July, 2010

Invivo and Invitro Tensile Properties of Orthodontic Elastomeric Chains

A Comparative Study

Dr.Rajkumar S Alle, MDS DNB and Dr.Pankaj Dixit, MDS

Abstract

Background & Objectives: Elastomeric chains were introduced to the orthodontic profession in the 1960s, and are now an integral part of most practices. They are used for correcting rotations, consolidating spaces, and retracting canines. Force decay in these materials is significant and has been a clinical problem. The polymers are not ideal elastic materials because their mechanical properties change with time and temperature. This study was to evaluate the permanent elongation and tensile strength of elastomeric chains in vivo and in vitro of three commonly available brands.

Methods: Two types (open & closed chains) of three brands of elastomeric modules producing six groups were included in the study. Specimens were measured by digital caliper and classified into four groups based on their ageing state: (a) as received; (b) subjected to a 24 hour steady strain in air determined as 50% of original length; (c) exposed intraorally for 24 hours; and (d) retrieved following 3 weeks of intraoral exposure. The final lengths of all the specimens were measured and mean percentage elongation was calculated for each group and was analysed with two-way ANOVA. For tensile strength, the specimens were subjected to the above said conditions and were subjected to tensile stress and their behavior was analysed with three-way ANOVA and Tukey’s multiple comparison tests.

Results: The results showed that open E-chains show more percentage elongation compared to closed types. The Alastik brand showed the most elongation after being subjected to oral conditions. The tensile strengths of the E-chains in all the groups decreased after being subjected to intraoral conditions. The decrease in the tensile strength was proportional to the time in the oral cavity.

Conclusion: Open E-chains have higher percentage elongation compared to closed E-chains in vitro but closed E-chains have higher percentage elongation compared to open E-chains in vivo. Among the different test conditions, the maximum percentage elongation was seen after 3 weeks intraoral stretching, followed by 24 hours intraoral and finally the least by the E-chains stretched 24 hours in air. The tensile strength was most in the in vitro conditions followed by 24 hours in the oral cavity and finally the least after three weeks stretching intraorally.

Keywords: Elastomeric chains, Force decay, Mechanical properties, Permanent elongation, Tensile strength

List of Tables

Sl. No.	FIGURES	PAGE No.
1.	Three brands of E-chains used.	24
2.	Digital caliper.	24
3.	Frame work and jigs.	25
4.	Sample stretched in frame work.	25
5.	Sample placed intraorally.	26
6.	Universal testing machine.	27
7.	Sample mounted on the testing configuration.	27
8.	Mean permanent elongation (percentage) of chain groups after 50% extension in air for 24 hours.	37
9.	Mean permanent elongation (percentage) of chaingroups after 50% extension intraorally for 24 hours.	37
10.	Mean permanent elongation (percentage) of chain groups after 50% extension intraorally for 3 weeks.	38
11.	Mean pattern of tensile strength of the chains 38 subjected to 50%extension of their original length (as received).	38
12.	Mean pattern of tensile strength of the chains subjected to 50% extension of their original length (24 hours extension in air).	39
13.	Mean pattern of tensile strength of the chains subjected to 50% extension of their original length (24 hours intraoral).	39
14.	Mean pattern of tensile strength of the chains subjected to 50% extension of their original length (3 weeks intraoral).	40

Introduction

Over the years orthodontists have relied on various force delivery systems to achieve tooth movement. Contemporary orthodontists advocate the use of light forces. Coil springs (NiTi and stainless), retraction springs, elastomeric chains, elastomeric threads, closing loop archwires and magnets are commonly used force delivery systems. It is difficult to maintain good oral hygiene while using springs, closing loop archwires and magnets. In contrast elastomeric chains are economical and are relatively hygienic.

The usage of elastic in the field of orthodontics can be traced back to 1846 when E. Backer used a thin sheet of Indian rubber for retraction of teeth. Begg was the first to use elastics in the form of gum rubber. In 1960, latex gave way for synthetic elastomers after the discovery of polyurethanes.

Orthodontic elastomeric module chains are polyurethanes, thermosetting polymer products of a step-reaction polymerization process. The elastomeric chains currently available are fabricated either by die cut stamping or injection moulding technique. The variation between nominally identical products with respect to time-dependent force decay (Ferriter et al [1]) may be attributed to: variations in module manufacturing techniques such as die cut stamping or injection moulding; effects due to additives; and different morphological (ellipsoid or circular modules) or dimensional characteristics (presence or absence of intermodular link) of the chains (Eliades et al [2]).

The mechanisms for permanent deformation of polymeric materials include molecular chain stretching, slippage between adjacent molecular chains, and molecular scission. During stretching there is a retarded elastic deformation, as well as irreversible viscous deformation, producing permanent elongation.

