Acute and chronic tears of the anterior cruciate ligament: differential features at MR imaging.

To evaluate the differential features of acute and chronic tears of the anterior cruciate ligament at magnetic resonance (MR) imaging, the authors performed a retrospective evaluation of findings in 81 MR examinations correlated with results at arthroscopy. Intact anterior cruciate ligaments (ACLs) were present in 29 patients; acute complete ACL tears, in 22; and chronic complete ACL tears, in 30. Acute tears were accurately distinguished from intact ligaments and were characterized by the presence of edema. Chronic tears had a more variable appearance: Nine (30%) were depicted at MR as intact bands with low signal intensity that bridged the expected origin and insertion of the ACL. This appearance is likely due to the presence of bridging fibrous scars within the intercondylar notch. Five of these nine cases were correctly characterized as chronically torn because of the presence of focal angulation. In four of these nine cases the scarred fragments produced a relatively straight band that mimicked an intact ligament. Although chronic and acute ACL tears usually have distinct findings at MR, a chronic tear will occasionally be difficult to distinguish from an intact ligament.