The effect of hydrogen ion concentration on the force-degradation rate of orthodontic polyurethane chain elastics James P. Ferriter, DDS,* Charles E. Meyers, Jr. DDS,** and Lewis Lorton, DDS*** Fort Meade, Md.
The effect of pH on the force-degradation rates of seven commercial orthodontic polyurethane chain elastics was evaluated in an in vitro study. The pH values of 4.95 and 7.26 were selected for testing because they represent values close to the reported extremes of plaque and saliva pH. Seven test elastic products were extended to (1) equal distances and (2) equal initial force levels, and the force-degradation rates were recorded over 4 weeks. All the test products yielded a significantly greater force-decay rate in the basic (pH 7.26) solution than in the acidic (pH 4.95) solution over 4 weeks. A hypothesis is presented that the decay rate of orthodontic polyurethane chain elastics is inversely proportional to the oral pH, with a corollary that basic pH levels are most hostile to polyurethane chain elastics. (AMJ ORTHOODENTOFACORTHOP1990;98:404-10.)
P o l y u r e t h a n e chain elastics are commonly used in orthodontics for intra-arch tooth movement. Placement and removal of chain elastics requires little chair time for the clinician and minimal patient cooperation during application, and the material is relatively compatible with the mucosa. These factors have contributed to a high degree of professional acceptance of chain elastics in orthodontics. A disadvantage of chain elastics is that they deteriorate rapidly in the oral environment and consequently do not produce the continuous forces most effective for tooth movement. ~ Proffitt states that the amount of tooth movement is directly proportional to the pressure (force per unit area of periodontal ligament) applied to a tooth when the pressure is above a minimal threshold and below the optimal force level. The force derived from chain elastics depends on the magnitude of the initial force, the length of time since activation, and the rate of force decay. Therefore, depending on the effects of these three factors, the chain elastic may be applying an ineffective force for some period before the patient returns for the next scheduled visit.
The views of the authors do not purport to reflect the position of the Department of the Army or the Department of Defense. *Major, US Army Dental Corps, Senior Orthodontic Resident, US Army Orthodontic Residency Program. **Colonel, US Army Dental Corps, Director, Orthodontic Residency, US Army Orthodontic Residency Program. ***Colonel, US Army Dental Corps, Chief, Bioengineering Branch, US Army Institute of Dental Research. 811111856
404
The clinician has some control over both the amount of the initial force applied and the time between elastic changes, but the variables infuencing the force-decay rate of chain elastics, as they relate to orthodontic treatment, are not completely understood or appreciated. This study evaluated seven commercial orthodontic chain elastics that had been activated and maintained in constant environments at different pH levels. REVIEW OF LITERATURE The pH of the oral environment that affects orthodontic chain elastics is influenced by the pH of both saliva and dental plaque. (Brawley2 found that the pH of saliva in 3405 cases ranged from 5.6 to 7.6, with a mean of 6.75.) Even when relatively strong solutions of acid and alkali are ingested, the salivary pH quickly reverts to the individual subject's baseline pH. 38 The initial research on orthodontic polyurethane elastics was done by Andreasen and Bishara, 9"t° who reported that the force of the polyurethane elastics decayed over time and that the decay rate increased with hydrolysis. They demonstrated that most of the force loss occurred in the first day, in fact, 55% loss was measured in the first hour, and much less occurred over the remaining 3 weeks. Kovatch and associates H and others lz reported that after the first 5 seconds the decay rate adheres to the following formula: Load (force) = Constant x (time)" The exponent n is a negative number, since the force decreases with the passage of time.
Volume 98 Number 5
Effect of pH on chain elastics
405
The polyurethane elastics have also been shown to decay at different rates in tests with various commercial products, n.~3.~7 The rate of elastic decay during tooth movement increased as the distance the elastic was stretched decreased. This was demonstrated by both Hershey and Reynolds ~s and De Genova and associatesJ 4 In summary, all studies involving polyurethane orthodontic elastics have reported that their forces decay with time. 922 Many of the studies have reported that the decay rate varies with the commercial product tested. IzqTA9
While few studies have compared various commercial products in an oral environment, even fewer studies have been conducted that isolate a single aspect of the oral environment to facilitate analysis of its potential impact on the force-decay rate of the chain elastics. This article reports on a study of one potential influence within the oral environment, pH, exploring its effect on the force-decay rate of several types of commercial orthodontic chain elastics.
EXTENDED LENGTH Fig. 1. Extension distance of typical chain elastic.
MATERIAL AND METHODS
The seven commercial polyurethane chain elastic products selected for this experiment, with their assigned three-letter acronyms in parenthesis, are listed below: 1. "A"-Company Force-A chain elastic, medium, gray (ACO). 2. American Orthodontics Memory Chain, short, gray (AMO). 3. GAC Chainette, gray, narrow (GAC). 4. Ormco Power Chain II, gray, close space (ORM). 5. RMO Energy Chain, narrow, gray (RMO). 6. TP Orthodontics E-Chain, medium, gray (TPO). 7. Unitek AlastiK Gray Spool Chain C-1 (UNI). The ACO, AMO, GAC, RMO, and TPO chain elastics were selected for testing because they were the latest chain elastic products available from each company. Ormco Generation II was not available at the time of the tests, so Power Chain II was used. Unitek AlastiK Spool Chain CI was selected because it has been available for the longest time, and is the most researched and referenced product in the literature. The products tested in this study were matched in size (both the distance between chain links and the size of the links) with Unitek AlastiK Spool Chain C1 as closely as possible for relative uniformity. Three-unit links of gray chain elastic were used for all products tested in this study (Fig. I). The chain elastics were maintained at activated lengths constructed by placing Whaledent International metal Pindex dowel pins into clear acrylic blocks (Great
ACTIVATED
LENGTH
Fig. 2. Measurement for chain extension.
