- Email: [email protected]
Force degradation of orthodontic elastomeric chains—A product comparison study
Force degradation of oqtihodontic elastomeric chains-A product comparison study
Dr. De Genova
David C. De Genova, D.D.S.,’ Pamela Mclnnes-Ledoux, B.D.S., M.Sc.(Dent.),** Roger Weinberg, Ph.D.,*‘* and Robert Shaye, D.D.S., Dr. Med. Dent.**** New Orleans. La. In the last 20 years, synthetic elastic modules have been introduced to the orthodontist. However, force decay of these materials has been a clinical problem and the purpose of this project was to evaluate the force decay patterns of three commercially available elastomeric products-Ormco Power Chain II, Rocky Mountain Energy Chain, and TP Elast-0 Chain-in a simulated oral environment. Thermal-cycled samples experienced less force decay over a 21-day period than samples stored at 37” C. Furthermore, statistical analysis confirmed that there was a highly significant difference (F < 0.01) between the mean force exerted by short modules and long modules for each material. Overall, modules producing higher initial forces (short modules) underwent less force decay after 21 days than did modules producing lower initial force values (long modules). All materials exerted 216 to 459 grams of force initially. After 21 days of simulated tooth movement, the force exerted by the elastic modules was 70 to 230 grams-a significant reduction (p < 0.001).
Key words: Elastomerics, load, force exarted, decay curves, thermal cycling
S
ynthetic elastomeric chains have been used by orthodontists since the 1960s. These polyurethane materials have largely replaced latex elastics for intraarch tooth movement. The plastic modules are designed for use in the correction of tooth rotations and the closure of space. Force decay in these materials is significant and has been a clinical problem. The synthetic elastics are amorphous polymers made from polyurethane materials; the exact composition is proprietary information. The polymers are not ideal elastic materials because their mechanical properties change with time and temperature. Polymer chains can slip past one another and/or they can stretch and uncoil when a force is applied to them. Chain slippage leads to viscous behavior that is slow and irreversible; chain stretching and uncoiling leads to elastic behavior that is quick and reversible. The orthodontic plastic modules undergo both behavior Funded by Louisiana State University School of Dentistry 1ntramur.d Research Grant. *Graduate SNdent in Orthodontics, Louisiana State University School of Dentistxy. **Assistant Professor of Biomaterials. ***Professor of Biometry, Louisiana State University Medical Center. ****Professor of Orthodontics, LSU School of Dentistry.
types, with chain slippage eventually predominating. Young and Sandrik’ studied the extension of plastic modules at various forces for loading and unloading. Upon unloading, the modules did not return to zero extension and a permanent deformation resulted. The difference between the loading and unloading curves, termed hysteresis, was significant. This indicated that it would be impossible to predict the force exerted by the plastic module and the period of time this force would be extended. Ware* stated earlier: “It could only be hoped that at some stage it would pass through the loading desirable for optimal movement.” Previous research into the properties of plastic modular products revealed a significant loss in the amount of available force delivered over varying periods of time. Andreasen and Bishara3 found that Unitek Alastik* chains lost approximately 74% of their initial force in the first day. To compensate for this loss, the authors noted that it might be necessary to apply four times the desired force to act on the tooth after the first day. Hershey and Reynolds4 determined that after 4 weeks, Unitek Alastik, TP Elast-0 Chain? and Ormco Wnitek Corporation, Monrovia, Calif. tTP Laboratories Incorporated, La Pose, Ind
377
378
De Genova et al.
Fig. 1. Polyester jig for providing varied extension of elastomeric chain products. Fifteen elastic samples were tested at a time. The modules were stretched between the protruding pins across the expansion joint.
Power Chain* modules retained only about 40% of their original force. Simulated tooth movement increased the rate of force decay whereby the materials averaged only 25% of their original force levels after 4 weeks. However, for these studies water was used at a constant temperature, limiting the study’s clinical application. These in vitro studies led to Ash and Nikolai’s in vivo comparison of synthetic chain materials.5 These investigators ascertained that force degradation was more rapid in the mouth than in air or water and that stress relaxation in the mouth remained significant throughout a 3-week test period. The effects of prestretching elastic chains were evaluated by Wong6 and by Young and Sandrik.’ They determined that plastic modules prestretched in air retained a greater percentage of their initial force over a period of time than did the unstretched control materials. Young and Sandrik’ proposed that prestretching the polymer chain decreased force decay. Brantley and associates’ showed a 5% improvement in retained force for specimens prestretched in air and water when compared to a control group. The research of Brooks and Hershey8 yielded approximately a 10% increase in force from prestretched modules. Kovatch and others’ recommended slowly stretching plastic modules to position to decrease the rate of stress relaxation. A review of the literature shows that most of the materials have been tested in water at constant temperature based on Bishara and Andreasen’s 1970 statement” that no significant difference existed when materials were tested in water or saliva. However, a significant difference was seen when intraoral tests were *Ormco
Corporation,
Glendora,
Calif.