In addition, tensile strength testing provides an assessment of the resistance to fracture of the chains across the direction of loading. This particular property may facilitate assessment of the probability of rupture of the chain as well as the impact of intraoral ageing (Eliades et al [3]) on the strength of the material. Hence, the purpose of this study is to assess the tensile strength and permanent elongation of elastomeric chains in vitro and in vivo which includes the following:

1. Permanent elongation- which determines the decay in the force generating capacity of the material.
2. Breaking force- which determines the resistance to fracture of the material across the direction of loading.

Aims and Objectives

1. To compare the mechanical properties of commonly available elastic chains, type- such as open (with an intermodular link) and closed (without intermodular link).
2. To compare the same mechanical properties among E-chain products of three different brands such as Alastik (3M/unitek, Monrovia, California, U.S.A.), Sunburst (GAC, Bohemia, New York,U.S.A.), OrthoOrganizers (San Marcos, California,U.S.A.).

The mechanical properties tested were:

1. Permanent elongation, tested from specimens which were measured and classified into four groups based on their ageing state:
   1. as received
   2. subjected to a 24 hours steady strain in air
   3. exposed intraorally for 24 hours and
   4. retrieved following 3 weeks of intraoral exposure.
      All the specimens were stretched to 50% of their original length.
2. Tensile strength tested from specimens of each of the six material groups exposed to the four conditions of the experiment were mounted on a calibrated universal testing machine and were subjected to tensile extension at a rate of 5mm/minute until failure.

Materials and Methods

The elastic products which were selected for the evaluation are:

Table 1. The elastomeric chains included in the study.

Product code	Brand (manufacturer)	Design
I	Alastik (3M/Unitek, Monrovia, California, USA)	Closed
II	Sunburst (GAC, Bohemia, New York, USA)	Closed
III	OrthoOrganizers (San Marcos, California,USA)	Closed
IV	Alastik	Open
V	Sunburst	Open
VI	OrthoOrganizers	Open

Two types (open and closed i.e., with and without an intermodular link) of three brands of elastomeric module producing six groups were included in the study (Fig 1).

For permanent elongation:

In vitro specimens were prepared by cutting multiple series of specimens of each type and brand containing ten samples each (total 60), having equal numbers of loops (6) from the spools, and in vivo specimens were taken depending on the patients’ needs, with the use of a sharp ligature cutter. Care was taken to avoid extended handling during cutting as this might have incorporated stresses in the material prior to testing.

The initial length (L₀) of the prepared specimens was measured with a digital caliper (Aerospace, China) (Fig 2). The specimens were classified into four groups based on their ageing state:

1. Elastomeric in the as received state;
2. Chains subjected to a 24 hour steady strain in air, determined as 50 per cent of its original length;
3. Modules elongated intraorally at approximately 50 per cent extension relative to the as received state. Specimens were retrieved following 24 hours of exposure in the oral cavity of the patient of good oral health under orthodontic treatment with edgewise, 0.022 inch, Roth prescription, not under any medication. This facilitated comparison between material alterations induced by in vitro stretching and intraoral extension during the same time period.
4. Chains retrieved after a 3 week exposure to the oral environment of patients receiving orthodontic therapy with brackets identical to the 24 hour retrieval experiment. The modules were again elongated approximately 50 per cent relative to the as received state.

All the in vitro specimens were stretched 50% of their original length in a framework consisting of a metallic box , screws (30nos) and nuts (60nos) and 1mm hard round stainless steel wire hooks soldered to the heads of the screws which were placed on either side of the metallic box (Fig 3). The distance between the two hooks was adjusted and measured with the help of a digital caliper to ensure that all the specimens were elongated 50% of their original lengths. Total 60 samples were placed in the framework, 15 at a time, to test the permanent elongation. The framework was placed at room temperature for 24 hours (Fig 4).

In vivo specimens were fabricated depending upon patients’ needs and all the intraorally exposed specimens (Fig 5a, b) were retrieved and rinsed with copious amounts of distilled water to remove the loosely bound intraorally formed integuments. The final length (L) of the specimens was measured at the end of the testing period. Because of the absorption of oral fluids, the resultant swelling of each modular unit and the different engagement pattern of elastomerics with brackets intraorally, the dimensions of retrieved modules were altered by a set of parameters additional to the stretching and, therefore, elongation measurements were only reliably performed for the specimens of the first group, i.e., stretched in air.

