Three-dimensional knee kinematics and stability in patients with a posterior cruciate ligament tear

The three-dimensional kinematics of the knee were studied from 5 months to 15 years after unilateral posterior cruciate ligament tears occurred in eig ht patients. All but two patients had signs of additional ligament injuries . Repeated radiostereometric examinations were conducted when the patients ascended a platform (step-up test) and during an instrumented anterior-post erior drawer test with the knee at 30 degrees of flexion. No changes in kne e kinematics were observed during the step-up test, whereas increased anter ior-posterior laxity (3.8-11.3 mm) was recorded for all patients. Four of t he patients had an increased side-to-side difference (more than 2 mm) in an terior as well as posterior laxity. A rupture of the posterior cruciate lig ament can be diagnosed at 30 degrees of knee flexion, but an increase in an terior laxity can erroneously be interpreted as an injury of the anterior c ruciate ligament. The unaffected kinematics of the knee suggest that factor s such as joint load and congruity and muscle activity can compensate for t he absent posterior cruciate ligament during static examinations.