conducted by Ash and Nikolai’ in 1978. The chemistry of salivary enzymes and temperature variations caused by the ingestion of hot and cold foods possibly accounted for this significant difference in results. To our knowledge, there has been no report published on the effect of thermal cycling on force decay in plastic modules. These temperature fluctuations may play a role in the relaxation and deformation of the polymer. An improvement in the material itself or in the manufacturing process was needed to provide the orthodontist with a more consistent means of delivering force over an extended period of time. It was for this reason, perhaps, that in 198 1 Ormco Corporation changed the polyurethane resin used in the manufacture of their power products. Rocky Mountain Orthodontics* entered the elastomeric chain market in 1982 with what they claimed to be a “tough elastic material . . that delivers a uniform continuous force over a long period of time.” The manufacturer also claims that “Energy Chain outperforms other similar ringlets in stress decay. aging time and elongation tests.” TP Laboratories markets Elast-0 Chain, which is said to provide a light continuous traction force. The manufacturer has eliminated the solid bar between the rings. This supposedly results in an increase in resiliency and provides lighter forces over a longer period of time. Hershey and Reynolds4 noted that modules manufactured by a die-cut stamping process were more consistent in the amount of force produced as compared to injection-molded materials. The Ormco Power Chain and the Rocky Mountain Energy Chain are examples of stamped modules; the Elast-0 Chain is injection molded. The recent addition of new synthetic elastic chain products by various manufacturers and the alteration of the polyurethane resin used by one of the manufacturers warrant an updated evaluation of the chain materials available today. The purpose of this study was to answer the following questions: 1. How do the forces produced by three types of plastic modules, stretched to a constant length, decay over a 3-week period? 2. How does thermal cycling affect force decay? 3. How does the initial load affect force decay at both constant and decreasing extension? 4. How is the force decay affected by an extension decrease of 0.5 mm per week (simulating tooth movement)? 5. How valid are the manufacturers’ claims? *Rocky
Mountain
Orthodontics,
Denver,
Co10
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MATERIALS AND METHODS Three elastomeric products were selected for evaluation: 1. Ormco Power Chain II (Product A) 2. Rocky Mountain Orthodontics Energy Chain (Product B) 3. TP Laboratories, Inc. Elast-0 Chain (Product C) The clinical uses of elastomeric chain products are varied. For testing purposes, the chain material was used to simulate canine retraction into the s.ite of an extracted premolar. Two different module lengths (long and short) of each material were used for the initial load-comparison study. Because of the size dil’ferences between the three materials, the number of units per module depended on the material. However, the overall unstretched length of all short modules and long modules was about the same. The long modules consisted of four units of Product A, three units of Product B, and five units of Product C. The short modules consisted of three units of Product A, two units of Product B, and four units of Product C. These length modules were chosen as being representative of what orthodontists would use to retract the canine into a premolar-extraction space. ‘r’he modules were stretched 20 mm. because this distance approximates the distance between the mesial wing of the canine bracket and the distal wing of the second premolar bracket (first premolar extracted). Polyester resin jigs 1 cm thick were constructed with the aid of a metal form. Two closing jigs (variable distance), each divided in half by orthodontic expansion screws (Fig. l), and two solid jigs (constant distance) were constructed. Fifteen pairs of stainless steel pins cut from O&IO-inch orthodontic wire were placed in rows on either side of the expansion joint or midline. Each elastic module was hooked on two pins, one from each row. The expansion screws varied the distance between the rows. All plastic modules were tested in a synthel:ic saliva medium, Oralube.* This formulation is similar to that recommended by Retief and associates. I1 This study evaluated force decay over 21 days at constant extension (Part I) and at decreasing extension (Part II). In Part I twenty randomly selected plastic modules from each of the three materials were randomly assigned to four groups of five each. By this arrangement, five long and five short modules of each material were tested either in conditions of constant temperature (37” C) or in a thermal-cycled environment to determine the effect of initial load and temperature on force decay. *Oral Disease Research Laboratory,
VA Medical Center, Houston,
Texas
Fig. 2. Electronic force gauge used to measure force exerted by the elastic modules. The modules were carefully removed from the testing jigs and attached to the hooks. After 5 seconds’ stabilization, force measurements were recorded in grams.
Thermal cycling between 15” C and 45” C was used. The rationale for choosing these temperatures was based on the work of Peterson, Phillips, and Swartz,” who demonstrated that during the drinking of liquids at two temperature extremes (hot coffee at 60” C and ice water at 0” C), the temperature at the tooth surface ranges from 15” C to 45” C. This discrepancy results because
380
De Genova et al.
iv, ./ , :rrhsi ,Miii I’JX>
Table 1. Mean percentage of remaining force for pooled long and short modules at constant length and varying temperature treatment I----..
--.
Time 30 min Treatment
Material Product
A
Product
B
Product
c
37” c Themalcycled 37” c Thennalcycled 37” c Thermalcycled
1 hr
8 hr
24 hr
7 day
14 da!
21 da))
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
62.7 65.6
4.1 1.2
58.9 61.2
4.4 2.0
52.5 52.3
3.8 1.7
46.7 47.9
3.7 1.9
42.0 44.2
3.3 1.6
41.7 42.3
3.1 1.3
39.1 42.5
3.9 1X
74.2 77.0
3.0 3.7
71.2 73.2
4.1 3.4
67.6 66.8
3.6 3.8
62.8 64.7
2.8 3.7
57.9 62.0
2.8 2.6
58.3 60.9
3.0 2.7
56.0 60.8
2.8 2.8
66.8 70.7
4.3 4.4
61.6 67.1
5.6 5.4
60.5 60.6
5.0 5.6
56.7 59.9
4.5 5.2
51.6 56.3
4.6 6.2
51.5 55.0
3.8 5.4
49.8 54.9
5.8 5.0
Table II. Mean initial and final force values (grams) exerted by long and short thermal-cycled modules at
constant length Force
(grams)
Initial Material Product
A
Product
B
Product
c
Final
(21 days)
Module length
Mean
SD
Mean
SD
Long Short Long Short Long Short
240.8 360.0 289.6 436.0 389.8 413.2
9.8 22.0 26.5 14.3 49.1 33.5
99.4 158.0 182.6 272.8 204.4 236.2
6.8 9.2 5.4 11.9 3.6 18.9
when these fluids are consumed they are not normally held in the mouth long. Therefore, the tooth surface does not reach the same temperature extremes. The thermal-cycled modules were subjected to twice-daily cycling for 30 minutes with a dwell time of 30 seconds. Force measurements were made at fourteen time intervals: initial, 5 minutes, 30 minutes, 1 hour, 8 hours, 24 hours, and 2, 3, 4, 5, 6, 7, 14, and 21 days. In Part II we investigated the effect of decreasing extension of 0.5 mm/wk in a thermal-cycled environment. Ten randomly selected plastic modules from each of the three products were randomly assigned to two groups of five long and five short modules. After 24 hours the initial extension of 20 mm was decreased by 0.5 mm to 19.5 mm. At the end of day 7, the extension was again decreased by 0.5 mm to 19 mm and further decreased after day 14 to 18.5 mm. The extension decreases were made following the force measurements for that time period. Force measurements were made at the same time intervals as in Part I. In both parts of this study, the modules were slowly stretched to the initial 20-mm extension. At the preselected times, the modules were removed from the
jigs, attached to an electronic force gauge,* and allowed to stabilize for 5 seconds before the recordings were made. This instrument (Fig. 2) permitted accurate force measurements and represented a significant improvement over the accuracy of the spring gauges used in previous studies. This gauge features a precision electronic load cell and a four-digit liquid crystal display that eliminates the parallax and the interpolation errors that are possible with dial force gauges. The gauge used was mounted on a test stand with a movable platform. Stainless steel hooks constructed of 0.40-inch orthodontic wire were mounted to the gauge and test stand. Metric vernier calipers were used to obtain the same extension distance as that to which the test material was subjected on the testing jig. Force readings were obtained for each of the samples, recorded on the data sheet, and the samples were returned to their respective jigs. In Part I a four-way analysis of variance (ANOVA) was used to test the effects of material, time interval, initial load, and temperature on force decay in ortho*Accu
Force II, Am&k,
Hunter Spring Division,
Hatfield,
Pa.
