Introduction
___.The examination of used orthodontic attachments demonstrates a preference for elastomeric ligatures. Their multitude, as well as the diversity exhibited by their properties, warrants their study inasmuch as to help the clinician select the best. Most studies, however, have used for this purpose sophisticated machines, often available only to universities. In addition, by the time of the publication of the related articles, their object may have been replaced or withdrawn from the market.
___.This preliminary study has attempted to find a simpler means to evaluate the elastomers’ force delivery in time. While widely used in quick, manual examination, the ultimate tensile strength and elongation cannot be used to provide an acceptable answer. Instead, a new and direct way to determine force delivery has been found.
Aging. The most suitable elastomer becomes useless in time. Military vehicles used during the Korean War, while left in good shape and properly stored, couldn’t be redeployed when the Vietnam War started: their tires were useless. The Challenger disaster, January 27, 1986, had as culprit a humble elastomeric O-ring that has lost its properties. Old rubber bands lose their elasticity within months; catheters become hard and brittle, while orthodontic ligatures may even decompose, Fig.1-3. Searching for reasons, scientists have discovered to their surprise that the main agent for such undesirable transformations is... ozone, an allotrope of oxygen. Having in its molecule three atoms of oxygen instead of two, ozone readily releases this excess. Despite that out of each 10 million air molecules, only three are ozone, the oxygen atom released behaves as a highly active free radical, attacking a variety of materials.
___.Common sources for elastomer degradation are also exposure to heat, ultra-violet light, water, chemicals, microorganisms and enzymes. The effects of aging can be seen in Fig. 4, where a standard chain is shown before and after their use.
___.Degradation can be either random, or due to the polymer’s depolymerization. The first type occurs when the carbon chain breaks at random points, leading to a significant decrease in molecular weight (i.e. the size of the macromolecule). The second is actually a release of monomeric units from the chain ends: being relatively small, this loss does not bring along a significant change in the polymer’s molecular weight. Both types of degradations may occur separately or in combination, varying with the nature of elastomer, type of cross links, nature of the antidegradant used and the conditions of service.
___.There are a variety of additives used to prolong the elastomers’ life. The most common are waxes and antioxidants. While the first generate an insulating, protective film over the polymer, the latter bonds the attacker (sacrificial agents). A third category blocks the effect of metal ions, well known oxidation catalysts. The blockers can be chelating agents, metal oxides or complexes capable of accepting the metal ions’ electrons. Polyurethane may degrade long before use, as the process may have started even from their manufacture, since temperatures of 150-200^oC are commonly used for their injection molding or extrusion. This interval is close to that of the decomposition of isocyanates. Traces of 4, 4’ diamino diphenyl methane (a toxic product resulting from the decomposition of the widely used MDI or 4, 4’ methylene diphenyl diisocyanate) have been found in both Pellethane 2363™ (Dow Chemical Co) and Biomer™ (Ethicon, now withdrawn), both elastomers designed for medical use¹. Polyurethane degradation products, as well as leaching additives, constitute a health problem.
___.It has been shown that polyurethane elastomers are sensitive to temperature (sterilization, hot beverages), pH variations, solvents (grease, oils) and light, in the last case becoming darker. Antioxidants and UV–absorbers may delay the decay, but cannot stop it. At the same time, in the oral environment and the presence of enzymes (pepsin, papayas, chemotrypsin, esterase) and microbes, especially fungi, polyurethane elastomers are subjected to hydrolysis that leads to a drastic depolymerization, the least stable being the polyether-Polyurethane,^1,2 . Another mechanism that leads to decay is the proximity to metals, the corrosion of which subjects the polymer to a metal ion-induced oxidation (MIO). Attempts to prevent the complex interaction metal-polyurethane-body have focused on using non-corrosive metals, coating these or incorporating antioxidants².
___.Not all elastomers age the same: in normal conditions, rubber degrades the fastest, followed by polyester-Polyurethane, nitride and styrene-butadiene-rubber (some 2 years). Butyl, neoprene and polyether- polyurethane elastomers may last 5 years, while silicone, fluorocarbon, polyacrylate and the polysulfide elastomers, some 10 years.
___.Perusing orthodontic sellers’ catalogues, it is interesting to note that only the worst-aging types of elastomeric materials are offered today: a “surgical” grade latex (a misnomer for rubber), and “polyurethane” (no further specification). To make the situation worse, no indications on their properties or expiration time are ever given.
Testing. The many studies having as purpose the evaluation of the orthodontic elastomers have used sophisticated, universal testing machines. These published prior to the last decade have been reviewed by Baty, Storie and v. Fraunhofer³, who also performed related tests⁴. In the last years, the degradation and deformation of elastomeric ligatures has been the object of articles by Taloumis, Smith, Hondrum and Lorton⁵, by Josell, Leiss and Rekow⁶, by v. Fraunhofer, Coffelt and Orbell⁷, by Stevenson and Kusy⁸ and by Lu and al.⁹
___.While the field has been intensely researched, it seems that the conclusion of Josell, Lewis and Rekow⁶ also applies today, despite the efforts and the sophisticated means used: “Unfortunately, there has been little consistency between tests, making comparison difficult”. As the need to evaluate elastomers persists, their quality being still far from perfect, the following simple tests may be of a real help.
