Axial skeleton · Vertebral column
Sacrum
The sacrum is the triangular bone at the base of the vertebral column, made of five fused sacral vertebrae. It is wedged between the two hip bones, transmits the weight of the body to the pelvis at the sacroiliac joints, and forms the back wall of the pelvis.
- Latin (TA)
- Os sacrum
- Type
- Irregular bone: a large triangular wedge formed by the fusion of the five sacral vertebrae
- Side
- Midline — single
- Model size
- 12.3 cm longest
Rotate the bone to inspect every surface. Blue dots are landmarks — tap one.
3D model: Z-Anatomy (CC BY-SA 4.0), derived from BodyParts3D © DBCLS (CC BY-SA 2.1 JP). Adapted by PharmaWallah. Credits & licence
Landmarks
20 of 20 marked on the modelTap a landmark to find it on the bone and read what it is.
The upper surface of the sacrum, formed by the first sacral vertebra.
Where: Top of the sacrum.
Why it matters: Its body joins the fifth lumbar vertebra through the lumbosacral intervertebral disc.
Related: Lumbosacral joint.
Marked on the model at the point Z-Anatomy places this label.
The projecting anterior edge of the body of the first sacral vertebra.
Where: Front of the base, in the midline.
Why it matters: Forms part of the boundary of the pelvic inlet (pelvic brim); used as a landmark when the pelvis is measured in obstetrics.
Viva: What is the sacral promontory part of?
Marked on the model at the point Z-Anatomy places this label.
The wing-like lateral part of the base on each side of the body of S1.
Where: Upper surface, lateral to the body of S1.
Why it matters: Forms part of the pelvic brim; crossed by structures passing into the pelvis.
Marked on the model at the point Z-Anatomy places this label.
An upward projection on each side behind the base, with a facet facing backwards and medially.
Where: Posterior part of the base, on each side of the sacral canal opening.
Why it matters: Articulates with the inferior articular process of L5 at the lumbosacral zygapophysial joint.
Related: Zygapophysial joint (L5–S1).
Marked on the model at the point Z-Anatomy places this label.
The smooth, concave anterior surface.
Where: Front of the sacrum, facing into the pelvis.
Why it matters: Forms the back wall of the pelvic cavity; piriformis arises from it.
Marked on the model at the point Z-Anatomy places this label.
Four pairs of openings on the pelvic surface.
Where: Pelvic surface, at the lateral ends of the transverse ridges.
Why it matters: Transmit the anterior (ventral) rami of the sacral spinal nerves, which contribute to the sacral plexus.
Viva: What passes through the anterior sacral foramina?
Marked on the model at the point Z-Anatomy places this label.
The rough, convex posterior surface.
Where: Back of the sacrum.
Why it matters: Carries the sacral crests and gives origin to back muscles and gluteus maximus.
Marked on the model at the point Z-Anatomy places this label.
A ridge with small tubercles in the midline of the dorsal surface.
Where: Midline of the dorsal surface, above the sacral hiatus.
Why it matters: Represents the fused spinous processes of the upper sacral vertebrae.
Viva: What does the median sacral crest represent?
Marked on the model at the point Z-Anatomy places this label.
Four pairs of openings on the dorsal surface.
Where: Dorsal surface, between the intermediate and lateral crests.
Why it matters: Transmit the posterior (dorsal) rami of the sacral spinal nerves.
Marked on the model at the point Z-Anatomy places this label.
The continuation of the vertebral canal within the sacrum.
Where: Runs the length of the sacrum, behind the fused bodies.
Why it matters: Contains the lower nerve roots of the cauda equina, the filum terminale and the lower end of the dural sac.
Marked on the model at the point Z-Anatomy places this label.
The gap at the lower end of the sacral canal on the dorsal surface.
Where: Lower dorsal surface, below the median sacral crest, between the sacral horns.
