Entheses are sites where tendons, ligaments, joint capsules or fascia attach to bone. Inflammation of the entheses or enthesitis is a well-known hallmark of the spondyloarthritis (SpA) which recently gained special attention because of its central role in disease pathogenesis. Despite the understanding of an enthesis as being a focal tendon, ligament or capsular insertion to the bone, the recent concepts of an enthesis organ and functional enthesis have been proposed. Both concepts are based on the fact that the enthesis itself is associated with other structures adjacent to it and are functionally related and thus the enthesis needs to be viewed as one unit with its associated structures which has significant implications for interpreting imaging findings in entheseal related diseases. Conventional x-rays and computed tomography are able to depict the chronic changes associated with enthesitis but are of very limited utility in early disease. In contrast, magnetic resonance imaging (MRI) has been shown to be sensitive for the detection of early signs of enthesitis and can evaluate both soft-tissue changes as well as intraosseous abnormalities of active enthesitis and thus represents a significant advance for the early diagnosis of enthesitis related arthropathies and for monitoring therapy. Indeed MRI of axial spondyloarthritis is commonly used for disease monitoring. In this article we review the current knowledge and typical MRI features of the most frequently involved entheses of the appendicular skeleton (e.g. shoulders, elbows, fingers, hips, knees and feet) in patients with SpA. The MRI appearances of inflammatory and degenerative enthesopathy are described. New options for imaging enthesitis including whole body MRI and high-resolution microscopy MRI are briefly discussed.
- peripheral joints