Force degradation
Volume 87 Number 5
of orthodontic
chains
381
ORMCO
1004 100
. ORMCO 90-
A RockyMau ntain
80-r
A T P Laboratories
I 7Q8 1s
elastomeric
A 0 8
60-
80
A Constant Extension l
3
DecreasingE&don
A 0 8
A A
A .
8
c !a5 Ea% : 40-
A
A
A
.
0
0
’
l
n
l
I
I
I
I
1
I
I
1
30RockyMountain
20lo0:
0
1 I 3Dm lh
1 8h
1 I I 1 24h 07 014 D21
Time Fig. 3. Graph illustrating mean percentage of force remaining at constant extension in a thermal-cycled environment for pooled long and short modules of three materials studied.
dontic plastic modules. In Part II a three-way ANOVA was used to test the effects of material, time interval, and initial load on force decay in thermal-cycled orthodontic plastic modules. The level of statistical significance chosen was P -=c0.05.
T P Laboratories 100 80I
RESULTS Effect of thermal cycling
Force measurements for each of the twenty samples of the three test materials were obtained at the specified time intervals during the 2 1-day test period. These measurements were used to determine the average percentage of remaining force for each material at 37” C and after thermal cycling. The mean percentage of remaining forces by time for pooled long and short modules at constant length is shown in Table I. After the first 30 minutes, 63% to 77% of the initial force exerted by the modules remained. After 21 days the percentage of remaining force ranged from 39.1% to 60.8%. The samples subjected to thermal cycling retained a significantly higher percentage of remaining force (P < 0.01) than the samples held at a constant temperature of 37” C. Because thermal cycling favored the production of higher force values (and because this treatment simulates oral conditions), the remaining force studies were conducted under thermal-cycled conditions.
0 31knlh 8hh
D7Dl4.021
Fig. 4. Graphs for three materials showing mean percentage of force remaining at both constant and decreasing extension. In all three cases decreasing extension produced significant reductions in the mean percentage of force remaining from day 14 through day 21. Comparison of three elastomeric chains
The decay patterns for each of the three materials at constant length and subjected to thermal cycling are illustrated in Fig. 3. Product B samples retained a
382
De Genova et al.
Table III. Mean percentage of remaining force for thermal-cycled long and short modules at constant length Time 30 min Material
Module length
Product
Long
Mean
I hr
8 hr
24 hr
7 day
14 duy
?I day
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Meun
SD
66.1
1.5
60.3
1.8
52.4
1.8
46.6
1.7
43.3
1.2
42.1
1.1
41.2
1.4
Short Long
65.2 14.9
0.1 3.6
62.1 71.4
1.8
52.1
1.8
49.1
1.0
42.6
64.4
3.6
61.8
2.6
45.2 60.4
1.4
3.8
2.1
59.1
1.6 2.0
43.8 59.0
2.1
Short Long
79.1
2.5 6.1
75.0 66.5
1.9 7.3
69.2 57.9
2.3 6.3
67.1 51.2
1.4
63.1
2.1
10.7
5.3
54.0
5.6
62.6 52.0
2.1 5.7
62.6 52.7
2.1 5.4
Short
70.7
2.4
61.1
3.4
63.3
3.6
62.6
4.0
58.5
4.1
58.0
3.3
51.3
4.0
A Product B Product
1.0
C
Table IV. Mean force values (grams) exerted by long and short thermal-cycled modules at decreasing extension (simulated tooth movement) Force Initial Material
Product A Product B Product C
Module length
Long Short Long Short Long Short
(20 mm)
7 day (19.5
(grams)
mm)
14 day (19 mm)
SD
Mean
SD
216.4
11.3
81.2
3.9
13.2
89.4 143.4
4.2
325.8
8.6 3.7
3.3 5.5
15.5 13.2
125.0 159.0
Ill.4
288.2 459.8
6.2 4.8
140.8
1.5
5.6
246.2
7.5
226.8
425.8 459.6
55.4 27.4
33.8 21.6
185.8 226.0
30.7 22.2
168.2 203.4
6.7 27.0
182.4
258.0
SD
mm)
Mean
280.2 222.2
Mean
21 day (18.5
Mean
SD
69.4
22.0
Table V. Mean percentage of remaining force for pooled long and short thermal-cycled modules at constant length and at decreasing extension Time 30 min Material
Extension
Product A Product B Product
c
Constant Decreasing Constant Decreasing Constant Decreasing
Mean
I hr
8 hr
24 hr
SD
Mean
SD
Mean
SD
Mean
65.6
1.2
61.2
61.7
2.1
58.4
2.0 2.4
52.3 50.9
1.7 2.0
71.0
73.2 74.8 67.1
3.4 4.3
66.8 69.7
3.8 3.8
70.7
3.1 4.8 4.4
5.4
60.6
68.6
4.3
65.8
2.8
61.9
11.5
significantly higher percentage of force (P < 0.01) throughout the 21-day test period than Product A or Product C samples. Product A modules retained the lowest percentage of remaining force of the three materials . Short modules stretched to a constant length exerted higher force values than longer modules stretched the same distance. This is illustrated by the mean initial and final force values for long and short modules as
7 day
I4 day
SD
Mean
SD
Mean
47.9
1.9
44.2
1.6
48.1 64.1
2.5 3.1
42.1 62.0
1.8
5.6
67.1 59.9
3.0 5.2
4.3
60.7
3.1
21 day
SD
Mean
SD
42.3
1.3 2.1
42.5 33.2
1.8
38.0
62.2 56.3
2.6 3.4 5.2
60.9 54.4 55.0
2.7 2.8 5.4
60.8 49.2
2.8 3.0
54.9
5.0
54.1
3.2
46.3
3.8
41.8
3.6
1.9
shown in Table II. There was a highly significant difference (P < 0.01) in the force exerted by the long and the short modules of each material over the 21-day period. (It should be noted that the mean initial force values have large standard deviations.) In addition, the short modules generally retained a higher mean percentage of remaining force for each material than the longer modules that were stretched the same distance (Table III). Overall, modules producing higher initial
Volume 87 Number 5
loads were left with higher percentage of remaining force. Force decay with simulated tooth movement
Force decay patterns with simulated tooth movement (decreasing extension by 0.5 mm per week) and pooled long and short modules are shown in Fig. 4. The plotted points are mean percentage of remaining force. Decreasing extension (simulated tooth movement) significantly decreased the mean percentage of remaining force at 14 days (1.0 mm decrease: and at 2 1 days ( 1.5 mm decrease) for each material. The mean force values (grams) for both long and short modules of each material are shown in Table IV. DISCUSSION
