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Mechanical and biological comparison of latex and silicone rubber bands
ORIGINAL ARTICLE
Mechanical and biological comparison of latex and silicone rubber bands Chung-Ju Hwang, DDS, PhD,a and Jung-Yul Cha, DDS, MSb Seoul, Korea Latex rubber bands are routinely used to supply orthodontic force. However, because the incidence of allergic reactions to latex is rising, the use of nonlatex alternatives is increasing, and assessing the mechanical properties of the replacement products is becoming more important. The purposes of this study were to compare the mechanical properties of latex and silicone orthodontic rubber bands through static testing under dry and wet conditions, and to compare their biologic (cytotoxic) properties. Three brands of latex and 1 brand of silicone rubber bands were tested. When extended to 300% of the lumen diameter, the silicone group had an initial force equal to 83% of the product specifications; this was the lowest of the 4 groups. All 4 brands showed notable amounts of force degradation at the 300% extension when subjected to saliva immersion; this approximated a 30% force decay over 2 days. The latex bands all followed a similar pattern of force degradation, whereas the silicone bands showed a greater increase in force decay as the extension length increased. The silicone bands were less cytotoxic than 2 of the 3 types of latex. Although the silicone bands showed the least discrepancy of force degradation between air and saliva conditions, the amount of the force decay was the greatest. Therefore, great improvements in the physical properties of the silicone band are required before they can be considered an acceptable replacement for latex. (Am J Orthod Dentofacial Orthop 2003;124:379-86)
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atex rubber bands, which are used to supply orthodontic force to teeth, are commonly used in orthodontic treatment. However, the protein content of latex is a known allergen. Studies have reported a prevalence of latex sensitization of 2.9% to 4.7% of hospital workers.1 In addition, 4% to 8% of the general population was reported to be positive to the natural rubber allergy by serologic testing with latex immunoglobulin E antibodies, whereas the skin-prick test showed a prevalence of less than 1% of the general population.2 Because latex allergy is prevalent of among occupationally exposed groups and patients, the need for nonlatex alternatives is increasing. To apply an ideal orthodontic force, an elastomer needs to maintain the appropriate force. However, conventional elastomers exhibit a high degree of irreversible force decay and are affected by environmental factors, such as the alkalinity of saliva, temperature From the Department of Orthdontics, College of Dentistry, Yonsei University, Seoul, Korea. a Professor. b Research fellow. This study was supported by the Korean Health Industry Development Institute Funds in 2001. Reprint requests to: Chung-Ju Hwang: Yonsei University, College of Dentistry, Department of Orthodontics, Seoul, Korea; e-mail, [email protected]. ac.kr. Submitted, March 2002; revised and accepted, November 2002. Copyright © 2003 by the American Association of Orthodontists. 0889-5406/2003/$30.00 ⫹ 0 doi:10.1016/S0889-5406(03)00564-X
changes, and prestretching. In addition, saliva and bacteria can infiltrate the weak molecular structures on the latex rubber surface, resulting in discoloration and expansion.3-6 For this reason, new materials with greater physical stability and biocompatibility, and without allergens, are needed. In 1960, Barnhart7 used the elastic, odorless, and biocompatible properties of silicon to reconstruct facial structures in facial and oral cancer surgery patients and reported it to be an excellent material for facial prosthetics. Thereafter, silicone was used to make dental products, such as denture liners and impression materials, because of its excellent plasticity, reproducibility, and biocompatibility. In this study, the physical properties of silicone rubber bands were evaluated by analyzing the amount of the force decay in relation to the extension length and the environmental changes. In addition, the biocompatibility of silicone bands was compared with that of latex rubber bands (Figs 1, 2). MATERIAL AND METHODS
Samples of latex and silicone orthodontic bands were obtained from Rocky Mountain Orthodontics (Energy Pak, Denver, Colo), Dentaurum (Olympia, Inspringen, Germany), TP Orthodontics (Plain Latex, Laporte, Ind), and JEPE (Silco, Seoul, Korea). All the elastics were reported to be of 6.25 mm (.25 in) internal diameter and medium weight (Table I). Samples were 379
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Fig 1. Latex rubber bands.
divided into 4 groups, by manufacturer. All samples had recent manufacturing dates and came in sealed plastic packages. Twenty-five specimens were taken from 5 batches of each group, making a total of 500 specimens. Loads were measured with a digital force gauge (IMADA, Northbrook, Ill) at 225%, 300%, and 450% of the internal diameter elongation. When measuring the initial forces, new elastics for each extension length were used. The bands were allowed to stabilize for 5 seconds before the recordings were made. Twenty-five new specimens taken from 5 different batches of 4 types of bands were tested for remaining force. Two hard stone dies (15 ⫻ 40 ⫻ 15 mm) with pins were constructed with an acrylic plate to maintain the correct elongation distance for 1 day. The pins were placed at intervals of 14.3, 19, and 28.4 mm. Elongated rubber bands were maintained in air (22°C ⫾ 3°C) and artificial saliva (37°C) conditions for 24 hours. The pH of the artificial saliva was 6.75, as suggested by Indiana University guidelines. One mol/L hydrochloric acid and 1 mol/L sodium hydroxide were triturated until the desired pH was reached. After 24 hours, the remaining force at each elongation distance was measured for each group. After sterilization of the elastics with ethylene oxide, the materials were exposed to air for 48 hours to eliminate the remaining gas. Then 4 g of each material was taken, washed with distilled water, and dried. Each dried specimen was completely soaked in 25 mL of phosphate-buffered saline (pH 6.6, for use in cell culture) and stored in an incubator at 37°C for 24 hours. Positive (polyurethane) and negative (tissue culture polystyrene) toxicity controls were included. We applied 3 parallel settings of elute system for each group. Each elute was sampled with a membrane having
Fig 2. Silicone rubber bands.
