Bacterial degradation of composite bonding materials.

The causes of bracket loss during the late phases of orthodontic treatment have hitherto been considered to be of mechanical nature (e.g. trauma, high forces applied). Recently, it was hypothesized that bacterial degradation of composite resin could be a reason for late bracket failure. This was based on the observation of apparently degraded composite on bracket bases sent to a recycling company, and on a weight loss of 0.1-1% of a limited number of composite resin foils incubated with bacteria. To further elucidate the basis for this hypothesis, we examined brackets, immediately after debonding, for signs of potential decay and tested the ability of selected bacterial species to degrade composites. Out of a total of 1056 brackets collected, 6.2% displayed signs of decay indicative of inclusion of air bubbles, corrosion of the bracket base and discoloration of the composite and comparable to the decay observed on brackets received from the recycling company. Composite discs of two different brands were each incubated with either of five strains representing different bacterial species. The incubation lasted 3 months or until the bacteria had died. There was no significant difference in weight change of the discs incubated with bacteria and control discs incubated without bacteria. Thus, our results do not support that bacterial degradation of composite resin bonding materials is a likely cause of bracket loss.