Influence of connector cross-sectional geometry on the load-bearing capacity under fatigue of implant-supported zirconia fixed partial prosthesis

Abstract

Statement of problem: Zirconia has been reported to be suitable for multiunit monolithic restorations. However, as the connector region is the weakest part of the system, studies are needed to determine the optimal connector geometry.

Purpose: The purpose of this in vitro study was to evaluate the load-bearing capacity under fatigue of implant-supported fixed partial prostheses made of monolithic zirconia with different connector cross-sectional geometries.

Material and methods: Three-unit monolithic fixed partial prostheses (from mandibular second premolar to the second molar) were fabricated in zirconia (Zenostar T) by computer-aided design and computer-aided manufacture (CAD-CAM). Different connector cross-sectional geometries, with an area of 9 mm², were tested (n=10): round, square with rounded angles, or trapezoid with rounded angles. The prostheses were screwed over 2 implants and inserted into acrylic resin bases. The specimens were submitted to a mechanical fatigue test until failure, with load applied to the pontic using the stepwise stress methodology (initial load of 100 N for 5000 cycles, followed by an increase to 200 N for 20 000 cycles, with a subsequent increase of 50 N each step). For data analysis, fatigue failure load (FFL) and cycles for fatigue failure (CFF) were recorded. Failed structures were analyzed by fractography. The Kaplan-Meier test followed by a log-rank Mantel-Cox post hoc test was used to analyze FFL and CFF (α=.05). Weibull analysis was also performed.

Results: The round (1065 N) and trapezoid (1010 N) groups presented higher FFL values than the square one (870 N) (P≤.05). For CFF, the round connector (358 777) was better than the square (280 310) (P=.006), and the trapezoid (337 773) was statistically similar to both (P>.05). No difference among groups was observed in Weibull modulus either for CFF or FFL data. All failures were catastrophic, originating at the base of the connector and propagating toward the occlusal surface of the pontic (region under loading).

Conclusions: The connector cross-sectional geometry significantly influenced the mechanical fatigue performance of implant-supported fixed partial prostheses made of monolithic zirconia.