Lakes Orthodontics, Ltd., Tonawanda, N.Y.) (Fig. 2). Pairs of dowel pins were placed in rows separated by distances of either 14.6, 15.0, or 19.3 mm. Extension distances (pin-to-pin circumference) were determined empirically to allow equal initial force values among loops of varying stiffness. Products with lower stiffness values (ACO, AMO, GAC, RMO) were tested at both greater and lesser extensions to allow evaluation at equal activation lengths. During the experiment, all chain elastics were handled in the same manner. No efforts were made to prestretch the elastics before activation or to stretch the elastics initially at an extremely slow rate. These factors were eliminated to reduce the number of variables involved so that the effect of the test variable, pH, could be isolated. The test solutions were made by buffering distilled water with 0.26% sodium phosphate monobasic solution (NaH2PO4HzO) and 2.17% sodium phosphate di-
406
Am. J. Orthod. Dentofac. Orthop. November 1990
Ferriter, Meyers, and Lorton
T a b l e I. F o r c e - r e s i s t a n c e v a l u e s (x _-_ SD) for elastics at 12.2 to 19.3 m m e x t e n s i o n
tested
Week 4 force (gin)
% Initial force
68 ± 4 53 ± 6
118±8 20±1
64±4 ll±I
127 ± II 94 ± 8
67 ± 5 52 ± 5
117 ± li 20±1
62 ± 5 ll±l
70 ± 4 69 ± 6
120 ± 7 87 ± 7
67 ± 4 50 ± 5
i12±5 20±2
63 ± 4 12± i
77 ± 7
134 ± I0
79 ± 8
118 -,- 8
74 ± 7 64 ± 6
127 ± l0 57±5
70 ± 7 31 ± 3
48 ± 4 56 ± 6
100 ± 10 97 ± 8
44 -=- 5 45 ± 4
85 ± 8 63 ± 5
37 ± 3
217 ± 15 198 ± 23
82 ± 7 81 ± 7
198 ± 15 153 --- 28
75 ± 5 63 ± 8
181 ± 20 80 ± 30
68 ± 7 33 ± I1
161 ± 10 159 + 12
65 ± 4 68 ± 5
150 ± il 140 ± 8
61 ± 4 60 ± 4
140 ± 8 87 ± 8
57 ± 4 37 ± 4
N
pH
tested (mm)
force (gm)
force (gm)
force
force (gin)
ACO ACO
29 29
4.95 7.26
15.1 ± 0.1 15.1 ± 0.2
185 ± I0 186 ± 9
132 --- 7 131 ± 10
72 ± 4 70 ± 6
126 ± 9 99 ± 9
AMO AMO
30 30
4.95 7.26
15.0 ± 0.2 15.1 ± 0.2
188 ± 12 179 ± 8
134 __. 14 125 ± 8
71 +-- 9 70 ± 5
GAC GAC
30 30
4.95 7.26
15.0 ± 0.2 15.1 ± 0.2
180 ± 9 174 ± 10
125 ± 7 119 ± 11
RMO RMO
29 29
4.95 7.26
15.0 ± 0.3 15.0 ± 0.3
183 ± 14 186 ± 14
139 ± 9 145 ± 8
ORM ORM
28 28
4.95 7.26
15.0 ± 0.3 15.0 ± 0.3
228 ± 12 213 ± l0
109 ± 10 120 ± 10
TPO TPO
26 26
4.95 7.26
14.6 ± 0.2 14.6 ± 0.2
266 ± 19 245 ± 28
UNI UNI
28 28
4.95 7.26
14.6 ± 0.2 14.6 ± 0.2
246 ± 11 235 ± 12
force
30 ± 3
T a b l e IL F o r c e - r e s i s t a n c e values (x -4- S D ) for elastics at 19.2 to 19.3 m m e x t e n s i o n
p.tested oduc,I N ISo,.,io,,I Length I Init,ol I WeekI I,nitio, I force Week2 I,nitiol pH tested (ram) force (gm) force (gm) force (gm) force
Week 4 [ % Initial force (gm) I force
ACO ACO
30 30
4.95 7.26
19.2 ± 0.2 19.3 --- 0.4
235 ± 11 237 ± 9
168 ± 6 154 ± 15
72 ± 4 65 ~ 7
156 ± 8 132 ± It
67 ± 4 56 ± 4
150±7 43±8
64±3 18±4
AMO AMO
29 29
4.95 7.26
19.3 ± 0.3 19.3 ± 0.3
245 ± 12 228 ± 9
175 ± 12 166 ± I1
72 ± 7 73 ± 6
160 ± 11 134 ± 11
66 ± 6 59 ± 6
149 ± 9 43±6
61 ± 4 19±2
GAC GAC
32 32
4.95 7.26
19.3 ± 0.3 19.3 ± 0.3
243 ± 9 221 ± 10
169 ± 4 149 ± I1
70 ± 4 67 ± 4
156 ± 9 126 ± 10
64 ± 4 57 ± 5
150 ± 9 50 ± 9
62 ± 4 23 ± 4
RMO RMO
31 31
4.95 7.26
19.3 ± 0.3 19.3 ± 0.3
236 ± 7 226 ± 13
179 ± 7 172 ± 10
76 ± 4 76 ± 5
165 _ 9 147 ___ 9
70 - 4 65 ± 5
158 ± 10 65 ± 7
67 - 4 29 --- 3
b a s i c h e p t a h y d r a t e s o l u t i o n (NaHPO4 • 7 H 2 0 ) and add-
daily to d e t e r m i n e p H stability. T h e p H m e t e r w a s cal-
ing 0 . 9 % NaCI. W e titrated I M HC1 a n d I M N a O H until the d e s i r e d p H was r e a c h e d . T h e t w o p H levels
ibrated daily w i t h s t a n d a r d solutions o f p H 4 . 0 and
s e l e c t e d w e r e 4 . 9 5 a n d 7 . 2 6 . T h e p H v a l u e o f 7 . 2 6 was
10.0. T h e acrylic b l o c k s w i t h elastics w e r e p l a c e d into
s e l e c t e d b e c a u s e it is s l i g h t l y b a s i c but w e l l w i t h i n the
the test s o l u t i o n s f o r 1 w e e k to m o n i t o r the p H stability
r a n g e o f n o r m a l saliva and p l a q u e p H . T h e p H value
o f the test s o l u t i o n s b e f o r e the elastic t e s t i n g . T h e solutions v a r i e d r a n d o m l y in p H around the d e s i r e d targets
o f 4 . 9 5 was s e l e c t e d b e c a u s e this level o f t e n c a n b e s e e n clinically in d e n t a l p l a q u e in the p r e s e n c e o f retained starchy f o o d s t u f f s . 46 T h e test s o l u t i o n s w e r e
( 7 . 2 6 __. 0 . 3 7 , 4 . 9 5 +__ 0 . 0 2 2 , x __- S D , n =
15). A f -
ter the stability o f the test solutions w a s c o n f i r m e d , n e w test elastics w e r e a c t i v a t e d on the j i g s and p l a c e d into
m a i n t a i n e d in p o r c e l a i n c o n t a i n e r s w i t h g l a s s c o v e r s to g u a r d against e v a p o r a t i o n and c o n s e q u e n t p H c h a n g e s .
the solutions. T h e test solutions w e r e m o n i t o r e d daily
A B e c k m a n Phi 71 p H m e t e r w i t h a m i c r o e l e c t r o d e ( A l l i e d F i s h e r S c i e n t i f i c , S p r i n g f i e l d , N . J . ) w a s used
for the first w e e k a n d t h e n w e e k l y f o r the n e x t 3 w e e k s for p H stability. T h e y w e r e stored in an i n c u b a t o r that
to m o n i t o r the p H o f the test s o l u t i o n s . T h e test solutions w e r e p r e p a r e d 2 w e e k s in a d v a n c e a n d m o n i t o r e d
h a d b e e n c a l i b r a t e d at 37 ° C for the d u r a t i o n o f the experiment.
