The sacrum is a triangular bone formed by the fusion of 5 vertebrae. It is positioned at the base of the spine between the two hip bones. The sacrum has four surfaces - pelvic, dorsal, and two lateral surfaces. It articulates superiorly with L5 and inferiorly with the coccyx. The sacrum contains the sacral canal which houses the cauda equina and filum terminale. Sexual dimorphism exists between male and female sacrums in features such as length, curvature, and auricular surface morphology. Congenital anomalies like sacralisation and lumbarisation can affect the number of sacral foramina.

1. Sacrum

2. Learning objectives
Sacrum:
• Introduction
• Anatomical position
• Attachments
• Sexual dimorphism
• Ossification
• Clinical Correlation

3. Introduction
• Wedge shaped.
• Triangular fusion of five vertebrae.
• Posterosuperior wall of pelvic cavity.
• Wedged between two hip bones.
• Base : Superior(Articulates with L5 at
lumbosacral angle)
• Apex: Inferior(Articulates with coccyx)
• Surfaces: Anterior/Pelvic
Posterior/Dorsal
2 Lateral

4. Anatomical Position
1. Sacrum is a midline bone plced between hip bones(on
each side), 5th Lumbar vertebra (Superiorly) and Coccyx
(Inferiorly).
2. Superior surface of the body of 1st sacral vertebra slopes
forward at an angle of 30 degree.
3. Anterior surface of sacrum faces downwards and
forwards.
4. The upper end of sacral canal is directed upwards.

5. General Features
Sacrum consists of:
1. Base
2. Apex
3. Four Surfaces : a) Pelvic
b) Dorsal
c) Right Lateral
d) Left Lateral
4. Sacral Canal

6. Base

7. Pelvic Surface
Concave surface:
Facing downwards
and forwards.
Features:
1. Transverse ridges.
2. Anterior sacral
foramina: Ventral
rami of upper 4
sacral nerves.
3. Lateral mass:
Lateral to foramina.

8. Pelvic Surface

9. Pelvic Surface
Muscle Attachments:
1. Piriformis
2. Coccygeus
3. Sacrospinal
ligament.

10. Dorsal Surface
Five vertical crests:
a)Median sacral crest.
b)2 intermediate sacral crests.
c)2 lateral sacral crests.
Sacral hiatus: U shaped gap at
lower end.

11. Dorsal Surface

12. Dorsal Surface
Muscle Attachments:
1. Erector spinae.
2. Multifidus.
3. Gluteus maximus.
4. Sacrotuberous
ligament.

13. Lateral Surface

14. Sacral Canal
Contents:
1. Lower part of cauda
equina.
2. Filum terminale.
3. Spinal meninges.
4. Lateral sacral
vessels.
** Dura and arachnoid
extents up S2.

15. Sexual Dimorphism in Sacrum
Features Male sacrum Female sacrum
1. Length More Less
2. Ratio between the transverse
width of body of 1st sacral
vertebra and the entire width of
sacral base.
More than 1/3rd. Less thn 1/3rd.
3. Auricular surface Relatively longer, upper three
segments.
Smaller, occupies only upper two
segments of sacrum.
4. Anterior surface of sacrum Shallower Deeper
5. Sacral Index
[Breadth of the base X 100]
Length
Lesser Greater
6. Width Relatively narrower Wider
7. Curvature Uniformly curved Flattened in the upper part but
sharply curved in the lower part.

17. Ossification
• Chondrification is initiated in
the 5th gestational week and
results in a cartilaginous
vertebral column.
• Primary or enchondral
ossification occurs in three
primary ossification centers
(central, neural, and costal)
and forms the axial skeleton.
• In the sacrum, the costal
ossification centers form a
portion of the lateral mass.
• A total of six centers produce
the sacral alae .

18. • Bilateral neural ossification centers
contribute to the neural arch and
the posterolateral vertebral body.
• The central ossification center
forms the midportion of the
vertebral body.
• With secondary ossification, two
epiphyseal plates provide accessory
ossification to the superior and
inferior portions of each sacral
vertebral body.
• Disks separate the sacral vertebrae
during childhood .
• The S3-4 and S4-5 disks fuse in late
adolescence, and the remaining
levels fuse during the 3rd decade of
life.

19. Clinical Correlation
Sacralisation:
• Congenital anomaly.
• Incorporation of the fifth lumbar (L5) or first
coccygeal vertebra (C1) in the sacrum.
• Number of sacral foramina is increased
unilaterally or bilaterally.

20. Lumbarisation:
• First sacral vertebra (S1) is separated from the
sacrum and fused with the fifth lumbar vertebra
(L5).
• Number of sacral foramina reduced to 3.