investigation elastics uv journal european

European Journal of Orthodonticsdoi:10.1093/ejo/cjt002

The Author 2013. Published by Oxford University Press on behalf of the European Orthodontic Society. All rights reserved. For permissions, please email: [email protected]

An investigation into UV light exposure as an experimental

model for artificial aging on tensile strength and force delivery of

elastomericchain

S. W.Wahab*, D.Bister** and M.Sherriff****Orthodontic Unit, National Dental Centre, Brunei, **Department of Orthodontics, Guy's and St Thomas' Dental Hospital, London, ***School of Oral & Dental Sciences, University of Bristol, UK

Correspondence to: D.Bister, Department of Orthodontics , Kings College London Dental Institute, 22nd Floor, Tower Wing, Guys Hospital, Great Maze Pond, London SE1 9RT, UK. E-mail: [email protected]

SUMMARy This study investigated the effect of ultraviolet type Alight (UVA) exposure on the tensile prop-erties of elastomeric chain. UVA light exposure was used as model for artificial aging, simulating pro-longed storage of elastomericchain.

Tensile strength (n=60) was measured after exposing Ormco, Forestadent and 3M chains to UVA light for 0, 2, 3, and 4weeks.

Force decay was measured (n=60) using chain exposed for 5, 10, and 14days. The chains were sub-sequently stretched at a constant distance and the resulting forces measured at 0, 1, 24 hours and 7, 14, 21, and 28days. This test simulated a clinical scenario of pre-stretching and subsequent shortening of elastomericchain.

Tensile strength had statistically significant difference and was directly related to the duration of ultravi-olet (UV) light exposure. Forestadent chain, which had the second highest value for the as received prod-uct, showed the most consistent values over time with the lowest degradation. Ormco showed the lowest values for as received as well as after UV exposure; 3M chain had the highest loss of tensile strength.

Force decay was also significantly different. UV light exposure of 10days or more appears to mark a watershed between products: 3M had most survivors, Forestadent chain had some survivors, depend-ing on the time the chain was stretched for. None of the Ormco product survived UV light exposure for more than 5days.

UVA light exposure may be used as a model for artificial aging as it reduces force delivery and tensile strength of exposed chains.

Introduction

The aim of this study was to investigate the effect of UV light exposure on the tensile properties of elastomeric chain and compare its effect with the new as received product. UV light exposure was used as model for artificial aging, simu-lating prolonged storage of elastomeric chain. This model should enable researchers/manufacturers to investigate the effect of aging on elastomeric elements without the necessity of awaiting natural aging. Three different chains from three manufacturers were exposed to UV light for a variety of time intervals to investigate whether the length of exposure affects the mechanical properties of elastomeric chain differently.

Elastomeric elements are used in orthodontics in a vari-ety of forms, such as separators, ligatures, and elastomeric chain. Latex rubber has been replaced by synthetic elas-tomers for the manufacture of those elements because of their improved characteristics and also as a precaution as there appears to be a rise in incidence of latex allergies among patients and health workers (Jacobsen and Hensten-Pettersen, 1989).

In orthodontics, space closure is often achieved by using auxiliaries that can be metal based (Nickel Titanium Coil - Spring) or elastomeric (Elastomeric chain, Olinks, Power-chain, etc.). Superelastic coil springs have the advantage over elastomers because of better physical characteristics such as superior elasticity and less force degradation. However, elastomers are still widely used because these are more cost efficient and close space both in straight line and along the arc and without impinging the mucosa (Andreasen and Bishara, 1970; Young and Sandrik, 1979; De Genova et al., 1985; Lu et al., 1993; Josell etal., 1997).

Most current synthetic elastomers are made of polyurethane, which is an organic polymer first discovered in 1937 by Otto Bayer and co-workers in Germany. The polymer is easily stretched and regains most of its shape on relaxation under normal conditions; however, it is not completely inert. It suffers from aging affects, and this may result in change of its properties such as colour and shape. Such speed of this aging phenomenon is affected by one or

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more environmental factors such as heat, light, exposure to chemicals (ozone), tension, and storage condition. In air, the free oxygen radicals readily degrade polyurethane overtime.

