Muscle adaptation during distraction osteogenesis in skeletally immature and mature rabbits

The lack of adaptation of muscle is thought to be a major source of complic ations during distraction osteogenesis (DO). Although adaptation to DO vari es with the regimen (lengthening rate >1 mm/day and increase in bone length >20%) muscle contractures associated with DO may be a function of age. We tested this idea by subjecting skeletally mature and skeletally immature ra bbits to an aggressive regimen of DO (1.4 mm/day with a 20% increase in tib ial length). By using immunofluorescence to assess the presence of neonatal myosin heavy chain in sections from the tibialis anterior, we observed tha t the generation of new muscle tissue in response to DO was vigorous in you ng animals (27% positive fibers), whereas it was more muted in adult animal s (9.9% positive fibers). This adaptive response was associated with a pron ounced proliferation of myoblasts in the young but not in the mature animal s. Adult tibialis anterior subjected to DO showed a 50% loss in tetanic and twitch tension whereas those in young animals did not. This correlated wit h partial denervation of adult but not young muscle, as judged by morpholog ical criteria. These experiments indicate that adaptation to DO depends not only on mechanical variables but also on skeletal maturity. (C) 2001 Ortho paedic Research Society. Published by Elsevier Science Ltd. All rights rese rved.