11. Prevention
_____To limit the risks, it was suggested that the debonding of ceramic brackets should be performed with the aid of a rubber dam17 and that a disclosure to patients becomes in this case appropriate.18 In what follows are shown ways to avoid both tooth damage and the breakage of ceramic brackets that may cause such problems as aspiration of fragments and injury by flying debris.37
_____Improvement of ceramics. The problems encountered when using brackets made of alumina have led to attempts to substitute it with zirconia and tricalcium phosphate. The first has been launched in Australia as a variation of the Begg brackets (Elipse) and then under the names Harmony (Hudson Ltd., Sheffield, U.K.), and Toray (Yamaura Corp., Tokyo, Japan). While cubic zirconia is widely used in jewelry as a substitute for diamond, it is unfit for dental applications. Monoclinic at room temperature, it can change when heated to the denser tetragonal form. This volume change creates cracks within its structures (low thermal shock resistivity). To counter this effect, it is added/stabilized with the oxides of Mg or Y leading to a mixture of cubic phase and monoclinic (or tetragonal)-phase that is stable at lower temperatures. Currently used in prostodontics (crowns), this partially stabilized zirconia (PSZ), takes a yellowish, tooth-like shade. Unfortunately, its hardness, which is almost as high as that of alumina, does not solve the problems.
_____A more promising alternative for ceramic brackets has been launched recently by the Japanese manufacturer Tomy under the name “Hyaline”. Its main constituent, tricalcium-phosphate (TCP), Ca3 (PO4)2 is a natural constituent of teeth and bone and has been used as an active bioceramic for teeth and bone replacement.35
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_____Adequate attachment’s shape. The multitude and variation of the brackets found in commerce renders difficult a correlation between their bonding failures and the orthodontic treatment. In addition, to the latter should be added the accidental causes ranging from pencil-biting to sport-related blows.
_____A similar situation is encountered in the field of ophthalmic lenses, which are subjected to a too-large array of breakages to allow for specific tests. In the United States alone, thousands of accidents are occurring each year where the glass fragments imperil sight. Like orthodontic brackets, eyeglasses are brittle; unlike them, the lenses are worn longer and by a larger number of people all over the world. To test their response to breakage, a single test has been selected as acceptably reflecting accidental impacts. Although the circumstances leading to the most common types of failure are different, both lenses and ceramic brackets depend on the type of material used, processing, treatment, geometry and dimensional characteristics. The wide experience accumulated in the field of optics makes the test interesting for the general evaluation of ceramic brackets when they are exposed to sudden, accidental shocks and/or orthodontic manipulations.
_____To detect the propensity for breakage of ceramic brackets,36 a standard designed to protect the eye lenses wearer was successfully adapted. (ANSI Z80.1-199522, 23) It uses a manual drop ball tester model 024 shown in Fig. 11.5, in which a steel ball (AISI E52100) is dropped from a height of 50 inches as close as possible onto the geometric center of the attachment which is supported by a circular anvil made of soft wood. The test is performed by placing the ball on the circular tray (b), that is raised until the ball slides, entering at the top of tube (c). The drop distance is precisely the same each time, and the path of the ball can be viewed through the clear acrylic tubing. The test is relatively inexpensive and can be used easily to predict which ceramic brackets would stand an accidental impact and which could not. The similarity between its results and those from both debonding and torquing tests is because the forces exerted are similar. With few exceptions, the alumina brackets tested broke in two modes. The real twin ones, i.e. these in which the tie wings were not connected, broke in various ways as shown in Fig, 11.6. In contrast, the semi-twin ones broke along the connection tie-wing-base, Fig. 11.7
_____The resulting data, along with the design, are presented in Table 11. and the shape characteristics considered as having an impact on the bracket’s resistance to impact are show in Fig. 11.8.
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The results allow drawing the following information:
_____a. Polycrystalline brackets are more impact resistant than the mono-crystalline ones due to a weaker structure
_____b. The real twin design is the least resistant
_____c. The “single” design per se does not present advantages vs. the semi twin type
_____d. Non-protruding tie-wings bases lead to in creased resistance
_____e. Thick tie-wings and bulky structures with stand blows better
_____f. Domed configurations seem to deflect blows.
_____g. Rough (non-glazed) surfaces lead to poorer resistance
_____At the time of the testing, the ceramic brackets most resistant to impact were 20/20® by American Orthodontics and Fascination® by Dentaurum. Lumina® by Ormco, Allure III® and Allure® by GAC, Transcend 2000® and Transcend® by Unitek/3M showed some resistance, the latter being worse than the other four. The least resistant were Illusion® by Ortho- Organizers, Intrigue® by Lancer Orthodontics, and Starfire TMB® by “A”-Co./Ormco.
_____The weakest bracket tested, the mono-crystalline Starfire TMB®, has also been found to be the most fragile when subjected to debonding,9-11 probably due to its design. The other two attachments least resistant to breakage, Illusion® and Intrigue® are similar.
_____The fact that the test does not depend on other variables allows its use for future generations of ceramic brackets, regardless of whether they are made of alumina, zirconia, or glass.
_____Weakness detection. As shown, ceramics have a poor fracture toughness which is exacerbated by stress concentration areas and flaws and fissures. A tiny pore, in especially the hard to inspect polycrystalline bracket, may lead to its breakage under torque, as shown in Fig. 11.9.
_____In an attempt to solve the problem, American Orthodontics claimed the ability to detect flaws that are smaller than 1/1000 of an inch in its 28 20/20® brackets.
_____Annealing. To avoid breakage, Dentaurum subjects its Fascination® brackets to a hardening process,21 a treatment that may be responsible for the lack of failures found also by Eliades et al.10 As in metallic alloys, annealing ceramics relieves these from stresses as a diagram from Dentaurum demonstrates, Fig.11. 10
_____While in all the cases studied the basic material was 99.9% alumina, the bracket processing should have brought a difference,12, 21, 29-32 knowing that the edgewise bracket has a multitude of stress concentration sites.
_____If the ceramic is attached to a softer support, the strain diminishes. Brittle alumina sheets, when bonded to plastic supports, are used to make composite windows for aircraft against armor- piercing projectiles.33,34 Interestingly, such a composite provides approximately 1.7 times the ballistic protection of standard steel armor of equal area density.30 The resilient backup material (plastic) absorbs the excess force, converting it in kinetic energy, and thus resisting penetration. If strong enough, ceramic brackets can withstand common stresses, allowing their removal in one piece. If fragile but just strong enough to withstand activation, ceramic brackets may break not only during removal, but also in an accidental shock, thus saving the enamel. When accidental blows are foreseeable as for athletes, brackets that shatter thus may be preferable to those specially treated to provide additional strength. In addition, if the patient has weak teeth, lessening and prolonging the activation and accepting the risk of a more tedious debonding are preferable to enamel fracture.
_____Conclusion
_____Aesthetic attachments will always be in high demand, and, if the problems discussed are taken care of, ceramics will be again desirable, as these do not stain teeth and exhibit a high biocompatibility, both results of their chemical inertia. Newer materials and procedures should lead to inconspicuous brackets that blend acceptable fracture strengths to a diminished hardness. Until then, ceramics will continue to be used as fillers or in composite brackets such as the InVu® by TP Orthodontics where the polymer base decreases the risk of bracket and enamel fracture.
_____References
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