Relationship between the surgical epicondylar axis and the articular surface of the distal femur: an anatomic study

Many authors presented the epicondylar axis as the fixed axis of rotation of the femoral condyles during flexion of the knee. Positioning of the femoral component of a total knee arthroplasty (TKA) based on the epicondyles has been proposed. This work is a critical analysis of this concept. Metallic bodies were inserted at the level of collateral ligament insertions on 16 dried femurs, allowing us to locate the surgical epicondylar axis. The dried femurs were studied using standard radiographs and CT-scan. CT cuts were made perpendicular to the epicondylar axis. The medial mechanical femoral angle and the epicondylar angle were measured on the radiographs. The posterior and distal epiphyseal rotations relative to the epicondylar axis (Posterior Condylar Angle, PCA, and Distal Condylar Angle, DCA, respectively) were measured on the CT-scans. PCA and DCA values were compared. The centre of the posterior femoral condyles was located on sagittal reconstructions using the tangent method and was confirmed with circular templates, and then compared to the location of the epicondyles. Circle-fitting of the entire femoral condylar contours centred on the epicondyles was also tried. The mechanical femoral axis was nearly perpendicular to the epicondylar axis but with important variations. The average PCA and DCA were 1.9° ± 1.8° and 3.1° ± 2.1°, respectively. No relationship could be established between the mechanical femoral angle and the PCA. The individual differences between the PCA and the DCA averaged 2.2°. A significant distance was found between the centre of the condylar contours and the epicondyles: 6.5 mm in average on the lateral side (range 2.3–11.3 mm) and 8.4 mm on the medial side (range 4.0–11.6 mm). Circle-fitting of the entire medial or lateral femoral condylar contours centred on the epicondyles was not possible. The centre of the posterior femoral condyles is significantly different from the epicondylar axis, thus refuting the conclusions of previous authors. Furthermore, considering the differences between the distal and posterior condylar angles shown here, as well as the difficulty of repeatably locating the epicondyles during surgery, using the epicondylar axis as the only landmark to position the femoral component during a first intention TKA is not recommended. The surgical epicondylar axis does not appear to be an adequate basis for the understanding of the shape of the distal femur.