ABSTRACT
Ankylosing spondylitis (AS) is the third most common form of chronic arthritis in the United States. It is an inflammatory condition affecting the hips, sacroiliac joints, and the spine. AS affects about 1-3 per 1000 of the general population (1). According to the Modified New York criteria, AS is characterized by back pain in a patient younger than 40 years persisting for at least 3 months, associated with morning stiffness and improving with exercise. There is limitation of movement of the lumbar spine in the sagittal and frontal planes and decreased chest expansion compared to the normals for age and sex. On plain films of the spine there will be unilateral or bilateral sacro-iliitis. Tissue typing is not included in any set of diagnostic criteria, although there is a high degree of association between HLA-B27 positivity and definite ankylosing spondylitis (2). The site of inflammation is at the junction of fibrous tissue and bone such as tendons and ligaments. This process leads to erosion of bone and ossification of the tendons and ligaments. In the spine this occurs at the insertion of the anulus fibrosis on the vertebral body. Erosion here leads to squaring of the vertebrae, while ossification leads to syndesmophytes and the classic ‘‘bamboo spine’’ caused by calcification of the longitudinal ligaments (3). This fusion of the spinal column causes a decrease in movement and elasticity resulting in altered biomechanics of the spine, rendering it susceptible to a variety of disorders including fracture, dislocation, progressive spinal deformity, atlanto-occipital or atlanto-axial subluxation, and spinal stenosis. The rigidity and immobility of the AS spine has a propensity to develop osteoporosis, further increasing the risk of fracture (4).