The percentage elongation was calculated using the formula ε = [(L − L₀)/ L₀].100

For tensile strength test:

In vitro specimens were fabricated by cutting 12 loops of each chain spool of the material included in the study; in this case the six central loops were subjected to stretching and the three loops on each side were used to alleviate the excessive stress concentration on the terminal loops.

In vivo specimens of each brand and type were taken depending on patient’s need. Ten specimens of the six material groups exposed to the four conditions of the experiment were mounted on a calibrated testing machine (LLOYD instruments, LR50K) (Fig. 6) and were subjected to tensile extension at a rate of 5mm/minute until failure (Fig 7). The testing configuration consisted of fabricating two hooks from 1mm diameter stainless steel wire and attaching the elastic chain to hooks. A choice of a large diameter round wire was to avoid the edges of the rectangular wire, which may have applied increased stress on the chain. A 1mm wire is also sufficiently stiff to exclude any absorption of stress during testing. The modules were subjected to tensile stress and the breaking force (N) was recorded for each specimen.

The results of the elongation experiment were analysed with a two-way ANOVA with brand and type (open or closed chains) serving as discriminating variables, while the results of the tensile strength measurement were statistically analysed with a three-way ANOVA with brand, design (closed versus open) and state (as-received, 24 hour stretched in air, 24 hours intraoral, and 3 weeks intraoral) variables. Further differences among groups were examined with Tukey’s multiple comparison test.

Statistical software: The Statistical software namely SPSS 11.0 and Systat 8.0 were used for the analysis of the data and Microsoft word and Excel have been used to generate graphs, tables etc.

Figure 1. Three brands of E-chains used.

Figure 2. Digital caliper

Figure 3. Frame work and jigs used in the study.

Figure 4. Samples placed in the frame work.

Figure 5a. Sample placed intraorally.

Figure 5b. Sample placed intraorally.

Figure 6. Universal testing machine. (LLOYD instruments, LR50K)

Figure 7. Sample mounted on the experimental configuration.

Study Design: A Comparative study consisting of three brands of elastomeric module (3M/Unitek, GAC and Ortho Organizers) and two types (Open and Closed) each with 10 samples is undertaken to study the permanent elongation and tensile strength.

Statistical analysis was performed for the obtained data. Data for percentage elongation was analysed using two-way ANOVA, and three-way ANOVA was used to analyse the data for tensile strength. The differences among the groups were examined with Tukey’s multiple comparisons test.

Table 2. Mean permanent elongation (percentage) of chain groups after 50% extension in air for 24 hours.

Groups	24 hours in Air
	Range	Mean ± SD
I	2.10-3.35	2.83 ± 0.44
II	2.55-3.60	3.10 ± 0.36
III	6.26-7.17	6.78 ± 0.33
IV	6.26-7.58	7.09 ± 0.34
V	3.76-4.52	4.15 ± 0.27
VI	8.19-10.05	9.30 ± 0.63
P value Two way ANOVA	Brand: P<0.001** Type: P<0.001**

Table 2 shows the results of mean permanent elongation measurements following extension in air for 24 hours. Group VI (9.30 ± 0.63) and Group IV (7.09 ± 0.34) underwent the highest elongation i.e., Orthoorganizers (open) and Alastik (open) respectively, followed by group III (6.78 ± 0.33) and Group V (4.15 ± 0.27) i.e., Orthoorganizers (closed) and GAC (open) respectively. The least elongation was seen in Group I (2.83 ± 0.44) and Group II (3.10 ± 0.36) i.e., Alastik (closed) and GAC (closed) respectively.

There were statistically significant differences in Groups when compared for brand and type.

Figure 8 shows the bar graph representation of mean permanent elongation (percentage) of chain groups after 50% extension in air for 24 hours.

Table 3. Mean permanent elongation (percentage) of chain groups after 50% extension intraorally for 24 hours.

Groups	24 hours intraoral
	Range	Mean ± SD
I	51.82-53.21	52.56 ± 0.52
II	35.89-37.11	36.52 ± 0.43
III	19.77-20.61	20.20 ± 0.29
IV	29.94-31.25	30.64 ± 0.46
V	16.37-17.24	16.77 ± 0.28
VI	15.47-16.43	15.95 ± 0.34
P value Two way ANOVA	Brand: P<0.001** Type: P<0.001**

Table 3 shows the results of mean permanent elongation measurements following extension intraorally for 24 hours. Group I (52.56 ± 0.52) and Group II (36.52 ± 0.43) underwent maximum elongation i.e., Alastik (closed) and GAC (closed) respectively, followed by Group IV (30.64 ± 0.46) and Group III (20.20 ± 0.29) i.e., Alastik (open) and Orthoorganizers (closed) respectively, where as Group V (16.77 ± 0.28) and Group VI (15.95 ± 0.34) i.e., GAC (open) and OrthoOrganizers (open) respectively, presented the least elongation.