0.045-m pore size; only 20 L was taken and used in the microtiter tetrazolium test (MTT) measurements. Eagle’s minimum essential medium, with the addition of 5% fetal blood serum and 1% antibiotic– antimycotic mixture and L-glutamate was used to culture mouse fibroblast (L-929, Koran Cell Line Bank) cells. The cells were reseeded twice weekly to ensure exponential growth of the cell line. The effect of elutes on cell proliferation was measured by an MTT test. The protocol reported by Ignatius et al8 with a slight modification was used to study these biomaterials. MTT (dimethylthiazol diphenyltetrazolium bromide) was added to the culture medium at a final concentration of 1 mg/L. After a 4-hour incubation to allow the yellow dye to be transformed into blue formazan crystals, the supernatant was replaced by 50 L of dimethyl sulfoxide, which was added to dissolve the formazan formed within the cell. The plates were shaken for 10 minutes to ensure the complete dissolution of the formazan. The optical densities were observed at a wavelength of 570 nm with an enzyme-linked immunosorbent assay reader (Dynatech, MRX, Dynatech Laboratories, Chantilly, Va). This test was conducted 3 different times. Statistical analysis was performed as follows. The remaining forces between all and saliva conditions were evaluated by paired t tests, and analysis of variance (ANOVA) was used for evaluating the rate of force decay for each group. For the cell inhibition test, a repeated ANOVA was used to compare the various means. When the F value was significant (P ⬍ .05), a Fisher test was used to compare the various means. RESULTS
Group III showed the largest initial load for each elongation distance, and the initial force value was 98% (125.6 g) of the manufacturer’s specification force at a
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Table I.
Group I II III IV
Specifications of tested materials Main composition
Brand (company) Plain Latex (TP) Energy Pak (Rocky Mountain Orthodontics) Olympia (Dentaurum) Experiment (JEPE)
Natural Natural Natural Natural
latex latex latex silicone
Size (in)
Oz pull*
.25 .25 .25 .25
4.5 3.5 4.5 3.5
*Force ratings based on specification of manufacturer at extension that is three times lumen size.
Table II.
Initial force and remaining force (mean ⫾ SD, in grams) at different extensions after 1 day % extension*
Group I
II
III
IV
Condition
225%
300%
450%
Initial Air Saliva Initial Air Saliva Initial Air Saliva Initial Air Saliva
76.1 ⫾ 5.6 64.9 ⫾ 5.4 55.5 ⫾ 3.5 81.8 ⫾ 6.9 67.8 ⫾ 5.7 56.9 ⫾ 4.0 89.9 ⫾ 3.8 76.1 ⫾ 5.2 61.6 ⫾ 3.5 52.1 ⫾ 3.2 44.4 ⫾ 3.5 43.3 ⫾ 2.0
108.1 ⫾ 7.1 91.3 ⫾ 7.2 83.5 ⫾ 3.9 115.1 ⫾ 9.7 100.2 ⫾ 7.0 83.0 ⫾ 7.6 125.6 ⫾ 5.9 106.6 ⫾ 5.5 89.7 ⫾ 5.4 82.2 ⫾ 4.7 62.5 ⫾ 6.2 60.0 ⫾ 2.6
159.6 ⫾ 7.5 138.1 ⫾ 8.3 122.4 ⫾ 6.2 166.9 ⫾ 10.4 149.7 ⫾ 6.8 126.5 ⫾ 11.1 185.0 ⫾ 9.6 161.9 ⫾ 11.6 135.1 ⫾ 6.2 160.0 ⫾ 8.9 106.9 ⫾ 7.3 104.4 ⫾ 5.2
*Percent extension lengths were measured by percent extension of internal lumen diameter of elastics.
300% extension length. Group II showed the largest discrepancy, with an initial force of 115.1 g, which was 20% greater than that of the manufacturer’s specification force (99.2 g) at a 300% elongation length. Group I showed 85% of the expected force, whereas group IV showed only 83%, the lowest load (Table II). The initial rate of force of group IV, which showed a more rapid rate of increase with greater elongation length than the other groups, rose to match the initial force of group I at the extension length of 450%. When the remaining force was measured after 1 day, a significant difference in remaining force between the bands in air and artificial saliva conditions was seen in all groups (P ⬍ .05). The remaining forces measured in air at the 300% extension length for groups I, II, III, and IV were 84.4%, 87.1%, 84.9%, and 76% of the initial force, respectively. In the artificial saliva, all the groups exhibited a weaker force, with groups I, II, III, and IV, showing 76.7%, 72.1%, 71.4%, and 73.0%, respectively (Table II). Group IV showed a continuous decrease of the remaining force as the elongation distance increased. At a 450% extension distance, it showed the lowest remaining force among the 4 groups, with a force of 65.3% (Figs 3 and 4). There was a discrepancy between the remaining
force (%) of each group when maintained in the air and artificial saliva conditions for 1 day. At a 300% extension length (19 mm), group I showed a discrepancy of 7.7% between the 2 conditions, group II showed a discrepancy of 15%, and group III showed a discrepancy of 13.5%. Whereas group II had the greatest discrepancy, group IV showed the least discrepancy (3%) (Figs 3-5) (P ⬍ .05). Relative viability was established by comparison with the viability of the polystyrene control, which was arbitrarily set at 100%. A significant difference (P ⬍ .05) was noted between all the samples tested and the polyurethane (positive cytotoxicity control). After 24 hours, the lowest viability (72.3% ⫾ 6.2%) corresponded to group II, whereas the viability of the polyurethane (negative control) was 38.3% ⫾ 6.3%. Groups II and III were noticeably more cytotoxic than the other groups (Fig 6). After 48 hours, a significant decrease in cell viability was observed in all groups except group I (Fig 7). Viability ranged from 66% to 89%, relative to the negative cytotoxicity control. Group II produced the lowest value (68.8% ⫾ 4.6%), whereas the viability of the polyurethane was 25.3% ⫾ 6.4%. The ANOVA attested to the superiority of the polystyrene control in
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Fig 3. Force-extension changes for each group in air and artificial saliva conditions (after 1 day). A, Group I; B, group II; C, group III; D, group IV.
