Effect of abutment screw channel angle and crown height on the screw stability of posterior monolithic zirconia crowns supported by tissue-level implants: An in vitro study.

Statement Of Problem
Angled screw channel titanium-base (Ti-base) abutments have been introduced for unfavorably positioned implants. However, knowledge on their screw stability in situations with compromised crown height space is lacking.Purpose
The purpose of this in vitro study was to evaluate the effect of abutment screw channel angle and crown height on the screw stability of monolithic zirconia crowns supported by a recently introduced implant-abutment connection, both after 24 hours and cyclic loading.Material And Methods
Forty tissue-level implants (TLX Standard Plus) were divided into subgroups according to Ti-base abutment screw channel angle (straight [S] or 25-degree angled [A]) and crown height (10 mm [S] or 15 mm [L]) (n=10). Monolithic zirconia screw-retained mandibular first molar crowns were milled for each configuration and cemented onto their corresponding Ti-base abutments. Initial torque values (ITVs) and removal torque values (RTVs) after 24 hours were recorded. The screws were then replaced with new ones, ITVs were recorded again, and the crowns were subjected to cyclic loading (2.4 million cycles at 98 N). Final RTVs were measured, and the percentage torque loss was calculated. All data were analyzed with 2-way analysis of variance tests to analyze the effect of abutment screw channel angle and crown height on ITVs, RTVs, and percentage torque loss (α=.05).Results
No mechanical complications (screw loosening, decementation, or component fracture) occurred during testing. There were no significant differences among subgroups when ITVs, RTVs, and percentage torque loss were considered (P≥.377). Mean ITVs ranged from 34.9 to 35.2 Ncm. Mean RTVs measured after 24 hours of passive settling (before cyclic loading) were 29.2 to 29.5 Ncm, whereas those measured after cyclic loading were 24.8 to 25.4 Ncm. Corresponding mean percentage torque loss was 15.9% to 16.4% at 24 hours and 27.7% to 29.3% after cyclic loading.Conclusions
No mechanical failures, screw loosening, or decementation occurred during cyclic loading, regardless of crown height or abutment screw channel angulation. These findings indicated the potential of tested implant-abutment connection and Ti-base abutment screw channel angles to be alternatives to treat situations with compromised crown height space up to 15 mm.