Biomechanical properties of orthodontic vacuum-formed retainers: A scoping review

Abstract

Objectives Orthodontic vacuum-formed retainers (VFRs) made from thermoplastic materials are widely used as retainers following active orthodontic treatment. The aim of this review was to comprehensively investigate the biomechanical properties and material changes of VFR materials under different environmental stressors. Methods The PUBMED, EMBASE, Web of Science, and Cochrane Databases were searched from 2003 to June 2024 and supplemented with manual searches of bibliographies concerning vacuum-formed retainers. Results A total of 31 studies (n = 3 in vivo and n = 28 in vitro) were included in this review, published between 2003 and 2024. Among these studies, 26 VFR commercial brands and 7 types of thermoplastic polymer materials were identified. Key biomechanical parameters were assessed, including dimension stability, wear resistance, biocompatibility, aesthetics and optical properties. The main influencing factors were identified as the thermoforming process, intraoral ageing, temperature and moisture fluctuations, and exposure to staining and chemical agents. Thermoforming and intraoral ageing processes showed a significant impact on the biomechanical properties of VFR materials, particularly influencing the surface roughness and translucency. Conclusions The biomechanical properties of VFRs varied significantly among different thermoplastic materials. The thermoforming process led to a reduction in material thickness and a compromise in mechanical performance, with variations depending on the material type. Intraoral ageing of VFRs caused an increase in the surface roughness and a decrease in the translucency, affecting both their aesthetics and functionality.