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Sacrum and Coccyx Anatomy

Introduction to Sacrum and Coccyx Anatomy:

Inferior to the fifth lumbar vertebra (L5) are the image descriptionsacrum and coccyx bones

Anterior and posterior views of the sacrum, coccyx, and pelvis bones.
Anterior and posterior views of the sacrum, coccyx, and pelvis bones with sacrum and coccyx highlighted
The sacrum and coccyx bones.
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The large, triangular-shaped sacrum (os sacrum) is image descriptioncomposed of five vertebrae (S1-S5) that fuse during early adulthood (18-30 years).

Anterior and posterior view of the sacrum and coccyx
Anterior and posterior view of the first sacral vertebra
Anterior and posterior view of the second sacral vertebra
Anterior and posterior view of the third sacral vertebra
Anterior and posterio view of the fourth sacral vertebra
Anterior and posterior view of the fifth sacral vertebra
The five sacral vertebra of the sacrum (S1-S5).

The smaller coccyx (os coccygis) articulates with S5 and is also known as the tailbone. It image descriptionconsists of three to five vertebrae (C1-C5) that are also usually fused.

Anterior and posterior view of the sacrum and coccyx
Anterior and posterior view of the first vertebra of the coccyx (C1)
Anterior and posterior view of the second vertebra of the coccyx (C2)
Anterior and posterior view of the third vertebra of the coccyx (C3)
Anterior and posterior view of the fourth vertebra of the coccyx (C4)
The four vertebra of the coccyx (C1-C4).

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 image descriptionbony pelvis.

anterior and posterior view of the coccyx, sacrum, and two os coxae forming the bony pelvis - animation slide
anterior and posterior view of the coccyx, sacrum, and two os coxae forming the bony pelvis - animation slide
anterior and posterior view of the coccyx, sacrum, and two os coxae forming the bony pelvis - animation slide
anterior and posterior view of the coccyx, sacrum, and two os coxae forming the bony pelvis - animation slide
anterior and posterior view of the coccyx, sacrum, and two os coxae forming the bony pelvis - animation slide
anterior and posterior view of the coccyx, sacrum, and two os coxae forming the bony pelvis - animation slide
anterior and posterior view of the coccyx, sacrum, and two os coxae forming the bony pelvis - animation slide
anterior and posterior view of the coccyx, sacrum, and two os coxae forming the bony pelvis - animation slide
anterior and posterior view of the coccyx, sacrum, and two os coxae forming the bony pelvis - animation slide
anterior and posterior view of the coccyx, sacrum, and two os coxae forming the bony pelvis - animation slide
anterior and posterior view of the coccyx, sacrum, and two os coxae forming the bony pelvis - animation slide
anterior and posterior view of the coccyx, sacrum, and two os coxae forming the bony pelvis - animation slide
anterior and posterior view of the coccyx, sacrum, and two os coxae forming the bony pelvis - animation slide
anterior and posterior view of the coccyx, sacrum, and two os coxae forming the bony pelvis - animation slide
The os coxae, sacrum, and coccyx forming 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 image descriptionsacral canal that extends the length of the bone.

Posterior view of the sacral canal - animation slide
Posterior view of the sacral canal - animation slide
Posterior view of the sacral canal - animation slide
Posterior view of the sacral canal - animation slide
Posterior view of the sacral canal - animation slide
Posterior view of the sacral canal - animation slide
Posterior view of the sacral canal - animation slide
Posterior view of the sacral canal - animation slide
Posterior view of the sacral canal - animation slide
The sacral canal of the sacrum.

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 image descriptionsacral foramina. [image descriptionanterior view/image descriptionposterior view]

Anterior view of the sacrum and coccyx
Anterior view of the spinal nerves passing through the anterior sacral foramina
Posterior view of the sacrum and coccyx
Posterior view of the spinal nerves passing through the posterior sacral foramina
The branches of the spinal nerves passing through the sacral foramina.
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Anterior Markings of the Sacrum:

