Rotatory laxity evaluation of the knee using modified Slocum's test in open magnetic resonance imaging

Purpose The authors have developed a quantitative method to evaluate rotatory laxity of the anterior cruciate ligament (ACL)-deficient knee using an open MRI. In this study, we evaluated the correlation between the pivot-shift test grading and values measured with this method. Furthermore, the articular contact area in the lateral compartment during the pivot shift was examined. Methods Ninety-two subjects were evaluated in this study. Among them, 36 subjects were evaluated before the ACL reconstruction, and 56 subjects were evaluated 1 year after the ACL reconstruction. Subjects were placed in the open MRI scanner in the posture based on the Slocum's ALRI test. Anterior translation of the tibia was measured in the lateral and medial compartments in the sagittal images of the MRI. The correlation between the grading of the pivot-shift test and either anterior translation of the lateral compartment, or the difference between translation of the medial compartment and the lateral compartment, was evaluated by logistic analysis. Furthermore, we examined the contact area of the articular surface in the lateral compartment with variation in the pivot-shift grading. Results The anterior translation of the lateral compartment had a significant correlation with the pivot-shift grading. A discriminant analysis showed that the lateral translation could predict the grade of the pivot-shift test more accurately than the lateral minus medial translation (84.8% vs. 70.7%, P < 0.001). The summit of the convex lateral tibial articular surface is located anterior to the closest point between the lateral femoral condyle and the tibial plateau in the pivot-shift-positive subjects, while it is located posterior to the femoral condyle in the pivot-shift-negative subjects. Conclusion The anterior translation in the lateral compartment measured with this method allows objective and quantitative evaluation of the rotatory laxity that causes the pivot-shift phenomenon. The grade of the pivot-shift test is determined by the magnitude of the movement of the lateral compartment when the femoral condyle surmounts the convex tibial articular surface, rather than by the deviation of the rotational angle of the tibia.