The module lengths of the various materials used in this investigation were selected because they represented realistic choices that a clinician might use to retract a canine into the space of an extracted first premolar. The wide range of initial forces produced (240.8 grams to 436.0 grams) is higher than the range of optimal force for retraction (100 grams to 250 grams) suggested by Storey and SmithI and by Reitan.14 Within the first 30 minutes, there was a loss of 23.0% to 37.3% of the initial force exerted by the materials. However, the remaining force exerted was still within the range for optimal tooth movement. The relatively large standard deviations within the materials indicated an unpredictability in force delivery. The decay curves for the three materials were similar in that an early and rapid loss of force occurred within the first hour and then a gradual reduction in the rate of force loss was seen throughout the 21-day test period. Product A had the lowest mean percentage of remaining force for pooled long and short mod.ules at constant length after 24 hours. Approximately 48% of the initial force remained. Product C retained about 60% of the initial force, and Product B retained 65% (Table V). Hershey and Reynolds4 reported that Elast-0 Chain retained only 5 1% of the initial force after 24 hours when tested in distilled water at a constant 37” C. The results obtained from this study for the same material tested in artificial saliva indicated that after 24 hours at 37” C 56% of the initial force remained, whereas after thermal cycling 60% force remained (Table I). The results obtained in this study for Product A are similar to those reported by Wong.6 He determined that approximately 50% of the initial force was lost after 24 hours for Ormco Power Chain stretched to a constant length and held at 37” C. Another study by Hershey
Force degradation
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and Reynolds4 reported that the average percentage of remaining force at 21 days for Elast-0 Chain and Power Chain samples was 41%. The Product A evaluated in this investigation produced similar percentages: 39.1% for the 37” C samples and 42.5% for those specimens subjected to thermal cycling. Statistical analysis to compare the results of the two studies was not possible because standard deviations were not reported by Hershey and Reynolds. Contrary to what was initially expected, those samples subjected to a thermal-cycled environment retained a higher mean percentage of remaining force than did those held at a constant temperature of 37” C. Possibly this was related to an increase in stiffness of the material caused by the temperature variations to which the samples were subjected in the 15” C and 45” C synthetic saliva baths. Since these conditions better simulate the oral environment, the results obtained from the samples subjected to thermal cycling were believed to be more representative of what would occur in vivo. As expected, the short modules produced significantly higher force levels throughout the 21-day test period compared to the longer samples for each material. The force ranges for the samples varied greatly. At day 21 Product A long samples produced the lowest mean force levels-perhaps too low for significant tooth movement. Product B and Product C materials produced high initial forces and retained values well within the clinically recommended levels at the end of the 21day period. For each material the shorter modules generally retained a higher percentage of remaining force than the longer modules. These results differ from those obtained by Hershey and Reynolds,4 who found no significant difference in the decay curves for those sample groups exhibiting different initial forces. The rate of tooth movement used in this study to determine its effect on force decay was based on an investigation by Boester and Johnston.15 Their data indicated that 0.5 mm per week is a reasonable rate of tooth movement into an extraction site. Reduction of the length over which the modules were stretched to simulate tooth movement had a highly significant (P < 0.01) influence over the amount of remaining force available at 14 days and at 21 days, compared to those modules held at constant length. The results after 21 days for Product C and especially for Product B appear better than any results reported previously for synthetic elastomeric chain products. REFERENCES 1. Young J, Sandrik JL: The influence of preloading on stress relaxation of orthodontic elastic polymers. Angle Orthod 49: 104108, 1979.
384 De Genova et al.
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2. Ware AL: Some properties of plastic modules used for tooth movement. Aust Orthod J2: 200-202, 1971. 3. Andreasen GF, Bishara S: Comparison of alastik chains with elastics involved with intra-arch molar to molar forces. Angle Orthod 40: 151-158, 1970. 4. Hershey HG, Reynolds WG: The plastic module as an orthodontic tooth-moving mechanism. AM J ORTHOD67: 554-562, 1975. 5. Ash JL, Nikolai RJ: Relaxation of orthodontic elastomeric chains and modules in vitro and in vivo. J Dent Res 57: 685-690, 1978. 6. Wong AK: Orthodontic elastic materials. Angle Orthod 46: 196205, 1976. 7. Brantley SA, Salander S, Myers CL, Winders RV: Effects of prestretching on force degradation characteristics of plastic modules. Angle Orthod 49: 37-43, 1979. 8. Brooks DG, Hershey HG: Effect of heat and time on stretched plastic orthodontic modules. J Dent Res 55B: 363, 1976. 9. Kovatch JS, Lautenschlager EP, Apfel DA, Keller JC: Loadextension-time behavior of orthodontic alastics. J Dent Res 55: 783-786, 1976. 10. Bishara S, Andreasen GF: A comparison of time related forces between plastic alastiks and latex elastics. Angle Orthod 40: 319-328, 1970.
