CLINICIANS' CORNER
The effect of clinical use and ste orthodontic arch wires
ilization o n s e l e c t e d
Glen A. Smith, DDS, MS," J. A. von Fraunhofer, MS, PhD, b and Glenn R. Casey, DDS, MS 9 Fort Lewis, Wash., attd Louisville and Fort Knox, Ky.
The effect of clinical use and various sterilization/disinfection protocols on three types of nickel-titanium, and one type each of 13-titanium and stainless steel arch wire was evaluated. The sterilization/disinfection procedures included disinfection alone or in concert with steam autoclave, dry heat, or cold solution sterilization. No clinically significant differences were found between new and used arch wires. The direction of load application to the arch wire and the particular segment of arch wire tested was found to cause substantial differences in generated loads for certain arch wire types. (AM J OFrrHoo DENTOFAC ORTHOP 1992;102:153-9.)
A t one time, precious metal arch wires were commonly employed in orthodontics, t but advances in metallurgy and wire manufacturing technology permitted the adoption of alternative alloy systems. For many years, arch wires were fabricated by extruding hardened austenitic stainless steel blanks through dies. These wires were of relatively low cost, but their mechanical properties were not ideal, particularly for initial leveling and alignment of jumbled dentitions, z Chrome-cobalt alloys were the next major improvement, having the advantage of being supplied in a more formable state, but being as stiff as regular stainless steel after heat treatment) Nickel-titanium alloy (Nitinol) wires became available for clinical use in the early 1970s, 4"5 and these were followed by 13-titanium alloys several years later, e-I~ The newer titanium alloys have several advantages over the precious metal and stainless steel wires in terms of physical properties. They are able to deliver lower, more constant forces over longer deflection distances without permanent deformation. This improvement in effective working range is especially important during the initial leveling and aligning of crowded teeth. ",t2 However, cost is a factor in the use of the newer materials since the titanium alloys cost 5 to 40 times as
This article is based on research submitted by the senior author in partial fulfillment of the requirements for Certificate of Training, U.S. Army Advanced Education 9 Program in Orthodontics, Fort Knox, Ky., and for the degree of Master of Science from the University of Louisville. The views, opinions, and findings in this article are those of the authors and do not represent official policy, position, or decision of the United States Army or the DeparZment of Defense. 9Lieutenant Colonel, U.S. Army Dental Corps, Fort Lev.is, Washington. bProfessor of Biomaterials Science, University of Louisville. ~Lieutenant Colonel, Assistant Director, U.S. Army Residency Program, Fort Knox, Ky. 811130363
much as stainless steel wires. This is an important consideration with regard to containing rising overhead costs, and is an acute problem within institutional practices such as the armed services, dental schools and public health clinics. The price differential between stainless steel and titanium arch wires has led to the suggestion that wires made of these alloys might be sterilized and reused. 13"t4 At the present time, there is considerable debate about whether patients should be treated with recycled and sterilized materials, particularly in such areas as orthopedic surgery and orthodontics. Reuse of materials would be of some economic benefit if recycling would not materially affect the properties of the devices in question. The viability of recycling arch wires is determined by both intraoral effects, such as corrosion, and those arising from the sterilization regimen. The literature offers little information on the possible recycling of orthodontic materials other than brackets, and the information on orthodontic arch wires tends to be contradictory and limited in scope. In vitro electrochemical studies indicate that Nitinol wires corrode when exposed to a chloride environment, and this effect is potentiated by contact with stainless steel. 15'~6 One study found the physical properties of nickel-titanium and B-titanium wires to be unaffecied by prolonged immersion in 1% sodium chloride. 17 In vivo evaluations have reported decreased performance of Nitinol t8 as well as surface interruption, oxidation, and pitting, ~9 whereas other clinical tests have shown no obvious visible difference in terms of surface corrosion. 20 Proper evaluation of the effect of corrosion in relation to the problem of recycling wires in orthodontic practice requires the addition of another variable, i.e., sterilization/disinfection. The effects of sterilization 153
154
Smith, yon Fraunhofer, and Casey
have received some study. Wentz 21 found a variable effect on certain wire properties dependent on the type of wire and the sterilization procedure. Two more recent studies on the effects of sterilization on unused straight lengths of rectangular nickel-titanium wire 22'2~ showed no significant effect. However, the joint effects of clinical use and subsequent sterilization on the properties of these wires was not addressed. Also, the physical characteristics of rectangular wire cannot be assumed to be identical to those of the round wires, commonly used in initial leveling and aligning. The only study that evaluated the combined effect of wear and sterilization was conducted by Huerter and Nikolai. 19 The 0.018-inch Nitinol wire was exposed to the oral environment for 12 weeks, sterilized by ethylene oxide at 130 ~ F, and then tested. They concluded that definite changes in mechanical properties had occurred, but that these wires could be recycled. Since there was only one wire type exposed to one sterilization protocol, and since the general orthodontic community was unaware that this study had been conducted, we felt that further research in this area was warranted. The present study was undertaken to evaluate the feasibility of recycling arch wires by determining whether the physical properties of arch wires are altered significantly by the combination of clinical use and sterilization. If clinical use and subsequent sterilization significantly a n d / o r unpredictably alter the mechanical properties of the wire, then use of recycled wires may cause the delivery of substandard care and thus be ethically wrong. However, if the effects of prior clinical use and sterilization are predictable or insignificant, then the decision on reuse of arch wires becomes a moral and ethical one over the relative importance of the conservation of resources versus the common practice of using virgin materials on every patient.
MATERIALS AND METHODS Three brands of nickel-titanium wire (Align, "A" Company, San Diego, Calif.; Nitinol, Unitek/3 Corp., Monrovia, Calif.; OSE, Gaithersburg, Md.), one brand of stainless steel (Permachrome Standard, Unitek, 3M Corp., Monrovia, CaliL), and a 13-titanium wire (TMA, Ormco Corp., Glendora, Calif.) were tested. The 0.016-inch wire was selected because of its popularity with clinicians for initial leveling and aligning, and because these are the most likely wires to be recycled. The nickel-titanium arch wires had ovoid arch forms, whereas the stainless steel and 13-titaniumhad standard arch form shapes. Forty wires of each type were to be used clinically to treat patients for 1 to 6 months. Five wires of each type were set aside in their as-received condition to serve as.!he cgntrol group (group NEW) during wire testing. Eight residents and two mentors from the University of Louisville Orthodontic
Am. J.
