Continuous Force Measurement in Limb Lengthening

Abstract

Complications of limb lengthening include stress on soft tissue, premature consolidation, and bone non-union. This study reports on the development and testing of instrumentation that continually measures distraction forces in rabbits with the aim of relating the forces developed to bone growth and muscle physiology. A full bridge strain gage resistor set mounted to a unilateral distractor measured distraction force and stored the data on a self-contained logger attached to a jacket worn by the rabbit. Two rabbits underwent limb lengthening on one hind leg. Lengthening was set at 0.5 mm/ day, twice per day for a total of 12 mm. Tibialis anterior muscle and proximal and distal tibial growth plates from the operated and control legs were harvested after euthanasia for histomorphometric analysis. Radiographs were taken periodically throughout lengthening. Force data show characteristic overall increase in force. Force steadily decays after each lengthening. Peak forces appear to correlate with time between lengthenings. Histologic data show that the operated leg had an increased preponderance of type I muscle fibers, a decrease in type-I and type-II muscle fiber diameters, and a decrease in endogenous bone growth at the distal growth plate. Histomorphometric analysis showed that the lengthening paradigm employed altered distal bone growth and muscle fiber physiology during lengthening.