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Effect of Physical Vapor Deposition on Cutting Efficiency of Nickel-Titanium Files
JOURNAL OF ENDODONTICS Copyright © 2002 by The American Association of Endodontists
Printed in U.S.A. VOL. 28, NO. 12, DECEMBER 2002
SCIENTIFIC ARTICLES Effect of Physical Vapor Deposition on Cutting Efficiency of Nickel-Titanium Files Edgar Scha¨fer, Prof, Dr
complex than that of stainless steel instruments, because the instruments have to be machined rather than twisted (1, 2). Due to the superelasticity of this alloy it is impossible to twist a nickeltitanium blank counterclockwise to produce a spiral, because nickel-titanium alloys underwent nearly no permanent deformation (1, 2). More likely they will fracture when being extensively twisted to produce a spiral. It is known that grinding of nickelbased alloys is quite difficult, because considerable wear of the milling head occurs within a short time (2). This leads to structural defects especially on the cutting edges of nickeltitanium instruments, which may compromise the cutting efficiency of these instruments (1, 2). Moreover, the cutting edges of nickel-titanium instruments have a microhardness ranging from 303 to 362 Vickers units, whereas stainless steel instruments showed a hardness ranging from 522 to 542 Vickers units (4). On account of the described surface irregularities and the low surface hardness, cutting efficiency of nickel-titanium files was less compared with most stainless steel instruments, as several authors have pointed out (5–7). Therefore, some surface engineering techniques have been used to improve the surface hardness and wear resistance of nickeltitanium instruments (8 –10). In some cases, the surface hardness of Nitinol was increased by ion implantation of boron (10) or nitrogen ions. Ionic implantation of nitrogen ions creates a surface layer of titanium nitride (TiN), which can significantly improve surface hardness, cutting efficiency, and wear resistance (8, 9, 11). Also by exposing Nitinol to thermal nitridation processes, surface hardness was successfully enhanced (9, 12). Another similar technique to deposit wear-resistant thin film coatings on surgical or dental instruments is the physical vapor deposition (PVD) technique, which was introduced to the medical device industry in the late 1980s (13). PVD includes reactive magnetron sputtering, ion plating, and arc evaporation (14). To achieve the best possible adhesion of the coating to the substrate’s surface, the cathodic arc evaporation technique is often used (13), creating hard coatings including TiN, TiC, TiCN, and TiAIN (14). Using this technique it is possible to deposit a fine-grained TiN film on the instruments at comparatively low temperatures (13). Coating thickness ranges from 1 to 7 m, and it is possible to obtain surface hardness of approximately 2200 Vickers units (13). Up to now, there are no studies available using PVD coatings
The purpose of this study was to investigate possible changes in cutting efficiency of nickel-titanium K-files that had undergone physical vapor deposition coating. Titanium nitride coatings were deposited using different process parameters. A total of 84 nickel-titanium K-files (size 35) were randomly divided into 7 groups of 12 instruments each. Groups A to F (experimental): instruments were coated with titanium nitride using different process parameters regarding substrate temperature, applied voltage, coating thickness, and ion bombardment. Group K (control): samples were not coated with titanium nitride. The cutting efficiency of all instruments was determined in a rotary working motion by means of a computerdriven testing device. Special plastic samples with a cylindrical canal were used, and the maximum penetration depth of the instruments into the lumen was the criterion for cutting efficiency. Instruments of groups A, F, and C achieved significantly greater penetration depths than the uncoated instruments of the control group (p < 0.05). Cutting efficiency of physical vapor deposition-coated nickel-titanium files was increased by up to 26.2% in comparison with uncoated instruments.
