Biomechanical analysis of canine bone lengthened by the callotasis method

The mechanical integrity of tibial bone treated by distraction osteogenesis (callotasis) using an Orthofix external fixator was evaluated in a group of 6 dogs. Diaphyseal corticotomies were performed on Day 0. Distraction was begun on Day 8, at a rate of 0.33 mm every 8 hours. On Day 34, the Orthofix frame was locked. On Day 56, the frames were removed and the dogs were euthanized. Radiographs were taken weekly to assess regenerate bone formation and extent of tibial bone lengthening, which averaged 12 mm (range, 5-20 mm). One dog showed incomplete bony union that was attributed to pin loosening. Torsional testing of lengthened and contralateral control tibiae resulted in spiral diaphyseal fractures. Five of 6 lengthened tibiae bones fractured within the diaphysis, but outside the regenerate area, suggesting that the strength of the lengthened segment exceeded that of the original diaphysis which was exposed to stress shielding by the external fixator. These observations have led to a method to test and evaluate lengthened bone at the time of frame removal and to monitor the effectiveness of this type of technique.