terms of viability. Again, significantly lower values were recorded for groups II and III when compared with the other groups. There was no significant difference between groups II and III. DISCUSSION
Although there have been a number of studies concerning dental elastomers and the degradation of strength with time, varying results have been reported. This inconsistency is the result of many different kinds
of materials and experimental methods, making it difficult to compare the different products.9 Therefore, in this experiment, products of the same size were used, and their physical properties were examined with standardized extensions and environments. Compared with latex rubber bands, silicone showed a small change in its original physical properties at both high and low temperatures, and it showed a maximum extension length of 1000% by maintaining a rubbery phase even at high temperatures.10 Moreover, unlike
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Fig 4. Mean percentage equivalents of force change at extension lengths in air and artificial saliva conditions (A, in air; B, in artificial saliva).
latex rubber, it has superior ozone-resistant and waterresistant qualities. Therefore, new silicone rubber bands have been developed with these distinctive features to benefit intraoral applications. The media in which elastics have been tested vary considerably as to results. For instance, Andreasen and Bishara6 carried out experiments in dry and simulated oral environments of 100% humidity conditions and reported no significant differences for the different conditions. Thomas et al11 also could not find a significant difference in the amount of force decay under dry and wet conditions. In contrast, Wong12 and Ash and Nikolai13 stated that greater force decay was observed in wet condition than dry conditions of the same temperature. Wong and Ash’s results corresponded with the results of this study. The latex rubber bands from Rocky Mountain Orthodontics and Dentaurum had a 13% to 15% discrepancy of force decay between saliva and air conditions, after 24 hours at 300% the extension length, showing a weakness in saliva, whereas the more stable silicone rubber band had only a 3% discrepancy in the remaining force after a day. Force decay in orthodontic elastics has been well documented, with more uniform results. Bell and Walter14 reported that 20% to 24% of elastic’s initial force was lost after 24 hours in an oral environment. Kanchana and Godfrey4 reported a large initial fall-off of force of elastic, averaging 29.9% during the first hour and increasing to 32.6% at 24 hours for orthodontic elastics in wet conditions. Russell et al15 reported
that the average force decay was 25% for latex bands and 40% for nonlatex bands. This present study produced similar results, showing a latex rubber band force decay of a 23% to 28%. In evaluating the initial forces corresponding to the extension length, the elastics were stretched to 3 times the internal diameter. Most elastics did not match the specified index during the dry tests. The Rocky Mountain Orthodontics products showed an especially higher load of 20% or more. This deviation of initial force was affected by the lack of uniformity in the rubber band size. The Rocky Mountain Orthodontics products showed the largest deviation in internal diameter, resulting in the largest range of initial force for this study. On the other hand, the silicone rubber band showed the lowest initial force of 82% of the manufacturer’s specifications. Therefore, improvements in the initial force are essential before silicone can be used in clinical applications. Kanchana and Godfrey4 measured the force of rubber bands corresponding to the extension length, using rubber bands of the same size as the present study. In their experiments, the initial load deflection curve in relation to the extension length was characterized by a smooth S-curve. Kanchana and Godfrey also observed similar patterns in their experiments as the rate of the force increase to the extension length slowed gradually up to 30 mm of extension length. In contrast, when the extension length of silicone bands exceeded 20 mm, the rate of the force increase increased abruptly (Fig 3). Thus, the silicone rubber band displayed a
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Fig 6. Viability after 24 hours.
Fig 5. Percentage of discrepancy of remaining force between air and artificial saliva conditions for each brand of rubber band.
greater change of force in relation to the change of extension length within a certain range; consequently, patients might feel uncomfortable with the change of vertical dimension during mouth-opening. Clinically, doctors would therefore also have difficulties in applying the elastics for a desired range of force. Wong12 and Taloumis et al16 reported that the greater the initial force of the rubber band, the greater the amount of force decay will increase. However, Hershey and Reynolds17 and Ware18 reported that the initial force had no relation to the amount of force decay. In the present experiment, no relationship was found between the initial force at a given extension length and the amount of force decay in the latex rubber band, whereas a relationship was found in the silicone rubber band. The silicone band, as observed in both wet and dry conditions, showed a dramatic decrease in remaining forces as extension length increased. This property of the silicone rubber band is a critical weak point if it needs to be clinically overextended. Although cytotoxicity and allergies to dental latex materials were mostly reported for latex gloves, some studies pointed to latex urinary catheters in the medical field.19-21 Although dental elastics are not invasive like artificial catheters, the cytoxicity test was necessary to prove the biocompatibility of the new material because orthodontic elastics are kept in the closed space of the oral cavity for several days. A comparison was made between silicone elastic and conventional biocompatible latex elastics. Although latex rubber has shown
Fig 7. Viability after 48 hours.
