The unstable SIJ

The sacro-iliac joint (SIJ) is a very stable structure connecting the spine to the pelvis. The stability is achieved by a system of active (force closure) and passive (form closure) stabilisers. 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 SIJ?

click for larger view

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 main role of the SIJ is to allow forces to be 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 is 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 also known as the core muscles. They act as a natural corset by providing that compression around the lower back and pelvic region.

Source: Gray’s Anatomy 20th ed
click for larger view

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 is being compromised.

WHAT CAUSES AN UNSTABLE 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.

TREATMENT

The only way to improve on the stability of the SIJ would be to increase the system of force closure. Thus, core stability exercises are recommended for an unstable SIJ to increase the compressive force. Sometimes, a sacro-iliac belt is prescribed to complement the core stability exercises.

This work by Musculoskeletal Consumer Review is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Singapore License. This article was contributed by Core Concepts - Musculoskeletal Health Group. In the spirit of promoting health education, you may copy, distribute and transmit the work under the conditions specified by the license. For articles re-printed with permission, copyright remains with the original copyright holder (author or publisher). MCR's Creative Commons License does not apply in such cases.

Comments

Linda Snyman
I am a 37 year old female and was recently diagnosed with unstable SIJ's. Although I fell down a flight of stairs more than 10 years ago, I only started having 'back ache' about three years ago. I visited so many physio's and orthopedic surgeons. Why did none of them consider an unstable SIJ? Not only was it expensive but also painful. Lieing down was agony. Since starting Pilates the improvement has been tremendous. I would strongly advise Pilates for unstable pelvis sufferers. Thank you for this page, it has been an excellent help.
chyetuan
Hi Linda,
Glad to hear you are much better now. I can understand your frustration with SIJ pain, it is commonly underdiagnosed. One reason is that some practitioners do not believe that there is such a condition. The SIJ is an inherently strong structure with strong ligaments holding the joint firmly, with very minimal movements available, hence usually a trauma will “destabilise” it which in your case was a fall.
All the best!
Maureen
My daughter broke her coccyx whilst giving birth to her first child, and 2 years on she is still suffering from severe lower back pain, and can only sit on the side of her hip. She has been told that the coccyx is now facing upward instead of down, and has been on a waiting list at a public hospital to have an operation to rectify this. Is there any exercises that could help her, apart from core strengthening, difficult to activate these muscsles as it places pressure on the broken coccyx and she also has separation of the rectus abdominus.
jim denike
four years ago, my wife took a pair of bad falls. Since then we have suspected SIJ instability and have even considered SIJ fusion surgery. But Linda complains about how lying down is agony. For my wife, sitting is agony and lying down is the only relief she gets. Any thoughts?