Orthod. Oentofac. Orthop. August 1992
Department and the Advanced Education Program in Orthodontics at Fort Knox were involved in the clinical phase of the study. These clinicians were given the freedom to choose what they thought were the appropriate clinical situations for the use of any of the test wires. The collection of used wires took place over a 10-month period of time. After clinical use, all wires were disinfected with an iodophor for 10 minutes, wiped down with an alcohol gauze, and allowed to air dry before being placed into separate paper storage envelopes. Not all wires that were collected were suitable for testing. Wires were dropped from the study if bends were placed in the wire by the clinician, had been cut shorter than 90 mm, or could not be positively identified by brand name to the investigator on their return. As a result, the sample size was reduced to slightly more than 20 per wire type. The sample size for the 13-titanium wire (10) was sufficiently large enough to test only two sterilization procedures. Equal numbers of each type of arch wire were placed into one of four sterilization/disinfection groups: group A, autoclave sterilized; group C, cold sterilization; group D, initial disinfection only; and group H, dry heat sterilized. Assignment of wires to each of the groups was based on a stratification procedure dependent on the length of clinical use. Twenty wires for each wire type were ranked according to how long they had been in the mouth. The four wires with the least amount of clinical use were randomly assigned to one of the four sterilization/disinfection groups. Then, the next four wires with the least amount of wear were randomly assigned to one of the four groups. This procedure was repeated until all the wires had been assigned, and each group contained five wires. By following this procedure, no one group was allowed to contain a sample that was significantly different in terms of length of clinical use than any other group. All group A wires were steam autoclaved at the same time. Each wire was individually wrapped in clear nylon sterilization tubing. Total cycle time was 15 minutes, while a sterilization temperature of 274 ~ F (134.4~ C) was maintained for 10 minutes. All wires in group C were sterilized together in freshly prepared sporocidin solution for 6.75 hours as per the manufacturer's recommendations. After sterilization, the wires were rinsed off in running water, laid on absorbent paper, and allowed to air dry. All group H wires were sterilized together in a dry heat sterilizer. The wires were laid on the sterilization pan of the unit and left uncovered. The unit was preheated, and a sterilization temperature of 375 ~ F (191 ~ C) was maintained for 10 minutes. The wires were allowed to cool down to near room temperature for approximately 15 minutes before being placed back into their storage envelopes. All wires were subjected to a load-deflection test, a tensile test, and a corrosion resistance test. Load-deflection data was gathered according to a similar approach to those of Miura, et al. :~ A specially designed bracket mount was used to secure the arch wire during testing. Standard 0.018-inch twin brackets were attached to cylindrical posts with epoxy. The bracket
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Fig. 1. Vertically positioned bracket mount for testing in-plane load behavior.
mount gave a consistent 14 mm interbracket distance, while allowing rotational adjustment of the bracket face so that curved sections of arch wire could be placed passively into full bracket slot engagement 9 Stainless steel wire ligatures were used to lightly secure the arch wire into the bracket slot. Support arms behind the bracket mounts were used to prevent arch wire rotation during loading so that the effects of direction of load application could be studied9 A universal testing machine (United Calibration Corp., Garden Grove, Calif.) with a 50 kg load cell was used for the transverse loading test. The bracket mount was clamped in place under the load cell in one of two orientations (vertical or horizontal) to evaluate the effect of arch wire orientation on load/deflection behavior (Figs. I and 2). Loads were applied at the midpoint of the interbracket span by a 5 mm diameter rod. Each arch wire was tested as four separate sectors. These sectors were identified by dye marks placed on one free end of the arch wire, at the midline, and at 25 mm to either side of the midline. The midline mark divided the wire into halves for cross-plane (free end marked with dye) and in-plane loading tests. The marks 25 mm to either side of the midline sectioned the wire into anterior and posterior segments. The resulting four combinations of load direction and wire segment were: cross-plane posterior segment (XP), cross-plane anterior segment (XA); in-plane anterior segment (IA): and in-plane posterior segment (IP). All wires were deflected at a rate of 2.5 mm per minute during both load application and reduction. Nickel-titanium was deflected over a total distance of 2 ram, stainless steel ! mm, and TMA 1.5 mm. These distances were chosen to avoid entering the plastic range of the wire and causing permanent deformation. Loading and unloading curves were generated on a Houston Instruments Onmigraphic 2000 Recorder (tiouston Instruments Div., Bausch and Laum, Austin, Texas). Force levels were identified at 1 mm deflections for all wires, and a t 1.5. mm for TMA, and 2.0 mm for the nickel-titanium wires. Tensile strength was tested by securing both ends of the
155
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wire in a cross-post cleat and loading to failure following a procedure reported previously. -'~ Corrosion resistance testing was performed to evaluate changes in corrosion potentials. These results will be reported on at a later dale. Analysis of variance was used to statistically analyze load/deflection and tensile test data. Follow up Tukey-Kramer (IISD) painvise analysis was done at the 95% level. A priori c~ of 0.05 was used and statements concerning statistical significance are therefore made at the p < 0.05 level. A paired t test was used in the comparisons of in-plane versus crossplane and anterior versus posterior load/deflection data. RESULTS
L o a d / d e f l e c t i o n data for each wire type are s h o w n in Figs. 3 through 7. T h e s e graphs are arranged to s h o w the effects o f different sterilization/disinfection procedures for the four separate wire s e g m e n t tests: crossplane posterior (XP) and anterior (XA), and in-plane anterior (IA) and posterior (IP). Only 3 o f the 72 s e g m e n t s tested g a v e statistically
156
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Fig. 7. TMA: mean load at 1.5 mm deflections.
