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The plastic module as an orthodontic tooth-moving mechanism
The plastic module as an orthodontic tooth-moving mechanism H. Garland Hershey, D.D.S., MS.,* and William G. Reynolds, D.D.S., M.S.
Chapel Hill, N. C.
T
he plastic module is a relatively new development in orthodontics and is currently in wide use as a tooth-moving mechanism. In spite of the modules’ common clinical use, however, little is known about their elastic properties. Studies by Andreasen and Bisharal, a and the descriptive information supplied by the manufacturers provide the only available information describing the tooth-moving force produced by stretched plastic modules. Further, Andreasen and Bishara limited their investigations to a description of the initial force produced and the rate of force decay over time with the modules held at, a fixed stretch distance. In a. clinical situation, however, teeth do not remain stationary as forces are applied to them but move in response to the applied force, Thus, for force decay data on plastic modules to be clinically useful, the data should be collected under conditions simulating actual tooth movement,. To our knowledge, information describing the changes in force levels produced by plastic modules in a simulated clinical situation has not been published. The purpose of this investigation was to provide the orthodontist with clinically useful data describing the behavior of plastic modules when used as orthodontic space-closing appliances. The specific ob,jectivcs were to answer the following questions : 1. How much does the force produced by a stretched plastic module decrease over a 4 or g-week period? 2. How much does this force decrease when the stretch dimension is decreased at a rate of 0.25 or 0.50 mm. per week (simulated tooth movement ) ? 3. What is the effect of initial force magnitude on the force decay curve of the modules? no the modules exhibit more desirable properties when stretched to high or low initial forces? 4. How much variability exists between modules of a given type? This investigation was supported by Kational Institutes of Health Grant RR-05333 from the Division of Research Facilities and Resources. *Associate Professor, Department of Orthodontics, Tiniversity of North Carolina.
554
Plastic module as tooth-moving mec,haksm
555
Fig. 1. Adjustable framework and force gauge used in the investigation. Tooth movement was simulated by decreasing interbracket distance across the framework.
5. Are the modules made by different manufacturers similar, or does one exhibit more desirable properties than the others? Materials and methods
A stainless steel framework, on which the modules were stretched, was constructed as shown in Fig. 1. A threaded mechanism separating the frame components allowed them to be adjusted to any given separation and permitted this separation to be decreased at any given rate to simulate tooth movement. Edgewise brackets were welded to the frame to allow attachment and simultaneous testing of 120 modules. Initial interbracket distance ranged from 12 to 34 mm. The brackets were welded in groups of ten at the same separation, with 2 mm. increments in separation between groups. Variation within any group was not more than 0.1 mm. and variation between groups was not more than 0.2 mm. A total of 540 plastic modules, representing three manufacturers, were tested: Alastik modules, both clear and gray type, manufactured by Unitek Corporation; Power Chain and Links, manufactured by Ormco Corporation; and Elast-0 Chain, manufactured by TP Laboratories. Formulas giving the composition of these modules are proprietary information. All modules were aged in triple distilled water at 3’7O C., as suggested by Andreasen and Bishara.l? 2 The framework was closed (simulating tooth movement) at rates of both 0.25 mm. per week and 0.50 mm. per week. All force measurements were made by two independent observers with calibrated Carpo gauges (Fig. 1). Each module was measured at each time interval by both observers, and any measurements disagreeing by more than 10 per cent were repeated. These two measurements were averaged, and the means were used to compute mea.n force level, standard deviation, and per cent of remaining force at each time interval. Intra- and interobserver variability was computed through the use of paired t tests from data reported in the study. Neither was significant at the 0.05 level.
5 5 6 Ilershey and Retpolds
A
I q 7r Iwk
I 2 wks
I
I
3 wkr
4 wkr
I 5 wks
I 6 wkr
1 hr 10 minr
Fig. 2. Curves illustrating the effect of time only (no tooth movement) on force decay of plastic modules. Curves are mean values for all 540 modules tested and are expressed as per cent of the initial force remaining at the indicated times. The curves illustrate that almost all of the loss of force occurs by the end of the first 24 hours, when approximately one half the initial force remains. During the period from 24 hours after placement to 6 weeks, the force is almost constant, decreasing only an additional 5 per cent of the original force. A, Insertion to 6 weeks. B, Insertion to 1 week to better illustrate the rapid initial loss in force.
The maximum time interval of 6 weeks was based on the assumption that orthodontists would see patients at intervals not exceeding this time span. Force magnitude was measured at the time of attachment of the module to the bracket wings and then at 10 minutes, 1 hour, 24 hours, and weekly from 1 to 6 weeks after placement.