biocompatibility as a dental material, many cytotoxic factors have been reported. Sulfur and zinc oxide, as preservatives, exhibit cytotoxicity, and dithiocarborates, N-nitrosodibutylamine, and N-nitrosopiperidine, which act as antioxidants, are also known to be cytotoxic factors.22,23 Holmes et al24 examined whether the dyes used in manufacturing colored latex bands might have any toxic effects. Their results showed that the plain and colored elastics have identical low-level toxic effects. Clinically, however, this effect is harmless. Although case reports about the harmful effects of cytotoxicity of latex materials have been rare, there have been many reports about latex allergies. Allergic reactions to latex materials have become both more prevalent with the increase of latex products and better recognized since the 1988 adoption of universal precautions.25 Most allergic reactions were related to the use of latex gloves, but 2 cases were related to the use of orthodontic elastics.26,27 In the cases related to
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orthodontic elastics, there were many small vesicles or acute swellings, and the patients complained of a burning, itching sensation. In certain cases, immunoglobulin E-mediated latex allergies carry a wide range of risk, including dermatologic reactions, respiratory reactions, and systemic reactions—anaphylactic shock in an extreme case.28,29 The most serious consequence of natural rubber latex allergy commonly takes place during the mucosal absorption of natural rubber latex proteins during intraoperative medical or dental procedures when health care workers or others already sensitized become patients.1 This latex allergy occurs because natural latex contains many kinds of proteins, and the glove’s powder coating functions as a carrier of those latex proteins. Therefore, the development and use of nonlatex products is becoming increasingly more important clinically. As a replacement for latex, silicone rubber has proved its safety and has not yet triggered allergic reactions; moreover, it has been used in rubber replica tests during the allergy testing of other materials.30 However, there have been a few reports about the cytotoxicity of silicone rubber. Byproducts produced during the condensation procedure of polymerization have been reported to be cytotoxic.31 However, these byproducts decrease in cytotoxicity after 1 hour and have a very low cytotoxic effect on intraoral soft tissues.32 The safety biocompatibility of silicone has been well proved through the use of mouth guards in dentistry.33 Although the protocol of immersion tests for rubber materials has yet to be standardized, a static immersion test was performed during the cytotoxicity test. Moreover, because the rubber bands were not identical (despite the uniform .25-in manufacturer’s specification), it was impossible to create the same volume setting for the different groups. Therefore, instead of testing by volume through by embedding an elastic band into the macro-well plates for each group, the same weight of elastics was placed into separate flasks filled with phosphate-buffered saline to extract an elute. Instead of a cell culture solution, phosphate-buffered saline was chosen as the soaking media to decrease any potential interaction between the cell culture media and elastics. Our experiment showed a difference in cytotoxicity between the silicone band and the traditional latex band during the 2-day experiment. The biocompatibility of the silicone rubber was comparable to the best of the 3 products currently being tested. The silicone rubber bands and the TP rubber bands exhibited low cytotoxicity during the 2 days. This is not to say that they are biologically stable materials for long-term use. Considering the long-term applications
of rubber bands in the oral cavity, a longer period for the cytotoxicity experiment would provide more useful results. In addition, because the talcum powder was removed before in vitro studies were performed, it is not known whether the talcum powder would have made any difference. In several cases, silicone rubber band failures were observed during yawning and mastication; notch defects of the elastics, possibly introduced during the manufacturing process, and the poor maintenance of elasticity were found to trigger failure. Therefore, if the silicone rubber band is to be used in the clinic, immediate improvements are required. Currently, lack of reinforcement of the manufacturer’s specified initial force, elastic stability, and poor design are the main defects. If these defects can be fixed, silicone rubber bands, which satisfy the esthetics and biocompatibility requirements of dental orthodontic materials, would be an ideal substitute for conventional latex rubber bands. CONCLUSIONS
The following results were obtained from the mechanical and biological tests of silicone compared with latex rubber bands: 1. The initial force of the silicone rubber band was 83% of product specifications, according to the standard extension of 3 times the lumen diameter; silicone had the lowest specified initial force of all bands tested. However, at the 450% extension length of the lumen diameter, the force of silicone rubber bands increased notably to match that of group I. Thus, it showed an extension-force pattern different from that of the latex rubber bands. 2. After 1 day in saliva conditions at 300% extension lengths, the force decay percentages of the latex rubber bands were about 23% to 28%, whereas the silicone rubber bands showed a force decay percentage of 27%. However, at the 450% extension length, the silicone rubber bands had a 33% force decay of initial force, showing an increasing rate of force decay in accordance with an increase of extension length. 3. On the second day of the cytotoxicity test, groups IV and I showed greater viability than the other groups; group II showed the lowest viability. In this study, silicone rubber bands were found to exhibit a low cytotoxicity and the smallest discrepancy of force level between the dry and wet conditions. However, in terms of the initial force level and the abrupt loss of remaining force with an increase in the extension length, great improvements in the silicone rubber band’s physical properties are required.
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REFERENCES 1. Sussman GL, Beezhold DH, Liss G. Latex allergy: historical perspective. Methods 2002;27:3-9. 2. Ma¨ kinen-Kiljunen S. Banana allergy in patients with immediatetype sensitivity to natural latex: characterization of cross-reacting antibodies and allergens. Allergy Clin Immunol 1994;93:990-6. 3. Ferriter JP, Meyers CE. The effect of hydrogen ion concentration on the force-degradation rate of orthodontic polyurethane chain elastics. Am J Orthod Dentofacial Orthop 1990;98:404-10. 4. Kanchana P, Godfrey K. Calibration of force extension and force degradation characteristics of orthodontic latex elastics. Am J Orthod Dentofacial Orthop 2000;118:280-7. 5. Brantley WA, Salander S, Myers CL. Effect of prestretching on force degradation characteristics of plastic modules. Ang1e Orthod 1979;49:37-43. 6. Andreasen GF, Bishara S. Comparison of alastik chains and elastics involved with intra-arch molar-to-molar forces. Angle Orthod 1970;40:151-8. 7. Barnhart GW. A new material and technique in the art of somatoprosthesis. J Dent Res 1960;39:836-44. 8. Ignatius AA, Claes LE. In vitro biocompatibility of bioresorbable polymers: poly(L,DL-lactide) and poly(L-lactide-co-glycolide). Biomaterials 1996;17:831-9. 9. Young J, Sandrik JL. The influence of preloading on stress relaxation of orthodontic elastic polymers. Angle Orthod 1979; 49:104-9. 10. Jung IN. The third generation of silicone chemistry. Seoul: Free-Academy Press; 1997. p. 104-6. 11. Thomas RB, Sapiro JC, Angle BC. Force extension characteristics of orthodontic elastics. Am J Orthod 1966;72:296-302. 12. Wong AK. Orthodontic elastic materials. Angle Orthod 1976;46: 196-205. 13. Ash J, Nikolai R. Relaxation of orthodontic elastic chains and modules in vitro and in vivo. J Dent Res 1978;57:685-90. 14. Bell WH, Walter R. A study of applied force as related to the use of elastics and coil springs. Angle Orthod 1951;21:151-4. 15. Russell KA, Milne AD, Khanna RA, Lee JM. In vitro assessment of the mechanical properties of latex and non-latex orthodontic elastics. Am J Orthod Dentofacial Orthop 2001;120:36-44. 16. Taloumis LL, Moles LJ, Hopkins CR. Instruction for wearing elastics. J Clin Orthod 1995;29:49. 17. Hershey GH, Reynolds WG. The plastic module as on orthodontic tooth moving mechanism. Am J Orthod 1975;67:554-5.