In recent years, nickel-titanium alloy has been successfully used in the manufacturing of endodontic instruments (1, 2). This alloy, named Nitinol, consists of approximately 55% nickel and 45% titanium by weight (1, 3, 4). The generic term for this alloy is 55-Nitinol (1). Owing to the substantially increased flexibility compared with stainless steel instruments, nickel-titanium– based instruments are reported to be particularly suitable for preparing curved root canals (1, 2). Nitinol alloys exhibit superelastic behavior, allowing the return to their original shape upon unloading after deformation (1, 2). Therefore, the manufacture of nickel-titanium instruments is more 800
Vol. 28, No. 12, December 2002
PVD Coating of NiTi
trying to enhance surface hardness and cutting efficiency of endodontic instruments. Therefore, the purpose of this study was to investigate possible changes in cutting efficiency of nickel-titanium K-files that had undergone PVD cathodic arc evaporation coating applying different process parameters. The hard coating used was TiN. MATERIALS AND METHODS Instruments A total of 84 new nickel-titanium K-files (Naviflex; Brasseler USA, Savannah, GA, USA) were examined. All size 35 instruments were taken from one single batch (P.O. #250459). The instruments were randomly divided into 7 groups of 12 instruments each. The instruments of the different groups were treated as follows: • Group A (experimental): argon ion sputtering (cleaning); bias ⫽ 100 V; substrate temperature 200°C; coating thickness 1 m. • Group B (experimental): argon ion sputtering (cleaning); bias ⫽ 60 V; substrate temperature 180°C; coating thickness 1 m. • Group C (experimental): argon ion sputtering (cleaning); bias ⫽ 60 V; substrate temperature 180°C; coating thickness 1.5 m. • Group D (experimental): argon ion sputtering (cleaning); bias ⫽ 100 V; substrate temperature 200°C; coating thickness 1.5 m. • Group E (experimental): argon and titanium ion sputtering (cleaning); bias ⫽ 60 V; substrate temperature ⬎ 250°C; coating thickness 1 m. • Group F (experimental): argon and titanium ion sputtering (cleaning); bias ⫽ 60 V; substrate temperature ⬎ 250°C; coating thickness 1.5 m. • Group K (control): samples were not coated with TiN. The instruments in all experimental groups were coated with TiN. They were fixtured in a vacuum-coating chamber and then the preheating cycle started. After the heating, an ion bombardment cycle started using either gas (argon) or metal (titanium) to clean the surface of the instruments before depositing the coating. During the argon cleaning the temperature of the substrates was less than 100°C, whereas metal ion cleaning leads to temperatures above 250°C. Cathodic arc evaporation was used to create a highly ionized plasma. After the cleaning process, an arc was struck on multiple titanium cathodes positioned inside. The arc flash evaporates titanium, which was attracted to the negatively biased instruments. After creation of a very thin layer of approximately 100 nm of pure titanium, which acts as an adhesive layer, a second
801
layer of TiN was created. Therefore, nitrogen gas was introduced to a low partial pressure into the coating chamber and reacted with the titanium to form titanium nitride.
Cutting Efficiency The spiral of a K-file establishes a cutting angle, e.g. an angle of the flutes to the long axis of the instrument, that is less than 45 degrees (2). Therefore, the cutting efficiency of all these instruments was determined in a rotary working motion, because these instruments are primarily designed to be used in this motion (2). 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 (5, 6, 15). 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 (5, 6, 15). The cylindrical lumen was 17-mm long and the diameter of this lumen was 0.40 mm (5, 15). Cutting efficiency was determined by using size 35 instruments; sample size was 12 instruments in all cases. Statistical analysis was carried out using commercial software (MedCalc 5.0; MedCalc Software, Mariakerke, Belgium), subjecting the data to ANOVA at a 0.05 significance level. A post-hoc Student-Newman-Keuls test was applied when ANOVA revealed statistical significant differences among the groups (p ⬍ 0.05).
RESULTS The mean maximum penetration depths and standard deviations are shown in Table 1. ANOVA revealed a significant difference in means (p ⫽ 0.009). Student-Newman-Keuls test was applied for pairwise comparisons, showing that instruments of groups A, F, and C achieved significantly greater penetration depths than the uncoated instruments of the control group (Fig. 1, Table 1).
