Heterogeneous group of arthritic diseases sharing clinical and radiologic features. They are associated with the HLA-B27 ANTIGEN and some with a triggering infection. Most involve the axial joints in the SPINE, particularly the SACROILIAC JOINT, but can also involve asymmetric peripheral joints. Subsets include ANKYLOSING SPONDYLITIS; REACTIVE ARTHRITIS; PSORIATIC ARTHRITIS; and others. [MeSH]