Many synthetic polymers are readily degraded by expo-sure to UV radiation, e.g. ambient light. The materials may crack, disintegrate, and/or discolour depending on extent and degree of exposure. Pigments and dyes integrated in the elastomers may also be affected. The UV degradation process involves the tertiary carbon chain that forms a free radical, which reacts with oxygen in the atmosphere, pro-ducing a carbonyl group. Intra-orally, the product ages and weakens further as it is exposed to saliva and its mineral contents, pH and temperature fluctuations, ingress of micro-organisms and its by-products, plaque formation, food, chewing forces, and abrasion during tooth brushing.

Artificial aging or accelerated aging can be simulated by enhanced exposure to environmental factors. Polymers are very sensitive to the effect of free-radical-generating sys-tems, notably, ozone and UV light. Thus, manufacturers add antioxidants and antiozonates in their elastomeric product to extend its shelf-life.

Numerous investigations on mechanical properties of polyurethane-based elastomeric chains have been under-taken. Previous studies investigated the efficiency of tooth movement for both in vivo and in vivo situations (Ash and Nikolai, 1978; Kuster et al., 1986; Eliades et al., 1999; Eliades, 2004), as well as the effect of different environ-ments on the elastic properties of the chains (De Genova et al., 1985; Ferriter et al., 1990; Lew, 1990; Stevensen and Kusy, 1995). A number of environmental effects on

the mechanical property of chains were investigated, such as the impact of storage media (Huget etal., 1990; Jeffries and Von Fraunhofer, 1991; von Fraunhofer, 1992; von Fraunhofer et al., 1992), exposure to foodstuffs (Nattrass etal., 1998), and the effect of ageing (Eliades etal., 2005). Baty etal. (1994), noted the impact of light exposure and suggested to keep the chains in the container provided by the manufacturer for protection from direct sunlight.

To our knowledge no investigator has systematically reviewed the effect of exposure of UV light type A(UVA) on the properties of elastomeric chain. Bulk purchase is often used in orthodontic practice to reduce prices, and long-term storage may become necessary. Orthodontists need to be reassured that products function reliably, and our investigation was designed to study the effect of artificial aging (UVA exposure) on the properties of elastomeric chains. We chose tensile testing to investigate the overall effect of UVA exposure of the final force level before fracture in the first instance and further tried to simulate the clinical scenario by measuring force decay levels of pre-stretched chain over various lengths of time.

Materials and methods

UVA light was used, which is the mildest form of UV light commercially used for tanning skin. Two units of the LAMPADA UVA light cure boxes (Bracon Limited, West Sussex) were used in this study and each unit was specifi-cally allocated for exposure for either force decay or ten-sile strength samples. The chains were laid flat in the box

Figure1 Plots of the estimated marginal means and 95 % confidence intervals for Ormco. Tensile force and ultraviolet type A(UVA) light exposure time before fracture.

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EFFECT OF UV LIGHT ON ELASTOMERIC CHAIN PROPERTIES Page 3 of 8

and regularly turned over. The UV-irradiation was produced by 4 units of fluorescent 9 Watt power lamps, which were ordered new from the supplier.

For the experiments, elastomeric chains from three differ-ent companies were used:

1. Ormco Generation II Power ChainOrmco 1332 South Lane Ormco 1332 South Lane Hill Avenue, Glendora CA 91740

2. Forestadent Happy Elastics normalForestadent USA 2315 Weldon Pkwy

3. 3M Unitek C-1 short chainMonrovia, CA 91016 USA

Clear chains were used to eliminate the effect of pig-mentation on the elastic property of the material (Lu etal., 1993; Baty etal., 1994).

The elastomeric chains from each company were pre-pared by a single operator (SW). Ten and 12 links were used

Figure2 Plots of the estimated marginal means and 95 % confidence intervals for Forestadent. Tensile force and ultraviolet type A(UVA) light exposure time before fracture.

Figure3 Plots of the estimated marginal means and 95 % confidence intervals for 3M Unitek. Tensile force and ultraviolet type A(UVA) light exposure time before fracture.


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in the force decay and tensile strength test, respectively. In both tests, only the middle six links of the specimens were stretched (Eliades etal., 2004) to 40 mm as this imitates the average width of an upper labial segment.

The tensile force test was undertaken first and the chains were stretched to their limits immediately after the exposure to UV light for 14, 21, and 28days.

The force decay samples were exposed to UV light for 5, 10, and 14 days. These samples were subsequently labelled and stretched and mounted on the jigs, which were stored in dry and dark conditions (a large cardboard box) for the designed amount of time before testing immediately afterwards.

