Hyo-Won Ahn

Assistant professor, Kyung Hee University

Esthetics is critical issue in orthodontic treatment, and invisible vacuum formed retainers (VFRs) using thermoplastic polymer sheets have become popular, replacing conventional Hawley or wrap-around retainers. However, from a clinical perspective, VFRs have major limitations related to their material properties and design. VFRs show poor wear resistance and durability along the incisal and occlusal surfaces with subsequent cracking after only a few months of use. Consequently, frequent replacement increases economic burden on patients. Moreover VFRs cause fulcrum point in the most posterior teeth which would be resulted in joint stress, mandibular positional changes, or intrusion of posterior teeth.
To overcome these disadvantages, a new-type of VFR composed of multi-layer hybrid materials was developed. They consist of three layers; an outer polyethylenterephthalate glycol modified (PETG) hard thermoplastic layer, middle thermoplastic polyurethane (TPU) soft thermoplastic layer, and an inner reinforced resin core. The resin core shows good wear resistance, mechanical strength and dissipates the occlusal force, which prevents unwanted distortion of the buccopalatal lateral wall during fitting and use. TPU is a ductile elastomer, which aids in improving elasticity, and absorbs impact while providing good wear sensation. The PETG has good formability, optical qualities, fatigue resistance, and dimensional stability which contribute to increased support from the mandibular dentition. The multi-layer VFRs showed improved mechanical strength and water absorption rate. They can be indicated to TMD or deep bite patients due to even occlusal contact with lower dentition.
To enhance site-specific functionalization, Nanofilms were firstly introduced on VFRs using the layer-by-layer (LbL) assembly technique, a simple method involving sequential adsorption of interactive molecules in aqueous solutions. Using this approach, nanofilms applied on occlusal surface of VFRs can enhance mechanical strength by durable materials and control optical properties while providing an extremely thin coating for patient's convenience. The inner surface of VFRs was coated for antimicrobial effect to reduce caries susceptibility.