SI Morphology

Question 1

What kind of joint (categorization) is the SI?

Answer 1

• Diarthrosis

- Synovial joint w/ fluid

Question 2

What kind of joint (categorization) is the pubic symphysis?

Answer 2

• fibrocartilaginous

- amphiarthrosis

Question 3

What is the shape of the SI joint?

Answer 3

An L (or a boot)

• upper half articulates at S1 level
• lower half is S2-S3 level

Question 4

What kind of cartilage is on the sacral surface of the SI joint?

Is it thicker or thinner than the iliac surface?

Answer 4

Hyaline cartilage

Three times thicker on sacral surface than iliac surface

Question 5

The sacral surface of the SI joint has 3 shape characteristics?

Answer 5

• central groove
• wedge shaped
• S shaped

Question 6

What kind of cartilage is on the iliac surface of the SI joint?

Is it thicker or thinner than the sacral surface?

Answer 6

Fibrocartilage

1/3 as thick as the sacral surface

Question 7

The iliac surface of the SI joint has what morphological feature to match the sacral surface?

Answer 7

A central ridge, to match the central groove of the sacrum

Question 8

During which life period do the surfaces of the SI joint begin to roughen?

Answer 8

Teenage years

Question 9

Which side of the SI joint see cartilage erosion sooner / faster?

Answer 9

The iliac surface - especially problematic since there is only 1/3 as much to begin with

Question 10

When do we start to see the beginning of SI joint surface erosion?

Answer 10

20 - 40 years old

Question 11

Which joint surface is likely to show signs of DJD first?

When is the soonest we expect to see that?

When do we expect to start seeing it on the other surface?

Answer 11

Iliac surface first, 20 -40 years old.

Sacral surface later, beginning 40 - 60 years old.

Question 12

Which part of the SI joint is more likely to have adhesions, upper or lower?

Answer 12

Upper

Question 13

What percentage of males do we expect to have bony ankylosis in the SI starting in the 60s?

And starting in the 80s?

Answer 13

60’s = 27%

80’s = 46%

Question 14

What is the distinction between intrinsic and extrinsic SI ligaments?

Answer 14

Both groups of ligaments affect the joint

Intrinsic SI ligs actually bind sacrum to ilium.

Extrinsic connect two other things - sacrum and ishium, for example.

Question 15

What are the 3 intrinsic SI ligaments?

Answer 15

• Posterior Sacroiliac Lig
  
  1. Interosseous part - strongest lig in body
  2. Dorsal lig - limit nutation at the base
• Anterior Sacroiliac Lig
• Joint capsule —> better developed anteriorly

Question 16

What are the 2 extrinsic sacroiliac ligaments?

Answer 16

• Sacrotuberous lig: limits nutation at the apex

- Sacrospinous lig: limits nutation at the apex

Question 17

Which 6 muscles load the SI joint?

Answer 17

1. Erector spinae
2. QL
3. Psoas
4. Iliacus
5. Piriformis
6. Gluts

Question 18

Which muscles cross the SI joint?

Answer 18

Trick question - NONE.

Question 19

What is the innervation of the SI joint?

Answer 19

We actually aren’t entirely sure, but the capsule and ligaments appear to be innervated for propio & nocioception

Question 20

The sacrum is the keystone in an arch.

What allows it to resist movement - inferiorly, anteriorly, and posteriorly?

Answer 20

• wedge shape resists inferior movement

- sacroiliac ligaments resist anterior and posterior displacement

Question 21

What is form closure?

Answer 21

The fact that the physical structure of the SI joint promotes stability and creates the end of the joint motion - like interlocking puzzle pieces

• wedge shape
• S shape
• convex/concave groove in the center

Question 22

What is force closure?

Answer 22

Tension in muscles, ligaments, and fascia that keep the joint together - especially L —> M force

Question 23

What is the consensus for total SI ROM?

Answer 23

Roughly 0 - 3 degrees

Question 24

Which patients likely have the most SI movement and which the least?

Answer 24

Young women are likely to have the most

Older men are likely to have the least

SI motion is less in males and decreases with age

Question 25

What is Illi’s model of locomotion?

Answer 25

Reciprocal motion between the sacrum and ilium:

Flexion of hip and ilium create PI movement of the PSIS

Is matched with

Ipsilateral anterioinferior movement of the sacral base - nutation