Sacroiliac Joint Anatomy

Sacroiliac joint anatomy (SI or sacro-iliac) is quite unlike any other joint in the body, because the joint surfaces are covered by two different kinds of cartilage.

Like all true joints, there is cartilage on both sides of the SI joint surfaces, but the articular surfaces have both hyaline cartilage (glassy) and fibro cartilage (spongy) surfaces that rub against each other. No other joints have this feature!

The joint also has many large ridges and depressions that fit together like pieces in a puzzle.

Unlike most other joints, the SI joint is not designed for large movements. The rocking movements made with every step are in fact very small. The SI joint usually only moves about two to four millimeters during weight bearing and forward flexion. It is a "viscoelastic joint", meaning that its major movement comes from giving or stretching.

Furthermore, it is common for the SI joint to become even more stiff and actually lock, usually due to injury, but also due to prolonged sitting, for example. This explains why manipulation is the treatment of choice for the very painful SI joint syndrome.

Mobilization may also be useful for keeping the joint loose using certain specific exercises and stretches that are important in the rehabilitative phase of the treatment.

Where is the Sacro-iliac Joint?

The Pelvis - a ring of three bones

X-ray of the Pelvis showing the sacroiliac joints

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