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 »  Home  »  Endodontic Articles 11  »  Effect of sterilization on the cutting efficiency of PVD-coated nickel-titanium endodontic instruments
Effect of sterilization on the cutting efficiency of PVD-coated nickel-titanium endodontic instruments
Discussion - References.



Discussion.
The PVD technique is a common method to deposit wear resistant thin film coatings on surgical or dental instruments (Grohmann & Mathey 1991, Schafer in press). PVD includes reactive magnetron sputtering, ion plating and arc evaporation (Smith1995). The cathodic arc evaporation technique is often used to create hard coatings including TiN, TiC, TiCN and TiAlN (Smith 1995). Using this technique it is possible to deposit a fine grained TiN film on instruments at comparatively low temperatures (Grohmann &Mathey1991, Schafer in press).Coating thickness ranges from1to 7 mm and it is possible to obtain surface hardness of about 2200 Vickers units (Grohmann & Mathey1991).
The test for determination of cutting efficiency employed in the present study has been described in detail (Schafer 1995, Tepel et al. 1995a,b). Tolerances in the diameter of the files or in the diameters of the specimens’ lumen did not affect the results because the lumen diameter of the cylindrical canal was 0.40 mm whilst the tapered instruments had a tip diameter of 0.35 mm. Thus, the instruments, which did not rotate at this time, could passively penetrate the lumen to a certain extent without contacting the canal wall. As soon as the instrument came into circumferential contact with the wall of the specimens, the motor of the testing device was switched on and the rotating instrument removed material and penetrated actively, i.e. deeper into the lumen until it was blunt and the maximum penetration depth was reached. Since only the passive penetration depth depends on the aforementioned tolerances, this was the reference (zero) line for the determination of the maximum penetration depth and, therefore, for determination of cutting efficiency in a rotary working motion (Tepel et al.1995a,b).Admittedly, the rotary motion generated by the testing device did not reflect a clinical situation in which the endodontic instrument is required to flex whilst it rotates, for example, when enlarging curved root canals. Thus, the results of the present investigation allow no prediction whether or not the TiN coating withstands repeated flexural stress, resulting in fatigue.
Nevertheless, according to the present results, the PVD coating technique increased the cutting efficiency of NiTi files. The TiN-coated instruments that were not sterilized (group B.C)h ad a significant increase (P < 0.001)i n cutting efficiency by up to 25.4% in comparison to the uncoated instruments of the same batch (group A.C)t o corroborate the results of previous studies (Rapisarda et al. 2000, Schafer in press).
Repeated sterilization of endodontic instruments using either steam sterilization or autoclave have been shown to have a minor influence on the cutting efficiency of stainless steel instruments (Schafer 1995, Haikel et al. 1996). Moreover, the bending and torsional properties of either NiTi or stainless steel instruments were not consistently affected by different sterilization procedures (Iverson et al. 1985, Silvaggio & Hicks 1997, Canalda-Sahli et al.1998, Svec & Powers1999).
The sterilized uncoated files (groups A.1 to A.3) had a statistically significant loss of cutting efficiency in comparison with instruments that were not sterilized (control group; P < 0.05). Files that underwent five cycles of sterilization experienced a 16.1% reduction in cutting efficiency compared with the control files (Fig. 2). After 10 cycles of sterilization, the cutting efficiency was reduced further by up to 50.8% (Table 1). The present findings are in good agreement with those of two other studies (Schafer1995, Rapisarda et al.1999).

Figure 2. Notched boxplots of the maximum penetration depths (mm) achieved by the uncoated instruments of the different groups.
Group A.1: Five cycles of sterilization; A.2: 10 cycles of sterilization; A.3: NaOCl treatment prior to five cycles of sterilization; A.C: Nonsterilized controls.

Notched boxplots of the maximum penetration depths achieved by the uncoated instruments of the different groups

The explanation for the undesirable effect of sterilization might be that sterilized instruments show changes in their outer surfaces and in their chemical composition (Rapisarda et al.1999). It was observed that sterilization produced an increase in titaniumoxide in the near surface layer of the instruments (Rapisarda et al.1999).
In the present investigation, two out of12 NaOCl-treated uncoated instruments separated during the determination of cutting efficiency. Since clinically, size 35 hand instruments rarely really separate, this might be owing to the torsional load in the specific test rather than due to an adverse effect of the sterilization procedures or the NaOCl treatment on the torsional properties of these instruments.
According to the present results, 30-min immersion of uncoated NiTi instruments in NaOCl followed by five cycles of sterilization (group A.3) resulted in a statistically significant loss of cutting efficiency (17.4%; P < 0.05)( Table 1). The results reveal that the maximum penetration depths of the files after immersion in NaOCl followed by repeated sterilization did not differ from those after five cycles of sterilization without NaOCl treatment (3.06 mm vs. 3.11mm; Table 1). Thus, the present results seem to confirm a previous study in that NaOCl treatment without any sterilization cycle seems to have no or only insignificant adverse effect on the cutting efficiency of NiTi instruments (Haikel et al. 1998). These findings are in good agreement with the results of Busslinger et al. (1998) who showed that NiTi was resistant to 1% solution of NaOCl, whilst a statistically significant amount of titanium was lost from the NiTi instruments after immersion times of 30 and 60 min in 5% NaOCl. Just as in the present study, the authors concluded that it is doubtful if this is of any clinical significance, since clinically, files do not have an ‘in situ’ contact time of 30 min with NaOCl (Busslinger et al.1998). Thus, according to these findings and the present results, it can be assumed that only the sterilization cycles displayed an adverse effect on the cutting efficiency of the files.
It has been stated that stainless steel hand files should be considered disposable instruments because they wear rapidly when used on dentine (Kazemi et al. 1995).These authors proposed that from a cost-effective point of view, files should be considered disposable because it is impossible to ensure that files are optimally efficient after use (Kazemi et al. 1995). Certainly, the same is true for uncoated NiTi hand instruments, which are also known to wear rapidly (Walia et al. 1989, Thompson 2000). However, further research is necessary to provide data on how often TiN-coated NiTi instruments can be reused without a considerable decline in efficiency.

References.

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