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Surface analysis of nickel-titanium archwire used in vivo
Dent Mater 13:163-167, May, 1997
Surface analysis of nickel-titanium archwire used in vivo Margret R. Grimsdottir, Arne Hensten-Petterseri NIOM, Scandinavian Institute of Dental Materials, Haslum, N O R W A Y
ABSTRACT
the archwire is ligated into steel brackets (Brown and Merritt, 1981; Clinard et al., 1981). Due to the high nickel content of nickel-titanium prevention of corrosion. This study was conducted to evaluate and asarchwires (54% Ni, 46% Ti), possible corrosion products sess the surface of as-received and used nickel-titanium archwires for are a concern to the o r t h o d o n t i s t t r e a t i n g a nickelevidence of corrosion, and to analyze possible corrosion products. sensitive patient. Surface characteristics can provide Methods. Round 0.4 mm and square 0.4 mm x 0.55 mm nickel-titanium archwires from two manufacturers were subjected to elemental i n f o r m a t i o n a b o u t the corrosion of u s e d a r c h w i r e s analysis, examined, and photographed in a scanning electron micro- compared to as-received wires (Rentler and Greene, 1975; scope with an EDAX unit. The used wires had been in service from 3 wk Pugh and Jaffe, 1977; Sarkar and Schwaninger, 1980; Edie to 4 mon. There were no systematic differences in surface topography et al., 1981; Schwaninger et al., 1982; Brune and Hultquist, 1985). The objective of this study was to evaluate and or composition between the as-received and used wires. Results. The examination revealed undulated surfaces with manufac- assess the surface of used and as-received nickel-titanium tural scratches and crevices. The surface quality within the same archwire archwires for evidence of corrosion, and to analyze varied slightly, with different smoothness in the anterior and posterior possible corrosion products. regions. No systematic discernible difference was found between used METHODS AND MATERIALS and as-received arch wires. The analyses of different areas on the used archwires revealed no differences in the metal composition. Nickel-titanium archwires from two manufacturers, NiTi, Significance. The surface defects found on the as-received wires were Ormco Corp., Glendora, CA, U S A and Sentalloy, GAC, evidently not large enough to act as sites for corrosion attack. Central Islip, NY, USA, were used in this study. The Ormco square wires were from Batch No. 9A75 (lower arch) and INTRODUCTION 1K82K (upper arch), and the round ones from Batch No. The use of nickel-titanium archwires in orthodontics has 1M59M (lower arch) and 1K168K (upper arch). The increased steadily since it was introduced in the early 1970's Sentalloy square wires were from Batch No. 520-12 and (Andreasen and Morrow, 1978). Low modulus of elasticity, the round ones from Batch No. 511-62. The test groups high resilience and moderately high strength are physical consisted often 0.44 mm round wires of each type and ten properties t h a t m a k e nickel-titanium the archwire of 0.4 mm x 0.55 mm square wires of each type. The distal ~ choice, especially in the leveling phase of t r e a t m e n t part of each end of the wires was cat off before insertion (Waters et al., 1981; Schwaninger et al., 1982). This and kept for comparison with the used wires. The archwires archwire can withstand extreme bends without deforma- were worn intraorally for 3 w k to 4 mon. All of the tion, a characteristic that earned it the name "super- patients were teenagers. They were not given any special elastic archwire" (Buehler and Cross, 1969;Andreasen and diet instructions during the treatment with the archwires. Morrow, 1978). In general, this quality leads to an extended The unused ends and three different areas on the used working time for nickel-titanium archwires compared to archwires were analyzed in a scanning electron microscope stainless steel wires. Thus, fewer archwire changes are (Philips XL 30, Eindhoven, The Netherlands) with an required so nickel-titanium archwires are often in the energy dispersive x-ray analyzer (EDAX DX4, Mahwah, mouth for several months (Burstone, 1981). Nickel- NJ, USA). As-received archwires with the same batch titanium archwires have a high corrosion resistance when numbers as those used in the study, were also analyzed on tested under static conditions in vitro (Grimsdottir et al., three corresponding areas. Scanning electron images were 1992). However, fretting corrosion can occur in vivo when viewed and photographed at 100x, 200x and 500x. Used
Objectives. The surface quality of an archwire is a critical factor in the
Dental Materials~May 1997163
Fig. 1. As-received, round 0.4 mm NiTi archwire, 10(O magnircation.
