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Elastics and elastomerics

INDIAN DENTAL ACADEMY Leader in continuing dental education

Introduction • Polymers “Poly” + “merors”

• n(CH2=CH2) (-CH2-CH2-)n Classification of polymers • Homopolymers and copolymers • Natural and synthetic polymers – Natural rubber is derived from Latex which is a polymer of 2-methyl buta-1,3-diene (isoprene)

• Linear, branched chain, cross-linked polymers • Based on type of reaction– Addition polymers (polyethylene, PVC) – Condensation polymers (nylon)

• Based on inter-particle force – Mechanical properties of macromolecules like TS, toughness, elasticity etc. depend on intermolecular forces – van der Waal’s forces and hydrogen bonds • • • •

Elastomers Fibers (nylon) Thermoplastics ( plasticizers) Thermosetting plastics

Elastomers • These type of polymers are held by weakest attractive forces • Amorphous in nature and highly elastic • These polymeric chains are randomly coiled with few cross links • When stresses are applied these randomly coiled structures straighten out and the polymer gets stretched. When released the weak intermolecular forces help in regaining the lost structure.

• Elastomer is a general term that encompasses materials that, after substantial deformation, rapidly return to their original dimensions. • Natural rubber (Incan and Mayan civilizations) was the first known elastomer. – unfavorable temperature behavior and water absorption properties.

• Charles Goodyear(1839)-vulcanization of natural rubber • “Vulcan” Roman God of Fire

Natural Rubber • Rubber is obtained from latex which is a suspension of rubber particles which oozes out of the rubber tree • Polymer of 2-methyl buta-1,3-diene (isoprene)

• • • •

They are derived from a number of plants “Hevea Brasiliensis” Chemical structure is Cis-1,4, polyisoprene One chain contains of 500 units but this may vary from plant to plant, region to region and season to season

• Highly resilient • Absorb water and swell • Sensitive to ozonization and free radical ionization

Synthetic rubber • Synthesized polymerization of “-dienes” other than isoprene. • The polymerization is carried out in the presence of “Zeigler-Natta ” catalyst

• 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. If the primary bonds are broken, the elastic limit has been exceeded and permanent deformation occurs.

• Synthetic polymers are very sensitive to the effects of free radical generating systems – ozone and ultraviolet light.

• The exposure to free radicals results in a “decrease in the flexibility and tensile strength” of the polymer. • Antioxidants and anti-ozonates are added to retard these effects and extend their shelf life.

• Elastomeric chains were introduced to the dental profession in the 1960’s. • Unitek  Alastiks (1968) • They are used to generate light continuous forces for : • • • •

canine retraction, diastema closure, rotational correction, arch constriction.

• Advantages: • • • •

Inexpensive Relatively hygienic Easily applied Require little or no patient cooperation.

• Disadvantages: – When extended and exposed to oral environment • Absorb water and saliva • Permanently stain • suffer a breakdown of internal bonds that leads to permanent deformation.

– They experience a rapid loss of force due to • Stress relaxation resulting in a gradual loss of effectiveness. This loss of force makes it difficult to determine the actual force transmitted to the dentition.

• Elastomerics used in dentistry are made of polyurethanes and are formed by a stepreaction (condensation) polymerisation. • Molecular wt. of 500,000

• {-(NH)-(C=O)-O-}  urethane linkage • Manufactured by extending a polyester \polyether glycol or a ‘diol’ with di-isocyanide

• Two main methods of manufacturing – Injection molding technique – Die stamping

• Pigmenting? (Tg) • Tg increase makes the polymer more rigid and hence increase in the modulus of elasticity • High tensile strength and modulus of elasticity

General properties of Elastomers • Elongations of 100% and more can be obtained on rapid stretching with no major loss of energy • Maximum values of Tensile strength and stiffness are obtained after full stretching • On removal of tensile load it returns to its original structure rapidly • Full recovery takes place as long as the elastic limit is not reached

Elastomeric ligatures • Conventional ligatures • Advantages over steel ligature: – – – – –

Ease of application Patient friendly Aesthetic appearance Possible release of flourides Decreased force delivery (almost equal to the steel ligatures when stretched around a twin bracket)

• According to a study done by Taloumis et al measuring force decay it can be assumed that elastic ligatures may be used during initial leveling and alignment phase but not for rotational correction as force decay is rapid • Huge et al have reported that water acts as a plasticizer and weakens the intermolecular forces leading to chemical degradation

• Synergistic effect of loading and water immersion leads hydrolysis of ester or ether linkages in polyurethanes • Hence one cannot expect the maintenance full engagement of the arch wire within the slot • This led to the introduction of E-modules with increased Total Diameter: Internal Diameter ratio (greater wall thickness)  greater initial force delivery

• Therefore in cases where full engagement of slot is critical the clinician should: – Use steel ligatures – Reduce the time interval for change of E-modules – Using Fig of ‘8’ configuration

