The sacrum is a large triangular bone formed by the fusion of five sacral vertebrae. It forms the posterior part of the bony pelvis and supports the vertebral column, transmitting body weight to the pelvis through sacroiliac joints. The sacrum has four surfaces: a base directed upwards, an anterior pelvic surface directed downwards, a dorsal surface directed upwards and backwards, and lateral surfaces. It contains the sacral canal which houses the cauda equina and forms various joints with surrounding bones.

1. The Sacrum
Dr. Mushfiqul Hoque
Lecturer
Dhaka National Medical College

3. The sacrum
• The sacrum is a large,
flattened, triangular bone
formed by the fusion of five
sacral vertebrae. It forms
the posterior superior part
of bony pelvis, articulating
on either side to the
corresponding hip bone at
the sacroiliac joint.

4. The sacrum
• Being a triangular bone
the sacrum has a base
and an apex as all
triangles have but the
base of the sacrum is
directed upwards and the
apex of the sacrum is
directed downwards.

5. The sacrum
• The upper part is huge
because it supports body
weight and transmits it to
the hip bones. The lower
part is free from weight,
and therefore tapers
rapidly.

6. Functions of the sacrum
1. Forms the posterior part of bony
pelvis
2. Supports the vertebral column
3. Transmits the bodyweight to the
pelvic girdle through sacroiliac joint

7. Anatomical points
1. Pelvic surface which is convex and
smooth is directed downwards and
forwards
2. The Dorsal surface is rough and
convex and directed upwards and
backwards
3. Base or upper surface is
directed upwards and forwards
4. Upper end of the sacral canal is
directed almost upward and slightly
backwards

8. Parts of Sacrum
1. Apex
2. 4 surfaces:
I. Base or upper
surface
II. Anterior surface
or pelvic surface
III. Posterior or
dorsal surface
IV. Lateral surface
3. Sacral canal

9. Anterior surface or pelvic surface
1. Directed forwards and
downwards
2. Concave
3. Median area is marked
by four transverse
ridges which indicate
the lines of fusion of
the bodies of the
sacral vertebrae
4. These ridges end on
either side at the four
pelvic sacral foramina

10. Anterior surface or pelvic surface
1. It has 4 pairs of pelvic
sacral foramina which
communicate through
intervertebral
foramina with sacral
canal
2. Bars of bone which
separate the foramina
from each other
represent the costal
element

11. Anterior surface or pelvic surface
• Surface lateral to sacral
foramina on each side
are formed by the
fusion of costal
elements to one
another

12. Anterior surface or pelvic surface
• Sacral foramen
transmits ventral rami
of upper 4 sacral
nerves and branch
from lateral sacral
artery

13. Anterior surface or pelvic surface
• The line of fusion of
the bodies of sacral
vertebrae are seen as 4
raised transverse
ridges

14. Attachments of Anterior surface or pelvic surface
• The piriformis muscle arises from
the bodies of middle 3 sacral
vertebrae in an E shaped fashion
• Origin of iliacus is partly from this
surface close to the sacroiliac
joint on the rough part of the Ala of
the sacrum
• Insertion of cuccygeus muscle in
the lower lateral part

15. Attachments of Anterior surface or pelvic surface
• The piriformis muscle arises from
the bodies of middle 3 sacral
vertebrae in an E shaped fashion
• Origin of iliacus is partly from this
surface close to the sacroiliac
joint on the rough part of the Ala of
the sacrum
• Insertion of cuccygeus muscle in
the lower lateral part

16. Ligaments on Anterior surface or pelvic surface
• Anterior Sacro iliac ligaments
• Sacro-spinous ligaments
• Sacro-coccygeal ligaments

17. Relations of Anterior surface or pelvic surface
1. Median sacral vessels in the
median plane
2. Sympathetic trunks along
medial margins of sacral
foramina
3. Upper two and half sacral
vertebrae are related to
parietal peritoneum
4. Lower two and half sacral
vertebrae are related to
rectum but separated from
S3 by bifurcation of superior
rectal artery

18. Relations of Anterior surface or pelvic surface
1. Median sacral vessels in the
median plane
2. Sympathetic trunks along
medial margins of sacral
foramina
3. Upper two and half sacral
vertebrae are related to
parietal peritoneum
4. Lower two and half sacral
vertebrae are related to
rectum but separated from
S3 by bifurcation of superior
rectal artery

19. Dorsal surface or posterior surface
1. Rough, irregular, convex
2. Faces backwards and
upwards
3. Raised and interrupted crest
in the midline known as
median sacral crest which
represents fused spines of
sacral vertebrae

20. Dorsal surface or posterior surface
• Sacral hiatus, an inverted U
shaped gap below the 4th
spinous tubercle in the
posterior wall of sacral canal
which is due to failure of 5th
sacral vertebrae to meet in
the median plane

