Al-Azhar University
Faculty of Dental Medicine
Orthodontic Department

Orthodontic elastics
By

El-Hassanein Hussein El-Ha...
I. Introduction
II. Composition
III. Advantage
IV. Disadvantage
V. Force delivery and force degradation
VI. Prestretching ...
XII. Uses and Clinical applications
1. Elastic ligatures
2. Elastomeric separators
3. Elastomeric module
4. Elastomeric ch...
Introduction
Materials used in an orthodontic office to apply forces to move teeth include archwire
loops, coil springs, l...
Natural rubber may be obtained from hundreds of different types of plants. The major
source, however, is the rubber tree (...
Rubber bands are also easier for a patient to remove and replace.10
The material is relatively compatible with the mucosa....
1. Chains
A) Initial force
Considerable variation in the initial force delivery of chains from different
manufacturers.5
A...
force values at 100% extension were constant for each individual material. They also noted
that all short filament chains,...
VI. Prestretching effects
1. Chains
Attempts to alleviate the large initial force degradation and improve the constancy of...
Oppenheim advocated the use of intermaxillary rubber dam elastics to be worn at night
only.6
In intermaxillary anchorage, ...
In other words, if an elastic is stretched to apply 100 g of force on a maxillary canine
bracket, the effective distal for...
While Baty et al., stated that In general, the colored chains of a particular manufacturer
behaved similarly to the gray c...
3. Air
Exposure of latex to air was found to cause a loss of force.5
4. Ozone
The most significant limitation of natural l...
and 45° C. They reported that the thermal-cycled chains displayed significantly less force
loss after 3 weeks. Starting wi...
11. Oral cavity
Ash and Nikolai,30 compared force decay of chains extended and stored in air, water,
and in vivo. They rep...
The approximate force generated when stretched dry, within the elastic limit, was 22
grams per millimeter for 3/16" heavy ...
On the other hand, stainless steel ligatures can be tied either too tight or too loose,
depending on the technique and nee...
b) Safe-T-Separators
These Safe-T-Separators have knobs on either side of each ring. These
extend beyond the inter-proxima...
4. Elastomeric chains
In orthodontics, polyurethane elastomeric chains are used extensively as tooth-moving
mechanisms.13
...
Fig.

3

Chain

interproximal

passed
contacts

through
before

both
Fig. 4 Labial button can be added to

being

prevent ...
To rotate a tooth distolingually, tie an elastomeric ligature in a figure-8 to the distal
wing of the bracket (Figs. A). A...
8. Midline discrepancies
A relatively common problem at the finishing stage of treatment is a
discrepancy in the midlines ...
canine. These techniques introduce significant side effects, such as tipping of the adjacent
teeth if the main archwire is...
11. Deep bite
In the growing patients where a clockwise rotation of the mandible is desirable,
extrusion of molars may be ...
Distal elastic has been added to change the direction
of force so that intrusion occurs parallel to the long axis
of the i...
14. Molar Correction with Inter-arch Elastics
Without extraction, Class II elastics produce molar correction largely by me...
XIII. Infection (Contamination) control
a) Gloves
Despite the most careful precautions, a certain percentage of gloves wil...
d)Avoiding Cross-Contamination of Elastomeric Ligatures
Cross-contamination in handling elastomeric ligatures is a serious...
9. Ferriter J, Meyers C, Lorton L. The effect of hydrogen ion concentration on the force
degradation rate of orthodontic p...
25. Storie DV, Regennitter F, von Fraunhofer JA. Characteristics of a fluoride-releasing
elastomeric chain. Angle Orthod.1...
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Elastics in orthodontics

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using for elastics in orthodontics clinic, uso de elasticos en clinica diaria de ortodoncia

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Elastics in orthodontics

  1. 1. Al-Azhar University Faculty of Dental Medicine Orthodontic Department Orthodontic elastics By El-Hassanein Hussein El-Hassanein (B.D.S.) Demonstrator in Orthodontic Department Faculty of Dental Medicine Al-Azhar University (Boys-Cairo) Under supervision of Dr. Ashraf Atia Ali El-Bedwehi Assistant Professor of Orthodontics Faculty of Dental Medicine Al-Azhar University (Boys-Cairo) 1428 H –2007 G 1
  2. 2. I. Introduction II. Composition III. Advantage IV. Disadvantage V. Force delivery and force degradation VI. Prestretching effects VII. Analyses of elastic force VIII. Changes induced by intermaxillary elastic force IX. Factors affecting orthodontic elastics and chains 1. Coloring 2. Fluoride 3. Air 4. Ozone 5. Disinfection and sterilization 6. Thermal-cycle 7. pH 8. Mastication 9. Daily diet 10. Staining 11. Oral cavity 12. Water X. Latex elastics and synthetic elastomers have certain similarities and differences. XI. Elastomeric Modules versus Stainless Steel Ligatures 2
  3. 3. XII. Uses and Clinical applications 1. Elastic ligatures 2. Elastomeric separators 3. Elastomeric module 4. Elastomeric chains 5. Derotation 6. Rubber bands 7. Dental Crossbite 8. Midline discrepancies 9. Impacted Canines 10. Canine retraction 11. Deep bite 12. Intrusion 13. Open bite 14. Molar Correction with Inter-arch Elastics XIII. Infection (Contamination)control XIV. References 3
  4. 4. Introduction Materials used in an orthodontic office to apply forces to move teeth include archwire loops, coil springs, latex elastics and synthetic elastomers. This seminar will give us an idea about the last two materials. These two materials which are considered as an essential part of any orthodontic office are "rubber elastics" and "alastiks" which are plastic (synthetic) elastics. 1 Although there are multiple surveys of natural rubber latex (latex) orthodontic elastics and other synthetic elastomeric materials (i.e., elastic ligatures, elastomeric chain), there is limited research on synthetic (nonlatex) orthodontic elastics. The latex and nonlatex elastics were not similar in their behavior. Furthermore, force delivery over time varied with the manufacturer.2 The majority of the orthodontic elastics on the market are latex elastics. Since the early 1990s synthetic products have been offered on the market for latex-sensitive patients and are sold as nonlatex elastics. 2 Elastomer is a general term that encompasses materials that, after substantial deformation, rapidly return to their original dimensions.3 After placement, the elastic chains are generally changed at 3- to 6-week intervals. Once the chains are activated, they begin to permanently elongate, thus decreasing the force that they can exert on the teeth.4 II. Composition At one time the word "rubber" connotated natural or tree rubber which is a hydrocarbon polymer of isoprene units. The synthetic rubbers which have been developed possess different chemical structures, but resemble tree rubber in many physical properties. Both natural and synthetic rubbers are composed of long, thread-like molecules. The characteristic property of reversible extensibility results from the randomly coiled structure of long, folded polymer chains. Upon extension, these randomly coiled chains are elongated into an ordered structure consisting of linear chains except when cross-linked. This tendency to revert to the original disordered state upon removal of elongation stress accounts for die elastic behavior.5 1. Natural rubber Natural rubber, probably used by the ancient Incan and Mayan civilizations, was the first known elastomer. It had limited use because of its unfavorable temperature behavior and water absorption properties.3 Calvin S. Case (1893) discussed the use of intermaxillary elastics at the Columbia Dental Congress.6 However; Henry A. Baker7 is credited with originating the use of intermaxillary elastics. Angle (1902) described the technic before the New York Institute of Stomatology.6 4
