Enzymatic adaptation in ligaments during immobilization

Ligaments are a composite of fibroblasts and collagen in a proteoglycan matrix. Seventy-five percent of the organic solid is collagen and 23% is proteoglycan. Fibroblasts are responsible for the overall composition of the ligament, that is the synthesis and the degradation of macromolecular components. Like muscle and bone, ligaments are dynamic, undergoing hypertrophy with exercise and atrophy with immobilization. This paper reviews the structure and composition of ligaments and discusses the cellular events responsible for atrophy of ligaments with immobilization. As an experimental model, one knee of New Zealand White rabbits was immobilized with a pin. After 2, 4, and 8 weeks of immobility, the medial collateral ligaments were isolated and enzyme analysis was performed. Gross and microscopic changes were apparent after 2 weeks. As for enzyme changes, lactic dehydrogenase and malic dehydrogenase decreased in activity. The lysosomal hydrolases responsible for glycosaminoglycan degradation increased in activity, suggesting that enzymatic adaptations mediate the physical and chemical changes in the ligament. The cells switch from an anabolic synthetic state to a catabolic, degradative state during immobility. It would seem from the biochemical viewpoint that, whenever possible, cast-bracing and functional splints may be preferable to rigid plasters in many sports-related ligamentous injuries.