Forces were measured using an Instron Universal Testing Machine (Model No. 1195, Instron Limited, High Wycombe, Bucks., UK) with 50 N tension load cell (type: 2511/1111). The testing was carried out at room tempera-ture throughout the experiment.

Tensile strengthtest

A total of 60 elastomeric chains were tested for each product. Fifteen were as-received (n=15), aged for 2 (n=15), 3 (n=15), and 4 weeks (n=15). In this test, the middle six-links were subjected to stretching, and the three loops at each end were not used as to alleviate the excessive stress concentration on the terminal loops (Eliades etal., 2004).

The first and sixth links of the mid section were hooked to the 1.0 mm stainless steel pin attached to the fixed and movable crosshead of the Instron machine. The two crossheads were positioned at the distance equal to the original length of test specimen. The chains were stretched at crosshead speed of 100 mm per minute, until the specimen fractured. The force at breakage was recorded for each specimen.

Force decaytest

A total of 60 sample chains from each product were tested. Ten were used as controls, 30 taken from samples aged for 14days, 10 from 10days, and 10 from 5days. The force delivered by each samples were recorded at the initial stretch after 1 and 24 hours and 7, 14, 21, and 28 days. The chains were stretched at fixed length of 55.5, 54.0 and 57 mm for product 1, 2, and 3, respectively. These dis-tances were made constant by storing the elements on a jig, of which three were used simultaneously. Each jig was made from two acrylic blocks of 1.5 1.5 20.0 cm held at a fixed distance (3 original length of tested elastomeric chains) by two stainless steel screws (see Figure 4). On each block, 30 stainless steel tubes (1.2 mm diameter) were positioned in place at 5 mm interval to each other. When not tested, the stretched chain was kept on the jig and stored in a cardboard box in dark and dry conditions and at room temperature.

Figure4 Clockwise direction, Top Left: Samples of elastomeric chains, Ormco on the right, Forestadent in the middle and 3M Unitek on the left; Top Right: Set up of stretched chain on test jig. Bottom: Storing jigs: Ormco on the right, Forestadent in the middle and 3M Unitek on the left.


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For the initial force value, the samples were first manu-ally extended to three times its original length. Using the extra two loops at both ends as engagement points during stretching eliminated additional stresses created by squeez-ing the material. The chains were then transferred to the Instron machine, without changing the length. The samples were then extended to four times their original length at a rate of 100 mm per minute and then held at that distance for 5 seconds. The specimens were then returned to three times their original length, and this length was maintained for another 30 seconds. In the end, the resulting force was recorded. The stretched length were then transferred to the storage jig again and kept at constant length for the rest of the experiment.

For the remaining testing at 1 and 24 hours and 7, 14, 21, and 28days, the same specimen were transferred from the storing jig to the testing jigs, and forces were measured as described above.

Data analysis

Data were analysed using Stata 12.0 with significance pre-determined at = 0.05. The null hypothesis was that there was no significant effect of treatment or time on the forcedecay.

The experimental design had repeated measures and was balanced for each time and treatment. Univariate summary statistics are shown in Table1, and it can be seen that there were a large number of failures resulting in empty cells, which complicates the data analysis. As a consequence the data were analysed separately for each system using linear mixed models, Stata xtmixed, and the effects of treatment were compared using contrasts with Sidaks method to adjust for multiple comparisons. Plots of the estimated mar-ginal means and 95 per cent confidence intervals are shown

in Figures 13. For all systems there was a significant effect of treatment, time, and a treatment-time interaction. The comparison of each treatment within each time for each system is summarized in Table2.

Results

Tensile strengthtest

Table 1 shows results of the tensile strength test for all chains. The results demonstrate reduction in tensile strength of aged elastomers from all products. Sites of weakening and fracture were commonly found in the cracks that are perpendicular to the direction of applied stress. This has also been referred to as ozone crack (Eliades etal., 1999; Eliades etal., 2003). From observation, the most frequent fracture sites for Ormco and 3M Unitek were found in the intermodular link. Forestadent chains exhibited sites of tearing in the loop itself. The differences in the findings were associated with the structural shape and dimension of the elastomers.