DISCUSSION According to the present results, the PVD cathodic arc evaporation technique seems to be suitable to increase the cutting efficiency of nickel-titanium files. However, it has also been demonstrated that the different process parameters of the PVD coating have a crucial influence on the outcome of this process. Although
TABLE 1. Mean penetration depths, standard deviation, median, upper and lower quartile, and statistical significance (n ⴝ 12 instruments in each group). Groups A–F: experimental groups with TiN-coated instruments; Group K: control group with uncoated instruments. Groups
Mean (mm)
SD
Median (mm)
Lower quartile (mm)
Upper quartile (mm)
Significance*
K E B D A F C
2.63 2.80 2.86 2.95 3.25 3.27 3.32
0.66 0.60 0.46 0.46 0.36 0.48 0.61
2.79 2.82 2.99 2.91 3.24 3.29 3.27
2.14 2.33 2.59 2.62 2.96 2.85 2.82
3.13 3.26 3.15 3.29 3.58 3.75 3.63
a ab ab ab bc bc bc
* Means with same letter are not significantly different (p ⬎ 0.05).
802
Scha¨ fer
FIG 1. Maximum penetration depths (mm) achieved by the instruments of the different groups. The values correspond to median and to 95% confidence intervals. Groups A–F: experimental groups with TiN-coated instruments. Group K: control group with uncoated instruments.
the TiN-coated instruments of groups A, F, and C showed a significant increase (p ⬍ 0.05) in cutting efficiency by up to 26.2% in comparison to the uncoated instruments of the control group (K), the cutting ability of the coated instruments of groups B, D, and E was only slightly and insignificantly increased compared to the control files (Fig. 1). Observation of the TiN-coated instruments using scanning electron microscopy (SEM) revealed that the comparatively thin coating thickness of approximately 1.5 m minimizes rounding of the cutting edges while obviously still increasing cutting efficiency. To produce PVD titanium nitride coating tailored to enhance cutting efficiency of nickel-titanium root canal instruments, it is necessary to characterize the TiN coating in terms of substrate temperature, applied voltage, coating thickness, and ion bombardment to clean the substrate’s surface before the coating process, because this allows for the optimization of the process parameters (11). The present results clearly indicate that ion bombardment of the instruments using gas (argon), a coating thickness of 1.5 m and an applied bias of 60 V resulted in a significant increase in cutting efficiency of the nickeltitanium files compared with the untreated results. Unfortunately, these results cannot be compared with already published data. Studies on cutting efficiency of PVD-coated endodontic instruments are not available yet. However, it has been shown that other surface engineering techniques increased the cutting efficiency of nickel-titanium instruments successfully. Thermal nitridation and nitrogen-ionic implantation treatment of nickel-titanium instruments resulted in an enhanced cutting efficiency and a higher wear resistance (8, 9). Compared with the thermal nitridation, which usually is performed at temperatures of approximately 500°C, the low deposition temperature of the PVD arc evaporation allows TiN coating without detrimental bulk alterations of the instrument (13). Nevertheless, this assumption warrants further investigations to study exactly the influence of TiN coating of nickel-titanium instruments on their torsional and bending properties. Moreover, a study should be initiated to assess the microhardness of TiN-coated nickeltitanium instruments, because it is assumed that surface layers of TiN increase the hardness of the cutting edges and thereby provide enhanced cutting efficiency (4, 9, 12, 13). According to some reports, TiN coating could reduce the corrosion susceptibility of titanium alloys (4, 16). This further improvement of the already unique physical properties of nickel-titanium instruments
Journal of Endodontics
seems to be very interesting, because so far a major disadvantage of these instruments is the reduction in cutting efficiency after repeated sterilization (17, 18). Therefore, further studies are currently in progress to investigate the effects of repeated sterilization on cutting efficiency of PVD-coated nickel-titanium instruments. The TiN coating used in this investigation has a gold color, thus it can be assumed that this coating could provide an indication of wear. As the cutting edges of the instruments wear, the TiN coating is removed, exposing the underlying nickel-titanium alloy. Therefore, attempts should be made to develop a method that makes possible the identification of when a root canal instrument has to be discarded due to wear. In conclusion, the PVD arc evaporation technology could provide TiN-coated nickel-titanium instruments that display significantly increased cutting efficiency compared with conventional, uncoated nickel-titanium instruments. Clinically, this improvement is very useful to shorten instrumentation time and probably to minimize the risk of instrument separation during enlargement (1, 2). PVD-coated Hedstro¨ m files, coated with TiN using the same processing parameters as applied to the instruments of group C investigated in the present study have been introduced into the dental market by Komet (Lemgo, Germany) most recently. The author thanks Komet (Lemgo, Germany) for providing the PVD-coated instruments used in this study. Dr. Scha¨fer is affiliated with Poliklinik fu¨r Zahnerhaltung, Mu¨nster, Germany. Address requests for reprints to Prof. Dr. Edgar Scha¨fer, Poliklinik fu¨r Zahnerhaltung, Waldeyerstr. 30, D-48149 Mu¨nster, Germany.
