Screw preload analysis of direct-to-abutment connections in complete arch zirconia frameworks before and after thermomechanical aging.

Statement Of Problem Direct-to-abutment connections have been increasingly used for implant-supported fixed complete dentures (ISFCDs). However, the biomechanical performance of these connections and their screws under simulated clinical conditions has not been adequately investigated.Purpose The purpose of this in vitro study was to investigate the reverse torque values (RTVs) of ISFCDs connected to transmucosal abutments with and without the use of a Ti-base interface before and after thermomechanical aging (TMA). In addition, the study aimed to compare RTV according to the implant position and location (straight anterior and angled posterior) within each group.Material And Methods Zirconia frameworks (N=24) for restorations supported by 4 implants were divided into 3 groups: frameworks with Ti bases and regular screws, direct-to-abutment frameworks with regular screws, and direct-to-abutment frameworks with modified screws. All frameworks were 3-dimensionally designed and subtractively manufactured. The RTVs of screws and preload efficiency (PE) were measured after tightening the screws. The frameworks were subjected to 1.2 million cycles at 90 N, 2 Hz with a dwell time of 30 seconds between 5 °C and 55 °C in distilled water with a total of 12 000 thermal cycles. RTV and PE measurements were repeated after TMA. Paired comparisons were performed using the Wilcoxon signed-rank test, and post hoc Mann-Whitney U tests with Bonferroni adjustment were used to evaluate RTV and PE loss after TMA. In addition, the Mann-Whitney U tests were used to compare the differences in preload loss between straight anterior and angled posterior abutments in each design. The Cohen r was used to estimate effect sizes for all group comparisons (α=.05).Results Before TMA, the median RTV was 13.3 Ncm, and the median PE was 88.3%. After TMA, the median RTV was 11.5, and the median PE was 76.7%, with significant loss of preload across the entire sample (z=6.06, P<.001). However, the Mann-Whitney U tests revealed that the losses were not significantly different between the standard Ti-base connection and either the direct-to-abutment connection with regular screws (P=.518) or the direct-to-abutment connection with modified screws (P=.664). Furthermore, no significant differences were observed between abutment positions in the Ti-Base connection with regular screws (P=.880) or between abutment positions in the direct-to-abutment connection with modified screws (P=.363). However, comparison of direct-to-abutment connections revealed significantly greater losses in angled posterior abutments with regular screws (z=-2.25, P=.025).Conclusions No significant differences in RTV and PE were observed between Ti-base and direct-to-abutment connections before and after TMA. Modified screw designs did not significantly affect preload before TMA in direct-to-abutment connections. However, they significantly minimized preload loss after TMA in angled posterior abutments only in the direct-to-abutment connection compared with that with regular screws in the Ti-base and the direct-to-abutment connections using regular screws.