There were statistically significant differences in Groups when compared for brand and type.

Figure 9 shows the bar graph representation of mean permanent elongation (percentage) of chain groups after 50% extension intraorally for 24 hours.

Table 4. Mean permanent elongation (percentage) of chain groups after 50% extension intraorally for 3 weeks.

Groups	3-weeks intraoral
	Range	Mean ± SD
I	85.11-66.22	65.74 ± 0.43
II	45.76-46.66	46.15 ± 0.34
III	28.91-29.61	29.22 ± 0.25
IV	40.09-41.12	40.68 ± 0.36
V	29.25-30.36	29.82 ± 0.37
VI	23.89-24.61	24.19 ± 0.24
P value Two way ANOVA	Brand: P<0.001** Type: P<0.001**

Table 4 shows the results of mean permanent elongation measurements following extension intraorally for 3 weeks. Group I (65.74 ± 0.43) and Group II (46.15 ± 0.34) presented the highest elongation i.e., Alastik (closed) and GAC (closed) respectively followed by Group IV (40.68 ± 0.36) and Group V (29.82 ± 0.37) i.e., Alastik (open) and GAC (open) respectively, where as Group III (29.22 ± 0.25) and Group VI (24.19 ± 0.24) presented with the least elongation i.e., OrthoOrganizers (closed) and OrthoOrganizers (open) respectively.

There were statistically significant differences in Groups when compared for brand and type.

Figure 10 shows the bar graph representation of mean permanent elongation (percentage) of chain groups after 50% extension intraorally for 3 weeks.

Table 5. Pair wise comparison of mean permanent elongation (percentage) for four treatments (as received, 24 hours stretched in air, 24 hours exposure intraorally and 3 weeks intraorally).

	Group I	Group II	Group III	Group IV	Group V	Group VI
Group I	-	<0.001**	<0.001**	<0.001**	<0.001**	<0.001**
Group II	-	-	<0.001**	<0.001**	<0.001**	<0.001**
Group III	-	-	-	<0.001**	<0.001**	<0.001**
Group IV	-	-	-	-	<0.001**	<0.001**
Group V	-	-	-	-	-	<0.001**
Group VI	-	-	-	-	-	-

Table 5 shows the statistical comparison between the groups for the four treatments i.e., as received, 24 hours stretched in air, 24 hours exposure intraorally and 3 weeks intraorally and the result indicates that there were statistically significant differences among the groups for all the four conditions.

Table 6. Mean pattern of Tensile strength (N) of the chains subjected to 50% extension of their original length.

Groups	As received		24 hours		24 hours intraoral		3 weeks intraoral
	Mean	SD	Mean	SD	Mean	SD	Mean	SD
I	23.45	0.56	23.81	0.34	17.08	0.36	21.95	0.28
II	22.39	0.67	23.12	0.51	18.34	0.74	17.15	0.79
III	20.38	0.53	19.40	0.61	18.16	0.81	19.58	0.65
IV	26.06	0.47	25.81	0.60	18.53	0.59	25.85	0.45
V	25.02	0.40	24.82	0.42	17.80	0.53	24.75	0.38
VI	21.24	0.47	18.71	0.61	18.46	0.41	18.47	0.42
P value	<0.001**		<0.001**		<0.001**		<0.001**

Table 6 shows the results of mean tensile strength measurements for all four treatment conditions i.e. as received, 24 hours stretched in air, 24 hours exposure intraorally and 3 weeks intraorally. For as received samples, the highest fracture values were recorded for Group IV (26.06 ± 0.47) and Group V (25.02 ± 0.40) i.e., Alastik (open) and GAC (open) respectively followed by Group I (23.45 ± 0.56) and Group II (22.39 ± 0.67) i.e., Alastik (closed) and GAC (closed) respectively, where as the least fracture values were recorded for Group VI (21.24 ± 0.47) and Group III (20.38 ± 0.53) i.e., OrthoOrganizers (open) and OrthoOrganizers (closed) respectively.

For samples after 24 hours extension in air, the highest fracture values were recorded for Group IV (25.81 ± 0.60) and Group V (24.82 ± 0.42) i.e., Alastik (open) and GAC (open) respectively, followed by Group I (23.81 ± 0.34) and Group II (23.12 ± 0.51) i.e., Alastik (closed) and GAC (closed) respectively, where as the least fracture values were recorded for Group III (19.40 ± 0.61) and Group VI (18.71 ± 0.61) i.e., OrthoOrganizers (closed) and OrthoOrganizers (open) respectively.

