Generality
The sacrum is the uneven, asymmetrical and triangular bone, which takes place between the lumbar tract and the coccygeal tract of the spine.
The sacrum forms 4 joints: the two sacroiliac joints, the joint with the last lumbar vertebra and the articulation with the coccyx.
The functions of the sacrum are two: providing protection to the sacral tract of the spinal cord and supporting the upper part of the human body, when an individual walks, runs etc.
What is the sacrum?
The sacrum is an uneven, asymmetric and triangular bone, which resides in the lower part of the spine, exactly between the lumbar tract and the coccyx.
In fact, the sacrum represents the posterior and central part of the pelvis (or pelvis ).
Bones of the pelvis: what are they?
In addition to the sacrum, the two iliac bones and the coccyx take part in the constitution of the pelvis.
Anatomy
Figure: sacrum and other pelvic bones. In the image, readers can recognize the sacral iliac joints, which have the important task of supporting the weight of the body during standing, walking, etc.
Concave inwards, the sacrum is, for the most part, the result of the fusion of the 5 sacral vertebrae of the vertebral column.
In describing the sacrum, anatomists recognize at least 6 extremely relevant regions: the so-called sacrum base, the so-called apex of the sacrum, the two lateral surfaces, the pelvic surface and the dorsal surface.
BASE OF THE SACRED
The base of the sacrum is the broad and flat bone region, projected upwards, which borders and articulates with the fifth lumbar vertebra. The fifth lumbar vertebra is the last vertebra of the lumbar spine.
The base of the sacred includes several bony portions of a certain importance, including: the so-called sacral promontory and the two lateral projections, identified with the term wings (or sacral wings ).
- Sacral promontory. Facing the inside of the human body and comprising part of the ileopettinea line and part of the terminal line, the sacral promontory is the bone portion that puts in communication and articulates the first sacral vertebra with the fifth lumbar vertebra.
The articulation existing between the first sacral vertebra and the last lumbar vertebra forms the so-called vertebral sacred angle .
APICE OF THE SACRED
The apex of the sacrum is the bone region projected downwards and has a flat and oval-shaped area ("oval facet"), which articulates with the coccyx; the coccyx is the last part of the spine.
PELVIC SURFACE
Sloping moderately downwards, the so-called pelvic surface is the region of the sacrum that looks anteriorly (therefore towards the interior of the human body). It is slightly curved, with the concavity, to which it gives rise, which is turned inwards.
On the pelvic surface four transverse bony ridges are recognizable, which represent the separation boundaries of the 5 sacral vertebrae.
With a top-down view, the first of the 5 sacral vertebrae has a very broad vertebral body; starting from the second sacral vertebra, the size of the various vertebral bodies is being reduced.
On the sides of where each transverse ridge ends, two holes are placed, called front sacral holes . If, in total, the transverse ridges are 4, the front sacral holes are altogether 8.
The role of the 8 anterior sacral holes is to allow the passage of the sacral nerves (in output) and the lateral sacral arteries (in entry).
DORSAL SURFACE
Sloping slightly upwards, the so-called dorsal surface is the region of the sacrum that looks posteriorly. In fact, it represents the posterior (or opposite) face of the pelvic surface. This means that it is also curved, but it is convex rather than concave.
On the dorsal surface, various elements are recognizable:
- In the center and with a top-down direction, there is the so-called medial sacral crest . Resulting from the fusion of the spinous processes of the sacral vertebrae, the median sacral crest gives birth to 3 or 4 tubercles and represents the coupling point of the supraspinatus ligament .
- On the two sides of the median sacral crest, the so-called intermediate sacral crests take place, one on the right and one on the left. Arising from the fusion of the articular processes of the sacral vertebrae, the intermediate sacral crests act as a point of attachment for the posterior sacral iliac ligaments .
The lower portions of the two intermediate sacral crests have two characteristic bone growths, which are called sacral horns . The sacral horns are connected to the horns of the coccyx.
- The so-called sacral canal develops inside the bony portion which includes the median sacral crest and the intermediate sacral crests. The sacral canal is nothing but the spinal canal formed by the sacral vertebrae. Inside it takes place the sacral section of the spinal cord .
The sacral canal generally terminates at the level of the fourth sacral vertebra, with a structure known as the sacral hiatus (hiatus sacrale).
