Introduction - Materials and methods.
E. SchaferDepartment of Operative Dentistry, University of Munster, Munster, Germany.Introduction.
In recent years, nickel-titanium (NiTi) alloy has been successfully used in the manufacture of endodontic instruments (Schafer 1997, Thompson 2000).This alloy, named Nitinol, consists of approximately 55% nickel and 45% titanium by weight (Schafer 1997, Thompson 2000). Owing to their substantially increased flexibility compared to stainless steel instruments, NiTi files are reported to be particularly suitable for preparing curved root canals (Pettiette et al. 1999, Thompson 2000, Bergmans et al.2001, Peters et al.2001). NiTi instruments were shown to have three times the elastic flexibility in bending and torsion of the stainless steel files (Walia et al.1989, Tepel et al.1997). Nitinol alloys exhibit superelastic behaviour, allowing them to return to their original shape upon unloading following deformation (Schafer1997, Thompson 2000).
Manufacturing of NiTi files is more complex than that of stainless steel instruments as the files have to be machined rather than twisted (Thompson 2000). Owing to its superelasticity, it is impossible to twist a NiTi blank counterclockwise in order to produce a spiral since NiTi alloys undergo almost no permanent deformation (Schafer 1997, Thompson 2000). More likely, they will fracture when being extensively twisted in order to produce a spiral. However, it is known that grinding of nickel-based alloys is difficult because considerable wear of the milling head occurs within a short time (Schafer 1997). This leads to structural defects especially on the cutting edges of NiTi instruments that may compromise the cutting efficiency of these instruments (Schafer 1997, Haikel et al. 1998, Thompson 2000).At the same time, metal flas hand surface heterogeneity might allow the NiTi to corrode (Serene et al. 1995).
Therefore, some surface engineering techniques have been used to improve the surface hardness and the corrosion resistance of NiTi instruments (Lee et al. 1996, Rapisarda et al. 2000; 2001). The surface hardness of Nitinol can be increased by ion implantation of boron (Lee et al.1996). Ionic implantation of nitrogen ions creates a surface layer of titanium nitride (TiN)t hat enhances surface hardness, cutting efficiency and wear resistance (Rapisarda et al. 2000; 2001). Furthermore, most recently it has been shown that a TiN hard coating created by a physical vapour deposition (PVD) cathodic arc evaporation technique increased the cutting efficiency of NiTi files significantly (Schafer in press).To date, no studies have examined the effects of repeated sterilization on the cutting efficiency of these TiN-coated NiTi instruments.
The purpose of the present study was to investigate potential alterations in cutting efficiency when conventional and TiN-coated NiTi K-files that had undergone PVD cathodic arc evaporation coating were exposed to repeated sterilization under autoclave. Moreover, the alterations in cutting of both uncoated and TiN coated NiTi files after exposure to sodium hypochlorite (NaOCl) and repeated autoclave sterilization were also evaluated. Materials and methods.Instruments.
A total of 96 new NiTi (size 35) hand K-files (Naviex, Brasseler, Savannah, GA, USA) we re examined. All instruments were taken from a single batch (081501515). The instruments were randomly divided into two groups (groups A and B)o f 48 instruments each. Whilst the instruments of group B were exposed to PVD coating creating a TiN layer with a thickness of 1.5 mm, as previously described (Schafer in press), the files of group A were not coated. Subsequently, the instruments of groups A and B were randomly divided into four subgroups of12 instruments each.
- Group A.1 (experimental): Uncoated instruments were exposed to five cycles of sterilization under autoclave.
- Group A.2 (experimental): Uncoated instruments were exposed to 10 cycles of sterilization under autoclave.
- Group A.3 (experimental): Uncoated instruments were immersed in 5.25% NaOCl for 30 min, rinsed for 10 min in tap water, allowed to dry and thereafter exposed to five cycles of sterilization under autoclave.
- Group A.C (control): Uncoated instruments were not exposed to the sterilization process.
- Group B.1 (experimental): TiN-coated instruments were exposed to five cycles of sterilization under autoclave.
- Group B.2 (experimental): TiN-coated instruments were exposed to 10 cycles of sterilization under autoclave.
- Group B.3 (experimental): TiN-coated instruments were immersed in 5.25% NaOCl for 30 min, rinsed for 10 min in tap water, allowed to air dry and thereafter exposed to five cycles of sterilization under autoclave.
- Group B.C (control): TiN-coated instruments were not exposed to the sterilization process.
The autoclave used was a Aesculap Automat 356 (Aesculap, Tuttlingen, Germany). Each cycle of sterilization was performed at 134 8C and 2 bar for 30 min without any corrosion protection. Between the single cycles of sterilization, the instruments were allowed to dry and cool to room temperature.
The TiN coating was created using a PVD technology described previously (Schafer in press).The instruments were fixtured in a vacuum coating chamber and preheated. After heating, an argon ion bombardment cycle commenced in order to clean the surface of the instruments prior to depositing the coating. The substrate temperature was 180 8C. Cathodic arc evaporation was used to create a highly ionized plasma. Following the cleaning process, an arc was struck on multiple titanium cathodes positioned inside. The arc flash evaporated titanium which was attracted to the negatively charged instruments (bias = 60 V). Following the creation of a thin layer of about100 nm of pure titanium, which acted as an adhesive layer, a second layer of TiN was created with a thickness of 1.5 mm. Thereafter, nitrogen gas was introduced at a low partial pressure into the coating chamber which reacted with the titanium to form titanium nitride. Cutting efficiency.
The spiral of a K-file establishes a cutting angle, i.e. an angle of the flutes to the long axis of the instrument, which is less than 458 (Schafer 1997). Therefore, the cutting efficiency of all instruments was determined in a rotary working motion since these instruments are primarily designed to be used in this way (Schafer 1997).
Cutting efficiency of all instruments was determined by means of a specially designed, computer-driven testing device. The function of this test apparatus has been described in detail in previous studies (Tepel et al. 1995a,b).Special plastic samples with a cylindrical canal having well-defined abrasive properties were used and the maximum penetration depth of the instruments into the lumen was the criterion for cutting efficiency and the basis for the comparison (Tepel et al. 1995a,b). The cylindrical lumen was 17 mm long and the diameter of this lumen was 0.40 mm (Tepel et al.1995a,b) .
The data established for uncoated and coated instruments were analyzed separately. Anova was used to compare maximum penetration depths across the four groups. Thereafter, Bonferroni-adjusted pair wise t-tests were used to evaluate which groups differed from the accompanying control group. The level of significance was set at P < 0.05.