  5. 5. Natural rubber may be obtained from hundreds of different types of plants. The major source, however, is the rubber tree (Hevea brasiliensis). The chemical structure of natural rubber is cis-1,4 polyisoprene which contains approximately 500 isoprene units in the average natural rubber polymer chain. This structure varies in molecular weight from plant to plant, region to region, and from season to season. The gray color found in some rubber bands denotes an inferior product with inclusions of impurities. Occasionally, the producer of the raw latex will attempt to bleach die latex resulting in die loss of some of its resilient properties. Latex elastics should be obtained from orthodontic supply houses that have adequate quality controls. To obtain optimal properties, the rubber latex which is used in die preparation of orthodontic elastics is a blend of carefully selected lots of the purest, high molecular weight latex.5 2. Synthetic rubber Synthetic rubber polymers, developed from petrochemicals in the 1920s, have a weak molecular attraction consisting of primary and secondary bonds. At rest, a random geometric pattern of folded linear molecular chains exists. On extension or distortion, these molecular chains unfold in an ordered linear fashion at the expense of the secondary bonds. Cross links of primary bonds are maintained at a few locations along the molecular chains. The release of the extension will allow for return to a passive configuration provided the distraction of the chains is not sufficient to cause rupture of these primary bonds.3 Synthetic polymers are very sensitive to the effects of free radical generating systems, notably, ozone and ultraviolet light. The exposure to free radicals results in a decrease in the flexibility and tensile strength of the polymer. Manufacturers have added antioxidants and antiozonates to retard these effects and extend the shelf life of elastomerics.5,8 Elastomeric chains were introduced to the dental profession in the 1960s and have become an integral part of many orthodontic practices.3 The composition of plastic elastomers such as Omolast, AlastiKs, Zing String, and Power Thread are proprietary secrets.5 Polyurethane rubber is a generic term given to the elastic polymers which contain the uremane linkage. They can be synthesized by extending a polyester or a polyether glycol or polyhydrocarbon diol with a di-isocyante. In either case the chosen diol that is used is up in the 500,000 molecular weight range. The basic repeating structure of this polymer leads to enormous varieties in physical properties for plastic and rubber. Depending upon the end use, a variety of means of processing and synthesizing may be employed. Polyurethane polyesters have been used for elastic ligatures. It has been found that they excel in strength and resistance to abrasion when compared with natural rubber. They tend to permanently distort, however, following long periods of time in the mouth and often lose their elastic properties.5 III. Advantage They are inexpensive, relatively hygienic and easily applied.3 Placement and removal of chain elastics requires little chair time for the clinician and minimal patient cooperation during application.9 5
  6. 6. Rubber bands are also easier for a patient to remove and replace.10 The material is relatively compatible with the mucosa.9 Rubber has the particularly valuable quality of a great elastic range, so that the extreme stretching produced when a patient opens the mouth while wearing rubber bands can be tolerated without destroying the appliance. 10 The major useful property of natural latex rubber is its resiliency. This property makes it useful intraorally for the application of tractive forces in the ranges up to 6 or 8 ounces. Greater forces would result in a large increase in the cross-sectional area of the rubber and would be difficult to be placed in orthodontic attachments. High quality latex more or less retains its resilience in water and under optimal conditions displays a minimal force decay.5 These factors have contributed to a high degree of professional acceptance of elastics in orthodontics. IV. Disadvantage Elastomeric chains, however, are not without their disadvantages. Gum rubber, which is used to make the rubber bands commonly used in households and offices, begins to deteriorate in the mouth within a couple of hours, and much of its elasticity is lost in 12 to 24 hours. Although orthodontic elastics once were made from this material, they have been largely superseded by latex elastics, which have a useful performance life 4 to 6 rimes as long. In contemporary orthodontics, only latex rubber elastics should be used.10 When extended and exposed to an oral environment, they absorb water and saliva, permanently stain, and suffer a breakdown of internal bonds that leads to permanent deformation.11 They also experience a rapid loss of force due to stress relaxation, resulting in a gradual loss of effectiveness.11, 12 This loss of force makes it difficult for orthodontists to determine the actual force transmitted to the dentition.3 V. Force delivery and force degradation Wong stated that Latex elastics are a source of continuous orthodontic forces.5 Baty et al., concluded that one characteristic of elastomeric chains is the inability to deliver a continuous force level over an extended period of time.3 While Profitt stated that it simply must be kept in mind that when elastomers are used, the force decay rapidly, and so can be characterized better as interrupted rather than continuous.10 6
  7. 7. 1. Chains A) Initial force Considerable variation in the initial force delivery of chains from different manufacturers.5 An initial force loss (Force decay) of 50% to 75% occurred in the first 24 hours.5,11 The greatest amount of force loss took place in the first 3 hours.5 After 24 hours of load, Alastiks suffered a 74% loss of force delivery capability, whereas latex elastics only lost 42%. After the first day, the force degradation declined in a relatively stable manner.12 Hershey and Reynolds,13 compared chains from three different companies. Their results showed no significant differences in the force degradation behavior of the chains, but there were substantial differences in the initial force delivery of the chains. The authors concluded that a force gauge should be used in a clinical setting to determine initial loads of the chains. B) Filament shape of chains Killiany and Duplessis14 reported on the force delivery and force decay characteristics of the Rocky Mountain “ Energy” chain (RMO) compared with those of a short loop chain from American Orthodontics. The initial force levels (330 gm) of the new “Energy” chain at 100% extension were lower than those of the short loop chain (375 gm). After 4 weeks of storage in a simulated oral environment, the “Energy” chain retained 66% of its initial force, whereas the short loop chain possessed only 33% of its original force. De Genova et al.15 showed that the short filament chains generally provided higher initial force levels and retained a higher percentage of the remaining force than the long filament chains. The longer filament chains will deliver a lower initial force at the same extension and exhibit a greater rate of force decay under load than the closed loop chain.3 Three types of elastomeric chains. Rock et al.16 tested 13 commercially available elastomeric chains for initial force extension characteristics. They reported that, regardless of the number of loops, the 7
  8. 8. force values at 100% extension were constant for each individual material. They also noted that all short filament chains, with the exception of Unitek AlastiKs, produced higher initial force level at 100% extension, the initial forces being in the range of 403 to 625 gm. This led Rock et al.16 to recommend extending chains to 50% to 75% of their original length to provide the desired force of approximately 300 gm. In general, the most force decay occurs at the first hour, and the greater the initial force, the greater the force decay of all chains.17 2. Elastics Stretching a rubber elastic of 7 mm. diameter to 20 mm. exerts a pressure as high as 168 grams. A 30 mm. stretching gives 243 grams and 40 mm. 350 grams.6 Most of the relaxation was shown to occur within the first 3–5 hours after extension, regardless of size, manufacturer, or force level of the elastics.18 In a study to evaluate latex elastic band fatigability, of three band sizes most commonly used for intermaxillary mechanics, both in-vivo and in-vitro over a period of 21 days. initial gram pulls were 320, 220 and 160 grams for elastic band sizes 3/16", 4/16" and 5/l6" successively, when stretched 30 mm., and 350, 260 and 190 grams when stretching the same bands 35 mm. successively.19 Both samples showed initial force drop at the end of the first day. The average drop was more in the vivo sample group, it was about 31% to 53% compared to the 26% to 47% force loss of the in vitro sample group. The second force drop was reported by the end of the 7th day (37% to 61%) for the in vivo sample. This percentage loss was more or less constant till the end of the 14th day, and then followed by third force drop by the end of the l5th day (58% to 63%), and fourth force drop at the end of the 21 days of the study ranging between 65% to 75 %. These findings were different from those obtained from the in vitro sample group in that; the second force drop at the end of the7th day was equal to 36% to 53%, and the third force drop (40% to 58%) was reported at the end of the 14th day, and then maintained its level till the end of the 21 days period.19 Force analysis, indicated that, residual remaining force after 14 days of intermaxillary mechanics, and is not of sufficient magnitude to produce effective tooth movement. 19 So for practical and clinical purposes, and to maintain the initiation of orthodontic tooth movement in the desired direction by intermaxillary mechanics, elastic bands must be changed at a maximum of two weeks interval.19 The distal driving force component is about 97.5% and 75% of the elastic force; with the mouth closed and widely opened respectively this factor must be considered during force analysis. 19 8
