Temporary muscle weakness in the early phase of distraction during femoral lengthening. Clinical and electromyographical observations.

Limb lengthening by distraction osteogenesis has a high complication rate. Much of the response of muscle and nerve to distraction is still unknown. Thirteen children, mean age 12.6 yr (8.4-17.3) were surgically treated by the Ilizarov procedure for acquired and congenital femoral limb-length discrepancy. All children showed a decrease in muscle strength in the quadriceps, shortly after the operation, followed by an improvement before distraction started. After an elongation in the early phase of distraction (1 to 2 cm), muscle weakness was again observed and the muscle strength gradually increased after ending of distraction. To provide an explanation for this clinical observation, in one patient (limb lengthening of 4.1 cm) muscle strength measurements were extended with investigations of Hoffman (H) reflex of m. vastus medialis and determination of muscle-fiber conduction velocity of m. vastus lateralis by using the invasive method (IMFCV). The examinations were performed every two weeks during 20 weeks and 12 weeks after removing the cast. A severe decrease in muscle strength of the corrected limb was found after 1.2 cm of distraction with a recovery in muscle strength before lengthening was ended. EMG study showed the same tendency. Denervation was observed as evidence by positive sharp waves and reduced IMFCV findings. Evidence for reinnervation before lengthening was ended, was found by an increased range of velocities consisting of a combination of slow potentials and gradual increase of the velocity of reinnervated fibers (increased Fast/Slow ratio). The latencies of M waves and H-M interval from both legs separated as well after 2.25 cm of distraction. At the end of the follow-up period, the H-M interval reached the preoperative value. It is suggested that these neurogenic changes are an effect of axonal dysfunction and the local effect due to intraoperative trauma and stretching might affect nerve blood flow adversely.