• Probable causes of change in structural and mechanical properties of E-ligatures: – Variation in pH and temperature – Accumulation of plaque (proteinacious film) – Calciumphosphate formation and possible calcification

Fluoride releasing Elastomerics • Elution of fluoride from elastomerics was studied in a different way compared to those done for other studies on other materials • The minimal release of fluoride inside the oral cavity is not as critical as the potential effect of this release has on their mechanical properties

• Storei et al have showed that fluoride releasing elastomerics were not able to deliver the required force for three weeks as conventional types • Hence caution should be exercised on the frequency of the patient revisit and the need for reactivation

Elastomeric Chains • The wide variation seen between E-chains and E-module although they are made from the same raw materials is because: – Manufacturing techniques – Additives incorporated – Morphological variation • Presence or absence of intermodular link • Ellipsoidal or circular links

• in vitro studies done to measure the rate of force decay of E-chains employed – Dry or wet testing states • Water • Simulated saliva • Fluoride media with varying temperatures

– Steady force application or release to simulate clinical conditions where tooth movement occurs – Acidic or neutral pH

• The general consensus showed – Steep decline in force ~ 40%-50% in 24hrs – Followed by a steady decline in the next 2-3weeks

• Ash and Nikolai have shown a greater decline in vivo than in vitro. • Stevenson and Kusy have employed a MaxwellWeichert model which fits the force degradation rate for elastomers that represents the two processes – Rapid loss of force initially – Slower rate that follows


• It has been postulated that since nearly 50% of the force is lost very early and then a steady decline is seen it would be logical to apply a heavier initial force which would eventually yield the desired force (3x-4x) • But this has been deleterious to the Periodontium as it may lead to early hyalinization and in effect would result in the same treatment time if not more.

• The use of elastomerics has significantly reduced over the years because of the advent of rare earth metals and super elastic coil NiTi’s that are capable of providing a more constant force over an extended period of time

In vivo aging phenomena • The effect of oral environment on the structure due to stress absorption is mainly on – Macromolecular chain orientation and elongation

• It may emanate on the surface as micro-tears that propagate from the margin to the centre – Fracture lines  perpendicular to the margins

• In open chains the residual strain correspond to the link extension pattern

• But in closed elastomeric modules the strain developed in the modular rings

• Eliades et al showed that after 24hrs of in vivo exposure, the surface of the modules were covered with non-continuous proteinacious film that was rich in alcohol groups and minimum Na & K mineralization • After 3weeks well-mineralized proteinacious films composed of Ca3(PO4)2 with carbonates and acid phosphate impurities were seen

• Probably due to the entropically favorable conformational changes that act as nuclei for microcrystalline growth (Na, K, Cl)


• One characteristic of elastomeric chains is the inability to deliver a continuous force level over an extended period of time.

• Andreasen and Bishara(1970) compared latex elastics and Unitek C-1 AlastiK modules (Unitek, Monrovia, Calif.) with respect to simulated intra-arch space closure and interarch forces. • They found that, after 24 hours of load, Alastiks suffered a 74% loss of force delivery capability, whereas latex elastics only lost 42%.

• Subsequent testing showed that after the first day, the force degradation declined in a relatively stable manner. These results led Andreasen and Bishara to recommend an initial extension of the chain of four times the desired force level to compensate for this inherent force loss.

• Bishara and Andreasen found a 50% force loss after the first day, with 40% of the original force remaining after 4 weeks. With simulated tooth movement of 0.25 mm and 0.5 mm per week, the amount of original force remaining after four weeks decreased to 25% and 33%, respectively.

• Their study also showed that consistent force was produced from chains manufactured by stamping process as compared with injection molded chains.

• In a study by Wong two manufacturer’s chains distracted to and maintained at 17mm while stored in water at 37° C were compared. • Greatest amount of force loss took place in the first 3 hours and initial force loss of 50% to 75% occurred in the first 24 hours. • Considerable variation in the initial force delivery of chains from different manufacturers was seen.

• Latex showed greatest amount of strength • Ormco power chains remained more constant in strength and resiliency than Unitek’s Alastik power chains • Ormco  342gms (12.0 oz.)  171 after 21 days • Unitek  641gms (22.5 oz.)  171 after 21 days

• Modulus of elasticity – Latex 22gms\mm – Ormco 46gms\mm – Unitek 89gms\mm

• Kovatch et al evaluated initial force values and force degradation of Unitek AlastiKs stretched to 30% of their original length at rates of 0.2”, 2.0”, and 20”\ min. • Rapidly extended chains showed greater initial force levels than those slowly stretched. • At 1 week the chains stretched at the slow rate exhibited less force decay. Therefore slowly stretching the modules to position is recommended.