Volume 98 Number 5
Effect of pH on chain elastics
AMO m
o 0 h
100
~|
407
ACO ~--v
o--o
pH 4.95 pH 7.26
X~
100
v~v pH4.95
0
pH 7.26
o--o
h
8O
E1
80
°--
~
"Z ~
60
60
o
N
40
40
2
O r-
~ o
20
20 o
L
~
o
Q-
0 Initiol wk 1 wk 2
0 Initiol wk t
wk 2
Figure 3
wk 4
Time
Figure 4
GAC
L,-
I
-6 :.:
I
"~
60 4
g
40
toot\
p.,.gs o__o p. 726 ~
\ ,~ - - - - . v 1 ~
Time
ORM
t0o, , I \
wk 4
pH49
I\ I\\
t.=..
o__o
p . 726
"6
T
"~~-~
o
"E
60
g
40
2
2o
o
0 Initial wk I Figure 5
o
20
I
0
wk 2
wk 4
Initiol wk I
Time
Figure 6
wk 2
wk 4
Time
Figs. 3-6. Force-resistance graphics for four tested elastics.
The elastics were placed in the test jigs and kept submerged in the test solutions at all times during the course of the test except when force measurements were taken. Force measurement readings were taken with a Correx gauge (Haag-Striet, Bern, Switzerland). The intial readings were made by attaching one end of the three-link chain elastic to one of the dowel pin pairs, slipping the end off its pin, and measuring the force required to stretch the elastic to its planned extended length. A 10-second delay between chain extension and force measurement allowed for initial relaxation and stabilization. Subsequent force readings were taken by removing one end of the elastic chain from one of the dowel pins and measuring the force required to extend the elastic chain back to that dowel pin. Again we used a 10-second stabilization period between extension and force readings. Care was taken to maintain the Correx gauge perpendicular to the chain elastics during all measurements.
Force readings were taken at initial activation and then at 1, 2, and 4 weeks of activation. RESULTS
The preactivation sizes of the three-link elastic chain segments of all seven test products, when measured from the internal surface of the most distant aspect of one terminal link to the same location on the other terminal link (Fig. 1), were as follows: Test Product ACO, AMO, GAC, RMO ORM TPO UNI
Distance ( m m ) 9.5 8.5 10.0 8.4
All data of the seven chain elastic test products, activated to a relatively equal distance over the 4-week test period, are presented with means and standard de-
408 Ferriter, Meyers, and Lorton
A m . J . Orthod. D e n t o f a c . Orthop. N o v e m b e r 1990
TPO
RMO I00~
o_o
pH
P
60-
0 n
v--v
o--o
~ 1
pH 4.95 pH 7.26
80 o
_c 60 40
40-
o E
I00
0 i,
?~
-6 "E
~--v
/\
L..
o E
? 20-
P
1
2O
n
13
0
Iniliol wk I Figure 7
wk 2
Inilial wk 1 wk 2
wk 4
Time
Figure 8
wk 4
Time
UNI 100
v--v o - - o
o
pH 4.95 pH 7.26
80 --~ 60 ~ o
40
?
E ~ n
20
o Initial wk 1 wk 2 Figure 9
wk 4
Time
Figs. 7-9. Force-resistance graphics for three tested elastics.
viations in Table I. The data from the test products ACO, AMO, GAC, and RMO, which were extended to 19.3 mm, are presented with means and standard deviations in Table II. The percentage values of the remaining force levels for each chain elastic product, tested at approximately 15 mm in both test solutions, are graphed by individual test product in Figs. 3 to 9. The response of the test products ACO, AMO, GAC, and RMO at two distances of activation are compared in both test solutions and graphed in Figs. 10 to 13. Two-way analysis of variance of the force, by brand and by pH of the solution, demonstrated a significant difference among all products and the solution. The results of each specific analysis are shown in Table III. DISCUSSION The basic test solution (pH 7.26) induced a significantly greater rate of force decay when compared with the acidic solution (pH 4.95). Both solutions were used to test all seven products.
The force-decay graphs show that the elastic decay curves did not adhere to the inverse exponential forcereduction equations suggested by both Kovatch and associates 6 and Brantley and associates.'2 Figs. 3 to 9 show that the force decay rates for six of the seven products (all except ORM) tested in the two test solutions were remarkably similar during the 4-week activation period. After the first week, the decay rates of the chain elastics in the acidic solution slowed significantly, while the force continued to decay at a more rapid rate in the basic solution. The ORM test product responded atypically, in that the force-decay rate was greater in the basic solution than in the acidic solution for the first week. Three of the four test products (ACO, AMO, GAC) that were extended to two distances (15.0 and 19.3 mm) in the test solutions (Figs. 10-13) had significantly different decay rates in the basic solution. The RMO test product had essentially the same force-decay behavior in both test solutions when activated to the two distances. One possible explanation for this difference in
Volume 9 8 Number 5
Effect of pH on chain elastics
ACO
AMO m 0 h
-6
100-
pH pH .... pH .... pH
-
~ , .
80.
4.95 7.26 4.95 7.26
(15 mm) (15 r a m ) (19 ram) (19 ram)
100t\ ta.
",..-,.,
° •
•
-....
/X
_/ 80 1
"N "'~,~
60
~
"'L'-,.
40. 0 r"
-
....
--~ 60-
409
pH 4.95 (15 ram) pHT.Z6(,Sm~) pH 4.95 (19 /'nm) pH 7.26 (19 ram)
....
"~.~.. "::5,,
40
0
-
f-
20
",.::.,
20 I1)
wk 2
Iniliol wk I
Figure 10
0 Initial wk 1 wk 2
wk 4
Figure 11
Time
GAC -.... -
pH 4.95 (15 ram) pH 7.26 (tS r~m) pH 4.95 (19 ram) - - - p H 7.26 (19 ram)
/\
/ "~
"-6 8 0 1 X "E 60
-,:...
-%'-,
40 0 u
Time
RMO
I001", h
wk 4
1oo L O It..