BOUND VOLUMES AVAIlABLE
11. Retief DH, Sorvas PG, Bradley EL, Taylor RE, Walker AR: In vitro fluoride uptake, distribution and retention by human enamel after l- and 24-hour application of various topical fluoride agents. J Dent Res 59: 573-582, 1980. 12. Peterson EA II, Phillips RW, Swartz ML: A comparison of the physical properties of four restorative resins. J Am Dent Assoc 73: 1324-1336, 1966. 13. Storey EE, Smith R: Force in orthodontics and its relation to tooth movement. Austral J Dent 56: 11-18, 1952. 14. Reitan K: Some factors determining the evaluation of forces in orthodontics. AM J ORTHOD43: 32-45, 1957. 15. Boester CH, Johnston LE: A clinical investigation of the concepts of differential and optimal force in canine retraction. Angle Orthod 44: 113-l 19, 1974. Reprint
requests
to:
Dr. Pamela McInnes-Ledoux Department of Biomaterials LSU School of Dentistry 1100 Florida Ave. New Orleans. LA 70119
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Dr. De Genova
David C. De Genova, D.D.S.,’ Pamela Mclnnes-Ledoux, B.D.S., M.Sc.(Dent.),** Roger Weinberg, Ph.D.,*‘* and Robert Shaye, D.D.S., Dr. Med. Dent.**** New Orleans. La. In the last 20 years, synthetic elastic modules have been introduced to the orthodontist. However, force decay of these materials has been a clinical problem and the purpose of this project was to evaluate the force decay patterns of three commercially available elastomeric products-Ormco Power Chain II, Rocky Mountain Energy Chain, and TP Elast-0 Chain-in a simulated oral environment. Thermal-cycled samples experienced less force decay over a 21-day period than samples stored at 37” C. Furthermore, statistical analysis confirmed that there was a highly significant difference (F < 0.01) between the mean force exerted by short modules and long modules for each material. Overall, modules producing higher initial forces (short modules) underwent less force decay after 21 days than did modules producing lower initial force values (long modules). All materials exerted 216 to 459 grams of force initially. After 21 days of simulated tooth movement, the force exerted by the elastic modules was 70 to 230 grams-a significant reduction (p < 0.001).
Key words: Elastomerics, load, force exarted, decay curves, thermal cycling
S
ynthetic elastomeric chains have been used by orthodontists since the 1960s. These polyurethane materials have largely replaced latex elastics for intraarch tooth movement. The plastic modules are designed for use in the correction of tooth rotations and the closure of space. Force decay in these materials is significant and has been a clinical problem. The synthetic elastics are amorphous polymers made from polyurethane materials; the exact composition is proprietary information. The polymers are not ideal elastic materials because their mechanical properties change with time and temperature. Polymer chains can slip past one another and/or they can stretch and uncoil when a force is applied to them. Chain slippage leads to viscous behavior that is slow and irreversible; chain stretching and uncoiling leads to elastic behavior that is quick and reversible. The orthodontic plastic modules undergo both behavior Funded by Louisiana State University School of Dentistry 1ntramur.d Research Grant. *Graduate SNdent in Orthodontics, Louisiana State University School of Dentistxy. **Assistant Professor of Biomaterials. ***Professor of Biometry, Louisiana State University Medical Center. ****Professor of Orthodontics, LSU School of Dentistry.
types, with chain slippage eventually predominating. Young and Sandrik’ studied the extension of plastic modules at various forces for loading and unloading. Upon unloading, the modules did not return to zero extension and a permanent deformation resulted. The difference between the loading and unloading curves, termed hysteresis, was significant. This indicated that it would be impossible to predict the force exerted by the plastic module and the period of time this force would be extended. Ware* stated earlier: “It could only be hoped that at some stage it would pass through the loading desirable for optimal movement.” Previous research into the properties of plastic modular products revealed a significant loss in the amount of available force delivered over varying periods of time. Andreasen and Bishara3 found that Unitek Alastik* chains lost approximately 74% of their initial force in the first day. To compensate for this loss, the authors noted that it might be necessary to apply four times the desired force to act on the tooth after the first day. Hershey and Reynolds4 determined that after 4 weeks, Unitek Alastik, TP Elast-0 Chain? and Ormco Wnitek Corporation, Monrovia, Calif. tTP Laboratories Incorporated, La Pose, Ind
377
378
De Genova et al.
Fig. 1. Polyester jig for providing varied extension of elastomeric chain products. Fifteen elastic samples were tested at a time. The modules were stretched between the protruding pins across the expansion joint.
Power Chain* modules retained only about 40% of their original force. Simulated tooth movement increased the rate of force decay whereby the materials averaged only 25% of their original force levels after 4 weeks. However, for these studies water was used at a constant temperature, limiting the study’s clinical application. These in vitro studies led to Ash and Nikolai’s in vivo comparison of synthetic chain materials.5 These investigators ascertained that force degradation was more rapid in the mouth than in air or water and that stress relaxation in the mouth remained significant throughout a 3-week test period. The effects of prestretching elastic chains were evaluated by Wong6 and by Young and Sandrik.’ They determined that plastic modules prestretched in air retained a greater percentage of their initial force over a period of time than did the unstretched control materials. Young and Sandrik’ proposed that prestretching the polymer chain decreased force decay. Brantley and associates’ showed a 5% improvement in retained force for specimens prestretched in air and water when compared to a control group. The research of Brooks and Hershey8 yielded approximately a 10% increase in force from prestretched modules. Kovatch and others’ recommended slowly stretching plastic modules to position to decrease the rate of stress relaxation. A review of the literature shows that most of the materials have been tested in water at constant temperature based on Bishara and Andreasen’s 1970 statement” that no significant difference existed when materials were tested in water or saliva. However, a significant difference was seen when intraoral tests were *Ormco
Corporation,
Glendora,
Calif.