  9. 9. VI. Prestretching effects 1. Chains Attempts to alleviate the large initial force degradation and improve the constancy of force delivery have led several investigators to look at the effects of prestretching the elastomeric chains before placement. Andreasen and Bishara recommend an initial extension of the chain of four times the desired force level to compensate for inherent force loss.12 Young and Sandrik8 conclude that extending chains three to four times the desired force would result in permanent deformation of the chain and subsequent reduction in the desired force level. And they recommended initial extension of the chains by 50% to 75% of the original length to provide an optimal force level. Wong,5 recommended prestretching the elastic chains a third of their original length to prestress the molecular polymeric bonds and improve the strength. Some products may require an extension of 100% to generate force levels of 300 gm, whereas others extended by this amount would provide excessive force levels.3 Kuster et al.,20 compared the chains of two companies stored in air and in vivo. Chains stored in air were extended to 82% and 115% of the original length and, after 4 weeks, had maintained 70% to 75% of their initial force level. Chains placed in vivo at approximately 100% extension retained 43% to 52% of their initial force level after 4 weeks. At 100% extension, the force levels of the two chains were 315 gm and 279 gm, respectively. Kovatch et al.21 reported that rapidly extended chains showed greater initial force levels than those slowly stretched. At 1 week, however, the chains stretched at the slow rate exhibited less force decay. Therefore they recommended slowly stretching the modules to position 2. Elastics The empirical rule of “3” indicating that the reported force level is achieved on extending the elastic three times its diameter, does not apply to all cases and shows remarkable variation, ranging from 2.7 to five.18 VII. Analyses of elastic force Intramaxillary elastics Intramaxillary elastics are used between two points in the same dental arch.6 (e.g., class I elastics from canine to first permanent molar) Intermaxillary elastics Intermaxillary elastics are used between the maxillary and the mandibular dental arches.6 9
  10. 10. Oppenheim advocated the use of intermaxillary rubber dam elastics to be worn at night only.6 In intermaxillary anchorage, elastics are used to retract the maxillary arch or to bring the mandibular arch forward when it is in lingual collapse. Elastics also may be used vertically to bring teeth in opposing arches into occlusion.6 Most of the fixed and some removable appliances rely on elastic force to varying extents. When intermaxillary elastics are used during the growth period, there is a greater tendency for the anchor teeth to move labially.6 Bien analyzed elastic force under various conditions. He found intermaxillary elastic strength of 4 oz. when the mouth is closed shows a distal driving force of 3.9 oz. or a loss of 2.5 per cent. When the mouth is open the distal force is 3 oz. or a loss of 25 per cent. When the headgear is used with the elastics parallel with the maxillary arch, the driving force is 4 oz. and shows no loss.6 FIG. FIG.. . intermaxillary intermaxillary elastics; mouth closed. elastics; mouth open. Bien found an upward displacement force on the mandibular molars of 2.6 oz. when the mouth is open and an intermaxillary elastic of 4-oz. tension is used. This force is present even under optimum anchorage in lingual appliances. When the mouth is closed there is only 0.9 oz. upward displacement of the mandibular molars. .6 When the headgear is used as indicated there is no upward or rotational force on the mandibular 1st molars. From a practical standpoint there is little difference between the force with the mouth closed and when the headgear is used.6 There is a rotational force at the apices of the maxillary and the mandibular molar roots when intermaxillary elastics are used. Rotational force can be minimized by using light elastics. When 4-oz. force elastics are used the rotational force in the mandibular molars is 5.4 oz. when the mouth is closed and 4.1 oz. against the mandibular 1st molar when the mouth is open.6 Class II intermaxillary traction from the distal of the mandibular first molar to the mesial of the maxillary canine results in a horizontal vector of 96% and a vertical vector of 27%.22 10
  11. 11. In other words, if an elastic is stretched to apply 100 g of force on a maxillary canine bracket, the effective distal force will be only 96 g. In situations where four premolar have been extracted and the space closed, the same Class II elastic, now stretch over a shorter span, has a horizontal vector of 93% but a significantly increased vertical vector of 37%. On the other hand, Class II elastics that are stretched over a long distance from the distal of the mandibular second molar to the mesial of the maxillary canine have a horizontal effect of 98% and a smaller vertical vector of 20%. There fore, to get the greatest distal effect of Class II elastics with the smaller collateral vertical vector, elastics should be applied from the farthest distal point, i.e., the distal of the mandibular second molar. The same concepts apply to Class III elastics, only in the opposite direction.22 VIII. Changes induced by intermaxillary elastic force are as follows: The changes effect upon the dentition during intermaxillary traction is essentially dentoalveolar in nature. Among the secondary effects of traction on the dentition is the emerging of the maxillary incisors and mandibular molars, causing tipping of the occlusal plane downward anteriorly, thereby increasing the vertical dimension of the lower face by opening the mandible. In addition, such forces applied over an extended period may cause the lower incisors to tip labially.23 Pressure is exerted on the incisors in a vertical direction, bringing them into supraclusion, or accentuating supraclusion already present.6 Tilting of the anchor teeth may occur.6 Many of the secondary effects of intermaxillary traction may be reduced or controlled to some extent by reducing the obliquity of the angle of the intermaxillary elastic force, by applying high-pull headgear to maxillary incisors and/or molars, or by adding counteractive torque forces to the brackets attached to the teeth. 23 The effects of intermaxillary traction on the teeth in anteroposterior jaw dysplasias depend on several factors including severity of the dysplasia, duration and direction of force application, age, and individual growth pattern of the subject. 23 IX. Factors affecting orthodontic elastics and chains Several investigators have attempted to determine the consequences of changing the environment with regard to initial force delivery and force decay of elastomeric chains. These attempts have looked at conditions that could exist within the oral cavity or might be used in sterilization of the chains before placement in the mouth. 1. Color The remaining force and the percentage of remaining force at each interval, except initial, of the Rocky Mountain energy chain were greater than that of the American Orthodontic chain including transparent and grey.17 When comparing the American Orthodontic elastomeric chains, the remaining forces and the percentage of remaining force of the transparent chains were greater than that of the grey chains.17 11
  12. 12. While Baty et al., stated that In general, the colored chains of a particular manufacturer behaved similarly to the gray chain of that manufacturer, the exception to this being the Ormco purple and green chains.24 2. Fluoride The physical properties and fluoride releasing capabilities of a recently introduced fluoride-containing elastomeric chain (Fluor-I-Chain) have been evaluated and compared to those of a standard gray elastomeric chain. Fluor-I-chain was found to require increased distraction to achieve 150g and 300g forces when immersed in liquid media. Gray chain only required an increase in displacement for a 150g and 300g force when immersed in artificial saliva (Oralube). The increase in displacement of gray chain is not felt to be clinically relevant, but the required increase in displacement of Fluor-I-Chain could be as much as one-third of its original length. 25 The initial force levels of Fluor-I-Chain and gray chain when stretched by 100% their original length were 316g and 280g respectively. After 1 week, Fluor-I-Chain’s force level had degraded to 43 g or 14% of its original force. This force level would not be adequate to retract a canine. Gray chain at one week had a force level of 107 g which remained fairly constant through the remaining 2 weeks. 25 When maintained at a constant distraction of 100%, Fluor-I-Chain was unable to deliver a force within the optimal range for tooth movement after one week. In contrast, the force delivery level of the standard gray chain remained adequate over the entire three-week test period. 