Kinematics of the knee joint in deep flexion: a radiographic assessment

The purpose of this study is to describe the kinematics of normal knees in vivo, assessed in deep flexion, using bi-planar radiographs. Antero-posterior and lateral views were obtained from five healthy males during three sequential positions of kneeling. In the first position, the subject knelt with the knees fully flexed (deep flexion between 150 and 165 degrees) and torso upright. In the second position, the subject bowed forward to an intermediate position (about 120 degrees of knee flexion). In the third position, the subject bowed further until his head touched the floor, supporting the upper torso with hands and with the knees flexed at about 90 degrees. The results show that past 135 degrees of knee flexion, the patella cleared the femoral groove and was in contact only with the condyles. For these particular postures, and during deep flexion, motion of the femur on the tibia did not reveal the classical femoral 'roll back'. Rather the lateral femoral condyle rolled further over the postero medial aspect of the lateral tibial plateau while contact of the medial femoral condyle occurred more anteriorly, but still in the posterior part of the medial plateau. This asymmetric rolling motion indicated an element of internal tibial rotation. Furthermore, the tibia was found to articulate with the femur at the most proximal points of the condyles in deep flexion. These data on the kinematics and contact characteristics of the tibio-femoral joint must be considered in any approach to design for a Deep Flexion Knee Implant.