Sacrum and coccyx
The sacrum (lat. os sacrum) together with the coccyx (lat. os coccygis) form the lowest part of the spine. In an adult person, the sacrum is the immobile part of the spine. The sacrum is joined to the coccyx. This bone combination has a triangular shape.
Anatomy of the sacrum
The sacrum is located between the last lumbar vertebrae and the first coccyx vertebrae. It originally consists of five separate sacral vertebrae (Vertebrae sacrales, S1-S5). These and the intervertebral discs (discus intervertebrales) between them fuse together after birth to form the immobile part of the spinal column. The sacrum has a triangular shape.
View from ventral
The ventral side of the sacrum is characterized by a concave, smooth surface with a hollow (Facies pelvica). This side is oriented towards the pelvis. Four transverse struts are visible. Above these struts horizontal lines (Lineae transversae) are to be seen. These show the area where the individual vertebrae have grown together. At the same level as these lines, four paired openings in the bone can be seen, the Foramina sacralia anteriora.
The upper surface of the first sacral vertebra (S1) is directed upwards, towards the last lumbar vertebra. This surface is called the base ossis sacri. The widely protruding edge of this surface is called the promontorium. A wedge-shaped intervertebral disc lies between the fifth lumbar vertebra and this base. The caudal end of the sacrum tapers to a point and ends with the apex ossis sacri. There it joins the coccyx.
===View from dorsal===,
The uneven, irregular, convex posterior surface (facies dorsalis) has three characteristic mountainous ridges. These lasts can vary in appearance. The spinous processes unite vertically in the middle to form a jagged bony ridge called the median sacral crest. In addition, the joint processes merge laterally and in pairs to form two medial sacral lines. Caudally, these ridges form the rudimentary articular processes of the fifth sacral vertebra, the Cornua sacralia. Between these rudiments, the spinal canal (Canalis sacralis) ends caudally at the lower opening of the sacrum (Hiatus sacralis). On the cranial side, they merge into the frontal articular processes (Procc. Articularis superior) for the fifth lumbar vertebra. The posterior sacral foramina can be seen on the lateral side of these groins. The transverse processes also create two groins by growing together; these are located laterally of the four paired bone openings and form the Crista sacralis lateralis.
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Furthermore, the fusion of the transverse processes with the rudiments of the ribs on both sides forms the body of the sacrum, the partes laterales. The ear-shaped joint surfaces (facies auriculares) for the iliac bones are attached to their sides.
Anatomy of the coccyx
The coccyx is the last section of the spine and consists of three to five vertebral rudiments. These are usually connected to each other as in the case of the sacrum. The coccyx connects directly under the sacrum. The specific vertebral form can only be recognized by the first vertebral rudiment. After caudally, the coccyx becomes narrower and narrower and ends as an apex (apex ossis coccygis). From the first coccygeal vertebra arise two bony articular processes, the so-called coccygeal horns (Cornua coccygea). The coccyx also has short transverse processes and is generally connected to the tip of the sacrum by a cartilaginous process (art. sacrococcygea). This enables a passive forward and backward movement of the coccyx. From the age of mid-20, mineralisation reduces this mobility.
Function
The sacrum is located in the pelvic area. Through this positioning, the sacrum provides the connection between the spine and the hip bones. This leads to a transfer of the load to the legs. Nerves are also conducted through the foramina sacralia.
The coccyx, on the other hand, serves as a point of attachment for muscles and ligaments of the pelvis, pelvic floor and hip joint. Since the pelvis is open at the bottom, these muscles and ligaments hold the organs in their intended position.
Furthermore, the female coccyx is involved in childbirth. Thus, the pressure exerted by the child's head during birth exploits the articulated connection to the sacrum. The coccyx shifts backwards by about two centimetres, thus enlarging the birth canal.
Entwicklung
Die Kreuz- und Steißbeinwirbel entstehen genau wie der Rest der Wirbelsäule im Mutterleib. Bei der Geburt treten das Steiß- und das Kreuzbein ebenfalls als einzelne bewegliche Wirbel auf, zwischen denen sich Bandscheiben befinden. Ab der Pubertät beginnen die Bandscheiben zwischen den Sakralwirbeln zu verknöchern. Die Verknöcherung führt zu einer Verschmelzung der Sakralwirbel, welche die Form des Kreuzbeines bedingt. In der Regel besitzt ein Mensch fünf Kreuzbeinwirbel die zum Kreuzbein verschmelzen. Es können an dieser Stelle aber auch vier oder sechs Wirbel auftreten.
Die Verknöcherung zieht verschiedene Varianten nach sich. Wird der erste Sakralwirbel nicht mit zum Kreuzbein verschmolzen, so spricht man von einer Lumbalisation. Der bleibende Sakralwirbel wird daraufhin als sechster Lendenwirbel angesehen. In anderer Richtung wird dieses Phänomen als Sakralisation bezeichnet. Dabei verknöchern nicht nur die fünf Sakralwirbel miteinander, sondern entweder der letzte Lendenwirbel oder der erste Steißbeinwirbel bilden ebenfalls eine Verbindung mit dem Kreuzbein aus.
