MECHANISMS OF JOINT CONTRACTURES DURING LIMB LENGTHENING

Abstract

Introduction and Aims: With a great progress in bone regeneration, muscle is currently regarded as a largest limiting factor for successful limb lengthening leading to joint contractures and fractures of distraction regenerate. The purpose of this study was to evaluate muscle architectural changes and potential mechanisms of joint contractures during limb lengthening.

Method: Nine mature goats underwent 20% unilateral tibial lengthening (0.25 mm x 3/day) and were sacrificed immediately upon completion of distraction. With the stifle (knee) and hock (ankle) joints fixed at similar angles, both limbs were disarticulated at the hip joint and submerged into 10% buffered formalin. Following tissue fixation, all tibial muscles were sequentially dissected and changes in muscle origin-to-incretion length, belly length, tendon length, myofibers length, and sarcomere length were analysed relative to the muscle measurements on the contralateral limb and bone lengthening. Muscle fiber length was assessed under stereoscopic magnification and sarcomere analysis was performed using laser diffraction.

Results: Thirteen muscles were identified for each limb. Anterior compartment consisted of two longitudinal and four pennate muscles, whereas posterior compartment had one longitudinal and six pennate muscles. Origin-to-insertion length measurements showed disproportion between the amounts of muscle and bone length increase with muscle-to-bone lengthening ratio ranging from 0.2 to 1.0. When assessed separately, muscle belly stretched more substantially (range, 11–24%) than muscle tendon (range, 3–14%). Longitudinal muscles showed better compliance to limb lengthening than pennate muscles. Origin-to-insertion, muscle belly, and tendon length increase for longitudinal muscles averaged 15%, 21%, and 11%, respectively, whereas for pennate muscles these parameters averaged 10%, 15%, and 6%, respectively. Although anterior pennate muscles showed higher proportion of muscle length increase than posterior pennate muscles, this difference was not significant. Lengthening of muscle fibers varied greatly, ranging from 0% to 88%. Fiber length of posterior muscles increased tremendously (average, 42%). This was associated with comparable increase in sarcomere length (average, 39%). Anterior muscles showed only 10% lengthening of the fibers. However, 12% reduction in sarcomere length indicated addition of new sarcomeres in series to accommodate increase in fiber length.

Conclusion: Different response of anterior and posterior muscles to distraction contributed greatly to the development of joint contractures. Posterior tibial muscles were predominantly pennate, larger in volume, and thus showed higher resistance to lengthening. Moreover, posterior muscle fibers incurred lengthening by sarcomere stretching, whereas anterior muscle fibers showed evidence of neosarcomerogenesis.