The SIJ is the joint between the base of the spine (sacrum) and part of the pelvis (ilium). It can be a cause of lower back pain, especially in pregnancy, or what seems to be sciatica.
The Role of the SIJ
This joint is not a hinge like the elbow, or a ball and socket like the hip. In fact, it is not capable of much movement at all. It makes a good shock absorber though, taking on the forces travelling down from the body and back up from impact with the floor. So it makes sense that jarring movements like stepping off a kerb can be particularly aggravating for an irritated SIJ.
The SIJ in Pregnancy
From early in the first trimester, the body starts producing the hormone "relaxin", which among other things is responsible for relaxing the ligaments around the pelvis. This prepares the pelvis for birth, at which point the sacrum needs to be able to move more than normal to allow the baby to move down the birth canal. In the meantime, it can play a role in pregnancy related lower back pain. In more severe cases, SPD can result from this increased movement.
The image above shows possible areas of referred pain from the SIJ. The route down the back of the legs makes it easy to mistake for sciatica, but symptoms and mechanism are different. Sciatica is caused by direct nerve irritation, such as when a disc or muscle put pressure on the nerve itself. Symptoms might include a shooting or stabbing pain, as well as numbness or weakness. In contrast, referred pain from the SIJ is more likely to be felt as a dull ache, and prodding the painful area is unlikely to aggravate the pain. Symptoms might be improved by certain positions that ease the load on the joint. Sometimes movement is the best thing for relieving pain, although it might only be temporary.
Managing SIJ Pain
Your osteopath will look at your whole case to work out exactly why you have the pain. If it is related to pregnancy, it may be appropriate to do some gentle strengthening exercises, or it might be a case of just doing what we can to keep the movement you have, and keep you comfortable through the rest of your pregnancy. For some mums, symptoms may persist (though often much less so) while breastfeeding, as the relaxin hormone stays in circulation until a few months after ceasing breastfeeding.
Sometimes, the SIJ is irritated because it is overloaded. This might be due to a previous injury, or because another area is not working as well as it should. Often the lower back is overworked due to reduced movement in the upper back, which often goes unnoticed. Treatment can be really simple here, releasing muscles and joints in the upper back to restore balance through the spine and pelvis.