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18. Ware AL. Some properties of plastic modules used for tooth movement. Aust J Orthod 1971;2:200-1. 19. Nattrass C, Ireland AJ, Lovell CR. Latex allergy in an orthognathic patient and implications for clinical management. Br J Oral Maxillofac Surg 1999;7:11-3. 20. Nacey JN, Delahunt B. Toxicity study of first and second generation hydrogel coated latex urinary catheters. Br J Urol 1991;67:314-6. 21. Ruutu M, Alfthan 0, Talja M. Cytotoxicity of latex urinary catheters. Br J Urol 1985;57:82-7. 22. Graham DT, Mark GE, Pomeroy AR. In vivo validation of a cell culture test for biocompatibility testing of urinary catheters. J Biomed Mater Res 1984;18:1125-35. 23. Fiddler W, Pensabene J, Sphon J, Andrzejewski D. Nitrosamine in rubber bands used for orthodontic purposes. Food Chem Toxicol 1992;30:325-6. 24. Holmes J, Barker MK, Walley EK, Tuncay OC. Cytotoxicity of orthodontic elastics. Am J Orthod Dentofacial Orthop 1993;104: 188-91. 25. Synder HA, Settle S. The rise in latex allergy: implications for the dentist. J Am Dent Assoc 1994;125:1089-97. 26. Everett FG, Hice TL. Contact stomatitis resulting from the use of orthodontic rubber elastics: report of case. J Am Dent Assoc 1974;88:1030-1. 27. Neiburger EJ. A case of possible latex allergy. J Clinic Orthod 1991;25:559-60. 28. Turjanmaa K, Alenius H, Ma¨ kinen-Kiljunen S, Reunala T, Palosuo T. Natural rubber latex allergy. Allergy 1996:51;593602. 29. Tomazic VJ, Withrow TJ, Fisher BR, Dillard SF. Latex-associated allergies and anaphylactic reaction. Clin Immunol Immunopathol 1992;64:89-97. 30. Peters K, Serup J. Papulo-vesicular count for the rating of allergic patch test reactions. A simple technique based on polysulfide rubber replica. Acta Derm Venereol 1987;67:491-5. 31. Ciapetti G, Granchi D, Stea S. Cytotoxicity testing of materials with limited in vivo exposure is affected by the duration of cell-material contact. J Biomed Mater Res 1998;15:485-90. 32. Polyzois GL, Hensten-Pettersen A. An assessment of the physical properties and biocompatibility of three silicone elastomers. J Prosthet Dent 1994;71:500-4. 33. Chauvel-Lebret DJ, Pellen-Mussi P. Evaluation of the in vitro biocompatibility of various elastomers. Biomaterials 1991;20: 291-9.
Mechanical and biological comparison of latex and silicone rubber bands Chung-Ju Hwang, DDS, PhD,a and Jung-Yul Cha, DDS, MSb Seoul, Korea Latex rubber bands are routinely used to supply orthodontic force. However, because the incidence of allergic reactions to latex is rising, the use of nonlatex alternatives is increasing, and assessing the mechanical properties of the replacement products is becoming more important. The purposes of this study were to compare the mechanical properties of latex and silicone orthodontic rubber bands through static testing under dry and wet conditions, and to compare their biologic (cytotoxic) properties. Three brands of latex and 1 brand of silicone rubber bands were tested. When extended to 300% of the lumen diameter, the silicone group had an initial force equal to 83% of the product specifications; this was the lowest of the 4 groups. All 4 brands showed notable amounts of force degradation at the 300% extension when subjected to saliva immersion; this approximated a 30% force decay over 2 days. The latex bands all followed a similar pattern of force degradation, whereas the silicone bands showed a greater increase in force decay as the extension length increased. The silicone bands were less cytotoxic than 2 of the 3 types of latex. Although the silicone bands showed the least discrepancy of force degradation between air and saliva conditions, the amount of the force decay was the greatest. Therefore, great improvements in the physical properties of the silicone band are required before they can be considered an acceptable replacement for latex. (Am J Orthod Dentofacial Orthop 2003;124:379-86)
L
atex rubber bands, which are used to supply orthodontic force to teeth, are commonly used in orthodontic treatment. However, the protein content of latex is a known allergen. Studies have reported a prevalence of latex sensitization of 2.9% to 4.7% of hospital workers.1 In addition, 4% to 8% of the general population was reported to be positive to the natural rubber allergy by serologic testing with latex immunoglobulin E antibodies, whereas the skin-prick test showed a prevalence of less than 1% of the general population.2 Because latex allergy is prevalent of among occupationally exposed groups and patients, the need for nonlatex alternatives is increasing. To apply an ideal orthodontic force, an elastomer needs to maintain the appropriate force. However, conventional elastomers exhibit a high degree of irreversible force decay and are affected by environmental factors, such as the alkalinity of saliva, temperature From the Department of Orthdontics, College of Dentistry, Yonsei University, Seoul, Korea. a Professor. b Research fellow. This study was supported by the Korean Health Industry Development Institute Funds in 2001. Reprint requests to: Chung-Ju Hwang: Yonsei University, College of Dentistry, Department of Orthodontics, Seoul, Korea; e-mail, [email protected]. ac.kr. Submitted, March 2002; revised and accepted, November 2002. Copyright © 2003 by the American Association of Orthodontists. 0889-5406/2003/$30.00 ⫹ 0 doi:10.1016/S0889-5406(03)00564-X
changes, and prestretching. In addition, saliva and bacteria can infiltrate the weak molecular structures on the latex rubber surface, resulting in discoloration and expansion.3-6 For this reason, new materials with greater physical stability and biocompatibility, and without allergens, are needed. In 1960, Barnhart7 used the elastic, odorless, and biocompatible properties of silicon to reconstruct facial structures in facial and oral cancer surgery patients and reported it to be an excellent material for facial prosthetics. Thereafter, silicone was used to make dental products, such as denture liners and impression materials, because of its excellent plasticity, reproducibility, and biocompatibility. In this study, the physical properties of silicone rubber bands were evaluated by analyzing the amount of the force decay in relation to the extension length and the environmental changes. In addition, the biocompatibility of silicone bands was compared with that of latex rubber bands (Figs 1, 2). MATERIAL AND METHODS
Samples of latex and silicone orthodontic bands were obtained from Rocky Mountain Orthodontics (Energy Pak, Denver, Colo), Dentaurum (Olympia, Inspringen, Germany), TP Orthodontics (Plain Latex, Laporte, Ind), and JEPE (Silco, Seoul, Korea). All the elastics were reported to be of 6.25 mm (.25 in) internal diameter and medium weight (Table I). Samples were 379
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Fig 1. Latex rubber bands.