significant differences between new (as received) wire and wires after clinical use and sterilization/disinfection. These differences were found for OSE groups A and H (IP), and TMA group A (XP). A comparison of the different wire types (nickeltitanium, 13-titanium, and stainless steel) was made for loads generated at the 1 mm deflection point. These anterior-in-plane comparisons are shown in Fig. 8. The most obvious differences are between stainless steel and the other wire types. For any given segment, the stainless steel wires generated about three times as much force for a 1 mm deflection as did any otherwire. An analysis of variance for nickel-titanium wires versus 13titanium gave statistically significant differences for all segments. An analysis of variance among nickel-titanium wires showed statistically significant differences between Align, Nitinol, and OSE, except for the IA load test for wire groups New, C, and D. Large differences between cross-plane (X) and inplane (1) force levels for stainless steel..(Fig. 6) prompted us to take a closer look at the results for different segments for all wire types. Pooled samples
of new and used wires (n = 25 except for 13-titanium where n = 15) were used to evaluate the effect of both load direction and anterior-versus-posterior segment test site. Force levels are for 1.0 mm deflections of stainless steel, 1.5 mm for [3-titanium, and 2.0 mm for nickel-titanium. Relatively large differences are evident between cross-plane and in-plane loading, with smaller differences between in-plane anterior and posterior loading of 13-titanium, and stainless steel (Fig. 9). The same pattern is seen with nickel-titanium wires, but to a much smaller degree. The effect of clinical use and sterilization/disinfection on ultimate tensile strength was evaluated through analysis of variance. For any given wire type, there was no statistically significant difference between new and used wire groups. Comparisons of stainless steel and 13-titanium with the nickel-titanium wires showed a significant difference. Tensile values for 13-titanium were smaller, and values for stainless steel were significantly higher than those of the nickel-titanium wires (Fig. 10). There were statistically significant differences among the nickel-titanium wire types for all sterilization/dis-
Volume 102 Number 2
Clinicians' corner
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In 1977, the American Dental Association (AOA) published specification no. 32 for orthodontic wires not containing precious metals.- Thts specification calls for tests on five specimens to determine flexural yield strength and modulus of stiffness, and five samples for resistance to 90 ~ bending. Tests are conducted on 3inch long specimens. Curved samples are straightened before testing. The standards, however, were written before nickel-titanium wires were widely used by the orthodontic specialty, and while the testing protocol is excellent for stainless steel wires, it is inadequate for nickel-titanium. Bending tests have no relevance for a wire that is not designed to be bent, and straightening a curved nickel-titanium arch wire for testing is impossible. The difficulty of designing a relevant testing procedure for nickel-titanium arch wires has led most researchers to use one of five main protocols. Comparisons between arch wires have been made using (1) theoretical models, 9n (2) the ADA cantilever test, 4'5'7"12"2~176 (3) a mandrel-wire wrapping test, '''3k32 (4) a dental arch template, ~7"33 and (5) a three-point bending test. 2224 Each method has its strengths and weaknesses, and unfortunately, they can yield conflicting results. The three-point bending test was used in this investigation because it provides easily quantified information from a model that closely mimics clinical conditions. For years, clinicians have been saying that the preformance of recycled nickel-titanium was clinically acceptable. The results of this investigtion add scien.dtic. credence to that position. These wires were subjected to various malocclusions, clinician handling tech-
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DISCUSSION
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niques, lengths of time in the mouth, and different sterilization/disinfection protocols. Yet there is no overall clinically significant difference in new-versusused wires. This finding agrees with the work Huerter and Nikolai ~9 did on 0.018-inch Nitinol. There are a few exceptions to the overall findings on the basis of statistical analysis. The OSE in-plane segment test for groups A and H, and TMA cross-plane segment test for group A gave statistically significant differences between new and used wires. It is possible that high temperatures from the autoclave or dry heat sterilizer has an effect on OSE and TMA arch wires. If this effect actually exists, its magnitude is small, and its effect is evident in only one of four segment tests. Threfore, we believe the clinical importance is negligible. The analysis of variance comparison of the three types of nickel-titanium wire showed statistically sig-
158
Am. 1. Orthod. Dentofac. Orthop. August 1992
Smith, yon Fraunhofer, and Case)'
niflcant differences between mean force values at 2 mm deflections. However, 95% prediction limits for individuals and for population means are considerably overlapped. Therefore, only the differences between nickel-titanium, 13-titanium, and stainless steel are clinically important, not the minor differences noted among nickel-titanium wires. The effect of direction of load application on the amount of force generated has been previously reported by Nikolai. ~9"~ In his study with Schaus, 34 he felt the cross-plane loads were slightly higher (2%) compared with the in-plane loads because of bracket slot clearance (0.016-inch wire in a 0.022 • 0.028 swing lock bracket). Huerter and Nikolai ~9 found approximately 60% higher stiffness values for in-plane loads versus cross-plane load, but used cantilever testing to get this result. Arch wires were ligated in place for our load/deflection tests to reduce the effect that bracket slot clearance might produce. Ligated wires should have given cross-plane loads approximately equal to in-plane loads if slot clearance was the major important variable. Since this was not the case, it is possible that the differences we found in force production are due to the interactions of bracket and arch wire geometry, and uneven distributions of strain hardening within the wire that result from the manufacturing process. The clinical perception of 0.016-inch TMA was that the wire had insufficient working range to be used for initial leveling and aligning. Tying this wire into poorly aligned teeth caused permanent deformation of the wire and produced little or no tooth movement. The reluctance of clinicians to use the wire after two or three trials left us with only enough arch wires to test two sterilization/disinfection protocols. Therefore the reuse of TMA after autoclave or sporocidin sterilization appears feasible on the basis of our data, but because of the lack of working range, the use of this size of TMA wire for initial leveling and aligning is not recommended. There were no statistically significant differences among sterilization/disinfection groups for any given wire type for stress to failure data. The affect of clinical wear and sterilization does not have an important effect on this arch wire property. The previously reported characteristic of a time dependent permanent set s,~ for Nitinol wire was not a clinically significant problem for our sample of nickeltitanium wires. There was little or no visible wire deformation after clinical use. The difference between prior testing results and our study may be the result of larger angular and linear activations in vitro than what this group of clinicians was willing to apply in vivo.
Care must be taken in applying the results of this study because of several limitations. The ADA guidelines on sample size for wire testing were met, but this is an extremely small sample compared with the number of arch wires used each day. Only one cycle of use and sterilization was tested. The effects of repeated clinical use and sterilization are still unknown. Cold sterilization products other than the one used in this study may affect arch wires to a greater or lesser degree because of their different chemical compositions. And finally, clinicians are reminded that even though the nickeltitanium wires in this study held up well to use and sterilization, the effect on the even more expensive superelastic nickel-titanium wires is at this time untested and unknown. SUMMARY Align, Nitinol, OSE, TMA, and stainless steel 0.016-inch arch wires were compared in their as-received state with wires that had been used for leveling and aligning for from 1 to 6 months. After their clinical use, the wires were sterilized/disinfected by one of four means: disinfection with Iodophor, autoclave sterilization, cold soak sterilization, or dry heat sterilization. Load/deflection tests, tensile tests, and corrosion potential test were performed on all groups to make before and after use comparisons. Load/deflection and tensile tests showed no clinically significant difference between as-received and used-then-disinfected/sterilized wires. These results suggest that nickel-titanium arch wires can be recycled at least once. A statistically significant difference was found between cross-plane and in-plane loading of arch wires. Cross-plane loading of nickel-titanium delivers approximately 10% more load for a given deflection than does an in-plane deflection directed against the outer curvature of the wire. The clinically significant finding is that this phenomenon is much greater for TMA and stainless steel wire. The use of 0.016-inch TMA is not recommended for initial leveling and aligning because of its relatively poor working range. Small differences were found among the different types of nickel-titanium wire. The clinical importance is negligible because of the small size of the difference and the large variability among nickel-titanium wires. The senior author thanks Dr. John Yancey for his instruction in statistical theory and practice, Mr. Randall Storey for his technical expertise and ingenious design of the testing apparatus, and the mentors and residents who participated in the clinical aspect of the study.
