The effect of static and dynamic testing on orthodontic latex and non latex elastics

Orthodontic Waves
ISSN: 1344-0241 (Print) 1878-1837 (Online) Journal homepage: https://www.tandfonline.com/loi/todw20
The effect of static and dynamic testing on
orthodontic latex and non-latex elastics
Ali S. Aljhani & Abdullah M. Aldrees
To cite this article: Ali S. Aljhani & Abdullah M. Aldrees (2010) The effect of static and dynamic
testing on orthodontic latex and non-latex elastics, Orthodontic Waves, 69:3, 117-122, DOI:
10.1016/j.odw.2010.04.003
To link to this article: https://doi.org/10.1016/j.odw.2010.04.003
Published online: 28 Nov 2019.
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orthodontic waves 69 (2010) 117–122
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Research paper
The effect of static and dynamic testing on orthodontic latex
and non-latex elastics
Ali S. Aljhani a, Abdullah M. Aldrees b,*
a
b
King Abdullah International Medical Research Centre, King Saud University for Health Sciences, Riyadh, Saudi Arabia
Department of Pediatric Dentistry and Orthodontics, College of Dentistry, King Saud University, P.O. Box 60169, Riyadh 11545, Saudi Arabia
article info
abstract
Article history:
The purpose of this study was to test the force decay properties of different kinds of
Received 1 September 2009
orthodontic elastics after subjecting them to static and cyclic testing. Latex and non-latex
Received in revised form
elastics obtained from GAC, American Orthodontics and Ortho-Organizers were used in a
12 April 2010
sample size of 10 elastics per group. Static testing involved stretching the elastics three
Accepted 24 April 2010
times the internal diameter, while in cyclic testing, the elastics were stretched up to 50 mm
Published on line 26 May 2010
to simulate maximum mouth opening. Elastic forces generated were measured using the
Instron testing machine and recorded in grams. Elastics on average lose 10% and 12% as a
Keywords:
result of static test and 30% and 35% as result of cyclic test for latex and non-latex brands
Elastics
respectively, and most of the force loss occurs during the first half hour and after the first 10
Latex
cycles. This difference in force loss between latex and non-latex elastics could be due to the
Non-Latex
different structure and composition of the polymer involved. There are no significant
differences between different groups of latex elastics in terms of force loss or even between
the different groups of the non-latex elastics under static testing, however, under cycling
testing differences between the groups were detected. Forces generated by the elastics are
different from the manufacturers’ labeled forces.
# 2010 Elsevier Ltd and the Japanese Orthodontic Society. All rights reserved.
1.
Introduction
Elastics have been used in orthodontics to move teeth for more
than a century [1,2]. Orthodontic elastic are inexpensive,
relatively hygienic and are easily applied. Elastics in the oral
cavity absorb water and saliva, permanently stain and
undergo both fatigue and creep [3]. They lose forces rapidly
due to stress relaxation which leads to gradual loss of their
effectiveness. The loss of force delivery and degradation of
orthodontic elastics are major defects that affect clinical
choice.
Andreason and Bishara found that in a period of 24 h, latex
elastics lost 42% of initial force, and the greatest percent of
force decay occurred during the first hour [4,5]. Kovatch and
Keller found that rapid stretching produced greater initial
forces compared to slow stretching [6], while Ash and Nikolai
concluded that more force loss may occur due to the effect of
mastication, oral hygiene, salivary enzymes and temperature
variation within the mouth (in vivo) [7]. A similar pattern of
force degradation in the wet environment was observed by
Wang et al. in vivo [8]. Bertl et al. found a significant force loss
generated by elastics within the first half an hour, which
continued to decrease up to 8 h reading [9]. To achieve the
* Corresponding author. Tel.: +966 1 4676057; fax: +966 1 4679017.
E-mail address: [email protected] (A.M. Aldrees).
