Effect of gamma-ray sterilization on phase transformation behavior and fatigue resistance of contemporary nickel-titanium instruments.

Objectives: The aim of this study was to compare body temperature cyclic fatigue resistance (CF) of heat-treated NiTi rotary instruments submitted to gamma-irradiation or autoclave sterilization.

Materials And Methods: A total of 176 instruments (88 Hyflex EDM (HEDM) and 88 TRUShape (TS)) were used. Forty-four of each were submitted to gamma-irradiation. Twenty-two in each group were further autoclaved, resulting in a total of eight groups (n = 22 each): TS-γ and HEDM-γ (gamma-irradiated)/TS-γ-ac and HEDM-γ-ac (gamma-irradiated and autoclaved)/TS-non-γ and HEDM-non-γ (non-sterilized controls, non-gamma-irradiated/non-autoclaved)/TS-non-γ-ac and HEDM-non-γ-ac (non-gamma-irradiated and autoclaved). Twenty in each were tested for CF. Weibull parameters were calculated and compared. The remainder was used for differential scanning calorimetry analysis.

Results: TS instruments lasted significantly longer if not submitted to gamma-irradiation (TS-non-γ mean-life = 101.5 s, 95 CI% 91.7-112.3; TS-γ mean-life = 83.2 s, 95 CI% 76-91.1); further autoclaving did not significantly affect CF. Gamma-irradiation did not affect HEDM (HEDM-γ mean-life = 717.9 s, 95 CI% 636.8-809.3; HEDM-non-γ: mean-life = 678.8 s, 95 CI% 595.1-744.2); further sterilization significantly decreased fatigue resistance (HEDM-γ-ac mean-life = 524.1 s, 95 CI% 476.1-576.8; HEDM-non-γ-ac mean-life = 570.6, 95 CI% 512.3-635.5). Gamma irradiation benefited HEDM conversion to martensite state and its maintenance, while further autoclave sterilization reverted the benefit.

Conclusions: Gamma-irradiation and autoclaving differentially affected fatigue life span and transformation temperatures of contemporary martensitic rotary instruments manufactured with different methods.

Clinical Relevance: Gamma-ray processing is increasingly utilized by manufacturers for clinician to use fully sterile packaged instruments; however, the sterilization method of choice should vary depending on the effects on the behavior of contemporary martensitic rotary instruments manufactured with different methods.