Sacrum: anatomy, function and treatment

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The sacrum is a single bone made up of five separate vertebrae that grow together in adulthood. It forms the base of the lower back and pelvis.

The sacrum is the concave sphenoid bone at the base of the spine. It looks like an inverted triangle: the widest part (base) is at the top, and the pointed end (top) is at the bottom.

The sides of the sacrum connect with the right and left hip ( ilium ) bones. The apex connects to the coccyx ( coccyx ). The base is connected to the largest and lowest of the lumbar vertebrae, L5.

Anatomy

The human sacrum is a strong bone that can withstand strong pressure and movement. It serves as an anchor point that holds the spine together with the pelvis. The sacrum and coccyx provide a stable platform for people to sit upright.

Humans need a larger sacrum than other mammals because we walk upright and we need additional stability to maintain balance and mobility. The size and orientation of the sacrum also affect a person's birth process.

People are born with four to six sacral vertebrae instead of a single bone. Fusion does not occur simultaneously for all sacral vertebrae: it begins with the fusion of S1 and S2.

As a person ages, the overall shape of the sacrum strengthens and the sacral vertebrae fuse into a single structure. This process usually begins in the mid-teens and ends around the age of 25, and is believed to begin earlier in women than in men.

The fusion time of the sacral vertebrae can be a useful tool to assess the age and sex of skeletal remains. For example , the female sacrum is wider, shorter, and has a more curved (concave) apex called the pelvic apex. entry. The male sacrum is longer, narrower, and flatter than the female sacrum.

Our understanding of what the sacrum does is still evolving. In humans, one of the main functions of the sacrum is to support the weight of the upper body when we are sitting or standing. However, it does not fulfill this function in mammals that walk on four legs (tetrapods) .

The human sacrum also has variability, although this is not yet well understood. For example, the number of bones that make up the sacrum and the course of the fusion process can vary from person to person .

Composition

The sacrum is an irregular bone (sphenoid) that forms the posterior (posterior) third of the pelvic girdle. The ridge that runs through the anterior (anterior) part of the S1 vertebra is called the sacral promontory.

On both sides of the sacrum, there are small holes (orifices) that remain after the fusion of individual vertebrae. Depending on the number of sacral vertebrae, there may be three to five sacral foramina on each side (although there are usually four) .

Each anterior foramen is usually wider than the corresponding posterior or dorsal (dorsal) foramen. Each sacral foramen (plural) is a conduit for sacral nerves and blood vessels.

Small ridges form between each fused sacral vertebra, called transverse ridges or transverse lines.

The median sacral crest, the ridge formed by the spinous processes of the sacral vertebrae, runs along the dorsal midline of the sacrum.

The sacral canal is a hollow space that runs from the apex (base) of the sacrum to the bottom (vertex). The sacral canal serves as the canal at the end of the spinal cord.

The sacrum connects (articulates) with the ilium on both sides at a point of attachment called the surface of the ear.

Just behind the surface of the atrium is a rough area called the sacral tubercle that serves as the point of attachment (point of attachment) for a complex network of ligaments that hold the pelvic girdle together.

The lowest part (bottom) of the sacrum is the narrowest point known as the apex. The apex of the sacrum connects to the tailbone (coccyx).

Location

The sacrum is at the level of the lower back, just above the gluteal cleft (better known as the gluteal cleft). The cleft begins approximately at the level of the coccyx or coccyx.

The sacrum is tilted forward (concave) and ends (ends) at the coccyx. The curvature is more pronounced in women than in men.

The base of the sacrum is the widest part. Although it is called the base, it is actually on the top (superior aspect) of the sacrum, not the bottom.

Here it connects to the L5 lumbar vertebra through the lumbosacral joint . The disc between these two lumbar vertebrae is a common source of back pain.

On each side of the lumbosacral joint are pterygoid structures (sacral wings) that connect to the iliac bone and form the apex of the sacroiliac joint.

The ilium is attached to both sides of the sacrum. These pelvic wings provide stability and strength when walking and standing.

Anatomical variations of the sacrum

The most common anatomical variation of the sacrum refers to the number of sacral vertebrae. Although five are the most common, the abnormalities reported in humans include the presence of four or six sacral vertebrae .

Other options are related to the surface and curvature of the sacrum. The curvature of the sacrum varies greatly from person to person. In some cases, the first and second sacral vertebrae do not fuse, but remain separately articulated .

The inability of the spinal canal to close completely during formation is a condition known as spina bifida that can occur due to the sacral canal.

Function

The human sacrum provides a solid foundation for the formation of the pelvis. Because humans walk on two legs (bipeds), the body needs a stable point to which the muscles and core of the legs can attach.

The human pelvis must also be wide enough to provide movement and balance, and to facilitate delivery. The human body can move and reproduce because the sacrum articulates with the surrounding bones and gives flexibility to the pelvic girdle.

If the entire pelvis were cohesive and rigid, the nuances of movement required for balance would be much more complex and require significantly more energy. In contrast, the sway observed when other primates walk upright is an example of the energy expenditure associated with a smaller and less flexible pelvis.

The sacrum provides an anchor point where the spine can join the pelvis and provide stability to the core of the body. It also serves as a platform for the column on which it sits.

Related conditions

The sacrum is often seen as a focus of low back pain. Forces applied to the sacrum and SI joint (which connects the sacrum to the ilium) can account for up to 27% of all low back pain discomfort .

One of the most common is sacroiliitis , an inflammation of the iliac joint. It is a diagnosis of exclusion, meaning that the doctor only makes a diagnosis when all other possible causes of pain have been ruled out.

Chordoma is a type of primary bone cancer. About half of all chordomas form in the sacrum, but tumors can also develop in other parts of the spine or at the base of the skull .

People can also be born with diseases that affect the sacrum. For example, spina bifida is a congenital disorder that can result from a malformation of the sacral canal.

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