Fig. 2. As-received, square 0.4 mm x 0.55 mm NiTi archwire, 100x magnification.
wires were rinsed in 1 N NaOH before examination to r e m o v e o r g a n i c debris. E l e m e n t a l a n a l y s i s w a s performed of t h e surface of eight selected areas and 17 spots of both as-received and used wires. Randomized SEM photographs depicting the surface topography of the as-received and used wires were examined for signs of corrosion.
!
RESULTS The EDS analyses of the archwires, expressed in weight percent (wt%) and rounded off to the nearest integer, revealed a Nifri ratio of 54/46. Both the square and the round nickel-titanium archwires were found to have rather smooth undulated surfaces, with many small, mainly longitudinal crevices (Figs. 1 and 2). There were no discernible visual differences between archwires from different manufacturers. Some variation was seen between archwires with the same batch number, and also on the same archwire in different locations (Fig. 3). The surface characteristics of these wires were similar, i.e., undulated surfaces with manufactural scratches and crevices along the wire. 164
Grimsdottir & Hensten-Petterse#Surface of NiTi archwire
Fig. 3. Three different locations on the same 0.4 mm-x 0.55 mm Nrri archwire, which had been in usa for 2.5 mon, 100x magnification. A) Molar region: marked longitudinal grooves; some pits and crevices, e) Cuspid region; longitudinal grooves almost obscured by numerous surface imperfections, C) Anterior region; smoother appearance with numerous small pits.
Visual examination of the overall surface of the used n i c k e l - t i t a n i u m a r c h w i r e s s h o w e d no d i s c e r n i b l e differences from the surface of unused wires, even though the archwire had been in service for up to 3 mon (Fig. 4). Areas were found on the surface of the used wires t h a t
Fig. 4. Comparison of wire condition as.received and after service. A) 0.4 mm NiTi as-received archwire, 200x magnification. B)The same archwire after 3 mon in service. C) 0.4 rnm Sentalloy as-received archwire, 200x magnification. D) The same archwire after 2.5 mon in service.
were smoother and had fewer and smaller crevices than the as-received surface. However, the opposite was also found (Fig. 5). There was no systematic change in the surface of the wires where there was contact with the brackets. Elemental analyses of different areas on the used wires revealed no difference in the Nifri ratio. However, spot a n a l y s e s in crevices, on both as-received and u s e d arc'hwires, revealed a decrease in the Nifri ratio, with higher titanium content than nickel. The average ratio in the crevices was 58]42. DISCUSSION
Corrosion r e s i s t a n c e in t h e oral e n v i r o n m e n t is an important factor for orthodontists to consider when they select metal appliances for their patients. The surface quality, expressed as the homogeneity of the archwire, has been shown to be the most critical factor in the prevention of pitting and crevice corrosion as well as fracture (Rentler and Greene, 1975; Pugh and Jaffe, 1977; Mohlin et al., 1991; Hartel et al., 1992). Rough surfaces, like manufactural imperfections, may serve as sites for localized corrosion attack and provide an i n c r e a s e d a r e a for m e t a l dissolution. S a r k a r a n d Schwaninger (1980) found pitting corrosion and corrosion
products rich in titanium on the surface of nickel-titanium wires which had been in the mouth for 3 wk to 5 mon. However, Edie et al. (1981) o b s e r v e d an u n d u l a t e d non-corroded smooth surface on nickel-titanium wires which had been in use for 1.5 mon to 1 y. They noted no discernible difference in surface characteristics on asreceived and used nickel-titanium archwires. Clinard et al. (1981) reported pitting corrosion on the surface of nickel-titanium wires during polarization, but they stated that it would not affect the mechanical properties of the wire. Schwaninger et al. (1982) found no difference in the physical properties between control and corroded nickeltitanium archwires. They bent the wires to fracture. The failure of the wires was due to the presence of surface imperfections generated during manufacturing, and not to the effects of corrosion. Mohlin et al. (1991) found that 28% of the Chinese NiTi archwire f r a c t u r e d during clinical use. Viewing the fractured wires in the scanning electron microscope revealed m a n y surface defects and inclusions of foreign material. Hartel et al. (1992) reported that surface roughness influenced both the corrosion behavior and the effectiveness of sliding mechanics. The present findings are in agreement with the results r e p o r t e d by Edie et al. (1981), in t h a t no a p p a r e n t Dental Materials~May 1997 165
have encouraged the m a n u f a c t u r e r s to improve their wire-drawing technique, making the wires smoother. However, the surface quality of the archwires varied depending on the location on the prefabricated wire; the anterior and cuspid region generally were not as smooth as the posterior region where the wire was straight. Fewer surface defects on wires manufactured today can be an e x p l a n a t i o n for t h e difference b e t w e e n t h e c u r r e n t findings and those of Sarkar and Schwaninger (1980) 14 years ago. The high titanium content found in crevices on t h e wires t e s t e d in this s t u d y is m o r e likely an indication o f a manufactural defect rather than corrosion, since it was found both on used and as-received archwires. The surface defects found on the wires were evidently not large enough to act as sites for corrosion. Since slight differences were found between archwires with the same batch numbers and on different locations on the same archwire in this study, the best way to evaluate surface corrosion is to examine exactly the s a m e area of the archwire before and after service (Edie et al., 1981). However, that was not possible in this investigation. The surface of the as-received NiTi wires exhibited multiple surface irregularities and minor defects. The surfaces did not appear to deteriorate appreciably during the 3 mon observation period of this study.