The data collected were used to determine the effects of time, rate of simulated tooth movement, initial force level, and composition (Blastik, Elast-0 Chain, or Power Chain) on force decay within the modules. To determine the effect of time on force decay, data obtained with the framework held at a constant distance were averaged for all modules at each measuring period. As seen in Fig. 2, A, all modules lost considerable force with time. The average per cent of initial force remaining at selected time intervals was ‘75 per cent after 10 minutes; 64 per cent after 1 hour; 47 per cent after 24 hours; and 42 per cent after 6 weeks. Fig. 2, B details the data from placement of the modules up to 1 week to better illustrate the initial, rather precipitous drop in force. The effect of simulated tooth movement (decrease in stretch dimension across
Plastic module as tooth-moving mechanism
557
B
“0 t tfl
30-
E
lo-
20-
,,
10 minr
c,
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I,
14 hrr
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Fig. 2, 6. For legend see opposite page.
the adjustable framework) on force decay was the next factor considered. Mean force remaining in all modules as a per cent of initial force was determined at each time period and for each rate of closure. After 4 weeks the modules retained 40 per cent of their initial force at the zero rate of closure, 32 per cent at the 0.25 mm. per week rate, and 25 per cent at the 0.50 mm. per week rate. After 6 weeks the modules retained 42 per cent at the zero rate and 28 and 18 per cent at the 0.25 mm. per week and 0.50 mm. per week rates of closure, respectively (Fig. 3). To determine the effect of initial force on force decay, the modules were divided into two groups representing those with higher (mean 573 grams) and lower (mean 284 grams) initial forces. All modules in these groups were subjected to the 0.50 mm. per week rate of closure. Although modules stretched to higher force levels retained lower percentages of their initial force after 24 hours, the force loss from 24 hours to 4 weeks was greater with lower initial forces. There was no significant difference in the decay curves of the two groups, indicating that their percentage force loss, at least for the two initial force values tested, was identical. The last factor considered was the effect of composition of the modules on force decay. Although Alastiks retained a higher per cent of their initial force after 24 hours than Power modules or Elast-0 Chains (51 versus 44 and 45 per cent), the force retained at 4 weeks was very similar. After 4 weeks at the 0.25 mm. per week rate of closure, Alastiks retained an average of 34 per cent of their initial force while Power modules and Elast-0 Chain retained 31 per cent. At the 0.50 mm. per week rate of closure, the Alastiks retained 26 per cent of their initial force, the Power modules 25 per cent, and the Elast-0 Chain 24 per cent. Although Power modules experienced an average of 7 per cent greater
YO RATE OF CLOSURE 0 mm, wk _-----
-
-
0 25 mm/rk 0 50 rnrn!Wk
Fig. 3. The effect of simulated tooth movement on force produced by the modules. The arrow indicates that after 1 month, the modules which were closed at the rate of 0.5
mm. per week (a representative rate of space closure) had approximately 25 per cent of their initial force remaining.
drop in force after 24 hours, their force loss from 24 hours to 4 weeks was 4 per cent less than that of the Alastiks. The variability of each type of module was csprcssed in quantitat,ivr terms by calculating the standard deviation of each group. Standard deviations for the Cl clear Alastiks ranged from + 21 grams to I? 123 grams and averaged + 80 grams with a mean initial forte of 566 grams. Other clear Alastik modules exhibited similar variation, and in some instances two identical modules s t r e t c h e d t o exaetlg the same c1istanc.c o n t h e s a m e t e s t seyucncc differed significantly in force levels, with one exerting twice t,he forc*e of t,hc other. The Power modules showed much less variation between modules of the same type. The average standard deviation for both higher (535 grams) and lower (376 grams) initial forcde levels was + 25 grams. The Elast-0 Chain also proved quite variable, with average standard deviations for the higher (736 grams) and lower (503 grams) initial forces of + 83 and + 75 grams, rcspect,ively. The variability of the gray Alastik modules was much lower (+ 29 grams), an amount almost identical to that of the Power modules. Discussion
The forec magnitudes and rates of simulated t,ooth movement. selected for use in this invrstigat,ion were based on earlier reports describing “optimum” tooth-moving forces. Starry and Smith” and X&an4 suggested that forces in the range of 100 to 250 grams are optimal for canine retraction, while Begg” stated that 300 grams was optimal for this type of movement. Hixon and as-
Vohme 67 Nwmbe, 5
Plastic module as tooth-moving mecChanism 559