  1. image descriptionBase of the sacrum (basis ossis sacri) is the broad, superior end of the sacrum.
Anterior view of the sacrum and coccyx
Anterior view of the base of the sacrum
The base of the sacrum.
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  1. image descriptionApex (apex ossis sacri) is the narrow, inferior end of the sacrum, which articulates with the first bone of the coccyx.
Anterior view of the sacrum and coccyx
Anterior view of the apex of the sacrum
The apex of the sacrum.
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  1. image descriptionBody of 1st sacral vertebra (corpus vertebrae ossis sacri) is a thick, central portion of the first sacral vertebra.
Anterior view of the sacrum and coccyx
Anterior view of the body of the first sacral vertebra
Body of the first sacral vertebra.
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  1. image descriptionSacral 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.
Anterior view of the sacrum and coccyx
Anterior view of the sacral promontory of the sacrum
The sacral promontory.
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  1. image descriptionSacral 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.
Anterior view of the sacrum and coccyx
Anterior view of the sacral ala
Sacral ala of the sacrum.
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  1. image descriptionTransverse lines (or ridges) (lineae transversae ossis sacri) are four horizontal ridges formed by the fusion of the sacral bodies.
Anterior view of the sacrum and coccyx
Anterior view of the transverse line of the sacrum
Transverse line of the sacrum.
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  1. image descriptionAnterior 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.
Anterior view of the sacrum and coccyx
Anterior view of an anterior sacral foramen
Anterior sacral foramen of the sacrum.
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Posterior markings of the Sacrum:

  1. image descriptionSacral 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.
Posterior view of the sacrum and coccyx
Posterior view of the sacral canal of the sacrum
The sacral canal of the sacrum.
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  1. image descriptionSuperior 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.
Posterior view of the sacrum and coccyx
Posterior view of the superior articular process of the sacrum
The superior articular facet of the sacrum.
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  1. image descriptionSacral 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).
Posterior view of the sacrum and coccyx
Posterior view of the sacral tuberosity of the sacrum
The sacral tuberosities of the sacrum.
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  1. image descriptionAuricular 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.
Posterior view of the sacrum and coccyx
Posterior view of the auricular surface of the sacrum
auricular surface of the sacrum.
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  1. image descriptionMedian 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 view of the sacrum and coccyx
Posterior view of the median saral crest of the sacrum
The median sacral crest of the sacrum.
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  1. image descriptionPosterior 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.
Posterior view of the sacrum and coccyx
Posterior view of the posterior sacral foramen
Posterior sacral foramen of the sacrum.
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  1.  image descriptionLateral 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.
Posterior view of the sacrum and coccyx
Posterior view of the lateral sacral crest
Lateral sacral crests of the sacrum.
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  1. image descriptionIntermediate 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.
Posterior view of the sacrum and coccyx
Posterior view of the intermediate sacral crest of the sacrum
Intermediate sacral crest of the sacrum.
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  1. image descriptionSacral 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.
Posterior view of the sacrum and coccyx
Posterior view of the sacral cornu of the sacrum
Sacral cornua of the sacrum.
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  1. image descriptionSacral 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 view of the sacrum and coccyx
Posterior view of the sacral hiatus of the sacrum
The sacral hiatus of the sacrum.
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Posterior Markings of the Coccyx:

  1. image descriptionCoccygeal 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.
Posterior view of the sacrum and coccyx
Posterior view of the coccygeal vertebra of the coccyx
The coccygeal vertebrae.
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  1. image descriptionCoccygeal 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.
Posterior view of the sacrum and coccyx
Posterior view of the coccygeal cornu of the coccyx
The coccygeal cornu of the sacrum.
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Test yourself:

Anterior view: [image descriptionShow/image descriptionHide answers]

Test yourself image with answers hidden.
Test yourself image for the anterior view of the sacrum showing answers: Body, sacral ala, sacral promontory, transverse line, apex, body of vertebra, and anterior sacral foramen
Anterior view of the sacrum and coccyx - test yourself.
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Test yourself:

Posterior view: [image descriptionShow/image descriptionHide answers]

Test yourself image for the posterior view of the sacrum
Test yourself image for the posterior view of the sacrum showing answers: Sacral canal, sacral tuberosity, auricular surface, lateral sacral crest, intermediate sacral crest, sacral cornu, first coccygeal vertebra, superior articular process, posterior sacral foramen, median sacral crest, sacral hiatus, and coccygeal vertebra
Posterior view of the sacrum and coccyx - test yourself.
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