Introducing the Lateral Femoral Condyle Index as a Risk Factor for Anterior Cruciate Ligament Injury: Response

Author's Response:

Thank you for your comment. We appreciate that you read our paper ¹ with great interest. Your comments are very helpful and constructive. We would like to clarify any given issues.

You are correct that one must be careful of potential errors of new measurements or indices. As you stated, the 6-o’clock position is not always unambiguous, and therefore several other factors were defined to allow a reproducible measurement. We defined the exact sagittal reference point of the index and additionally the congruence of the circle to the subchondral bone. Combining these factors, as precisely stated in the paper, allows a reproducible, valid measurement of the lateral femoral condyle index (LFCI) and minimizes errors. Changing the 6-o’clock position would result in a violation of the congruence of the circle to the subchondral bone and vice versa.

Furthermore, we analyzed the robustness of the LFCI and could demonstrate that the LFCI remained robust, even if a parasagittal slide was used 3 mm medial or lateral to the midsagittal plane (intraclass correlation coefficient [ICC] 0.83). Therefore, we do not agree that it is “difficult to draw an extension circle and a flexion circle accurately.”

As stated in the paper, as a limitation of our findings, our cohort consists of a variety of anterior cruciate ligament (ACL) injuries and our findings cannot be extrapolated to noncontact ACL injuries solely. However, previously known anatomic risk factors (lateral tibial height, lateral tibial slope) could be reproduced in our study, which indicates a representative cohort in our opinion.

Future research should focus on investigating the role of the anatomy of the lateral compartment on ACL injury, based on our findings. The LFCI is an additional MRI measurement to the previously established risk factors in conventional radiography. These risk factors may be validated in 3D models to help aid in our understanding of the anatomy of the knee and its potential biomechanical impact in ACL injury.

Pfeiffer et al ² also analyzed the shape of the lateral femoral condyle and defined the “lateral femoral condyle ratio.” The ICC for the lateral femoral condyle ratio was 0.77 for intraobserver reliability and 0.80 for interobserver reliability. In contrast, the ICCs for the LFCI were 0.89 and 0.96, respectively. Furthermore, the index described by Pfeiffer et al depends on an arbitrarily defined long axis of the distal part of the femur, in our opinion. The robustness of this index against a shift of the center of the circles along the femur or against the curvature of the femur was not analyzed. Therefore, without further analysis or evidence, we would not agree that the index defined by Pfeifer et al is more accurate than the LFCI.

Sandro Hodel, MD Zurich, Switzerland