Why it matters: Results from the laminae of the lowest sacral vertebra (S5, sometimes also S4) failing to meet. It is the entry point for a caudal epidural injection of local anaesthetic.
Related: Caudal epidural anaesthesia.
Viva: Why is the sacral hiatus clinically important?
Marked on the model at the point Z-Anatomy places this label.
A small projection on each side of the sacral hiatus.
Where: Lower dorsal surface, bordering the sacral hiatus.
Why it matters: Represents the inferior articular processes of S5; connected to the coccygeal horns. Felt through the skin to locate the hiatus.
Marked on the model at the point Z-Anatomy places this label.
An ear-shaped articular surface on each side.
Where: Upper lateral part of the sacrum.
Why it matters: Articulates with the auricular surface of the ilium at the sacroiliac joint, transmitting body weight to the pelvis.
Related: Sacroiliac joint.
Viva: Why is it called the auricular surface?
Marked on the model at the point Z-Anatomy places this label.
The narrow lower end of the sacrum.
Where: Inferior end.
Why it matters: Articulates with the coccyx at the sacrococcygeal joint.
Related: Sacrococcygeal joint.
Marked on the model at the point Z-Anatomy places this label.
Classification & region
Irregular bone: a large triangular wedge formed by the fusion of the five sacral vertebrae.
- Base
- The upper surface, formed by the first sacral vertebra: body with the promontory, alae on each side, superior articular processes behind, and the opening of the sacral canal.
- Apex
- The narrow lower end, which articulates with the coccyx.
- Pelvic surface
- The smooth, concave front surface, crossed by transverse ridges and pierced by the anterior sacral foramina.
- Dorsal surface
- The rough, convex back surface, carrying the median, intermediate and lateral sacral crests and the posterior sacral foramina.
- Lateral parts
- The sides, each with the ear-shaped auricular surface for the ilium and the sacral tuberosity behind it.
- Sacral canal
- The continuation of the vertebral canal through the sacrum, opening below at the sacral hiatus.
Hold it in the anatomical position
- The broad base is above and the narrow apex below.
- The smooth concave pelvic surface faces forwards (and downwards); the rough convex dorsal surface with its crests faces backwards.
- The base is tilted forwards, so the promontory projects into the pelvis.
- The auricular surfaces face laterally towards the ilia.
Orientation (a midline bone has no side)
- Superior vs inferior: the wide base with the body of S1 is superior; the narrow apex is inferior.
- Anterior vs posterior: the smooth, concave surface with transverse ridges is anterior (pelvic); the rough, convex surface with crests is posterior (dorsal).
- Confirm with the sacral hiatus, which opens on the dorsal surface near the lower end.
The sacrum is a single midline bone with symmetrical halves, so it has no side; you orient it instead.
Articulations
- With the fifth lumbar vertebra (body) at the Lumbosacral intervertebral disc joint (symphysis) — Body of S1 on the base..
- With the fifth lumbar vertebra (inferior articular processes) at the Lumbosacral zygapophysial joint, synovial plane joint — Superior articular processes..
- With the hip bone (ilium) at the Sacroiliac joint, synovial anteriorly with a strong fibrous (syndesmosis) part posteriorly — Auricular surface..
- With the coccyx at the Sacrococcygeal joint, secondary cartilaginous joint — Apex..
Muscle and ligament attachments
| Structure | Type | Where on the bone |
|---|---|---|
| Piriformis | origin | Pelvic surface, around the anterior sacral foramina. |
| Gluteus maximus | origin | Dorsal surface (lateral part), with the ilium, coccyx and sacrotuberous ligament. |
| Erector spinae and multifidus | origin | Dorsal surface. |
| Iliacus (some fibres) | origin | Ala. |
| Coccygeus | insertion | Lateral margin of the lower sacrum (and coccyx). |
| Sacrotuberous and sacrospinous ligaments | ligament | Lateral margins of the lower sacrum (and coccyx). |
| Interosseous sacroiliac ligament | ligament | Sacral tuberosity. |
| Anterior and posterior sacroiliac ligaments | ligament | Pelvic and dorsal surfaces next to the sacroiliac joint. |
Functions
- Transmits the weight of the upper body from the vertebral column to the hip bones.
