Acrylic denture distortion following double processing with microwaves or heat

The present study compared the fit of acrylic resin bases subjected to two curing cycles in a water-bath or in a microwave oven. Resin bases were processed on duplicate stone casts prepared from a cobalt chromium master die. Ten bases, composed of conventional polymethylmethacrylate, were cured in a water-bath for 7 h at 70 degrees C and 3 h at 100 degrees C, and a further 10 cured in a domestic microwave oven at 65 W for 25 min. Ten bases composed of polymethylmethacrylate modified by means of a chemical activator were cured at 100 degrees C in a water-bath for 20 min. All bases were then subjected to a second curing cycle following addition of new acrylic resin to the alveolar ridge areas. Following each curing cycle the fit of the posterior border of each base was evaluated. A silicone impression material was used to form indices between the bases and the master die, these were then invested in stone and sectioned to allow cross-sectional measurement by means of an eyepiece micrometer. A one-way analysis of variance and unpaired Student's t tests were employed to compare the differences in distortion of the three groups. The differences in distortion between the first and second curing cycles were significant for all curing methods (P less than 0.001). However, microwave curing caused significantly less distortion of the denture base than short water-bath curing, especially in the centre of the palate (P less than 0.001).