sociates6, 7 noted that higher forces, up to 1,000 grams, tended to produce more rapid tooth movement. With 300 grams of force used for canine retraction, they reported that tooth movement occurred at a more or less continuous rate of 0.4 mm. per week, beginning after the twenty-fifth day. Boester and Johnston8 in an investigation relating rate of tooth movement to applied forces ranging between 55 and 310 grams, reported that although a 55 gram force produced significantly slower canine retraction than a 310 gram force, there was no significant difference in rate of tooth movement when forces of 140, 225, and 310 grams were used. Their data indicate that 0.5 mm. per week is a very typical figure for rate of first premolar space closure. In the present investigation, the initial forces of approximately 300 and 500 grams were selected because they were expected to decrease by approximately 50 per cent in the first 24 hours and thus fall to within the “desirable” ranges reported by the previous investigators. As an aside, for those who are accustomed to thinking in terms of ounces, a reasonably close conversion from grams to ounces can be made by dividing the grams by 30; for example, 300 grams equals 10 ounces. All modules tested produced similar decay curves. The rapid loss of slightly more than one half of the initial force in the first 24 hours, if uncompensated for, may explain why some clinicians fail to achieve the tooth movement they desire when using plastic modules. Although the performance of different modules was similar, looking only at the mean force decay values for all modules is somewhat misleading in that there were substantial differences between modules made by different manufacturers. These differences are large enough to make comparisons of different manufacturers’ modules desirable, although knowledge of the properties of the particular module being used would allow the clinician to compensate by applying higher or lower initial forces and/or different replacement intervals. Alastik modules tended to retain slightly more of their initial force than did the Power modules or Elast-0 Chain at all time intervals. Decay of the Elast-0 Chain for the first 24 hours, at the zero rate of closure, was very similar to the other two, except for the loss of force in the first hour, which was 20 to 24 per cent greater. After 24 hours the Elast-0 Chain retained 45 per cent of its initial force as compared with 51 per cent for the Alastiks and 44 per cent for Power modules. This difference in decay pattern for the Elast-0 Chain (a very rapid initial force loss in the first hour) could increase patient comfort and initial tissue response without affecting tooth-moving ability. This would seem to be a desirable property, assuming that the force loss in the first 24 hours is an inherent property of the material and cannot be eliminated by any of the manufacturers. It should be emphasized, however, that differences in the decay curves of the three types of products were in the range of a few percentage points and all produced force decay curves which are probably clinically indistinguishable. A factor which may serve to distinguish between these modules in clinical use is the degree of variability in force produced when a given module is stretched a given distance. As described under “Results,” this variability was quantified by calculating the standard deviations for each sample of identical
and s h o w e d that ~‘OIVCT ~nodulcs a n d g r a y 31astik were much more consistent in the amount of force protinccd thau were the clear Xlastik 01 Blast-0 Chain modules. The clinical implication of this finding is that, for modules wit,h little variability, the amount of force produc~l by stretching an individual module a 1illOWn distance could bc recorded mtl used in subsequent similar clinical situations. For modules with high variability, l~oweve~~, every individual placement would require checking with a forcr gauge in order to quantify the force produced. It is int,eresting to note that both Power modules and gray Alastik are apparently produce~l by a stamping process, rather than being molded as clear Alastik and ElasttO Chain appear to be. a factor to he considered in contemplating possible future improvrmcnts in plastic moclnlcs. As expected, simulated tooth movement (decrease in stretch distance ) increased rates of force decay in all modules. As the rate of tooth movement increased, the rate of decay also incrcasctl, the increase becoming more evident, beginning with the second week of the test period. After the fourth week at the 0.50 mm. per week rate of closure, the average force remaining was 25 per cent of the initial force. Knowledge of this large amount of force loss with time and t,ooth movement indicates that when plastic modules arc placed they should have force magnit,udes substantially larger than what the clinician views as an “optimum” tooth-moving force. Although initially this loss of force, approaching 75 per cent over the period of a usual orthodontic appointment, seems rather large, it should be viewed from a perspective gained from an examination of forces produrcd b- other commonly used tooth-moving devices. Extraoral forces vary from a pound or more to zero force (removal of the headgear) several times a day. An 0.0215 by 0.025 inch Bull loop arch wire activated the proverbial “thin dime” exerts forces greater than 1 kg.’ (2.2 pountfs) and drops to near zero force over a l-month period. By comparison, a typical drop in force by a plastic module of from 300 grams at placement to 75 grams at the end of I month shows a much more continuous force application. As is true in many other arcas in orthodontics, knowledge of the “big picture” is necessary bcforc one can accurately evaluate the desirability of using plastic modules as a tooth-moving mechanism. As is apparent from the data prcscnted in this study, plastic moclulcs provide a continuous application of tooth-moving force throughout i\ 4- to G-week pcriotl. The common clinical assumption that plastic modules cscrt little or no f’orcc after the first few hours in place is certainly inaccuratt~. Similarly, the wnccrn t h a t plastic modules are undesirable because one must, place a heavy- init,ial force on the tooth does not appear significant,, especially after a comparison of their force magnitudes to time-linked forces generated by an arch wire or cxtraoral appliances. Several additional observations of clinical significance were madr in the study. Clear Alastiks were less colorfast than the I’owcr modules or Mast-0 Chain and tended to discolor. Heavy Alastik modules proved difficult to manipulate, especially the “K” modules, although this would not, be a factor if t,hey were stretched between hooks rather tjlmn bracket wings. The rounded Alastik and Elast-0 Chain modules seemed to bc more easily attachecl to the brackets ~nodnles
