Is Lateral Femoral Notch Depth Associated with Rotatory Instability in ACL Deficient Knees: A Quantitative Pivot Shift Analysis

OBJECTIVES: Persistent rotatory knee instability after anterior cruciate ligament (ACL) reconstruction is relatively common. While the causes of this persistent instability are multifactorial, bony morphologic characteristics have been proposed to play a role. Therefore, the purpose of this study was to evaluate the relationship between the well-described lateral femoral notch (LFN) depth and quantitative measures of rotatory knee stability. We hypothesized that greater LFN depth would be associated with increased rotatory knee instability. METHODS: A consecutive series of patients undergoing primary ACL reconstruction at our university medical center from June 2014 to April 2016 were analyzed. Inclusion criteria included primary ACL tear, no concurrent ligamentous or bony injury requiring operative treatment, no history of previous knee injury or surgery to the ACL-injured extremity, and no history of injury or surgery to the contralateral knee. A standardized pivot shift test was performed by the senior surgeon preoperatively under anesthesia in both knees and quantified using tablet image analysis software and accelerometer sensors as previously described and validated. Lateral knee radiographs and sagittal magnetic resonance images (MRI) of the injured knee were evaluated for depth of the LFN as previously described. A line tangent to the lateral femoral condyle articular surface was drawn across the notch. Notch depth was measured perpendicular from this line to the deepest point of the LFN. Pearson correlation coefficient was used to analyze correlations between continuous variables. Chi-square test was used to analyze relationships between notch depth and presence/absence of medial or lateral meniscus tears. Analyses were performed with SPSS 22.0 and significance was set at a p<0.05. RESULTS: Fifty patients met inclusion criteria and were included in this study (mean age 24 years, range 13-45; 28 females, 22 males). Mean LFN depth as measured via x-ray was 0.8 mm (SD=0.63, n=50) and via MRI was 1.0 mm (SD=0.73, n=47). Twenty-two (44%) patients had a medial meniscus tear and 27 (54%) had a lateral meniscus tear. LFN on x-ray had moderate but significant positive correlations with ipsilateral lateral compartment acceleration (r=0.402, p=0.004) and acceleration side-to-side differences (r=0.407, p=0.003). LFN depth on MRI had moderate but significant positive correlations with ipsilateral lateral compartment acceleration (r=0.334, p=0.022) and acceleration side-to-side differences (r=0.363, p=0.012). LFN depth on x-ray was significantly associated with the presence of a lateral meniscus tear (p=0.014). There were no significant associations between LFN depth (x-ray or MRI) on ipsilateral or contralateral lateral compartment translation, contralateral lateral compartment acceleration, or the presence of medial meniscus tears. CONCLUSION: The results from this study demonstrated that a well described bony morphologic feature - LFN depth - was correlated with higher lateral compartment acceleration as measured by quantitative pivot shift analysis. Furthermore, greater LFN depth was associated with an increased incidence of lateral meniscus tears, which supports findings from previous studies. Assessment of LFN depth may help clinicians identify patients with greater rotatory instability prior to ACL reconstruction and potentially direct surgical treatment to account for additional rotatory knee instability.