Anatomy

The sacrum is a block of bone at the base of the vertebral column. The sacrum is the largest vertebral element, consisting of five fused vertebrae and is the place where the spine meets the pelvis vis-a-vis the SI joint. It supports the spine and transmits load to the lower extremities. It has a triangular shape, which is broader cephalad and tapers caudally. There are foramina anteriorly and posteriorly. It provides two roles with respect to dispersing forces longitudinally and transversely. The lateral margins of the sacrum have two regions. One area is an auricle-shaped (ear-like) surface, which is smooth and provides the articulation for the SI joint. The other area is ligamentous, which is rough and provides the interosseous portion of the SI joint. The SI joint itself has a small range of motion. There is no muscle that executes active movements across the SI joint. All of the movements are passive and thus it is considered a "stress relieving" type of articulation. Nevertheless, it is a complex set of movements. The axis of movement passes obliquely across the pelvis. In flexion, the axis passes backward from the pubic symphysis to the sciatic notch. In extension, the axis passes from the pubic symphysis through the pelvis between the ischium and coccyx. There are normal age-related (senescent) changes of the SI joint. In the embryo, the SI joint starts as a strip of mesenchyme that then goes on to cavitate. During the first decade of life the joint enlarges and the surface is flat. During the second decade there begins corrugation of the joint surfaces. During the fifth and sixth decades, osteophytes develop. By the eighth decade, large interdigitizing osteophytes may be identified. These imaging findings may be part of the normal aging process and there is no correlation between degenerative type findings of the SI joints and symptomatology (particularly over the age of 50).

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