Correlation between spatial inclination of Blumensaat line and anterior cruciate ligament injury based on EOS biplanar 3D imaging system

Abstract

To explore correlation between femoral mechanical axis and Blumensaat line (FMBL) angle of knee joint (angle between Blumensaat line and femoral mechanical axis), α angle (angle between Blumensaat line and axis of distal femur in sagittal plane) on EOS biplane imaging and non-contact anterior cruciate ligament(ACL) injury, and evaluate angle for its accuracy in predicting the populations prone to non-contact ACL injury. From February 2018 to October 2020, EOS imaging and clinical data from 88 patients (176 knees) with unilateral non-contact ACL injury were retrospectively analyzed, including 53 males and 35 females, aged from 18 to 45 years old with an average of (30.3±6.2) years old, 48 patients on the left side and 40 patients on the right side. The patients were divided into ACL-affected group and ACL-health group according to side of ACL injuries, and 51 patients (51 knees) with non-ACL identified from EOS database were included in normal control group, including 28 males and 23 females, aged from 20 to 44 years old with an average of (31.6±5.5) years old, 26 patients on the left side and 25 patients on the right side. Full-length EOS imaging of skeleton extremitatis inferioris among three groups were reconstructed to 3D images of skeletal system with EOS software, and then FMBL angle and α angle were measured on the images. Univariate binary Logistic regression analysis was performed to determine the influence of the univariate(FMBL angle or α angle) on ACL status(normal or torn). And the angle cutoff value for univariate was selected based on receiver operating characteristics curve (ROC) to got the best accuracy. There was no statistically significant difference in age, gender and side distribution between ACL-injured group and normal control group(P>0.05). Statistical analyses (one-way ANOVA) indicated no significant difference in FMBL angle between ACL-injured knee group (32.8±2.3)° and ACL-injured contralateral knee group(32.5±2.3)°(P>0.05), but the values between two groups were significantly lower than that in normal control group (37.0±2.0)°(P<0.001). There was no statistically significant difference in α angle among three groups (P>0.05). Univariate binary Logistic regression analysis demonstrated that FMBL angle was risk factor for non-contact ACL injury[OR=0.433, 95%CI(0.330, 0.569), P<0.001]. The area under ROC curve for FMBL angle was 0.909[95%CI(0.861, 0.958), P<0.001], and the sensitivity and specificity were 70.5% and 98.0% respectively, cut-off value was 33.7°. FMBL angle formed by Blumensaat line and femoral mechanical axis is one of the risk factors for non-contact ACL injury and has good predictive accuracy. The general population with FMBL angle below 33.7° may be increased risk for ACL injury.