ACKNOWLEDGMENTS We express our gratitude to Ketil Kvam at NIOM for the work with the scanning electron microscope and EDAXanalysis, and to Arni Thordarson, private practitioner in Iceland, for the collection of used wires. Received March 3, 1993 / Accepted February 26, 1995 Address correspondence and reprint requests to: Arne Hensten-Pettersen NIOM, Boks 70, N-1344 Haslum NORWAY Phone: +47-67-58-01-00 Fax: +47-67-59-15-30 era: [email protected]
REFERENCES
Fig, 5. Comparison of wire condition as-received and after service. A) 0.4 mm x 0.55 mm NiTi as-received archwire, 200x magnification. B)The same archwire after 3 mon in service. C) The same archwire after 3 mort in service, different location. The variations in surface topography on different parts of the wires may mask the evaluation of in vivo formed surface defects due to corrosion.
difference between the surface characteristics of used and as-received nickel-titanium archwires was found. The published fact that manufactural imperfections impair the corrosion resistance of nickel-titanium archwires seems to 166
Gnmsdottir & Hensten-PetterseffSurface of NiTi archwire
Andreasen GF, Morrow RE (1978). Laboratory and clinical analyses of nitinol wire. A m J Orthod 73:142-151. Brown SA, Merritt K (1981). Fretting corrosion in saline and serum. J Biomed Mater Res 15:479-488. Brune D, Hultquist G (1985). Corrosion.of a stainless steel w i t h low nickel c o n t e n t u n d e r s t a t i c conditions. B iomaterials 6:265-268. Buehler WJ, Cross WB (1969). 55-nitinol-unique wire alloy with a memory. Wire J 2:41. Burstone CJ (1981). Variable-modulus orthodontics. A m J Orthod 80:1-16. Clinard K, von Fraunhofer JA, Kuftinec MM (1981). The corrosion susceptibility of modern orthodontic spring wires. J Dent Res 60A:628, Abstr. No. 1277.
Edie JW, Andreasen GF, Zaytoun MP (1981). Surface corrosion of nitinol and stainless steel under clinical conditions.Angle Orthod 51:319-324. Grimsdottir MR, Gjerdet NR, Hensten-Pettersen A (1992). Composition and in vitro corrosion of orthodontic appliances. A m J Orthod Dentofac Orthop 101:525532. Hartel A, Bourauel C, Drescher D, Schmuth GPF (1992).Die Oberfl~ichenrauheit orthodontischer Dr~ihte - eine Laseroptische und Profilometrische untersuchung. Schweiz Monatsschr Z a h n m e d 102:1195-1202. Mohlin B, Miiller H, 6 d m a n J, Thilander B (1991). Examination of Chinese NiTi wire by a combined clinical and laboratory approach. Eur J Orthod 13:386391.
Pugh JW, Jaffe WL (1977). Clinically observed corrosion of surface defects of wires. J Biomed Mater Res 11:625628. Rentler RM, Greene ND (1975). Corrosion of surface defects in fine wires. J B i o m e d M a t e r Res 9:597610. Sarkar NK, Schwaninger B (1980). The in vivo corrosion of nitinol wire. J D e n t Res 59A:528, Abstr. No.1035. Schwaninger B, Sarkar NK, Foster BE (1982). Effect of l o n g - t e r m i m m e r s i o n corrosion on t h e flexural properties of nitinol. A m J Orthod 82:45-49. Waters NE, Houston WJB, St ephens CD (1981). The characterization Ofarch wires for the initial alignment of irregular teeth. A m J Orthod 79:373-389.
