Posterior cruciate ligament (PCL)-deficient patients usually display few functional disabilities during activities of daily living (ADL), even in the presence of significant objective knee laxity. This suggests that the magnitude of posterior instability occurring in ADL (dynamic instability) does not parallel the knee laxity detected in clinical examinations. The present study analyzed kinematics of the knee joint during stair descent in 14 isolated PCL-deficient patients and ten healthy volunteers using fluoroscopy. Factors influencing dynamic instability were investigated. In addition, magnitude of posterior tibial translation occurring during stair descent was measured and compared with static knee laxity measured on posterior stress radiography. Increased posterior tibial translation was observed in early swing phase (52.5 ± 5.6%) in PCL-deficient knees compared with normal knees (48.2 ± 8.6%). Almost the same magnitude of posterior instability was observed at early swing phase during stair descent using fluoroscopy and on posterior stress radiography. These results indicate that in PCL-deficient patients, posterior instability does not occur when weight is loaded onto the knee, but occurs when weight-bearing is released during stair descent.
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