John O'Daniel, age 13, of Columbia, Tenn., for his question:

WHERE IS THE SACROILIAC?

The sacroiliac joint is a fibrous joint in the lower back between the lumbar vertebrae and the coccyx, the bones of the lower end of the spinal column. It is composed of five fused vertebrae that form a solid triangular bone (the sacrum) and the ilium parts of the two hip bones.

Once believed to be the site of sprain and lower back pain, it is in fact bound by powerful ligaments that avert injury. Such pain is more generally caused by a displaced vertebral disk.

The sacroiliac joint is subject to a number of diseases embraced by the term spondylitis, an inflammation of the vertebrae that is often rheumatic in character. Spondylitis is frequently accompanied by ankylosis, a chronic disease that progresses up the spine, fusing the vertebrae.

The different forms of spondylitis are controlled by early recognition, diagnosis by X ray and treatment by deep radiation therapy, cortisone, exercise and orthopedic procedures.