Abstract
Aims and Objectives:
Interindividual variations in knee laxity are poorly understood. Utilizing novel pivot shift arthrometers and patients in our ACL registry the magnitude of the pivot shift was found to vary between 0-14 mm of anterior translation in the lateral compartment. Increased tibial slope was found to influence the magnitude of the pivot shift, however, it is unclear if femur morphology might influence the magnitude of the pivot shift. Therefore, the purpose of this study was to determine the relationship between anterior translation of the lateral compartment during a quantitative pivot shift test and the posterior femoral offset, quantified as the “tomahawk ratio”, in patients with complete ACL rupture. It was hypothesized that the “tomahawk ratio” would correlate with increased anterior translation of the lateral compartment of the knee.
Materials and Methods:
Consecutive patients with no history of prior knee surgeries undergoing primary ACL reconstruction were analyzed. A standardized pivot shift test was performed preoperatively under anesthesia on both knees and quantified using tablet technology as previously described. Radiographic measurements were made using standard lateral knee radiographs. The long axis of the femur shaft was determined by a line through the center of two circles centered on the femoral shaft. The axis of the femoral condyle was determined by a line between the most posterior and most anterior points of the lateral condyle. The distance from the intersection of these lines to the posterior end of the condyle was divided by the total anteroposterior length of the condyle. This ratio was defined as “tomahawk ratio”. Pearson correlation coefficient was used to analyze correlations (p- value < 0.05).
Results:
Data sets were successfully obtained for 32 female and 25 male patients. The mean anterior translation of the lateral compartment during the pivot shift test was found to be 3.96 ±2.38 mm and 1.27±0.89 mm for the involved and uninvolved knees, respectively, with a mean side-to-side difference of 2.70±2.25 mm. The mean length of the lateral femoral condyle on x-ray was 68.36±5.31 mm, and the mean tomahawk ratio was 63.16±4.53%. There was a significant correlation between the tomahawk ratio and the absolute quantitative (r = 0.370 p < 0.05) and side-to-side differences in anterior translation of the lateral compartment (r =0.419 p < 0.05).
Conclusion:
The main finding from this study is that ACL-deficient patients with larger posterior femoral offsets quantified as the “tomahawk ratio” were found to have higher lateral translations of the knee during the pivot shift test. This suggests that variation in knee instability may be significantly affected by femoral bony morphology characterized by a larger posterior portion of the lateral condyle. This study may assist clinicians in evaluating ACL injuries and identifying patients at greater risk for increased rotational knee instability.