SI Joints: Form: Sacroiliac / SI Joints Anatomy, Pain & Issues
- See also: Anatomy of the Pelvic Girdle, Pelvis & Hips Introduction.
- The sacroiliac (SI) joints are located where the sacrum joins the ilium (hip) bones.
- “Extremely resilient, tough ligaments… surround and support the SI joints from all sides, holding them firmly in place… The ligaments of the SI joint include some of the strongest ligaments in the human body!” (Jenni Rawlings)
- Muscles and fascia also lend stability to the joint. (Ray Long MD)
- The image above clearly shows the location of the joint, but the images here and here also show the ligaments, providing a more real and dramatic view that demonstrates the stability provided by the ligaments.
Sitting vs. Standing
- When standing, the sacrum wedges into the pelvis. When sitting, the sacrum is no longer wedged into the pelvis and therefore the joints are less stable.
Generally Immobile, Capacity for Small Movements
Even though the sacroiliac joints are synovial joints, their opposing surfaces usually fit together tightly enough for every movement of the pelvis to affect the sacrum (and therefore the spine as a whole), and for every movement of the sacrum to affect the pelvis. This view—that the sacroiliac joints are essentially immobile—has practical value… but it is an oversimplification: the synovial structure of the healthy sacroiliac joint is now known to provide its groove-and-rail architecture with the capacity for a small amount of slippage—movements that have been called nutation and counternutation. – H. David Coulter
Stable Joint—Except in Advanced Yoga Practitioners
As gravity wedges the triangular sacrum firmly down between the inclined auricular surfaces of the ilium bones, it tends to force the ilium bones apart, but strong ligaments prevent them from moving. This wedging action and the resistance of the ligaments combine to form a stable joint… In most people, the sacroiliac joints don’t move much, if at all… In more advanced yoga students and teachers, on the other hand, it appears that these joints often move quite a bit, and they frequently get hurt in the process. – Roger Cole
When Sitting, Stability is Lost
Held together by strong yet pliable anterior sacroiliac ligaments, the SI joint is designed to lock in place in standing positions with the sacrum bone wedging itself into the pelvic joint due to the weight of the trunk. This compact sacrum-pelvis connection together with the psoas and illiacus muscles creates a firm base for the entire spinal column. However, when you sit, this stability is lost because the sacrum is no longer wedged into the pelvis. – Ram Rao
Muscles are Also SI Joint Stabilizers
Note that the piriformis is a muscular stabilizer of the sacroiliac joint. Imbalances between the piriformis muscles can contribute to subtle asymmetries within the pelvis. – Ray Long MD