25 Water and artificial saliva (Oralube) significantly affected Fluor-I-Chain’s initial force displacement beginning at 4 hours of immersion. 25 Approximately 3 mg of fluoride is released from the Fluor-I-Chain (a single four-loop piece of chain) over a 3 week test period, 50% of which is leached out in the first 24 hours and 90% within the first week. 25 Fluor-I-Chain does release fluoride over a 3 week period at a level that could have the potential to inhibit demineralization and promote remineralization.25 Elastomeric chains exhibit good elastic behavior when distracted to an initial force of less than 300g. When forces exceeded 300g, permanent deformation occurred and the force delivery was less predictable. Exposure to artificial saliva and topical fluoride affected the elastic properties of the elastomeric chains and increased the distraction required to deliver both the 150g and 300g force. The increase in distraction for a force of 150g, however, was relatively small and probably insignificant in the clinical setting. The distraction required to produce 300g was significantly larger and appeared to be clinically significant. Prestretching the elastomeric chains by 100% of their initial length was not found to be advantageous in terms of the load relaxation behavior. There was less load relaxation found in chains that were immersed in distilled water and Acidulated Phosphate Fluoride than in chains exposed only to air. The observed relaxation may be a problem in the clinical situation only when the module is required to deliver high forces, >300g, or if there is prolonged exposure to fluoride media. Pre-stretching appeared to have an overall beneficial effect only for Ormco Generation II power chain.26 12
  13. 13. 3. Air Exposure of latex to air was found to cause a loss of force.5 4. Ozone The most significant limitation of natural latex is its enormous sensitivity to the effects of ozone or other free radical generating systems such as sunlight or ultraviolet light that produces cracks. The ozone breaks down the unsaturated double bonds at the molecular level as the water molecule is absorbed. This weakens the latex polymer chain. The swelling and staining is due to the filling of the voids in the rubber matrix by fluids and bacteria debris. In clinical use the latex elastics are replaced before this stage is reached. Antiozone and antioxidant agents are added at the time of manufacture of the latex tubing. However, when it is chopped into individual latex bands, the surface area is increased and ozone can diffuse more rapidly into the bands. This sharply limits the shelf life of the latex elastics. Out-of-date elastics may break after a few elongation-relaxation cycles. Usually, this type of break is due to crack-propagation which occurs somewhere in the elastics due to ozone effects. An additional manifestation of ozone attacks on latex bands is the reduced force values which may be seen after a short period of two or three months after manufacture. Commonly, one may see elastics, which are rated at four ounces when manufactured, show force value of 2.5 to 3 ounces after a few months of storage. The tensile strength, therefore, is unpredictable and is more critical in higher force range applications.5 5. Disinfection and sterilization Disinfection (short-term exposure) and/or sterilization (long-term exposure) may become a common procedure for elastomeric chains. The effect of two proprietary alkaline gluteraldehyde solutions on the strength (failure load) and the required displacement or stretching to achieve a force of 500g was studied for six types of elastomeric chains. 27 Exposure to gluteraldehyde solution affected the strength and the distention required to deliver a force of 500g of certain elastomeric chains. However, the resultant changes were relatively small and are probably insignificant in the clinical setting. 27 The use of alkaline gluteraldehyde solutions for this purpose may have no deleterious effects when the clinical use of the elastomeric chains is considered. In particular, the displacement required to produce a 500g force increased by 5mm, at most, following longterm exposure to gluteraldehyde solution; the force to break the chains only decreased by approximately 20 to 100g under the same conditions. Discoloration of some chains occurred in the sterilizing solutions but this change appeared to have no effect on the chain properties. The findings suggest that cold disinfection and/or sterilization via gluteraldehyde solutions (Sporicidin and Cidex-7) may be an effective and convenient approach for elastomeric chains.27 6. Thermal-cycle De Genova et al.,15 investigated force degradation of chains from three companies that were maintained at a constant length and stored in artificial saliva. In the first study, one set of specimens was maintained at 37° C and another was thermal cycled between 15° C 13
  14. 14. and 45° C. They reported that the thermal-cycled chains displayed significantly less force loss after 3 weeks. Starting with an initial force level of 300 to 400 gm, this difference, however, was only 7 to 10 gm. and reported some minor improvement in the retention of force after 3 weeks. 7. pH Oral pH almost certainly has a significant influence on the decay rate of orthodontic polyurethane chain elastics.9 All the test products yielded a significantly greater force-decay rate in the basic (pH 7.26) solution than in the acidic (pH 4.95) solution over 4 weeks.9 A hypothesis is presented that the decay rate of orthodontic polyurethane chain elastics is inversely proportional to the oral pH, with a corollary that basic pH levels (above neutral) are most hostile to polyurethane chain elastics, thus increasing their force-decay rates.9 Clinically, it would seem that an oral pH lower than 7.26 would retard the force-decay rate of the chain elastics. Before this study, we did not expect to find that decreased pH associated with dental plaque in the presence of carbohydrates may actually decrease the force-decay rate of the chain elastics and thus potentially enhance their effectiveness. The clinician does not have an ability to control the patient's oral environment, but he or she must have knowledge of the individual elements within it that can affect the mechanics plan selected.9 8. Mastication The forces of mastication and the intraoral environment cause natural rubber to break down by formation of knotty tearing mechanisms.5 9. Daily diet At the various levels of simulated daily dietary challenge/patient compliance, the latex elastics maintain their applied force over a day of wear.28 No differences were found between daily diet/patient compliance levels. Except for band breakage or recommended reasons of oral hygiene, beyond the once-per-day experience, there may be no need to change elastics during the day.28 10. Staining The foods appeared to fall into three distinct categories29 A. No staining (Coca-Cola even after 72 hours and, presumably, most colorless foodstuffs) B. Gradual staining (chocolate drink, Lea & Perrins sauce, red wine, tomato ketchup) C. Rapid staining (coffee produced severe staining after only six hours, tea) 14
  15. 15. 11. Oral cavity Ash and Nikolai,30 compared force decay of chains extended and stored in air, water, and in vivo. They reported that chains exposed to an in vivo environment exhibited significantly more force decay after 30 minutes than those kept in air. No difference was noted between the chains maintained in water and those in vivo until 1 week. However, after 3 weeks, the chains stored in vivo had a greater force loss than those stored in water, but both still maintained force levels of more than 160 gm. They postulated that the effects of mastication, oral hygiene, salivary enzymes, and temperature variations within the mouth influenced the degradation rates of in vivo chains. Although they stated that their initial extension was too much. In the oral cavity, elastics absorb water and saliva, which cause a breakdown of the internal bonds and permanent deformation of the material. In addition, the elastics swell and stain due to the filling of the voids in the rubber matrix by fluids and bacterial debris. These lead to a loss in force delivered to the tooth. To minimize such side effects, orthodontists recommend that patients change their elastics twice daily, but this requires faithful patient adherence. Elastomeric chains gained in popularity because they were more in the control of the clinician. They too experience a rapid loss of force as a result of stress relaxation.22 12. Water Huget et al.,31 concluded that the load decay associated with elastomeric chains for 1 and 7 days of water storage may be the result of water sorption and the concurrent formation of hydrogen bonds between the water molecules and macromolecules of the elastomers. A gas chromatography test was performed on the water in the storage vials to establish the presence of any organic materials leached from the chains, organic material did not appear in the storage media until the fourteenth day of immersion. X. Latex elastics and synthetic elastomers have certain similarities and differences. The latex elastics showed a greater amount of loss in strength than plastic elastomers when stretched over a 21 day period.5 There is a great variability, as much as 50%, in the tensile strength of the plastic materials taken from the same batch and stretched under the same conditions. 5 The force decay of synthetic elastomers, stretched over a specific length and time, exhibited a great loss in force. This loss could be as great as 73% during the first day. The decay of force continued at a slower rate during the rest of the 21 day period. 5 The Ormco Power Chain was more resilient than the Unitek AlastiK chain. The Unitek AlastiKs had more force and stretched less. 5 Unitek AlastiK C2 double links, when stretched 17 millimeters, had a higher initial force averaging 641 grams (22.5 ounces) than the Ormco Power Chain which averages 342 grams (12.0 ounces). In one day the force was reduced to 171 grams (6.0 ounces) for both materials. 5 15