Volume 102 Number 2
REFERENCES i. Graber TM. Orthodontics principles and practice. Philadelphia: WB Saunders, 1972. 2. Waters NE, Houston WJ, Stephens CD. The characterization of arch wires for the initial alignment of irregular teeth. AM J OR'roOD 1981;79:373-89. 3. Proffit WR, Fields HW, Ackerman JL, Thomas PM, Tulloch JF. Contemporary orthodontics. St. Louis: CV Mosby, 1986:248. 4. Andreasen GF, Morrow RE. Laboratory and clinical analyses of nitinol wire. A.',t J ORTIlOD 1978;73:i42-51. 5. Lopez I, Goldberg AJ, Burstone CJ. Bending characteristics of nitinol wire. AM J OR'ntOD 1979;75:569-75. 6. Burstone CJ, Goldberg AJ. Beta titanium: a new orthodontic alloy. AM J OR'rHOD 1980;77:121-32. 7. Goldberg AJ, Burstone CJ. An evaluation of beta titanium alloys for use in orthodontic appliances. J Dent Res 1979;58:593-600. 8. Goldberg AJ, Burstone CJ. Status report on beta titanium orthodontic wires. J Am Dent Assoc 1982;105:684-5. 9. Kusy RP. Comparison of nickel-titanium and beta titanium wire sizes to conventional arch wire materials. AM J ORTtIOD 1981 ;79:625-9. 10. Kusy RP, Greenberg AR. Comparison of the elastic properties of nickel-titanium and beta titanium arch wires. AM J OR'I'HOD 1982;82:199-205. I 1. Burstone CJ. Variable-modulus orthodontics. AM J ORTHOD 1981;80:1-16. 12. Burstone CJ, Goldberg AJ. Maximum forces and deflections from orthodontic appliances. AM J ORTttOD 1983;84:95-103. 13. Andreasen GF, Atha E, Fahl J. Arch leveling and alignment effectiveness of two types of wire(1): a quantitative study. Quint Int 1984;15:49-57. 14. Ackerman JL, Chanda LH, Creekmore TD, Meyer M, Nelson GD. Round table: nitinol wire. J Clin Orthod 1978;12:479-85. 15. Sarkar NK, Redmond W, Schwaninger B. The chloride corrosion behavior of four orthodontic wires. J Oral Rehab 1983;10: 121-8. 16. Clinard K, von Fraunhofer JA, Kuftinec MM. The corrosion susceptibility of modem orthodontic spring wires. J Dent Res 1981 ;628. 17. Schwaninger B, Sarkar NK, Foster BE. Effect of long-term immersion corrosion on the flexural properties of nitinol. AM J ORTHOD 1982;82:45-9. 18. Harris EF, Newman SM, Nicholson JA. Nitinol arch wire in a simulated oral environment: changes in mechanical properties. AM J OR'I-HODDEm'OFACOR'mOP 1988;93:508-13. 19. Huerter TJ, Nikolai RJ. On the mechanical behavior of recycled Nitinol orthodontic wire. St. Louis: Department of Orthodontics, St. Louis University Medical Center, 1980.
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20. Edie JW, Andreasen GF, Zaytoun MP. Surface corrosion of nitinol and stainless steel under clinical conditions. Angle Orthod 1981;51:319-24. 21. Wentz CE. The effect of different sterilization techniques on the bending properties of Nitinol, TMA, and Stainless Steel arch wires. New Orleans: Department of Orthodontics, Louisiana State University School of Dentistry, 1985. 22. MayhewMJ, Kusy RP. Effects of sterilization on the mechanical properties and the surface topography of nickel-titanium arch wires. AM J ORTHODDENTOFACORTHOP 1988;93:3:232-6. 23. Buckthal JE, Kusy RP. Effects of cold disinfectants on the mechanical properties and the surface topography of nickel titanium arch wires. AM J ORrHOD DEN'rOFACOR'rHOP 1988;4:2:117-22. 24. Miura F, Mogi M, Ohura Y, Hamanaka H. The superelastic properties of the Japanese NiTi alloy wire for use in orthodontics. AM J ORTHODDENTOFACORTItOP 1986;90:1-10. 25. yon Fraunhofer JA, Storey RS, Stone IK, Masterson BJ. Tensile strength of suture material. J Biomed Mater Res 1985;19:595600. 26. Council on Dental Materials and Devices. New American Dental Association specification no. 32 for orthodontic wires not containing precious metals. J Am Dent Assoc 1977;95:1169-71. 27. Brantley WA, Augat WS, Myers CL, Winders RV. Bending deformation studies of orthodontic wires. J Dent Res 1978;57:609-15. 28..Burstone CJ, Qin B, Morton JY, Chinese NiTi wire - a new orthodontic alloy. AM J ORTIIOD 1985;87:445-52. 29. Goldberg AJ, Morton J, Burstone CJ. The flexure modulus of elasticity of orthodontic wires. J Dent Res 1983;26:856-8. 30. Asgharnia MK, Brantley WA. Comparison of bending and tension tests for orthodontic wires. Arq J ORTHOD 1986;89:228-36. 31. Waters NE. An improved method for the yield in bending of straight orthodontic wires. Br J Orthod 1981;8:89-98. 32. Ingram SB, Gipe DP, Smith RJ. Comparative range of orthodontic wires. AM J ORTHOD DEN'rOFACORTHOP 1986;90:296307. 33. Barrowes KJ. Arehwire flexibility and deformation. J Clin Orthod 1982;16:803-1 I. 34. Schaus JG, Nikolai RJ. Localized, transverse, flexural stiffnesses of continuous arch wires. AM J ORTHOD 1986;89:407-14. Reprint requests to:
LTC Glen A. Smith USA DENTAC Ft. Lewis, WA 98431
The effect of clinical use and ste orthodontic arch wires
ilization o n s e l e c t e d
Glen A. Smith, DDS, MS," J. A. von Fraunhofer, MS, PhD, b and Glenn R. Casey, DDS, MS 9 Fort Lewis, Wash., attd Louisville and Fort Knox, Ky.
