Clinical evaluation of fiber-reinforced-plastic bonded orthodontic retainers

IntroductionThis clinical study was designed to evaluate the efficacy of innovative fiber-reinforced-thermoplastic (FRP) bonded orthodontic retainers.MethodsAnterior lingual retainers were formed by using a 2-step process from preimpregnated unidirectional long glass fibers (volume fraction, 0.25) in a thermoplastic resin matrix of either poly(ethylene terephthalate glycol) or polycarbonate. Seventy-six canine-to-canine retainers were placed in 56 patients by using the acid-etch technique over a 34-month period. They were evaluated for clinical acceptability to function as a retainer, structural integrity of the FRP, and integrity of the bonding. Variables examined included material composition, design factors, and mechanism of failure of the retainers. The results were analyzed by using the univariate Kaplan-Meier survival method and the multivariate Cox regression model.ResultsThe overall median survival time was 7.6 months, with 33% surviving after 12 months. The retainer with the longest service was in use for over 24 months. The variables with the most significant effects and improved survival rates were FRP formulation (polycarbonate), retainer thickness (1.02 mm), and number of teeth overlapped (all 6 teeth). Failure was primarily due to bond failure at the enamel-adhesive or adhesive-FRP interface.ConclusionsWith improved survival times, FRP retainers could be a viable alternative to metal retainers.