80
O
"~
60
~
40
~". ~ . ",
-.... -
pH 4.95 (I,5 mm) pH 7.26 (15 ram) pH 4.95 (19 turn) 7.26 (,g ram)
0
13.
20
~
20
I3_
0
0 Inilial wk I
wk 2
Figure 12
wk 4
wk 4
Initial wk 1 wk 2
Time
Figure 13
Time
Figs. 10-13. Force-resistance graphics for elastics at two distances.
T a b l e III. Two-way analysis of variance by pH and by type of elastic
so,,,ce Week I (n = 646} Elastic Interaction pH Error Week 2 (n = 646) Elastic Interaction pH Error Week 4 (n = 646) Elastic Interaction pH Error
I
loFl
,eoo'
l
F ratio
I
P
3.218 0.000 0.194 1.995
10 1 I0 624
0.322 0.000 0.019 0.003
100.685 0.081 6.061
0.000 0.776 0.000
2.473 1.282 0.495 1.603
10 1 l0 624
0.247 1.282 0.049 0.003
96.292 499.307 19.265
0.000 0.000 0.000
2.003 23.624 2.760 1.194
10 1 I0 624
0.200 23.624 0.276 0.002
104.631 12342.485 144.207
0.0(30 0.000 0.000
responses is that the polymer chains of the polyurethane chain elastics are less susceptible to environmental influences such as basic pH when they are stretched than when they are more relaxed.
In view of the apparent effect pH can have over force decay rates of polyurethane chain elastics, the results of previous studies that have compared various products under common environments should be
410
Am. J. Orthod. Dentofac. Orthop. November 1990
Ferriter, Meyers, and Lorton
v i e w e d with the understanding that, in a particular individual during a particular period o f time, these chain elastics m a y not respond in the s a m e m a n n e r o r even with the s a m e force d e c a y rate c u r v e , c o m p a r e d with other chain elastic products. T h e r e f o r e , w e r e c o m m e n d that the results o f this study not be used to assert that one product is superior to another. Clinically, it w o u l d s e e m that an oral p H l o w e r than 7 . 2 6 w o u l d retard the f o r c e - d e c a y rate o f the chain elastics. Before this study, we did not e x p e c t to find that decreased p H associated with dental plaque in the presence o f carbohydrates m a y actually d e c r e a s e the f o r c e - d e c a y rate o f the chain elastics and thus potentially e n h a n c e their effectiveness. The clinician does not h a v e an ability to control the p a t i e n t ' s oral envir o n m e n t , but he or she must h a v e k n o w l e d g e o f the individual e l e m e n t s within it that can affect the mechanics plan selected. Oral pH almost certainly has a significant influence on the decay rate o f orthodontic polyurethane chain elastics. Further study o f the effect o f p H at several levels w o u l d be the next logical step in understanding the force d e c a y o f orthodontic chain elastics. W e hypothesize that the d e c a y rate o f polyurethane orthodontic chain elastics is inversely proportional to the pH o f the oral e n v i r o n m e n t , with a corollary that the p H levels a b o v e neutral are m o s t hostile to the polyurethane chain elastics, thus increasing their f o r c e - d e c a y rates. REFERENCES 1. Proffit WR. Contemporary orthodontics. St. Louis: CV Mosby 1986:236-8,260, 432. 2. Brawley RE. Studies of the pH of normal resting saliva: variations with age and sex. J Dent Res 1935;15:55-62. 3. Brobeck JR. Best and Taylor's physiological basis of medical practice. Baltimore: Williams & Wilkins. 1973;2-23. 4. Kleinberg I. Studies on dental plaque. I. The effects of different concentrations of glucose on the pH of plaque in vivo. J Dent Res 1961;40:1087-111. 5. Mormann JE, Mahlmann HR. Oral starch degradation and its influence on acid production in human dental plaque. Caries Res 1981;15:166-75. 6. lensen hiE, Schachtele CF. The acidogenic potential of reference
7. 8.
9.
10.
I 1.
12.
13. 14.
15. 16.
foods and snacks at interproximal sites in the human dentition. J Dent Res 1982;62:889-92. Kleinberg I, Jenkins GW, Denepitiya L, Chatterjee R. Diet and dental plaque. Front Oral Physiol 1981;3:88-107. Bibby BG, Mundorff SA, Zero DT, Almokinder KJ. Oral food clearance and the pit of plaque and saliva. J Am Dent Assoc 1986;112:333-7. Andreasen GF, Bishara SE. Comparison of Alastik chains with elastics involved with intra-arch molar to molar forces. Angle Orthod 1970;40:151-8. Bishara SE, Andreasen GF. A comparison of time related forces between plastic AlastiKs and latex elastics. Angle Orthod 1970;67:554-62. Kovatch JS, Lautenschlager EP, Apfel DA, Keller JC. Loadextension-time behavior of orthodontic elastics. J Dent Res 1976;55:783-6. Brantley WA, Salander S, Myers CL, Winders RV. Effects of prestretching on force degradation characteristics of plastic modules. Angle Orthod 1979;49:37-43. Wong AK. Orthodontic elastic materials. Angle Orthod 1976; 46:196-205. De Genova DC, Mclnnes-Ledous P, Weinberg R, Shaye R. Force degradation of orthodontic elastomeric chains-a product comparison study. AM J ORTIIOD1985;87:377-84. Killiany DM, Duplessis J. Relaxation of elastomeric chains. J Clin Orthod 1985;19:592-3. Kuster R, Ingervall B, Buergin W. Laboratory and intra-oral tests of the degradation of elastic chains. Eur J Orthod
1986;8:202-8. 17. Chang H. Effects of instantaneous prestretching on force degradation characteristics of orthodontic plastic modules. Proc Natl Sci Counc Repub China IBI 1987;11:45-53. 18. Hershey HG, Reynolds WG. The plastic module as an orthodontic tooth-moving mechanism. AM J OR'I]IOD 1975;67:55462. 19. Rock WP, Wilson HJ, Fisher SE. Force reduction of orthodontic elastomeric chains after one month in the mouth. Br J Orthod 1986;13:147-50. 20. Brooks DG, Hershey HG. Effects of heat and time on stretched plastic orthodontic modules lAbstract]. J Dent Res 1976; 55(B):385. 21. Ash JL, Nikolai RJ. Relaxation of orthodontic elastomeric chains and modules in vitro and in vivo. J Dent Res 1978;57:685-91. 22. Young J, Sandfik JL. The influence of preloading on stress relaxation of orthodontic elastic polymers. Angle Orthod 1979; 49:104-9. 23. Sonis AL, Van der Plas E, Gianelly A. A comparison of elastomeric auxiliaries versus elastic thread on premolar extraction site closure: an in vivo study. AM J OR'nqOD 1986;89:73-8.
