A Study of Application of Shape Memory Plastic as Elastic Material in Clinical Dentistry and Orthodontics.

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Polynorbornen, a shape memory plastic developed in Japan, has a glass transitional point of 35^ﾟC. Once the environmental temperature exceeds the critical point, this plastic will begin to display an elastic property, then return to its original shape, if deformed. We examined whether the force generated during the elastic phase of polynorbornen could be used to displace human teeth. We found that the shape memory plastic wire of 1mm in diameter stretched to two to three times of its original length at a temperature of 50^ﾟC and a speed of 0.5mm/sec would exert a relatively stable continuous light force of 119-156 grams to move the teeth. This new material, compared with conventional elastic modules used in orthodontic therapy, exhibited a laser degree of force degradation at a body temperature of 37^ﾟC for a long period, and can be manufactured to near the tooth color required. These advantages make feasible clinical application of the shape memory plastic in orthodontics. However, a combination with conventional metal wires cannot be avoided for practical purposes, and its clinical efficiency has to be further studied.