Lumbar Vertebrae Anatomy
Sacrum and Coccyx Anatomy
Introduction to Sacrum and Coccyx Anatomy:
Inferior to the fifth lumbar vertebra (L5) are the sacrum and coccyx bones
The large, triangular-shaped sacrum (os sacrum) is composed of five vertebrae (S1-S5) that fuse during early adulthood (18-30 years).
The smaller coccyx (os coccygis) articulates with S5 and is also known as the tailbone. It consists of three to five vertebrae (C1-C5) that are also usually fused.
Lateral to the sacrum and coccyx are the os coxae or hip bones. Together, the two os coxae, sacrum, and coccyx form a basin-shaped ring of bones called the bony pelvis.
The bony pelvis protects the bladder, lower colon, rectum, and reproductive organs. It also supports the weight of the trunk and is an attachment point for several muscles that move the trunk, hip, and thigh.
The sacrum is concavely curved, and each vertebra contains a large central opening or foramen. The fused foramina form a chanal called the sacral canal that extends the length of the bone.
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Spinal nerves that arise from the end of the spinal cord at L1 pass through the canal. Branches (rami) of these spinal nerves enter and exit the sacral canal through anterior and posterior sacral foramina. [anterior view/posterior view]
Anterior Markings of the Sacrum:
- Base of the sacrum (basis ossis sacri) is the broad, superior end of the sacrum.
- Apex (apex ossis sacri) is the narrow, inferior end of the sacrum, which articulates with the first bone of the coccyx.
- Body of 1st sacral vertebra (corpus vertebrae ossis sacri) is a thick, central portion of the first sacral vertebra.
- Sacral promontory (promunturium ossis sacri) is an anterior projection into the pelvic cavity from the base of the sacrum. It is formed by the anterior (ventral) lip of the body of S2. This bulge serves as a landmark to help determine the size of the pelvic cavity.
- Sacral alae; singular, sacral ala (ala ossis sacri) are large triangular, wing-like, lateral extensions from the body of S1, near the base of the sacum. The lateral margins of the alae articulate with the os coxae at the sacroiliac joints.
- Transverse lines (or ridges) (lineae transversae ossis sacri) are four horizontal ridges formed by the fusion of the sacral bodies.
- Anterior sacral foramina (foramina sacralia anteriora) are four pairs of openings located lateral to the fused sacral bodies. The foramina are arranged in two vertical rows and communicate with the sarcal canal. They give passage to the anterior primary branches (rami) of the sacral nerves and lateral sacral arteries and veins.
Posterior markings of the Sacrum:
- Sacral canal (canalis sacralis) is a large, triangular opening that extends the length of the bone. It is an inferior extension of the vertebral canal (foramen) and contains the sacral coccygeal spinal nerves, which descend from the end of the spinal cord at L1 as a part of the cauda equina.
- Superior articular process (processus articularis superior) are oval-shaped, superior projections located on both sides of the sacral canal. On the posterior surface of each process is a concave facet. The facets articulate with inferior articular facets on the fifth lumbar vertebra (L5) above.
- Sacral tuberosities (tuberositas ossis sacri) are irregular surfaces found on the upper lateral edges of the bone. Each sacral tuberosity contains depressions that are attachment points for the ligaments that hold the sacrum to the os coxae (posterior sacroiliac ligament).
- Auricular surfaces (facies auricularis) are ear-shaped surfaces on the lateral sides of the bone that articulate with the ilia of the os coxae at the sacroiliac joints.
- Median sacral crest (crista sacralis mediana) is a tubercular, midline ridge formed by the fusion of the first three to four sacral spinous processes. It is an attachment point for the multifidus and erector spinae muscles.
- Posterior sacral foramina (foramenina sacralia posteriora) are eight irregular shaped openings located lateral to the bodies of the sacral vertebrae. They communicate with the sacral canal and transmit the posterior (dorsal) primary branches (rami) of the sacral nerves.
- Lateral sacral crests (Crista sacralis lateralis) are vertical, tuberular, ridges located lateral to the sacral foramina. They are composed of the fused transverse processes of the sacral vertebrae and are the attachment points for some of the erector spinae muscles and the interosseous sacroiliac ligaments.
- Intermediate sacral crests (crista sacralis intermedia) are vertical, tubercular ridges located between the median sacral crests and the sacral foramina. The are composed of the fused articular processes of the sacral vertebrae.
- Sacral cornua (cornu sacrale) are forn-like inferior extensions of the intermediate sacral crest. Each sacral cronu is a remnant of an inferior articular process of the fifth sacral vertebra and articulates with the coccygeal cornu below.
- Sacral hiatus (hiatus sacralis) is a U-shaped opening at the inferior end of the sacral canal that is bordered laterally by the two sacral cornua. It results when the two opposing lamina of the 5th sacral vertebrae do not fuse. The 5th sacral nerve and the coccygeal nerve pass through the opening, which is covered by the sacrococcygeal ligament.
Posterior Markings of the Coccyx:
- Coccygeal vertebrae (vertebra coccygeae) is a trangular-shaped arrangement of 3 to 5 vertebrae that attach to the sacrum by fibrocartilage. All the vertebrae lack pedicles, laminae, and spinous processes. The first coccygeal bone is the largest and has rudimentary transverse processes that extend laterally from the body. The transverse processes form the inferior portion of the fifth sacral foramina.
- Coccygeal cornua (cornu coccygeum) are small, knob-like, upward extensions from the first coccygeal vertebra. Each coccygeal cornu is a remnant of a superior articulate process and articulates with the sacral cornu above.