Agreement Between Two Methods for Computing the Anterior–Posterior Positions of Native Femoral Condyles Using Three-Dimensional Bone Models With and Without Articular Cartilage and Smoothing

Knowledge of anterior–posterior (AP) movement of the femoral condyles on the tibia in healthy knees serves to assess whether an artificial knee restores natural movement. Two methods for identifying AP positions and hence condylar movements include: (1) the flexion facet center (FFC) and (2) the lowest point (LP) methods. The objectives were to determine (1) agreement between the two methods and (2) whether addition of articular cartilage and/or smoothing significantly affects AP positions. Magnetic resonance (MR) images of healthy knees were obtained from eleven subjects, who subsequently performed a dynamic, weight-bearing deep knee bend under fluoroscopy. Four different types of MR models of the distal femur were created: femur, smoothed femur, femur with articular cartilage, and femur with smoothed articular cartilage. In the medial and lateral compartments for the femur with smoothed articular cartilage at 0 deg flexion, mean AP positions of the LPs were 7.7 mm and 5.4 mm more anterior than those of the FFCs, respectively (p < 0.0001, p = 0.0002) and limits of agreement were ±5.5 mm. In the flexion range 30 deg to 90 deg, differences in mean AP positions were 1.5 mm or less and limits of agreement were bounded by ±2.4 mm. Differences in mean AP positions between model types were <1.3 mm for both LPs and FFCs. Since omitting articular cartilage from three-dimensional (3D) models of the femur minimally affected AP positions, faster and less expensive imaging techniques such as computed-tomography (CT) can be used to generate 3D bone models for kinematic analysis. In addition, the LP method is preferred over the FFC method because of its inherent accuracy in indicating the AP position of the instant center of curvature of the femoral condyles which varies with the knee in extension versus flexion.