Effect of heat sterilization on surface characteristics of two different Niti rotary systems – A SEM study

Abstract

Rotary nickel-titanium instruments have attained great popularity because of their high flexibility, increased elastic limits and better resistance to bending and torsional failure compared to stainless steel instruments. Endodontic instruments come into contact with saliva, blood and infected pulp tissue and it is inevitable that they are sterilized after each use to avoid cross-infection between patients. The effect of heating and cooling cycle’s occurring during heat sterilization on the physical and mechanical properties of few brands of endodontic instruments has been investigated in the past.

Aims and objectives:

The aim and objective of this study is to investigate the surface characteristics of Protaper and Race NiTi instruments at 0, 3 and 5 cycles of moist heat sterilization by using scanning electron microscopy.

Materials and Methods:

A total of 60 NiTi rotary instruments, 30 instruments belonging to two different brands, Protaper F2, 25 mm, tip size 25 NiTi rotary files (Dentsply – Tulsa dental) and  Race 6 % 25 mm, tip size 25 NiTi rotary files (FKG Dentaire)  were selected for this study. Of the 30 samples from each group, 10 samples were kept for observation in as received condition (control group / sub group a), 10 samples were sterilized for 3 cycles in autoclave (sub group b) and 10 samples were sterilized for 5 cycles in autoclave (sub group c). The sterilization cycles were conducted in an autoclave (Uniqueclave C-79, Confident dental equipments, India) at 121 degree C under 15 psi for 20 minutes. Samples were allowed to cool to room temperature for at least 30 minutes between cycles. All the samples were examined in a Stereoscan 440 SEM (Leica Cambridge Inc.) from 200 X to 1000 X magnification. The level of surface roughness of the specimens was rated and scored on four appearances, using a predefined scale and selected SEM pictures. The results were statistically analysed using ANOVA followed by Student Newman Keuls test.

Results:

In this study, the outer surface of all new Protaper and Race rotary instruments manifested structural defects, debris, pitting, metal strips and milling works. From the study it was found that, after 5 cycles of moist heat sterilization there is a statistically significant increase in surface roughness  of both Protaper and Race NiTi instruments  compared to as received samples, but no significant difference in between the two brands.

Conclusion:

The surface characteristics of the endodontic files as seen by SEM act as a very significant factor in failure and initiation of fracture. The presence of defects on the cutting edges of these instruments may be a reason for their relatively low cutting efficiency and may diminish their corrosion resistance and may make them more susceptible to fracture. These defects can act as areas of stress concentration at which crack propagation starts. Thus NiTi instruments, especially small sizes, should be used with caution. The consequences of these surface imperfections in terms of durability and cutting efficiency in clinical use are difficult to assess and further studies are required.

Keywords: (endodontics, endodontic instruments, niti files, niti rotary, protaper, autoclave sterilization, scanning electron microscopy, surface characterization, surface roughness)