For samples after 24 hours exposure intraorally, the highest fracture values were recorded for Group IV (18.53 ± 0.59) and Group VI (18.46 ± 0.41) i.e., Alastik (open) and OrthoOrganizers (open) respectively, followed by Group II (18.34 ± 0.74) and Group III (18.16 ± 0.81) i.e., GAC (closed) and OrthoOrganizers (closed) respectively, where as the least fracture values were recorded for Group V (17.80 ± 0.53) and Group I (17.08 ± 0.36) i.e., GAC (open) and Alastik (closed) respectively.

For samples after 3 weeks exposure intraorally, the highest fracture values were recorded for Group IV (25.85 ± 0.45) and Group V (24.75 ± 0.38) i.e., Alastik (open) and GAC (open) respectively, followed by Group I (21.95 ± 0.28) and Group III (19.58 ± 0.65) i.e., Alastik (closed) and OrthoOrganizers (closed) respectively, where as the least fracture values were recorded for Group VI (18.47 ± 0.42) and Group II (17.15 ± 0.79) i.e., Orthoorganizers (open) and GAC (closed) respectively.

Figures 11, 12, 13 and 14 show the bar graph representation of the mean tensile strengths for the four different treatment conditions i.e., as received, 24 hours stretched in air, 24 hours exposure intraorally and 3 weeks intraorally respectively.

Table 7. Three way ANOVA table for tensile strength.

Source	Type III Sum of Squares	df	Mean Square	F	Sig.
Corrected Model	2213.984	23	96.260	317.069	<0.001**
Intercept	108507.641	1	108507.6	357410.5	<0.001**
TYPE	179.124	1	179.124	590.014	<0.001**
BRAND	514.590	2	257.295	847.498	<0.001**
STATE	924.629	3	308.210	1015.204	<0.001**
TYPE * BRAND	107.966	2	53.983	177.812	<0.001**
TYPE * STATE	80.769	3	26.923	88.681	<0.001**
BRAND * STATE	281.877	6	46.979	154.744	<0.001**
TYPE * BRAND * STATE	125.030	6	20.838	68.639	<0.001**
Error	65.576	216	.304
Total	110787.201	240
Corrected Total	2279.560	239

a R Squared = .971 (Adjusted R Squared = .968)

Table 7 shows the results of the mean tensile measurements with a three-way ANOVA to compare between brand, type (closed and open) and state (as received, 24 hours stretched in air, 24 hours exposure intraorally and 3 weeks intraorally) variables.

All the comparisons made presented statistically significant differences between the variables (P<0.001).

Table 8. Pair wise comparison of mean tensile strength (As received, 24 hours stretched in air, 24 hours exposure intraorally and 3 weeks intraorally).

	Group I	Group II	Group III	Group IV	Group V	Group VI
Group I	-	<0.001**	<0.001**	<0.001**	<0.001**	<0.001**
Group II	-	-	<0.001**	<0.001**	<0.001**	<0.001**
Group III	-	-	-	<0.001**	<0.001**	<0.001**
Group IV	-	-	-	-	<0.001**	<0.001**
Group V	-	-	-	-	-	<0.001**
Group VI	-	-	-	-	-	-

Table 8 shows the statistical comparison between the groups for the four treatments i.e., as received, 24 hours stretched in air, 24 hours exposure intraorally and 3 weeks intraorally, and the result indicates that there are statistically significant differences among the groups for all the four conditions.

Statistical Methods: Analysis of variance has been used to find the significant difference of study parameters between groups. Two-way ANOVA for permanent elongation has been used to find the significance between brand and type. Three-way analysis of variance has been used to find the significance of tensile strength between three brands, two types and four conditions.

Significant figures:

+ Suggestive significance 0.05<P<0.10
* Moderately significant 0.01<P £ 0.05
** Strongly significant P£0.01

Statistical software: The Statistical software namely SPSS 11.0 and Systat 8.0 were used for the analysis of the data and Microsoft word and Excel have been used to generate graphs, tables etc.

Figure 8. Mean permanent elongation (percentage) of chain groups after 50% extension in air for 24 hours.

Figure 9. Mean permanent elongation (percentage) of chain groups after 50% extension intraorally for 24 hours.

Figure 10. Mean permanent elongation (percentage) of chain groups after 50% extension intraorally for 3 weeks.

Figure 11. Mean pattern of tensile strength of the chains subjected to 50% extension of their original length ( as received ).

Figure 12. Mean pattern of tensile strength of the chains subjected to 50% extension of their original length ( 24 hours extension in air ).

Figure 13. Mean pattern of tensile strength of the chains subjected to 50% extension of their original length ( 24 hours intraoral ).