- Laterally to each intermediate sacral crest, they locate the so-called 4 posterior sacral holes, which have the task of allowing the passage of the spinal nerves.
- Externally to the posterior sacral holes, both on the right and on the left, the transverse processes of the sacral vertebrae develop, which give rise to the so-called lateral sacral crests .
The lateral sacral crests represent the point of attachment for the posterior sacral iliac ligaments and sacrotuberous ligaments .
SIDE SURFACES
The two lateral surfaces are the regions of the sacrum that articulate with the right iliac bone and the left iliac bone, giving rise to the two so-called sacrum iliac joints .
The side surfaces are large at the top and narrow at the bottom.
The most important anatomical elements of the lateral surfaces are:
- The rough surface that makes contact with the iliac bone. It is the true architect of the sacred iliac joint on each side of the sacrum;
- Sacral tuberosity . Located behind the aforementioned rough surface, it represents a point of attachment for the posterior sacral iliac ligaments;
- The attachment points for sacrotuberous ligaments and sacrospinosis.
JOINTS
The sacrum takes part in 4 joints:
- The two joints with the iliac bones, the right one and the left one. The joint that the sacrum establishes with one of the two iliac bones is the aforementioned sacral iliac articulation ;
- The joint with the last lumbar vertebra;
- The joint with the first coccygeal vertebra.
MUSCLES
On the pelvic surface and on the dorsal surface, different muscles originate and end, both in the lower limbs and in the back.
The muscle elements associated with the pelvic surface of the sacrum are:
- The piriformis muscle : it originates in the sacrum, precisely in the space between the second and fourth sacral vertebrae. It ends by attaching itself to the great trochanter of the femur.
It allows the human being to rotate externally, abduct, extend and stabilize the hip joint.
- The coccygeal muscle : it originates in the lower part of the sacrum and ends at the level of the coccyx. Provides support to the pelvic cavity and allows the coccygeal region of the spine to flex slightly.
- The iliac muscle : has a shared origin between the iliac fossa of the iliac bone and the wings of the sacrum (base of the sacrum). Ends on the small trochanter of the femur.
It has two functions: it stabilizes the hip joint and allows you to flex the leg up to the hip.
Instead, the muscular elements associated with the dorsal surface of the sacrum are:
- The lumbar multifidus muscle : the fibers that compose it originate in the area next to the posterior sacral holes, therefore they pass over these, oriented obliquely, and terminate at the level of the spinous process of the upper vertebra.
The lumbar multifidus muscle helps stabilize the spine.
- The erector muscle of the column : it originates in the intermediate sacral crest (NB: there is one on each side) and ends at various points in the column and beyond.
Allows extension and flexion of the head and spine.
BLOOD VESSELS
Arterial vessels that relate to the sacrum are median sacral arteries and lateral sacral arteries .
The median sacral arteries are a posterior continuation of the abdominal aorta. They supply blood to the posterior section of the rectum, the coccygeal glome, the meninges of the sacral tract of the spinal cord and the sacrum.
The lateral sacral arteries, instead, derive from the posterior division of the internal iliac artery. They supply blood to the meninges of the sacral tract of the spinal cord, the sacrum and the surrounding muscles.
DEVELOPMENT
The formation of the sacral vertebrae takes place on the 29th day of embryogenesis.
Their definitive fusion is a process that, in the human being, occurs between the ages of 18 and 30 years of life.
Function
The functions of the sacrum are two: to provide protection to the sacral tract of the spinal cord and support the weight of the upper part of the human body, when an individual stands up, walks, runs etc.
The protective function belongs to the sacral vertebrae fused together. The protective properties of the sacral vertebrae are a common point in all the other vertebrae of the spine.
The supporting function, on the other hand, is the sacral iliac joint, the result of the interaction between the sacrum and the iliac bone.
Associated pathologies
The most important issues that can affect the sacrum are bone fractures and an inflammatory condition known as sacroileitis .
Bone fractures are generally traumatic injuries due to accidental falls, motor vehicle accidents and similar circumstances.
Sacroiliitis, on the other hand, is the inflammation of the joints that connect the sacrum to the iliac bone. The main causes of sacroileitis include traumatic injuries, arthritis, pregnancy and various types of infections.
