Sacrum

From the spineToward the hip bonesA connected ring

Selected focus: From the spine

1. 1L5 vertebraThe last lumbar vertebra lies immediately above the sacral base.The last lumbar vertebra lies immediately above the sacral base.
2. 2Lumbosacral discThe disc separates the L5 body from the superior sacral endplate.The disc separates the L5 body from the superior sacral endplate.
3. 3IliumThe broad upper region of each hip bone receives load through the sacroiliac connection.The broad upper region of each hip bone receives load through the sacroiliac connection.
4. 4Sacroiliac articulationThe sacrum meets the ilium at either side; strong surrounding ligaments support limited joint movement.The sacrum meets the ilium at either side; strong surrounding ligaments support limited joint movement.
5. 5SacrumThe fused central bone links the spine with both hip bones.The fused central bone links the spine with both hip bones.
6. 6CoccyxA separate small segmental bone continues below the sacral apex.A separate small segmental bone continues below the sacral apex.
7. 7Pubic symphysisThe hip bones also connect in front, helping complete the pelvic ring.The hip bones also connect in front, helping complete the pelvic ring.

Enlarge A spinal base within a pelvic ring ↗

Teaching notes

L5 vertebra

The last lumbar vertebra lies immediately above the sacral base.

Lumbosacral disc

The disc separates the L5 body from the superior sacral endplate.

Ilium

The broad upper region of each hip bone receives load through the sacroiliac connection.

Sacroiliac articulation

The sacrum meets the ilium at either side; strong surrounding ligaments support limited joint movement.

Sacrum

The fused central bone links the spine with both hip bones.

Coccyx

A separate small segmental bone continues below the sacral apex.

Pubic symphysis

The hip bones also connect in front, helping complete the pelvic ring.

A spinal base within a pelvic ring · anatomy detail

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1. 1L5 vertebraThe last lumbar vertebra lies immediately above the sacral base.The last lumbar vertebra lies immediately above the sacral base.
2. 2Lumbosacral discThe disc separates the L5 body from the superior sacral endplate.The disc separates the L5 body from the superior sacral endplate.
3. 3IliumThe broad upper region of each hip bone receives load through the sacroiliac connection.The broad upper region of each hip bone receives load through the sacroiliac connection.
4. 4Sacroiliac articulationThe sacrum meets the ilium at either side; strong surrounding ligaments support limited joint movement.The sacrum meets the ilium at either side; strong surrounding ligaments support limited joint movement.
5. 5SacrumThe fused central bone links the spine with both hip bones.The fused central bone links the spine with both hip bones.
6. 6CoccyxA separate small segmental bone continues below the sacral apex.A separate small segmental bone continues below the sacral apex.
7. 7Pubic symphysisThe hip bones also connect in front, helping complete the pelvic ring.The hip bones also connect in front, helping complete the pelvic ring.

Original AI-assisted normal bony pelvic-ring illustration, anterior view. Gold arrows are qualitative loading cues, not measured force vectors or joint motion. Ligaments, muscles and hidden neural structures are not drawn. Separate native model specimens are not aligned by this illustration.

Open full-resolution illustration ↗ · Image only; editable labels remain in this lesson.

L5 → disc → sacral base

The upper joint receives spinal loading. The front promontory is an edge of the sacral body, not a separate support under L5.

Sacrum → two sacroiliac joints → ilia

The lateral joints and their supporting ligaments share load with the hip bones. The arrows show a relationship, not an isolated force calculation.

A posterior bridge and an anterior connection

The pubic symphysis joins the hip bones in front. The whole ring participates; the sacrum is neither an immobile block nor a freely swinging hinge.

These are teaching emphases on one unchanged bone illustration. No stress map, movement amplitude, ligament attachment or patient-specific loading is simulated.

Teaching references: OpenStax · Vertebral column · OpenStax · Pelvic girdle and pelvis · Cho and Kwak · Sacroiliac-joint movement review

The adult sacrum usually combines five vertebral segments. Its superior base is broad; the inferior apex narrows toward the coccyx. Broad upper lateral regions, the alae, spread away from the first sacral body.

Above it, L5 articulates with the sacrum across the lumbosacral disc and paired posterior joints. At either side, an ear-shaped auricular surface meets the ilium at a sacroiliac joint. These are different connections with different orientations.

