Sacroiliitis is inflammation of the sacroiliac (SI) joint. The SI joint is the joint located on either side of the sacrum (the lower spine) that connects the iliac bone to the hip. Until recently, many people's lower back pain was attributed to herniated discs in connection with degenerative disc disease, but today inflammation of the sacroiliac joint is recognized as the cause of pain in the lower back and buttocks.

Sacroiliitis is not usually caused by osteoarthritis, but by and inflammatory condition called ankylosing spondylitis. Ankylosing spondylitis is a chronic disease that causes inflammation of the joints between the spinal bones, and the joints between the spine and the pelvis. Ankylosing spondylitis is the most common spondyolarthropathy, a collection of inflammatory diseases that primarily affects the spine and SI joints. Sacroiliitis is a chronic inflammatory disease. Most patients with ankylosing spondylitis are HLB27 positive and rheumatoid factor negative. The etiology of sacroiliitis is not known, and this condition tends to affect young, adult, white males.

Sacroiliitis is a hallmark of the disease of ankylosing spondylitis, and is the usual site of initial involvement. The synovial portion of SI joints is that portion of the joint that is most frequently involved, and that is the inferior and anterior portion of the joint. Other enthesopathies like (Diffuse Idiopathic Skeletal Hyperostosis) DISH can cause bridging of the upper, non-synovial part of the joint.

Ankylosing spondylitis begins with widening of the sacroiliac joint and erosions, and later goes on to form ankylosis and fusion, and it is bilaterally symmetrical. On X-Ray radiography, you don't usually see ankylosing spondylitis in its initial phase, when the SI joint is widened. You usually see it in the healing phase, when the SI joint bilaterally are fused (and where you sometimes see a fusion of the synthesis pubis). You can usually see a fusion of the SI joints bilaterally on and X-Ray or a CT scan.

Ankylosing Spondylitis. Ankylosing spondylitis of the spine usually begins at the thoracolumbar or lumbosacral junctions. It extends symmetrically without skips areas, unlike other inflammatory joint conditions such as psoriasis and Reitter's which are characteristically asymmetric and have skip areas. The hallmark lesion in the spine is marginal syndesmophyte formation, which are thin vertical dense spicules of bone bridging the vertebral bodies, that represent ossification of the outer fibers of the annulus fibrosis. On an X-Ray, you can see a fusion of the SI joints and the thin bridging syndesmophyte on the spine.

Early in the disease. There may be straightening or squaring of the anterior vertebral margins early in the disease, which is due to an osteitis of the anterior corners of the vertebral bodies. Bamboo spine is the appearance that's been described on the frontal view of the spine because of the undulating contour of the spine due to the syndesmophytes.

There may be reactive sclerosis at the corners of the vertebral bodies, which is called the shiny corner sign. And patients with ankylosing spondylitis of the spine are prone to fracture of the spine which may lead to pseudoarthrosis.