21. Dorsal surface or posterior surface
• Dorsal sacral foramina, 4 in
number lying on each side of
the median crest. They
communicate with the sacral
canal through intervertebral
foramen and transmit dorsal
rami of sacral nerves

22. Dorsal surface or posterior surface
• Intermediate sacral crest,
these are 4 small articular
tubercles lying just medial to
the dorsal sacral foramina.
These represent the articular
processes fused together
• Lateral sacral crest, lies
lateral to the dorsal sacral
foramina, formed by fusion
of transverse processes of
the sacral vertebrae

23. Dorsal surface or posterior surface
• Sacral corua, free and
projecting part of the lower
part of 5th sacral vertebrae
on the sides of the sacral
hiatus; it represents the
inferior articular processes of
the 5th sacral vertebrae

24. Attachments of Dorsal surface
• Erector spinae muscle arises
from an elongated U shaped
linear area involving the
continuous rows of spinous
and transverse tubercle

25. Attachments of Dorsal surface
• Multifidus arises from the
area enclosed by the U
shaped origin of Erector
spinae muscle
• Gluteus maximus arises from
lower and lateral part of this
surface
• Sacrotuberous ligament
attached to the lower and
lateral part below the
attachment of gluteus
maximus

26. Lateral surface
• Formed by fused transverse
process and costal elements
• Broader above and narrower
below
• Upper ear shaped part is
called the articular surface
which articulate with the
ilium to form the sacroiliac
joint. This articular surface is
formed entirely by costal
element of the sacral
vertebrae

27. Lateral surface
• The area behind the articular
surface is rough for
attachments of ligaments

28. Lateral surface
• At the lower part of the lateral surface it suddenly bends and forms an angle
which is called inferior lateral angle.
• Below the angle the lateral surface becomes very narrow and almost like a
border.

29. Attachments of Lateral surface
• The rough area behind the
articular surface gives
attachment to Interosseous
Sacro-iliac ligament
• Lower narrow part below the
articular surface gives
attachment to 4 structures
from behind to forwards:
1.Gluteus Maximus
2.Sacrotuberous and
sacrospinous ligaments
3.coccygeus

30. Attachments of Lateral surface
• Inferior lateral angle gives
attachment to lateral
sacrococcygeal ligament

31. Base
• Directed upwards and
forwards
• Formed by superior surface
of 1st sacral vertebrae
• It articulates with 5th lumbar
vertebrae to form sacro-
vertebral angle
• Divided into three parts
1.Median part
2.Right lateral part
3.Left lateral part

32. Base
• It has the parts of a typical
vertebrae in modified form
1. Body:
I. Transverse measurement
is more that anterior-
posterior measurement
II. Sacral promontory:
anterior projecting part of the
body
2. Vertebral foramen: lies behind
the body and leads to sacral
canal
3. Superior articular process: to
articulate with the Lumbar 5
vertebrae

33. Base
• Transverse process and costal
elements are modified to
form the Ala of the sacrum
• Ala of the sacrum forms
lateral part of the base
• Divided into two parts
1. Smooth medial part
2. rough lateral part

34. Attachments of Base
• Rough lateral part of the Ala
of the sacrum gives origin to
iliacus muscle anteriorly and
gives attachment to
lumbosacral ligaments
posteriorly

35. Relations of Base
• Smooth medial surface of the
Ala of sacrum is related to
following 4 structures from
medial to lateral side:
1. Sympathetic chain
2. Lumbosacral trunk
3. Iliolumbar artery
4. Obturator nerve

37. Joints formed with sacrum
• Between the upper surface
of sacrum and body of
lumbar 5th vertebrae,
secondary cartilaginous joint
• Between apex of sacrum and
coccyx, secondary
cartilaginous joint
• Between lateral articular
surface of sacrum and ilium
of hip bone on both sides,
plane variety of synovial
joint

38. Sacral canal
• Formed by vertebral
foramina of sacral vertebrae
• Triangular in shape
• Lateral wall of the canal has 4
intervertebral foramens
which communicates in front
with pelvic sacral foramina
and in the back with dorsal
sacral foramina

39. Sacral canal
Contents:
1. Cauda equina including filum
terminale
2. Spinal meninges
3. Sacral and coccygeal nerve
roots
Sacral hiatus transmits:
1. 5th pair of sacral nerves
2. Coccygeal nerves
3. Filum terminale

40. Sacral canal
Contents:
1. Cauda equina including filum
terminale
2. Spinal meninges
3. Sacral and coccygeal nerve
roots
Sacral hiatus transmits:
1. 5th pair of sacral nerves
2. Coccygeal nerves
3. Filum terminale

41. Sacral canal
Contents:
1. Cauda equina including filum
terminale
2. Spinal meninges
3. Sacral and coccygeal nerve
roots
Sacral hiatus transmits:
1. 5th pair of sacral nerves
2. Coccygeal nerves
3. Filum terminale

42. End of discussion
Any questions??