Dental Materials~May 1997 167
Surface analysis of nickel-titanium archwire used in vivo Margret R. Grimsdottir, Arne Hensten-Petterseri NIOM, Scandinavian Institute of Dental Materials, Haslum, N O R W A Y
ABSTRACT
the archwire is ligated into steel brackets (Brown and Merritt, 1981; Clinard et al., 1981). Due to the high nickel content of nickel-titanium prevention of corrosion. This study was conducted to evaluate and asarchwires (54% Ni, 46% Ti), possible corrosion products sess the surface of as-received and used nickel-titanium archwires for are a concern to the o r t h o d o n t i s t t r e a t i n g a nickelevidence of corrosion, and to analyze possible corrosion products. sensitive patient. Surface characteristics can provide Methods. Round 0.4 mm and square 0.4 mm x 0.55 mm nickel-titanium archwires from two manufacturers were subjected to elemental i n f o r m a t i o n a b o u t the corrosion of u s e d a r c h w i r e s analysis, examined, and photographed in a scanning electron micro- compared to as-received wires (Rentler and Greene, 1975; scope with an EDAX unit. The used wires had been in service from 3 wk Pugh and Jaffe, 1977; Sarkar and Schwaninger, 1980; Edie to 4 mon. There were no systematic differences in surface topography et al., 1981; Schwaninger et al., 1982; Brune and Hultquist, 1985). The objective of this study was to evaluate and or composition between the as-received and used wires. Results. The examination revealed undulated surfaces with manufac- assess the surface of used and as-received nickel-titanium tural scratches and crevices. The surface quality within the same archwire archwires for evidence of corrosion, and to analyze varied slightly, with different smoothness in the anterior and posterior possible corrosion products. regions. No systematic discernible difference was found between used METHODS AND MATERIALS and as-received arch wires. The analyses of different areas on the used archwires revealed no differences in the metal composition. Nickel-titanium archwires from two manufacturers, NiTi, Significance. The surface defects found on the as-received wires were Ormco Corp., Glendora, CA, U S A and Sentalloy, GAC, evidently not large enough to act as sites for corrosion attack. Central Islip, NY, USA, were used in this study. The Ormco square wires were from Batch No. 9A75 (lower arch) and INTRODUCTION 1K82K (upper arch), and the round ones from Batch No. The use of nickel-titanium archwires in orthodontics has 1M59M (lower arch) and 1K168K (upper arch). The increased steadily since it was introduced in the early 1970's Sentalloy square wires were from Batch No. 520-12 and (Andreasen and Morrow, 1978). Low modulus of elasticity, the round ones from Batch No. 511-62. The test groups high resilience and moderately high strength are physical consisted often 0.44 mm round wires of each type and ten properties t h a t m a k e nickel-titanium the archwire of 0.4 mm x 0.55 mm square wires of each type. The distal ~ choice, especially in the leveling phase of t r e a t m e n t part of each end of the wires was cat off before insertion (Waters et al., 1981; Schwaninger et al., 1982). This and kept for comparison with the used wires. The archwires archwire can withstand extreme bends without deforma- were worn intraorally for 3 w k to 4 mon. All of the tion, a characteristic that earned it the name "super- patients were teenagers. They were not given any special elastic archwire" (Buehler and Cross, 1969;Andreasen and diet instructions during the treatment with the archwires. Morrow, 1978). In general, this quality leads to an extended The unused ends and three different areas on the used working time for nickel-titanium archwires compared to archwires were analyzed in a scanning electron microscope stainless steel wires. Thus, fewer archwire changes are (Philips XL 30, Eindhoven, The Netherlands) with an required so nickel-titanium archwires are often in the energy dispersive x-ray analyzer (EDAX DX4, Mahwah, mouth for several months (Burstone, 1981). Nickel- NJ, USA). As-received archwires with the same batch titanium archwires have a high corrosion resistance when numbers as those used in the study, were also analyzed on tested under static conditions in vitro (Grimsdottir et al., three corresponding areas. Scanning electron images were 1992). However, fretting corrosion can occur in vivo when viewed and photographed at 100x, 200x and 500x. Used
Objectives. The surface quality of an archwire is a critical factor in the
Dental Materials~May 1997163
Fig. 1. As-received, round 0.4 mm NiTi archwire, 10(O magnircation.
