Immobilization and retraining of cruciate ligaments in the rat.

Fifty male rats, aged 40 days, were assigned to one of five groups to examine the influence of immobilization and retraining on the strength and elastic stiffness of the anterior and posterior cruciate ligaments. Following the experimental treatments, the anterior ligament from the right leg and the posterior ligament from the left leg were tested in tension at a separation speed of 50 mm/min. After 4 weeks of cast immobilization, the separation force of the anterior ligament declined by one fourth and elastic stiffness of both ligaments declined by one fourth and one third, respectively. No changes occurred in the separation force of the posterior ligament. Following a 6-week retraining swimming program, the separation force and elastic stiffness values had returned to control levels. Additionally, there was a transition from avulsion type failures in the immobilized groups of animals to insertion site and intraligament failures in animals subjected to rehabilitative swimming exercise. This was indicative of a return in the strength of the osseous component of the bone-ligament-bone complex following immobilization.