Orthodontic thermoformed retainers: a two-arm laboratory study into post-fabrication outcomes

Abstract

Background: Retainers are commonly used to maintain post-orthodontic occlusion stability. We aimed to determine post-fabrication thickness and thinning rate of thermoformed retainers. Methods: Forty-eight retainers were fabricated from polyethylenterepthalat-glycol copolyester or polypropylene blanks, using vacuum- or pressure-thermoforming. Retainer thickness was measured at multiple locations. Results: Thinning rate had a broad range: the mid-labial incisor region of 1 mm polyethylenterepthalat-glycol copolyester pressure-thermoformed mandibular retainers had the greatest thinning rate (68.25 ± 8.26%) and smallest mean post-fabrication thickness (0.32 ± 0.08 mm). Polyethylenterepthalat-glycol copolyester retainers were 0.11 mm thinner than polypropylene (P=0.0222), and polypropylene retainers were 0.21 mm thicker, when pressure-thermoformed (P<0.0001). The interaction of manufacturing method and material used, and tooth type, explained over a third of the variability in the post-fabrication thickness of these retainers. Maxillary retainers made from 1 mm blanks were 0.04 mm thicker in the incisor region compared with the molar region (P=0.0492). Conclusions: Thermoformed retainers do no thin uniformly against individual teeth and have variable intra- and inter-arch post-fabrication thicknesses. There is no clear benefit in using a specific type of thermoforming machine or material for increasing post-fabrication thickness or reducing thinning rate. Blank thickness and tooth morphology influence these outcomes.