divided into 4 groups, by manufacturer. All samples had recent manufacturing dates and came in sealed plastic packages. Twenty-five specimens were taken from 5 batches of each group, making a total of 500 specimens. Loads were measured with a digital force gauge (IMADA, Northbrook, Ill) at 225%, 300%, and 450% of the internal diameter elongation. When measuring the initial forces, new elastics for each extension length were used. The bands were allowed to stabilize for 5 seconds before the recordings were made. Twenty-five new specimens taken from 5 different batches of 4 types of bands were tested for remaining force. Two hard stone dies (15 ⫻ 40 ⫻ 15 mm) with pins were constructed with an acrylic plate to maintain the correct elongation distance for 1 day. The pins were placed at intervals of 14.3, 19, and 28.4 mm. Elongated rubber bands were maintained in air (22°C ⫾ 3°C) and artificial saliva (37°C) conditions for 24 hours. The pH of the artificial saliva was 6.75, as suggested by Indiana University guidelines. One mol/L hydrochloric acid and 1 mol/L sodium hydroxide were triturated until the desired pH was reached. After 24 hours, the remaining force at each elongation distance was measured for each group. After sterilization of the elastics with ethylene oxide, the materials were exposed to air for 48 hours to eliminate the remaining gas. Then 4 g of each material was taken, washed with distilled water, and dried. Each dried specimen was completely soaked in 25 mL of phosphate-buffered saline (pH 6.6, for use in cell culture) and stored in an incubator at 37°C for 24 hours. Positive (polyurethane) and negative (tissue culture polystyrene) toxicity controls were included. We applied 3 parallel settings of elute system for each group. Each elute was sampled with a membrane having
Fig 2. Silicone rubber bands.
0.045-m pore size; only 20 L was taken and used in the microtiter tetrazolium test (MTT) measurements. Eagle’s minimum essential medium, with the addition of 5% fetal blood serum and 1% antibiotic– antimycotic mixture and L-glutamate was used to culture mouse fibroblast (L-929, Koran Cell Line Bank) cells. The cells were reseeded twice weekly to ensure exponential growth of the cell line. The effect of elutes on cell proliferation was measured by an MTT test. The protocol reported by Ignatius et al8 with a slight modification was used to study these biomaterials. MTT (dimethylthiazol diphenyltetrazolium bromide) was added to the culture medium at a final concentration of 1 mg/L. After a 4-hour incubation to allow the yellow dye to be transformed into blue formazan crystals, the supernatant was replaced by 50 L of dimethyl sulfoxide, which was added to dissolve the formazan formed within the cell. The plates were shaken for 10 minutes to ensure the complete dissolution of the formazan. The optical densities were observed at a wavelength of 570 nm with an enzyme-linked immunosorbent assay reader (Dynatech, MRX, Dynatech Laboratories, Chantilly, Va). This test was conducted 3 different times. Statistical analysis was performed as follows. The remaining forces between all and saliva conditions were evaluated by paired t tests, and analysis of variance (ANOVA) was used for evaluating the rate of force decay for each group. For the cell inhibition test, a repeated ANOVA was used to compare the various means. When the F value was significant (P ⬍ .05), a Fisher test was used to compare the various means. RESULTS
Group III showed the largest initial load for each elongation distance, and the initial force value was 98% (125.6 g) of the manufacturer’s specification force at a
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Table I.
Group I II III IV
Specifications of tested materials Main composition
Brand (company) Plain Latex (TP) Energy Pak (Rocky Mountain Orthodontics) Olympia (Dentaurum) Experiment (JEPE)
Natural Natural Natural Natural
latex latex latex silicone
Size (in)
Oz pull*
.25 .25 .25 .25
4.5 3.5 4.5 3.5
*Force ratings based on specification of manufacturer at extension that is three times lumen size.
Table II.
Initial force and remaining force (mean ⫾ SD, in grams) at different extensions after 1 day % extension*
Group I
II
III
IV
Condition
225%
300%
450%
Initial Air Saliva Initial Air Saliva Initial Air Saliva Initial Air Saliva
76.1 ⫾ 5.6 64.9 ⫾ 5.4 55.5 ⫾ 3.5 81.8 ⫾ 6.9 67.8 ⫾ 5.7 56.9 ⫾ 4.0 89.9 ⫾ 3.8 76.1 ⫾ 5.2 61.6 ⫾ 3.5 52.1 ⫾ 3.2 44.4 ⫾ 3.5 43.3 ⫾ 2.0
108.1 ⫾ 7.1 91.3 ⫾ 7.2 83.5 ⫾ 3.9 115.1 ⫾ 9.7 100.2 ⫾ 7.0 83.0 ⫾ 7.6 125.6 ⫾ 5.9 106.6 ⫾ 5.5 89.7 ⫾ 5.4 82.2 ⫾ 4.7 62.5 ⫾ 6.2 60.0 ⫾ 2.6
159.6 ⫾ 7.5 138.1 ⫾ 8.3 122.4 ⫾ 6.2 166.9 ⫾ 10.4 149.7 ⫾ 6.8 126.5 ⫾ 11.1 185.0 ⫾ 9.6 161.9 ⫾ 11.6 135.1 ⫾ 6.2 160.0 ⫾ 8.9 106.9 ⫾ 7.3 104.4 ⫾ 5.2
*Percent extension lengths were measured by percent extension of internal lumen diameter of elastics.