Load from the trunk reaches the sacrum and is shared with the hip bones through the sacroiliac region. Strong ligaments help restrain separation and shear. The anterior pelvic ring also contributes to a connected structure; a simple downward arrow is only a qualitative orientation cue.

Normal sacroiliac movement is limited rather than absent. Reported values depend on the study method, position and structures measured. No fixed angle, ideal posture or individual movement diagnosis can be read from this illustration.

Teaching references: OpenStax · Vertebral column · OpenStax · Pelvic girdle and pelvis · Cho and Kwak · Sacroiliac-joint movement review

1. 1Base and promontoryThe broad superior base supports L5 through a disc. Its projecting front rim is the sacral promontory.The broad superior base supports L5 through a disc. Its projecting front rim is the sacral promontory.
2. 2Sacral alaA broad wing extends lateral to the upper sacral body, toward the pelvic articulation.A broad wing extends lateral to the upper sacral body, toward the pelvic articulation.
3. 3Anterior sacral foraminaFour pairs of bony passages transmit ventral primary rami and vessels; an opening is not a nerve root.Four pairs of bony passages transmit ventral primary rami and vessels; an opening is not a nerve root.
4. 4Transverse fusion ridgesThese ridges record fused body junctions; they are not remaining mobile discs.These ridges record fused body junctions; they are not remaining mobile discs.
5. 5Sacral apexThe narrow inferior end meets the coccyx, which is omitted in this isolated view.The narrow inferior end meets the coccyx, which is omitted in this isolated view.

Enlarge Sacrum · anterior pelvic surface ↗

Teaching notes

Base and promontory

The broad superior base supports L5 through a disc. Its projecting front rim is the sacral promontory.

Sacral ala

A broad wing extends lateral to the upper sacral body, toward the pelvic articulation.

Anterior sacral foramina

Four pairs of bony passages transmit ventral primary rami and vessels; an opening is not a nerve root.

Transverse fusion ridges

These ridges record fused body junctions; they are not remaining mobile discs.

Sacral apex

The narrow inferior end meets the coccyx, which is omitted in this isolated view.

Sacrum · anterior pelvic surface · anatomy detail

Close

1. 1Base and promontoryThe broad superior base supports L5 through a disc. Its projecting front rim is the sacral promontory.The broad superior base supports L5 through a disc. Its projecting front rim is the sacral promontory.
2. 2Sacral alaA broad wing extends lateral to the upper sacral body, toward the pelvic articulation.A broad wing extends lateral to the upper sacral body, toward the pelvic articulation.
3. 3Anterior sacral foraminaFour pairs of bony passages transmit ventral primary rami and vessels; an opening is not a nerve root.Four pairs of bony passages transmit ventral primary rami and vessels; an opening is not a nerve root.
4. 4Transverse fusion ridgesThese ridges record fused body junctions; they are not remaining mobile discs.These ridges record fused body junctions; they are not remaining mobile discs.
5. 5Sacral apexThe narrow inferior end meets the coccyx, which is omitted in this isolated view.The narrow inferior end meets the coccyx, which is omitted in this isolated view.

Original AI-assisted gross-bone illustration, anterior view. The complete superior base is intact; the front upper rim forms the promontory. Fine bone pores and exact proportions are illustrative, not density measurements or clinical level mapping.

Open full-resolution illustration ↗ · Image only; editable labels remain in this lesson.

Four transverse ridges mark junctions between the five typical fused bodies. They are bony landmarks, not four remaining movable intervertebral discs.

Four pairs of anterior sacral foramina usually flank the midline. Ventral primary rami and accompanying vessels pass through them. A visible hole is an opening through bone; it is not itself a nerve, ganglion or sensory receptor.

The promontory is the projecting anterior margin of the superior first sacral body. The broad base includes the upper body and adjacent alae; the promontory is not a separate bone or the whole upper surface.

Teaching references: OpenStax · Vertebral column · UAMS · Lower-limb bone anatomy table

1. 1Upper sacral canalA posterior bony passage continues the vertebral canal, behind the sacral body.A posterior bony passage continues the vertebral canal, behind the sacral body.
2. 2Median sacral crestFused posterior elements form this raised midline landmark.Fused posterior elements form this raised midline landmark.
3. 3Posterior sacral foraminaFour pairs of openings transmit dorsal primary rami; they differ from the anterior passageways.Four pairs of openings transmit dorsal primary rami; they differ from the anterior passageways.
4. 4Sacral hiatusA separate low midline opening reflects an incomplete posterior arch. It is not the spinal-cord ending.A separate low midline opening reflects an incomplete posterior arch. It is not the spinal-cord ending.