VoZwne 67 Nuna her 5
Plastic module as tooth-moving meohanism 561
than the gray Alastik or Power modules, which had a rectangular crosssectional shape. This may have resulted, however, from our longer-term clinical familiarity with manipulating the round cross-sectional modules. None of these factors involving ease of manipulation would apply to the use of the modules on lingual attachments, a situation for which plastic modules are ideally suited, as Anderson’ has shown. The method of dispensing some modules from a spool, rather than using individual precut lengths, produced less waste and prevented mixing of the modules. The form of the Elast-0 Chain, a series of connected loops without intervening straight lengths of material, may allow the clinician more versatility in choosing initial forces, a trait shared by the CK Alastik module and the middle section of the Power links. All modules sustained the testing manipulation without breaking, although, of course, no forces of occlusion were operating. This has been substantiated clinically in that breakage of plastic modules in actual use is not frequent. One factor which could contribute to an increase in loss of the module is the decreased force magnitude (approximately one fourth of the initial force) noted a.fter several weeks in place. This may increase the chance of displacement from the bracket by occlusal forces and thus “failure” of the module. Summary and conclusions
A framework was constructed on which 120 plastic modules could be simultaneously stretched to predetermined distances between edgewise brackets. Using calibrated Carpo gauges, measurements of force exerted by the modules while stretched on the framework were made by two independent observers at the time of attachment to the frame, and after 10 minutes, 1 hour, 24 hours, and weeklp to 6 weeks. Measurements were made with the modules held at a constant stretch distance and with interbracket distances decreased at rates of 0.25 and 0.50 mm. per week to simulate tooth movement. A total of 540 plastic modules representing three manufacturers were tested. Modules were compared and contrasted to determine the effects of time, composition, initial force, and rate of closure on the force decay curve. Analysis of the data justifies the following conclusions : 1. All modules lost considerable force with time, with the largest loss (approximately 50 per cent) occurring within the first 24 hours. The modules retained an average of 40 per cent of their initial force after 4 weeks at the zero rate of closure and almost exactly the same after 6 weeks, 2. Simulated tooth movement (decrease in stretch dimension across the adjustable framework) increased the rate of force loss. In situations in which a rate of tooth movement of 0.25 mm. per week is anticipated, the clinician can expect a plastic module to retain approximately one third the initial force after 1 month. When a rate of tooth movement of 0.50 mm. per week is predicted, the force remaining after 1 month will be approximately one fourth the initial force. 3. Percentage of force loss was similar for modules stretched to high and 10~ initial forces. Decay characteristics were not related to magnitude of initial force.
562 Hershey and Reynolds 4. Modules produced by three different manufacturers exhibited similar force decay curves and were judged to be clinically equivalent. 5. Variability among modules of the same type was found to be large for clear Alastiks, somewhat less for Elast-0 Chain, and much less for Power modules and gray Iilastiks. 6. Force decay data exhibited by plastic modules indicate that they can produce effective tooth-moving force throughout a 4- to 6-week period. REFERENCES
1. Andreason, G. F., and Bishara, S. E.: Comparison of Alastik chains with elastics involved with intra-arch molar to molar forces, Angle Orthod. 40: X1-158, 1970. 2. Andreasen, G. F., and Bishara, S. E.: A comparison of time related forces between plastic Alastiks and latex elastics, Angle Orthod. 40: 319-328, 1970. 3. Storey, E. E., and Smith, R.: Force in orthodontics and its relation to tooth movement, Aust. J. Dent. 56: 11-18, 1952. 4. Reitan, K.: Some factors determining the evaluation of forces in orthodontics, AM. J. ORTHOD. 43: 32-45,1957. 5. Begg, P. R.: Begg orthodontic theory and technique, Philadelphia, 1965, W. B. Saunders Company. 6. Hixon, E. H., Aasen, T. O., Arango, J., Clark, R. A., Klosterman, R., Miller, S. S., and Odom, W. M.: On force and tooth movement, AM. J. ORTHOD. 57: 467-489, 1970. 7. Hixon, E. H., Atikian, H., Callow, G. E., McDonald, H. W., and Tacy, R. J.: Optimal force, differential force, and anchorage, A M. J. ORTHOD. 55: 437-457, 1969. 8. Boester, C. H., and Johnston, L. E.: A clinical investigation of the concepts of differential and optimal force/in canine retraction, Angle Orthod. 44: 113-119, 1974. 9. Anderson, R. M.: A return to large nonresilient straight arch wires, AM. J. ORTHOD. 66: 9-39, 1974.