The effect of clinical use and various sterilization/disinfection protocols on three types of nickel-titanium, and one type each of 13-titanium and stainless steel arch wire was evaluated. The sterilization/disinfection procedures included disinfection alone or in concert with steam autoclave, dry heat, or cold solution sterilization. No clinically significant differences were found between new and used arch wires. The direction of load application to the arch wire and the particular segment of arch wire tested was found to cause substantial differences in generated loads for certain arch wire types. (AM J OFrrHoo DENTOFAC ORTHOP 1992;102:153-9.)
A t one time, precious metal arch wires were commonly employed in orthodontics, t but advances in metallurgy and wire manufacturing technology permitted the adoption of alternative alloy systems. For many years, arch wires were fabricated by extruding hardened austenitic stainless steel blanks through dies. These wires were of relatively low cost, but their mechanical properties were not ideal, particularly for initial leveling and alignment of jumbled dentitions, z Chrome-cobalt alloys were the next major improvement, having the advantage of being supplied in a more formable state, but being as stiff as regular stainless steel after heat treatment) Nickel-titanium alloy (Nitinol) wires became available for clinical use in the early 1970s, 4"5 and these were followed by 13-titanium alloys several years later, e-I~ The newer titanium alloys have several advantages over the precious metal and stainless steel wires in terms of physical properties. They are able to deliver lower, more constant forces over longer deflection distances without permanent deformation. This improvement in effective working range is especially important during the initial leveling and aligning of crowded teeth. ",t2 However, cost is a factor in the use of the newer materials since the titanium alloys cost 5 to 40 times as
This article is based on research submitted by the senior author in partial fulfillment of the requirements for Certificate of Training, U.S. Army Advanced Education 9 Program in Orthodontics, Fort Knox, Ky., and for the degree of Master of Science from the University of Louisville. The views, opinions, and findings in this article are those of the authors and do not represent official policy, position, or decision of the United States Army or the DeparZment of Defense. 9Lieutenant Colonel, U.S. Army Dental Corps, Fort Lev.is, Washington. bProfessor of Biomaterials Science, University of Louisville. ~Lieutenant Colonel, Assistant Director, U.S. Army Residency Program, Fort Knox, Ky. 811130363
much as stainless steel wires. This is an important consideration with regard to containing rising overhead costs, and is an acute problem within institutional practices such as the armed services, dental schools and public health clinics. The price differential between stainless steel and titanium arch wires has led to the suggestion that wires made of these alloys might be sterilized and reused. 13"t4 At the present time, there is considerable debate about whether patients should be treated with recycled and sterilized materials, particularly in such areas as orthopedic surgery and orthodontics. Reuse of materials would be of some economic benefit if recycling would not materially affect the properties of the devices in question. The viability of recycling arch wires is determined by both intraoral effects, such as corrosion, and those arising from the sterilization regimen. The literature offers little information on the possible recycling of orthodontic materials other than brackets, and the information on orthodontic arch wires tends to be contradictory and limited in scope. In vitro electrochemical studies indicate that Nitinol wires corrode when exposed to a chloride environment, and this effect is potentiated by contact with stainless steel. 15'~6 One study found the physical properties of nickel-titanium and B-titanium wires to be unaffecied by prolonged immersion in 1% sodium chloride. 17 In vivo evaluations have reported decreased performance of Nitinol t8 as well as surface interruption, oxidation, and pitting, ~9 whereas other clinical tests have shown no obvious visible difference in terms of surface corrosion. 20 Proper evaluation of the effect of corrosion in relation to the problem of recycling wires in orthodontic practice requires the addition of another variable, i.e., sterilization/disinfection. The effects of sterilization 153
154
Smith, yon Fraunhofer, and Casey
have received some study. Wentz 21 found a variable effect on certain wire properties dependent on the type of wire and the sterilization procedure. Two more recent studies on the effects of sterilization on unused straight lengths of rectangular nickel-titanium wire 22'2~ showed no significant effect. However, the joint effects of clinical use and subsequent sterilization on the properties of these wires was not addressed. Also, the physical characteristics of rectangular wire cannot be assumed to be identical to those of the round wires, commonly used in initial leveling and aligning. The only study that evaluated the combined effect of wear and sterilization was conducted by Huerter and Nikolai. 19 The 0.018-inch Nitinol wire was exposed to the oral environment for 12 weeks, sterilized by ethylene oxide at 130 ~ F, and then tested. They concluded that definite changes in mechanical properties had occurred, but that these wires could be recycled. Since there was only one wire type exposed to one sterilization protocol, and since the general orthodontic community was unaware that this study had been conducted, we felt that further research in this area was warranted. The present study was undertaken to evaluate the feasibility of recycling arch wires by determining whether the physical properties of arch wires are altered significantly by the combination of clinical use and sterilization. If clinical use and subsequent sterilization significantly a n d / o r unpredictably alter the mechanical properties of the wire, then use of recycled wires may cause the delivery of substandard care and thus be ethically wrong. However, if the effects of prior clinical use and sterilization are predictable or insignificant, then the decision on reuse of arch wires becomes a moral and ethical one over the relative importance of the conservation of resources versus the common practice of using virgin materials on every patient.
MATERIALS AND METHODS Three brands of nickel-titanium wire (Align, "A" Company, San Diego, Calif.; Nitinol, Unitek/3 Corp., Monrovia, Calif.; OSE, Gaithersburg, Md.), one brand of stainless steel (Permachrome Standard, Unitek, 3M Corp., Monrovia, CaliL), and a 13-titanium wire (TMA, Ormco Corp., Glendora, Calif.) were tested. The 0.016-inch wire was selected because of its popularity with clinicians for initial leveling and aligning, and because these are the most likely wires to be recycled. The nickel-titanium arch wires had ovoid arch forms, whereas the stainless steel and 13-titaniumhad standard arch form shapes. Forty wires of each type were to be used clinically to treat patients for 1 to 6 months. Five wires of each type were set aside in their as-received condition to serve as.!he cgntrol group (group NEW) during wire testing. Eight residents and two mentors from the University of Louisville Orthodontic
Am. J.
