The current study found that an increased LFCR, as measured by MRI, was associated with an increased risk of noncontact ACL injury. The LFCR threshold for predicting noncontact ACL rupture was 0.61, with a sensitivity of 0.79 and specificity of 0.67, which confirmed the hypothesis of the study. Combining the two most predictive factors (LFCR> 0.61 and LTH > 2.35 mm) improved the predictive diagnostic performance for noncontact ACL injury. The AUC was 0.85, sensitivity was 0.83, and specificity was 0.76. These parameters can help clinicians identify patients at risk for ACL injury.
Many studies have shown that the shape of the lateral femoral condyle exerts an important influence on the rotational stability of the knee [9, 14]. Pfeiffer reported that the mechanism of the influence of lateral femoral condyle shape on knee rotation stability was an increase in the depth of the lateral femoral posterior condyle, which may affect the movement of the femur in relation to the tibia and lead to changes in gait and load mechanics [8]. The increase in the depth of the lateral posterior condyle of the femur alters the shape of the lateral condyle of the femur to be more elliptical and not equidistant, which may lead to an increase in ligament relaxation when the knee joint is nearly fully extended [8]. It may also lead to a reduction in the contact area between the femur and tibia, thereby increasing rotational relaxation of the knee [8]. The results of this study are consistent with the aforementioned results.
A steeper tibial slope was significantly correlated with an increase in axial displacement [15,16,17] and with the risk of ACL injury [15,16,17]. Interestingly, in this study, only a significant increase in tibial slope was found in the noncontact ACL injury group compared to the control group. However, there have also been literature reports that found no association between steeper tibial slopes and a higher risk of ACL injury [18, 19].
In this study, we found that the combination of the two factors (LFCR> 0.61, LTH > 2.35 mm) was more predictive for noncontact ACL injuries than either factor alone. This confirms previous findings that bone morphology of both the femoral and tibial lateral ventricles contributes to axial displacement of the knee. That is, increased depth of the lateral posterior condyle of the femur and height of the lateral tibial plateau lead to increased rotation [9, 20]. These differences in bone morphology affect the biomechanics of the knee, causing greater traction of the ACL and increasing the risk of injury [21].
In this study, there was no significant difference in LFCR between women and men. In previous studies, sex differences in the morphology of the distal femur were reported. Some studies found that the anatomical morphology of femurs in females was inconsistent with that of femurs in males [11, 22], while other studies found that there was no significant difference in the morphology of distal femurs between males and females [10].
Previous studies have used different measurement methods to describe the bony morphology of the lateral femoral condyle and further found that bone morphology changes in the lateral femoral condyle could increase the probability of ACL injury. Pfeiffer quantitatively measured the osseous morphology of the lateral condyle of the femur according to the ratio of the lateral condyle on X-ray [11]. This method was simple and relatively accurate. The exclusion of a large number of patients because of malrotated radiographs could introduce bias. The study by Voleti et al. also demonstrated that radiographs underestimate posterior condylar depth measurements when compared with MRI [23]. The use of MRI would have allowed the authors to accurately quantify the posterior condylar depths of the lateral femoral condyles, assess their influence on the risk of ACL injury and reduce measurement imprecision and patient exclusion because of malrotated radiographs. Hodel et al. used the LFCI to quantitatively measure the skeletal morphology of the lateral femoral condyle on MRI [10]. In other words, the flexion circle and extension circle were drawn before and after the lateral femoral condyle, respectively, and the ratio of the radius of the two circles was considered the lateral femoral condyle index [10]. However, Li et al. indicated that the two circles had great uncertainty and randomness, possibly leading to large errors of this measurement method [24].
This study offers some strengths but also presents limitations. The main advantage of this study is that the LFCR was measured by MRI, and this method is simple to apply. Reducing the exclusion of large numbers of patients due to poor X-ray rotation may introduce bias. Considering several risk factors, a diagnostic threshold was proposed. The primary limitation of this study is that only conventional MRI was used to measure the morphological parameters of this study, instead of 3D-MRI examination. When morphological parameters are measured, the ideal sagittal section should be perpendicular to the line connecting the posterior condyle of the femur [25], which is difficult to ensure with conventional MRI [26]. Conventional MRI with excessive section thickness will make it difficult to accurately identify the sections and points of interest, resulting in errors [27]. Compared with conventional MRI, 3D-MRI can perform high-resolution isotropic acquisition through unspaced thin layer scanning [28] and create multi-plane recombination images on any angle plane while reducing some volume artefacts, thus solving the problems of image presentation. The second limitation of this study is that the control group included a group of people who visited the hospital for anterior knee pain, not healthy people. Therefore, they could have higher risk of knee sprain with respect to the general population, potentially leading to underestimation of the investigated parameters.
