The sacrum (os sacrum) is the fusion of the five sacrum vertebrae in adults, usually between the ages of 20-25. Its shape resembles a triangle with its top pointing downwards. It is also compared to a wedge because it is wedged between the pelvic bones with which it forms a strong bone ring called the pelvis. The popular so-called back pain often concerns this bone or its vicinity.

The sacrum is part of the pelvic girdle , which is the part of the skeleton responsible for connecting the lower limb with the axial skeleton. Children have minimal mobility in this section of the spine, which disappears with age, as the sacral vertebrae fuse into the sacrum.

Sacrum - structure

The base of the sacrum ( basis ossis sacri ) is its upper, widest part, while the lower is called the vertex ( apex ossis sacri ) . The triangular shape of the sacrum is influenced by the rapid reduction of the size of the sacral vertebrae downwards. The apex has an oval, downward-facing surface thatconnects to the coccyx , andthe base has all the typical features of a vertebra(including: upper the surface of the body of the first sacral vertebra, two symmetrical bone lamellae forming the vertebral arch at the back, at their junction a tubercle corresponding to the spinous process of other vertebrae, and at the base of the arches - symmetrical superior articular processes that touch the fifth lumbar vertebra), whichenables connection with the lumbar spine .

Since the lumbar spine is convex to the front (lumbar lordosis) and the sacral spine is convex to the back, they connect at theangle called lumbosacral(angulus lumbosacralis ) whose vertex is pointing forward and the extent ofis between 120 and 170 degrees .The apex of this angle is the mound of the last intervertebral disc , which isbetween the fifth lumbar vertebrae and the first sacral vertebra . Lateral to the bodies of the sacral vertebrae are the so-called wings of the sacrum ( ala sacralis ).

The arch and body of the first sacral vertebrae form the entrance to the sacral canal( canalis sacralis ) which ends at the bottom cross hiatus( hiatus sacralis ). The transverse processes and vestigial ribs of the individual sacral vertebrae fuse together to form the lateral portions of the sacrum. In the upper-lateral part, they are crowned with articular surfaces, the so-calleduvoid surfaces( facies auricularis ), which are usedto connect to the hip bones . Towards the back there is a strongly lumpy surface, the so-called sacral tuberosity ( tuberositas sacralis ), to which the interosseous sacroiliac ligaments attach.

The anterior surface of the sacrum, also called the pelvic ( facies pelvina ), is usually smooth and concave. It clearly showstransverse lines( linea transversa ), i.e. lines where the sacral vertebrae fuse. These lines are crowned on the sides by four pairsof sacral holes( foramen sacrale pelvinum ), which form the boundary between the medial and lateral parts of the sacrum.

On the dorsal surface, i.e. the posterior ( facies dorsalis ) of the sacrum, which is usually convex and full of unevenness, you can see the correspondingdorsal sacral holes( foramen sacrale dorsale ). They are located between thelateral cross-crest( crista sacralis lateralis ), which was formed as a result of fusing the transverse processes, and themedial sacral crest( crista sacralis media ), which are the fused spinous processes of the four upper sacral vertebrae. There is also a less convexity between the medial cross crest ( crista sacralis media ) and the dorsal sacral openings ( foramen sacrale dorsale )intermediate cross crest( crista sacralis intermedia ), which form the fused articular processes of the sacral vertebrae. The crest is crowned from the top by the superior articular processes of the first sacral vertebra, and from the bottom by the characteristicsacral cones( cornua sacralia ), which enable connection with the coccyx.

Sacrum - sex differences

The sacrum can vary greatly depending on the gender.The male's sacrum is narrower and longerthan the female's sacrum. It also has a more flattened upper part and sharper bends to the front.The curvature of the female sacrum is smoother , more evenly distributed along the entire sacrum.

In a female body, the sacrum lies more horizontal than in men, and has ashorter ovoid surface(two vertebrae long, while in men it istwo and a half or three circles in length).

Sacrum - Functions

The main function of the sacrum is thatcarries the weight of the upper body and transfers it to the lower limbsvia the girdle of the lower limb.Also protects organs, vessels and other structureslocatedinside the pelvisand the holes in them are the perfect location for anterior cruciate nerves and spinal twigs lateral cruciate artery.

Numerous ligaments, tendons and musclesare attached to the sacrum bone , incl. pear-shaped muscle. The mobility of the sacroiliac joint is low, usually 1 to 3 degrees. The sacrum, however, has an important stabilizing function and supports the body in maintaining a sitting position .

Sacrum - anomalies, dysfunctions and pain

The sacrum is the result of the evolutionary fusion of the sacral vertebrae, a process that repeats itself in humans throughout childhood and adolescence. In some adults, however, it happens thatthe first sacral vertebrae does not fuse with the sacrumand begins to function as an additional, sixth lumbar vertebra. This anomaly is calledlumbalizing S1 . Sometimes an intervertebral disc is even produced between this additional lumbar vertebra (S1 / L6) and the anatomically second sacral vertebra (S2). An inverse ailment in which the anatomically last, , fifth lumbar vertebra (L5) fuses with the sacrum , is calledsacralization of the L5 vertebra .

One of the most common dysfunctions of the sacrum is misalignment. Whenis leaning too much forward (in nutation) , it causes deepening of the lumbar lordosis. When too muchis leaning backwards (in coronation) , the lumbar lordosis becomes shallow. If the sacrum is slightlylaterally inclined , it causes an asymmetry in the position of the iliac plates, which often leads to a lateral curvature of the spine. A similar effect can be achieved byrotation of the sacrumto the right or left, which leads to dysfunction of the intervertebral joints and scoliosis.

The incorrect, asymmetrical position of the sacrum is usually the result of overly tense or overstretched muscles. Shortenedilio-lumbar muscleswill trigger nutation, and too muchtension of the posterior band- counter-nutation of the sacrum. The lateral tilt of the sacrum is due to the asymmetricaltension of the gluteus muscles, pear-shaped and broad fascia muscles .

Sacral pain can also occur under the influence of many other diseases andfactors, not necessarily structural or anatomical. It can be a symptom of a serious illness, such as bone cancer, or it can be the result of the usualoverloadcaused by a sedentary lifestyle. A common cause of prolonged pain in this area is a fall injury to the back, which results in a bruiseor a fracture of the sacrum bone . Sometimes it is necessary to perform an operation to remove the fragments after such fracture. Pain in this area is also the result of discopathy,neuralgia (neuralgia) of the sacral plexusoroverload of the pear-shaped muscle . The sources of pain areTarlov cystsandcoccyx hair cyst , as well as constipation, pregnancy andexcessive exercisewhich overloads the muscles that have trailers on the pelvis.

Bibliography :

Bochenek A., Reicher M .: "Human anatomy. Volume I. General anatomy, bones, joints and ligaments, muscles ”. PZWL Medical Publishing, Warsaw, 2006

Category:

Anatomy