Differential metabolic responses of periarticular ligaments and tendon to joint immobilization

Parameters of collagen metabolic behavior were analyzed in the periarticular connective tissues, i.e., medial collateral ligament (MCL), anterior cruciate ligament (ACL), and patellar tendon (PT), of control and immobilized rabbit knees. Two periods of immobilization were studied: 9 and 12 wk. Collagen turnover and collagen cross-links were quantitatively assessed in the three tissues. The results showed that after 9 wk both synthesis and degradation were significantly increased in the MCL and ACL, whereas the PT showed lesser effects. After 12 wk all three tissues experienced significant losses of collagen mass, which resulted in tissue atrophy. The concentrations of the reducible collagen cross-links dihydroxylysinonorleucine and hydroxylysinonorleucine in the immobilized MCL and ACL were greater than their respective controls, indicating an increase in collagen synthesis, whereas concentrations of the nonreducible cross-link hydroxypyridinoline were observed to be decreased in these tissues. Of the reducible cross-links in the PT, only hydroxylysinonorleucine was found to be increased over control, whereas hydroxypyridinoline was slightly less concentrated. These results taken together have demonstrated that the ligamentous tissues are more susceptible to the effects of stress deprivation secondary to joint immobilization than the PT, and, in particular, the ACL of the three tissues studied appears to be most vulnerable.