Lateral plain X-ray of the thoraco-lumbar spine. Straightening of the spinal column and loss of the normal lordosis are apparent. Squaring of the vertebral bodies with sclerosis at the vertebral corners is visible. Courtesy Dr. N. Haramati.

may help detect decreased lumbar mobility. A mark is made on the skin overlying the low back region at two points while the patient is in the upright position. The lower point is placed between the dimples of Venus—at the level of the posterior superior iliac spines—and the more proximal point is placed ten centimeters cranially. The distance between the skin marks is measured again following forward bending. If the distance between the skin markings increases by more than five centimeters, it attests to normal lumbar motion (Figure 8-3). In patients with AS most of the forward bending would occur at the hip joints and the distance between the skin marks will increase only minimally, less than three centimeters.

Imaging Studies

Initially, enthesopathic changes, erosions followed by sclerosis, appear at the corners of the vertebral bodies and lead to squaring of the vertebrae. These changes can be easily detected on plain films (Figure 8-4). Erosions and whiskering appear in the sacroiliac joints and in the pubic region. These can be detected on CT and MRI and help reach a diagnosis at an early stage of the disease. Although CT excels in erosion detection and finding surface irregularities, the MRI may show signal changes (decreased on T1 and increased on T2) around these erosions and the corners of the vertebral bodies. Initially, widening of the sacroiliac joint space occurs and later on blurring of the sac-roiliac joint margins will be seen. Over the course of months or years

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