Sacral Spine — Sacrum
The sacral spine or sacrum refers to the large irregular triangular shaped bone made up of the five fused vertebrae below the lumbar region. There is a wedge-shaped intervertebral disc between the base of the last lumbar vertebra and the sacrum called the lumbosacral disc. The spinal canal extends into the sacrum and the sacral nerves exit the canal through bony foramina.
The sacrum is inserted like a wedge between the two iliac or pelvis bones and is held together by the two sacroiliac joints. Many back problems occur where the lumbar and sacral region of the spine connect because this region of the spine is subjected to a large amount of stress with certain activities.
The Sacral Ala are the "wings" of the sacrum. They are an important part of the connection between the sacrum and pelvis and they are often used during spine surgery as a point of attachment for instrumentation that helps to stabilize the lumbosacral junction.
The coccyx is the terminal part of the spine that is commonly referred to as the tailbone, which is made up of four and sometimes five vertebrae fused together.
The sacroiliac joints occur where the sacrum is joined with the ilium, or top portion of the pelvis on both sides of the back. These joints bear the weight of the twists and turns of the trunk of the body. The sacroiliac joints have been the source of considerable controversy for many decades as well as the subject of many studies. Anthropologists have long used the joint as a skeletal target to determine the age of the specimen. The joint is an extremely stable structure because of its bony configuration and ligamentous support. The ligaments of this joint are among the strongest in the human body. *
The characteristics of the sacroiliac joint change as we age. The joint's surface remains flat until sometime after puberty. In our thirties and forties there is an increase in the size and number of elevations and depressions on the sacral and iliac surfaces. There is an obliteration of a true joint space over time, which occurs earlier in males and after menopause in females.
* Porterfield, JA, DeRosa, C. Mechanical Low Back Pain: Perspectives in Functional Anatomy. Philadelphia, PA: W.B. Saunder's Co.; 1976.
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