The effect of pH on the force-degradation rates of seven commercial orthodontic polyurethane chain elastics was evaluated in an in vitro study. The pH values of 4.95 and 7.26 were selected for testing because they represent values close to the reported extremes of plaque and saliva pH. Seven test elastic products were extended to (1) equal distances and (2) equal initial force levels, and the force-degradation rates were recorded over 4 weeks. All the test products yielded a significantly greater force-decay rate in the basic (pH 7.26) solution than in the acidic (pH 4.95) solution over 4 weeks. A hypothesis is presented that the decay rate of orthodontic polyurethane chain elastics is inversely proportional to the oral pH, with a corollary that basic pH levels are most hostile to polyurethane chain elastics. (AMJ ORTHOODENTOFACORTHOP1990;98:404-10.)
P o l y u r e t h a n e chain elastics are commonly used in orthodontics for intra-arch tooth movement. Placement and removal of chain elastics requires little chair time for the clinician and minimal patient cooperation during application, and the material is relatively compatible with the mucosa. These factors have contributed to a high degree of professional acceptance of chain elastics in orthodontics. A disadvantage of chain elastics is that they deteriorate rapidly in the oral environment and consequently do not produce the continuous forces most effective for tooth movement. ~ Proffitt states that the amount of tooth movement is directly proportional to the pressure (force per unit area of periodontal ligament) applied to a tooth when the pressure is above a minimal threshold and below the optimal force level. The force derived from chain elastics depends on the magnitude of the initial force, the length of time since activation, and the rate of force decay. Therefore, depending on the effects of these three factors, the chain elastic may be applying an ineffective force for some period before the patient returns for the next scheduled visit.
The views of the authors do not purport to reflect the position of the Department of the Army or the Department of Defense. *Major, US Army Dental Corps, Senior Orthodontic Resident, US Army Orthodontic Residency Program. **Colonel, US Army Dental Corps, Director, Orthodontic Residency, US Army Orthodontic Residency Program. ***Colonel, US Army Dental Corps, Chief, Bioengineering Branch, US Army Institute of Dental Research. 811111856
404
The clinician has some control over both the amount of the initial force applied and the time between elastic changes, but the variables infuencing the force-decay rate of chain elastics, as they relate to orthodontic treatment, are not completely understood or appreciated. This study evaluated seven commercial orthodontic chain elastics that had been activated and maintained in constant environments at different pH levels. REVIEW OF LITERATURE The pH of the oral environment that affects orthodontic chain elastics is influenced by the pH of both saliva and dental plaque. (Brawley2 found that the pH of saliva in 3405 cases ranged from 5.6 to 7.6, with a mean of 6.75.) Even when relatively strong solutions of acid and alkali are ingested, the salivary pH quickly reverts to the individual subject's baseline pH. 38 The initial research on orthodontic polyurethane elastics was done by Andreasen and Bishara, 9"t° who reported that the force of the polyurethane elastics decayed over time and that the decay rate increased with hydrolysis. They demonstrated that most of the force loss occurred in the first day, in fact, 55% loss was measured in the first hour, and much less occurred over the remaining 3 weeks. Kovatch and associates H and others lz reported that after the first 5 seconds the decay rate adheres to the following formula: Load (force) = Constant x (time)" The exponent n is a negative number, since the force decreases with the passage of time.
Volume 98 Number 5
Effect of pH on chain elastics
405
The polyurethane elastics have also been shown to decay at different rates in tests with various commercial products, n.~3.~7 The rate of elastic decay during tooth movement increased as the distance the elastic was stretched decreased. This was demonstrated by both Hershey and Reynolds ~s and De Genova and associatesJ 4 In summary, all studies involving polyurethane orthodontic elastics have reported that their forces decay with time. 922 Many of the studies have reported that the decay rate varies with the commercial product tested. IzqTA9
While few studies have compared various commercial products in an oral environment, even fewer studies have been conducted that isolate a single aspect of the oral environment to facilitate analysis of its potential impact on the force-decay rate of the chain elastics. This article reports on a study of one potential influence within the oral environment, pH, exploring its effect on the force-decay rate of several types of commercial orthodontic chain elastics.
EXTENDED LENGTH Fig. 1. Extension distance of typical chain elastic.
MATERIAL AND METHODS
The seven commercial polyurethane chain elastic products selected for this experiment, with their assigned three-letter acronyms in parenthesis, are listed below: 1. "A"-Company Force-A chain elastic, medium, gray (ACO). 2. American Orthodontics Memory Chain, short, gray (AMO). 3. GAC Chainette, gray, narrow (GAC). 4. Ormco Power Chain II, gray, close space (ORM). 5. RMO Energy Chain, narrow, gray (RMO). 6. TP Orthodontics E-Chain, medium, gray (TPO). 7. Unitek AlastiK Gray Spool Chain C-1 (UNI). The ACO, AMO, GAC, RMO, and TPO chain elastics were selected for testing because they were the latest chain elastic products available from each company. Ormco Generation II was not available at the time of the tests, so Power Chain II was used. Unitek AlastiK Spool Chain CI was selected because it has been available for the longest time, and is the most researched and referenced product in the literature. The products tested in this study were matched in size (both the distance between chain links and the size of the links) with Unitek AlastiK Spool Chain C1 as closely as possible for relative uniformity. Three-unit links of gray chain elastic were used for all products tested in this study (Fig. I). The chain elastics were maintained at activated lengths constructed by placing Whaledent International metal Pindex dowel pins into clear acrylic blocks (Great
ACTIVATED
LENGTH
Fig. 2. Measurement for chain extension.
