Imaging and differential diagnosis of masses within a joint.

Careful analysis of conventional radiographs remains the first step in the investigation of intra-articular abnormalities. However, other imaging techniques, such as ultrasonography, arthrography, computed tomography and magnetic resonance imaging, may add critical information that allows the correct diagnosis. The authors discuss the evaluation of masses within a joint on the basis of plain film analysis, as well as the contribution of additional imaging techniques to the differential diagnosis.