Sacrum

In human anatomy, the sacrum is a big, triangular bone at the bottom of the spine formed by the fusion of sacral vertebrae S1–S5 between the ages of 18 and 30.

The sacrum is located between the two wings of the pelvis in the top, the left and right sacrum, and the rear region of the pelvic cavity. It joins four other bones to form joints. The alae (wings) are two projections on the margins of the sacrum that engage with the ilium at the L-shaped sacroiliac joints. The sacrum attaches to the last lumbar vertebra (L5) just on the upper side and the coccyx (tailbone) on the bottom side through the sacral and coccygeal cornua.

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The sacrum bone is made up of three separate surfaces that are designed to fit around the pelvic structures. It has a concave shape overall (curved upon itself). The sacrum's base, the broadest and uppermost section, is inwardly inclined forward as the sacral promontory. The middle section is bent outward toward the back, creating more space for the pelvic cavity.  The pelvic vertebrae of most other quadrupedal animals go through a similar developmental process of creating a sacrum in adulthood, even though the bone tail (caudal) vertebrae stay unfused. There are a few differences in the number of sacral vertebrae. For example, a horse's S1–S5 vertebrae will merge, a dog's S1–S3 vertebrae will merge, and a rat's four pelvic vertebrae would merge between the lumbar and caudal vertebrae of its own tail.

The "posterior braincase" of the Stegosaurus dinosaur was distinguished by a considerably expanded neural canal in the sacrum.

Sacrum Anatomy

The sacrum bone is a complicated structure that supports the spine while also giving space for the spinal nerves. This also connects to the hip bones. The sacrum does have a dorsal, pelvic, and lateral surface, as well as a base and an apex. The broad and enlarged base of the sacrum is pointed upward and forward. A wide projection called an ala of the sacrum is seen on each side of the base, and all these alae (wings) articulate well with sacroiliac joints. The lumbosacral trunk and the psoas major muscles, that link the lumbar and sacral plexuses, are supported by the alae. The alae are connected to the iliac fossa in the articulated pelvis. Every ala is a bit concave from sideways and convex out from the back, and it connects to very few iliacus muscle fibres. The transverse process is represented by the posterior quarter of the ala, while the costal process of the first sacral segment is represented by the anterior three-quarters. Every ala also functions as a portion of the pelvic brim's boundary. The bottom of the lumbosacral triangle is formed by the alae. The ala is connected to the iliolumbar and lumbosacral ligaments.

A broad oval articular surface, the top portion of the very first sacral vertebra's body, is located in the centre of the base and is linked to the undersurface of the last lumbar vertebra's body via intervertebral fibrocartilage. The sacral canal has a huge triangular opening behind it, that is completed by the first sacral vertebra's spinous and lamina process. On each side, superior articular processes protrude; they are concave, oval, and oriented backward and medial-ward, similar to the superior articular processes of a lumbar vertebra. Short thick pedicles connect the body of the very first sacral vertebra and each ala; a vertebral notch on the top surface of each pedicle forms the bottom section of the foramen between the first sacral vertebrae and the last lumbar.

The apex of the coccyx is pointed downward and has an oval facet for articulation. The sacral canal runs the length of the sacrum and is a continuation of the vertebral canal. The sacral angle is the intersection of the true conjugate and the two sacral components. It is usually warmer than 60 degrees. A smaller sacral angle indicates that the pelvis is funnelling.

Promontory

The linea terminalis and the iliopectineal line are elements of the sacral promontory, which forms the core of the pelvic inlet's border. The sacral promontory expresses with the final lumbar vertebra to generate the sacrovertebral inclination, a 30-degree angle from the horizontal axis which can be used as a guide for sling implant placement.

Surfaces

The sacrum's pelvic region is concave from top to bottom and bent gently from the side- to- side. Four transverse ridges run in the centre, corresponding to the initial planes of division amongst the five sacral vertebrae. The first segment's body is broad and shaped like a lumbar vertebra; the next segment's bodies are shorter, flattened from the back, and bent to mould themselves to the sacrum, becoming concave in the front and convex beyond. The four anterior sacral foramina are located at each end of the transverse ridges, decreasing in size as the vertebral bodies get smaller. 

The anterior divisions of the sacral nerves depart through the foramina, while the lateral sacral arteries enter through the foramina. The anterior divisions of the sacral nerves are lodged in four broad, deep grooves that run along the sidewalls of the foramina. They're separated by noticeable bone ridges that provide the piriformis muscle its start. A sagittal segment via the centre of the sacrum reveals the bodies to be joined at respective circumferences by bone, with broad spaces left centrally, that are occupied by the intervertebral discs in the fresh form.

