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 eight 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-posterior drawer test with the knee at 30 degrees of flexion. No changes in knee kinematics were observed during the step-up test, whereas increased anterior-posterior laxity (3.8-11.3 mm) was recorded for all patients. Four of the patients had an increased side-to-side difference (more than 2 mm) in anterior as well as posterior laxity. A rupture of the posterior cruciate ligament can be diagnosed at 30 degrees of knee flexion, but an increase in anterior laxity can erroneously be interpreted as an injury of the anterior cruciate ligament. The unaffected kinematics of the knee suggest that factors such as joint load and congruity and muscle activity can compensate for the absent posterior cruciate ligament during static examinations.