  16. 16. The approximate force generated when stretched dry, within the elastic limit, was 22 grams per millimeter for 3/16" heavy latex elastics. The Unitek AlastiK C2 gave a force of 89 grams per millimeter, while the Ormco Power Chain had a value of 46 grams per millimeter. The modulus of elasticity of all of the materials was much lower after immersion in the water bath. 5 The force decay under constant force application to latex, elastic, polymer chains, and tied loops showed that the greatest amount of force decay occurred during the first three hours in the water bath. The forces remained relatively the same throughout the rest of the test period. 5 The elastic materials undergo permanent deformation in shape. The synthetic elastomers exhibited plastic deformation when the elastomers were stretched 17 millimeters for 21 days. In the dry condition the force decay was 63% for the Unitek chains and 42% for the Ormco Power Chain. 5 The synthetic elastomers should be prestretched before being placed in the mouth. The elastomers should be used within their resilient ranges. 5 Clinical treatment procedures should take into consideration the rapid initial force decay of elastic materials that occurs during the first day and the residual forces remaining. 5 American Orthodontics latex elastics (0.25 inch, 4.5 ox, 6.25 mm, 127.5 g) retained significantly more force over time than their nonlatex equivalents. 2 Cyclic testing (repeated stretching )of orthodontic elastics caused significantly more force loss than static testing but this effect was seen early in testing and did not change the rate of force decay after this. 2 Because of higher rates of force loss that continued throughout testing, it is more important that nonlatex elastics be changed at regular intervals not exceeding 6-8 hours. 2 Because of variability in force delivery, it is advisable for practitioners to test a sample of their elastics before using them or purchasing large quantities to ensure that the force levels are in the expected range. 2 Cyclic testing caused significantly more force loss and this difference occurred primarily within the first 30 minutes. For statically tested elastics the percentage of initial force remaining at 4, 8, and 24 hours was 87%, 85%, 83%, and 83%, 78%, 69% for latex and nonlatex elastics, respectively. For cyclically tested elastics the percentage of initial force remaining at 4, 8, and 24 hours was 77%, 76%, 75%, and 65%, 63%, 53% for latex and nonlatex elastics, respectively.2 XI. Elastomeric Modules versus Stainless Steel Ligatures The normal force exerted by the ligature has a significant influence in determining the frictional resistance developed within an orthodontic system. This force has been estimated to be between 50 and 300 g and up to 735 g.22 16
  17. 17. On the other hand, stainless steel ligatures can be tied either too tight or too loose, depending on the technique and needs of the clinician.22 Elastomeric ligatures tied in a figure 8 pattern were responsible for significantly greater mean static frictional forces than any other ligation technique. There were no significant differences between the frictional resistances offered by the conventionally tied elastomeric modules and stainless steel ligatures. Teflon-coated ligatures produced the lowest mean static frictional forces under both testing conditions.22 XII. Uses and Clinical applications 1. Elastic ligatures Elastic ligatures of light, medium and heavy thickness may be used for separating teeth before fitting bands, rotating teeth, space closure, tooth alignment, canine retraction and for bringing teeth into the line of occlusion. The elastic ligature is drawn through the interproximal space of the teeth with a fine ligature wire and tied under tension.6 Cotton thread 2. Elastomeric separators a) Elastomeric ring Elastomeric ring ("doughnuts"), which surround the Contact point and squeeze the teeth apart over a period of several days.10 From the patient's perspective, separating springs are the easiest to tolerate, both when they are being placed and removed, and as they separate the teeth, typically opening enough space for banding in approximately 1 week. These separators tend to come loose and may fall out as they accomplish their purpose, which are their main disadvantage and the reason for leaving them in place only a few days. Brass wire left in place for 5 to 7 days. Brass wire and elastomeric separators are more difficult to insert, but are usually retained well when they are around the contact, and so may be left in position for somewhat longer periods. Because elastomeric separators are radiolucent, a serious problem can arise if one is lost into the interproximal space. It is wise to use a brightly colored elastomeric material to make a displaced separator more visible, and these separators should not be left in place for more than 2 weeks.10 Separation with an elastomeric ring or "doughnut." A, The elastomeric ring is placed over the beaks of a special pliers; B, the ring is stretched, then one side is snapped through the contact; C, the separator in place; D, an alternative to the special .pliers is two loops of dental floss, placed so they can be used to stretch the ring. The dental floss is snapped through the contact; E, the doughnut is placed underneath the contact point, then; F, the doughnut is snapped into position. At that point, the dental floss is removed.10 17
  18. 18. b) Safe-T-Separators These Safe-T-Separators have knobs on either side of each ring. These extend beyond the inter-proximal area out past the gingiva. Their presence prevents this separator from submerging into the sub-gingival area. Available in two styles: Standard (purple) and Slim (blue). The Slim separator is thinner with less force for use in situations needing only minimal space opening. Each come on star shaped sticks with 80 separators each. Standard - Purple and Slim - Blue c) Dumbbel Separators White, individual rubber separators can be stretched and slipped between contacts for in-office separations. Contracting action of separators achieves ample separation for banding within minutes.32 d) Stick Separators e) Durasep Separators 3. Elastomeric module Elastomeric modules are used to ligate archwires to brackets, Mini modules 18 Ligature-forceps
  19. 19. 4. Elastomeric chains In orthodontics, polyurethane elastomeric chains are used extensively as tooth-moving mechanisms.13 Polyurethane chain elastics are commonly used in orthodontics for intra-arch tooth movement.9 They are used to generate light continuous forces for canine retraction, diastema closure, rotational correction, and arch constriction.15 5. Derotation a) Derotating a Tooth with a Lingual Rotation Tie The patient in the early or late mixed dentition who presents with a severely rotated tooth may not require immediate full treatment, but esthetic considerations usually dictate correction of the problem. Will describes a simple method of derotating a single tooth, utilizing a fixed lingual or palatal arch in conjunction with the "rotation tie" commonly used in lingual orthodontics.32 Solder a lingual or palatal arch to molar bands and cement it to the first permanent molars. Take care to position the arch in close proximity to the lingual aspect of the tooth to be derotated. The arch will thus act as a stop, preventing the tooth from moving lingually.32 Bond a button to the lingual surface of the rotated tooth in a location with respect to the height of the gingival margin, so that the gingiva will not be damaged when the rotation tie is placed.32 Slip one end of a length of elastic chain under the archwire (Fig. 1). Thread the end of the chain through the last link in the direction toward which the tooth is to be rotated. Pull tight, tying the chain to the archwire (Fig. 2).32 Bring the chain around the labial side of the tooth, passing through both interproximal contacts, and attach it to the button (Fig. 3). An additional bonded button or composite resin can be placed on the labial surface (Fig. 4) to prevent any incisal slippage of the chain, which would render the technique useless.32 Fig. 1 Length of elastic chain Fig. 2 End of chain threaded through slipped under archwire last link and pulled tight. 19
