Magnetic resonance imaging of anterior cruciate ligament (ACL) tears: diagnostic value of ACL-tibial plateau angle.

Tears of the anterior cruciate ligament (ACL) are often difficult to diagnose on magnetic resonance imaging (MRI), as it is often difficult to interpret the images. We developed a quantitative method for diagnosing ACL tears on MRI; the method involves measuring the angle formed by the intersection between a straight line drawn along the anterior border of the ACL and a straight line drawn along the medial tibial plateau (AT angle) on MRI. The AT angle was measured by two orthopedic surgeons approved by the Japanese Orthopedic Association and one orthopedic resident, none of whom had any prior knowledge of the physical and arthroscopic findings of the patients. Measurements were performed in 30 knees with ACL tears and 57 knees without tears. There was no significant difference between the AT angles measured by the three examiners in either the group with ACL tears or that without ACL tears. The mean value of the AT angle in the torn ACL group (33.9 +/- 1.1 degrees) was significantly smaller than that in the intact ACL group (52.3 +/- 1.0 degrees; mean +/- SE). When 45 degrees or less was set as the criterion defining a tear of the ACL, the diagnostic accuracy of our method was 87.4%; sensitivity was 93.3%, and specificity, 84.2%.