7.4 SI Joint

Question 1

What are the two primary functions of the sacroiliac (SI) joint?

Answer 1

Stress relief and stability.

Question 2

What is the SI joint the main interface between?

Answer 2

The axial skeleton and the appendicular skeleton of the lower extremity.

Question 3

Where is the SI joint located anatomically?

Answer 3

Between the sacrum and the ilium of the pelvis.

Question 4

Can you directly palpate the SI joint?

Answer 4

No, it cannot be palpated directly, but it can be approximated near the PSIS.

Question 5

How can you locate the PSIS to approximate the SI joint?

Answer 5

Place hands on hips and march in place to feel the PSIS move under the thumbs; the SI joint lies just anterior to it.

Question 6

What percentage of chronic low back pain cases are attributed to the SI joint?

Answer 6

Approximately 25%.

Question 7

Why is the SI joint often subject to stress and torsion?

Answer 7

Because humans are bipedal and activities like running or kicking create repetitive unilateral torsion forces.

Question 8

What is the pelvic ring composed of?

Answer 8

Sacrum, two SI joints, ilium, ischium, pubis, and pubic symphysis.

Question 9

What is considered the keystone of the pelvic ring?

Answer 9

The sacrum.

Question 10

What are the two key movements at the SI joint?

Answer 10

Nutation and counternutation.

Question 11

What does the term ‘nutation’ mean?

Answer 11

A forward nodding motion or anterior tilt of the sacral promontory relative to the ilium.

Question 12

What happens to the ilium during nutation?

Answer 12

It tilts posteriorly relative to the sacrum.

Question 13

What is counternutation?

Answer 13

Posterior tilt of the sacral promontory and anterior tilt of the ilium.

Question 14

What is another way to describe nutation and counternutation?

Answer 14

Nutation ≈ flexion; counternutation ≈ extension (though not identical).

Question 15

What are the two functional roles of the SI joint?

Answer 15

Stress relief and stability.

Question 16

During which activities does the SI joint provide stress relief?

Answer 16

Walking, running, and during pregnancy.

Question 17

How does the SI joint provide stress relief during gait?

Answer 17

It dissipates torsional forces created by descending gravity and ascending ground reaction forces.

Question 18

Why is SI joint mobility important during pregnancy?

Answer 18

Ligamentous laxity allows nutation, which increases the size of the pelvic outlet for childbirth.

Question 19

What two forces act on the pelvis during standing?

Answer 19

Gravity (descending) and the ground reaction force (ascending through the acetabulum).

Question 20

What motion does gravity cause at the sacral promontory during standing?

Answer 20

Anterior tilt or nutation.

Question 21

What position provides the greatest SI joint stability?

Answer 21

Nutation (the close-packed position).

Question 22

Why is nutation considered the close-packed position of the SI joint?

Answer 22

Because it maximizes ligament tension and joint congruence, increasing stability.

Question 23

What can cause posterior tilt of the innominate on the sacrum?

Answer 23

The ground reaction force acting through the acetabulum.

Question 24

What happens if the SI joint were a solid, immobile ring?

Answer 24

It would lose its ability to absorb torsional forces, leading to excessive stress and pain.