Figure 14. Mean pattern of tensile strength of the chains subjected to 50% extension of their original length ( 3 weeks intraoral ).

Discussion

This study was done in the Department of Orthodontics and Dentofacial Orthopedics, The Oxford Dental College, Bangalore. This study was both an in vitro and an in vivo, conducted on elastomeric modules. The purpose of this study was to compare the mechanical properties among E-chain products of three different brands such as 3M/Unitek, GAC and OrthoOrganizers. It also aimed to compare the mechanical properties between the open (with an intermodular link) and closed (without intermodular link) elastomeric chains. The percentage elongation of the E-chains was compared between the three different brands and also between the open and closed type. The samples were divided into three groups based on their commercial brands and then each brand was further subdivided into an open E-chain group and a closed E-chain group, giving a total of six groups (Table 1), which were then tested under the following four test conditions.

They were:

1. as received
2. subjected to a 24 hours steady strain in air (in vitro)
3. exposed intraorally for 24 hours and
4. retrieved following 3 weeks of intraoral exposure.

This gave four ageing states for each brand. The E-chains were tested after being stretched to 50% of their original lengths. Along with the percentage elongation the tensile strengths of the E-chains were also assessed at all the four test conditions as above.

The E-chains were subjected to the testing conditions and then measured for both percentage elongation as well as their tensile strengths. The data was recorded, tabulated and then subjected to statistical analyses, two-way ANOVA for percentage elongation and three-way ANOVA for the tensile strength.

The results of the present study can be discussed under the following headings:

1. Comparison of the percentage elongation between the groups.
2. Comparison of the tensile strengths between the groups.

I. Comparison of the percentage elongation between the groups.

1. Comparison of the percentage elongation between the groups following 50% elongation for 24 hours in air.

   After 50% elongation for 24 hours in air (in vitro), the percentage elongations were 2.83, 7.09, 3.10, 4.15, 6.78 and 9.30 for Groups I, II, III, IV, V and VI respectively. The maximum percentage elongation of 9.30 ± 0.63 was seen in the Group VI (open E-chain, OrthoOrganizers). The minimum percentage elongation of 2.83 ± 0.44 was seen in the Group I (closed E-chain, Alastik). The range of elongation for the samples (all groups) after 24 hours in air was from 2.10 to 10.05. This indicated that OrthoOrganizers open E–chain is elongated the most after 24 hours in air and Alastik closed E-chain the least. This is in accordance with the findings of Kuster et al [19], who demonstrated that 50% to 75% loss of initial force occurs in these products over 3 to 4 weeks, with the most of the loss within the first hour.

   On comparison of the percentage elongation within the brands it was seen that the open E-chains showed more percentage elongation than the closed types in both Alastik and OrthoOrganizers. But in the GAC group the closed E-chain showed more elongation when compared to the closed type of the same brand (7.09 closed vs. 6.78 open). This indicated that on the whole open E-chains are more susceptible to permanent elongation following plastic prestrain.

   Comparing between the different brands for open E-chains it was seen that Alastik company E-chains showed the least elongation with 4.15 ± 0.27 and the OrthoOrganizers the most with 9.30 ± 0.63. Among the closed E-chains the maximum elongation (7.09 ± 0.34) was in the GAC group and the least (2.83 ± 0.44) in the Alastik group. This indicated that Alastik E-chains showed the least permanent deformation following 50% elongation after 24 hours in the air.

   This is in agreement with the findings of Andreasen and Bishara [5,6] who compared the percentage elongation between latex elastics and Unitek C-1 Alastik modules in a simulated interarch space closure and interarch forces and reported that Alastiks suffered a 74% loss of force degradation after 24 hours where as latex elastics lost 42% only. Hershey et al [7] in contrast to Andreasen and Bishara [5] found 50% force loss after the first day, with 40% of the original force remaining after 4 weeks for the Alastik E-chains. Brooks et al [6] claimed that 50% of the force degradation can be reduced by a combination of pre stretching and heat application from their study.

   All the above differences in percentage elongation were statistically significant (p<0.001) except for that between Groups I and III (closed E-chains of the Alastik and the OrthoOrganizers) and that between Groups II and V (GAC closed and GAC open).

