Two Million Cycle Fatigue Performance of Custom and Stock Conical-Hex Abutments: A Removal Torque and SEM Study

Introduction

Screw loosening remains a frequent mechanical complication in implant-supported prostheses, primarily caused by the gradual loss of abutment-screw preload. The aim of this study was to evaluate the mechanical performance of CAD-CAM custom and stock abutments by measuring removal torque values (RTV) at multiple time points and assessing surface morphology by scanning electron microscopy (SEM), following prolonged loading up to 2 × 10⁶ cycles.

Methods

Forty-four implant-abutment assemblies with an internal conical-hex connection were divided into two groups: Stock abutments (SA) and custom abutments (CA). After initial tightening, baseline RTVs were recorded. The samples underwent cyclic loading following ISO-14801 standards. RTVs were measured after 50 000, 1 × 10⁶, 1.5 × 10⁶ and 2 × 10⁶ cycles and after post-fatigue re-tightening. SEM analysis was performed at baseline, after 1 × 10⁶ and 2 × 10⁶ cycles.

Results

Baseline RTVs were higher in SA than CA. Both groups exhibited a progressive decrease in RTVs until 1 × 10⁶ cycles, with significantly lower values in the CA. Thereafter, a partial recovery was observed up to 2 × 10⁶ cycles, with no significant difference between groups. SEM images revealed more extensive surface wear in the SA group, while the CA group demonstrated localized adaptations; however, thread integrity was maintained in all samples.

Conclusion

Custom abutments showed lower baseline removal-torque values yet maintained preload as effectively as stock abutments after two-million cycles, confirming the mechanical suitability of both designs for functional loading. The greatest preload loss occurred between 50 000 and 1 × 10⁶ cycles; therefore, retightening the abutment screws during the early post-insertion period is recommended to maintain preload stability.