Osteoid osteoma with severe spinal stiffness simulating ankylosing spondylitis.

A highly atypical case of osteoid osteoma of L1 is reported. The patient pr esented with severe spinal stiffness and bilateral alternating pain in the upper part of the buttocks extremely responsive tb nonsteroidal antiinflamm atory therapy. There was-no scoliosis. The first diagnosis considered was a nkylosing spondylitis, although plain films of the spine and sacroiliac joi nts were normal.-Careful analysis of physical findings with routine examina tion for painful spinal stiffness with or without the classical spinal scol iosis is essential to the diagnosis of osteoid osteoma. When plain radiogra phs are normal, as is often the case, whole body radionuclide bone scanning is valuable for identifying the site of this benign tumor, which can then be further investigated using other imaging techniques.