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he plastic module is a relatively new development in orthodontics and is currently in wide use as a tooth-moving mechanism. In spite of the modules’ common clinical use, however, little is known about their elastic properties. Studies by Andreasen and Bisharal, a and the descriptive information supplied by the manufacturers provide the only available information describing the tooth-moving force produced by stretched plastic modules. Further, Andreasen and Bishara limited their investigations to a description of the initial force produced and the rate of force decay over time with the modules held at, a fixed stretch distance. In a. clinical situation, however, teeth do not remain stationary as forces are applied to them but move in response to the applied force, Thus, for force decay data on plastic modules to be clinically useful, the data should be collected under conditions simulating actual tooth movement,. To our knowledge, information describing the changes in force levels produced by plastic modules in a simulated clinical situation has not been published. The purpose of this investigation was to provide the orthodontist with clinically useful data describing the behavior of plastic modules when used as orthodontic space-closing appliances. The specific ob,jectivcs were to answer the following questions : 1. How much does the force produced by a stretched plastic module decrease over a 4 or g-week period? 2. How much does this force decrease when the stretch dimension is decreased at a rate of 0.25 or 0.50 mm. per week (simulated tooth movement ) ? 3. What is the effect of initial force magnitude on the force decay curve of the modules? no the modules exhibit more desirable properties when stretched to high or low initial forces? 4. How much variability exists between modules of a given type? This investigation was supported by Kational Institutes of Health Grant RR-05333 from the Division of Research Facilities and Resources. *Associate Professor, Department of Orthodontics, Tiniversity of North Carolina.
554
Plastic module as tooth-moving mec,haksm
555
Fig. 1. Adjustable framework and force gauge used in the investigation. Tooth movement was simulated by decreasing interbracket distance across the framework.
5. Are the modules made by different manufacturers similar, or does one exhibit more desirable properties than the others? Materials and methods
A stainless steel framework, on which the modules were stretched, was constructed as shown in Fig. 1. A threaded mechanism separating the frame components allowed them to be adjusted to any given separation and permitted this separation to be decreased at any given rate to simulate tooth movement. Edgewise brackets were welded to the frame to allow attachment and simultaneous testing of 120 modules. Initial interbracket distance ranged from 12 to 34 mm. The brackets were welded in groups of ten at the same separation, with 2 mm. increments in separation between groups. Variation within any group was not more than 0.1 mm. and variation between groups was not more than 0.2 mm. A total of 540 plastic modules, representing three manufacturers, were tested: Alastik modules, both clear and gray type, manufactured by Unitek Corporation; Power Chain and Links, manufactured by Ormco Corporation; and Elast-0 Chain, manufactured by TP Laboratories. Formulas giving the composition of these modules are proprietary information. All modules were aged in triple distilled water at 3’7O C., as suggested by Andreasen and Bishara.l? 2 The framework was closed (simulating tooth movement) at rates of both 0.25 mm. per week and 0.50 mm. per week. All force measurements were made by two independent observers with calibrated Carpo gauges (Fig. 1). Each module was measured at each time interval by both observers, and any measurements disagreeing by more than 10 per cent were repeated. These two measurements were averaged, and the means were used to compute mea.n force level, standard deviation, and per cent of remaining force at each time interval. Intra- and interobserver variability was computed through the use of paired t tests from data reported in the study. Neither was significant at the 0.05 level.
5 5 6 Ilershey and Retpolds
A
I q 7r Iwk
I 2 wks
I
I
3 wkr
4 wkr
I 5 wks
I 6 wkr
1 hr 10 minr
Fig. 2. Curves illustrating the effect of time only (no tooth movement) on force decay of plastic modules. Curves are mean values for all 540 modules tested and are expressed as per cent of the initial force remaining at the indicated times. The curves illustrate that almost all of the loss of force occurs by the end of the first 24 hours, when approximately one half the initial force remains. During the period from 24 hours after placement to 6 weeks, the force is almost constant, decreasing only an additional 5 per cent of the original force. A, Insertion to 6 weeks. B, Insertion to 1 week to better illustrate the rapid initial loss in force.
The maximum time interval of 6 weeks was based on the assumption that orthodontists would see patients at intervals not exceeding this time span. Force magnitude was measured at the time of attachment of the module to the bracket wings and then at 10 minutes, 1 hour, 24 hours, and weekly from 1 to 6 weeks after placement.