1344-0241/$ – see front matter # 2010 Elsevier Ltd and the Japanese Orthodontic Society. All rights reserved.
doi:10.1016/j.odw.2010.04.003
118
orthodontic waves 69 (2010) 117–122
reported force of the latex elastics, an extension of 2.7 and five
times the original length was found to be required by Gioka et
al. [10].
Orthodontic elastics are not subjected to static stresses
only when they are in the patient’s mouth. They also undergo
repeated stretching as the patient talks, eats, and yawns.
Effects from these movements had not been investigated until
1993 when Liu et al. studied this elastic property. The authors
found that cyclic stress will significantly lead to force loss.
However, most of the effect occurs in the first 200 cycles [11].
Due to the recent increase in awareness of the health risks
of latex products [12], several companies have started
marketing synthetic non-latex elastics and are offered as an
alternative to latex elastics. Russell et al. compared the
mechanical properties of latex and non-latex orthodontic
elastics [13]. They reported that after 24 h, the relationship
between the different groups was significantly different [13].
In their study, Kersey et al. reported that the latex elastics lost
25% of their force after 24 h while the non-latex elastics lost
nearly 50% of their force over 24 h, and they concluded that
latex elastics are superior to non-latex elastics [14]. Kersey
et al. also compared the effect of cyclic fatigue on force decay
properties of four different brands of non-latex elastics [15].
The authors concluded that all elastics in the study are
comparable in terms of their mechanical properties. The rapid
loss of force of the orthodontic elastics due to static and cyclic
stress relaxation, resulting in a gradual loss of effectiveness
was a major drawback. Bertoncini et al. compared latex to
non-latex elastics produced by a single manufacturer and they
confirmed previous studies that reported that latex elastics
undergo less loss of force than the non-latex elastics [16]. The
same researchers has also investigated these two groups
under cyclic testing, and concluded that the latex-free elastics
must be replaced more frequently than conventional latex
elastics [17].
The extensive body of literature regarding this property
has been difficult to evaluate because of the different
investigation methods and the different types of elastics
available. The mechanical properties of the elastics varied
considerably with the type of material and the manufacturer.
Clinicians using orthodontic elastics need to know the forces
applied to teeth at certain extensions and how these forces
decline over time. The literature lacks a comparison between
different types of latex and non-latex elastics supplied from
different manufacturers with respect to static and cyclic
stress. This study aimed to compare the force decay pattern of
different latex and non-latex elastics from different manufacturers subjected to static and cyclic stresses. Also, another
objective was to determine the differences from a single
supplier and between different suppliers. Furthermore, to
determine the accuracy of initial forces generated by different
latex and non-latex elastics as stated on by the suppliers and
determine the variation in force produced in each group
(package).
2.
Materials and methods
Orthodontic latex and non-latex elastics from three manufacturers were tested: GAC International (Islanda, NY) 6 mm
(1/4 in.) 4 oz (113 g), American Orthodontics (Sheboygan, Wis.)
6 mm (1/4 in.) 4 1/2 oz (127 g), and Ortho-Organizers (San
Marcos, Calif.) 6.4 mm (1/4 in.) 4 1/2 oz (127 g). All elastics were
similar in terms of structure. Samples obtained were within
their expiration dates and stored as recommended in sealed
bags in a dark environment at room temperature.
2.1.
Testing apparatus
An Instron testing machine (Model 4202, Instron Corporation,
Canton, Mass.) with load cell capacity of 100 Newton was used.
Elastics were stretched on the testing machine using two
hooks made of 1.3 mm diameter stainless steel (Fig. 1a). The
loads were recorded in Newton and then converted to grams.
The load cell was calibrated before each data collection to
confirm the validity of reading.
Fig. 1 – (a) Elastic stretched three times the internal diameter with the hooks attached to the Instron testing machine. (b)
Elastic stretched in an environmental chamber.