Results showed generalized reduction in the tensile strength of artificially aged chain. The effect of UV Light in degrading the polymer resulted in reduced stress resist-ance of the chain; the longer the exposure, the weaker they became. Sidaks method for multiple comparisons for each time period and each system are shown in Table2. It dem-onstrated that the differences between all combinations were statistically significant, except for 2 week aging and 3 week aging for Forestadent. Overall, the Forestadent chain showed the least relative reduction of tensile strength fol-lowed by 3M Unitek and Ormco (see Figures 13). Please note that the time scale on the x-axis of the figures is not proportionate but is equally spaced so that the differences are shown more clearly.

Table1 Univariate summary statistics.

Treatment System and time

Ormco Forestadent 3M Unitek

0 1 2 3 4 5 6 0 1 2 3 4 5 6 0 1 2 3 4 5 6

As received 10 10 10 10 10 10 10 10 10 10 9 7 7 6 10 10 10 10 10 10 101.92 1.50 1.19 1.28 1.28 1.52 1.33 2.17 1.73 1.49 1.57 1.58 1.46 1.21 1.76 1.34 1.09 1.15 1.15 0.93 0.890.11 0.08 0.06 0.07 0.15 0.10 0.09 0.07 0.06 0.13 0.12 0.06 0.06 0.42 0.07 0.09 0.06 0.08 0.08 0.07 0.07

5day aged 5 5 4 4 3 2 10 7 4 10 10 10 10 10 10 101.69 1.30 1.20 1.27 1.36 1.25 1.99 1.58 1.29 1.87 1.50 1.32 1.03 0.91 0.99 0.890.11 0.05 0.04 0.04 0.06 0.04 0.08 0.06 0.32 0.08 0.08 0.09 0.09 0.09 0.07 0.12

10day aged 6 6 4 3 2 10 10 10 9 9 8 81.77 1.47 1.39 1.26 1.20 1.56 1.19 1.16 0.96 1.21 1.16 0.960.13 0.14 0.17 0.08 0.21 0.14 0.11 0.09 0.20 0.18 0.11 0.09

14day aged 12 4 14 7 4 3 2 21.41 0.93 1.51 1.08 0.85 0.86 0.73 0.560.09 0.20 0.08 0.13 0.05 0.03 0.02 0.09


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Force decaytest

The numbers of samples that survived aging and initial stretching to three times the original length were subse-quently used. The failure or fracture of chain in the initial stage was associated with the first stretch to 300 per cent of the original length, when placing them on the test jigs. Subsequent failure during the pre-stretching to 400 per cent the original length also occurred and is included in the figures.

After 5days of aging some of the Ormco samples failed to mechanical stresses of stretching during the initial loading on to the test jig. The results showed consistently lower num-bers of survival with increasing duration of exposure to UV light. Only half the chain aged for 5days survived the initial stretch for up to 24 hours. From 24 hours to 7days the num-ber of chain reduced to 40 per cent, followed by 30 per cent at 14days and 20 per cent at 21days. None of the chain sur-vived the initial stretching for chains aged 10 and 14days. Double aging in the form of UV exposure and mechanical stress of stretching does not agree with this product.

Some of as received (control, before UVA exposure) chain from Forestadent demonstrated failure after 7 days of loading: 10 per cent of the stretched chain fractured and only 60 per cent of the chain survived until 28days. Test chain aged for 5 days demonstrated 40 per cent survival after 24 hours; then all failed when stretched for more than 7days. Chain aged for 10days showed better survival than chain exposed to 5days, but this was not statistically signifi-cant: 40 per cent survival after 24 hour, which then reduced to 20 per cent at 14days, and none were left at 21days. For

chains aged for 14days, 12 out of 30 survived the initial stretch, which then reduced to four after 1 hour followed by 100 per cent failure at 24hours.

Sample chains from 3M Unitek demonstrated 100 per cent survival throughout test period for both as received and 5day aging. The 10day aged chain showed 100 per cent survival for 24 hours and this reduced to 80 per cent after stretching for 14days and beyond. After 14days of aging 14 out of 30 survived the initial stretch of 300 per cent origi-nal length; this number was reduced to 3 when stretched between 7 and 21 days and all failed thereafter.

