There have been few, if any, clinical reports on the use of an internal distractor to cure mandibular symphyseal defects. Compared with the currently-used external distractor, an internally implanted distractor may be more comfortable for patients, so we designed an internal trifocal distractor and tested its efficacy in animal models. We removed the mandibular symphyseal bones from mongrel dogs, producing a broken site 25-30 mm wide. Two transport discs, each roughly 10mm wide, were placed at the two ends of the broken bone, followed by assembly of the self-made internal trifocal distractor on the support of the two discs. A week after the procedure we initiated traction at a speed of 0.5mm/12h. The whole process lasted for 13-16 days until the two transfer discs had joined together at the centre of the mandible. Occlusal films were taken at the first, fourth, eighth, and twelfth week after traction. The animals were killed at the twelfth week, and the mandibles were examined histologically to assess the new bone formation. During the twelfth week after the initiation of traction, the physical continuity of the broken bones was restored. Histological results showed that the new bone formed in the traction area was abundant and had fully matured. We also found the formation of external rough callus, which indicated osteal concretion. Our results suggest that a well-designed internal trifocal distractor may be useful in the treatment of mandibular symphyseal defects.