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orthodontic waves 69 (2010) 117–122
Table 1 – Comparison of initial forces and force loss over time for latex and non-latex elastics between manufacturers
(static test). Data are presented as means W standard deviation (range) in grams.
Time (h)
GAC International
American Orthodontics
Ortho-Organizers
Initial
Latex
Non-latex
104.2* 3.6 (100–110)
139** 6.9 (112–129)
117.5* 5.1 (110–126)
120.5 5.6 (124–148)
109.4 8.5 (101–126)
137.6*** 8.9 (100–149)
0.5
Latex
Non-latex
95.9 3.3 (90–101)
124.9 7.3 (115–135)
109.6 5.2 (103–120)
110.53 5.9 (104–120)
102.6 7.4 (95–115)
125.6 9.7 (112–137)
1.0
Latex
Non-latex
94.8 3.0 (90–99)
122.8 7.0 (113–132)
97.9 5.0 (110–117)
107.2 5.8 (101–116)
100.5 7.6 (93–113)
123.2 9.2 (110–134)
1.5
Latex
Non-latex
93.9 2.9 (88–97)
121.7 7.1 (112–131)
106.6 5.0 (101–115)
104.9 5.6 (101–11)
99.15 7.7 (91–111)
120.6 9.0 (110–134)
2.0
Latex
Non-latex
93.4 2.9 (87–97)
121.1 6.7 (112–131)
107.0 5.0 (101–116)
103.6 5.3 (96–112)
98.14 7.6 (90–110)
119.5 7.7 (107–130)
*
Significantly different from manufacturer’s published value at p < 0.05.
GAC non-latex elastics are significantly different from GAC latex elastics at p < 0.05.
***
Ortho Organizer non-latex elastics is significantly different from Ortho Organizer latex elastics at p < 0.05.
**
2.2.
Study design
A sample size of 10 elastics per group was used. The static test
contained six groups from three different manufacturers (3
latex and 3 non-latex groups). Elastics were stretched by the
use of two hooks from a resting position to three times the
internal diameter. The lower hook was attached to a fixture,
which was secured to the base of the Instron machine. The
upper hook is the movable part and it was attached to the load
cell which moved at crosshead speed of one cycle per minute.
The cross head speed was 36 mm per minute (1 cycle/min).
Forces generated were recorded immediately at the end of the
following hold time: 0.5, 1, 1.5, and 2 h.
Another six groups were used for the cyclic tests. In the
cyclic tests the elastics were stretched from three times the
internal diameter to 50 mm and returned back to the previous
length which is considered as one cycle. The speed was 2
cycles per minute, a good representation of the average cycle
speed in the oral cavity. Forces generated were recorded at
zero cycles and after 10, 25, 50, 75, and 100 cycles. At the end of
each experiment, elastics were stretched until they broke and
the breaking force was recorded. Elastics were also weighed
before the beginning of each experiment using an electronic
scale (Mettler Toledo, Switzerland).
All the above tests were made in a dry environment at room
temperature. These tests have been repeated at 37 8C in dry
condition in an environmental chamber (incubator) (Fig. 1b).
The same tests have been performed after the elastics were
soaked in water for 24 h to test the effect of water sorption. No
differences have been noticed between the different environmental conditions on either absolute force values or the force
decay pattern.
2.3.
for type-I error, the Scheffe method was used. Calculations
were performed using the computer program Statistical
Package for the Social Sciences (SPSS Inc., Chicago, IL).
3.
Results
The initial force levels for the different types of elastics are
mentioned in Table 1. GAC and American Orthodontics latex
elastics showed significantly less force compared to the
manufacturer’s value ( p < 0.05). GAC and Ortho-Organizers
latex and non-latex elastics were also significantly different.
Furthermore, the range of forces produced by these elastics
was large (Table 1). The weight of these elastics also varied,
however, in smaller range than the force produces. There was
a significant difference in the weight between GAC latex
(21.3 1.7 g) and non-latex elastics (42.5 3.8 g).