• They also calculated a formula that predicted the force values of a chain at a given time because, after the first 5 seconds of force decay, the force decay rate followed a straight line on a log-log graph. • This formula is a parabolic equation of the form: load = constant x (time)-n where n is a fixed exponent for a given set of variables.

• In 1978 Ash and Nikolai compared force decay of chains extended and stored in air, water, and in vivo. Chains exposed to an in vivo environment exhibited 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.

• After 3 weeks, the chains stored in vivo had a greater force loss than those stored in water, but both still a force of 160gm was maintained. 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.

• De Genova et al(1985) investigated force degradation of chains from 3 companies that were maintained at a constant length and stored in artificial saliva. – Ormco Power Chain ll – Rocky Mountain Energy Chain – TP Elast-O Chain

• In the first study, one set of specimens was maintained at 37° C and another was thermal cycled between 15° C and 45° C. • Results thermal-cycled chains displayed significantly less force loss after 3 weeks. • Initially force level of 300 to 400 gm for all three specimens • Difference of only 7 – 10gms was seen between them at the end of the test

• A second study compared force decay rates of thermal-cycled chains held at a constant length to those subjected to simulated tooth movement of 0.25 mm per week. The chains subjected to tooth movement retained 9% to 13% less force than those held at a constant length.

• Rock et al tested commercially available elastomeric chains for initial force extension characteristics and reported that, regardless of the number of loops, the force values at 100% extension were constant for each individual material. • Hence it is recommended to extend chains to 50% to 75% of their original length to provide the desired force of approximately 300 gm.

• Killiany and Duplessis (1986) studied the force delivery and force decay characteristics of the Rocky Mountain “ Energy” chain (RMO, Denver, Colo.) compared with 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.

• Storie and von Fraunhofer investigated the initial force delivery and force degradation of a gray chain and a recently marketed fluoridereleasing chain from Ortho Arch. • Fluoride-releasing chain possessed a higher initial force level at 100% extension • Gray chain retained 38% of its initial force

• Fluoride-releasing chain delivered only 14% after 1 week in 37° C distilled water. After 3 weeks only 6% of the original force level was observed. • Evaluation of the flouride release capacity showed

– Single four-loop piece of chain  3 mg of fluoride during the 3-week testing period. – 50% of the total fluoride released(24hrs) – 90% had been leached out in 1 week of fluid immersion.

Colour coded chains • The initial force delivery and effects of fluid immersion of colored chains were studied (Baty and von Fraunhofer). They compared three colors of elastomeric chains with the standard gray chain from three different manufacturers, and the data indicated that the coloring of the chains had little effect on the initial force delivery levels of the chains.

• Force Degradation in Elastomeric Chains Stuart D. Josell, Jeffrey B. Leiss, and E. Dianne Rekow

• TP Orthodontics closed chain and Rocky Mountain Orthodontics closed and open chains maintained the highest percentage of initial force. • Dentaurum's closed and open chain decayed to the lowest percentage of initial force. • There were significant differences between closed and open chains in five of the six companies investigated when comparing 28day mean forces (RMO's closed and open chains were not different).


• 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.

• Pre-stretching was done to eliminate the force loss • Two modes of pre-stretching have been proposed

– Instantaneous pre-stretching (Sandrik, Chang & Young) – Extended-time technique of pre-stretching (Brantley et al)

Extended-time technique of prestretching Lexan plastic

0.880�~ 22.4mm

• Samples tested were – Unitek Alastik Chain – Ormco power chain

• 5 batches with each batch containing ten samples • Group A, B, C, D, E • Group A- control batch • Groups B & C- 370 distilled water • Groups E & F- air

Results • Three week pre-stretching  nearly constant forces on immediate usage • Pre-stretching in air not effective • Unitek vs Ormco • Force = constant x (time)-n

• Kuster et al (1986) compared the chains of two companies stored in air and in vivo. Chains stored in air were extended to 82% and 115% their 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. These results do not recommend the extending the chains by 50% to 75% of the original length to provide an optimal force level.

• Williams and von Fraunhofer ďƒ prestretching effects on force decay at 1 week, prestretching chains to 100% of their original length for 10 seconds before loading. Their results displayed a statistically significant difference in some prestretched chains compared with the controls. But this improvement was only 4% to 6% and clinically importantance is questinable.

• Prestretching of elastomeric chains has been suggested as a means of reducing the rapid force decay rate and providing for a more constant and consistent force delivery. • The increased residual force at 3 weeks is generally about 5%. Therefore, with a 50% to 75% reduction in the initial force, it is questionable whether this improvement is of any clinical benefit.

Conclusion • All chains delivered reduced force over time. • The shape of the degradation curve was constant for all types of chains and for chains from all suppliers. • The force dropped rapidly for the first 2 to 4 days then remained approximately constant.

• There was a difference between the amount of – Initial force delivered – Percentage of degradation from initial to final force delivered. Chains delivering the highest initial forces delivered higher forces after degradation.

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