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An antimicrobial agent, benzalkonium chloride (BAC), was added to a chemically cured composite resin, and the antimicrobial benefits and physical properties of the modified composite were evaluated. BAC was added to Reliance Phase II composite to create modified composites with BAC concentrations ranging from 0.25 to 2.50 wt%. Specimen disks of the modified composite were incubated with Streptococcus mutans for 48 hours, and an agar disk diffusion assay was used to measure zones of bacterial inhibition. Larger disks were suspended in brain-heart infusion medium containing 2 x 10(4) colony-forming units/mL Streptococcus sobrinus (10 mL, 2 wt% sucrose, 24 hours) to measure bacterial adherence to the adhesive; adherent cells were removed from the surface with 1 N NaOH, and the optical density of the cells was measured at 550 nanometers. Traction hooks were bonded to bovine teeth with the modified composite, and tensile bond strength was evaluated with a universal testing machine. Diametral tensile stress was also measured. The modified composite samples showed that antimicrobial activity increased with higher BAC content; no antimicrobial activity was measured for the original compound in either the disk diffusion or the bacterial adherence test. There were no significant differences (P <.05) in either tensile bond strength or diametral tensile stress among the modified composite groups and the original product. The incorporation of BAC in composite material added antimicrobial properties to the original compound without altering its mechanical properties.

While the beginning of orthodontics was marked by an acute scarcity in materials, the modern clinician has to face a large array of devices made of metals, plastics, ceramics and combinations thereof. Each of these materials have been developed for other purposes than orthodontics, and then transplanted "as is" in a field which has its own demands. During the years, theses devices, as well as the materials these are made of, have been modified most of time to the benefit of both the patient and the clinician, and some time to that of the manufacturer who has tried to save, or bypass limiting patients. Among the latter are the harder, but less corrosion resistant steels, the efficient but corrosion prone brazing, the inexpensive but weaker molded attachments, the aesthetic but fragile ceramics. Understanding the trends and basics of materials science should help the clinician to avoid being sued iatrogenic inflicted affections.

The overall resistance to accidental blows of the many ceramic brackets that are sold today has not been explored. Facing a similar diversity, the eyeglasses industry has chosen to standardize the testing of lenses by subjecting them to the drop of a steel ball. By slightly modifying this test, 10 brands of ceramic brackets were examined. In most cases, the findings coincided with those found by other authors when duplicating debonding. Thus, polycrystalline ceramics with bulkier structures and glazed surfaces were found to be more resistant to impact than the monocrystalline brackets, the loftier real "twins," and the less dense attachments. Protruding tie wings and bases were liabilities, and domed configurations seemed to deflect the blows. Bulkier "single" designs alone did not offer a guarantee of impact resistance when not accompanied by an appropriate microstructure and a smooth surface. The ceramic brackets most resistant to impact were found to be 20/20 by American Orthodontics and Fascination by Dentaurum. Medium resistance was displayed by Lumina by Ormco, Allure III and Allure by GAC, Transcend 2000 and Transcend by Unitek/3M; the last was not as good as the other four. The least resistant were Illusion by Ortho-Organizers, Intrigue by Lancer Orthodontics, and Starfire TMB by "A"-Co. Probably because of its real twin design, the last bracket lends itself to the highest probability for accidental breakage. Although resistance to impact and accidental debonding is desirable from the point of view of treatment, the advantage should be weighted against the chance of enamel fracture. Indeed a weak bracket attached with a soft adhesive may be preferable when the chance of an increased exposure to accidental blows is probable. In such cases, the ceramic may take the brunt of the force, instead of the tooth.

Because the strength of direct bonding brackets is both important and difficult to measure, a related property, microhardness, has been investigated. Twelve popular U.S. brands of direct bonding brackets have been tested with a Vickers-Hanemann microhardness apparatus. The tests have shown a consistently wide difference in hardness between brands, the highest values being exhibited by those using the precipitation hardened steel known as PH 17-4, and the lowest by those using the austenitic 316L. As the first steel proved to be significantly less corrosion resistant than the last one, it seems that today there are too few attachments that are both strong and chemically resistant.

The slot angulations that are built into currently manufactured brackets are the same as those recommended more than 20 years ago. Because of a continuous trend toward miniaturization, the bracket slot length has diminished considerably, fact that should have had an impact on the characteristics of the slot. This impact has not yet taken place. A decrease in the overall bracket size and the shortening of the slot length invite unaccounted tooth rotation, which happens when an attempt is made to adhere to traditional angulations. If only a pure translation of a tooth is wanted, such a rotation becomes undesirable. To reduce its action, power arms are used. Unfortunately, these do not solve the problem. Indeed, to overcome this effect, a modification of the values of standard angulations is needed. To illustrate the point, maxillary canine retraction is considered, because it is both common and involves an already high bracket slot angulation. With standard solid mechanics and statics, new angulations are proposed as functions of the bracket slot length and arch wire rigidity. In addition, an estimation of the minimum useful size of the slot is made.

Composites used as orthodontic direct bonding adhesives have a polymeric matrix that can host and nurture a variety of aerobic and anaerobic microorganisms acting alone or in combination (consortia). Their accumulation can lead to the weakening of the bond and possibly the attacking of the tooth. A number of microorganisms have been identified as present on the removed direct bonding brackets. Their action has been duplicated in vitro. Although some are specific to the oral environment, others are "opportunistic," i.e., have external sources. The results of attempts to detect the origin of the latter suggests a contamination of iatrogenic origin. Thus a high percentage of attacked adhesive can be traced to specific orthodontic offices, a fact that could be indicative of a breach in the sterilization procedures. While the attack of microorganisms on orthodontic adhesives is spectacular, less obtrusive inroads are likely to occur whenever similar acrylic composites are used in general dentistry, i.e., in restorations, veneers, and crowns. Indeed, it has been found that neither these polymeric products, nor some of the ingredients used to make them, exhibit a bacteriostatic activity, being metabolized instead. Results of rendering the adhesive microbe-resistant by adding a bactericide have shown to be encouraging.

Despite the fact that retention has been widely discussed in many specialty papers, there is still a lack of information regarding it. The author of the following article happens to have been both a designer or orthodontic adhesives for a major manufacturer and a principal scientist of a Fortune 200 company (in charge of adhesion projects like the substitution of riveting through bonding for airplanes and advanced composites metal/plastics). Based upon his experience, he's trying to relate knowledge which is common in the bonding industry to the needs of the orthodontist.