Latex and non-latex elastics lose force over time (Table 1).
No statistical differences were noticed within the groups from
the same materials ( p > 0.05) (intra group; latex brands or non-
Statistical analysis
The difference between groups was first analyzed by ANOVA.
If a statistically significant difference was observed, the
difference between the two groups was analyzed by post
hoc multiple comparisons to determine significant differences
between the elastics and different testing methods. To control
Fig. 2 – Force decay pattern of static test: mean percent
remaining of initial force loss over time for the latex
elastics.
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orthodontic waves 69 (2010) 117–122
Fig. 3 – Force decay pattern of static test: mean percent
remaining of initial force loss over time for the non-latex
elastics.
latex brands). However, the non-latex groups lose force
significantly higher than the latex groups. The percentage of
force loss over time is almost the same for the different latex
elastics. The decay pattern starts by losing the big amount of
force at the first half hour. The elastics continue to lose forces
until 2 h and then a stable curve is created (Fig. 2). The nonlatex elastics mimic the same behavior, however, the amount
of force loss during the first half hour is significantly more
(Fig. 3).
The loss of force over number of cycles for both latex
elastics and non-latex elastics is displayed in Table 2. Elastics
lose more force under the cyclic test. Most of this force loss
occurs in the first 10 cycles (Fig. 4). The different brands of the
latex elastics act the same. The non-latex elastics have the
same pattern with more force loss occurring in the first 10
cycles (Fig. 5). The latex elastics are significantly superior in
retaining forces than the non-latex ones. In the latex elastics
groups, Ortho-Organizers is significantly different than the
other latex groups ( p < 0.05). In the non-latex groups, GAC
elastics are statistically different from the others ( p < 0.05).
Similar to the results of the static part of the study, statistical
analysis showed that there were significant differences
Fig. 4 – Force decay pattern of cyclic test: mean percent
remaining of initial force loss over number of cycles for the
latex elastics.
between any latex elastic group and non-latex elastic group
regardless of the manufacturer at p < 0.05.
Fig. 6 shows the breaking forces of the different types of
orthodontic elastics. GAC latex elastics’ breaking forces are
considered statistically higher than the other groups.
4.
Discussion
The environments in which the elastics have been tested in
the literature vary considerably. Latex and non-latex orthodontic elastics have been tested in a dry environment [4,5,17],
wet environment at 37 8C [18,19], in artificial saliva [16,19,20],
in saline [9,17], and in vivo [7,8]. Most authors reported no
difference in the behavior of elastics in different environments. However, few mentioned that greater force relaxation
is associated with a wet environment but this is after a
minimum of 6 h up to three weeks of testing [7–10,19,21–23].
This study examined the effect of one day of exposure to oral
environment. The testing conditions in this study were
selected after reviewing the results obtained from three
different environments which showed that there was no
statistical difference among them. For this reason, the
Table 2 – Initial force loss over number of cycles for latex and non-latex elastics (cyclic test). Data are presented as
means W standard deviation (range) in grams.
Cycles
GAC International
American Orthodontics
Ortho-Organizers
10
Latex
Non-latex
86.7 4.6 (81–94)
112.2 3.7 (104–116)
97.3 6.1 (87–105)
96.1 6.8 (85–109)
87.5 7.4 (79–102)
109.8 6.1 (98–117)
25
Latex
Non-latex
81.4 2.9 (77–85)
105.0 2.3 (100–107)
91.5 5.9 (83–102)
87.3 6.6 (74–99)
80.9 6.7 (75–94)
102.4 5.1 (98–117)
50
Latex
Non-latex
78.5 3.2 (73–82)
102.5 2.5 (97–106)
88.3 5.5 (80–79)
81.3 5.8 (69–91)
76.3 5.9 (70–87)
93.8 5.1 (86–104)
75
Latex
Non-latex
77.1 2.9 (73–82)
101.2 3.4 (93–105)
86.7 5.6 (79–96)
77.1 4.8 (67–85)
75.7 6.0 (70–87)
91.1 3.9 (84–97)
100
Latex
Non-latex
76.0 2.7 (72–81)
100.3 3.1 (93–104)
85.1 5.6 (78–94)
75.0 4.3 (67–82)
75.4 6.0 (70–86)
90.8 3.9 (84–96)
orthodontic waves 69 (2010) 117–122
Fig. 5 – Force decay pattern of cyclic test: mean percent
remaining of initial force loss over number of cycles for the
non-latex elastics.
