Physical and mechanical properties of clear plastic retainers: A scoping review

Physical properties refer to a material's observable properties, and mechanical properties relate to a material's response to external forces. This scoping review aims to collate and synthesize the evidence concerning the physical and mechanical properties of clear plastic retainers (CPRs).

Six databases and a grey literature source were searched with key terms regarding CPRs and their physical and mechanical properties. The study followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Extension for Scoping Reviews. The findings were categorized into seven sections by mechanical and/or physical property.

The initial search yielded 1086 studies with forty-three satisfying selection criteria. Five related to three-dimensional (3D)-printed CPRs. The remainder concerned 'traditionally' fabricated, thermoplastic CPRs (T-CPRs). Nine thermoplastic materials and three 3D-printed materials were evaluated with a total of 36 commercial brands being investigated. 3D-CPRs generally displayed inferior optical and mechanical properties compared to T-CPRs although post-processing generally appeared to enhance relevant properties. Curry and coffee were frequently associated with staining. Cleaning agents containing sodium hypochlorite and tooth-whitening agents containing peroxides appeared to negatively affect surface roughness and mechanical properties.

3D-CPRs displayed inferior optical and mechanical properties compared to T-CPRs. Patients should be advised to avoid using bleach-containing agents to clean their CPRs and avoid using their CPRs as receptacles for tooth-whitening materials. They should be made aware of the staining-potential of coffee and curry on the appliance. Further relevant clinical research over the long term is required.