Combined Effects of Instrumentation Geometry and Sealer Type on Mandibular Premolar Root Fracture Resistance

Aim The aim of this study was to evaluate the effects of different root canal preparation parameters (file size and taper) and sealer types on the fracture resistance of endodontically treated mandibular premolar roots. Methods Ninety extracted single-rooted mandibular premolars were randomly assigned to eight experimental groups (n = 10 per group) and one control group (n = 10). Experimental groups were prepared using FKG RACE EVO nickel-titanium rotary files with sizes 25 or 30 and tapers of 0.04 or 0.06 and obturated using a single-cone technique with either TotalFill BC Sealer (bioceramic) or AH Plus (epoxy resin-based). Control specimens remained untreated. After seven days of storage at 37°C and 100% humidity, fracture resistance was measured using a universal testing machine at a crosshead speed of 1 mm/min until fracture occurred. Data were analyzed using the Kruskal-Wallis and Mann-Whitney U tests, with statistical significance set at α = 0.05. Results No statistically significant differences were found among the eight experimental groups (χ² = 11.64, df = 8, p = 0.076, η² = 0.045), although a small effect size was observed. Consistent trends were noted: bioceramic sealer groups demonstrated 10.7% higher mean fracture resistance (420.13 N; 89.8% of control) compared with epoxy resin-based groups (379.42 N; 81.2% of control). Size 25 preparations showed an 8.5% advantage (416.06 N; 89.0% of control) over size 30 (383.50 N; 82.0% of control), and 0.04 taper showed a 6.0% advantage (411.51 N; 88.0% of control) over 0.06 taper (388.04 N; 83.0% of control). The weakest group retained 67.7% of the control strength (316.35 N vs 467.58 N). High within-group variability (CV: 14-42%) likely precluded detection of statistical significance. Conclusions Within the limitations of this study, preparation parameters and sealer type did not produce statistically significant differences in fracture resistance. However, consistent directional trends favoring smaller preparations and bioceramic sealers suggest potential clinical relevance. These findings support the principles of minimally invasive endodontics.