Surface defect burden as determinant of cyclic fatigue fragmentation in counterfeit endodontic files: SEM-based comparative study

Introduction. Counterfeit nickel-titanium endodontic instruments may present manufacturing defects that compromise their resistance to cyclic fatigue; however, the quantitative contribution of SEM-objectified surface irregularities to file separation remains insufficiently defined. This study aimed to determine whether counterfeit origin and formally categorized SEM-based surface defects are statistically associated with reduced time to cyclic fatigue-induced fragmentation of endodontic files.

Methods. Forty Soco SC Pro endodontic instruments were examined, including 20 original files and 20 counterfeit replicas. Surface morphology was assessed by scanning electron microscopy, and detected irregularities were categorized using adapted SEM-based defect classification systems. Cyclic fatigue testing was performed in standardized endodontic plastic blocks with simulated canal curvatures of different radii (7 mm and 5 mm). Time to separation was recorded as the primary outcome. Associations between fragmentation time, instrument authenticity and SEM-based defect category were evaluated using correlation and regression analyses.

Results. The between-group difference was statistically significant (p = 0.000167), indicating markedly reduced cyclic fatigue resistance among counterfeit instruments. Correlation analysis showed moderate negative associations between time to separation and counterfeit status (r = −0.561), and SEM-based defect category (r = −0.405). Regression analysis further confirmed the independent contribution of these factors: counterfeit origin reduced time to separation by 74.24 s (95% CI: −109.55 to −38.93; p < 0.001), and each increase in SEM defect category was associated with a 17.20 s reduction in fracture time (95% CI: −33.05 to −1.34; p = 0.034). However, none of the examined files was characterized with defect-free surface based on provided SEM observations.

Conclusion. Counterfeit status, and SEM-quantified surface defects are statistically significant predictors of earlier cyclic fatigue-induced file separation. These findings indicate that counterfeit instruments not only fracture faster on average but also demonstrate clinically relevant and statistically measurable deterioration of mechanical reliability.