- Forms the posterior wall of the pelvis and part of the pelvic brim.
- Protects the lower nerve roots in the sacral canal and lets the sacral nerves leave through its foramina.
- Gives attachment to gluteal, back and pelvic muscles and to strong ligaments that stabilise the pelvis.
Clinical & practical relevance
- Caudal epidural anaesthesia: local anaesthetic is injected through the sacral hiatus, found by feeling the sacral horns, into the epidural space.
- Variations of the lumbosacral junction are common: the fifth lumbar vertebra may be partly fused to the sacrum (sacralisation), or the first sacral segment may be separate (lumbarisation).
- The sacrum is generally wider and shorter in females, which contributes to a wider pelvis.
Identification tips
- Smooth and concave = front (pelvic surface). Rough with crests = back.
- Crests from medial to lateral follow the processes from back to side: spinous, articular, transverse.
- Ear-shaped auricular surface = sacroiliac joint.
- Feel the two sacral horns: the hiatus lies between them.
Common student mistakes
- Mixing up the anterior and posterior foramina: anterior rami go through the anterior (pelvic) foramina.
- Calling the sacral hiatus an opening on the pelvic surface: it is on the dorsal surface.
- Thinking the sacroiliac joint is wholly synovial: it has a fibrous (syndesmosis) part behind.
- Placing the promontory on the dorsal surface: it is on the front of the base.
Common exam questions
How many vertebrae fuse to form the sacrum?Show answer
Five sacral vertebrae.
Name the crests on the dorsal surface and what they represent.Show answer
Median (fused spinous processes), intermediate (fused articular processes) and lateral (fused transverse processes).
What passes through the anterior and posterior sacral foramina?Show answer
Anterior rami of the sacral nerves through the anterior foramina; posterior rami through the posterior foramina.
What is the sacral hiatus and why is it important?Show answer
The lower opening of the sacral canal on the dorsal surface, from failure of the S5 laminae to fuse; used for caudal epidural injection.
With which bones does the sacrum articulate?Show answer
L5 above, the two hip bones (ilia) at the sacroiliac joints, and the coccyx below.
What is the sacral promontory?Show answer
The projecting anterior edge of the body of S1, part of the pelvic inlet boundary.
How do you orient a sacrum?Show answer
Wide base up, narrow apex down; smooth concave surface forwards; rough convex surface with crests backwards.
Revision summary
- Five fused sacral vertebrae; triangular, base up and apex down.
- Base: body of S1 with the promontory, alae, superior articular processes.
- Pelvic surface: transverse ridges and four pairs of anterior sacral foramina.
- Dorsal surface: median, intermediate and lateral crests; posterior sacral foramina; sacral hiatus with sacral horns.
- Auricular surface for the ilium at the sacroiliac joint; sacral tuberosity behind it.
- Caudal epidural block through the sacral hiatus.
Test yourself on the sacrum
Spotting questions on bones, sides and landmarks — untimed, with explanations.
Joints of the sacrum
References
- Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students. 5th ed. Philadelphia: Elsevier; 2023.
- Moore KL, Dalley AF, Agur AMR. Moore's Clinically Oriented Anatomy. 9th ed. Philadelphia: Wolters Kluwer; 2023.
- Federative International Programme for Anatomical Terminology (FIPAT). Terminologia Anatomica. 2nd ed. FIPAT.library.dal.ca; 2019. fipat.library.dal.ca/TA2
- Kervyn G, Zielinski M, Vinent L, et al. Z-Anatomy: the libre 3D atlas of anatomy. Models derived from BodyParts3D, The Database Center for Life Science (CC BY-SA 2.1 JP). CC BY-SA 4.0. www.z-anatomy.com/
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