Force feedback in limb lengthening.

A new variable-rate distraction system using a motorized distractor driven by feedback from the distraction force was designed. The distractor was mounted on a unilateral fixator and attached to the tibiae of 6 sheep that underwent distraction osteogenesis. The sheep were divided equally into 3 groups. In group 1, the forces were recorded but were not used to drive the lengthening rate. In group 2, force feedback was used and the desired distraction force level was set to 300 N and the initial rate was 1 mm/day. Group 3 also underwent force feedback with the desired force limit at 300 N, but the rate change was initiated earlier, at 200 N. The distraction force was recorded at 15 second intervals throughout the distraction phase and stored onboard the distractor.