Lakes Orthodontics, Ltd., Tonawanda, N.Y.) (Fig. 2). Pairs of dowel pins were placed in rows separated by distances of either 14.6, 15.0, or 19.3 mm. Extension distances (pin-to-pin circumference) were determined empirically to allow equal initial force values among loops of varying stiffness. Products with lower stiffness values (ACO, AMO, GAC, RMO) were tested at both greater and lesser extensions to allow evaluation at equal activation lengths. During the experiment, all chain elastics were handled in the same manner. No efforts were made to prestretch the elastics before activation or to stretch the elastics initially at an extremely slow rate. These factors were eliminated to reduce the number of variables involved so that the effect of the test variable, pH, could be isolated. The test solutions were made by buffering distilled water with 0.26% sodium phosphate monobasic solution (NaH2PO4HzO) and 2.17% sodium phosphate di-
406
Am. J. Orthod. Dentofac. Orthop. November 1990
Ferriter, Meyers, and Lorton
T a b l e I. F o r c e - r e s i s t a n c e v a l u e s (x _-_ SD) for elastics at 12.2 to 19.3 m m e x t e n s i o n
tested
Week 4 force (gin)
% Initial force
68 ± 4 53 ± 6
118±8 20±1
64±4 ll±I
127 ± II 94 ± 8
67 ± 5 52 ± 5
117 ± li 20±1
62 ± 5 ll±l
70 ± 4 69 ± 6
120 ± 7 87 ± 7
67 ± 4 50 ± 5
i12±5 20±2
63 ± 4 12± i
77 ± 7
134 ± I0
79 ± 8
118 -,- 8
74 ± 7 64 ± 6
127 ± l0 57±5
70 ± 7 31 ± 3
48 ± 4 56 ± 6
100 ± 10 97 ± 8
44 -=- 5 45 ± 4
85 ± 8 63 ± 5
37 ± 3
217 ± 15 198 ± 23
82 ± 7 81 ± 7
198 ± 15 153 --- 28
75 ± 5 63 ± 8
181 ± 20 80 ± 30
68 ± 7 33 ± I1
161 ± 10 159 + 12
65 ± 4 68 ± 5
150 ± il 140 ± 8
61 ± 4 60 ± 4
140 ± 8 87 ± 8
57 ± 4 37 ± 4
N
pH
tested (mm)
force (gm)
force (gm)
force
force (gin)
ACO ACO
29 29
4.95 7.26
15.1 ± 0.1 15.1 ± 0.2
185 ± I0 186 ± 9
132 --- 7 131 ± 10
72 ± 4 70 ± 6
126 ± 9 99 ± 9
AMO AMO
30 30
4.95 7.26
15.0 ± 0.2 15.1 ± 0.2
188 ± 12 179 ± 8
134 __. 14 125 ± 8
71 +-- 9 70 ± 5
GAC GAC
30 30
4.95 7.26
15.0 ± 0.2 15.1 ± 0.2
180 ± 9 174 ± 10
125 ± 7 119 ± 11
RMO RMO
29 29
4.95 7.26
15.0 ± 0.3 15.0 ± 0.3
183 ± 14 186 ± 14
139 ± 9 145 ± 8
ORM ORM
28 28
4.95 7.26
15.0 ± 0.3 15.0 ± 0.3
228 ± 12 213 ± l0
109 ± 10 120 ± 10
TPO TPO
26 26
4.95 7.26
14.6 ± 0.2 14.6 ± 0.2
266 ± 19 245 ± 28
UNI UNI
28 28
4.95 7.26
14.6 ± 0.2 14.6 ± 0.2
246 ± 11 235 ± 12
force
30 ± 3
T a b l e IL F o r c e - r e s i s t a n c e values (x -4- S D ) for elastics at 19.2 to 19.3 m m e x t e n s i o n
p.tested oduc,I N ISo,.,io,,I Length I Init,ol I WeekI I,nitio, I force Week2 I,nitiol pH tested (ram) force (gm) force (gm) force (gm) force
Week 4 [ % Initial force (gm) I force
ACO ACO
30 30
4.95 7.26
19.2 ± 0.2 19.3 --- 0.4
235 ± 11 237 ± 9
168 ± 6 154 ± 15
72 ± 4 65 ~ 7
156 ± 8 132 ± It
67 ± 4 56 ± 4
150±7 43±8
64±3 18±4
AMO AMO
29 29
4.95 7.26
19.3 ± 0.3 19.3 ± 0.3
245 ± 12 228 ± 9
175 ± 12 166 ± I1
72 ± 7 73 ± 6
160 ± 11 134 ± 11
66 ± 6 59 ± 6
149 ± 9 43±6
61 ± 4 19±2
GAC GAC
32 32
4.95 7.26
19.3 ± 0.3 19.3 ± 0.3
243 ± 9 221 ± 10
169 ± 4 149 ± I1
70 ± 4 67 ± 4
156 ± 9 126 ± 10
64 ± 4 57 ± 5
150 ± 9 50 ± 9
62 ± 4 23 ± 4
RMO RMO
31 31
4.95 7.26
19.3 ± 0.3 19.3 ± 0.3
236 ± 7 226 ± 13
179 ± 7 172 ± 10
76 ± 4 76 ± 5
165 _ 9 147 ___ 9
70 - 4 65 ± 5
158 ± 10 65 ± 7
67 - 4 29 --- 3
b a s i c h e p t a h y d r a t e s o l u t i o n (NaHPO4 • 7 H 2 0 ) and add-
daily to d e t e r m i n e p H stability. T h e p H m e t e r w a s cal-
ing 0 . 9 % NaCI. W e titrated I M HC1 a n d I M N a O H until the d e s i r e d p H was r e a c h e d . T h e t w o p H levels
ibrated daily w i t h s t a n d a r d solutions o f p H 4 . 0 and
s e l e c t e d w e r e 4 . 9 5 a n d 7 . 2 6 . T h e p H v a l u e o f 7 . 2 6 was
10.0. T h e acrylic b l o c k s w i t h elastics w e r e p l a c e d into
s e l e c t e d b e c a u s e it is s l i g h t l y b a s i c but w e l l w i t h i n the
the test s o l u t i o n s f o r 1 w e e k to m o n i t o r the p H stability
r a n g e o f n o r m a l saliva and p l a q u e p H . T h e p H value
o f the test s o l u t i o n s b e f o r e the elastic t e s t i n g . T h e solutions v a r i e d r a n d o m l y in p H around the d e s i r e d targets
o f 4 . 9 5 was s e l e c t e d b e c a u s e this level o f t e n c a n b e s e e n clinically in d e n t a l p l a q u e in the p r e s e n c e o f retained starchy f o o d s t u f f s . 46 T h e test s o l u t i o n s w e r e
( 7 . 2 6 __. 0 . 3 7 , 4 . 9 5 +__ 0 . 0 2 2 , x __- S D , n =
15). A f -
ter the stability o f the test solutions w a s c o n f i r m e d , n e w test elastics w e r e a c t i v a t e d on the j i g s and p l a c e d into
m a i n t a i n e d in p o r c e l a i n c o n t a i n e r s w i t h g l a s s c o v e r s to g u a r d against e v a p o r a t i o n and c o n s e q u e n t p H c h a n g e s .
the solutions. T h e test solutions w e r e m o n i t o r e d daily
A B e c k m a n Phi 71 p H m e t e r w i t h a m i c r o e l e c t r o d e ( A l l i e d F i s h e r S c i e n t i f i c , S p r i n g f i e l d , N . J . ) w a s used
for the first w e e k a n d t h e n w e e k l y f o r the n e x t 3 w e e k s for p H stability. T h e y w e r e stored in an i n c u b a t o r that
to m o n i t o r the p H o f the test s o l u t i o n s . T h e test solutions w e r e p r e p a r e d 2 w e e k s in a d v a n c e a n d m o n i t o r e d
h a d b e e n c a l i b r a t e d at 37 ° C for the d u r a t i o n o f the experiment.
