A Tomahawk Shaped Femur is a Risk Factor for Anterior Cruciate Ligament Injury

AIMS AND OBJECTIVES: Bony morphologic characteristics have been demonstrated to increase the risk of anterior cruciate ligament (ACL) injury. The purpose of the study was to examine distal femoral morphology relative to ACL injury, reconstruction failure, and contralateral ACL injury. It was hypothesized that an increased posterior femoral condylar depth, quantified as the tomahawk ratio, would correlate with increased risk of primary ACL ruptures, ACL reconstruction failures, and contralateral ACL injuries. MATERIALS AND METHODS: Consecutive patients undergoing arthroscopic knee surgery 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 tomahawk 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 tomahawk ratio cut-off for detecting increased risk for ACL injury RESULTS: 175 patients met inclusion criteria. The mean tomahawk ratios in the control group, primary ACL injury group, failed ACL reconstruction group, and contralateral ACL injury group 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 tomahawk ratio compared to the control group (p<0.008). ROC analysis demonstrated a tomahawk ratio of 63% or greater to be associated with an increased risk for ACL injury with a sensitivity of 83% and a specificity of 71%. CONCLUSION: The data from this study show that an increased posterior femoral condylar depth, or tomahawk ratio, is associated with increased risk of ACL injury, including primary ACL injury, failed ACL reconstruction, and contralateral ACL injury. Readily identifiable risk factors, such as an increased tomahawk ratio, could assist clinicians in identifying at-risk individuals who may experience greater benefit from targeted ACL injury prevention counseling and intervention. The presence of the tomahawk-shaped femur could also be used to guide treatment decisions and identify ACL reconstruction patients who may benefit from additional surgical procedures such as extra articular tenodesis.