An ACL tear causes an anterior subluxation of the tibial bone which rotates with its fulcrum on the medial collateral ligament; consequently, the lateral femoral condyle impacts on the external tibial plateau. The presence of a subcortical lesion of the spongiform bone in the posterior external tibial plateau is an indirect sign of an ACL tear. On MR images, traumatic changes are depicted as changes in bone marrow signals. To assess the diagnostic capabilities of CT in demonstrating the bone lesion subsequent to ACL trauma, 23 patients with positive MR exams of the knee were submitted to CT. Fifteen patients had complete ACL tears, 8 had partial tears and 20 exhibited an occult fracture of the posterior portion of the external tibial plateau. This study pointed out a significant correlation between ACL tears and changes in the spongiform structure of the posterolateral tibial plateau. Such changes are depicted not only by MRI but also by CT: the latter method is also a valuable tool to study these conditions long after the traumatic event. No more than 5 slices, 1-1.5 mm thick, acquired at the tibial plateau allowed the trabecular structure and its abnormal changes to be studied without markedly lengthening examination time.