Immobilization alters cell metabolism in an immature ligament

This investigation examined the metabolic and cellular effects of immobilization on immature rabbit medial collateral ligaments (MCLs). Thirty-two three-month-old rabbits had their right knee pinned in flexion for intervals of two weeks, one month, two months, or three months before being killed for comparison with MCLs from nonpinned contralateral controls and MCLs from 16 normal age-matched controls. Twelve animals were studied by gait analysis after either one month or three months of immobilization and were killed with the others for in vitro analysis of ligament mass, collagen synthesis, plasminogen activator secretion, or DNA content. Immobilized hindlimbs carried less weight than did contralaterals, and MCLs from those hindlimbs synthesized less collagen and secreted less plasminogen activator than did controls at all intervals. Contralaterals also showed some signs of altered cell metabolism compared with normal controls, suggesting that they are not normal by these assessment criteria. With increasing periods of knee immobilization, immobilized MCLs switched progressively from an anabolic to a more catabolic state, apparently contributing to the failure of immobilized ligaments to increase normally in mass. Knee immobilization alters the metabolic behavior of MCLs in both knees in growing rabbits, and, based on these results, either knee joint motion or hindlimb load bearing could be necessary for normal rabbit medial collateral ligament growth and maturation.