Enlarge Sacrum · posterior dorsal surface ↗

Teaching notes

Upper sacral canal

A posterior bony passage continues the vertebral canal, behind the sacral body.

Median sacral crest

Fused posterior elements form this raised midline landmark.

Posterior sacral foramina

Four pairs of openings transmit dorsal primary rami; they differ from the anterior passageways.

Sacral hiatus

A separate low midline opening reflects an incomplete posterior arch. It is not the spinal-cord ending.

Sacrum · posterior dorsal surface · anatomy detail

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1. 1Upper sacral canalA posterior bony passage continues the vertebral canal, behind the sacral body.A posterior bony passage continues the vertebral canal, behind the sacral body.
2. 2Median sacral crestFused posterior elements form this raised midline landmark.Fused posterior elements form this raised midline landmark.
3. 3Posterior sacral foraminaFour pairs of openings transmit dorsal primary rami; they differ from the anterior passageways.Four pairs of openings transmit dorsal primary rami; they differ from the anterior passageways.
4. 4Sacral hiatusA separate low midline opening reflects an incomplete posterior arch. It is not the spinal-cord ending.A separate low midline opening reflects an incomplete posterior arch. It is not the spinal-cord ending.

Original AI-assisted gross-bone illustration, posterior view. Four paired foraminal rows and the separate low midline hiatus are shown. Exact sacral-cornua boundaries, hidden roots and individual variations are not mapped.

Open full-resolution illustration ↗ · Image only; editable labels remain in this lesson.

Fused posterior elements form raised longitudinal features on the dorsal surface. The median sacral crest follows the midline. Posterior sacral foramina lie on either side and transmit dorsal primary rami.

The low dorsal sacral hiatus is an opening where the posterior bony arch is incomplete. Its size and shape vary. It is not an extra anterior foramen, the spinal-cord ending or a feature that can be safely located in an individual from a generic drawing.

An auricular articular surface faces laterally toward the ilium. An isolated straight front or back view cannot display the entire joint surface and all of its ligament attachments at once.

Teaching references: OpenStax · Vertebral column · UAMS · Lower-limb bone anatomy table · OpenStax · Pelvic girdle and pelvis

In adults the spinal cord usually ends in the upper lumbar region, with individual variation. Descending lumbar, sacral and coccygeal nerve roots continue below it as the cauda equina. The bony sacral canal is a continuation of the vertebral canal, not evidence that spinal cord tissue reaches the coccyx.

Anterior and posterior roots join to form a spinal nerve. That nerve then divides into anterior and posterior primary rami. Anterior rami contribute to pelvic and lower-limb pathways; posterior rami supply posterior regional tissues. Root and ramus are not interchangeable labels.

The bone illustrations deliberately do not invent a complete hidden nerve map. The anterior and posterior foramina can be named from gross bone landmarks without claiming that a generated rootlet, vessel or individual clinical level has been verified.

Teaching references: UTHealth · Anatomy of the spinal cord · UAMS · Lower-limb bone anatomy table

A cortical shell and internal trabecular bone contribute to the sacrum’s mechanical behaviour. The surface texture in the teaching plates is illustrative; it is not a measurement of bone density, age or disease.

The usual five-segment description is an orientation, not a guarantee for every person. Transitional anatomy and fusion patterns can change the apparent boundary between lumbar spine and sacrum. Clinical level numbering requires appropriate individual imaging and professional interpretation.

Sex-labelled source models describe their particular source specimens. They do not establish an individual’s function, pelvic dimensions or reproductive capacity, and separate source datasets must not be treated as perfectly aligned specimens.

Teaching references: OpenStax · Vertebral column · OpenStax · Pelvic girdle and pelvis

A male sacrum representation and the separate female skeleton reference are different source specimens. A compound source mesh that includes the coccyx is labelled as related sacrum knowledge, not an exact sacrum-only match. No individual source opening is assigned a clinical label without verification.

The Return control above retains the originating reference and selection. The illustrations explain normal gross anatomy; they do not replace the selected mesh or add unseen nerves to it.