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Have you missed a fracture associated with an ACL injury? Plain radiography generally fails to detect subchondral fractures around the knee associated with ACL tears. 1 However, those fractures are frequently seen on MRI in patients with acute ACL tear.
When radiologists report 'fractures' based on MRI findings, they refer to a very different entity than bone bruises, traumatic bone marrow oedema or contusion, which all refer to an identical image finding-an ill-defined signal alteration without definite evidence of a fracture line. On MRI, a fracture is defined as a dark (low signal intensity) line on the different pulse sequences acquired, commonly surrounded by bone marrow oedema. These 'dark lines' are usually better depicted using T1-weighted image without fat suppression ( figure 1 ). When using MRI to assess the knee, such fractures are present in as many as 60%-72% of knees after an acute ACL tear. 2 3
Different types of fractures include cortical depression fractures and trabecular fractures. Cortical depression fractures are defined as depressed cortical bone with or without cortical discontinuity. A trabecular fracture is defined as a line of low T1 and T2 signal intensities usually extending from the cortex into the trabecular bone with surrounding bone marrow oedema. 2 Dr Kijowski and colleagues reported that 81 of the 114 patients in their study group (71%) had at least one fracture within the knee joint, including 54 non-articular cortical depression fractures in the lateral tibial condyle and 40 articular cortical depression fractures. Regarding type of fractures, cortical depression fractures (81 patients: 71%) were much more common than trabecular fractures (six patients: 5%). 2 Another study showed 89 MRI-detected radiographically occult fractures in 56 knees with acute ACL tear. 1 While pure bone contusions healed without sequelae, fractures (subchondral or osteochondral) were associated with cartilage abnormalities at follow-up in more than 50%...