The Unstable Sacro-Iliac Joint (SIJ)

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The Sacro-Iliac Joint (SIJ) a very stable structure connecting the spine to the pelvis. A system of active (force closure) and passive (form closure) stabilisers makes the joint stable. Its main role is to helps absorb vertical forces through the body. An unstable SIJ can compromise the effectiveness of transmitting forces, causing symptoms like low back pain or sometimes radiating pain down the legs. Core stability exercises and a sacro-iliac belt can help with an unstable SIJ.

What Is The Sacro-Iliac Joint?

Sacro-Iliac Joint (SIJ)

The sacrum sits in between the two iliac bones as they connect to form the SIJ. The sacrum is the tail end of your spine and the iliac bone forms part of the pelvis.

The Sacro-Iliac Joint (SIJ) allows forces to transmitted vertically through the body. The SIJ acts like a very tough suspension that absorbs the impact from the legs to the spine during activities like walking, running and jumping.

The SIJ is an inherently strong and stable joint that permits very little movement due to its natural construction. This structural strength provides passive stability or form closure. The rough, groove-like connecting surfaces of the iliac bones and sacrum interlock and help stabilise the joint; like fixing two Lego pieces together. In addition, strong and tight ligaments hold the SIJ firmly together.

Another feature that stabilises the SIJ described as the active stabilisers or force closure. Imagine wrapping your fingers around the two Lego pieces. This is similar to your muscles surrounding the SIJ, actively contracting to create a compressive force over the SIJ, gripping the joint firmly together. These muscles are the core muscles. They act as a natural corset by providing that compression around the lower back and pelvic region.

Sacro-Iliac Joint (SIJ)

This system of form and force closure ensures stability in the SIJ to perform its role optimally. However, an unstable SIJ can happen when the integrity of this system being compromised.

What Causes An Unstable Sacro-Iliac Joint (SIJ)?

Passive instability can be compromised when the ligaments joining the SIJ becomes loose or lax. This happens when you fall and land on to the buttocks, or miss a step. The impact going through the SIJ overstretches the ligaments causing it to become loose or lax.

Another cause of SIJ instability is pregnancy. Hormones released during pregnancy relax the ligaments of the body to allow the pelvis to enlarge. At the same time, muscles around the pelvis get overstretched and weakened. This contributes to the loss of form and force closure.


The only way to improve on the stability of the Sacro-Iliac Joint would be to increase the system of force closure. To increase the compressive force, perform core stability exercises. A sacro-iliac belt also prescribed to complement the core stability exercises