Fig. 2. As-received, square 0.4 mm x 0.55 mm NiTi archwire, 100x magnification.
wires were rinsed in 1 N NaOH before examination to r e m o v e o r g a n i c debris. E l e m e n t a l a n a l y s i s w a s performed of t h e surface of eight selected areas and 17 spots of both as-received and used wires. Randomized SEM photographs depicting the surface topography of the as-received and used wires were examined for signs of corrosion.
!
RESULTS The EDS analyses of the archwires, expressed in weight percent (wt%) and rounded off to the nearest integer, revealed a Nifri ratio of 54/46. Both the square and the round nickel-titanium archwires were found to have rather smooth undulated surfaces, with many small, mainly longitudinal crevices (Figs. 1 and 2). There were no discernible visual differences between archwires from different manufacturers. Some variation was seen between archwires with the same batch number, and also on the same archwire in different locations (Fig. 3). The surface characteristics of these wires were similar, i.e., undulated surfaces with manufactural scratches and crevices along the wire. 164
Grimsdottir & Hensten-Petterse#Surface of NiTi archwire
Fig. 3. Three different locations on the same 0.4 mm-x 0.55 mm Nrri archwire, which had been in usa for 2.5 mon, 100x magnification. A) Molar region: marked longitudinal grooves; some pits and crevices, e) Cuspid region; longitudinal grooves almost obscured by numerous surface imperfections, C) Anterior region; smoother appearance with numerous small pits.
Visual examination of the overall surface of the used n i c k e l - t i t a n i u m a r c h w i r e s s h o w e d no d i s c e r n i b l e differences from the surface of unused wires, even though the archwire had been in service for up to 3 mon (Fig. 4). Areas were found on the surface of the used wires t h a t
Fig. 4. Comparison of wire condition as.received and after service. A) 0.4 mm NiTi as-received archwire, 200x magnification. B)The same archwire after 3 mon in service. C) 0.4 rnm Sentalloy as-received archwire, 200x magnification. D) The same archwire after 2.5 mon in service.
were smoother and had fewer and smaller crevices than the as-received surface. However, the opposite was also found (Fig. 5). There was no systematic change in the surface of the wires where there was contact with the brackets. Elemental analyses of different areas on the used wires revealed no difference in the Nifri ratio. However, spot a n a l y s e s in crevices, on both as-received and u s e d arc'hwires, revealed a decrease in the Nifri ratio, with higher titanium content than nickel. The average ratio in the crevices was 58]42. DISCUSSION
Corrosion r e s i s t a n c e in t h e oral e n v i r o n m e n t is an important factor for orthodontists to consider when they select metal appliances for their patients. The surface quality, expressed as the homogeneity of the archwire, has been shown to be the most critical factor in the prevention of pitting and crevice corrosion as well as fracture (Rentler and Greene, 1975; Pugh and Jaffe, 1977; Mohlin et al., 1991; Hartel et al., 1992). Rough surfaces, like manufactural imperfections, may serve as sites for localized corrosion attack and provide an i n c r e a s e d a r e a for m e t a l dissolution. S a r k a r a n d Schwaninger (1980) found pitting corrosion and corrosion
products rich in titanium on the surface of nickel-titanium wires which had been in the mouth for 3 wk to 5 mon. However, Edie et al. (1981) o b s e r v e d an u n d u l a t e d non-corroded smooth surface on nickel-titanium wires which had been in use for 1.5 mon to 1 y. They noted no discernible difference in surface characteristics on asreceived and used nickel-titanium archwires. Clinard et al. (1981) reported pitting corrosion on the surface of nickel-titanium wires during polarization, but they stated that it would not affect the mechanical properties of the wire. Schwaninger et al. (1982) found no difference in the physical properties between control and corroded nickeltitanium archwires. They bent the wires to fracture. The failure of the wires was due to the presence of surface imperfections generated during manufacturing, and not to the effects of corrosion. Mohlin et al. (1991) found that 28% of the Chinese NiTi archwire f r a c t u r e d during clinical use. Viewing the fractured wires in the scanning electron microscope revealed m a n y surface defects and inclusions of foreign material. Hartel et al. (1992) reported that surface roughness influenced both the corrosion behavior and the effectiveness of sliding mechanics. The present findings are in agreement with the results r e p o r t e d by Edie et al. (1981), in t h a t no a p p a r e n t Dental Materials~May 1997 165
have encouraged the m a n u f a c t u r e r s to improve their wire-drawing technique, making the wires smoother. However, the surface quality of the archwires varied depending on the location on the prefabricated wire; the anterior and cuspid region generally were not as smooth as the posterior region where the wire was straight. Fewer surface defects on wires manufactured today can be an e x p l a n a t i o n for t h e difference b e t w e e n t h e c u r r e n t findings and those of Sarkar and Schwaninger (1980) 14 years ago. The high titanium content found in crevices on t h e wires t e s t e d in this s t u d y is m o r e likely an indication o f a manufactural defect rather than corrosion, since it was found both on used and as-received archwires. The surface defects found on the wires were evidently not large enough to act as sites for corrosion. Since slight differences were found between archwires with the same batch numbers and on different locations on the same archwire in this study, the best way to evaluate surface corrosion is to examine exactly the s a m e area of the archwire before and after service (Edie et al., 1981). However, that was not possible in this investigation. The surface of the as-received NiTi wires exhibited multiple surface irregularities and minor defects. The surfaces did not appear to deteriorate appreciably during the 3 mon observation period of this study.
