The effects of immobilization on the ultrastructure and mechanical properties of the medial collateral ligament of rats.

Rat medial collateral ligaments from one group of animals were tensile tested following immobilization. Ligaments from a second group of animals were examined using transmission electron microscopy to determine collagen fibril cross-sectional area and density. Immobilized and contralateral control ligaments were compared to determine the effect of immobilization on these parameters. There was a significant decrease in the linear stress, maximum stress, and stiffness of immobilized ligaments. There was no significant difference in the strain values reported. There was a significant decrease in the proportion of smaller, cross-sectional area fibrils (0-10,000 nm2) following immobilization. There was also a significant increase in the proportion of larger (30,000-40,000 nm2 and 40,000-50,000 nm2) fibrils following immobilization. This finding was attributed to a decreased synthesis and decreased degradation of collagen during immobilization; post-immobilization, the density of collagen fibrils in the ligament decreased. There may be a relationship between ultrastructural and mechanical alterations following immobilization. Significant clinical implications are apparent in the altered mechanical properties following immobilization.