Deformation and fracture of K3 rotary nickel-titanium endodontic instruments after clinical use

The aim was to evaluate the incidence and type of defects that occurred with K3 rotary nickel-titanium instruments during routine clinical use.

A total of 2397 K3 (G-PACKS, SybronEndo, West Collins, Orange, CA, USA) instruments were collected from a graduate endodontic clinic over 21 months. All the instruments were limited to a maximum use of 30 canal preparations. The collected instruments were measured by a digital caliper to determine whether any fractures had occurred and then were visually inspected for deformation and fracture under a stereomicroscope. The surfaces of fractured instruments were further evaluated under a scanning electron microscope. Data were analysed using chi-square test and Kruskal-Wallis test.

The incidence of instrument defect was 5.63%, consisting of 3.59% fractures and 2.05% deformations. The defect rates of 0.04 and 0.06 files were statistically higher than the other taper groups (P < 0.003) except for 0.08 files (P > 0.05). For the fractured instruments, 63.95% failed from flexural fatigue, whilst 36.05% failed from torsion.

Flexural fracture was the major mode of fracture for instruments with larger taper. A routine check for instrument integrity particularly for 0.04 and 0.06 files at high magnification is recommended after each clinical use.