Particular difficulties arise in the interpretation of the above data because of two sources of variability: within and between samples. Within each sample, the forces measured differ because they were taken at different times. Furthermore, inconsistency of the number of survived samples at various time points made it impossible to analyse the overall force decay. However, the available data were still analysed and significance was pre-determined when probability is greater than 0.05. The analyses indicated a complex situation where all main terms (time point, aging condition and elastomeric chain type) and interactions were significant, so each elasto-meric chain was analysed separately to evaluate the effect of time and interaction of aging andtime.

Figures 13 show the effect of time for all conditions of chains. Analyses showed that there is statistically signifi-cant interaction between as received and Ormco chain, which survived 5day aging. The percentage of force that remained after 24 hours was between 60 and 75 per cent of the initialforce.

There was no significant difference between the as received and 10day aged Forestadent chain. The remaining

Table2 Sidak adjusted probability for the pairwise comparison of treatements within each time period for each system.

System Comparison Probability

Time period

0 1 2 3 4 5 6

Ormco 21 0.001 0.001 0.796 0.861 0.147 0.0013141324243

Forestadent 21 0.043 0.196 0.09931 0.001 0.004 0.64441 0.001 0.00132 0.031 0.710 0.96242 0.001 0.00143 0.001 0.001

3M Unitek 21 0.069 0.003 0.001 0.061 0.001 0.783 1.00031 0.001 0.006 0.560 0.001 0.709 0.001 0.71841 0.001 0.001 0.001 0.001 0.001 0.00132 0.001 0.001 0.004 0.518 0.001 0.005 0.74742 0.001 0.001 0.001 0.027 0.100 0.00143 0.736 0.069 0.001 0.435 0.001 0.001


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force found after 24 hours was between 65 and 79 per cent for the survivor from as received, 5day aged, and 10day aged chain.

3M Unitek chain showed a significant effect between the as received, 5day aged, and 10day aged chain. But there was no significant difference between 14 day aged chain and the previous three. About 6075 per cent of initial force remained after 24 hours of stretching.

Discussion

The results of this study have shown that polymers degrade after experimental UVA light exposure; tensile strength is affected as well as force decay after pre-stretching.

The tensile strength of all exposed chains demonstrated a lower maximum load. Ormco chain had the lowest tensile strength compared with the other two elastomers tested. The material had become brittle after aging for 2 weeks ormore.

Tensile strength of chain from Forestadent showed least reduction in tensile strength after aging. Interestingly there was no difference in strength of chain aged for 2 or 3 weeks. This phenomenon may be attributed to irregularities during production of the chain or method error inherent in this study, the latter being less likely than the former.

Aged 3M Unitek chain had lower tensile strength than Forestadent chain, despite its good performance in the force decay test. This finding may be explained by the different chemical composition of the chain (such as antiozonates). Aging for 2 weeks or more is, at least theoretically, a critical length of time before the antiozonate of the material fails to protect the molecular structure.

Differences in force decay and tensile strength of differ-ent brands of chains can be attributed to either variations in manufacturing technique (die stamping or injection mold-ing), differences in the constituent of basic polyurethanes, various incorporated additives, and in morphology / dimen-sional characteristics such as ellipsoidal or circular shapes or presence or absence of an intermodular links (Eliades etal., 2005).

The Ormco chain demonstrated highest vulnerability to the aging process despite sufficient force delivery (above 1 N after force relaxation), it had the highest failure rate: 50 per cent of chain that had been exposed to UV light for 5days or more fractured.

Chains from Forestadent exhibited a mixture of failure and survival for all conditions, including as received and aged conditions. The observed failure of the chain from as received samples indicate the unfavourable response to stretching of more than three times its original length. It appears that at this distance (or more), the chains had reached its limit of elasticity/transition point (Rock et al., 1985). While pre-stretching up to two times its original length is often performed clinically, the effects of mechani-cal stretching of more than three times its original length had

a profound effect on this chain. Rock etal. (1985) investi-gated 13 commercially available elastomeric products. They looked at the strength / stiffness / initial force of each elas-tomeric chain when stretched to 100 per cent of its original length. The authors suggested that 5075 per cent of exten-sion should be sufficient to generate reliable orthodontic force. However, Andreasen and Bishara (1970) suggested stretching up to four times the original length would be nec-essary to counteract the effect of force decay associated with the material and aging effect of intra-oral environment.

3M Unitek chain aged for 5 days behaved similarly to the as received. All chains survived pre-stretching. This material hence performed best in terms of mechanical and environmental stress and the same holds for all other exposuretimes.

