Impact of implant abutment connection designs and cyclic loading on screw stability in dental implants: A systematic review and meta-analysis.

STATEMENT OF PROBLEM: Screw loosening is a common mechanical complication that can compromise the long-term success of implant-supported restorations. Implant-abutment connection design and cyclic loading influence load transfer, preload retention, and joint stability; however, their specific contributions to screw loosening under functional conditions remain unclear.

PURPOSE: The purpose of this systematic review and meta-analysis was to evaluate the effect of implant-abutment connection geometry on abutment screw stability under cyclic mechanical loading.

MATERIAL AND

METHODS

This manuscript followed the Preferred Reporting Items for the Systematic Review and Meta-Analyses (PRISMA) statement. Electronic searches were conducted in the PubMed, Scopus, and Web of Science databases for in vitro studies assessing screw loosening after cyclic loading. Studies that evaluated implant-abutment assemblies with clearly specified connection designs using commercially available implants were included. Two reviewers independently screened and extracted data and assessed methodological quality. Random-effects meta-analyses were performed to estimate the mean removal torque loss ratio for each connection type. Meta-regression explored potential sources of heterogeneity. Designs not suitable for meta-analysis were evaluated qualitatively.

RESULTS

A total of 21 in vitro studies met the inclusion criteria. Most applied cyclic loading ranging from 50 000 to 3 000 000 cycles, with forces between 100 and 300 N. Meta-analysis of studies on external hexagon connections revealed a significant pooled torque loss of 23.9% (P=.016) with high heterogeneity (I²=87.6%). The pooled torque loss was 19.2% for the internal hexagon (P=.074), 16.6% for Morse taper (P=.057), and 16.0% for conical connections (P>.05). Narrative synthesis indicated that hybrid conical connections retained preload more effectively than conventional internal hexagon designs. Angled abutments increased torque loss, and Morse taper hybrids generally showed superior stability.

CONCLUSIONS

All connection types experienced preload loss under cyclic loading, with external hexagon systems showing the greatest average loss. Conical and Morse taper designs preserved preload better than flat-to-flat hexagon connections. Mechanical stability was affected by connection geometry, abutment angulation, and screw material.