Secondary signs on static stress MRI in anterior cruciate ligament rupture

The MRI diagnosis of ACL rupture based on primary signs has variable rates of diagnostic accuracy. These signs observed on direct visualisation of the ACL include discontinuity of fibres, abnormal ligament contour and increased intrasubstance signal intensity. Secondary radiological signs of ACL rupture are increasingly being used. These indirect ancillary signs include PCL angle, translocation of the tibia relative to the femur and displacement of the posterior horns of the menisci. The aim of this study was to investigate if the application of static stress force to the knee will accentuate the secondary signs of ACL rupture on MRI. One hundred and forty-eight subjects (50 subjects with arthroscopically confirmed ACL rupture, 48 subjects with arthroscopically confirmed intact ACL and 50 subjects with clinically confirmed normal knees) underwent MRI of their knee with the application of specially designed fibreglass leg splints which exert a translational force on the knee joint. Five secondary radiological signs were evaluated and all were found to be significantly accentuated following the application of the splints (p<0.001). The MRI diagnosis of ACL rupture demonstrated a sensitivity of 95.9%, specificity of 91.7% and accuracy of 93.8%. The static stress technique improved the accuracy of diagnosing ACL ruptures on MRI and illustrated the abnormal tibiofemoral kinematics.