The data collected were used to determine the effects of time, rate of simulated tooth movement, initial force level, and composition (Blastik, Elast-0 Chain, or Power Chain) on force decay within the modules. To determine the effect of time on force decay, data obtained with the framework held at a constant distance were averaged for all modules at each measuring period. As seen in Fig. 2, A, all modules lost considerable force with time. The average per cent of initial force remaining at selected time intervals was ‘75 per cent after 10 minutes; 64 per cent after 1 hour; 47 per cent after 24 hours; and 42 per cent after 6 weeks. Fig. 2, B details the data from placement of the modules up to 1 week to better illustrate the initial, rather precipitous drop in force. The effect of simulated tooth movement (decrease in stretch dimension across
Plastic module as tooth-moving mechanism
557
B
“0 t tfl
30-
E
lo-
20-
,,
10 minr
c,
1 hr
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14 hrr
lwk
Fig. 2, 6. For legend see opposite page.
the adjustable framework) on force decay was the next factor considered. Mean force remaining in all modules as a per cent of initial force was determined at each time period and for each rate of closure. After 4 weeks the modules retained 40 per cent of their initial force at the zero rate of closure, 32 per cent at the 0.25 mm. per week rate, and 25 per cent at the 0.50 mm. per week rate. After 6 weeks the modules retained 42 per cent at the zero rate and 28 and 18 per cent at the 0.25 mm. per week and 0.50 mm. per week rates of closure, respectively (Fig. 3). To determine the effect of initial force on force decay, the modules were divided into two groups representing those with higher (mean 573 grams) and lower (mean 284 grams) initial forces. All modules in these groups were subjected to the 0.50 mm. per week rate of closure. Although modules stretched to higher force levels retained lower percentages of their initial force after 24 hours, the force loss from 24 hours to 4 weeks was greater with lower initial forces. There was no significant difference in the decay curves of the two groups, indicating that their percentage force loss, at least for the two initial force values tested, was identical. The last factor considered was the effect of composition of the modules on force decay. Although Alastiks retained a higher per cent of their initial force after 24 hours than Power modules or Elast-0 Chains (51 versus 44 and 45 per cent), the force retained at 4 weeks was very similar. After 4 weeks at the 0.25 mm. per week rate of closure, Alastiks retained an average of 34 per cent of their initial force while Power modules and Elast-0 Chain retained 31 per cent. At the 0.50 mm. per week rate of closure, the Alastiks retained 26 per cent of their initial force, the Power modules 25 per cent, and the Elast-0 Chain 24 per cent. Although Power modules experienced an average of 7 per cent greater
YO RATE OF CLOSURE 0 mm, wk _-----
-
-
0 25 mm/rk 0 50 rnrn!Wk
Fig. 3. The effect of simulated tooth movement on force produced by the modules. The arrow indicates that after 1 month, the modules which were closed at the rate of 0.5
mm. per week (a representative rate of space closure) had approximately 25 per cent of their initial force remaining.
drop in force after 24 hours, their force loss from 24 hours to 4 weeks was 4 per cent less than that of the Alastiks. The variability of each type of module was csprcssed in quantitat,ivr terms by calculating the standard deviation of each group. Standard deviations for the Cl clear Alastiks ranged from + 21 grams to I? 123 grams and averaged + 80 grams with a mean initial forte of 566 grams. Other clear Alastik modules exhibited similar variation, and in some instances two identical modules s t r e t c h e d t o exaetlg the same c1istanc.c o n t h e s a m e t e s t seyucncc differed significantly in force levels, with one exerting twice t,he forc*e of t,hc other. The Power modules showed much less variation between modules of the same type. The average standard deviation for both higher (535 grams) and lower (376 grams) initial forcde levels was + 25 grams. The Elast-0 Chain also proved quite variable, with average standard deviations for the higher (736 grams) and lower (503 grams) initial forces of + 83 and + 75 grams, rcspect,ively. The variability of the gray Alastik modules was much lower (+ 29 grams), an amount almost identical to that of the Power modules. Discussion
The forec magnitudes and rates of simulated t,ooth movement. selected for use in this invrstigat,ion were based on earlier reports describing “optimum” tooth-moving forces. Starry and Smith” and X&an4 suggested that forces in the range of 100 to 250 grams are optimal for canine retraction, while Begg” stated that 300 grams was optimal for this type of movement. Hixon and as-
Vohme 67 Nwmbe, 5
Plastic module as tooth-moving mecChanism 559