Fig. 6 – The mean breaking force of the orthodontic elastics
tested.
selected testing condition is considered representative of the
clinical use of the orthodontic elastics and the test results of
this study are expected to provide guidelines for choosing
orthodontic elastics for clinical use. In the literature, there is a
wide range of extensions ranging from 20 to 40 mm [4–
6,18,21,24–26]. In this study, the extension of the elastics in the
static experiment was three times the internal diameter in
order to simulate the clinical use of elastics. This also
corresponds well to Gioka et al. study which found that in 6
out of 7 types of elastics tested, an extension of 2.6–3 times the
original diameter is enough to achieve the reported force [10].
In the cyclic experiment, the extension of the elastics from
three times the internal diameter to 50 mm was chosen to mimic
normal maximum mouth opening [27]. Although no statistical
differences were noticed, the decision was made to select a
speed of 2 cycles/minute to reflect a balance between chewing
(higher frequency) and other times such as sleeping (lower
frequency). One of the advantages of this testing protocol is that
the elastics are tested and the results are recorded immediately
without the need of transferring the elastics to another machine
for reading which will affect the actual result [7,9].
The elastics tested produced a wide range of forces. The
trend observed in this study was that GAC and OrthoOrganizers latex elastics initial forces were lower and the
non-latex ones were higher than the manufacturer marketed
121
values. This result was not reported by Kersey et al., however,
American Orthodontics non-latex elastics produced initial
forces that were close to those reported by Kersey et al. [14,15].
One explanation for that is the relationship with the weight of
the elastics, i.e., the heavier the elastics the higher the force.
This was true for all non-latex elastics. They are heavier and
stiffer compared with the latex elastics, and they produce
higher forces than their latex counterpart. The arguments
about how the forces produced in this study differ from those
labeled by the manufacturers are not similar until the testing
protocol used by those companies is known.
One interesting observation is the wide range of forces
produced by different elastics from the same bag. This study is
in general agreement with other studies that reported
variability between samples in the same batch [6,19,21,24].
However, the variation did not coincide with the type but
rather with the manufacturer. GAC were found to be more
consistent with respect to labeled force whether it is latex or
non-latex followed by American Orthodontic elastics and then
Ortho Organizer elastics. Clinically, there is no important
significance to this observation since the variation is still
within acceptable limits and force production is clinically
acceptable for the desired tooth movement because it falls
within the reported range of optimal force for retraction (100–
250 g) suggested by Storey and Smith [28] and by Reitan [29].
The percentage of the initial force loss of elasticity was
compared rather than the actual values due to the wide
variability of force production by the elastics. Force degradation for the different brands of elastics in this study exhibits an
almost identical pattern. In the static part, the three latex
groups lost most of their force in the first half hour and
maintained an almost steady curve afterwards. This steady
curve will continue for up to 12 h. The main difference
between the latex and non-latex elastics at this point is that
the non-latex material continues to lose strength slowly after
the first half hour while the force from the latex material
remains steady after the initial drop. The pattern of force
decay observed is very much in agreement with the pattern
observed in previous studies [6,18,24]. Even as a percent of
force loss it is similar to Russell et al., Kersey et al., Gioka et al.,
and Wang et al. [8,10,13–15]. Bertoncini et al. were able to
detect significant difference between latex and non-latex
elastics only after 24 h of stretching, and this may be due to the
interruptions in their force measurement protocol [16].