The sacrum has a convex dorsal surface that is shorter than the pelvic surface. The median sacral crest lies somewhere in the middle, topped by three or four tubercles, which are the rudimentary spinous processes of the top three or four sacral vertebrae. A shallow sacral groove runs along either side of the median sacral crest, giving rise to the multifidus muscle.

The joined laminae of the adjacent sacral vertebrae create the groove's floor. The laminae of the fifth sacral vertebra, and occasionally those of the fourth, may not connect at the rear, leading in a fissure in the posterior side of the sacral canal referred to as the sacral hiatus. The sacral canal is an extension of the spinal canal, and it covers the length of the sacrum. It has a triangle shape over the sacral hiatus. The sacral nerves are lodged in the canal through the anterior and posterior sacral foramina.

The fusion of the articular processes produces a linear sequence of tubercles on the lateral face of the sacral groove that collectively creates the indistinct medial sacral crest. The first sacral vertebra's articular processes are big and shaped like an oval. Their facets appear concave from sideways, face to back and middle, and articulate with the facets on the fifth lumbar vertebra's inferior processes.

Articulations

The Sacrum is Made up of Four Bones:

• Above is the last lumbar vertebra.

• Below is the coccyx (tailbone).

• Along either side, the ilium part of the hip bone.

Nutated Sacrum:- "Nutation" (from the Latin word nutatio, which also implies "nodding") refers to rotation of the sacrum superiorly and anteriorly while the coccyx progresses posteriorly relative to the ilium, and "counter-nutation" refers to the opposite, posteroinferior movement of the sacrum comparative to the ilium while the coccyx shifts anteriorly. The sacrum can move slightly independently all along the sagittal plane in upright vertebrates. The top (bottom) of the sacrum goes forward towards relation to the ilium while bending backward; while bending forward, the top slips back.

All of the components of the sacrum are referred to as the sacrum. When referring to the separate parts, they are known as sacral vertebrae.

Variations

The sacrum may be made up of six components or decreased to four in certain situations. It's possible that the first and second vertebrae's bodies will not fuse together.

Development

From head to tail all along the path of the notochord, the somites which lead to the vertebral column start to form. The very first four pairs of somites emerge in the eventual occipital bone area on day 20 of development. The next 8 pairs form within cervical region to evolve into the cervical vertebrae; the very next 12 pairs shape the thoracic vertebrae; the very next five combinations form the lumbar vertebrae; and so by about day 29, the sacral somites would then seem to to establish into the sacral vertebrae; and eventually, the very last three pairs shape the coccyx on day 30.

Clinical Significance

Congenital Disorders - Spina bifida is a congenital condition caused by a faulty embryonic neural tube, which is defined via an incomplete closure of the vertebral arch or the top of the spinal canal. The lumbar and sacral regions seem to be the most major areas for spina bifida abnormalities. 

Caudal regression syndrome, commonly termed sacral agenesis, is yet another congenital condition. This is defined via aberrant underdevelopment of the lower spine embryo (by the seventh week). Sometimes a portion of the coccyx is gone, or the lower vertebrae are gone, or a little section of the spine is lacking with no visible signs.

Fracture - Sacral fractures are infrequent, but they are frequently accompanied by neurological problems. The majority of the time, when neurological indications are present, they are addressed with surgical fixation.

Cancer - The sacrum has been one of the primary sites for the formation of chordomas, which are sarcomas formed from the embryonic notochord remains.

Fractured Sacrum

A sacral fracture is a break throughout the lower spine that occurs between the two hip bones on the posterior side of the pelvic ring. 

Trauma or stress fractures seem to be the most common causes of sacral fractures. Overuse in athletes and the military causes the latter, which is caused by repetitive, submaximal pressure on the bone, that eventually gave rise. It's also found in those who have osteoporotic bones. Because of the dense network of nerves that flow through the sacrum, a fracture in this area might result in nerve damage, which can have major neurological repercussions.

Classification

Sacral Fracture is Classified on the Basis of the Area Where the Break Occurs:

• Zone I: Impingement of the L5 nerve root may be linked with a fracture from across sacral ala, lateral to the foramina (in about 5 percent of patients it is associated with neurological injury). The much more popular kind of sacral fracture is this one.

• Zone II: Fracture across the neural foramina is linked to neurologic dysfunction; surgery is required to remove bone fragments (debridement), then reduce and internally fix the fracture (posterior approach).

• Zone III: A sacral area of body fracture poses a significant risk of neurologic damage (bladder, bowel, and sexual dysfunction) in more than 60% of patients. Transversal (severe nerve dysfunction) or U-type (commonly known as spinopelvic dissociation) fractures may occur (caused by axial loading).