Magnetic resonance imaging of 3-dimensional in vivo tibiofemoral kinematics in anterior cruciate ligament-reconstructed knees.

PURPOSE: The purpose of this study was to use magnetic resonance imaging (MRI) to determine 3-dimensional knee kinematics after anterior cruciate ligament (ACL) reconstruction.

METHODS: Nine ACL-reconstructed and contralateral knees were tested 12 +/- 8 months after surgery. MRI was performed at full extension and 40 degrees of knee flexion under simulated weight-bearing conditions. Femoral condyle positions, tibial rotation, contact area, and contact location were analyzed by use of MRI-based 3-dimensional models.

RESULTS: When knees were fully extended, tibiae in ACL-reconstructed knees were externally rotated by 3.6 degrees +/- 4.2 degrees compared with contralateral knees. The external rotation was due to anterior subluxation of the medial side of the tibia. At 40 degrees of knee flexion, tibiae in ACL-reconstructed knees and contralateral knees were both internally rotated by 5.3 degrees. There were no significant differences in contact area or contact location between ACL-reconstructed and contralateral knees. When moving from extension to flexion, ACL-reconstructed knees exhibited 3.5 degrees +/- 5.9 degrees more internal tibial rotation than contralateral knees.

CONCLUSIONS: Reconstruction of the ACL restored normal motion on the lateral side of the knee but not on the medial side, resulting in increased internal tibial rotation when moving from full extension to 40 degrees of flexion. These results suggest that ACL reconstruction does not restore normal kinematics on the medial side of the knee, which may lead to early cartilage degeneration.

LEVEL OF EVIDENCE: Level IV, therapeutic case series.