3. Sacroiliac (SI) Joint :
• Articulations between
Left and right articular surfaces on sacrum and left and
right iliac bones
INTRODUCTION
3
4. • Plane synovial joint → modified amphiarthrodial joint
• Stable, rigid; relatively immobile; allowing effective load
transfer
• Each of two SI joints are about 1-2 mm wide
4
5. • Connects spine to pelvis
• Absorbs vertical forces from spine and transmitting
them to pelvis and lower extremities
5
6. SACRUM
• Large flattened triangular bone
• Formed by fusion of five sacral verebrae
• Forms postero-superior part of bony pelvis
OSTEOLOGY
6
7. • Upper part (base) of sacrum is
massive
• Supports body weight &
transmits it to hip bones
• Lower part (apex) is free from
weight and therefore tapers
rapidly
• Sacrum has pelvic, dorsal, &
right & left lateral surface
7
9. ILIUM
• Upper expanded plate like part of
hip bone
Parts:
• Upper end called iliac crest
• Lower end which is smaller & fused with pubis &
ischium at acetabulum
9
10. • Three borders: anterior, posterior & medial
• Three surfaces: gluteal, iliac fossa & sacropelvic surface
10
11. SACRUM:
• Auricular (C)-shaped on sides of fused sacral vertebrae
• Covered with hyaline cartilage
• Thicker than iliac cartilage
ILIA:
• Covered with fibrocartilage
• Type II collagen, typical of hyaline cartilage, has been
identified
ARTICULATING SURFACES
11
12. • Flat and smooth in foetus
• Postpubertal : marked by a central groove or surface
depression
• Rough irregular surface with many large ridges and
depression
• Form an interlocking mechanism with the ilium, fitting
together like pieces of a puzzle
12
13. • 6 times more resistant to lateral forces than lumbar
spine
• 1/20 resistance to forces in axial compression
• 1/2 resistance to rotational forces compared to lumbar
spine
13
14. Primary Ligaments: Secondary Ligaments:
a. Anterior sacroiliac a. Sacrotuberous
b. Posterior sacroiliac b. Sacrospinous
c. Interosseous
LIGAMENTS
14
15. 1. Anterior Sacroiliac Ligament
• Iliac crests to tubercles of first
four sacral vertebrae
• Join ilia to sacrum
• Thickening of part of capsule
• Thin, not very strong
2. Interosseous SI ligament
• Strong & massive
• Superficial & Deep: Superior band
Inferior band
15
16. 3. Posterior Sacroiliac Ligament
• Stronger than anterior ligament and connects sacrum to
PSIS.
• Categorized into two sets;
- short (superior) posterior SI ligament; horizontal
- long (inferior) posterior SI ligament; vertical
• Short & long
16
17. 4. Sacrospinous Ligament
• Connects ischial spines to lateral borders of sacrum and
coccyx
• Forms inferior border of greater
sciatic notch
5. Sacrotuberous Ligament
• Connects the ischial tuberosities to posterior spines at
ilia and lateral sacrum and coccyx
• Forms inferior border of lesser sciatic notch.
17
18. • Cartilaginous joint
• Joins 2 ends of pubic bones
• 3 ligaments associated are
- superior pubic ligament
- inferior pubic ligament
- posterior ligament
SYMPHYSIS PUBIS
18
19. • Branches of posterior division of internal iliac artery :
• Iliolumbar
• Lateral sacral &
• Superior gluteal arteries
Blood supply
19
20. • Superior gluteal nerve
– Ventral rami &
– Lateral branches of dorsal rami of first & second
sacral nerve
NERVE SUPPLY
20
22. • Stability is primary requirment of joint
• Maintained by:
- Interlocking of articular surfaces
- Thick & strong interosseous & posterior sacroiliac
ligaments
- Vertebropelvic ligaments
- With advancing age partial synostosis of joint takes
place which further reduces movements
FACTORS PROVIDING STABILITY
22
23. • Very slight motion is available
• The SIJs are linked to symphysis pubis in a closed
kinematic chain
• Any motion at symphysis pubis is accompanied by
motion at SIJs and vice versa
• Rotational motion : 0.2 – 2⁰
KINEMATICS
23
24. • Translation motion : 1 – 2 mm
• Rom increase during pregnancy in which all ligaments
of pelvis become loose under influence of hormones, to
facilitate delivery of foetus
24
25. NUTATION
COUNTER
NUTATION
• Movement of sacral
promontory anteriorly
& inferiorly
• Posterior ilium-on-
sacrum rotation
rotation
• Anterior tip of sacral
promontory moves
posteriorly & superiorly
• Anterior ilium-on-
sacrum rotation
25
26. • Coccyx moves posteriorly
in relation to ilium
• AP diameter of pelvic
brim is ↓
• AP diameter of pelvic
outlet is ↑
• Coccyx moves anteriorly
in relation to ilium
• AP diameter of pelvic
brim is ↑
• AP diameter of pelvic
outlet is ↓
26
27. • SIJs and symphysis pubis are closely linked functionally
to hip joint
• Affects and gets affected by movements of trunk and
lower extremities
Hip flexion in supine position
Posterior tilting of ilium
Nutation at SIJs
↑ diameter of pelvic outlet
Facilitates delivery of Foetal head
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28. Hip flexion in supine
Anterior tilting of ilium
Counternutation at SIJs
↑ diameter of pelvic brim
Descent of foetal head in pelvis
28
29. • Hip extended position is favored early in birthing
process to facilitate descent of fetal head into pelvis
• Hip flexed position is used during delivery
29
30. Movements of Ilium
• Posterior Rotation
• Anterior Rotation
• Motion of innominate relative to sacrum occurs
about a coronal axis
30
31. Posterior Rotation
• In Single leg standing:
Both weight bearing and non
weight bearing innominates,
posteriorly rotates relative to
sacrum which is relatively nutated
• SIJ is thus in closed packed position
31
32. • It is also associated
with side flexion of
pelvis.
a) Non-weight-bearing
innominate: antero-
superior relative to
the sacrum.
b) Weight-bearing side:
posterior and superior
relative to the
sacrum.
32
34. 2 main functions of SIJs:
a. Stress relief in pelvic ring
• During walking
• During child birth
b. Stable means for transfer of load between axial skeleton
and lower extremities
• SIJ plane is nearly vertical
• Susceptible to slipping
FUNCTIONAL CONSIDERATIONS
34
35. • Nutation ↑ stability by increasing compression and
frictional forces
• Closed pack position = Full Nutation
• Forces that create nutation torque include:
- Gravity
- Passive tension in stretched ligaments
- Muscle tension
35
38. Muscles that reinforce and stabilize SIJ:
• Erector Spinae
• Lumbar multifidi
• Abdominal muscles: External & Internal obliques
Rectus abdominis
Transversus abdominis
• Hamstrings such as biceps femoris
38
39. • Lumbosacral trunk & ventral ramus of s1 nerve crosses
pelvic surface of joint & may involved in area of their
distribution
• During pregnancy pelvic joints & ligaments are relaxed,
& locking mechanism becomes less efficient, it naturally
puts greater strain on ligaments, the sacroiliac strain
thus produced may persist even after pregnancy
CLINICAL ANATOMY
39
40. • After childbirth ligaments are tightned up again, so
that locking mechanism returns to its original
efficiency
• Sometimes locking occurs in rotated position of hip
bones adopted during pregnancy
• This results in subluxation of joint, causing low
backache due to strain on ligaments
40
