[Occult post-traumatic osteochondral changes in the knee. Assessment with magnetic resonance]

Occult posttraumatic osteochondral injuries of the knee are bone and/or cartilage abnormalities which cannot be detected on plain radiographs. This study was aimed at investigating MR capabilities in the detection and characterization of occult bone and/or cartilage injuries in symptomatic patients with previous musculoskeletal trauma of the knee. We retrospectively selected 60 patients (45 men and 15 women; mean age: 33.1 +/- 16.3 years; range: 12-70 years) from our patients submitted to MRI of the knee during a 3-year period. These patients had a history of previous acute musculoskeletal trauma, negative conventional radiographs and MR signal intensity changes of the osteochondral structures. MR protocol included: SE T1-, PD and T2-weighted images and GRE T2-weighted images; imaging planes were the sagittal, the axial and the coronal planes. Plain radiographs and MR images were interpreted by three experienced readers in musculoskeletal radiology. Our reviewers confirmed normal conventional radiographic findings in all the patients enrolled in the study. According to morpho-topographic and signal intensity patterns, we identified three types of occult posttraumatic injuries: bone contusions or bone bruises or occult subcortical fractures (n 30), osteochondral injuries (n 26) and chondral injuries (n 4). A bone contusion was defined as a typical subcortical area of signal loss, with various shapes, on T1-weighted images and increased signal intensity on T2-weighted images. The cortical bone and articular cartilage below were normal in all cases. On the contrary, osteochondral lesions presented an association of cartilage and bone injuries with the same M(R) signal abnormalities. Thinning and focal interruptions of the cortical bone were demonstrated in all cases. Chondral lesions were characterized by a sudden discontinuity and irregularity of cortical bone outline and by small high-intensity spots in cartilage thickness on T2-weighted images, in the absence of any subchondral bone abnormality. Finally, 42 of 60 patients (70%) had an anterior cruciate ligament tear and 28 (45%) had a medial meniscus tear. In conclusion, MRI appears a very useful tool in the detection and characterization of the different types of occult bone and/or cartilage injuries of the knee in the patients with previous acute trauma. Moreover, the correct and early diagnosis of an osteochondral lesion does affect prognosis.