The force decay pattern observed in the cyclic test is the
same as that in the static part but with respect to number of
cycles instead of elapsed time. The first 50 cycles are the most
significant ones, with the largest amount of force loss. After
that, the curve starts flattening indicating little force loss. The
difference between the elastics was in the first 50 cycles where
the non-latex elastics lost more force than the latex ones
ending their hundred cycles with lower remaining force. This
was in contrast to the findings of Kersey et al. and Gandini
et al. and who reported that the loss of force noted for the nonlatex elastics was significant only after a high number of
opening/closing cycles [14,17]. The current study reported
significant differences between the tested brands in the cyclic
test and this was not indicated by Kersey et al. who reported
that GAC, American Orthodontics and Ortho-Organizers nonlatex elastics behaved similarly under cyclic testing [15].
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orthodontic waves 69 (2010) 117–122
Differences in testing conditions may explain the discrepancy
in the behavior of non-latex elastics in these studies.
Bertran reported that intermaxillary elastics are used
mainly over a distance of 20–40 mm, producing forces of 60–
300 g [30]. In their study, Bales et al. stretched 3/16 in. heavy
elastics a distance of 30–40 mm and the forces produced did
not exceed 318 g [21]. The lowest breaking force recorded in
this study was above 2700 g, and this indicates that the tested
elastics’ breaking forces exceed in magnitude any force
expected from mouth activity
Between all the six groups, the three latex groups are
statistically superior to the non-latex ones which means they
retain more forces. There was also some recovery of forces
generated by the elastics after the cycling testing. This
recovery ranges from 4% to 6% in ten minutes period after
stopping the cyclic stress. Kersey et al. was not able to observe
this phenomenon [14,15]. The difference in force loss between
latex and non-latex elastics could be due to the different
structure and composition of the polymer involved. The nonlatex elastics, synthetic polymer, may rely more on molecular
entanglement for structural integrity rather than the covalent
cross-linking that is present in the natural rubber used in the
latex elastics. These structural differences may lead to the
long term poorer behavior of the non-latex elastics [13].
5.
Conclusions
1. Elastics’ decay pattern starts by losing a big amount of force
at the first half hour, and more force is lost in the first 10
cycles in the cyclic test.
2. Latex elastics are significantly superior in retaining forces
than the non-latex ones, and this may indicate that nonlatex elastics need to be changed more frequently.
3. In the static test, there are no significant differences between
the tested different groups of latex elastics or even between
the different groups of the non-latex elastics, however, under
cycling testing, significant difference were detected.
4. Forces generated by the elastics do not consistently equal
the loads specified by the manufacturers.
references
[1] Angle EH. Treatment of malocclusion of the teeth.
Philadelphia: The S. S. White Dental Manufacturing
Company; 1907.
[2] Case CS. Original use and misuse of the intermaxillary
force, and its relation to occipital and other anchorage
forces in orthodontia. Dental Cosmos 1904;345–62.
[3] Baty DL, Storie DJ, von Fraunhofer JA. Synthetic elastomeric
chains: a literature review. Am J Orthod Dentofacial Orthop
1994;105:536–42.
[4] Andreasen GF, Bishara S. Comparison of alastik chains with
elastics involved with intra-arch molar to molar forces.
Angle Orthod 1970;151–8.
[5] Bishara SE, Andreasen GF. A comparison of time related
forces between plastic alastiks and latex elastics. Angle
Orthod 1970;40:319–28.
[6] Kovatch JS, Lautenschlager EP, Apfel DA, Keller JC. Loadextension-time behavior of orthodontic alastiks. J Dent Res
1976;55:783–6.
[7] Ash JL, Nikolai RJ. Relaxation of orthodontic elastomeric
chains and modules in vitro and in vivo. J Dent Res
1978;57:685–90.
