Enthesitis.

Enthesitis is a distinctive pathological feature of spondyloarthritis and may involve synovial joints, cartilaginous joints, syndesmoses and extra-articular entheses. This review focuses on peripheral extra-articular enthesitis, which is a clinical hallmark of spondyloarthritis. The entheses of the lower limbs are more frequently involved than those of the upper limbs, and heel enthesitis is the most frequent. Entheseal pain may be mild or moderate as well as severe and disabling. Peripheral enthesitis may be observed in all forms of spondyloarthritis, including the undifferentiated forms, and may, for a prolonged period, be the only longstanding clinical manifestation of the B27-associated disease process. The conceptual understanding of spondyloarthritis and the ability to image sites of skeletal inflammation accurately, i.e. ultrasound and magnetic resonance imaging, confirm that enthesitis is the primary lesion of spondyloarthritis. This advance has been occurring simultaneously with the therapeutic advances in spondyloarthritis due to the introduction of anti-tumour necrosis factor-alpha agents.