References 1. Thompson SA. An overview of nickel-titanium alloys used in dentistry. Int Endod J 2000;33:297–300. 2. Scha¨fer E. Root canal instruments for manual use: a review. Endod Dent Traumatol 1997;13:51– 64. 3. Lautenschlager EP, Monaghan P. Titanium and titanium alloys as dental materials. Int Dent J 1993;43:245–53. 4. Serene TP, Adams JD, Saxena A. Nickel-titanium instruments. Applications in endodontics. St. Louis: Ishiyaku EuroAmerica Inc., 1995. 5. Tepel J, Scha¨fer E, Hoppe W. Root canal instruments for manual use: cutting efficiency and instrumentation of curved canals. Int Endod J 1995;28:68–76. 6. Tepel J, Scha¨fer E, Hoppe W. Properties of endodontic hand instruments used in rotary motion. Part 1. Cutting efficiency. J Endodon 1995;21:418 –21. 7. Walia H, Costas J, Brantley W, Gerstein H. Torsional ductility and cutting efficiency of the Nitinol file [Abstract]. J Endodon 1989;15:174. 8. Rapisarda E, Bonaccorso A, Tripi TR, Condorelli GG, Torrisi L. Wear of nickel-titanium endodontic instruments evaluated by scanning electron microscopy: effect of ion implantation. J Endodon 2001;27:588 –92. 9. Rapisarda E, Bonaccorso A, Tripi TR, Fragalk I, Condorelli GG. The effects of surface treatments of nickel-titanium files on wear and cutting efficiency. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2000;89:363– 8. 10. Lee DH, Park JB, Saxena A, Serene TP. Enhanced surface hardness by boron implantation in Nitinol alloy. J Endodon 1996;22:43– 6. 11. Brading HJ, Morton PH, Bell T, Earwaker LG. The structure and composition of plasma nitrided coatings on titanium. Nucl Instruments Methods Phy Res 1992;66:230 – 6. 12. Torrisi L. Ion implantation and thermal nitridation of biocompatible titanium. Biomed Mater Engl 1996;6:379 – 88. 13. Grohmann FR, Mathey Y. PVD coating in dentistry. Zahna¨rztl Mitt 1991;81:30 –1. 14. Smith DL. Thin film deposition: principles and practice. New York: McGraw Hill, 1995. 15. Scha¨fer E. Wurzelkanalinstrumente fu¨r den manuellen Einsatz: Schneidleistung und Formgebung gekru¨mmter Kanalabschnitte. Mu¨nster: Habilitationsschrift, 1996. 16. Hendry JA, Pilliar RM. The fretting corrosion resistance of PVD surfacemodified orthopedic implant alloys. J Biomed Mater Res 2001;58:156 – 66. 17. Rapisarda E, Bonaccorso A, Tripi TR, Condorelli GG. Effect of sterilization on the cutting efficiency of rotary nickel-titanium endodontic files. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1999;88:343–7. 18. Scha¨fer E. Effects of various sterilization procedures on the cutting efficiency of root canal instruments. Dtsch Zahna¨rztl Z 1995;50:150 –3.
