Sacral and Pelvic Biomechanics

Question 1

What are the four common pain referral patterns of the SI joint?

Answer 1

i) bilateral SI region
ii) bilateral SI region and anterior hip
iii) lateral leg/buttock and anterior hip
iv) lateral leg/buttock

Question 2

What are three possible MOIs for SI joint dysfunction?

Answer 2

i) trauma
ii) repetitive torsions
iii) postural abnormalities (ex. LLD)

Question 3

What are the bony components of the pelvic ring?

Answer 3

i) R innominate
ii) L innominate
iii) sacrum (keystone - integrity depends on tight fit!)
iv) pubic symphysis

Question 4

What is the function of the SI joint?

Answer 4

Transfer loads between lower limb and vertebrae while maintaining stability.

Question 5

Describe the arthrology of the SI joint.

Answer 5

i) S1-3 vertebrae
ii) hyaline cartilage at sacrum, fibrocartilage at ilium
iii) C or L shaped

Question 6

Describe how the arthrology of the SI joint changes with age.

Answer 6

i) diarthrodal -> symphhysis (fibrocartilage)
ii) smooth -> rough
iii) mobile -> immobile
iv) osteophytes and fibrotic fusion is later life

Question 7

Describe the borders of the SI joint capsule.

Answer 7

i) superior - iliolumbar ligament extension
ii) inferior - blends with sacrospinous ligament
iii) posterior - interosseous ligament and long dorsal ligament

Question 8

Describe the osteokinematic movements of the innominate at the SI joint.

Answer 8

i) anterior rotation (sagittal)

ii) posterior rotation (sagittal)

Question 9

Describe the osteokinematic movements of the sacrum at the SI joint.

Answer 9

i) nutation (anterior rotation of base)

ii) counternutation (posterior rotation of base)

Question 10

Under which conditions does the innominate rotate anteriorly on the sacrum?

Answer 10

i) thorax extension

ii) hip extension non-WBing

Question 11

Describe the arthrokinematics of anterior rotation of the innominate on the sacrum.

Answer 11

Inferior, posterior slide of the innominate.

Question 12

Under which conditions does the innominate rotate posteriorly on the sacrum?

Answer 12

i) thorax flexion
ii) hip flexion
iii) non WBing leg

Question 13

Describe the arthrokinematics of posterior rotation of the innominate on the sacrum.

Answer 13

Superior, anterior slide of the innominate.

Question 14

Describe the arthrokinematics of sacral nutation.

Answer 14

Inferior, posterior glide of the sacrum.

Question 15

Describe the arthrokinematics of sacral counternutation.

Answer 15

Superior, anterior glide of sacrum.

Question 16

What is the close packed position of the SI joint?

Answer 16

Sacral nutation.

Question 17

How does sacral nutation contribute to stability at the SI joint?

Answer 17

i) increases compression and shear forces at joint surface

ii) base within column mechanically

Question 18

Explain what is meant by sacral form closure.

Answer 18

Joint surfaces align to nutate the sacrum thereby increasing stability or self-locking the joint. No other forces are required to maintain stability.

Question 19

Explain what is meant by sacral force closure.

Answer 19

Sacral stability and compression at the joint surfaces that is aided by external forces (ex muscles and ligaments).

Question 20

Using the neutral zone model, what is the effect of form and force closure respectively?

Answer 20

i) form closure: is the shape of the bowl

ii) force closure: reduces the neutral zone of the bowl..

Question 21

Under which conditions are form and force closure indicated?

Answer 21

i) form closure, or nutation torque from gravity and weightbearing, is adequate under SMALL and STATIC loading (ex. sitting, standing)
ii) force closure, or tensioning of ligaments and use of muscle to further nutate, is necessary for LARGER and DYNAMIC loads

Question 22

Describe the sacral stabilizing effect of gravity in standing.

Answer 22

i) COM anterior to sacrum, therefore provides a nutation torque.
ii) femoral head compression in acetabulum provides a superiorly directed force causing the innominate to rotate posteriorly.
- The combined rotations locks the joint.

Question 23

Describe the sacral stabilizing effect of ligaments.

Answer 23

i) nutation tightens the posterior ligaments (sacrotuberous, sacrospinous, interosseous) therefore increasing compression in the SI joint.
ii) Tension in the sacrotuberous ligament can be increased by posterior rotation of the innominate or contraction of muscles that attach to it (ex. biceps femoris)

Question 24

Describe the effect of counternutation on sacral ligaments.

Answer 24

Counternutation tensions the long dorsal sacroiliac ligament. If this becomes static or regular posture, it can become a source for pain.

Question 25

Contractile forces of muscle contribute to sacral stability by which mechanisms?

Answer 25

i) contribute to full sacral nutation
ii) pull on passive structures (ex. ligaments) that reinforce the joint
iii) generating active compression forces across the joint surface.
iv) produces slings of force that assist in load transfer.

Question 26

Name the inner unit core muscles.

Answer 26

i) transversus abdominis
ii) pelvic floor
iii) multifidus
iv) diaphragm

Question 27

What is the effect of multifidus contraction on the thoracolumbar fascia?

Answer 27

i) broadens TLF

ii) increases tension in TLF

Question 28

Name the components of the posterior oblique sling.

Answer 28

i) contralateral latissimus dorsi
ii) glute max
iii) thoracolumbar fascia

Question 29

Which outer unit sling contributes to force closure in rotation activities (ex. walking, golfing etc.)?

Answer 29

Posterior oblique sling.

Question 30

Name the components of the deep longitudinal sling.

Answer 30

i) erector spinae
ii) multifidus attaching to sacrum
iii) deep lamina of TLF
iv) sacrotuberous ligament
v) biceps femoris

Question 31

Name the components of the anterior oblique sling.

Answer 31

i) external oblique
ii) contralateral internal oblique
iii) contralateral adductors
iv) intervening anterior abdominal fascia

Question 32

Name the components of the lateral sling system.

Answer 32

i) glute med/min
ii) tensor fascia lata
iii) contralateral adductors of the hip
- significant for pelvic girdle in standing and walking.