Sacral Illiac Joint Mechanics Part II

Sacral Illiac Joint Mechanics Part II

Part one of SI mechanics stopped at the unloaded pelvis- we begin with the loaded pelvis.


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5. Loaded Pelvis

a.  Loading of the Superincumbant weight causes
b. Ligaments are loaded
c. Si joint is separated and placed in a loose pack position
d. SI joint sinks caudally or inferiorly
e. Sacrum rotates anteriorly and supports by posterior interouseus ligaments

i. Increases lordosis
ii. Increases sacral angle

f. Anterior rotation of sacrum is limited by the sacrotuberous and sacrospinous ligaments

i. Creates equal force coupling with posterior interouseous ligament
ii. Force coupling prevents excessive sacral angle and shearing at L5/SI and L4/L5

g. Secondary to the force coupling the illium rotates posteriorly bilaterally

- When the sacrum is loaded force form and closure is eliminated as the sacrum sinks caudally acting as a reverse keystone, opens the joint and allows for movement

6. Sacral Axis

a. Axis in which the sacrum rotates anteriorly and posteriorly
b. Majority of the motion occurs at S3
c. Asymmetrical axis
d. Sacral axis at S3 is “forced” secondary the joint and boney landmarks
e. The forced axis creates the couple force which limits the sacral angle and sheering forces at L5/S1 and L4/L5



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