A study by Eliades etal. (2004) showed no correlation in tensile strength with as received, stretched in air and samples retrieved from intra-oral environment. However, our study found that the tensile strength is affected by artifi-cial environmental aging: chains exposed to UVA light have significantly lower tensile strength than as received chains. The differences in findings may be due to the differences of applied strain: stretching the material to different lengths and different aging methods.

Previous studies have shown the effect of thermal cycling on elastomeric properties. De Genova etal., (1985) dem-onstrated that elastomeric properties of chains subjected to thermal cycling (15 and 45C) over a 3 week period had less force decay than those stored at 37C. We recognized that we were unable to hold the temperature of our sam-ples constant at room temperature when the chains were exposed to the UVA light. However, the temperature in the LAMPADA UVA light cure boxes was identical for all products and should have hence had little effect on the results of our study.

Conclusions

1. Exposure of elastomeric chain to UVA light ages elas-tomeric chain as it had a significant impact on force decay and tensile strength of the three products tested. Different makes of elastomeric chains were affected in different ways.

2. For tensile strength tests we found statistically signifi-cant differences, which were related to the duration of UV light exposure. The Forestadent chain that had the second highest value for the as received product showed the most consistent values over time with the lowest degradation of tensile strength. The Ormco prod-uct showed the lowest values at as received as well as after UV exposure, while the 3M product had the highest net force decay over time.

3. For the force decay test results were complex: after 5days of aging all of the Ormco chain fractured after they had been stretched for more than 3 weeks. Forestadent


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chain should not be pre-stretched more than three times their original length as this induced failure even for as received products; after 5 days of exposure all chains had fractured after 1 week of stretching; for the 3M product, however, all samples survived 5days of expo-sure. The results for 10days of aging were as follows: all Ormco chains failed; some of the Forestadent chain survived initial stretch but all eventually failed after 3 weeks of stretching; again the 3M product did best with 80 per cent survivors even after more than 3 weeks of stretching. After 10 days of UVA exposure all Ormco chains had fractured, 60 per cent of the Forestadent chain survived initially, but all remainders failed after 14days of stretching, only the 3M products showed a somewhat better survival with 80 per cent surviving even 28days of stretching. After 14days of aging only 3M showed any noteworthy survival, but no chain survived more than 3 weeks of stretching.

4. Force delivery of the survivors of the force decay test showed that there was little further force decay after the initial force decay until failure occurred. In other words, the chains will deliver reasonably constant force delivery once they have survived pre-stretching.

Acknowledgements

Our thanks go to Mr Richard Mallett for his permission to use the Instron Universal Testing Machine (Model No. 1195, Instron Limited, High Wycombe, Bucks., UK).

References

Andreasen G F, Bishara S 1970 Comparison of alastik chains with elastics involved with intra-arch molar to molar forces. The Angle Orthodontist 40: 151158

Andreasen G F, Bishara S E 1970 Relaxation of orthodontic elastomeric chains and modules in vitro and in vivo. The Angle Orthodontist 40: 319328

Ash J L, Nikolai R J 1978 Relaxation of orthodontic elastomeric chains and modules in vitro and in vivo. Journal of Dental Research 57: 685690

Baty D L, Storie D J, von Fraunhofer J A 1994 Synthetic elastomeric chains: a literature review. American Journal of Orthodontics and Dentofacial Orthopedics 105: 536542

Baty D L, Volz J E, von Fraunhofer J A 1994 Force delivery properties of colored elastomeric modules. American Journal of Orthodontics and Dentofacial Orthopedics 106: 4046

Brantley W A, Salander S, Myers C L, Winders R V 1979 Effects of pre-stretching on force degradation characteristics of plastic modules. The Angle Orthodontist 49: 3743

De Genova D C, McInnes-Ledoux P, Weinberg R, Shaye R 1985 Force deg-radation of orthodontic elastomeric chainsa product comparison study. American Journal of Orthodontics and Dentofacial Orthopedics 87: 377384

Eliades T, Eliades G, Watts D C 1999 Structural conformation of in vitro and in vivo aged orthodontic elastomeric modules. European Journal of Orthodontics 21: 649658

Eliades G, Eliades T, Brantley W A, Watts D C 2003 Aging of orthodontic utilities and auxiliaries. In: Eliades G (ed.) Dental Materials. In Vivo Aging and Related Phenomena, Quintessence Books, pp. 165170

