The name ankylosing spondylitis (or AS for short) is derived from Greek roots angkylos = ”bent” (although the word ankylosis now means joint stiffening or fusion) and spondylos = ”spinal vertebra”.
It is a chronic, inflammatory rheumatic disease with a prevalence of about 0.5% in epidemiologic studies in Europe, but the prevalence can vary in other populations.
It is the prototype of a group of diseases that share clinical and pathogenetic features and are grouped under the term "Spondyloarthropathies" (SpA) that include, in addition to AS, reactive SpA, psoriatic SpA, enteropathic SpA (associated with Crohn's disease and ulcerative colitis) and undifferentiated SpA.
These diseases can occur within families because of a strong genetic predisposition highlighted by its strong association with HLA-B27, a normal gene that is also present in a small percentage of the general population.