Printed in U.S.A. VOL. 28, NO. 12, DECEMBER 2002
SCIENTIFIC ARTICLES Effect of Physical Vapor Deposition on Cutting Efficiency of Nickel-Titanium Files Edgar Scha¨fer, Prof, Dr
complex than that of stainless steel instruments, because the instruments have to be machined rather than twisted (1, 2). Due to the superelasticity of this alloy it is impossible to twist a nickeltitanium blank counterclockwise to produce a spiral, because nickel-titanium alloys underwent nearly no permanent deformation (1, 2). More likely they will fracture when being extensively twisted to produce a spiral. It is known that grinding of nickelbased alloys is quite difficult, because considerable wear of the milling head occurs within a short time (2). This leads to structural defects especially on the cutting edges of nickeltitanium instruments, which may compromise the cutting efficiency of these instruments (1, 2). Moreover, the cutting edges of nickel-titanium instruments have a microhardness ranging from 303 to 362 Vickers units, whereas stainless steel instruments showed a hardness ranging from 522 to 542 Vickers units (4). On account of the described surface irregularities and the low surface hardness, cutting efficiency of nickel-titanium files was less compared with most stainless steel instruments, as several authors have pointed out (5–7). Therefore, some surface engineering techniques have been used to improve the surface hardness and wear resistance of nickeltitanium instruments (8 –10). In some cases, the surface hardness of Nitinol was increased by ion implantation of boron (10) or nitrogen ions. Ionic implantation of nitrogen ions creates a surface layer of titanium nitride (TiN), which can significantly improve surface hardness, cutting efficiency, and wear resistance (8, 9, 11). Also by exposing Nitinol to thermal nitridation processes, surface hardness was successfully enhanced (9, 12). Another similar technique to deposit wear-resistant thin film coatings on surgical or dental instruments is the physical vapor deposition (PVD) technique, which was introduced to the medical device industry in the late 1980s (13). PVD includes reactive magnetron sputtering, ion plating, and arc evaporation (14). To achieve the best possible adhesion of the coating to the substrate’s surface, the cathodic arc evaporation technique is often used (13), creating hard coatings including TiN, TiC, TiCN, and TiAIN (14). Using this technique it is possible to deposit a fine-grained TiN film on the instruments at comparatively low temperatures (13). Coating thickness ranges from 1 to 7 m, and it is possible to obtain surface hardness of approximately 2200 Vickers units (13). Up to now, there are no studies available using PVD coatings
The purpose of this study was to investigate possible changes in cutting efficiency of nickel-titanium K-files that had undergone physical vapor deposition coating. Titanium nitride coatings were deposited using different process parameters. A total of 84 nickel-titanium K-files (size 35) were randomly divided into 7 groups of 12 instruments each. Groups A to F (experimental): instruments were coated with titanium nitride using different process parameters regarding substrate temperature, applied voltage, coating thickness, and ion bombardment. Group K (control): samples were not coated with titanium nitride. The cutting efficiency of all instruments was determined in a rotary working motion by means of a computerdriven testing device. Special plastic samples with a cylindrical canal were used, and the maximum penetration depth of the instruments into the lumen was the criterion for cutting efficiency. Instruments of groups A, F, and C achieved significantly greater penetration depths than the uncoated instruments of the control group (p < 0.05). Cutting efficiency of physical vapor deposition-coated nickel-titanium files was increased by up to 26.2% in comparison with uncoated instruments.