conducted by Ash and Nikolai’ in 1978. The chemistry of salivary enzymes and temperature variations caused by the ingestion of hot and cold foods possibly accounted for this significant difference in results. To our knowledge, there has been no report published on the effect of thermal cycling on force decay in plastic modules. These temperature fluctuations may play a role in the relaxation and deformation of the polymer. An improvement in the material itself or in the manufacturing process was needed to provide the orthodontist with a more consistent means of delivering force over an extended period of time. It was for this reason, perhaps, that in 198 1 Ormco Corporation changed the polyurethane resin used in the manufacture of their power products. Rocky Mountain Orthodontics* entered the elastomeric chain market in 1982 with what they claimed to be a “tough elastic material . . that delivers a uniform continuous force over a long period of time.” The manufacturer also claims that “Energy Chain outperforms other similar ringlets in stress decay. aging time and elongation tests.” TP Laboratories markets Elast-0 Chain, which is said to provide a light continuous traction force. The manufacturer has eliminated the solid bar between the rings. This supposedly results in an increase in resiliency and provides lighter forces over a longer period of time. Hershey and Reynolds4 noted that modules manufactured by a die-cut stamping process were more consistent in the amount of force produced as compared to injection-molded materials. The Ormco Power Chain and the Rocky Mountain Energy Chain are examples of stamped modules; the Elast-0 Chain is injection molded. The recent addition of new synthetic elastic chain products by various manufacturers and the alteration of the polyurethane resin used by one of the manufacturers warrant an updated evaluation of the chain materials available today. The purpose of this study was to answer the following questions: 1. How do the forces produced by three types of plastic modules, stretched to a constant length, decay over a 3-week period? 2. How does thermal cycling affect force decay? 3. How does the initial load affect force decay at both constant and decreasing extension? 4. How is the force decay affected by an extension decrease of 0.5 mm per week (simulating tooth movement)? 5. How valid are the manufacturers’ claims? *Rocky
Mountain
Orthodontics,
Denver,
Co10
Volume 87 Number 5
Force degradation
of orthodontic
elastomeric
chains
379
MATERIALS AND METHODS Three elastomeric products were selected for evaluation: 1. Ormco Power Chain II (Product A) 2. Rocky Mountain Orthodontics Energy Chain (Product B) 3. TP Laboratories, Inc. Elast-0 Chain (Product C) The clinical uses of elastomeric chain products are varied. For testing purposes, the chain material was used to simulate canine retraction into the s.ite of an extracted premolar. Two different module lengths (long and short) of each material were used for the initial load-comparison study. Because of the size dil’ferences between the three materials, the number of units per module depended on the material. However, the overall unstretched length of all short modules and long modules was about the same. The long modules consisted of four units of Product A, three units of Product B, and five units of Product C. The short modules consisted of three units of Product A, two units of Product B, and four units of Product C. These length modules were chosen as being representative of what orthodontists would use to retract the canine into a premolar-extraction space. ‘r’he modules were stretched 20 mm. because this distance approximates the distance between the mesial wing of the canine bracket and the distal wing of the second premolar bracket (first premolar extracted). Polyester resin jigs 1 cm thick were constructed with the aid of a metal form. Two closing jigs (variable distance), each divided in half by orthodontic expansion screws (Fig. l), and two solid jigs (constant distance) were constructed. Fifteen pairs of stainless steel pins cut from O&IO-inch orthodontic wire were placed in rows on either side of the expansion joint or midline. Each elastic module was hooked on two pins, one from each row. The expansion screws varied the distance between the rows. All plastic modules were tested in a synthel:ic saliva medium, Oralube.* This formulation is similar to that recommended by Retief and associates. I1 This study evaluated force decay over 21 days at constant extension (Part I) and at decreasing extension (Part II). In Part I twenty randomly selected plastic modules from each of the three materials were randomly assigned to four groups of five each. By this arrangement, five long and five short modules of each material were tested either in conditions of constant temperature (37” C) or in a thermal-cycled environment to determine the effect of initial load and temperature on force decay. *Oral Disease Research Laboratory,
VA Medical Center, Houston,
Texas
Fig. 2. Electronic force gauge used to measure force exerted by the elastic modules. The modules were carefully removed from the testing jigs and attached to the hooks. After 5 seconds’ stabilization, force measurements were recorded in grams.
Thermal cycling between 15” C and 45” C was used. The rationale for choosing these temperatures was based on the work of Peterson, Phillips, and Swartz,” who demonstrated that during the drinking of liquids at two temperature extremes (hot coffee at 60” C and ice water at 0” C), the temperature at the tooth surface ranges from 15” C to 45” C. This discrepancy results because
380
De Genova et al.
iv, ./ , :rrhsi ,Miii I’JX>
Table 1. Mean percentage of remaining force for pooled long and short modules at constant length and varying temperature treatment I----..
--.
Time 30 min Treatment
Material Product
A
Product
B
Product
c
37” c Themalcycled 37” c Thennalcycled 37” c Thermalcycled
1 hr
8 hr
24 hr
7 day
14 da!
21 da))
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
62.7 65.6
4.1 1.2
58.9 61.2
4.4 2.0
52.5 52.3
3.8 1.7
46.7 47.9
3.7 1.9
42.0 44.2
3.3 1.6
41.7 42.3
3.1 1.3
39.1 42.5
3.9 1X
74.2 77.0
3.0 3.7
71.2 73.2
4.1 3.4
67.6 66.8
3.6 3.8
62.8 64.7
2.8 3.7
57.9 62.0
2.8 2.6
58.3 60.9
3.0 2.7
56.0 60.8
2.8 2.8
66.8 70.7
4.3 4.4
61.6 67.1
5.6 5.4
60.5 60.6
5.0 5.6
56.7 59.9
4.5 5.2
51.6 56.3
4.6 6.2
51.5 55.0
3.8 5.4
49.8 54.9
5.8 5.0
Table II. Mean initial and final force values (grams) exerted by long and short thermal-cycled modules at
constant length Force
(grams)
Initial Material Product
A
Product
B
Product
c
Final
(21 days)
Module length
Mean
SD
Mean
SD
Long Short Long Short Long Short
240.8 360.0 289.6 436.0 389.8 413.2
9.8 22.0 26.5 14.3 49.1 33.5
99.4 158.0 182.6 272.8 204.4 236.2
6.8 9.2 5.4 11.9 3.6 18.9
when these fluids are consumed they are not normally held in the mouth long. Therefore, the tooth surface does not reach the same temperature extremes. The thermal-cycled modules were subjected to twice-daily cycling for 30 minutes with a dwell time of 30 seconds. Force measurements were made at fourteen time intervals: initial, 5 minutes, 30 minutes, 1 hour, 8 hours, 24 hours, and 2, 3, 4, 5, 6, 7, 14, and 21 days. In Part II we investigated the effect of decreasing extension of 0.5 mm/wk in a thermal-cycled environment. Ten randomly selected plastic modules from each of the three products were randomly assigned to two groups of five long and five short modules. After 24 hours the initial extension of 20 mm was decreased by 0.5 mm to 19.5 mm. At the end of day 7, the extension was again decreased by 0.5 mm to 19 mm and further decreased after day 14 to 18.5 mm. The extension decreases were made following the force measurements for that time period. Force measurements were made at the same time intervals as in Part I. In both parts of this study, the modules were slowly stretched to the initial 20-mm extension. At the preselected times, the modules were removed from the
jigs, attached to an electronic force gauge,* and allowed to stabilize for 5 seconds before the recordings were made. This instrument (Fig. 2) permitted accurate force measurements and represented a significant improvement over the accuracy of the spring gauges used in previous studies. This gauge features a precision electronic load cell and a four-digit liquid crystal display that eliminates the parallax and the interpolation errors that are possible with dial force gauges. The gauge used was mounted on a test stand with a movable platform. Stainless steel hooks constructed of 0.40-inch orthodontic wire were mounted to the gauge and test stand. Metric vernier calipers were used to obtain the same extension distance as that to which the test material was subjected on the testing jig. Force readings were obtained for each of the samples, recorded on the data sheet, and the samples were returned to their respective jigs. In Part I a four-way analysis of variance (ANOVA) was used to test the effects of material, time interval, initial load, and temperature on force decay in ortho*Accu
Force II, Am&k,
Hunter Spring Division,
Hatfield,
Pa.