Materials and methods
___.A variety of elastomeric ligatures, chains and modules purchased at the 102^nd Annual Meeting of AAO, May 2002, were tested for strength, both immediately and after eight months. The samples were from Adenta GmbH (Gutemberg-strasse 9, Gilching, 82205, Germany); Dynaflex (10246 Bach Blvd., St. Louis MO 63132, USA); Forestadent (2301 Weldon Pkwy, St. Louis MO 63146, USA); OSE Co. (7851 Airpark Rd #202, Gaithersburg, MD 20879, USA); Ormco Corp. & A+ (1717 W. Collins Ave., Orange, CA 92867, USA); Ortho-Arch Co. (1185 Tower Rd., Schaumburg, IL 60173); OrthoSource (13343 Sherman Way, N. Hollywood, CA 91605, USA); Tiger Orthodontics (4570 Progress Dr., Columbus, IN 47201, USA); H. Schein (5 Harbor Park Dr., Port Washington, NY 11050). The measurements were performed on average from ten to twenty samples. As the study’s purpose was not to provide precise data, but to suggest a simple ways of testing, the results are presented only as arithmetical averages (between arrows), also showing, however the extreme values (the space above and below these).
___.Three simple devices were used, all being easy to assemble in a laboratory, or easy to get. The only exceptions are the electric valves, open and closed, both from Bio-Chem Valve Inc., 85 Fulton Street, Boonton, NJ 07005.
Tensile strength. Each elastomeric ring was expanded till breakage between two hooks, each of these having a diameter of 2.30 mm. While one of these was hanging firm, the other was pulled down by a controllable force, i.e. water pouring from a funnel till the ring broke. The water flow (1 ml/sec) was stopped first manually, as the metal container fell about half an inch to land on foam, Fig. 5a. Later, the water flow was stopped automatically by the contact the fallen metal vessel established between two electrodes embedded in the foam support, Fig. 5b. The closed circuit acted on an electric valve (P/N 075P2NO12-026, 15 psi, 12V DC) that halted the water flow. The force needed to break the ring was obtained by measuring the volume of the water transferred in the container (1cc = 1g) to which the tare was added (container, additional weights and hook).
a. Accelerated aging. In an attempt to simulate the elastomers’ exposure to both mechanic and chemical stress, ten rings of each elastomeric ligatures were pulled over two steel bars (AISI 304) having a diameter of 6.25mm, and then immersed at room temperature for ten days in an aqueous solution of 1% lactic acid and 1% sodium chloride, Fig. 6. The above solution was used to enhance the release of heavy metal ions, and is recommended by ISO for testing stainless steel’s accelerated corrosion resistance¹⁰. The force/weight needed to break each type of elastomer was recorded before and after the test.
b. Shelf life. Twenty rings of each elastomeric ligature were tested as above for the force needed to break them, two weeks after these were purchased. The test was repeated after an eight months environment exposure in a non-air-conditioned room in S. Florida.
Maximum elongation. The strain (maximum elongation) was measured using a vise to which two pointy steel wires were brazed, Fig. 7. While the vise was tightly closed, an elastomeric ring was pulled over both wires. The vise was then slowly expanded till the ring broke. Knowing the wire diameter and the distance between the latter at breaking point, all measured with a caliper, the inner circumference of the ring was calculated.
Force delivery. The installation used (a variation of the one shown in Fig. 5) measured the force necessary to separate two hooks over which an elastomeric ring was stretched, Fig. 8. At an excessive force (tare + water volume/weight), the hooks cease to make contact, as the circumference of the ring is expanded above a certain length C. The latter has been taken to equal to the circumference of a ligature tying an arch wire inserted in a standard sized upper central bracket, see Fig.9. The lack of electrical contact closes a normally open electric valve (075 P2NC-12v-02S), interrupting the water flow. For a better reproducibility, the hooks were brazed with gold.
___.The installation, comprising an extra circuit signaling with an electric bulb the passage of a current, is shown in Fig. 10a, and its simplified sketch in Fig.10b.
Results
Tensile strength a. Accelerated aging. Measured after two weeks since purchased, the total weight (force) at which twenty rings of each of the purchased elastomers broke is shown in Fig.11. Used as reference, the diagram shows both the arithmetic average (between arrow points) and the extreme values (lower and upper spaces).The forces at which a certain elastomeric ring breaks after ten days exposure, i.e. stretched on steel bars and immersed into the ISO recommended solution, are shown in Fig.12.
Tensile strength b. Shelf life. After eight months of exposure in a non-air-conditioned room in S. Florida, twenty samples each of the elastomers tested in Fig.11 were tested again for their resistance to breakage. The arithmetic average of the forces needed and the percentage of decrease in strength are shown in Fig. 13.