Eliades T, Eliades G, Silikas N, Watts D C 2004 Tensile properties of ortho-dontic elastomeric chains. European Journal of Orthodontics 26: 157162

Eliades T, Eliades G, Silikas N, Watts D C 2005 In vitro degradation of polyurethane orthodontic elastomeric modules. Journal of Oral Rehabilitation 32: 7277

Ferriter J P, Meyers C E Jr, Lorton L 1990 The effect of hydrogen ion concentration on the force-degradation rate of orthodontic polyure-thane chain elastics. American Journal of Orthodontics and Dentofacial Orthopedics 98: 404410

Hershey H G, Reynolds W G 1975 The plastic module as an orthodon-tic tooth-moving mechanism. American Journal of Orthodontics and Dentofacial Orthopedics 67: 554562

Hanson M, Lobner D 2004 In vitro neuronal cytotoxicity of latex and nonlatex orthodontic elastics. American Journal of Orthodontics and Dentofacial Orthopedics 126: 6570

Huget E F, Patrick K S, Nunez L J 1990 Observations on the elastic behavior of a synthetic orthodontic elastomer. Journal of Dental Research 69: 496501

Jacobsen N, Hensten-Pettersen A 1989 Occupational health problems and adverse patient reactions in orthodontics. European Journal of Orthodontics 11: 254264

Jeffries C L, von Fraunhofer J A 1991 The effects of 2% alkaline gluta-raldehyde solution on the elastic properties of elastomeric chain. The Angle Orthodontist 61: 2530

Josell S D, Leiss J B, Rekow E D 1977 Force degradation in elastomeric chains. Seminars in Orthodontics 3: 189197

Kovatch J S, Lautenschlager E P, Apfel D A, Keller J C 1976 Load-extension-time behavior of orthodontic Alastiks. Journal of Dental Research 55: 783786

Kuster R, Ingervall B, Brgin W 1986 Laboratory and intra-oral tests of the degradation of elastic chains. European Journal of Orthodontics 8: 202208

Lew K K K 1990 Staining of clear elastomeric modules from certain foods. Journal of Clinical Orthodontics 24: 472474

Lu T C, Wang W N, Tarng T H, Chen J W 1993 Force decay of elasto-meric chaina serial study. Part II. American Journal of Orthodontics and Dentofacial Orthopedics 104: 373377

Matthews J N, Altman D G, Campbell M J, Royston P 1990 Analysis of serial measurements in medical research. BMJ (Clinical research ed.) 300: 230235

Nattrass C, Ireland A J, Sherriff M 1998 The effect of environmental factors on elastomeric chain and nickel titanium coil springs. European Journal of Orthodontics 20: 169176

Petrie A, Bulman J S, Osborn J F 2003 Further statistics in dentistry. Part 9: Bayesian statistics. British Dental Journal 194: 129134

Reitan K 1957 Some factors determining the evaluation of forces in ortho-dontics. American Journal of Orthodontics and Dentofacial Orthopedics 43: 3245

Rock W P, Wilson H J, Fisher S E 1985 A laboratory investigation of ortho-dontic elastomeric chains. British Journal of Orthodontics 12: 202207

Stevensen J S, Kusy R P 1995 Structural degradation of polyurethane-based elastomeric modules. Journal of Material Science, Materials in Medicine 6: 377384

Storey E E, Smith R 1952 Force in orthodontics and its relation to tooth movement. Australian Journal of Dentistry 56: 1118

Turjanmaa K 1987 Incidence of immediate allergy to latex gloves in hospi-tal personnel. Contact Dermatitis 17: 270275

von Fraunhofer J A, Coffelt M T, Orbell G M 1992 The effects of artificial saliva and topical fluoride treatments on the degradation of the elastic properties of orthodontic chains. The Angle Orthodontist 62: 265274

Von Fraunhofer J A, Coffelt M T, Orbell G M 1993 The effects of artificial saliva and topical fluoride treatments on the degradation of the elastic properties of orthodontic chains. The Angle Orthodontist 62: 265274

Wong A K 1976 Orthodontic elastic materials. The Angle Orthodontist 46: 196205

Young J, Sandrik JL 1979 The influence of preloading on stress relaxa-tion of orthodontic elastic polymers. The Angle Orthodontist 49: 104109


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