ACKNOWLEDGMENTS We express our gratitude to Ketil Kvam at NIOM for the work with the scanning electron microscope and EDAXanalysis, and to Arni Thordarson, private practitioner in Iceland, for the collection of used wires. Received March 3, 1993 / Accepted February 26, 1995 Address correspondence and reprint requests to: Arne Hensten-Pettersen NIOM, Boks 70, N-1344 Haslum NORWAY Phone: +47-67-58-01-00 Fax: +47-67-59-15-30 era: [email protected]
REFERENCES
Fig, 5. Comparison of wire condition as-received and after service. A) 0.4 mm x 0.55 mm NiTi as-received archwire, 200x magnification. B)The same archwire after 3 mon in service. C) The same archwire after 3 mort in service, different location. The variations in surface topography on different parts of the wires may mask the evaluation of in vivo formed surface defects due to corrosion.
difference between the surface characteristics of used and as-received nickel-titanium archwires was found. The published fact that manufactural imperfections impair the corrosion resistance of nickel-titanium archwires seems to 166
Gnmsdottir & Hensten-PetterseffSurface of NiTi archwire
Andreasen GF, Morrow RE (1978). Laboratory and clinical analyses of nitinol wire. A m J Orthod 73:142-151. Brown SA, Merritt K (1981). Fretting corrosion in saline and serum. J Biomed Mater Res 15:479-488. Brune D, Hultquist G (1985). Corrosion.of a stainless steel w i t h low nickel c o n t e n t u n d e r s t a t i c conditions. B iomaterials 6:265-268. Buehler WJ, Cross WB (1969). 55-nitinol-unique wire alloy with a memory. Wire J 2:41. Burstone CJ (1981). Variable-modulus orthodontics. A m J Orthod 80:1-16. Clinard K, von Fraunhofer JA, Kuftinec MM (1981). The corrosion susceptibility of modern orthodontic spring wires. J Dent Res 60A:628, Abstr. No. 1277.
Edie JW, Andreasen GF, Zaytoun MP (1981). Surface corrosion of nitinol and stainless steel under clinical conditions.Angle Orthod 51:319-324. Grimsdottir MR, Gjerdet NR, Hensten-Pettersen A (1992). Composition and in vitro corrosion of orthodontic appliances. A m J Orthod Dentofac Orthop 101:525532. Hartel A, Bourauel C, Drescher D, Schmuth GPF (1992).Die Oberfl~ichenrauheit orthodontischer Dr~ihte - eine Laseroptische und Profilometrische untersuchung. Schweiz Monatsschr Z a h n m e d 102:1195-1202. Mohlin B, Miiller H, 6 d m a n J, Thilander B (1991). Examination of Chinese NiTi wire by a combined clinical and laboratory approach. Eur J Orthod 13:386391.
Pugh JW, Jaffe WL (1977). Clinically observed corrosion of surface defects of wires. J Biomed Mater Res 11:625628. Rentler RM, Greene ND (1975). Corrosion of surface defects in fine wires. J B i o m e d M a t e r Res 9:597610. Sarkar NK, Schwaninger B (1980). The in vivo corrosion of nitinol wire. J D e n t Res 59A:528, Abstr. No.1035. Schwaninger B, Sarkar NK, Foster BE (1982). Effect of l o n g - t e r m i m m e r s i o n corrosion on t h e flexural properties of nitinol. A m J Orthod 82:45-49. Waters NE, Houston WJB, St ephens CD (1981). The characterization Ofarch wires for the initial alignment of irregular teeth. A m J Orthod 79:373-389.
Dental Materials~May 1997 167