Comparative Evaluation of Screw Loosening in Zirconia Restorations with Different Abutment Designs

Background: Screw loosening is considered a leading mechanical complication in implant-supported restorations. Hybrid abutments, combining a titanium base with a ceramic mesostructured, were proposed to enhance stability and esthetics. Objective: We aimed to evaluate screw-loosening behavior in implant-supported zirconia restorations fabricated with various abutment designs. Methods: Thirty-six implant analogs were divided into three groups: (A) a one-piece hybrid abutment crown, (B) a two-piece hybrid zirconia abutment with a separated crown, (C) and a stock abutment with zirconia crown. Restorations were fabricated with CAD/CAM, bonded using a dual cure resin cement, and torqued to 35 Ncm to the analogs. The initial removal torque (RTV1) was measured, followed by thermal cycling and mechanical loading (500 cycles, 120,000 load cycles). The post-aging removal torque (RTV2) was measured and the torque loss percentage was calculated. Paired t-tests, ANOVA, and Tukey’s test were used (p < 0.05). Results: All groups demonstrated significant torque loss following aging (p < 0.001). Group A showed the highest torque loss (12.0%), while Groups B and C exhibited lower loss (7.6% and 7.9%, respectively). The between-group difference was statistically significant (p < 0.001), except for between Groups B and C (p = 0.53). Conclusions: Within the limitations of this in vitro study, the abutment configuration affected screw preload stability. The one-piece hybrid abutment crown showed greater torque loss after aging, while the two-piece and stock abutment designs maintained comparatively better stability. Further clinical studies are required to confirm these findings.