Assessment of Wear Characteristics, Longevity and Stiffness of Essix-type Retainers

Abstract

Objective: To compare four commercially-available Essix-type retainers in terms of longevity, wear characteristics, stiffness, and their range of rigidity. Design and Setting: An in vitro study conducted at Queen Mary University of London. Materials and Methods: Four groups of thermoplastic materials were included: Duran (PETG), Essix C+ (Polypropylene), Vivera, and Zendura (Polyurethane). A working typodont was fabricated to evaluate surface wear characteristics using a wear machine with a customised jig. Retainers were measured for tensile test and water absorption was measured at five different time points up to 6 months after initial immersion in two different physical states and two different solutions. Hydrolytic degradation was also evaluated using FTIR spectroscopy. Results: Essix C+ was the most flexible retainer with Vivera the stiffest material. Zendura and Essix C+ had the most surface wear and highest absorption rates. Only Essix C+ displayed signs of degradation following water absorption. Conclusions: All materials displayed characteristic levels of flexibility and were susceptible to water absorption. Duran 1.5mm performed similarly to Vivera in relation to stiffness and wear properties. While Zendura and Vivera have similar chemical structures, they exhibited differences concerning wear resistance and water absorption. Further clinical research evaluating the clinical relevance of these laboratory findings is required. Clinical relevance: Characteristic patterns of wear and rigidity of four commercially-available Essix-type retainers were observed. This information should help in the tailoring of retainer material based on a case-by-case basis.