Force degradation
Volume 87 Number 5
of orthodontic
chains
381
ORMCO
1004 100
. ORMCO 90-
A RockyMau ntain
80-r
A T P Laboratories
I 7Q8 1s
elastomeric
A 0 8
60-
80
A Constant Extension l
3
DecreasingE&don
A 0 8
A A
A .
8
c !a5 Ea% : 40-
A
A
A
.
0
0
’
l
n
l
I
I
I
I
1
I
I
1
30RockyMountain
20lo0:
0
1 I 3Dm lh
1 8h
1 I I 1 24h 07 014 D21
Time Fig. 3. Graph illustrating mean percentage of force remaining at constant extension in a thermal-cycled environment for pooled long and short modules of three materials studied.
dontic plastic modules. In Part II a three-way ANOVA was used to test the effects of material, time interval, and initial load on force decay in thermal-cycled orthodontic plastic modules. The level of statistical significance chosen was P -=c0.05.
T P Laboratories 100 80I
RESULTS Effect of thermal cycling
Force measurements for each of the twenty samples of the three test materials were obtained at the specified time intervals during the 2 1-day test period. These measurements were used to determine the average percentage of remaining force for each material at 37” C and after thermal cycling. The mean percentage of remaining forces by time for pooled long and short modules at constant length is shown in Table I. After the first 30 minutes, 63% to 77% of the initial force exerted by the modules remained. After 21 days the percentage of remaining force ranged from 39.1% to 60.8%. The samples subjected to thermal cycling retained a significantly higher percentage of remaining force (P < 0.01) than the samples held at a constant temperature of 37” C. Because thermal cycling favored the production of higher force values (and because this treatment simulates oral conditions), the remaining force studies were conducted under thermal-cycled conditions.
0 31knlh 8hh
D7Dl4.021
Fig. 4. Graphs for three materials showing mean percentage of force remaining at both constant and decreasing extension. In all three cases decreasing extension produced significant reductions in the mean percentage of force remaining from day 14 through day 21. Comparison of three elastomeric chains
The decay patterns for each of the three materials at constant length and subjected to thermal cycling are illustrated in Fig. 3. Product B samples retained a
382
De Genova et al.
Table III. Mean percentage of remaining force for thermal-cycled long and short modules at constant length Time 30 min Material
Module length
Product
Long
Mean
I hr
8 hr
24 hr
7 day
14 duy
?I day
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Meun
SD
66.1
1.5
60.3
1.8
52.4
1.8
46.6
1.7
43.3
1.2
42.1
1.1
41.2
1.4
Short Long
65.2 14.9
0.1 3.6
62.1 71.4
1.8
52.1
1.8
49.1
1.0
42.6
64.4
3.6
61.8
2.6
45.2 60.4
1.4
3.8
2.1
59.1
1.6 2.0
43.8 59.0
2.1
Short Long
79.1
2.5 6.1
75.0 66.5
1.9 7.3
69.2 57.9
2.3 6.3
67.1 51.2
1.4
63.1
2.1
10.7
5.3
54.0
5.6
62.6 52.0
2.1 5.7
62.6 52.7
2.1 5.4
Short
70.7
2.4
61.1
3.4
63.3
3.6
62.6
4.0
58.5
4.1
58.0
3.3
51.3
4.0
A Product B Product
1.0
C
Table IV. Mean force values (grams) exerted by long and short thermal-cycled modules at decreasing extension (simulated tooth movement) Force Initial Material
Product A Product B Product C
Module length
Long Short Long Short Long Short
(20 mm)
7 day (19.5
(grams)
mm)
14 day (19 mm)
SD
Mean
SD
216.4
11.3
81.2
3.9
13.2
89.4 143.4
4.2
325.8
8.6 3.7
3.3 5.5
15.5 13.2
125.0 159.0
Ill.4
288.2 459.8
6.2 4.8
140.8
1.5
5.6
246.2
7.5
226.8
425.8 459.6
55.4 27.4
33.8 21.6
185.8 226.0
30.7 22.2
168.2 203.4
6.7 27.0
182.4
258.0
SD
mm)
Mean
280.2 222.2
Mean
21 day (18.5
Mean
SD
69.4
22.0
Table V. Mean percentage of remaining force for pooled long and short thermal-cycled modules at constant length and at decreasing extension Time 30 min Material
Extension
Product A Product B Product
c
Constant Decreasing Constant Decreasing Constant Decreasing
Mean
I hr
8 hr
24 hr
SD
Mean
SD
Mean
SD
Mean
65.6
1.2
61.2
61.7
2.1
58.4
2.0 2.4
52.3 50.9
1.7 2.0
71.0
73.2 74.8 67.1
3.4 4.3
66.8 69.7
3.8 3.8
70.7
3.1 4.8 4.4
5.4
60.6
68.6
4.3
65.8
2.8
61.9
11.5
significantly higher percentage of force (P < 0.01) throughout the 21-day test period than Product A or Product C samples. Product A modules retained the lowest percentage of remaining force of the three materials . Short modules stretched to a constant length exerted higher force values than longer modules stretched the same distance. This is illustrated by the mean initial and final force values for long and short modules as
7 day
I4 day
SD
Mean
SD
Mean
47.9
1.9
44.2
1.6
48.1 64.1
2.5 3.1
42.1 62.0
1.8
5.6
67.1 59.9
3.0 5.2
4.3
60.7
3.1
21 day
SD
Mean
SD
42.3
1.3 2.1
42.5 33.2
1.8
38.0
62.2 56.3
2.6 3.4 5.2
60.9 54.4 55.0
2.7 2.8 5.4
60.8 49.2
2.8 3.0
54.9
5.0
54.1
3.2
46.3
3.8
41.8
3.6
1.9
shown in Table II. There was a highly significant difference (P < 0.01) in the force exerted by the long and the short modules of each material over the 21-day period. (It should be noted that the mean initial force values have large standard deviations.) In addition, the short modules generally retained a higher mean percentage of remaining force for each material than the longer modules that were stretched the same distance (Table III). Overall, modules producing higher initial
Volume 87 Number 5
loads were left with higher percentage of remaining force. Force decay with simulated tooth movement
Force decay patterns with simulated tooth movement (decreasing extension by 0.5 mm per week) and pooled long and short modules are shown in Fig. 4. The plotted points are mean percentage of remaining force. Decreasing extension (simulated tooth movement) significantly decreased the mean percentage of remaining force at 14 days (1.0 mm decrease: and at 2 1 days ( 1.5 mm decrease) for each material. The mean force values (grams) for both long and short modules of each material are shown in Table IV. DISCUSSION