2. Comparison of the percentage elongation between the groups following 50% elongation for 24 hours intraoral.

   After 50% elongation for 24 hours intraorally (in vivo), the percentage elongations were 52.56, 30.64, 36.52, 16.77, 20.20 and 15.95 for Groups I, II, III, IV, V and VI respectively. The maximum percentage elongation of 52.56 ± 0.52 was seen in Group I (closed E-chain, Alastik). The minimum percentage elongation of 15.95 ± 0.34 was seen in Group VI (open E-chain, OrthoOrganizers). The range of elongation for the samples (all groups) after 24 hours intraorally was from 15.47 to 53.21. This indicated that OrthoOrganizers open E–chain is elongated the least after 24 hours intraorally and Alastik closed E-chain the most. This is in agreement with the findings of Kuster et al [19] that E-chains stored in air were extended to 82% and 115% and retained higher initial force when compared with E-chains placed in vivo at 100% extension and concluded that initial extension of E-chains from its original length differs between the products to provide an optimal force level.

   On comparison of the percentage elongation within the brands it was seen that the closed E-chains showed more percentage elongation than the open types in the groups. This indicated that on the whole closed E-chains are more susceptible than

   open E-chains to permanent elongation after 24 hours in the oral cavity. Kovatch et al [9] suggested that clinically these modules should be stretched slowly to position. They demonstrated that the load-extension curves of E-chains were found to be sensitive to both the degree and the rate of extension. When halting at a constant extension, the exponential load decay was also dependent on the initial deformation rate.

   Comparing between the different brands for open E-chains it was seen that OrthoOrganizers company’s E-chains showed the least elongation with 15.95 ± 0.34 and the GAC brand the most with 20.20 ± 0.29. Among the closed E-chains the maximum elongation (52.56 ± 0.52) was in the Alastik group and the least (30.64 ± 0.46) in the GAC group. This indicated that GAC closed E-chains showed the least permanent deformation following 50% elongation after 24 hours intraorally. This agrees with the study by Renick et al [46] who demonstrated that there may be significant differences in clinical force-degradation behavior between the products of different brands.

   All the above differences in percentage elongation were statistically significant (p<0.001) except for that between Groups I and III (closed E-chains of the Alastik and the OrthoOrganizers) and that between Groups II and V (GAC closed and GAC open).

3. Comparison of the percentage elongation between the groups following 50% elongation for three weeks intraoral.

   After 50% elongation for three weeks intraorally (in vivo) the percentage elongations were 65.74, 40.68, 46.15, 29.82, 29.22 and 24.19 for Groups I, II, III, IV, V and VI respectively. The maximum percentage elongation of 65.74 ± 0.43 was seen in the Group I (closed E-chain, Alastik). The minimum percentage elongation of 24.19 ± 0.24 was seen in the Group VI (open E-chain, OrthoOrganizers). The range of elongation for the samples (all groups) after three weeks intraorally was from 23.89 to 66.22. This indicated that OrthoOrganizers, open E–chain was elongated the least after three weeks intraorally and Alastik closed E-chain the most.

   On comparison of the percentage elongation within the brands, it was seen that the closed E-chains showed more percentage elongation than the open types in all the groups. This indicated that on the whole closed E-chains are more susceptible than open E-chains to permanent elongation after three weeks in the oral cavity. This is in agreement with the findings of Ash et al [11] who showed that the force decay in vivo is significantly greater than in air.

   Comparing between the different brands for open E-chains, it was seen that the OrthoOrganizers company’s E-chains showed the least elongation with 24.19 ± 0.24 and the Alastik brand the most with 29.82 ± 0.37. Among the closed E-chains the maximum elongation (65.74 ± 0.43) was in the Alastik group and the least (40.68 ± 0.36) in the GAC group. This indicated that the GAC closed E-chains showed the

   least permanent deformation following 50% elongation after three weeks intraorally. This is in agreement with the findings of Eliades T. et al [47] who demonstrated that E-chains showed the most degradation characteristics after 3 weeks in the oral cavity. They found that the structural changes identified were the same in all the products tested and may indicate degradation mechanisms that could potentially hold intraorally.

   All the above differences in percentage elongation were statistically significant (p<0.001) except for that between Groups I and III (closed E-chains of the Alastik and the OrthoOrganizers) and that between Groups II and V (GAC closed and GAC open).

   So it can be concluded that:

   1. Open E-chains have higher percentage elongation compared to closed E-chains in vitro but closed E-chains have higher percentage elongation compared to open E-chains after 24 hours and three weeks intraorally.
   2. While OrthoOrganizers brand of E-chain showed more percentage elongation after 24 hours in vitro prestretching, the percentage elongation after 24 hours and three weeks intraorally was most in the Alastik brand. Hence Alastik brand can be said to be the most affected by the intra oral conditions on its force decay characteristics.
   3. Among the different test conditions, the maximum percentage elongation was seen after 3 weeks intraoral stretching, followed by 24 hours intraoral and finally the least by the E-chains stretched 24 hours in air.