  20. 20. Fig. 3 Chain interproximal passed contacts through before both Fig. 4 Labial button can be added to being prevent incisal slippage of chain. attached to lingual button. Fig. 5 Case 1. After 16 weeks of derotation with rotation tie. Compared to fixed appliances, the rotation tie is a fast and easy way to correct rotations in the early or late mixed dentition. Chair time is minimal, both at initial and follow-up visits. Patient cooperation is not as critical as with removable appliances or elastics. From an esthetic standpoint, the appliance is virtually invisible.32 Fig. 6 Case 2. Elastic chain tied to palatal arch. b) Correction of Individual Tooth Rotations with Elastomeric Ligatures Elastomeric ligatures can be used to correct individual tooth rotations in conjunction with fixed appliances. I have found them just as effective and less costly than rotational wedges.33 20
  21. 21. To rotate a tooth distolingually, tie an elastomeric ligature in a figure-8 to the distal wing of the bracket (Figs. A). After placing the archwire, tie the mesial wing of the bracket to the archwire with a ligature wire or an elastic tie (Figs. B).33 c) Rotation Wedges( Elastic-Rotator) 6. Rubber bands Rubber bands were used from the beginning, in orthodontics to transmit force from the upper arch to the lower.10 7. Dental Crossbite Rarely is one tooth alone tipped. In most cases, its antagonist in the opposite arch is out of position also. Thus, the maxillary first molar may be tipped lingually and the mandibular first molar is tipped slightly buccally, so both teeth must be moved.34 Simple through-the-bite elastics are sometimes effective for molars in crossbite when both teeth are out of position and there is adequate space for them. If the problem is largely a matter of tipping a single tooth rather than reciprocal tipping of two teeth, an acrylic plate with auxiliary spring will serve well. Lingual archwires with recurved auxiliary springs also are satisfactory in some cases. On occasion, a labial archwire is the most efficient.34 21
  22. 22. 8. Midline discrepancies A relatively common problem at the finishing stage of treatment is a discrepancy in the midlines of the dental arches. This condition can result either from a preexisting midline discrepancy that was not completely resolved at an earlier stage of treatment or an asymmetric closure of spaces within the arch. Minor midline discrepancies at the finishing stage are no great problem, but it is quite difficult to correct large discrepancies after extraction spaces have been closed and occlusal relationships have been nearly established.10 As with any discrepancy at the finishing stage, it is important to establish as clearly as possible exactly where the discrepancy arises. Although coincident dental midlines are an important component of functional occlusion—all other things being equal, a midline discrepancy will be reflected in how the posterior teeth fit together—it is undesirable esthetically to displace the maxillary midline, bringing it around to meet a displaced mandibular midline. A correct maxillary midline is important for good facial esthetics, while a small displacement of the mandibular midline creates no esthetic difficulty.10 If a midline discrepancy results from a skeletal asymmetry, it may be impossible to correct it orthodonotically, and treatment decisions will have to be made in the light of camouflage vs. surgical correction. Fortunately, midline discrepancies in the finishing stage usually are not this severe and are caused only by lateral displacements of maxillary or mandibular teeth accompanied by a mild Class II or Class III relationship on one side.10 In this circumstance, the midline often can be corrected by using asymmetric Class II (or Class III) elastic force. As a general rule, it is more effective to use Class II or Class III elastics bilaterally with heavier force on one side than to place a unilateral elastic. However, if one side is totally corrected while the other is not, the patient usually tolerates a unilateral elastic reasonably well. It is also possible to combine a Class II or Class III elastic on one side with a diagonal elastic anteriorly, to bring the midlines together (Figure). Coordinated steps in the arch-wires also can be used to shift the teeth of one arch more than the other.10 An important consideration in dealing with midline discrepancies is the possibility of a mandibular shift contributing to the discrepancy. This can arise easily if a slight discrepancy in the transverse position of posterior teeth is present. For instance, a slightly narrow maxillary right posterior segment can lead to a shift of the mandible to the left on final closure, creating the midline discrepancy. The correction in this instance, obviously, must include some force system (usually careful coordination of the maxillary and mandibular archwires, perhaps reinforced by a posterior cross-elastic) to alter the transverse arch relationships.10 9. Impacted Canines a) Placing a Straight Wire in a High Buccal Canine or a Palatal Canine. Eruption of high buccal canines has been achieved using a number of different techniques. Chain elastics or elastic threads are used to deliver a single erupting force to the canine and are attached directly to a main archwire that bypasses the 22
  23. 23. canine. These techniques introduce significant side effects, such as tipping of the adjacent teeth if the main archwire is deflected, and provide only poor control of the movement of the canine. This appliance design also has a high load deflection rate due to the rapid decay of the force delivered by the elastic and the necessity of using a very rigid arch-wire to avoid deflection.22 The placement of a continuous wire into the bracket of a high buccal canine is made possible by incorporating loops into a stainless steel wire (0.016 stainless steel, for example) or by using a multistrand wire to decrease the load deflection rate. Superelastic wires are able to withstand great deflections without significant permanent deformation and can be placed directly into the bracket of a high, unerupted buccal canine. Careful analysis of the force system between the brackets of the canine, premolar, and lateral incisor shows that the eruption of the canine is achieved with significant and predictable side effects on the adjacent teeth. Tipping forward of the premolars and tipping back of the lateral incisors are observed simultaneously with their intrusion, creating an open bite in the canine region. Vertical elastic, often used concurrently to avoid the bite opening in the canine region, requires good patient cooperation.22 b) Palatally impacted canines Palatally impacted canines have traditionally been moved into the arch with elastic chains or elastic threads extending from the canine to the main buccal archwire. A palatally impacted canine needs movement in two directions. An eruptive force is necessary to bring the tooth to the level of the occlusal plane, and a buccal force to bring the tooth into alignment in the arch. The conventional use of elastics to arch wires is often accompanied by undesirable side effects on the adjacent teeth. In the horizontal plane, the buccal movement of the canine is accompanied by a lingual displacement of the adjacent teeth, mesial-in rotation of the premolar, and distal-in rotation of the lateral incisor. In the vertical plane, the eruption of a palatally impacted canine is associated with similar side effects as discussed for erupting high buccal canines. Canine bypasses associated with overlaid superelastic wires can be used successfully to erupt palatally placed canines.22 10. Canine retraction In Rocky Mountain clear chain, 40 mm stretch length was optimal for clinic use. The effective canine retraction may be no more than 3 weeks, meaning that it may be changed about 3 weeks during clinical usage.17 Sonis et al.,35 compared in vivo canine retraction by using two elastomeric chains and a nylon covered latex thread. All the materials were extended sufficiently to produce 350 to 400 gm of initial force. Patients were seen at 3-week intervals to measure the amount of space closure and to change the elastic modules. No significant difference in tooth movement was noted for any of the products. The elastomeric auxiliaries were found to be more hygienic and required less chair time to apply than did the elastic thread. 23