Volume 98 Number 5
Effect of pH on chain elastics
AMO m
o 0 h
100
~|
407
ACO ~--v
o--o
pH 4.95 pH 7.26
X~
100
v~v pH4.95
0
pH 7.26
o--o
h
8O
E1
80
°--
~
"Z ~
60
60
o
N
40
40
2
O r-
~ o
20
20 o
L
~
o
Q-
0 Initiol wk 1 wk 2
0 Initiol wk t
wk 2
Figure 3
wk 4
Time
Figure 4
GAC
L,-
I
-6 :.:
I
"~
60 4
g
40
toot\
p.,.gs o__o p. 726 ~
\ ,~ - - - - . v 1 ~
Time
ORM
t0o, , I \
wk 4
pH49
I\ I\\
t.=..
o__o
p . 726
"6
T
"~~-~
o
"E
60
g
40
2
2o
o
0 Initial wk I Figure 5
o
20
I
0
wk 2
wk 4
Initiol wk I
Time
Figure 6
wk 2
wk 4
Time
Figs. 3-6. Force-resistance graphics for four tested elastics.
The elastics were placed in the test jigs and kept submerged in the test solutions at all times during the course of the test except when force measurements were taken. Force measurement readings were taken with a Correx gauge (Haag-Striet, Bern, Switzerland). The intial readings were made by attaching one end of the three-link chain elastic to one of the dowel pin pairs, slipping the end off its pin, and measuring the force required to stretch the elastic to its planned extended length. A 10-second delay between chain extension and force measurement allowed for initial relaxation and stabilization. Subsequent force readings were taken by removing one end of the elastic chain from one of the dowel pins and measuring the force required to extend the elastic chain back to that dowel pin. Again we used a 10-second stabilization period between extension and force readings. Care was taken to maintain the Correx gauge perpendicular to the chain elastics during all measurements.
Force readings were taken at initial activation and then at 1, 2, and 4 weeks of activation. RESULTS
The preactivation sizes of the three-link elastic chain segments of all seven test products, when measured from the internal surface of the most distant aspect of one terminal link to the same location on the other terminal link (Fig. 1), were as follows: Test Product ACO, AMO, GAC, RMO ORM TPO UNI
Distance ( m m ) 9.5 8.5 10.0 8.4
All data of the seven chain elastic test products, activated to a relatively equal distance over the 4-week test period, are presented with means and standard de-
408 Ferriter, Meyers, and Lorton
A m . J . Orthod. D e n t o f a c . Orthop. N o v e m b e r 1990
TPO
RMO I00~
o_o
pH
P
60-
0 n
v--v
o--o
~ 1
pH 4.95 pH 7.26
80 o
_c 60 40
40-
o E
I00
0 i,
?~
-6 "E
~--v
/\
L..
o E
? 20-
P
1
2O
n
13
0
Iniliol wk I Figure 7
wk 2
Inilial wk 1 wk 2
wk 4
Time
Figure 8
wk 4
Time
UNI 100
v--v o - - o
o
pH 4.95 pH 7.26
80 --~ 60 ~ o
40
?
E ~ n
20
o Initial wk 1 wk 2 Figure 9
wk 4
Time
Figs. 7-9. Force-resistance graphics for three tested elastics.
viations in Table I. The data from the test products ACO, AMO, GAC, and RMO, which were extended to 19.3 mm, are presented with means and standard deviations in Table II. The percentage values of the remaining force levels for each chain elastic product, tested at approximately 15 mm in both test solutions, are graphed by individual test product in Figs. 3 to 9. The response of the test products ACO, AMO, GAC, and RMO at two distances of activation are compared in both test solutions and graphed in Figs. 10 to 13. Two-way analysis of variance of the force, by brand and by pH of the solution, demonstrated a significant difference among all products and the solution. The results of each specific analysis are shown in Table III. DISCUSSION The basic test solution (pH 7.26) induced a significantly greater rate of force decay when compared with the acidic solution (pH 4.95). Both solutions were used to test all seven products.
The force-decay graphs show that the elastic decay curves did not adhere to the inverse exponential forcereduction equations suggested by both Kovatch and associates 6 and Brantley and associates.'2 Figs. 3 to 9 show that the force decay rates for six of the seven products (all except ORM) tested in the two test solutions were remarkably similar during the 4-week activation period. After the first week, the decay rates of the chain elastics in the acidic solution slowed significantly, while the force continued to decay at a more rapid rate in the basic solution. The ORM test product responded atypically, in that the force-decay rate was greater in the basic solution than in the acidic solution for the first week. Three of the four test products (ACO, AMO, GAC) that were extended to two distances (15.0 and 19.3 mm) in the test solutions (Figs. 10-13) had significantly different decay rates in the basic solution. The RMO test product had essentially the same force-decay behavior in both test solutions when activated to the two distances. One possible explanation for this difference in
Volume 9 8 Number 5
Effect of pH on chain elastics
ACO
AMO m 0 h
-6
100-
pH pH .... pH .... pH
-
~ , .
80.
4.95 7.26 4.95 7.26
(15 mm) (15 r a m ) (19 ram) (19 ram)
100t\ ta.
",..-,.,
° •
•
-....
/X
_/ 80 1
"N "'~,~
60
~
"'L'-,.
40. 0 r"
-
....
--~ 60-
409
pH 4.95 (15 ram) pHT.Z6(,Sm~) pH 4.95 (19 /'nm) pH 7.26 (19 ram)
....
"~.~.. "::5,,
40
0
-
f-
20
",.::.,
20 I1)
wk 2
Iniliol wk I
Figure 10
0 Initial wk 1 wk 2
wk 4
Figure 11
Time
GAC -.... -
pH 4.95 (15 ram) pH 7.26 (tS r~m) pH 4.95 (19 ram) - - - p H 7.26 (19 ram)
/\
/ "~
"-6 8 0 1 X "E 60
-,:...
-%'-,
40 0 u
Time
RMO
I001", h
wk 4
1oo L O It..
80
O
"~
60
~
40
~". ~ . ",
-.... -
pH 4.95 (I,5 mm) pH 7.26 (15 ram) pH 4.95 (19 turn) 7.26 (,g ram)
0
13.
