Biomechanics of the knee: methodological considerations in the in vivo kinematic analysis of the tibiofemoral and patellofemoral joint.
The purpose of this review article is twofold: to report on the use of intracortical pins to measure three-dimensional tibiofemoral and patellofemoral joint kinematics and highlight methodological concerns associated with this procedure. Tibiofemoral and patellofemoral kinematics has been extensively investigated using reflective markers attached to the surrounding soft tissue of the calf and thigh. However, surface markers may not adequately represent true anatomical locations and skin movement artefacts present the most critical source of measurement error. Consequently, knowledge about skeletal tibiofemoral kinematics is limited, in particular abduction-adduction and internal-external rotations. Considerable questions remain regarding what constitutes normal motion of the knee. A way to avoid the problem of surface markers is use invasive markers to directly measure skeletal motion. To date, many co-ordinate systems have been used to describe three-dimensional skeletal kinematics of the lower limb in vivo. They include helical axes, finite helical axes, instantaneous helical axes, and the joint co-ordinate system based on local anatomic landmarks. Although each method accurately describes the relative motion in 6 d. of f., the differences in how the motion is partitioned may account for the differences across investigations. Additionally, the problem of defining the anatomical co-ordinate system makes comparisons across subjects and studies difficult since subtle differences may be caused by small deviations in the anatomical reference alignment. Cross talk is also a primarily a concern. For joints that articulate principally about one axis, the primary flexion/extension that is registered will be cross-talked into ab/adduction and internal/external rotations.