Rheumatoid arthritis is a chronic and progressive inflammatory disease of the joints. It affects approximately 1% of the general population, with incidence being three times higher in women than men. The areas most commonly involved are the metacarpophalangeal joints, the proximal interphalangeal joints, and the feet.

Small-parts ultrasound has become increasingly important for the early diagnosis of rheumatoid arthritis. Clinical findings, including swelling and joint tenderness, can be supported by an ultrasound examination that documents the characteristic pattern of inflammation.

Early stages of disease primarily affect the synovium. Acute synovial inflammation (synovitis) is characterized by increased synovial vascularity (neoangiogenesis) and is regarded as a predictive marker of erosive progression.^1,2 The chronic inflammatory process causes synovial proliferation and the development of the aggressive pannus that can undermine articular cartilage and cause irreversible erosions at the osteochondral junction (bare areas). Adjacent structures, such as the capsule, cartilage, ligaments, tendons (Figure 1), and adjacent bone, become involved during the course of the disease.