Magnetic resonance imaging and axial involvement in spondylarthropathies. Delineation of the spinal entheses.

We used magnetic resonance imaging in a prospective cross-sectional study to evaluate the components of axial involvement in spondylarthropathies, to determine whether the entire intervertebral disk is an enthesis and to gauge how useful this imaging technique is in detecting enthesitis of the spine. Thirty-one patients with spondylarthropathies and 14 controls with mechanical spinal disease were included. Images of the thoracic and lumbar spine were obtained using plain radiography, radionuclide bone scanning, and magnetic resonance imaging (sagittal sections, T1-weighted sequences before and after gadolinium injection and fat saturation and T2-weighted sequences). Magnetic resonance imaging signal abnormalities reflected inflammation and hypervascularization of the subchondral bone underlying the affected entheses (low signal enhancing after gadolinium and fat saturation on T1 images, high signal on T2 images). These abnormalities were often visible early in the disease process, at a time when there were not yet any clinical manifestations or radiographic or bone scan changes. In addition to showing involvement of the classic spinal entheses, magnetic resonance imaging also demonstrated evidence of inflammation and hypervascularization of the central part of the vertebral endplates and intervertebral disks, confirming that the center of the disk is an enthesis and that inflammatory enthesitis is the mechanism underlying at least some cases of discitis seen in patients with spondylarthropathies.