Ultimate failure strengths of human tibial collateral and anterior and
posterior cruciate ligaments were determined at two different loading rates
(12.5 and fifty centimeters per minute) using an Instron Tension Analyzer.
The posterior cruciate ligament was significantly stronger than the tibial
collateral and anterior cruciate ligaments, which were of equal strength.
At ultimate failure the ligaments were intact macroscopically but electron
microscopy revealed widespread disruption of the collagen fibrils. Only
after further application of stress did actual macroscopic disruption
occur, suggesting that microscopic failure of the collagen fibrils in
grossly intact ligaments may be a significant cause of clinical
instability.