Practical application of infection control in endodontics

Influence of sterilization on mechanical properties and fatigue resistance of nickel–titanium rotary endodontic instruments

AIM

To evaluate the effect of repeated sterilization cycles in dry oven or autoclave, on the mechanical behaviour and fatigue resistance of rotary endodontic Ni-Ti instruments.

METHODOLOGY

New Ni-Ti instruments were subjected to five consecutive sterilization cycles in a dry oven or steam autoclave. Microhardness was measured in the nonmachined parts of the shanks of instruments using a Vickers indenter. Specimens of Ni-Ti wires were submitted to the same sterilization protocol and tensile tested until rupture. A group of instruments were fatigued to one half of their average fatigue life and then sterilized. New and sterilized instruments were fatigue tested until rupture. anova tests at alpha = 0.05 were used for statistical analysis.

RESULTS

Sterilization procedures resulted in no significant changes in Vickers microhardness, nor in the parameters describing the mechanical behaviour of the wires. However, the number of cycles to failure was statistically higher for all instruments after dry heat or autoclave sterilization cycles. In the instruments previously fatigued to one half of their fatigue life, autoclave sterilization gave rise to an increase of 39% in the remaining number of cycles to failure.

CONCLUSIONS

Changes in the mechanical properties of Ni-Ti endodontic instruments after five cycles of commonly used sterilization procedures were insignificant. The sterilization procedures are safe as they produced a significant increase in the fatigue resistance of the instruments.

Surface analysis of ProFile instruments by scanning electron microscopy and X-ray energy-dispersive spectroscopy: a preliminary study

AIM

The purpose of this preliminary study was to analyze surface irregularities on ProFile nickel-titanium (NiTi) rotary instruments before use, after sterilization by two different methods and after instrumentation of molar root canals.

METHODOLOGY

Five new ProFile NiTi rotary instruments with 0.04 taper, size 20, five instruments with 0.04 taper, size 25 and five with 0.06 taper, size 20 were analyzed by scanning electron microscopy (SEM) and X-ray energy-dispersive spectroscopy (EDS). The instruments were then divided into two groups: three of each taper and size were sterilized in a dry heat sterilizer, whilst two of each taper and size were sterilized in an autoclave. After inspection by the same techniques, the files were used to instrument six molar root canals, and again analyzed by SEM/EDS.

RESULTS

Together with the usual machining defects, a variable amount of material containing mainly carbon and sulphur was found on the surface of new instruments. This material was not removed by standard cleaning and sterilization procedures. The use of the files for instrumentation of root canals eliminated a considerable amount of the adhered substance, but deposits rich in calcium and phosphorus accumulated on the remaining impurities.

CONCLUSIONS

The adherence of dentine on surface deposits found on new ProFile instruments requires special cleaning, prior to sterilization.

Cleaning of endodontic files, Part I: The effect of bioburden on the sterilization of endodontic files

Ninety-two new endodontic files were randomly assigned to five groups with varying parameters of contamination, cleaning method, and sterilization (steam or chemical). Files were instrumented in bovine teeth to accumulate debris and a known contaminant, Bacillus stearothermophilus. Positive controls produced growth on both T-soy agar plates and in T-soy broth. Negative controls and experimental files (some with heavy debris) failed to produce growth. The results showed that there was no significant difference between contaminated files that were not cleaned before sterilization and contaminated files that were cleaned before sterilization. Bioburden present on endodontic files does not appear to affect the sterilization process.

Effects of cleaning, disinfection, and sterilization procedures on the cutting efficiency of endodontic files

The effects of various cleaning, chemical disinfection, and sterilization procedures on the cutting efficiency of endodontic instruments Unifile (De Trey, Bois Colombes, France), (Flexofile Maillefer, Ballaigues, Switzerland), and H-File (Maillefer)) were investigated. The cross-infection control treatment procedures investigated were as follows: chemical disinfection--NaOCl (2.5%) for 12 and 48 h, and NH4 (5%) for 1 and 4 h; ultrasonic cleaning for 4 and 16 cycles of 15 min; and sterilization methods with chemiclave for 5 and 10 cycles of 20 min, Poupinel for 5 and 10 cycles of 120 min at 180 degrees C and glass beads for 10 and 40 cycles of 40 at 250 degrees C. Cutting efficiency was evaluated as the mass of Plexiglas cut per unit of energy expended by the instrument in microgram/Joule. The cutting efficiency decreased from 1 to 77%, depending on the file design and type of treatment procedures. Heat sterilization (Poupinel) did not modify the cutting efficiency of Unifile and Flexofile. The decrease in cutting efficiency was independent of frequency and duration of treatment procedures.