The muscles in chronic ligamentous instability of the knee: an experimental study

Several authors have formulated the idea that insufficiency of the anterior cruciate ligament (ACL) might lead to a biological adaptation, consisting of atrophy of the quadriceps and other extensors (antagonists of the ACL) and relative preservation of the hamstrings and other flexors (agonists of the ACL). We studied the modifications in hindlimb muscles that spontaneously occurred in 36 rabbits with surgically-induced chronic ligamentous instability of the right knee. The unoperated left hindlimb served as a control. Three months after surgery the rabbits were sacrificed and the muscles dissected for macroscopic and microscopic evaluation. In the anterior cruciate ligament-deficient limbs the extensors (quadriceps, glutei and extensor digitorum longus) showed significantly greater atrophy than the other muscles (p < 0.05). Six hundred twenty-two fibers from atrophic muscles, 373 from hypertrophic muscles and 879 from control muscles were evaluated by microscopy. The atrophic muscles showed a significantly lower percentage of type I fibers (responsible for maintenance of posture) and a reduction in the diameter of both type I and II (responsible for rapid movement) fibers with respect to the controls (p < 0.05). The hypertrophic muscles showed a significantly greater proportion of type I fibers and an increase in the diameter of type II fibers with respect to the controls (p < 0.05). Our results support the idea that there is spontaneous adaptation of the muscles to compensate for ligamentous instability.