Ankylosing spondylitis (AS) is
a rheumatic disease involving the SIJ with a potential concomitant involvement
of spinal joints. The disease symptoms in the form of back pain and stiffness typically
start between 20 and 30 years of age, but may occur earlier or later.
The disease is due to a rheumatic inflammation primarily involving the
sacroiliac and spinal joints in addition to areas where tendons, ligaments and
joint capsules are attached to bone, areas containing fibrocartilage and
Untreated, the disease often results in increasing spinal stiffness due to
fusion of the joints when the inflammatory changes heal.
The aetiology of AS is yet unknown, but there is, among other things, a
hereditary factor, since many patients have a hereditary tissue type HLA B27.