[8] Wang T, Zhou G, Tan X, Dong Y. Evaluation of force
degradation characteristics of orthodontic latex elastics in
vitro and in vivo. Angle Orthod 2007;77:688–93.
[9] Bertl WH, Droschl H. Forces produced by orthodontic
elastics as a function of time and distance extended. Eur J
Orthod 1986;8:198–201.
[10] Gioka C, Zinelis S, Eliades T, Eliades G. Orthodontic latex
elastics: a force relaxation study. Angle Orthod 2006;76:475–9.
[11] Liu CC, Wataha JC, Craig RG. The effect of repeated stretching
on the force decay and compliance of vulcanized cispolyisoprene orthodontic elastics. Dent Mater 1993;9:37–40.
[12] The dental team and latex hypersensitivity. ADA Council
on Scientific Affairs. J Am Dent Assoc 1999;130:257–64.
[13] Russell KA, Milne AD, Khanna RA, Lee JM. In vitro
assessment of the mechanical properties of latex and nonlatex orthodontic elastics. Am J Orthod Dentofacial Orthop
2001;120:36–44.
[14] Kersey ML, Glover KE, Heo G, Raboud D, Major PW. A
comparison of dynamic and static testing of latex and
nonlatex orthodontic elastics. Angle Orthod 2003;73:181–6.
[15] Kersey ML, Glover K, Heo G, Raboud D, Major PW. An in
vitro comparison of 4 brands of nonlatex orthodontic
elastics. Am J Orthod Dentofacial Orthop 2003;123:401–7.
[16] Bertoncini C, Cioni E, Grampi B, Gandini P. In vitro
properties’ changes of latex and non-latex orthodontic
elastics. Prog Orthod 2006;7:76–84.
[17] Gandini P, Gennai R, Bertoncini C, Massironi S.
Experimental evaluation of latex-free orthodontic elastics’
behaviour in dynamics. Prog Orthod 2007;8:88–99.
[18] Barrie WJ, Spence JA. Elastics – their properties and clinical
applications in orthodontic fixed appliance therapy. Br J
Orthod 1974;1:167–71.
[19] De Genova DC, McInnes-Ledoux P, Weinberg R, Shaye R.
Force degradation of orthodontic elastomeric chains – a
product comparison study. Am J Orthod 1985;87:377–84.
[20] Baty DL, Volz JE, von Fraunhofer JA. Force delivery
properties of colored elastomeric modules. Am J Orthod
Dentofacial Orthop 1994;106:40–6.
[21] Bales TR, Chaconas SJ, Caputo AA. Force-extension
characteristics of orthodontic elastics. Am J Orthod
1977;72:296–302.
[22] Newman GV. Biophysical properties of orthodontic rubber.
J N J Sate Dent Soc 1963;35:95–103.
[23] Paulish F. Measuring orthodontic forces. Am J Orthod
1939;25:817–47.
[24] Hershey HG, Reynolds WG. The plastic module as an
orthodontic tooth-moving mechanism. Am J Orthod
1975;67:554–62.
[25] Ware AL. A survey of elastics for control of tooth movement.
1. General properties. Aust Orthod J 1970;2:99–108.
[26] Wong AK. Orthodontic elastic materials. Angle Orthod
1976;46:196–205.
[27] Zawawi KH, Al-Badawi EA, Lobo SL, Melis M, Mehta NR. An
index for the measurement of normal maximum mouth
opening. J Can Dent Assoc 2003;69:737–41.
[28] Storey EE, Smith R. Force in orthodontics and its relation to
tooth movement. Aust J Dent 1952;56:11–8.
[29] Reitan K. Some factors determining the evaluation of forces
in orthodontics. Am J Orthod 1957;43:32–45.
[30] Von Bertran C. Die Krafte der orthodontischen
gummuligatur. Fortschr Orthod 1931;605–9.