In recent years, nickel-titanium alloy has been successfully used in the manufacturing of endodontic instruments (1, 2). This alloy, named Nitinol, consists of approximately 55% nickel and 45% titanium by weight (1, 3, 4). The generic term for this alloy is 55-Nitinol (1). Owing to the substantially increased flexibility compared with stainless steel instruments, nickel-titanium– based instruments are reported to be particularly suitable for preparing curved root canals (1, 2). Nitinol alloys exhibit superelastic behavior, allowing the return to their original shape upon unloading after deformation (1, 2). Therefore, the manufacture of nickel-titanium instruments is more 800
Vol. 28, No. 12, December 2002
PVD Coating of NiTi
trying to enhance surface hardness and cutting efficiency of endodontic instruments. Therefore, the purpose of this study was to investigate possible changes in cutting efficiency of nickel-titanium K-files that had undergone PVD cathodic arc evaporation coating applying different process parameters. The hard coating used was TiN. MATERIALS AND METHODS Instruments A total of 84 new nickel-titanium K-files (Naviflex; Brasseler USA, Savannah, GA, USA) were examined. All size 35 instruments were taken from one single batch (P.O. #250459). The instruments were randomly divided into 7 groups of 12 instruments each. The instruments of the different groups were treated as follows: • Group A (experimental): argon ion sputtering (cleaning); bias ⫽ 100 V; substrate temperature 200°C; coating thickness 1 m. • Group B (experimental): argon ion sputtering (cleaning); bias ⫽ 60 V; substrate temperature 180°C; coating thickness 1 m. • Group C (experimental): argon ion sputtering (cleaning); bias ⫽ 60 V; substrate temperature 180°C; coating thickness 1.5 m. • Group D (experimental): argon ion sputtering (cleaning); bias ⫽ 100 V; substrate temperature 200°C; coating thickness 1.5 m. • Group E (experimental): argon and titanium ion sputtering (cleaning); bias ⫽ 60 V; substrate temperature ⬎ 250°C; coating thickness 1 m. • Group F (experimental): argon and titanium ion sputtering (cleaning); bias ⫽ 60 V; substrate temperature ⬎ 250°C; coating thickness 1.5 m. • Group K (control): samples were not coated with TiN. The instruments in all experimental groups were coated with TiN. They were fixtured in a vacuum-coating chamber and then the preheating cycle started. After the heating, an ion bombardment cycle started using either gas (argon) or metal (titanium) to clean the surface of the instruments before depositing the coating. During the argon cleaning the temperature of the substrates was less than 100°C, whereas metal ion cleaning leads to temperatures above 250°C. Cathodic arc evaporation was used to create a highly ionized plasma. After the cleaning process, an arc was struck on multiple titanium cathodes positioned inside. The arc flash evaporates titanium, which was attracted to the negatively biased instruments. After creation of a very thin layer of approximately 100 nm of pure titanium, which acts as an adhesive layer, a second
801
layer of TiN was created. Therefore, nitrogen gas was introduced to a low partial pressure into the coating chamber and reacted with the titanium to form titanium nitride.
Cutting Efficiency The spiral of a K-file establishes a cutting angle, e.g. an angle of the flutes to the long axis of the instrument, that is less than 45 degrees (2). Therefore, the cutting efficiency of all these instruments was determined in a rotary working motion, because these instruments are primarily designed to be used in this motion (2). 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 (5, 6, 15). 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 (5, 6, 15). The cylindrical lumen was 17-mm long and the diameter of this lumen was 0.40 mm (5, 15). Cutting efficiency was determined by using size 35 instruments; sample size was 12 instruments in all cases. Statistical analysis was carried out using commercial software (MedCalc 5.0; MedCalc Software, Mariakerke, Belgium), subjecting the data to ANOVA at a 0.05 significance level. A post-hoc Student-Newman-Keuls test was applied when ANOVA revealed statistical significant differences among the groups (p ⬍ 0.05).
RESULTS The mean maximum penetration depths and standard deviations are shown in Table 1. ANOVA revealed a significant difference in means (p ⫽ 0.009). Student-Newman-Keuls test was applied for pairwise comparisons, showing that instruments of groups A, F, and C achieved significantly greater penetration depths than the uncoated instruments of the control group (Fig. 1, Table 1).