The module lengths of the various materials used in this investigation were selected because they represented realistic choices that a clinician might use to retract a canine into the space of an extracted first premolar. The wide range of initial forces produced (240.8 grams to 436.0 grams) is higher than the range of optimal force for retraction (100 grams to 250 grams) suggested by Storey and SmithI and by Reitan.14 Within the first 30 minutes, there was a loss of 23.0% to 37.3% of the initial force exerted by the materials. However, the remaining force exerted was still within the range for optimal tooth movement. The relatively large standard deviations within the materials indicated an unpredictability in force delivery. The decay curves for the three materials were similar in that an early and rapid loss of force occurred within the first hour and then a gradual reduction in the rate of force loss was seen throughout the 21-day test period. Product A had the lowest mean percentage of remaining force for pooled long and short mod.ules at constant length after 24 hours. Approximately 48% of the initial force remained. Product C retained about 60% of the initial force, and Product B retained 65% (Table V). Hershey and Reynolds4 reported that Elast-0 Chain retained only 5 1% of the initial force after 24 hours when tested in distilled water at a constant 37” C. The results obtained from this study for the same material tested in artificial saliva indicated that after 24 hours at 37” C 56% of the initial force remained, whereas after thermal cycling 60% force remained (Table I). The results obtained in this study for Product A are similar to those reported by Wong.6 He determined that approximately 50% of the initial force was lost after 24 hours for Ormco Power Chain stretched to a constant length and held at 37” C. Another study by Hershey
Force degradation
of orthodontic
elastomeric
chains
383
and Reynolds4 reported that the average percentage of remaining force at 21 days for Elast-0 Chain and Power Chain samples was 41%. The Product A evaluated in this investigation produced similar percentages: 39.1% for the 37” C samples and 42.5% for those specimens subjected to thermal cycling. Statistical analysis to compare the results of the two studies was not possible because standard deviations were not reported by Hershey and Reynolds. Contrary to what was initially expected, those samples subjected to a thermal-cycled environment retained a higher mean percentage of remaining force than did those held at a constant temperature of 37” C. Possibly this was related to an increase in stiffness of the material caused by the temperature variations to which the samples were subjected in the 15” C and 45” C synthetic saliva baths. Since these conditions better simulate the oral environment, the results obtained from the samples subjected to thermal cycling were believed to be more representative of what would occur in vivo. As expected, the short modules produced significantly higher force levels throughout the 21-day test period compared to the longer samples for each material. The force ranges for the samples varied greatly. At day 21 Product A long samples produced the lowest mean force levels-perhaps too low for significant tooth movement. Product B and Product C materials produced high initial forces and retained values well within the clinically recommended levels at the end of the 21day period. For each material the shorter modules generally retained a higher percentage of remaining force than the longer modules. These results differ from those obtained by Hershey and Reynolds,4 who found no significant difference in the decay curves for those sample groups exhibiting different initial forces. The rate of tooth movement used in this study to determine its effect on force decay was based on an investigation by Boester and Johnston.15 Their data indicated that 0.5 mm per week is a reasonable rate of tooth movement into an extraction site. Reduction of the length over which the modules were stretched to simulate tooth movement had a highly significant (P < 0.01) influence over the amount of remaining force available at 14 days and at 21 days, compared to those modules held at constant length. The results after 21 days for Product C and especially for Product B appear better than any results reported previously for synthetic elastomeric chain products. REFERENCES 1. Young J, Sandrik JL: The influence of preloading on stress relaxation of orthodontic elastic polymers. Angle Orthod 49: 104108, 1979.
384 De Genova et al.
Am. J. Orrhod Mm 1985
2. Ware AL: Some properties of plastic modules used for tooth movement. Aust Orthod J2: 200-202, 1971. 3. Andreasen GF, Bishara S: Comparison of alastik chains with elastics involved with intra-arch molar to molar forces. Angle Orthod 40: 151-158, 1970. 4. Hershey HG, Reynolds WG: The plastic module as an orthodontic tooth-moving mechanism. AM J ORTHOD67: 554-562, 1975. 5. Ash JL, Nikolai RJ: Relaxation of orthodontic elastomeric chains and modules in vitro and in vivo. J Dent Res 57: 685-690, 1978. 6. Wong AK: Orthodontic elastic materials. Angle Orthod 46: 196205, 1976. 7. Brantley SA, Salander S, Myers CL, Winders RV: Effects of prestretching on force degradation characteristics of plastic modules. Angle Orthod 49: 37-43, 1979. 8. Brooks DG, Hershey HG: Effect of heat and time on stretched plastic orthodontic modules. J Dent Res 55B: 363, 1976. 9. Kovatch JS, Lautenschlager EP, Apfel DA, Keller JC: Loadextension-time behavior of orthodontic alastics. J Dent Res 55: 783-786, 1976. 10. Bishara S, Andreasen GF: A comparison of time related forces between plastic alastiks and latex elastics. Angle Orthod 40: 319-328, 1970.
BOUND VOLUMES AVAIlABLE
11. Retief DH, Sorvas PG, Bradley EL, Taylor RE, Walker AR: In vitro fluoride uptake, distribution and retention by human enamel after l- and 24-hour application of various topical fluoride agents. J Dent Res 59: 573-582, 1980. 12. Peterson EA II, Phillips RW, Swartz ML: A comparison of the physical properties of four restorative resins. J Am Dent Assoc 73: 1324-1336, 1966. 13. Storey EE, Smith R: Force in orthodontics and its relation to tooth movement. Austral J Dent 56: 11-18, 1952. 14. Reitan K: Some factors determining the evaluation of forces in orthodontics. AM J ORTHOD43: 32-45, 1957. 15. Boester CH, Johnston LE: A clinical investigation of the concepts of differential and optimal force in canine retraction. Angle Orthod 44: 113-l 19, 1974. Reprint
requests
to:
Dr. Pamela McInnes-Ledoux Department of Biomaterials LSU School of Dentistry 1100 Florida Ave. New Orleans. LA 70119
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