Root canal preparation with FlexMaster: asessment of torque and force in relation to canal anatomy

To investigate physical parameters of FlexMaster nickel-titanium instruments while preparing curved canals in maxillary molars in vitro.

A torque-testing platform was used to prepare root canals in 11 extracted human maxillary molars with FlexMaster rotary instruments. Peak torque and force was registered along with numbers of rotations required to shape the canals. Canals were divided into 'wide' and 'constricted' groups depending on canal volumes assessed by microcomputed tomography. Resistance to cyclic fatigue was also tested. Mean scores for each instrument type were calculated and statistically compared using anova and Scheffé post hoc tests.

Mean torque varied between 0.1 +/- 0.1 and 0.8 +/- 0.5 N cm while mean force ranged from 4.2 +/- 2.0 to 7.3 +/- 3.5 N. Mean numbers of rotations totalled up to 18. All three variables registered showed weak correlations to preoperative canal volumes (P < 0.01) and differed significantly between 'wide' and 'constricted' canals (P < 0.001). Numbers of rotations to fracture in a cyclic fatigue test were between 348 and 1362.

FlexMaster instruments generated low torque scores and were highly resistant to cyclic fatigue, whilst three instruments fractured in extremely narrow canals. Consequently, more research is required to limit fracture incidence and to optimize instrumentation guidelines.