DISCUSSION According to the present results, the PVD cathodic arc evaporation technique seems to be suitable to increase the cutting efficiency of nickel-titanium files. However, it has also been demonstrated that the different process parameters of the PVD coating have a crucial influence on the outcome of this process. Although
TABLE 1. Mean penetration depths, standard deviation, median, upper and lower quartile, and statistical significance (n ⴝ 12 instruments in each group). Groups A–F: experimental groups with TiN-coated instruments; Group K: control group with uncoated instruments. Groups
Mean (mm)
SD
Median (mm)
Lower quartile (mm)
Upper quartile (mm)
Significance*
K E B D A F C
2.63 2.80 2.86 2.95 3.25 3.27 3.32
0.66 0.60 0.46 0.46 0.36 0.48 0.61
2.79 2.82 2.99 2.91 3.24 3.29 3.27
2.14 2.33 2.59 2.62 2.96 2.85 2.82
3.13 3.26 3.15 3.29 3.58 3.75 3.63
a ab ab ab bc bc bc
* Means with same letter are not significantly different (p ⬎ 0.05).
802
Scha¨ fer
FIG 1. Maximum penetration depths (mm) achieved by the instruments of the different groups. The values correspond to median and to 95% confidence intervals. Groups A–F: experimental groups with TiN-coated instruments. Group K: control group with uncoated instruments.
the TiN-coated instruments of groups A, F, and C showed a significant increase (p ⬍ 0.05) in cutting efficiency by up to 26.2% in comparison to the uncoated instruments of the control group (K), the cutting ability of the coated instruments of groups B, D, and E was only slightly and insignificantly increased compared to the control files (Fig. 1). Observation of the TiN-coated instruments using scanning electron microscopy (SEM) revealed that the comparatively thin coating thickness of approximately 1.5 m minimizes rounding of the cutting edges while obviously still increasing cutting efficiency. To produce PVD titanium nitride coating tailored to enhance cutting efficiency of nickel-titanium root canal instruments, it is necessary to characterize the TiN coating in terms of substrate temperature, applied voltage, coating thickness, and ion bombardment to clean the substrate’s surface before the coating process, because this allows for the optimization of the process parameters (11). The present results clearly indicate that ion bombardment of the instruments using gas (argon), a coating thickness of 1.5 m and an applied bias of 60 V resulted in a significant increase in cutting efficiency of the nickeltitanium files compared with the untreated results. Unfortunately, these results cannot be compared with already published data. Studies on cutting efficiency of PVD-coated endodontic instruments are not available yet. However, it has been shown that other surface engineering techniques increased the cutting efficiency of nickel-titanium instruments successfully. Thermal nitridation and nitrogen-ionic implantation treatment of nickel-titanium instruments resulted in an enhanced cutting efficiency and a higher wear resistance (8, 9). Compared with the thermal nitridation, which usually is performed at temperatures of approximately 500°C, the low deposition temperature of the PVD arc evaporation allows TiN coating without detrimental bulk alterations of the instrument (13). Nevertheless, this assumption warrants further investigations to study exactly the influence of TiN coating of nickel-titanium instruments on their torsional and bending properties. Moreover, a study should be initiated to assess the microhardness of TiN-coated nickeltitanium instruments, because it is assumed that surface layers of TiN increase the hardness of the cutting edges and thereby provide enhanced cutting efficiency (4, 9, 12, 13). According to some reports, TiN coating could reduce the corrosion susceptibility of titanium alloys (4, 16). This further improvement of the already unique physical properties of nickel-titanium instruments
Journal of Endodontics
seems to be very interesting, because so far a major disadvantage of these instruments is the reduction in cutting efficiency after repeated sterilization (17, 18). Therefore, further studies are currently in progress to investigate the effects of repeated sterilization on cutting efficiency of PVD-coated nickel-titanium instruments. The TiN coating used in this investigation has a gold color, thus it can be assumed that this coating could provide an indication of wear. As the cutting edges of the instruments wear, the TiN coating is removed, exposing the underlying nickel-titanium alloy. Therefore, attempts should be made to develop a method that makes possible the identification of when a root canal instrument has to be discarded due to wear. In conclusion, the PVD arc evaporation technology could provide TiN-coated nickel-titanium instruments that display significantly increased cutting efficiency compared with conventional, uncoated nickel-titanium instruments. Clinically, this improvement is very useful to shorten instrumentation time and probably to minimize the risk of instrument separation during enlargement (1, 2). PVD-coated Hedstro¨ m files, coated with TiN using the same processing parameters as applied to the instruments of group C investigated in the present study have been introduced into the dental market by Komet (Lemgo, Germany) most recently. The author thanks Komet (Lemgo, Germany) for providing the PVD-coated instruments used in this study. Dr. Scha¨fer is affiliated with Poliklinik fu¨r Zahnerhaltung, Mu¨nster, Germany. Address requests for reprints to Prof. Dr. Edgar Scha¨fer, Poliklinik fu¨r Zahnerhaltung, Waldeyerstr. 30, D-48149 Mu¨nster, Germany.
