Influence of increased crown height space on reverse torque value in extra-narrow–diameter implants: insights from a 5-year laboratory simulation

The biomechanical effects of different crown height space (CHS) on stability in extra-narrow–diameter implants (ENDIs) remain unclear. Therefore, this in vitro study aimed to evaluate the reverse torque value (RTV) of 2.9-mm ENDIs restored with increased CHS. Thirty-three ENDIs were allocated into three groups based on CHS: Group C, 8 mm (control); Group L, 11 mm (long); and Group E, 14 mm (extra-long). The specimens were subjected to thermocycling and cyclic loading to simulate 5 years of clinical function. The reverse torque values (RTVs, in Ncm) were recorded at baseline, after 200,000 cycles (1 year), and after 1,000,000 cycles (5 years) and compared among the three groups. The reverse torque loss (RTL) ratios (in %) at these time points were also computed. Data was analyzed using one-way analysis of variance (ANOVA), repeated measures ANOVA followed by Tukey and Bonferroni post hoc test. The RTL ratio was analyzed using Kruskal-Wallis test (α = 0.05). There was no significant difference among groups at all evaluation intervals (P > 0.05). However, the comparison of overall mean RTV across different measurement intervals within each group was statistically significant (p < 0.05). The post-hoc comparison showed significant differences between 1-year vs. 5-years (p = 0.02) for group C, initial vs. 1-year (p < 0.001) and 1-year vs. 5-years (p = 0.04) for group L, and initial vs. 1-year (p = 0.02) and 1-year vs. 5-years (p = 0.006) for group E. Regarding RTL%, no significant differences were observed in % initial RTL (p = 0.13), % post-load RTL1 (p = 0.13), and % post-load RTL5 (p = 0.25). Within the limitations of this in vitro study, increasing crown height space from 8 to 14 mm did not significantly affect reverse torque values or torque loss in extra-narrow diameter implants subjected to thermomechanical aging.