Sacroiliac Joint is Different from Other Joints

Most joints have two smooth surfaces that glide smoothly against each other when in motion. The sacroiliac joints (SI joints) do 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 your advantage. The "matching up" of the irregular surfaces lend greater stability to the joint. It's when the surfaces don't align correctly that SI joint dysfunction can occur.

All the interlocking bones of the sacroiliac joints 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" of one or both SI joints 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.

The sacroiliac joints sit between the sacrum and the iliac bones in the lower back where the spine meets the pelvis. Alterations in function of the sacroiliac joints are a common cause of lower back pain. Photo Source: 123RF.com.

Why the SI joints are so important

Every time you walk, your SI joints distribute the shock of motion across the pelvis, thereby reducing strain on the spine. The joints also help stabilize your body, enabling you to maintain an upright posture while walking or running.

Walking, step-by-step

Here's a simplified explanation of the role of the SI joints 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 of the sacroiliac joints. Certain conditions (eg, osteoarthritis, trauma, pregnancy) can cause too much or too little movement of the SI joints. When this occurs, the abnormal motion may cause lower back pain and alterations in mobility.