Functional properties of knee ligaments and alterations induced by immobilization: a correlative biomechanical and histological study in primates.

The effect of immobility on the biomechanical properties of an anterior cruciate bone-ligament-bone unit of rhesus monkey is reported. Wild primates were used and the captivity time was minimized to lessen disuse changes from cage confinement. The ligament-bone specimens were tested in tension to failure under high strain-rate conditions. After 8-weeks total-body plaster immobilization of the primate, the ligament units showed significant decreases in maximum failure load and energy absorbed to failure (39 per cent and 32 per cent respectively). A dramatic change in ligament load-elongation behavior occurred. There was a statistically significant decrease in stiffness (slope of the load-deformation curve) of the ligament unit which may also be expressed as an increase in ligament compliance (elongation per unit load). The change in mechanical properties following immobility indicated a significant alteration in the projected functional capacity of the ligament unit to resist loading and to resist elongation; factors which relate directly to the ligament's ability to provide joint stability. The mechanisms of failure of the ligament unit, where an increase in osseous avulsion fractures occurred after immobilization correlated with histological changes of bone resorption in the cortex immediately beneath the ligament insertion site. Ultimate failure occurred in many, but not in all specimens, through the weakened bone. This emphasizes the potential for such effects in clinical conditions characterized by reduced bone mass. Alterations in the strength of the cortical bone may affect the integrity of ligament units as a whole. The effect of immobility on ligament units about the knee depended on histological characteristics of the ligament-bone attachment. The anterior cruciate ligament-bond junction, through zones of fibrocartilage, was little affected. In contrast, the tibial insertion of the superficial tibial collateral ligament, directely into bone in relation to the periosteum without well-defined zones of catilage, showed marked interruption after immobility due to bone resorption in the subperiosteal and outer cortical regions. In places the ligament was attached only to the overlying periosteum. In a second and third group of animals, the long-term effects of disuse were investigated. After 5 months' resumed activity following total-body-immobilization, there was only partial recovery in ligament strength although ligament stiffness and compliance parameters had returned to control values. It required up to 12 months for the complete recovery of ligament strength parameters.