Maximum elongation. The elastomeric rings’ inner circumference at breaking point is shown in Fig. 14.
Force delivery. The weight (force) at which the gold-brazed hooks cease to make contact (see Fig. 9 and 10a and b) is equivalent to the force with which the ligature presses the arch wire into the slot. Measurements performed on twenty various rings shows an array of values, Fig. 15, that can be used for comparison purposes.

Discussion
___.Any attempt to duplicate in vitro the behavior of elastomeric ligatures during the orthodontic treatment has to be taken with a grain of salt. A number of factors that all contribute to the degradation of polymers¹¹ (enzymes, microorganisms, fluorides, variation of pH, chewing, air and light), are very unlikely to be duplicated in vitro. As only seldom is the ligature seller also its manufacturer, accurate information is scarce, depriving the user from knowing at least how old these are, or their composition.
___.From the user’s point of view, what counts most are the forces delivered and their persistence. Such data, Fig. 15, determine the arch wire’s effectiveness, as well as friction. Indeed, as shown in Fig. 16^12,13, the latter is determined in part by the force with which the ligature presses over the inserted arch wire.
___.The attempts to predict an elastomer ring’s delivery force by evaluating the force at which it breaks or its maximum elongation do not show a reliable relationship, as shown in the diagrams presented in Fig. 17 and 18 where these values are represented together. The reason can be understood by examining Hooke’s stress/strain diagram, Fig. 19. In it, for discussion purposes, such incomplete data for two elastomeric rings, 1and 2, are sketched along with their respective resiliency domain, shaded (ability to recover the initial shape). Their coordinates are the values F₁ and F₂ which represent the breaking forces and E₁ and E₂ their maximum elongations. By using only these ultimate values, the shape of the related curves is ignored.
___.As a result, attempts to correlate the breaking force with the delivery force (see ratio above columns in Fig. 17) or the maximum elongation with the delivery force (Fig. 18) are doomed from start. Confirming the fact is the average breaking force of the worn and deformed chain shown in Fig. 4. After an obvious intense in vivo exposure, this force was of about 1600g, i.e. higher than some of the brand new chains, sees Fig. 11.
___. In contrast, reliable are the values of the delivery forces F₁ and F₂, i.e. these needed to detach each ring from the circumference C_M (taken to be the largest, matching the ring’s path around the arch wire and an upper central bracket¹⁴, Fig. 9). Each of these forces is equal with these that are pressing the arch wire on the slot’s bottom.
___.If a similar measurement is taken while using the smallest circumference exhibited by the system wire/bracket used, C_m (e.g. when the latter is a bicuspid or lower incisor), a second point on the Hooke’s diagram can be obtained. The line drawn between C_M and C_m should approximate with litttle error the stress/strain curve, comprising between these extremes all the circumferences exhibited by the bracket system used. The larger these circumferences and the sharper the angle a formed by the ligature above the arch wire, the stronger the pressure exerted and the higher the generated friction.

___.While not allowing to predict the behavior of an elastomeric ring by just stretching it, the tests show that even after eight months of their improper storage (to which also the unknown time till the ligature’s sale should be added), forces of over 2.5 kg (5 lbs) are still needed to break some of them. In contrast, the measurements of the breaking force in conditions of either a) accelerated steel corrosion, or b) storage in improper environment leads to a decrease in strength varying between 0.4 and 21% for the first case (a), and between 2 and 14% for the second (b), in average 6.1% vs. 7.3%, respectively.
___.The sharp drop in strength encountered in the relatively short, but accelerated aging experiment, is very likely due not only to the exposure to moisture, dissolved air and mechanical stress, but also to the action of the several heavy metal ions. The combination between the later two has not yet been considered and should be further researched. Indeed, oxidation, an important factor in polymer degradation, is known to be catalyzed by metal ions. As a result, the conditions used in the accelerated aging decrease breaking resistance to a level similar to that generated by an eight months’ of exposure in open air at subtropical temperatures (shelf life).
___.The method used to determine force delivery is promising and should be used in further research on aging or exposure to various environments.
Conclusions
___.The simple examination of the limit properties of an elastomer (breaking force and maximum elongation) are not enough to judge and even less to predict the in vivo behavior of an elastomeric ligature, as other, important data are missing.
___. While no reliable information can be thus drawn, the experiments performed can provide some information. Thus, a ten days-stretching in an aqueous solution that contained heavy metal ions leads to a degradation similar to that occurring after an eight months on a shelf in an improper environment (open, and in subtropical temperatures). However, after eight months (to which the time till the ligature’s sale should be added) and even after an in vivo use, some ligature rings stand expansion forces of over 3lbs (1.5 kg).
___.In contrast, the method used to evaluate the force delivery exerted by a ligature over an arch wire inserted in a bracket is not only simple, but can provide, in conditions simulating the oral environment, reliable data for comparing elastomeric ligatures.
References
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