The effects of increasing the reverse curve of Spee in a lower archwire examined using a dynamic photo-elastic gelatine model

This paper describes the development and testing of a dynamic in vitro phot oelastic model for evaluating the effects of orthodontic mechanics on an en tire arch of teeth. A model of a mandibular arch was made and the teeth wer e embedded in a gelatine material with a high level of mechanical creep whi ch permitted tooth movement in response to orthodontic forces. The excellen t photo-elastic properties of this material also facilitated the analysis o f the stress distribution around the roots of the teeth. The model of a man dibular arch was used to investigate the tooth movements and stress distrib utions produced by increasing the reverse curve of Spee in a 0.018 x 0.025- inch stainless steel archwire. The results revealed that a l-mm reverse curve of Spee increased the arch l ength by 1.6 mm, but increasing the reverse curve of Spee to 5 mm did not i ncrease arch length further. Photo-elastic analysis showed an increased str ess distribution around the roots of the incisors and molars as the reverse curve of Spee was increased in the archwire.