II. Comparison of the tensile strengths between the groups.

1. Comparison of the tensile strengths between the groups when received

   The tensile strengths for the E-chains were 23.45, 22.39, 20.38, 26.06, 25.02 and 21.24 for Groups I, II, III, IV, V and VI respectively. The Group IV showed the maximum tensile strength, 26.06 N and the Group III the least tensile strength 20.38 N after 50% elongation in air for 24 hours. This indicates that open E-chains of the Alastik brand have the most tensile strength and the closed E-chain of the OrthoOrganizers the least before any prestretching. This agrees with the findings of Kim et al [48] who demonstrated that the effects of prestretching on force decay of elastomeric chains were noted mainly in the first hour. Thus according to them the clinical value of prestretching a synthetic elastomeric chain is questionable.

2. Comparison of the tensile strengths between the groups after 24 hours in air (in vitro)

   The tensile strengths for the E-chains were 23.81, 23.12, 19.40, 25.81, 24.82 and 18.71 for Groups I, II, III, IV, V and VI respectively. The Group IV showed the maximum tensile strength, 25.81 N and the Group III the least tensile strength 19.40 N after 50% elongation in air for 24 hours. This indicates that open E-chains of the Alastik brand has the most tensile strength and the closed E-chain of the OrthoOrganizers the least following prestretching for 24 hours in air. This is in accordance with the findings of Kovatch et al [9] who demonstrated that open E-chains have a very high tensile strength under clinical conditions. This also agrees with the findings of Nightingale et al [44] who showed that initial forces resulted in high force decay.

3. Comparison of the tensile strengths between the groups after 24 hours intraorally (in vivo)

   The tensile strengths for the E-chains were 17.08, 18.34, 18.16, 18.53, 17.80 and 18.46 for Groups I, II, III, IV, V and VI respectively. Group IV showed the maximum tensile strength, 18.53 N and Group III the least tensile strength 17.08 N after 50% elongation in air for 24 hours. This indicates that open E-chains of the Alastik brand has the most tensile strength and the closed E-chain of the OrthoOrganizers the least following stretching for 24 hours intraorally. This is in accordance with the study by Gioka et al [50] where they assessed the force relaxation of latex elastics occurring within 24 hours of extension and found that latex elastics show force relaxation in the order of 25%, which consists of an initial high slope component and a latent part of decreased rate.

   They proposed that most of the relaxation occurs within the first 3–5 hours after extension, regardless of the size, the manufacturer, or the force level of the elastic.

4. Comparison of the tensile strengths between the groups after three weeks intraorally (in vivo)

   The tensile strengths for the E-chains were 21.95, 17.15, 19.58, 25.85, 24.75 and 18.47 for Groups I, II, III, IV, V and VI respectively. Group IV showed the maximum tensile strength, 25.85 N and Group II the least tensile strength 17.15 N after 50% elongation in air for 24 hours. This indicates that open E-chains of the Alastik brand has the most tensile strength and the closed E-chain of the OrthoOrganizers the least following stretching for three weeks intraorally.

   Statistical analyses showed a statistically significant difference (p<0.001) between the tensile strengths of both open and closed E-chains of the three bands, in all the test conditions. This is in agreement with the findings of Ash et al [11] who showed that after 3 weeks the Alastiks, at the given level of initial activation; lose 20% more force intraorally than in the air. In contrast to this are the findings of Eliades T. et al [47] who found no correlation between specimen treatment and the tensile strength of elastomers.

   So it can be concluded that all the different groups of E-chains exhibited high tensile strengths in the air, which gradually reduced after subsequent stretching in the oral cavity for 24 hours and three weeks.

Conclusions

The conclusions that can be drawn from the present study are as follows:

1. Open E-chains have higher percentage elongation compared to closed E-chains in vitro but closed E-chains have higher percentage elongation compared to open E-chains after 24 hours and three weeks intraorally.
2. While OrthoOrganizers brand of E-chain showed more percentage elongation after 24 hours in vitro pre stretching, the percentage elongation after 24 hours and three weeks intraorally was most in the Alastik brand. Hence Alastik brand can be said to be the most affected by the intra oral conditions on its force decay characteristics.
3. Among the different test conditions, the maximum percentage elongation was seen after 3 weeks intraoral stretching, followed by 24 hours intraoral and finally the least by the E-chains stretched 24 hours in air.
4. The tensile strengths of the E-chains of all the groups were reduced following stretching in the oral environment. The tensile strength was most in the in vitro conditions followed by 24 hours in the oral cavity and finally the least after three weeks stretching intraorally.

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Contributed by:

Dr.Rajkumar S Alle, MDS DNB

Dr.Pankaj Dixit, MDS

Tags: elastomeric chains