sociates6, 7 noted that higher forces, up to 1,000 grams, tended to produce more rapid tooth movement. With 300 grams of force used for canine retraction, they reported that tooth movement occurred at a more or less continuous rate of 0.4 mm. per week, beginning after the twenty-fifth day. Boester and Johnston8 in an investigation relating rate of tooth movement to applied forces ranging between 55 and 310 grams, reported that although a 55 gram force produced significantly slower canine retraction than a 310 gram force, there was no significant difference in rate of tooth movement when forces of 140, 225, and 310 grams were used. Their data indicate that 0.5 mm. per week is a very typical figure for rate of first premolar space closure. In the present investigation, the initial forces of approximately 300 and 500 grams were selected because they were expected to decrease by approximately 50 per cent in the first 24 hours and thus fall to within the “desirable” ranges reported by the previous investigators. As an aside, for those who are accustomed to thinking in terms of ounces, a reasonably close conversion from grams to ounces can be made by dividing the grams by 30; for example, 300 grams equals 10 ounces. All modules tested produced similar decay curves. The rapid loss of slightly more than one half of the initial force in the first 24 hours, if uncompensated for, may explain why some clinicians fail to achieve the tooth movement they desire when using plastic modules. Although the performance of different modules was similar, looking only at the mean force decay values for all modules is somewhat misleading in that there were substantial differences between modules made by different manufacturers. These differences are large enough to make comparisons of different manufacturers’ modules desirable, although knowledge of the properties of the particular module being used would allow the clinician to compensate by applying higher or lower initial forces and/or different replacement intervals. Alastik modules tended to retain slightly more of their initial force than did the Power modules or Elast-0 Chain at all time intervals. Decay of the Elast-0 Chain for the first 24 hours, at the zero rate of closure, was very similar to the other two, except for the loss of force in the first hour, which was 20 to 24 per cent greater. After 24 hours the Elast-0 Chain retained 45 per cent of its initial force as compared with 51 per cent for the Alastiks and 44 per cent for Power modules. This difference in decay pattern for the Elast-0 Chain (a very rapid initial force loss in the first hour) could increase patient comfort and initial tissue response without affecting tooth-moving ability. This would seem to be a desirable property, assuming that the force loss in the first 24 hours is an inherent property of the material and cannot be eliminated by any of the manufacturers. It should be emphasized, however, that differences in the decay curves of the three types of products were in the range of a few percentage points and all produced force decay curves which are probably clinically indistinguishable. A factor which may serve to distinguish between these modules in clinical use is the degree of variability in force produced when a given module is stretched a given distance. As described under “Results,” this variability was quantified by calculating the standard deviations for each sample of identical
and s h o w e d that ~‘OIVCT ~nodulcs a n d g r a y 31astik were much more consistent in the amount of force protinccd thau were the clear Xlastik 01 Blast-0 Chain modules. The clinical implication of this finding is that, for modules wit,h little variability, the amount of force produc~l by stretching an individual module a 1illOWn distance could bc recorded mtl used in subsequent similar clinical situations. For modules with high variability, l~oweve~~, every individual placement would require checking with a forcr gauge in order to quantify the force produced. It is int,eresting to note that both Power modules and gray Alastik are apparently produce~l by a stamping process, rather than being molded as clear Alastik and ElasttO Chain appear to be. a factor to he considered in contemplating possible future improvrmcnts in plastic moclnlcs. As expected, simulated tooth movement (decrease in stretch distance ) increased rates of force decay in all modules. As the rate of tooth movement increased, the rate of decay also incrcasctl, the increase becoming more evident, beginning with the second week of the test period. After the fourth week at the 0.50 mm. per week rate of closure, the average force remaining was 25 per cent of the initial force. Knowledge of this large amount of force loss with time and t,ooth movement indicates that when plastic modules arc placed they should have force magnit,udes substantially larger than what the clinician views as an “optimum” tooth-moving force. Although initially this loss of force, approaching 75 per cent over the period of a usual orthodontic appointment, seems rather large, it should be viewed from a perspective gained from an examination of forces produrcd b- other commonly used tooth-moving devices. Extraoral forces vary from a pound or more to zero force (removal of the headgear) several times a day. An 0.0215 by 0.025 inch Bull loop arch wire activated the proverbial “thin dime” exerts forces greater than 1 kg.’ (2.2 pountfs) and drops to near zero force over a l-month period. By comparison, a typical drop in force by a plastic module of from 300 grams at placement to 75 grams at the end of I month shows a much more continuous force application. As is true in many other arcas in orthodontics, knowledge of the “big picture” is necessary bcforc one can accurately evaluate the desirability of using plastic modules as a tooth-moving mechanism. As is apparent from the data prcscnted in this study, plastic moclulcs provide a continuous application of tooth-moving force throughout i\ 4- to G-week pcriotl. The common clinical assumption that plastic modules cscrt little or no f’orcc after the first few hours in place is certainly inaccuratt~. Similarly, the wnccrn t h a t plastic modules are undesirable because one must, place a heavy- init,ial force on the tooth does not appear significant,, especially after a comparison of their force magnitudes to time-linked forces generated by an arch wire or cxtraoral appliances. Several additional observations of clinical significance were madr in the study. Clear Alastiks were less colorfast than the I’owcr modules or Mast-0 Chain and tended to discolor. Heavy Alastik modules proved difficult to manipulate, especially the “K” modules, although this would not, be a factor if t,hey were stretched between hooks rather tjlmn bracket wings. The rounded Alastik and Elast-0 Chain modules seemed to bc more easily attachecl to the brackets ~nodnles
