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

 

 

Upon physical examination, the earliest abnormality is usually tenderness in the joints where the spine joins the pelvis (the sacroilliac joints). Over time, pain and stiffness may progress along the spine up to the neck region. Ultimately, the sacroiliac joints can fuse, causing various degrees of immobility as the normal flexibility of the spine is diminished. As AS progresses, the length of the spinal column can become fused and rigid. Often, scoliosis, when the spine assumes an abnormal curvature, affects how the spine gives the body its posture. It's common that the neck is stooped in a forward position. These manifestations are common though are not present in everyone with AS. (Fortunately, my

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vertebras have fused in a fashion that gives a typical curvature to my spine. My neck though does have moderate stoop and the joints where my head join my neck (the atlanto-occipital joints) are almost entirely fused, allowing very little mobility at that joint.)

The fusion can also cause joints associated with the spine, such as the rib cage, to suffer a loss of some mobility. As the joints that connect the ribs to the spine (the vertebrocostral joints) fuse, full chest expansion is compromised to the degree of the bones' fusions. The lungs are restrained in their ability to fully expand during inhalation. This lessened ability for lung expansion can bring less oxygen to the blood as it passes through the lungs when the body demands more oxygen. This can cause shortness of breath and increased fatigue during times of physical exertion.

The typical vertebral joint

 

 

consists of two vertebrae, one above and one below, a disk between them made of fibrocartilage that provides tensile strength and absorbs compression shock throughout the spine, ligaments that configure the joint, and muscles that maintain the structure of the joint. Normally, the ligaments retain their connective tissue composition, and the bone remains simply as one of the chief structural component of the joint.

However, in the AS condition, the ligaments and bone of the vertebral joint lose their defined purpose. Chemical changes at the rim of the vertebra that sandwich the disk cause ossification beyond the edge of the vertebra. That is, bone growth begins to re-occur at the rim of the vertebra where it had stopped growing years before when the vertebra had finished developing. This abnormal bone growth slowly builds bony protrusions along some of the ligaments that connect the vertebras. Ossification, the growth of bone,

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