until failure with the highest force at the point of failure being recorded. Results: The mean axial load at failure for the molar crown without a screw access channel was 2032 N while the molar crown with an access channel was 1505 N. Comparatively, the mean axial load at failure for the premolar crown without a screw access channel was 1338 N while the premolar crown with an access channel was 964.5 N . Using a one-way ANOVA, the presence of a screw access channel in molar restorations led to a significant decrease in the axial load needed to fracture the restorations (P<.05); however, the screw access channel did not significantly affect the premolar restorations (P=.12). Molar restorations also required significantly more axial load to fracture relative to premolar restorations in specimens without a screw access channel (P<.01) and those with a screw access channel (P<.05). Conclusions: Metal-ceramic molar crowns, with or without a screw access channel, required a significantly higher axial load to fail than metal-ceramic premolar crowns. The presence of a screw access channel resulted in a significantly lower axial load force required for failure in the molar restorations; however, the difference was not statistically significant in the premolar restorations.