A Cam Shaped Femur Might Be A Risk Factor for ACL Injuries

OBJECTIVES: Bony morphologic characteristics have been demonstrated to increase the risk of anterior cruciate ligament (ACL) injury. While posterior femoral condyle condylar offset is an aspect of distal femoral bony morphology that has been reported to influence range of motion and other aspects of knee joint kinematics, it remains unclear whether this characteristic influences the risk of ACL injury. The purpose of the study was to examine the relationship between distal femoral morphology and risks of ACL injury, reconstruction failure, and contralateral ACL injury. It was hypothesized that increased posterior femoral condylar depth, quantified as the cam ratio, would correlate with increased risk of primary ACL injuries, ACL reconstruction failures, and contralateral ACL injuries. METHODS: Consecutive patients undergoing evaluation for knee complaints at an academic medical center from 2012-2016 with minimum 24-month follow-up were retrospectively reviewed. Subjects were stratified into four groups: a control group consisting of patients with no ACL injuries and three groups of patients with a primary ACL injury, failed ACL reconstruction, or previous ACL injury with subsequent contralateral ACL injury. Using lateral radiographs, the ratio of posterior condylar depth over total condylar distance was defined as the cam ratio. Analysis-of-variance (ANOVA) and post-hoc testing were used to test for differences in the mean tomahawk ratio between study groups (p<0.05). Receiver Operating Characteristic (ROC) analysis was performed to determine the optimal cam ratio cut-off for detecting increased risk for ACL injury. RESULTS: One hundred and seventy-five patients met inclusion criteria. The mean cam ratios in the control, primary ACL injury, failed ACL reconstruction, and contralateral ACL injury groups were 61.1% (± 2.1), 64.2% (± 3.8), 64.4% (± 3.6), and 66.9% (± 4.0), respectively. Patients with a primary ACL injury, failed ACL reconstruction, or contralateral ACL injury had a significantly higher cam ratio compared to the control group (p<0.008). ROC analysis demonstrated a cam ratio of 63% or greater to be associated with an increased risk for ACL injury with a sensitivity of83% and a specificity of 71%. CONCLUSION: The data from this study show that an increased posterior femoral condylar depth, or cam ratio, might be associated with increased risk of ACL injury, including primary ACL injury, failed ACL reconstruction, and contralateral ACL injury. The data from this study may help clinicians identify patients at greater risk of ACL injury and re-injury. Future prospective studies will be helpful in investigating the mechanism by which an increased cam ratio increases ACL injury risk and potential strategies to mitigate the increased risk posed by this bony characteristic.