The sacrum (from Latin os sacrum, sacred bone) is a bone that joins the spine to the pelvis.

The sacrum is a bone made from the fusion of five sacral vertebrae. It has an inverted pyramid shape whose upper base is the promontory and the tip is the apex.It is located between the spine and the coccyx and forms the posterior wall of the bony pelvis.

It has three joints:

• The lumbosacral joint formed on its upper surface with the last lumbar vertebra of the spine.
• The sacroiliac joint, located laterally between the wings of the sacrum and the two iliac bones (hip bones). The union of the hip bone, the sacrum and the coccyx forms the bony pelvis of which the sacrum constitutes the posterior wall.
• The sacrococcygeal joint formed at its lower part with the coccyx.

The sacrum does not house the spinal cord as such in its sacral canal because it does not descend to the bottom of the spine (it stops at the level of the second lumbar vertebra). But a root of spinal nerves escapes from its lower part called a ponytail. Some nerves emerge from the sacrum through holes: the sacral-dorsal foramina. These sacral nerves control the lower limbs and organs in the pelvis.

Several muscles involved in the hip joint, such as the gluteus maximus, iliacus, and piriformis, insert themselves into the surface of the sacrum. These muscles ensure the stability of the body when standing.

The sacrum is part of the pelvis whose main role is to support the weight of the upper body. It is also the link between the latter and the spine.

The sacrum, thanks to its sacral canal, also protects the spinal nerves.

Sacral fracture : infrequent, fractures of the sacrum generally occur spontaneously in the elderly (fragility of the bones) or following a major trauma in the young subject. In 40 to 45% of cases, they are associated with a pelvic fracture and only 5 to 10% are isolated fractures (4). Depending on its location, a fracture can damage the sacral nerves and lead to significant neurological damage.

Sacrococcygeal tumors

• Saccrococcygeal teratoma: rare disease, it is characterized by the development of a tumor, most often benign, during embryonic life. It particularly affects girls up to 75% (5). Diagnosis usually takes place before birth. Surgery is done in the first week of life to quickly remove the tumor.
• Chordoma: rare malignant tumor found mainly in adults. The sacrum is one of the main places where these tumors are located. It is characterized by slow growth associated with local destruction of bone and spread to adjacent tissues.

Lumbarization : congenital malformation of the first sacral vertebra (S1). It is not united to the other vertebrae of the sacrum but separated from the second vertebra (S2) by a true intervertebral disc. S1 is therefore free.

Abnormality of shape and position of the sacrum: the sacrum may present a defect in length (sacrum with 4 vertebrae), asymmetric development or spina bifida at the level of the first sacral vertebra. Its curvature can also vary; the sacrum may in fact be flat or take the form of a hook. In women, these abnormalities can have an impact on the progress of childbirth by hindering the passage of the baby.

Pelvic movements during childbirth

Specific movements in the sacroiliac joints that allow vaginal delivery:

• Counter-nutation movement: a verticalization of the sacrum (retreat and elevation of the promontory) occurs simultaneously with an advance and a lowering of the coccyx and a separation of the iliac wings. These movements have the effect of enlarging the upper strait * and reducing the lower strait **.
• Nutation movement: the reverse movement occurs: advancement and lowering of the promontory of the sacrum, retreat and elevation of the coccyx and approximation of the iliac wings. These movements have the consequence of enlarging the lower strait and narrowing the upper strait.

* The upper strait corresponds to the upper opening of the basin.

 ** The lower strait corresponds to the lower opening of the basin.

Sacral pathologies are mainly due to trauma, tumors and congenital factors. They are therefore difficult to prevent.

However, fractures can occur due to bone fragility. Eating foods rich in calcium and vitamin D to strengthen bones can help ward off the risk of fractures.

Physical examination : carried out by the doctor, it first includes the questioning (general, on the causes of the consultation). It is followed by a physical examination of the sacrum (examination of the sacroiliac joints, mobility tests, etc.) which is supplemented by an examination of the lumbar region, pelvis and lower limbs.

X-ray : medical imaging technique that uses X-rays. Frontal and lateral radiography mainly detects fractures and tumors (chordoma). The analysis of the sacrum is sometimes difficult in radiography because of the complexity of its anatomy; in these cases, healthcare professionals resort to MRI and CT scans (9).

Ultrasound : imaging technique based on the use of ultrasound to visualize the internal structure of an organ. Ultrasound of the sacrum is the main examination for the diagnosis, treatment decision and monitoring of sacrococcygeal teratomas.

MRI (magnetic resonance imaging) : medical examination for diagnostic purposes carried out using a large cylindrical device in which a magnetic field and radio waves are produced. Where radiography does not allow it, it reproduces very precise images. It is used in particular in cases of tumors to characterize it and determine its extent.

Scanner : diagnostic imaging technique which consists of “scanning” a given region of the body in order to create cross-sectional images, thanks to the use of an X-ray beam. The term “scanner” is actually the name of the medical device, but it is commonly used to refer to the exam. We also speak of computed tomography or computed tomography. In the case of the sacrum, a CT scan can be used in cases of tumors to determine the composition of the tumor and its extent.

The word sacrum appeared in the 10th century. Among the Greeks, it was referred to by the terms “herion osteon”, or the temple bone. We therefore find there this idea of ​​sacredness. This may be due to its role during reproduction or the fact that it was part of the bones of animals given in sacrifice to the Gods (XNUMX).