Sacroiliac Joint is Different from Other Joints
Distributes shock and stabilizes the body
Most joints have two smooth surfaces that, in healthy individuals, glide smoothly against each other when in motion. The sacroiliac joint (SI joint) does not. Instead, the surfaces where the sacrum and pelvis meet are rough and irregular. This was not a mistake of nature; in fact, it works to our advantage. The "matching up" of the irregular surfaces lend greater stability to the joint. It's when the surfaces don't align correctly that sacroiliac joint dysfunction can occur.
All of the interlocking bones of the sacroiliac joint and pelvis stay in place because strong ligaments support and stabilize the area, while at the same time, permitting motion. Were this not the case, walking, running, and giving birth would not be possible. However, it is important to note that the amount of "give" in the SI joint is extremely small. The movement that does occur consists of tilting, sliding and rotation, and is limited to a few millimeters or degrees, at most.
SI joint – Why it's so important
Every time you walk, the sacroiliac joint distributes the shock of motion across the pelvis, thereby reducing strain on the spine. It also stabilizes the body, enabling you to maintain an upright position while walking or running.
Here's a simplified explanation of the role of the SI joint in walking/running:
- Leg moves forward; one heel hits the ground
- Weight shifts from the heel and moves through the foot to the toes
- SI joint on the same side as the leg that's moving absorbs the shock
- SI joint on the opposite side "locks" to keep pelvis in position
- The body is propelled either forward or backward; both SI joints pivot in concert with the motion
The ligaments in/around the SI joints prevent the sacrum and pelvis from excessive tilting, forward or backward.
SI joint axes of motion include:
- Anterior/posterior tilt of the pelvis
- Forward tilt of the sacrum (nutation)
- Backward tilt of the sacrum (counter-nutation)
- Forward tilt of one side in conjunction with a backward tilt of the opposite side
Each side of the pelvis moves in opposition during the gait sequence. This requires slight movement on each side of the sacroiliac joint. Certain conditions (eg, osteoarthritis, trauma, pregnancy) can cause too much or too little movement of the SI joint. When this occurs, the abnormal motion may cause lower back pain and alterations in mobility.