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Regarding Ankylosing Spondylitis (AS)
Basic bone growth and AS
More AS implications
AS pathological physiology
View animations of AS development



follows the ligaments at the rim of the upper and lower vertebra and can eventually encapsulate the disk.

The growth of these bony protrusions may fully bridge the vertebras where ligament once was along side the disk. This advanced condition is termed "bamboo spine," as an x-ray displays this vertebral column as a continuous structure of bone shaped like bamboo with vertebra and bulbous growth of bone between them. Other lesser developed ossification is termed "postage stamp" because of the x-ray visible bony protrusions that follow the ligament's fibrous strands that run between the vertebra. This abnormal ossification is what fuses the vertebral column and gives it its rigid nature.

The typical vertebral joint consists of the vertebras, the disk, the ligaments, and muscle. Among the ligaments

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necessary are those associated with the outer portion of the vertebral disk that are called the anulus fibrosi. This ligament is vital in the disk's attachment to the vertebrae and spans every intradisk space.

Prior to ossification, the ligament's attachment at the rim of the vertebra becomes inflamed. Lesions form on the vertebra rim at the sites of this inflammation, a deteriating process known as enthesopathy. The reason for this inflammation is unknown. This inflammatory condition exists sporadically for many years, causing pain and some stiffness. This enthesopathy is known to occur along the anterior rim of the vertebra rim (the side


facing the abdomen). The inflammation eventually causes bone to erode at the rim where the inflamed ligament is attached.

Findings indicate that bone cells fill in the fibrous tissue of the ligament to repair the defect caused by the eroded bone. This new bone is woven into the inflammed fibrous ligament tissue, joining the underlying bone of the vertebra with the fibrous tissue of the ligament, thus causing the bone surface to protrude outside the original vertebral surface. The result of the inflammation's healing process are bony protrusions at the vertebra's original ligament attachment. Over time, these protrusions grow and tend to fuse with one another along the length of the spine. This process usually occurs at several places on the vertebra and may also develop some distance away from the ligament's attachment at the vertebra rim.

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