The effect of cleaning and sterilization on the torsional properties of protaper rotary Nickel Titanium endodontic instrument


Background: The torsional properties of rotary nickel titanium endodontic instrument may be affected by cleaning
and sterilization. This study was conducted to evaluate the effect of the number of cleaning procedures involving
immersion in NaOCl with two different concentrations (1% and 5.25%) and sterilization procedures (autoclave and
dry heat) on the torsional properties of protaper rotary nickel titanium instrument when subjected to clockwise
torsional load). The types of defects for the fractured instruments also were studied.
Material and methods: A total of ninety rotary protaper finisher F3 instruments were divided into 3 groups. Group 1
comprises of ten instruments were subjected to neither cleaning nor sterilization cycles and assigned as the control
group. Group 2 and group 3 were subjected to five and ten cleaning and sterilization cycles respectively. Each
group comprises of forty instruments, twenty of them soaked within 1% NaOCl and the other twenty were soaked
within 5.25% NaOCl, ten of each these twenty instruments, were sterilized with autoclave while, the other ten
instruments were sterilized with dry heat oven. Then, all instruments were subjected to a clockwise torsional load until
fracture, the torsional moment and angular deflection at fracture were calculated. Then the fractured instruments
examined to detect the signs of distortion.
Results: The results show a statistically significant reduction in the torsional strength after cleaning and sterilization
cycles regardless the concentration of NaOCl, type and number of sterilization cycles. For the angular deflection,
there was a highly significant reduction, where, the number of cycles was the most influencing factor for reduction
of the angular deflection whereas, the concentration of NaOCl and the type of sterilization were show no significant
influence on the angular deflection. The examination of the fractured instruments show a signs of plastic deformation
of the spirals, unwinding, reverse winding, reverse winding with tightening of the flutes of the instruments or a
combination of all these signs.
Key words: Torsional strength, angular deflection, NaOCl, nickel titanium. (J Bagh Coll Dentistry 2009; 21(4):53-59)