Orthod. Oentofac. Orthop. August 1992
Department and the Advanced Education Program in Orthodontics at Fort Knox were involved in the clinical phase of the study. These clinicians were given the freedom to choose what they thought were the appropriate clinical situations for the use of any of the test wires. The collection of used wires took place over a 10-month period of time. After clinical use, all wires were disinfected with an iodophor for 10 minutes, wiped down with an alcohol gauze, and allowed to air dry before being placed into separate paper storage envelopes. Not all wires that were collected were suitable for testing. Wires were dropped from the study if bends were placed in the wire by the clinician, had been cut shorter than 90 mm, or could not be positively identified by brand name to the investigator on their return. As a result, the sample size was reduced to slightly more than 20 per wire type. The sample size for the 13-titanium wire (10) was sufficiently large enough to test only two sterilization procedures. Equal numbers of each type of arch wire were placed into one of four sterilization/disinfection groups: group A, autoclave sterilized; group C, cold sterilization; group D, initial disinfection only; and group H, dry heat sterilized. Assignment of wires to each of the groups was based on a stratification procedure dependent on the length of clinical use. Twenty wires for each wire type were ranked according to how long they had been in the mouth. The four wires with the least amount of clinical use were randomly assigned to one of the four sterilization/disinfection groups. Then, the next four wires with the least amount of wear were randomly assigned to one of the four groups. This procedure was repeated until all the wires had been assigned, and each group contained five wires. By following this procedure, no one group was allowed to contain a sample that was significantly different in terms of length of clinical use than any other group. All group A wires were steam autoclaved at the same time. Each wire was individually wrapped in clear nylon sterilization tubing. Total cycle time was 15 minutes, while a sterilization temperature of 274 ~ F (134.4~ C) was maintained for 10 minutes. All wires in group C were sterilized together in freshly prepared sporocidin solution for 6.75 hours as per the manufacturer's recommendations. After sterilization, the wires were rinsed off in running water, laid on absorbent paper, and allowed to air dry. All group H wires were sterilized together in a dry heat sterilizer. The wires were laid on the sterilization pan of the unit and left uncovered. The unit was preheated, and a sterilization temperature of 375 ~ F (191 ~ C) was maintained for 10 minutes. The wires were allowed to cool down to near room temperature for approximately 15 minutes before being placed back into their storage envelopes. All wires were subjected to a load-deflection test, a tensile test, and a corrosion resistance test. Load-deflection data was gathered according to a similar approach to those of Miura, et al. :~ A specially designed bracket mount was used to secure the arch wire during testing. Standard 0.018-inch twin brackets were attached to cylindrical posts with epoxy. The bracket
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Fig. 1. Vertically positioned bracket mount for testing in-plane load behavior.
mount gave a consistent 14 mm interbracket distance, while allowing rotational adjustment of the bracket face so that curved sections of arch wire could be placed passively into full bracket slot engagement 9 Stainless steel wire ligatures were used to lightly secure the arch wire into the bracket slot. Support arms behind the bracket mounts were used to prevent arch wire rotation during loading so that the effects of direction of load application could be studied9 A universal testing machine (United Calibration Corp., Garden Grove, Calif.) with a 50 kg load cell was used for the transverse loading test. The bracket mount was clamped in place under the load cell in one of two orientations (vertical or horizontal) to evaluate the effect of arch wire orientation on load/deflection behavior (Figs. I and 2). Loads were applied at the midpoint of the interbracket span by a 5 mm diameter rod. Each arch wire was tested as four separate sectors. These sectors were identified by dye marks placed on one free end of the arch wire, at the midline, and at 25 mm to either side of the midline. The midline mark divided the wire into halves for cross-plane (free end marked with dye) and in-plane loading tests. The marks 25 mm to either side of the midline sectioned the wire into anterior and posterior segments. The resulting four combinations of load direction and wire segment were: cross-plane posterior segment (XP), cross-plane anterior segment (XA); in-plane anterior segment (IA): and in-plane posterior segment (IP). All wires were deflected at a rate of 2.5 mm per minute during both load application and reduction. Nickel-titanium was deflected over a total distance of 2 ram, stainless steel ! mm, and TMA 1.5 mm. These distances were chosen to avoid entering the plastic range of the wire and causing permanent deformation. Loading and unloading curves were generated on a Houston Instruments Onmigraphic 2000 Recorder (tiouston Instruments Div., Bausch and Laum, Austin, Texas). Force levels were identified at 1 mm deflections for all wires, and a t 1.5. mm for TMA, and 2.0 mm for the nickel-titanium wires. Tensile strength was tested by securing both ends of the
155
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wire in a cross-post cleat and loading to failure following a procedure reported previously. -'~ Corrosion resistance testing was performed to evaluate changes in corrosion potentials. These results will be reported on at a later dale. Analysis of variance was used to statistically analyze load/deflection and tensile test data. Follow up Tukey-Kramer (IISD) painvise analysis was done at the 95% level. A priori c~ of 0.05 was used and statements concerning statistical significance are therefore made at the p < 0.05 level. A paired t test was used in the comparisons of in-plane versus crossplane and anterior versus posterior load/deflection data. RESULTS
L o a d / d e f l e c t i o n data for each wire type are s h o w n in Figs. 3 through 7. T h e s e graphs are arranged to s h o w the effects o f different sterilization/disinfection procedures for the four separate wire s e g m e n t tests: crossplane posterior (XP) and anterior (XA), and in-plane anterior (IA) and posterior (IP). Only 3 o f the 72 s e g m e n t s tested g a v e statistically
156
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Fig. 7. TMA: mean load at 1.5 mm deflections.
