Partial tear of the anterior cruciate ligament of the knee: injury patterns on MR imaging.

PURPOSE: To describe rupture patterns of partial anterior cruciate ligament (ACL) tears on magnetic resonance (MR) imaging.

METHODS: MR images of 51 patients with a surgically confirmed partial ACL tear were retrospectively and independently interpreted by 2 experienced, blinded radiologists. Using previously described MR criteria, ACLs were categorized as follows: complete tear, partial tear, isolated anteromedial or posterolateral bundle tear, mucoid degeneration or normal ACL. MR interpretations were compared with the arthroscopic results as the standard of reference. Inter- and intraobserver agreements were determined using kappa (к) coefficients.

RESULTS: On MR imaging, ACL injuries were categorized as complete tears (16-23%), partial tears (20-47%), mucoid degeneration (12-27%) or normal ACLs (18-23%). Isolated ACL bundle tears were diagnosed on MR in 6% of our patients. Accuracy of MR for the diagnosis of partial ACL tears was 25-53%. Interobserver agreement was moderate (к = 0.48-0.56). Intraobserver agreement was good (к = 0.72-0.76).

CONCLUSION: MR diagnosis of a partial ACL tear is difficult because various tear patterns may be seen. Many partial tears demonstrate MR features that are indistinguishable from complete ACL tear, mucoid ACL degeneration or normal ACL. An isolated ACL bundle tear is infrequently detected on MR images.