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Biomechanics Of

Sacroiliac Joints

Venus Pagare

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• Introduction • Osteology • Articulating surfaces• Ligaments• Blood supply• Nerve supply• Factors promoting • Stability• Kinematics• Functional consideration• Clinical anatomy• SI Dysfunction

FLOW OF THE LECTURE

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Sacroiliac (SI) Joint : • Articulations between Left and right articular surfaces on sacrum

and left and right iliac bones

INTRODUCTION

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• 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

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• Connects spine to pelvis

• Absorbs vertical forces from spine and

transmitting them to pelvis and lower

extremities

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SACRUM• Large flattened triangular bone• Formed by fusion of five sacral verebrae• Forms postero-superior part of bony pelvis

OSTEOLOGY

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• 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

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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

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• Three borders: anterior, posterior & medial• Three surfaces: gluteal, iliac fossa &

sacropelvic surface

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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

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• 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

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• 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

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Primary Ligaments: Secondary Ligaments:a. Anterior sacroiliac a. Sacrotuberousb. Posterior sacroiliac b. Sacrospinousc. Interosseous

LIGAMENTS

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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

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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

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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.

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• Cartilaginous joint• Joins 2 ends of pubic bones• 3 ligaments associated are - superior pubic ligament - inferior pubic ligament - posterior ligament

SYMPHYSIS PUBIS

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• Branches of posterior division of internal iliac artery :

• Iliolumbar

• Lateral sacral &

• Superior gluteal arteries

Blood supply

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• Superior gluteal nerve

– Ventral rami &

– Lateral branches of dorsal rami of first & second sacral nerve

NERVE SUPPLY

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• 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

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• 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

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• 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

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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

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• 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 ↓

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• 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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Hip flexion in supine

Anterior tilting of ilium

Counternutation at SIJs

↑ diameter of pelvic brim

Descent of foetal head in pelvis

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• Hip extended position is favored early in birthing process to facilitate descent of fetal head into pelvis

• Hip flexed position is used during delivery

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Movements of Ilium• Posterior Rotation• Anterior Rotation• Motion of innominate relative to sacrum occurs

about a coronal axis

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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

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• 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.

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Anterior Rotation• Occurs during extension of the freely

swinging leg

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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

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• 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

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Gravity

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Stretched Ligaments

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Muscles that reinforce and stabilize SIJ:• Erector Spinae• Lumbar multifidi• Abdominal muscles: External & Internal

obliques Rectus abdominis Transversus abdominis• Hamstrings such as biceps femoris

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• 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

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• 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

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The systemic causes of sacroiliac dysfunction:– Inflammatory conditions• Ankylosying spodylitis, Rheumatoid

Arthritis– Joint infections• Brucellosis, Tuberculosis

–Metabolic disorders• Gout, Hyper parathyroidism

–Miscellaneous• Osteitis condensans illi, Paget’s disease

SACROILIAC DYSFUNCTION

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Doubts??

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NUTATION

COUNTER NUTATION

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