significant differences between new (as received) wire and wires after clinical use and sterilization/disinfection. These differences were found for OSE groups A and H (IP), and TMA group A (XP). A comparison of the different wire types (nickeltitanium, 13-titanium, and stainless steel) was made for loads generated at the 1 mm deflection point. These anterior-in-plane comparisons are shown in Fig. 8. The most obvious differences are between stainless steel and the other wire types. For any given segment, the stainless steel wires generated about three times as much force for a 1 mm deflection as did any otherwire. An analysis of variance for nickel-titanium wires versus 13titanium gave statistically significant differences for all segments. An analysis of variance among nickel-titanium wires showed statistically significant differences between Align, Nitinol, and OSE, except for the IA load test for wire groups New, C, and D. Large differences between cross-plane (X) and inplane (1) force levels for stainless steel..(Fig. 6) prompted us to take a closer look at the results for different segments for all wire types. Pooled samples
of new and used wires (n = 25 except for 13-titanium where n = 15) were used to evaluate the effect of both load direction and anterior-versus-posterior segment test site. Force levels are for 1.0 mm deflections of stainless steel, 1.5 mm for [3-titanium, and 2.0 mm for nickel-titanium. Relatively large differences are evident between cross-plane and in-plane loading, with smaller differences between in-plane anterior and posterior loading of 13-titanium, and stainless steel (Fig. 9). The same pattern is seen with nickel-titanium wires, but to a much smaller degree. The effect of clinical use and sterilization/disinfection on ultimate tensile strength was evaluated through analysis of variance. For any given wire type, there was no statistically significant difference between new and used wire groups. Comparisons of stainless steel and 13-titanium with the nickel-titanium wires showed a significant difference. Tensile values for 13-titanium were smaller, and values for stainless steel were significantly higher than those of the nickel-titanium wires (Fig. 10). There were statistically significant differences among the nickel-titanium wire types for all sterilization/dis-
Volume 102 Number 2
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In 1977, the American Dental Association (AOA) published specification no. 32 for orthodontic wires not containing precious metals.- Thts specification calls for tests on five specimens to determine flexural yield strength and modulus of stiffness, and five samples for resistance to 90 ~ bending. Tests are conducted on 3inch long specimens. Curved samples are straightened before testing. The standards, however, were written before nickel-titanium wires were widely used by the orthodontic specialty, and while the testing protocol is excellent for stainless steel wires, it is inadequate for nickel-titanium. Bending tests have no relevance for a wire that is not designed to be bent, and straightening a curved nickel-titanium arch wire for testing is impossible. The difficulty of designing a relevant testing procedure for nickel-titanium arch wires has led most researchers to use one of five main protocols. Comparisons between arch wires have been made using (1) theoretical models, 9n (2) the ADA cantilever test, 4'5'7"12"2~176 (3) a mandrel-wire wrapping test, '''3k32 (4) a dental arch template, ~7"33 and (5) a three-point bending test. 2224 Each method has its strengths and weaknesses, and unfortunately, they can yield conflicting results. The three-point bending test was used in this investigation because it provides easily quantified information from a model that closely mimics clinical conditions. For years, clinicians have been saying that the preformance of recycled nickel-titanium was clinically acceptable. The results of this investigtion add scien.dtic. credence to that position. These wires were subjected to various malocclusions, clinician handling tech-
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DISCUSSION
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niques, lengths of time in the mouth, and different sterilization/disinfection protocols. Yet there is no overall clinically significant difference in new-versusused wires. This finding agrees with the work Huerter and Nikolai ~9 did on 0.018-inch Nitinol. There are a few exceptions to the overall findings on the basis of statistical analysis. The OSE in-plane segment test for groups A and H, and TMA cross-plane segment test for group A gave statistically significant differences between new and used wires. It is possible that high temperatures from the autoclave or dry heat sterilizer has an effect on OSE and TMA arch wires. If this effect actually exists, its magnitude is small, and its effect is evident in only one of four segment tests. Threfore, we believe the clinical importance is negligible. The analysis of variance comparison of the three types of nickel-titanium wire showed statistically sig-
158
Am. 1. Orthod. Dentofac. Orthop. August 1992
Smith, yon Fraunhofer, and Case)'
niflcant differences between mean force values at 2 mm deflections. However, 95% prediction limits for individuals and for population means are considerably overlapped. Therefore, only the differences between nickel-titanium, 13-titanium, and stainless steel are clinically important, not the minor differences noted among nickel-titanium wires. The effect of direction of load application on the amount of force generated has been previously reported by Nikolai. ~9"~ In his study with Schaus, 34 he felt the cross-plane loads were slightly higher (2%) compared with the in-plane loads because of bracket slot clearance (0.016-inch wire in a 0.022 • 0.028 swing lock bracket). Huerter and Nikolai ~9 found approximately 60% higher stiffness values for in-plane loads versus cross-plane load, but used cantilever testing to get this result. Arch wires were ligated in place for our load/deflection tests to reduce the effect that bracket slot clearance might produce. Ligated wires should have given cross-plane loads approximately equal to in-plane loads if slot clearance was the major important variable. Since this was not the case, it is possible that the differences we found in force production are due to the interactions of bracket and arch wire geometry, and uneven distributions of strain hardening within the wire that result from the manufacturing process. The clinical perception of 0.016-inch TMA was that the wire had insufficient working range to be used for initial leveling and aligning. Tying this wire into poorly aligned teeth caused permanent deformation of the wire and produced little or no tooth movement. The reluctance of clinicians to use the wire after two or three trials left us with only enough arch wires to test two sterilization/disinfection protocols. Therefore the reuse of TMA after autoclave or sporocidin sterilization appears feasible on the basis of our data, but because of the lack of working range, the use of this size of TMA wire for initial leveling and aligning is not recommended. There were no statistically significant differences among sterilization/disinfection groups for any given wire type for stress to failure data. The affect of clinical wear and sterilization does not have an important effect on this arch wire property. The previously reported characteristic of a time dependent permanent set s,~ for Nitinol wire was not a clinically significant problem for our sample of nickeltitanium wires. There was little or no visible wire deformation after clinical use. The difference between prior testing results and our study may be the result of larger angular and linear activations in vitro than what this group of clinicians was willing to apply in vivo.
Care must be taken in applying the results of this study because of several limitations. The ADA guidelines on sample size for wire testing were met, but this is an extremely small sample compared with the number of arch wires used each day. Only one cycle of use and sterilization was tested. The effects of repeated clinical use and sterilization are still unknown. Cold sterilization products other than the one used in this study may affect arch wires to a greater or lesser degree because of their different chemical compositions. And finally, clinicians are reminded that even though the nickeltitanium wires in this study held up well to use and sterilization, the effect on the even more expensive superelastic nickel-titanium wires is at this time untested and unknown. SUMMARY Align, Nitinol, OSE, TMA, and stainless steel 0.016-inch arch wires were compared in their as-received state with wires that had been used for leveling and aligning for from 1 to 6 months. After their clinical use, the wires were sterilized/disinfected by one of four means: disinfection with Iodophor, autoclave sterilization, cold soak sterilization, or dry heat sterilization. Load/deflection tests, tensile tests, and corrosion potential test were performed on all groups to make before and after use comparisons. Load/deflection and tensile tests showed no clinically significant difference between as-received and used-then-disinfected/sterilized wires. These results suggest that nickel-titanium arch wires can be recycled at least once. A statistically significant difference was found between cross-plane and in-plane loading of arch wires. Cross-plane loading of nickel-titanium delivers approximately 10% more load for a given deflection than does an in-plane deflection directed against the outer curvature of the wire. The clinically significant finding is that this phenomenon is much greater for TMA and stainless steel wire. The use of 0.016-inch TMA is not recommended for initial leveling and aligning because of its relatively poor working range. Small differences were found among the different types of nickel-titanium wire. The clinical importance is negligible because of the small size of the difference and the large variability among nickel-titanium wires. The senior author thanks Dr. John Yancey for his instruction in statistical theory and practice, Mr. Randall Storey for his technical expertise and ingenious design of the testing apparatus, and the mentors and residents who participated in the clinical aspect of the study.
