Factors influencing the fracture of nickel-titanium rotary instruments

Aim To evaluate the effect of rotational speed and the angle and radius of curvature of root canals on the fracture of two types of nickel-titanium rotary instruments: K3((R)) and ProTaper((R)) . Methodology A total of 240 root canals of extracted human maxillary and mandibular molars were divided into two groups of 120, according to the angle of the canal curvature (group A: <30degrees, group B: >30degrees). Each group was then divided into two subgroups of 60 canals in order to perform instrumentation using K3((R)) and ProTaper((R)) rotary instruments at three different rotational speeds: 150, 250 and 350 r.p.m. (20 canals at each rotational speed). Each instrument was used a maximum of 20 times and at one rotational speed only. The angle and radius of canal curvature were measured in the only group in which fractures actually took place (group B). Results There were a total of 22 instrument fractures; all of these occurred in canals with curves >30degrees. In a multivariate analysis, it was demonstrated that the files used at a rotational speed of 350 r.p.m. were more likely to fracture than those used at 250 r.p.m. (OR: 1113.88; 95% CI: 2.36-526420.05) and than those used at 150 r.p.m. (OR: 13531.33; 95% CI: 5.37-34120254.00). A decrease in the angle of curvature of the canal also significantly reduced the likelihood of fracture (OR: 0.2083; 95% CI: 0.068-0.6502). These relationships remained significant after being adjusted for the potential interactions between the remaining variables. No significant differences were found between the files or the radii of the canals. Conclusions Instrument fracture was associated with rotational speed and the angle of curvature of the canal.