Knee kinematics in-vivo of kneeling in deep flexion examined by bi-planar radiographs.

Squatting and kneeling are important daily activities for Middle and Far East cultures that require positioning the knee in deep flexion. In these activities, the limb becomes fully flexed with a knee flexion angle reaching between 150 and 160 degrees and the heel reaching the posterior surface of the upper thigh. Existing knee prostheses do not allow a full return to normal activities for this large population since they are limited to achieving knee flexion of about 120 degrees. Also, there is very limited information on knee kinematics and/or forces in the range beyond 120 degrees. The purpose of this study is to describe the kinematics of normal knees in-vivo, assessed in deep flexion, using bi-planar radiographs. A-P and lateral views were obtained from 5 healthy subjects during three sequential positions of kneeling. In the 1st position, the subject knelt with the knees fully flexed (deep flexion between 150 degrees and 160 degrees) and torso upright. In the 2nd position, the subject bowed forward to an intermediate position (about 120 degrees of knee flexion). In the 3rd position, the subject bowed further until his/her head touched the floor, supporting the upper torso with hands and attaining a knee flexion of about 90 degrees. The results show that past 135 degrees of knee flexion, the patella was found to clear the femoral groove and was in contact only with the condyles. The results also show that the classical femoral "roll back" does not appear to occur in deep flexion. It seems that the lateral femoral condyle rolls over the postero medial aspect of the lateral tibial plateau while contact of the medial femoral condyle occurs more anteriorly, but still in the posterior aspect of the medial tibial plateau. This asymmetric rolling motion implies 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.