‘Sigh’ Joint

Last month a class member asked me about the pelvic joints. A friend of theirs had injured themselves skiing in Sweden and had been told by the physio there to rest to give the joint time to recover. Upon returning to the UK they were told that the joint was not moveable and so to be active. The question was simple – was the joint at the back of the pelvis moveable. The answer is not so simple.

The pelvis is such a crucial bony girdle for our upright posture that it is maybe not surprising that people have lots of different ideas about it. It starts its life as a number of separate bones. The ilium, the ishium and the pubis – this set of three on each side and then the sacrum and coccyx at the rear (there is a philosophical debate as to whether the sacrum and coccyx are part of the pelvic girdle or part of the spine but for now lets consider the pelvic girdle to be the complete ring and include these bones).

We will know the ilium from the ridges at the top of our hips or the anterior superior iliac spine (ASIS) that we are told to point in certain directions in exercise classes. We will know the ishium from the ishial tuberosities or sit bones on which we are told to sit. All 3 bones come together to make the acetabulum or socket for the ball and socket hip joint. It is not until the early 20’s that all of these bones are fully fused together and it is hormonal levels that affect the shape, with oestrogen giving the wider pelvic openings required for childbirth.

At the back of the pelvis, the iliums join the sacrum at the sacroiliac joint (SI joint – said as two separate letters not as my exasperation has spelt it above).

The joints are highlighted here in yellow.

They are synovial joints with cartilage on both side (different types of cartilage which is quite unusual). Strong ligaments abound in the pelvis as you would expect from something that is weight bearing. The pelvis is usually depicted as a bowl but in life the sacrum is almost parallel to the ground giving the bowl a forward tilt.

Can those joints move? If you ask an anatomist then the answer will probably be no. They are very strong joints, tightly bound and weight bearing. If they could move at birth (and these are all separate bones at birth so I am not sure that is a particularly good argument) then they fuse at some point. It is the weight bearing joint in the body – would you want it to move.

However – we have the motion of nutation and counter nutation. These are the names given to the movement of the sacral bone in relation to the pelvic bones. Why would someone name something that doesn’t exist? If you have found discussion about these movements then you are probably on a yoga or manual therapy website. The movement claimed is in the region of 2mm.

If you have severe lower back and pelvic pain there is an option of SI joint fusion surgery. Why fuse something that doesn’t move.

So – does the SI joint move? The answer, as with so many things in anatomy, is it depends. I think it probably does move to begin with. I think if you keep moving it it probably retains its movement for longer than if you don’t. I think different people have different ranges of movement and I suspect that most of the bodies donated to anatomists may well have a very limited range of motion if any due to their age.

If you want a clear yes or no answer then you won’t get it studying something as diverse as the human body.

Author: Anatomy Fundamentals

Janet Philp has spent a lifetime exploring fitness and wellbeing. Starting in group exercise, travelling through rugby to representing the UK at martial arts before including Yoga, meditation, Budokon and personal instruction. Her passion is anatomical function and educating people to use their bodies to their full potential.

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