Introduction: Many attempts have been made to describe the flexion axis of the knee based on landmarks or simple geometric representations of the anatomy. An alternative approach is to use kinematic data to describe the axis of motion of the joint. The helical axis is one kinematic parameter that can accomplish this. The purpose of this study was to compare the correlation between kinematic and anatomic axes of motion.
Methods: Six cadaver lower extremities were skeletonized except for the knee joint. Passive navigation markers were implanted, and CT scans obtained. The limbs were then placed in an open-chain lower extremity rig that allows full range of knee motion. Threedimensional kinematic data were recorded using a camera and the helical axis of motion was calculated. Anatomic landmarks were placed on CT derived CAD models of the extremities consisting of spherical and cylindrical fits of the femoral condyles and a trans-epicondylar axis. Data for the normal knee was processed, by comparison of the helical axis to the landmark axes over varying ranges of flexion and the variation in helical axis direction within that range was also calculated.
Results: The flexion range with the minimum variation of anatomic parameters to the helical axis was 30–100°. Helical axis variation in this range was 5.489 ± 1.173, while variation between the helical axis and those axis defined by spherical, TEA, and cylindrical landmarks were 5.115 ± 2.129°, 3.127 ± 2.029°, and 5.111 ± 1.710°, respectively. A students t-test was performed on each data set with the null hypothesis that the angular difference between the anatomically defined axes and the helical axis is zero. All axes were found to be significantly different from the average helical axis in the range of 30–100° (P= 0.002, 0.013, and 0.001, respectively). The tightest variation in the helical axis occurred at 40–50° of flexion 2.89 ± 0.722.
Conclusion/Discussion/Summary: None of the anatomic landmarks considered in this study represent a consistently valid approximation of the kinematic flexion axis of the knee. The TEA represents the closest approximation of the three with a 95% CI between 0.998 and 5.256°. The range of 30–100° represented the tightest variation over the largest range of flexion. Extension was defined at approximately 30° based on kinematic profiles of internal/external rotation which show a “screw-home” tendency beginning at 30° through extension. This behavior is consistent with an increase in helical axis variation in ranges that were less than 30° of flexion. In a previous open-chain model, both compartments of the joint were spinning around 45 degrees of flexion, which is consistent with the smallest helical axis variation observed in the 40–50° range.
