Purpose: To determine the accuracy of secondary magnetic resonance (MR) imaging signs of anterior cruciate ligament (ACL) insufficiency in predicting clinical anterior translational knee laxity, in the presence of an intact graft, after ACL reconstruction.
Materials and methods: Institutional review board approval was obtained, and the requirement to obtain informed consent was waived. Fifty-two patients with ACL reconstruction and no injury to the contralateral knee were included. Three patients with visible ACL graft tears at MR imaging were excluded. All patients underwent MR imaging of the affected knee, functional assessment with use of the International Knee Documentation Committee (IKDC) score, and arthrometric testing of both the affected and uninjured knee. A side-to-side difference of more than 3 mm at 133 N was considered to be indicative of knee laxity. Two radiologists independently evaluated all MR images for seven signs of anterior knee laxity. The sensitivity, specificity, negative predictive value (NPV), and positive predictive value (PPV) were calculated for each MR imaging sign.
Results: Ten patients demonstrated knee laxity at arthrometric testing. Patients with knee laxity at arthrometric testing had significantly lower IKDC scores (P < .03). Sensitivities for all signs were low (0%-50%). Anterior translation of the tibia of more than 7 mm, a posterior cruciate ligament (PCL) angle of less than 100°, and a PCL curvature ratio of more than 0.39 demonstrated high specificity (range, 82%-90%). Uncovering of the posterior horn of the lateral meniscus and the posterior femoral line had a specificity of 97%-100% and a sensitivity of 0%. All signs had a low PPV and high NPV for laxity. All MR imaging signs demonstrated near-perfect interobserver agreement.
Conclusion: Although MR imaging signs of knee laxity in the presence of an intact ACL graft have a high specificity, the low PPV means that MR imaging is of little value in predicting anterior knee laxity as demonstrated with mechanical testing.