  24. 24. 11. Deep bite In the growing patients where a clockwise rotation of the mandible is desirable, extrusion of molars may be the treatment of choice to correct a deep bite. This can be achieved by means of a bite plate, leaving the molars free to erupt. This effect can also be achieved by means of a cervical headgear. The more the outer bow arms are angulated upward, the more extrusion can be expected. Moreover, the length of the outer bow arms as well as their inclination may define the amount of tipping of the upper first molar. Although Class II elastics may cause extrusion of the upper incisors, they also attempt to overerupt lower molars. The use of an anchorage bend in the upper jaw as well as in the lower jaw in combination with Class II elastics may cause overeruption of the molars and may help to correct a dental deep bite. Nanda warns against the use of a reverse curve of Spee when edgewise appliances are used: there is lack of control of this wire in the edgewise brackets, causing undesirable changes in the axial inclination of the buccal teeth and flaring of the incisors.22 Extrusion of molars might be fortified by means of elastics, which attempt to overerupt the molars in both the upper and lower jaws. To obtain this goal, the use of box elastics may also be helpful.36 "check elastic" in place. Forces on a lower molar produced by a Class II elastic hooked on the distal end of the arch wire (a) and a mesial hook (b). The moment arm, and hence the moment tending to tip the molar mesially, may be much greater with distal elastic attachment. Using long buccal tubes and angulating them mesiogingivally, distoocclusally are other factors important in maximizing this moment.36 12. Intrusion Many Class II patients have deep overbite, so one may take advantage of simultaneously intruding incisors while posterior teeth are tipped back. Figure shows a three-piece tip-back mechanism. A rigid anterior segment fits the four incisors that are to be intruded. A posterior tip-back spring produces a tip-back moment, tipping the first molar distally. Note that the spring is free to slide distally on the posterior extension of the anterior segment. Transseptal fibers or figure eight ties keep the bicuspids moving distally along with the molar. Since individual tipping of teeth is required, no archwire is placed in the buccal segment. By placing the intrusive hooks on the anterior segment either anteriorly or posteriorly, one can either flare the incisors or retract them simultaneously during the tip back of the posterior teeth. The same type of spring can be used to move the anterior teeth distally and simultaneously intrude them. The three-piece intrusion arch has intruded and retracted the upper incisors. During treatment, the intrusive force was placed along the long axis and through the center of resistance and later moved lingually to produce the lingual movement of the upper incisors.22 Three-piece intrusion arch used for distal movement of molars. A, Passive tip-back spring. B, Activated spring. The moment tips back the molar. The hook slides along the anterior segment. The reciprocal effect is the intrusion of incisors.22 24
  25. 25. Distal elastic has been added to change the direction of force so that intrusion occurs parallel to the long axis of the incisors. Placing the hook distal to the center of resistance of the anterior segment produces incisor retraction. A, Appliance; B, tracing showing incisor intrusion and retraction.22 13. Open bite Anterior Open Bite As with deep bite, it is important to analyze the source of the difficulty if an anterior open bite persists at the finishing stage of treatment. Only rarely is a persistent open bite caused by lack of eruption of the upper incisors, so elongating these teeth usually is undesirable. If the open bite results from excessive eruption of posterior teeth, whether from a poor growth pattern or improper use of interarch elastics, correcting it at the finishing stage can be extremely difficult.10 If no severe problems with the pattern of facial growth exist, however, a mild open bite at the finishing stage of treatment usually is due to an excessively level lower arch. This condition is managed best by elongating the lower but not the upper incisors, thereby creating a slight curve of Spee in the lower arch. Because of the stiffness of the rectangular archwires used for finishing, even with 18-slot edgewise it is futile to use vertical elastics without altering the form of the archwires to provide a curve of Spee in the lower arch. Moreover, it is preferable to replace a heavy rectangular lower archwire with a lighter round wire before using anterior vertical elastics.10 Anterior vertical elastics used with light arch-wires to close a mild anterior open bite at the end of the treatment A 16 mil round wire with a curve of Spec is used in the lower arch, where most of the tooth movement should occur. A full dimension rectangular wire often is preferred in the upper arch. For this patient, two small elastics are being used on the right and left sides. An alternative is to use single larger elastic in an anterior box configuration.10 The preferred approach is to place a light round wire (16 or 18 mil steel) in the lower arch, with a slight curve of Spee and any vertical steps necessary to correct marginal ridge discrepancies, while retaining a full-dimension rectangular archwire in the upper arch. Posterior marginal ridge discrepancies may also contribute to the open bite and should be eliminated with small vertical steps in the archwires. Light elastic force is then used to augment the action of the archwires, elongating the lower incisors to close the open bite. Elongating lower anterior teeth in this way, of course, is no substitute for controlling posterior vertical development. If carried to an extreme, this will produce an esthetically unacceptable relationship even if proper occlusion is achieved.10 As a general guideline, mildly excessive overbite at the finishing stage usually is treated best by slightly intruding the maxillary incisors, using an auxiliary depressing arch and segmenting the main archwire; but mild open bite at the end of treatment usually is treated best by elongating the lower but not the upper incisors. This is both more esthetic and more stable than elongating the upper incisors.10 25
  26. 26. 14. Molar Correction with Inter-arch Elastics Without extraction, Class II elastics produce molar correction largely by mesial movement of the mandibular arch, with only a small amount of distal positioning of the maxillary arch. When this pattern of tooth movement is desired, the amount of force varies with the amount of tipping allowed in the mandibular arch. With a wellfitting rectangular wire in the lower arch that is somewhat constricted posteriorly (to prevent rolling the lower molars facially and control the inclination of the lower incisors), approximately 300 gm per side is needed to displace one arch relative to the other. With a lighter round wire in the lower arch, not more than half that amount of force should be used. Incorporating the lower second molars in the appliance and attaching the elastics to a mesial hook on this tooth increase the anchorage and give a more horizontal direction of pull than hooking the elastic to the first molar.10 It is important to keep in mind that with or without extraction, Class II elastics produce not only anteroposterior and transverse effects but also a vertical force. This force elongates the mandibular molars and the maxillary incisors, rotating the occlusal plane up posteriorly and down anteriorly. If the molars extrude more than the ramus grows vertically, the mandible itself will be rotated downward. Class II elastics are therefore contraindicated in nongrowing patients who cannot tolerate some downward and backward rotation of the mandible. The rotation of the occlusal plane, in and of itself, facilitates the desired correction of the posterior occlusion, but even if elongation of the lower molars can be tolerated because of good growth, the corresponding extrusion of the maxillary incisors can be unsightly.10 Class II elastics, in short, may produce occlusal relationships that look good on dental casts but are less satisfactory when viewed from the perspective of skeletal relationships and facial esthetics.10 Because of their vertical effects, prolonged use of Class II elastics, particularly with heavy forces, is rarely indicated. Using Class II elastics for 3 or 4 months at the completion of treatment of a Class II patient to obtain good posterior interdigitation is often acceptable. Applying heavy Class II force for 9 to 12 months as the major method for correcting a Class II malocclusion is rarely good treatment.10 Class III elastics also have a significant extrusive component, tending to elongate the upper molars and the lower incisors. Elongating the molars enough to rotate the mandible downward and backward is disastrous in Class II treatment but, within limits, can help treatment of a Class III problem. If Class III elastics are used to assist in retracting mandibular incisors, high-pull headgear to the upper molars worn simultaneously with the elastics can control the amount of elongation of the upper molars. Elongation of the lower incisors, however, still can be anticipated.10 26
