Experimental evaluation of new rotary instruments with innovative manufacturing treatments potentially affecting clinical performance. Part 2: torsional resistance

Abstract

The mechanical behavior of nickel-titanium (NiTi) rotary instruments is strongly influenced by manufacturing technologies, including proprietary heat treatments and surface modifications. While increased flexibility is generally associated with improved cyclic fatigue resistance, torsional resistance has traditionally been correlated with instrument stiffness and cross-sectional mass. In a previous investigation (Part 1), significant differences in flexibility were observed among three size 15 rotary instruments. The aim of the present study was to compare the torsional resistance of the same instruments and to investigate whether modern manufacturing treatments modify the traditional relationship between flexibility and torsional strength. Three groups (n = 10) of size 15 rotary instruments were evaluated using a modified ISO 3630-1 torsional testing protocol. The tested instruments were ZARC 15/.04 (Simit, Italy), ONYX 15/.03, and ONYX 15/.04 (EdgeEndo, USA). Maximum torque at fracture (N·cm) and time to fracture (s) were recorded. Statistical analysis was performed using one-way analysis of variance, with significance set at P < .05. Mean torsional resistance values were 1.28 N·cm for ZARC 15/.04, 1.36 N·cm for ONYX 15/.03, and 1.69 N·cm for ONYX 15/.04. Mean times to fracture were 0.75 s, 0.92 s, and 1.49 s, respectively. Statistical analysis demonstrated significant differences among the instruments, particularly between the two .04 taper instruments (P < .05). Despite exhibiting significantly greater flexibility in the previous study (Part 1), ONYX 15/.04 demonstrated the highest torsional resistance. These findings challenge the traditional assumption that greater rigidity is necessarily associated with superior torsional strength and suggest that contemporary manufacturing and thermomechanical processing can substantially influence the mechanical behavior of NiTi instruments.