300% extension length. Group II showed the largest discrepancy, with an initial force of 115.1 g, which was 20% greater than that of the manufacturer’s specification force (99.2 g) at a 300% elongation length. Group I showed 85% of the expected force, whereas group IV showed only 83%, the lowest load (Table II). The initial rate of force of group IV, which showed a more rapid rate of increase with greater elongation length than the other groups, rose to match the initial force of group I at the extension length of 450%. When the remaining force was measured after 1 day, a significant difference in remaining force between the bands in air and artificial saliva conditions was seen in all groups (P ⬍ .05). The remaining forces measured in air at the 300% extension length for groups I, II, III, and IV were 84.4%, 87.1%, 84.9%, and 76% of the initial force, respectively. In the artificial saliva, all the groups exhibited a weaker force, with groups I, II, III, and IV, showing 76.7%, 72.1%, 71.4%, and 73.0%, respectively (Table II). Group IV showed a continuous decrease of the remaining force as the elongation distance increased. At a 450% extension distance, it showed the lowest remaining force among the 4 groups, with a force of 65.3% (Figs 3 and 4). There was a discrepancy between the remaining
force (%) of each group when maintained in the air and artificial saliva conditions for 1 day. At a 300% extension length (19 mm), group I showed a discrepancy of 7.7% between the 2 conditions, group II showed a discrepancy of 15%, and group III showed a discrepancy of 13.5%. Whereas group II had the greatest discrepancy, group IV showed the least discrepancy (3%) (Figs 3-5) (P ⬍ .05). Relative viability was established by comparison with the viability of the polystyrene control, which was arbitrarily set at 100%. A significant difference (P ⬍ .05) was noted between all the samples tested and the polyurethane (positive cytotoxicity control). After 24 hours, the lowest viability (72.3% ⫾ 6.2%) corresponded to group II, whereas the viability of the polyurethane (negative control) was 38.3% ⫾ 6.3%. Groups II and III were noticeably more cytotoxic than the other groups (Fig 6). After 48 hours, a significant decrease in cell viability was observed in all groups except group I (Fig 7). Viability ranged from 66% to 89%, relative to the negative cytotoxicity control. Group II produced the lowest value (68.8% ⫾ 4.6%), whereas the viability of the polyurethane was 25.3% ⫾ 6.4%. The ANOVA attested to the superiority of the polystyrene control in
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Fig 3. Force-extension changes for each group in air and artificial saliva conditions (after 1 day). A, Group I; B, group II; C, group III; D, group IV.
terms of viability. Again, significantly lower values were recorded for groups II and III when compared with the other groups. There was no significant difference between groups II and III. DISCUSSION
Although there have been a number of studies concerning dental elastomers and the degradation of strength with time, varying results have been reported. This inconsistency is the result of many different kinds
of materials and experimental methods, making it difficult to compare the different products.9 Therefore, in this experiment, products of the same size were used, and their physical properties were examined with standardized extensions and environments. Compared with latex rubber bands, silicone showed a small change in its original physical properties at both high and low temperatures, and it showed a maximum extension length of 1000% by maintaining a rubbery phase even at high temperatures.10 Moreover, unlike
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Fig 4. Mean percentage equivalents of force change at extension lengths in air and artificial saliva conditions (A, in air; B, in artificial saliva).
latex rubber, it has superior ozone-resistant and waterresistant qualities. Therefore, new silicone rubber bands have been developed with these distinctive features to benefit intraoral applications. The media in which elastics have been tested vary considerably as to results. For instance, Andreasen and Bishara6 carried out experiments in dry and simulated oral environments of 100% humidity conditions and reported no significant differences for the different conditions. Thomas et al11 also could not find a significant difference in the amount of force decay under dry and wet conditions. In contrast, Wong12 and Ash and Nikolai13 stated that greater force decay was observed in wet condition than dry conditions of the same temperature. Wong and Ash’s results corresponded with the results of this study. The latex rubber bands from Rocky Mountain Orthodontics and Dentaurum had a 13% to 15% discrepancy of force decay between saliva and air conditions, after 24 hours at 300% the extension length, showing a weakness in saliva, whereas the more stable silicone rubber band had only a 3% discrepancy in the remaining force after a day. Force decay in orthodontic elastics has been well documented, with more uniform results. Bell and Walter14 reported that 20% to 24% of elastic’s initial force was lost after 24 hours in an oral environment. Kanchana and Godfrey4 reported a large initial fall-off of force of elastic, averaging 29.9% during the first hour and increasing to 32.6% at 24 hours for orthodontic elastics in wet conditions. Russell et al15 reported
that the average force decay was 25% for latex bands and 40% for nonlatex bands. This present study produced similar results, showing a latex rubber band force decay of a 23% to 28%. In evaluating the initial forces corresponding to the extension length, the elastics were stretched to 3 times the internal diameter. Most elastics did not match the specified index during the dry tests. The Rocky Mountain Orthodontics products showed an especially higher load of 20% or more. This deviation of initial force was affected by the lack of uniformity in the rubber band size. The Rocky Mountain Orthodontics products showed the largest deviation in internal diameter, resulting in the largest range of initial force for this study. On the other hand, the silicone rubber band showed the lowest initial force of 82% of the manufacturer’s specifications. Therefore, improvements in the initial force are essential before silicone can be used in clinical applications. Kanchana and Godfrey4 measured the force of rubber bands corresponding to the extension length, using rubber bands of the same size as the present study. In their experiments, the initial load deflection curve in relation to the extension length was characterized by a smooth S-curve. Kanchana and Godfrey also observed similar patterns in their experiments as the rate of the force increase to the extension length slowed gradually up to 30 mm of extension length. In contrast, when the extension length of silicone bands exceeded 20 mm, the rate of the force increase increased abruptly (Fig 3). Thus, the silicone rubber band displayed a
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Fig 6. Viability after 24 hours.
