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. The force profile data showed a drop in the force levels associated with an increase in the lengthening rate. The drop in these cases is thought to be due to separation of the callus as indicated in the radiographs. The study demonstrates the feasibility of using force feedback to vary the rate of lengthening in response to distraction force. It will be important to determine whether the forces that develop originated from soft tissue or callus. Collectively, the results support the use of the force-driven distraction system and demonstrate the need for using the force data to regulate the distraction rate in order to achieve optimal clinical outcomes.