Enthesitis

Enthesitis is inflammation at the entheses, the sites where tendons, ligaments, or joint capsules insert into bone. It is a common feature of seronegative spondyloarthropathies and can involve any entheseal site, with certain locations such as the Achilles tendon insertion, plantar fascia near the heel, the iliac crest, and the medial epicondyle being frequently affected.

Entheses are part of the broader enthesis organ, which includes adjacent bone, bursa, and fat pad. Inflammation may arise from immune-mediated processes, mechanical stress, or a combination of factors. The pathophysiology involves inflammatory pathways such as TNF and the IL-23/IL-17 axis, and is influenced by genetic predisposition and repetitive microtrauma. Both inflammatory and mechanical mechanisms contribute to symptom development and tissue changes.

Clinically, enthesitis presents as focal tenderness and pain at the insertion of a tendon or ligament, sometimes with swelling or reduced function of the nearby joint. Pain is often aggravated by activity and may be accompanied by morning stiffness. Distinguishing inflammatory enthesitis from noninflammatory tendinopathy or fascial irritation can be challenging; the presence of additional features suggestive of a spondyloarthropathy (eg, inflammatory back pain, peripheral arthritis, psoriasis, inflammatory bowel disease) supports an inflammatory etiology.

Diagnosis relies on clinical assessment supported by imaging. Ultrasound may show thickened, hypoechoic entheses with calcifications or enthesophytes and adjacent bursitis. MRI can reveal bone marrow edema at the enthesis and surrounding soft tissue inflammation. Laboratory tests may help identify associated conditions (eg, HLA-B27 positivity in some spondyloarthropathies) but are not specific for enthesitis.

Treatment aims to control the underlying disease and relieve symptoms. NSAIDs, physical therapy, and targeted injections for focal sites are common first steps. Inflammatory enthesitis related to spondyloarthropathies may require disease-modifying antirheumatic drugs or biologics (such as TNF or IL-17 inhibitors). Management also focuses on reducing mechanical stress and addressing comorbid risk factors. The prognosis varies with the underlying condition and response to therapy.