References 1. Thompson SA. An overview of nickel-titanium alloys used in dentistry. Int Endod J 2000;33:297–300. 2. Scha¨fer E. Root canal instruments for manual use: a review. Endod Dent Traumatol 1997;13:51– 64. 3. Lautenschlager EP, Monaghan P. Titanium and titanium alloys as dental materials. Int Dent J 1993;43:245–53. 4. Serene TP, Adams JD, Saxena A. Nickel-titanium instruments. Applications in endodontics. St. Louis: Ishiyaku EuroAmerica Inc., 1995. 5. Tepel J, Scha¨fer E, Hoppe W. Root canal instruments for manual use: cutting efficiency and instrumentation of curved canals. Int Endod J 1995;28:68–76. 6. Tepel J, Scha¨fer E, Hoppe W. Properties of endodontic hand instruments used in rotary motion. Part 1. Cutting efficiency. J Endodon 1995;21:418 –21. 7. Walia H, Costas J, Brantley W, Gerstein H. Torsional ductility and cutting efficiency of the Nitinol file [Abstract]. J Endodon 1989;15:174. 8. Rapisarda E, Bonaccorso A, Tripi TR, Condorelli GG, Torrisi L. Wear of nickel-titanium endodontic instruments evaluated by scanning electron microscopy: effect of ion implantation. J Endodon 2001;27:588 –92. 9. Rapisarda E, Bonaccorso A, Tripi TR, Fragalk I, Condorelli GG. The effects of surface treatments of nickel-titanium files on wear and cutting efficiency. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2000;89:363– 8. 10. Lee DH, Park JB, Saxena A, Serene TP. Enhanced surface hardness by boron implantation in Nitinol alloy. J Endodon 1996;22:43– 6. 11. Brading HJ, Morton PH, Bell T, Earwaker LG. The structure and composition of plasma nitrided coatings on titanium. Nucl Instruments Methods Phy Res 1992;66:230 – 6. 12. Torrisi L. Ion implantation and thermal nitridation of biocompatible titanium. Biomed Mater Engl 1996;6:379 – 88. 13. Grohmann FR, Mathey Y. PVD coating in dentistry. Zahna¨rztl Mitt 1991;81:30 –1. 14. Smith DL. Thin film deposition: principles and practice. New York: McGraw Hill, 1995. 15. Scha¨fer E. Wurzelkanalinstrumente fu¨r den manuellen Einsatz: Schneidleistung und Formgebung gekru¨mmter Kanalabschnitte. Mu¨nster: Habilitationsschrift, 1996. 16. Hendry JA, Pilliar RM. The fretting corrosion resistance of PVD surfacemodified orthopedic implant alloys. J Biomed Mater Res 2001;58:156 – 66. 17. Rapisarda E, Bonaccorso A, Tripi TR, Condorelli GG. Effect of sterilization on the cutting efficiency of rotary nickel-titanium endodontic files. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1999;88:343–7. 18. Scha¨fer E. Effects of various sterilization procedures on the cutting efficiency of root canal instruments. Dtsch Zahna¨rztl Z 1995;50:150 –3.