The procedure of limb lengthening is a common method for correcting limb length inequality. The lengthening procedure, however, can result in serious complications such as premature consolidation of bone and soft-tissue contractures. To avoid adverse effects, it is important to determine the safe limits of the lengthening procedure in terms of forces applied, rate of distraction, overall length added and effects of the procedure on associated tissue. In this study, we investigate the relationship between distraction speed and callus formation in the fracture gap and how this affects the distraction resisting force (DRF) and muscle adaptation. A new distraction system was designed to continuously measure the DRF during tibial lengthening using a unilateral fixator. Measurements were made in 2 sheep for 51 days and 38 days respectively. The first sheep acquired 5cm (25% of its original length) of new bone with various rates of distraction. The second sheep acquired 3.8cm (15% of its original length) of new bone with 1 mm/day fixed rate of distraction. During the lengthening procedure, the DRF steadily increased as the callus was being formed in the distraction gap. In sheep 1 the lengthening rate was increased from 1mm/day to 1.25mm/day at the lengthening day 36 to manage what was interpreted to be signs of early premature consolidation. The DRF decreased to 310 N from 400N at the lengthening day 43. Radiographs taken at day 43 showed separation in the callus. During the 1.25mm/day lengthening the sheep lost ankle joint movement completely. The major muscles, long digital extensor (LDE), cranial tibial (CT), gastrocnemius and deep digital flexor (DDF) were collected from the operated and non-operated hind limbs. The result showed that LDE and CT were damaged while gastrocnemius and DDF were intact. In sheep2, the distraction rate was maintained at 1mm/day and it showed a similar increase of DRF up to 550N at the end of distraction (lengthening day 38). There was no visible joint restriction observed. None of the four muscles collected showed signs of the profound tissue damage noted in sheep 1. Comparison of the DRF profile with radiological interpretation confirmed that the degree of callus formation was appropriate at DRF of 300 ～ 500N. Additionally the ability of the muscle to adapt to the given length could be affected by the time when the rate was changed. Additional studies are underway to determine the optimal regimen including appropriate rates and forces.
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