Fig 5. Percentage of discrepancy of remaining force between air and artificial saliva conditions for each brand of rubber band.
greater change of force in relation to the change of extension length within a certain range; consequently, patients might feel uncomfortable with the change of vertical dimension during mouth-opening. Clinically, doctors would therefore also have difficulties in applying the elastics for a desired range of force. Wong12 and Taloumis et al16 reported that the greater the initial force of the rubber band, the greater the amount of force decay will increase. However, Hershey and Reynolds17 and Ware18 reported that the initial force had no relation to the amount of force decay. In the present experiment, no relationship was found between the initial force at a given extension length and the amount of force decay in the latex rubber band, whereas a relationship was found in the silicone rubber band. The silicone band, as observed in both wet and dry conditions, showed a dramatic decrease in remaining forces as extension length increased. This property of the silicone rubber band is a critical weak point if it needs to be clinically overextended. Although cytotoxicity and allergies to dental latex materials were mostly reported for latex gloves, some studies pointed to latex urinary catheters in the medical field.19-21 Although dental elastics are not invasive like artificial catheters, the cytoxicity test was necessary to prove the biocompatibility of the new material because orthodontic elastics are kept in the closed space of the oral cavity for several days. A comparison was made between silicone elastic and conventional biocompatible latex elastics. Although latex rubber has shown
Fig 7. Viability after 48 hours.
biocompatibility as a dental material, many cytotoxic factors have been reported. Sulfur and zinc oxide, as preservatives, exhibit cytotoxicity, and dithiocarborates, N-nitrosodibutylamine, and N-nitrosopiperidine, which act as antioxidants, are also known to be cytotoxic factors.22,23 Holmes et al24 examined whether the dyes used in manufacturing colored latex bands might have any toxic effects. Their results showed that the plain and colored elastics have identical low-level toxic effects. Clinically, however, this effect is harmless. Although case reports about the harmful effects of cytotoxicity of latex materials have been rare, there have been many reports about latex allergies. Allergic reactions to latex materials have become both more prevalent with the increase of latex products and better recognized since the 1988 adoption of universal precautions.25 Most allergic reactions were related to the use of latex gloves, but 2 cases were related to the use of orthodontic elastics.26,27 In the cases related to
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orthodontic elastics, there were many small vesicles or acute swellings, and the patients complained of a burning, itching sensation. In certain cases, immunoglobulin E-mediated latex allergies carry a wide range of risk, including dermatologic reactions, respiratory reactions, and systemic reactions—anaphylactic shock in an extreme case.28,29 The most serious consequence of natural rubber latex allergy commonly takes place during the mucosal absorption of natural rubber latex proteins during intraoperative medical or dental procedures when health care workers or others already sensitized become patients.1 This latex allergy occurs because natural latex contains many kinds of proteins, and the glove’s powder coating functions as a carrier of those latex proteins. Therefore, the development and use of nonlatex products is becoming increasingly more important clinically. As a replacement for latex, silicone rubber has proved its safety and has not yet triggered allergic reactions; moreover, it has been used in rubber replica tests during the allergy testing of other materials.30 However, there have been a few reports about the cytotoxicity of silicone rubber. Byproducts produced during the condensation procedure of polymerization have been reported to be cytotoxic.31 However, these byproducts decrease in cytotoxicity after 1 hour and have a very low cytotoxic effect on intraoral soft tissues.32 The safety biocompatibility of silicone has been well proved through the use of mouth guards in dentistry.33 Although the protocol of immersion tests for rubber materials has yet to be standardized, a static immersion test was performed during the cytotoxicity test. Moreover, because the rubber bands were not identical (despite the uniform .25-in manufacturer’s specification), it was impossible to create the same volume setting for the different groups. Therefore, instead of testing by volume through by embedding an elastic band into the macro-well plates for each group, the same weight of elastics was placed into separate flasks filled with phosphate-buffered saline to extract an elute. Instead of a cell culture solution, phosphate-buffered saline was chosen as the soaking media to decrease any potential interaction between the cell culture media and elastics. Our experiment showed a difference in cytotoxicity between the silicone band and the traditional latex band during the 2-day experiment. The biocompatibility of the silicone rubber was comparable to the best of the 3 products currently being tested. The silicone rubber bands and the TP rubber bands exhibited low cytotoxicity during the 2 days. This is not to say that they are biologically stable materials for long-term use. Considering the long-term applications
of rubber bands in the oral cavity, a longer period for the cytotoxicity experiment would provide more useful results. In addition, because the talcum powder was removed before in vitro studies were performed, it is not known whether the talcum powder would have made any difference. In several cases, silicone rubber band failures were observed during yawning and mastication; notch defects of the elastics, possibly introduced during the manufacturing process, and the poor maintenance of elasticity were found to trigger failure. Therefore, if the silicone rubber band is to be used in the clinic, immediate improvements are required. Currently, lack of reinforcement of the manufacturer’s specified initial force, elastic stability, and poor design are the main defects. If these defects can be fixed, silicone rubber bands, which satisfy the esthetics and biocompatibility requirements of dental orthodontic materials, would be an ideal substitute for conventional latex rubber bands. CONCLUSIONS
The following results were obtained from the mechanical and biological tests of silicone compared with latex rubber bands: 1. The initial force of the silicone rubber band was 83% of product specifications, according to the standard extension of 3 times the lumen diameter; silicone had the lowest specified initial force of all bands tested. However, at the 450% extension length of the lumen diameter, the force of silicone rubber bands increased notably to match that of group I. Thus, it showed an extension-force pattern different from that of the latex rubber bands. 2. After 1 day in saliva conditions at 300% extension lengths, the force decay percentages of the latex rubber bands were about 23% to 28%, whereas the silicone rubber bands showed a force decay percentage of 27%. However, at the 450% extension length, the silicone rubber bands had a 33% force decay of initial force, showing an increasing rate of force decay in accordance with an increase of extension length. 3. On the second day of the cytotoxicity test, groups IV and I showed greater viability than the other groups; group II showed the lowest viability. In this study, silicone rubber bands were found to exhibit a low cytotoxicity and the smallest discrepancy of force level between the dry and wet conditions. However, in terms of the initial force level and the abrupt loss of remaining force with an increase in the extension length, great improvements in the silicone rubber band’s physical properties are required.
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