  27. 27. XIII. Infection (Contamination) control a) Gloves Despite the most careful precautions, a certain percentage of gloves will always fail. It also appears that, to date, latex is a more reliable material than vinyl for gloves that are worn to avoid the spread of infection. Furthermore, since it appears that a number of gloves will be punctured in use, it is important that other aspects of cross-infection control, such as hepatitis B vaccination for those placed at risk and effective sterilization and disinfection procedures, must be emphasized. Most punctures occurred in the left thumb and forefinger, despite the fact that all the clinicians were right-handed. A number of the gloves investigated had more than one puncture.37 b) Elastomeric Module Changer Although ceramic brackets have made orthodontic treatment acceptable to many adults, staining of clear elastomeric ligature modules by certain foods can still create an unesthetic appearance. a simple tool that allows the patient to change stained modules at home. (Always tie in rotated teeth with stainless steel ligatures.)38 The changer is made of .028" stainless steel wire with one end bent into a handle and the other bent into a hook. The patient is shown how to engage the hook under the elastomeric module and remove it. Each patient is given a length of closed elastomeric chain and shown how to place an individual link under the bracket wings and cut it from the chain with a scissor.38 c) Presectioning Elastomerics to Avoid Cross-Contamination One of the weakest links in the barrier control chain has been individual and chain elastomeric ligatures. If they are removed from trees, canes, or spools at chairside, the unused ligatures become contaminated. Ten minutes of “cold sterilization” serves only to disinfect, not to sterilize. Even the disinfection requires considerable rinsing and drying and may result in degradation of the material.39 Uses an 18-section tackle box with the top removed, cover the box at night and leave it uncovered during office hours.39 One section is filled with individual elastomeric ligatures. Other sections hold cut-up trees of two, four, six, eight, and 10 ligatures, and another contains presectioned chain sections of two, four, six, eight, 10, and 12. Various separators and rotation wedges are also in the box. Elastomerics are removed as needed with a sterilized tweezer, reducing the risk of contamination.39 Keep clear and gray ligatures in separate sections; the clear chains seem to discolor faster once they are removed from the spool. Try to use all the ligatures in a section before adding new ones.39 27
  28. 28. d)Avoiding Cross-Contamination of Elastomeric Ligatures Cross-contamination in handling elastomeric ligatures is a serious concern in the orthodontic office, since cold sterilization can damage the elastomeric material. In addition, most manufacturers produce strips with enough ligatures for both arches of a single patient. If an entire strip is not used at once, the remaining ligatures are either wasted or re-exposed to potential cross-contamination.40 Purchase clear tubing with a lumen of about 5/16" at a hardware store, cut it into 3-4" sections, and cold-sterilize the sections. Cut strips of elastomeric ligatures into sections about ½'' longer than the tubes. Insert the ligature sections into the tubes, leaving the ends of the ligature sticks protruding.40 During archwire placement, the operator contacts only the outside tubing while removing ligatures.40 The used section of ligatures is cut off and discarded after ligation.40 And the remaining section is inserted into a clean and sterilized tube. The used tube is placed in a cold-sterilizing solution.40 XIV. References 1. Bishara SE, Andreasen GF. A comparison of time related forces between plastic Alastiks and latex elastics. Angle Orthod.1970; 40:319–328. 2. Kersey ML, Glover KE, Heo G, Raboud D, Major PW. A comparison of dynamic and static testing of latex and nonlatex orthodontic elastics. Angle Orthod.2003; 73:181– 186. 3. Baty DL, Storie DJ, von Fraunhofer JA: Synthetic elastomeric chains: A literature review. Am J Orthod Dentofac Orthop.1994; 536-542. 4. Stevenson JS, Kusy RP. Force application and decay characteristics of untreated and treated polyurethane elastomeric chains. Angle Orthod.1994; 6: 455-467. 5. Wong A. Orthodontic elastic materials. Angle Orthod.1976; 46:196-205. 6. Salzmann, J.A.: Practice of orthodontics. J.B. Lippincott Company, U.S.A. 1966. 7. Baker H. Treatment of protruding and receding jaws by the use of intermaxillary elastics. Int Dent J 1904; 25:344-56. 8. Young J, Sandrik J. Influence of preloading on stress relaxation of orthodontic elastic polymers. Angle Orthod.1979; 49:104-9. 28
  29. 29. 9. Ferriter J, Meyers C, Lorton L. The effect of hydrogen ion concentration on the force degradation rate of orthodontic polyurethane chain elastics. Am J Orthod Dentofac Orthop.1990;98:404-10. 10. Profitt WR, Fields HW Jr, eds. Contemporary Orthodontics. 3rd ed. St Louis, Mo: Mosby Inc; 2000. 11. Andreasen GF, Bishara SE. Comparison of alastik chains and elastics involved with intra-arch molar to molar forces. Angle Orthod.1970;40:151-8. 12. Andreasen GF, Bishara SE. Relaxation of orthodontic elastomeric chains and modules in vitro and in vivo. Angle Orthod.1970; 40:319-28. 13. Hershey G, Reynolds W. The plastic module as an orthodontic tooth moving mechanism. Am J Orthod Dentofac Orthop.1975; 67: 554-662. 14. Killiany D, Duplessis J. Relaxation of elastomeric chains. J Clin Orthod 1985; 19:592-3. 15. De Genova DC, McInnes-Ledoux P, Weinberg R, Shaye R. Force degradation of orthodontic elastomeric chains— a product comparison study. Am J Orthod Dentofac Orthop.1985; 87:377-84. 16. Rock W, Wilson H, Fisher S. A laboratory investigation of orthodontic elastomeric chains. Br J Orthod 1985; 12:202-7. 17. Lu T, Wang W, Tarng T, Chen J: Force decay of elastomeric chain— A serial study.Part II. Am J Orthod Dentofac Orthop.1993 ; 373-7. 18. Christiana Gioka; Spiros Zinelis; Theodore Eliades; George Eliades Gioka C, Zinelis S, Eliades T, Eliades G. Orthodontic Latex Elastics:A Force Relaxation Study. Angle Orthod. 2006; 76: 475–9. 19. Abdel-Kader HM. Elastic band fatigability during intermaxillary mechanics. Egypt Orthod J.1987; 1, 59-71. 20. Kuster R, Ingervall B, Burgin W. Laboratory and intraoral test of the degradation of elastic chains. Eur J Orthod. 1986; 8:202-8. 21. Kovatch J, Lautenschlager D, Keller J. Load extension-time behavior of orthodontic alastiks. J Dent Res 1976; 55:783-6. 22. Nanda R; Biomechanics in Clinical Orthodontics. Philadelphia; W.R. Saunders company; 1996. 23. Jacobson A. Radiographic cephalometry from basics to videoimaging.chicago; Quintessence publishing co, inc.1995. 24. Baty DJ, Volz JE, and von Fraunhofer JA: Force delivery properties of colored elastomeric modules. Am J Orthod Dentofac Orthop.1994; 106:40-6. 29
  30. 30. 25. Storie DV, Regennitter F, von Fraunhofer JA. Characteristics of a fluoride-releasing elastomeric chain. Angle Orthod.1994; 3:199-210. 26. von Fraunhofer J. A.,Coffelt M-T. P., Orbell G.M.The effects of artificial saliva and topical fluoride treatments on the degradation of the elastic properties of orthodontic chains. Angle Orthod.1992; 4: 265-74. 27. Jefferies C, von Fraunhofer J. The effects of 2% alkaline gluteraldehyde solution on the elastic properties of elastomeric chain. Angle Orthod.1991; 61:25-30. 28. Beattie S: An In Vitro Study Simulating Effects of Daily Diet and Patient Elastic Band Change Compliance on Orthodontic Latex Elastics. Angle Orthod.2004; 74:234-9. 29. Lew K. Staining of Clear Elastomeric Modules from Certain Foods. J Clin Orthod. 1990; 1990: 472 –4. 30. Ash J, Nikolai R. Relaxation of orthodontic elastic chains and modules in vitro and in vivo. J Dent Res 1978; 57:685-90. 31. Huget E, Patrick K, Nunez L. Observations on the elastic behavior of a synthetic orthodontic elastomer. J Dent Res 1990; 69; 496-501. 32. Van Heerden PW, Roux JP. Derotating a Tooth with a Lingual Rotation Tie. J Clin Orthod. 1991:160 - 162 33. Abrahamin A. Technique Clinic Correction of Individual Tooth Rotations with Elastomeric Ligatures. J Clin Orthod. 1993:163 – 163. 34. Moyers RE. Handbook of Orthodontics. 4th ed., Chicago. Year Book Medical Publishers; 1988. 35. Sonis A, Van der Plas E, Gianelly A. A comparison of elastomeric auxiliaries versus elastic thread on premolar extraction site closure: an in vivo study. Am J Orthod Dentofac Orthop.1986; 89:73-7. 36. Hocevar RA. Orthodontic force systems: Technical refinements for increased efficiency. Am J Orthod Dentofac Orthop.1982 ;1-11. 37. Burke FJT, Lewis HG, Wilson NHF. The incidence of puncture in gloves worn during orthodontic clinical practice. Am J Orthod Dentofac Orthop.1991 ; 477-81. 38. Counihan DR. Elastomeric Module Changer for Patient Use. J Clin Orthod. 1996;575 - 575 39. Baron MA.Clinical Aid Presectioning Elastomerics to Avoid Cross-Contamination. J Clin Orthod. 1990; 746-746. 40. Schneeweiss DM.Clinical Aid Avoiding Cross-Contamination of Elastomeric Ligatures. J Clin Orthod. 1993;538 – 538. 30

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