OBJECTIVE : To investigate the in vivo femoral condyle motion and synergistic function of the ACL/PCL along the weight-bearing knee flexion. METHODS : Twenty-two healthy human knees were imaged using a combined MRI and dual fluoroscopic imaging technique during a single-legged lunge (0°-120°). The medial and lateral femoral condyle translation and rotation (measured using geometric center axis-GCA), and the length changes of the ACL/PCL were analyzed at: low (0°-30°), mid-range (30°-90°) and high (90°-120°) flexion of the knee. RESULTS : At low flexion (0°-30°), the strains of the ACL and the posterior-medial bundle of the PCL decreased. The medial condyle showed anterior translation and lateral condyle posterior translation, accompanied with a sharp increase in external GCA rotation (internal tibial rotation). As the knee continued flexion in mid-range (30°-90°), both ACL and PCL were slack (with negative strain values). The medial condyle moved anteriorly before 60° of flexion and then posteriorly, accompanied with a slow increase of GCA rotation. As the knee flexed in high flexion (90°-120°), only the PCL had increasingly strains. Both medial and lateral condyles moved posteriorly with a rather constant GCA rotation. CONCLUSIONS : The ACL and PCL were shown to play a reciprocal and synergistic role during knee flexion. Mid-range reciprocal anterior-posterior femoral translation or laxity corresponds to minimal constraints of the ACL and PCL, and may represent a natural motion character of normal knees. The data could be used as a valuable reference when managing the mid-range "instability" and enhancing high flexion capability of the knee after TKAs. METHODS : Level IV.
