Enthesitis in seronegative spondyloarthropathies with special attention to the knee joint by MRI: a step forward toward understanding disease pathogenesis.

Seronegative spondyloarthropathies are a unique group of disorders sharing similar clinical characteristics (e.g., inflammatory back pain, spondylitis, sacroiliitis, uveitis, inflammatory bowel disease, skin rashes, and enthesitis). Clinical and genetic similarities suggest that they also share similar causes or pathophysiologies. Rheumatoid factor (RF) is characteristically negative in this group of disorders, hence collectively termed seronegative spondyloarthropathies (SpA). They include psoriatic arthritis, ankylosing spondylitis, reactive arthritis, ulcerative colitis, and Crohn's disease. "Enthesitis", the term used to describe inflammation at tendon, ligament, or joint capsule insertions, is considered a common feature in this domain and was included in the European Spondyloarthropathy Study Group criteria for the classification of SpA. Evaluation of entheseal-related changes at different joints by MRI became an important item on the research agenda in both differentiated and undifferentiated arthritis. Most of the research focused on MRI findings in the hand and wrist joints among patients with RA and SpA and support two patterns of inflammation "RA" phenotype where synovial involvement is the primary target of inflammation and "SpA" pattern where enthesitis comes first followed by synovitis. In this review, we summarize the literature on enthesitis in SpA and focus on MRI findings in the knee joint in the SpA group of disorders and subclinical synovitis among patients with skin psoriasis.