Volume 102 Number 2
REFERENCES i. Graber TM. Orthodontics principles and practice. Philadelphia: WB Saunders, 1972. 2. Waters NE, Houston WJ, Stephens CD. The characterization of arch wires for the initial alignment of irregular teeth. AM J OR'roOD 1981;79:373-89. 3. Proffit WR, Fields HW, Ackerman JL, Thomas PM, Tulloch JF. Contemporary orthodontics. St. Louis: CV Mosby, 1986:248. 4. Andreasen GF, Morrow RE. Laboratory and clinical analyses of nitinol wire. A.',t J ORTIlOD 1978;73:i42-51. 5. Lopez I, Goldberg AJ, Burstone CJ. Bending characteristics of nitinol wire. AM J OR'ntOD 1979;75:569-75. 6. Burstone CJ, Goldberg AJ. Beta titanium: a new orthodontic alloy. AM J OR'rHOD 1980;77:121-32. 7. Goldberg AJ, Burstone CJ. An evaluation of beta titanium alloys for use in orthodontic appliances. J Dent Res 1979;58:593-600. 8. Goldberg AJ, Burstone CJ. Status report on beta titanium orthodontic wires. J Am Dent Assoc 1982;105:684-5. 9. Kusy RP. Comparison of nickel-titanium and beta titanium wire sizes to conventional arch wire materials. AM J ORTtIOD 1981 ;79:625-9. 10. Kusy RP, Greenberg AR. Comparison of the elastic properties of nickel-titanium and beta titanium arch wires. AM J OR'I'HOD 1982;82:199-205. I 1. Burstone CJ. Variable-modulus orthodontics. AM J ORTHOD 1981;80:1-16. 12. Burstone CJ, Goldberg AJ. Maximum forces and deflections from orthodontic appliances. AM J ORTttOD 1983;84:95-103. 13. Andreasen GF, Atha E, Fahl J. Arch leveling and alignment effectiveness of two types of wire(1): a quantitative study. Quint Int 1984;15:49-57. 14. Ackerman JL, Chanda LH, Creekmore TD, Meyer M, Nelson GD. Round table: nitinol wire. J Clin Orthod 1978;12:479-85. 15. Sarkar NK, Redmond W, Schwaninger B. The chloride corrosion behavior of four orthodontic wires. J Oral Rehab 1983;10: 121-8. 16. Clinard K, von Fraunhofer JA, Kuftinec MM. The corrosion susceptibility of modem orthodontic spring wires. J Dent Res 1981 ;628. 17. Schwaninger B, Sarkar NK, Foster BE. Effect of long-term immersion corrosion on the flexural properties of nitinol. AM J ORTHOD 1982;82:45-9. 18. Harris EF, Newman SM, Nicholson JA. Nitinol arch wire in a simulated oral environment: changes in mechanical properties. AM J OR'I-HODDEm'OFACOR'mOP 1988;93:508-13. 19. Huerter TJ, Nikolai RJ. On the mechanical behavior of recycled Nitinol orthodontic wire. St. Louis: Department of Orthodontics, St. Louis University Medical Center, 1980.
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20. Edie JW, Andreasen GF, Zaytoun MP. Surface corrosion of nitinol and stainless steel under clinical conditions. Angle Orthod 1981;51:319-24. 21. Wentz CE. The effect of different sterilization techniques on the bending properties of Nitinol, TMA, and Stainless Steel arch wires. New Orleans: Department of Orthodontics, Louisiana State University School of Dentistry, 1985. 22. MayhewMJ, Kusy RP. Effects of sterilization on the mechanical properties and the surface topography of nickel-titanium arch wires. AM J ORTHODDENTOFACORTHOP 1988;93:3:232-6. 23. Buckthal JE, Kusy RP. Effects of cold disinfectants on the mechanical properties and the surface topography of nickel titanium arch wires. AM J ORrHOD DEN'rOFACOR'rHOP 1988;4:2:117-22. 24. Miura F, Mogi M, Ohura Y, Hamanaka H. The superelastic properties of the Japanese NiTi alloy wire for use in orthodontics. AM J ORTHODDENTOFACORTItOP 1986;90:1-10. 25. yon Fraunhofer JA, Storey RS, Stone IK, Masterson BJ. Tensile strength of suture material. J Biomed Mater Res 1985;19:595600. 26. Council on Dental Materials and Devices. New American Dental Association specification no. 32 for orthodontic wires not containing precious metals. J Am Dent Assoc 1977;95:1169-71. 27. Brantley WA, Augat WS, Myers CL, Winders RV. Bending deformation studies of orthodontic wires. J Dent Res 1978;57:609-15. 28..Burstone CJ, Qin B, Morton JY, Chinese NiTi wire - a new orthodontic alloy. AM J ORTIIOD 1985;87:445-52. 29. Goldberg AJ, Morton J, Burstone CJ. The flexure modulus of elasticity of orthodontic wires. J Dent Res 1983;26:856-8. 30. Asgharnia MK, Brantley WA. Comparison of bending and tension tests for orthodontic wires. Arq J ORTHOD 1986;89:228-36. 31. Waters NE. An improved method for the yield in bending of straight orthodontic wires. Br J Orthod 1981;8:89-98. 32. Ingram SB, Gipe DP, Smith RJ. Comparative range of orthodontic wires. AM J ORTHOD DEN'rOFACORTHOP 1986;90:296307. 33. Barrowes KJ. Arehwire flexibility and deformation. J Clin Orthod 1982;16:803-1 I. 34. Schaus JG, Nikolai RJ. Localized, transverse, flexural stiffnesses of continuous arch wires. AM J ORTHOD 1986;89:407-14. Reprint requests to:
LTC Glen A. Smith USA DENTAC Ft. Lewis, WA 98431
