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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
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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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