VoZwne 67 Nuna her 5
Plastic module as tooth-moving meohanism 561
than the gray Alastik or Power modules, which had a rectangular crosssectional shape. This may have resulted, however, from our longer-term clinical familiarity with manipulating the round cross-sectional modules. None of these factors involving ease of manipulation would apply to the use of the modules on lingual attachments, a situation for which plastic modules are ideally suited, as Anderson’ has shown. The method of dispensing some modules from a spool, rather than using individual precut lengths, produced less waste and prevented mixing of the modules. The form of the Elast-0 Chain, a series of connected loops without intervening straight lengths of material, may allow the clinician more versatility in choosing initial forces, a trait shared by the CK Alastik module and the middle section of the Power links. All modules sustained the testing manipulation without breaking, although, of course, no forces of occlusion were operating. This has been substantiated clinically in that breakage of plastic modules in actual use is not frequent. One factor which could contribute to an increase in loss of the module is the decreased force magnitude (approximately one fourth of the initial force) noted a.fter several weeks in place. This may increase the chance of displacement from the bracket by occlusal forces and thus “failure” of the module. Summary and conclusions
A framework was constructed on which 120 plastic modules could be simultaneously stretched to predetermined distances between edgewise brackets. Using calibrated Carpo gauges, measurements of force exerted by the modules while stretched on the framework were made by two independent observers at the time of attachment to the frame, and after 10 minutes, 1 hour, 24 hours, and weeklp to 6 weeks. Measurements were made with the modules held at a constant stretch distance and with interbracket distances decreased at rates of 0.25 and 0.50 mm. per week to simulate tooth movement. A total of 540 plastic modules representing three manufacturers were tested. Modules were compared and contrasted to determine the effects of time, composition, initial force, and rate of closure on the force decay curve. Analysis of the data justifies the following conclusions : 1. All modules lost considerable force with time, with the largest loss (approximately 50 per cent) occurring within the first 24 hours. The modules retained an average of 40 per cent of their initial force after 4 weeks at the zero rate of closure and almost exactly the same after 6 weeks, 2. Simulated tooth movement (decrease in stretch dimension across the adjustable framework) increased the rate of force loss. In situations in which a rate of tooth movement of 0.25 mm. per week is anticipated, the clinician can expect a plastic module to retain approximately one third the initial force after 1 month. When a rate of tooth movement of 0.50 mm. per week is predicted, the force remaining after 1 month will be approximately one fourth the initial force. 3. Percentage of force loss was similar for modules stretched to high and 10~ initial forces. Decay characteristics were not related to magnitude of initial force.
562 Hershey and Reynolds 4. Modules produced by three different manufacturers exhibited similar force decay curves and were judged to be clinically equivalent. 5. Variability among modules of the same type was found to be large for clear Alastiks, somewhat less for Elast-0 Chain, and much less for Power modules and gray Iilastiks. 6. Force decay data exhibited by plastic modules indicate that they can produce effective tooth-moving force throughout a 4- to 6-week period. REFERENCES
1. Andreason, G. F., and Bishara, S. E.: Comparison of Alastik chains with elastics involved with intra-arch molar to molar forces, Angle Orthod. 40: X1-158, 1970. 2. Andreasen, G. F., and Bishara, S. E.: A comparison of time related forces between plastic Alastiks and latex elastics, Angle Orthod. 40: 319-328, 1970. 3. Storey, E. E., and Smith, R.: Force in orthodontics and its relation to tooth movement, Aust. J. Dent. 56: 11-18, 1952. 4. Reitan, K.: Some factors determining the evaluation of forces in orthodontics, AM. J. ORTHOD. 43: 32-45,1957. 5. Begg, P. R.: Begg orthodontic theory and technique, Philadelphia, 1965, W. B. Saunders Company. 6. Hixon, E. H., Aasen, T. O., Arango, J., Clark, R. A., Klosterman, R., Miller, S. S., and Odom, W. M.: On force and tooth movement, AM. J. ORTHOD. 57: 467-489, 1970. 7. Hixon, E. H., Atikian, H., Callow, G. E., McDonald, H. W., and Tacy, R. J.: Optimal force, differential force, and anchorage, A M. J. ORTHOD. 55: 437-457, 1969. 8. Boester, C. H., and Johnston, L. E.: A clinical investigation of the concepts of differential and optimal force/in canine retraction, Angle Orthod. 44: 113-119, 1974. 9. Anderson, R. M.: A return to large nonresilient straight arch wires, AM. J. ORTHOD. 66: 9-39, 1974.
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