SI Morphology Flashcards
What kind of joint (categorization) is the SI?
- Diarthrosis
- Synovial joint w/ fluid
What kind of joint (categorization) is the pubic symphysis?
- fibrocartilaginous
- amphiarthrosis
What is the shape of the SI joint?
An L (or a boot)
- upper half articulates at S1 level
- lower half is S2-S3 level
What kind of cartilage is on the sacral surface of the SI joint?
Is it thicker or thinner than the iliac surface?
Hyaline cartilage
Three times thicker on sacral surface than iliac surface
The sacral surface of the SI joint has 3 shape characteristics?
- central groove
- wedge shaped
- S shaped
What kind of cartilage is on the iliac surface of the SI joint?
Is it thicker or thinner than the sacral surface?
Fibrocartilage
1/3 as thick as the sacral surface
The iliac surface of the SI joint has what morphological feature to match the sacral surface?
A central ridge, to match the central groove of the sacrum
During which life period do the surfaces of the SI joint begin to roughen?
Teenage years
Which side of the SI joint see cartilage erosion sooner / faster?
The iliac surface - especially problematic since there is only 1/3 as much to begin with
When do we start to see the beginning of SI joint surface erosion?
20 - 40 years old
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?
Iliac surface first, 20 -40 years old.
Sacral surface later, beginning 40 - 60 years old.
Which part of the SI joint is more likely to have adhesions, upper or lower?
Upper
What percentage of males do we expect to have bony ankylosis in the SI starting in the 60s?
And starting in the 80s?
60’s = 27%
80’s = 46%
What is the distinction between intrinsic and extrinsic SI ligaments?
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.
What are the 3 intrinsic SI ligaments?
- Posterior Sacroiliac Lig
- Interosseous part - strongest lig in body
- Dorsal lig - limit nutation at the base
- Anterior Sacroiliac Lig
- Joint capsule —> better developed anteriorly
What are the 2 extrinsic sacroiliac ligaments?
- Sacrotuberous lig: limits nutation at the apex
- Sacrospinous lig: limits nutation at the apex
Which 6 muscles load the SI joint?
- Erector spinae
- QL
- Psoas
- Iliacus
- Piriformis
- Gluts
Which muscles cross the SI joint?
Trick question - NONE.
What is the innervation of the SI joint?
We actually aren’t entirely sure, but the capsule and ligaments appear to be innervated for propio & nocioception
The sacrum is the keystone in an arch.
What allows it to resist movement - inferiorly, anteriorly, and posteriorly?
- wedge shape resists inferior movement
- sacroiliac ligaments resist anterior and posterior displacement
What is form closure?
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
What is force closure?
Tension in muscles, ligaments, and fascia that keep the joint together - especially L —> M force
What is the consensus for total SI ROM?
Roughly 0 - 3 degrees
Which patients likely have the most SI movement and which the least?
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
What is Illi’s model of locomotion?
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
