Often many people wonder why the SacroIliac Joint (SIJ) is designed the way it is? The SIJ is in an area where a lot of forces are transferred from the lower body to the upper body and vice versa. Due to this reasoning one may wonder why the SIJ has such flat articulation. The SIJ is designed to deal with compressive and bending forces but not shear forces. This is why it would make more functional sense for the SIJ to be glued together.
So how does the SIJ compensate?
But if the SIJ was molded perfectly it would have no mobility?
This is why the SIJ needs to find a perfect equilibrium between movement and stability. With the muscles, fascia and ligaments around it, the SIJ now finds a way to create what's called Force Closure. Using anatomical structures to create force and allow the SIJ to move and be stable.
How does the SIJ achieve this "SelfLocking" or "SelfBracing Mechanism"?
- Dorsal Sacral Ligaments (Nutation=Tension)
-When the sacrum experiences nutation it uses the dorsal sacral ligaments like a hammock. Resting on these ligaments now creates tension and thus provides friction/stability to the SIJ. Often this ligament can be dysfunctional or tender amongst pregnant women. This relationship explains how now a more mobile pelvis has to use ligaments around it for stability.
- Sacrotuberous Ligaments (Loading = Decreased Mobility)
- Studies have shown that by loading the sacrotuberous ligament mobility of the SIJ will be decreased. This ligament will become loaded by tension in the muscles that attach to it such as the Bicep Femoris, Glut Max, Piriformis and Tharacolumbar Fascia. Again this connection can be supported by the correlation of pregnant women having tight hamstrings. The hamstrings attempt to load the sacrituberous joint and work harder to keep this now mobile SIJ stable.
- Multiple muscles attach to the thoracolumbar fascia allowing it to the act as a connection point between muscles of the lower back, pelvic and proximal aspect of the lower extremities. This fascia creates stability by producing tension when these muscle contract. The Multifidus may contract and balloon up into the fascia allowing it to tighten and thus brace its surroundings.
3. Musculature Slings
- There are multiple slings of muscles which envelope the SIJ allowing it to be stabilized. The anatomical pattern of these slings gives premise for rehabilitation to be performed on these muscles in an attempt to create SIJ stability. If one creates a rehabilitation program to strengthen these muscles the movement and pain from the SIJ may be reduced.
Pool-Goudzwaard AL, Vleeming A, Stoeckart R, Snijders CJ, & Mens JM (1998). Insufficient lumbopelvic stability: a clinical, anatomical and biomechanical approach to 'a-specific' low back pain. Manual therapy, 3 (1), 12-20 PMID: 11487296