Effect of different restorative approaches and broken instrument removal techniques on the fracture resistance of root canal-treated teeth: an in vitro study

This study aimed to evaluate the effect of three different restorations—a prefabricated glass-fiber post and zirconia crown (PGF/Zc), a polyethylene woven-fiber post and zirconia crown (PWF/Zc), or a zirconia endocrown (ZE)—on the fracture resistance of teeth after broken file removal using either an ultrasonic or a micro-retrieve and repair (MR&R) system. Ninety mandibular premolar root canals were instrumented. Endodontic rotary files were intentionally fractured in the middle third of 80 teeth. Ten teeth served as a positive-control group. The broken fragments were removed from the 80 teeth and divided into two main groups: ultrasonic and MR&R. The removal times and the amount of dentin loss were measured. Following removal and single-cone obturation, the teeth were divided into four subgroups (n = 10 per group): PGF/Zc, PWF/Zc, ZE, and a negative control. All zirconia restorations were cemented with a dual-curing cement. Fracture resistance was measured using a universal testing machine with force applied at a 45° angle. Statistical analysis used the non-parametric Kruskal–Wallis test with (α = 5%). No significant differences were observed between the two removal methods for file removal time or weight loss ( p > 0.05). The fracture resistance was numerically higher in the ultrasonic group; however, this difference was not statistically significant compared to the MR&R group ( p = 0.317). Among the restored groups, ZE restorations consistently provided higher fracture resistance than PGF/Zc or PWF/Zc restorations ( p = 0.035). Notably, PGF/Zc exhibited the lowest resistance in MR&R-treated samples. The zirconia endocrown (ZE) was the structurally superior restorative choice across all groups following fractured instrument removal.