Introduction and purpose: To assess the diagnostic usefulness of MRI to diagnose ACL lesions using quantitative instead of qualitative parameters.

Materials and methods: A retrospective study was made of the MRIs of a group of 50 patients with an athroscopically confirmed ACL tear and a control group of 50 patients with meniscal lesions and with an arthroscopic confirmation that they had a normal ACL. Multiple MRI findings were studied in order to evaluate their sensitivity and Specificity to detect an ACL lesion. Special emphasis was placed on 3 quantitative parameters, including a simplified method to measure the angle between the ACL and the tibial plateau.

Results: Using 45° as a cutoff value, the ACL/tibial plateau angle yielded a sensitivity and Specificity of 100%. With a 0° cutoff value, the angle together with Blumensaat’s line showed a sensitivity of 90% and a Specificity of 98%. With a cutoff value of 115°, the PCL angle showed a sensitivity of 70% and a Specificity of 82%

Conclusions: The quantitative parameters studied are valuable to predict ACL lesions and can increase both the sensitivity and Specificity of MRI images. The ACL/ tibial plateau angle can be measured easily using a single MRI image and can be regarded as the best clue to diagnose ACL lesions.