20
~
20
I3_
0
0 Inilial wk I
wk 2
Figure 12
wk 4
wk 4
Initial wk 1 wk 2
Time
Figure 13
Time
Figs. 10-13. Force-resistance graphics for elastics at two distances.
T a b l e III. Two-way analysis of variance by pH and by type of elastic
so,,,ce Week I (n = 646} Elastic Interaction pH Error Week 2 (n = 646) Elastic Interaction pH Error Week 4 (n = 646) Elastic Interaction pH Error
I
loFl
,eoo'
l
F ratio
I
P
3.218 0.000 0.194 1.995
10 1 I0 624
0.322 0.000 0.019 0.003
100.685 0.081 6.061
0.000 0.776 0.000
2.473 1.282 0.495 1.603
10 1 l0 624
0.247 1.282 0.049 0.003
96.292 499.307 19.265
0.000 0.000 0.000
2.003 23.624 2.760 1.194
10 1 I0 624
0.200 23.624 0.276 0.002
104.631 12342.485 144.207
0.0(30 0.000 0.000
responses is that the polymer chains of the polyurethane chain elastics are less susceptible to environmental influences such as basic pH when they are stretched than when they are more relaxed.
In view of the apparent effect pH can have over force decay rates of polyurethane chain elastics, the results of previous studies that have compared various products under common environments should be
410
Am. J. Orthod. Dentofac. Orthop. November 1990
Ferriter, Meyers, and Lorton
v i e w e d with the understanding that, in a particular individual during a particular period o f time, these chain elastics m a y not respond in the s a m e m a n n e r o r even with the s a m e force d e c a y rate c u r v e , c o m p a r e d with other chain elastic products. T h e r e f o r e , w e r e c o m m e n d that the results o f this study not be used to assert that one product is superior to another. Clinically, it w o u l d s e e m that an oral p H l o w e r than 7 . 2 6 w o u l d retard the f o r c e - d e c a y rate o f the chain elastics. Before this study, we did not e x p e c t to find that decreased p H associated with dental plaque in the presence o f carbohydrates m a y actually d e c r e a s e the f o r c e - d e c a y rate o f the chain elastics and thus potentially e n h a n c e their effectiveness. The clinician does not h a v e an ability to control the p a t i e n t ' s oral envir o n m e n t , but he or she must h a v e k n o w l e d g e o f the individual e l e m e n t s within it that can affect the mechanics plan selected. Oral pH almost certainly has a significant influence on the decay rate o f orthodontic polyurethane chain elastics. Further study o f the effect o f p H at several levels w o u l d be the next logical step in understanding the force d e c a y o f orthodontic chain elastics. W e hypothesize that the d e c a y rate o f polyurethane orthodontic chain elastics is inversely proportional to the pH o f the oral e n v i r o n m e n t , with a corollary that the p H levels a b o v e neutral are m o s t hostile to the polyurethane chain elastics, thus increasing their f o r c e - d e c a y rates. REFERENCES 1. Proffit WR. Contemporary orthodontics. St. Louis: CV Mosby 1986:236-8,260, 432. 2. Brawley RE. Studies of the pH of normal resting saliva: variations with age and sex. J Dent Res 1935;15:55-62. 3. Brobeck JR. Best and Taylor's physiological basis of medical practice. Baltimore: Williams & Wilkins. 1973;2-23. 4. Kleinberg I. Studies on dental plaque. I. The effects of different concentrations of glucose on the pH of plaque in vivo. J Dent Res 1961;40:1087-111. 5. Mormann JE, Mahlmann HR. Oral starch degradation and its influence on acid production in human dental plaque. Caries Res 1981;15:166-75. 6. lensen hiE, Schachtele CF. The acidogenic potential of reference
7. 8.
9.
10.
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12.
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foods and snacks at interproximal sites in the human dentition. J Dent Res 1982;62:889-92. Kleinberg I, Jenkins GW, Denepitiya L, Chatterjee R. Diet and dental plaque. Front Oral Physiol 1981;3:88-107. Bibby BG, Mundorff SA, Zero DT, Almokinder KJ. Oral food clearance and the pit of plaque and saliva. J Am Dent Assoc 1986;112:333-7. Andreasen GF, Bishara SE. Comparison of Alastik chains with elastics involved with intra-arch molar to molar forces. Angle Orthod 1970;40:151-8. Bishara SE, Andreasen GF. A comparison of time related forces between plastic AlastiKs and latex elastics. Angle Orthod 1970;67:554-62. Kovatch JS, Lautenschlager EP, Apfel DA, Keller JC. Loadextension-time behavior of orthodontic elastics. J Dent Res 1976;55:783-6. Brantley WA, Salander S, Myers CL, Winders RV. Effects of prestretching on force degradation characteristics of plastic modules. Angle Orthod 1979;49:37-43. Wong AK. Orthodontic elastic materials. Angle Orthod 1976; 46:196-205. De Genova DC, Mclnnes-Ledous P, Weinberg R, Shaye R. Force degradation of orthodontic elastomeric chains-a product comparison study. AM J ORTIIOD1985;87:377-84. Killiany DM, Duplessis J. Relaxation of elastomeric chains. J Clin Orthod 1985;19:592-3. Kuster R, Ingervall B, Buergin W. Laboratory and intra-oral tests of the degradation of elastic chains. Eur J Orthod
1986;8:202-8. 17. Chang H. Effects of instantaneous prestretching on force degradation characteristics of orthodontic plastic modules. Proc Natl Sci Counc Repub China IBI 1987;11:45-53. 18. Hershey HG, Reynolds WG. The plastic module as an orthodontic tooth-moving mechanism. AM J OR'I]IOD 1975;67:55462. 19. Rock WP, Wilson HJ, Fisher SE. Force reduction of orthodontic elastomeric chains after one month in the mouth. Br J Orthod 1986;13:147-50. 20. Brooks DG, Hershey HG. Effects of heat and time on stretched plastic orthodontic modules lAbstract]. J Dent Res 1976; 55(B):385. 21. Ash JL, Nikolai RJ. Relaxation of orthodontic elastomeric chains and modules in vitro and in vivo. J Dent Res 1978;57:685-91. 22. Young J, Sandfik JL. The influence of preloading on stress relaxation of orthodontic elastic polymers. Angle Orthod 1979; 49:104-9. 23. Sonis AL, Van der Plas E, Gianelly A. A comparison of elastomeric auxiliaries versus elastic thread on premolar extraction site closure: an in vivo study. AM J OR'nqOD 1986;89:73-8.
