Anterior-posterior stability of the knee by an MR image subtraction method.

The purpose of our study was to test the hypothesis that when a shear force was applied posteriorly to the loaded knee in vivo, there would be no relative motion between the tibia and the medial femoral condyle. Siemens 7 Tesla high-resolution MRI machine was used to scan eight healthy male volunteers with the knee at 15° of flexion. Two scans were obtained: the first with a compressive force of 660 N along the tibial long axis and a second with the compressive force and a posterior shear force of 36 N applied to the tibia. Solid models were created of the femur, tibia, and menisci for both loading conditions. The tibial models were superimposed enabling the displacements of the femur and menisci to be determined, relative to a fixed tibia. On average, the lateral femoral condyle displaced anteriorly by 0.66 mm but the medial femoral condyle displaced posteriorly by 0.36 mm. This indicated an axial rotation with a center between the lateral and medial condyles, but closer to the medial. The menisci displaced with the femoral condyles, but there was no indication that the medial meniscus was contributing to the pivoting action. This study supported the concept of medial anterior-posterior stability under weight-bearing conditions, but with structures other than the medial meniscus providing the stability. This study has application to the treatment of knee injuries and to knee arthroplasty design.