Imaging

Plain radiographs are the most useful imaging tool for the initial evaluation of hip complaints. Radiographs can reveal degenerative disease, bony lesions, dysplastic changes, the presence of loose bodies, and impingement. An MRI allows improved visualization of the soft tissues, early degenerative changes, and osteonecrosis [12,13,18]. Plain MRI does not accurately identify labral or chondral defects primarily because the lack of joint distension makes it more difficult to assess the cartilage surface. Sekiya et al found that plain MRI is not adequate for measuring the articular cartilage of the hip joint in avascular necrosis (AVN) when compared with hip arthroscopy [19]. They suggested that either direct visualization by arthrotomy or arthroscopy of the hip joint is required for accurate evaluation and staging of cartilage, especially in Marcus-Enneking stage IV AVN [19].

MRI arthrogram allows improved visualization of intraarticular structures. Dilute gadolinium is injected into the joint of interest, which distends the capsule and allows better visualization of the articular cartilage. Labral tears can be identified by an abnormal linear extension of contrast solution into the labrum. Chondral lesions are better visualized when the cartilage defect is outlined by gadolinium. Keeney compared MRI arthrogram with arthroscopy of the hip and showed that the MRI arthrogram detected 76% of the acetabular labral tears [20]. Articular cartilage findings on the MRI arthrogram were confirmed using arthroscopy only 62.7% of the time. In Keeney's study [20], the MRI arthrogram had a sensitivity of 47%, specificity of 89%, a positive predictive value of 84%, and negative predictive value of 59%. Although MRI arthrogram offers improved evaluation of the chondral surface, this technique has been shown to have a high false-negative rate, thus limiting its usefulness in identifying true articular surface damage [18,20].

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