The sacrum is the lower portion of the spine where several vertebrae are fused together. The sacrum is a triangular shaped bone that has a joint on either side with the ilium, the back part of the pelvis. The pubic rami, the front part of the pelvis, fit together in the front forming the pubic symphysis. The sacroiliac joints are L shaped in contour with a shorter upper and longer lower arm. Normally the sacroiliac joint is configured in such a way that the bones have an interlocking structure, which assists in keeping them properly aligned. In some cases the opposing joint surfaces are quite flat. This type of joint is much less stable and can lead to a shearing or sliding malalignment. Some sacroiliac joints reverse the normal concave-convex ‘locking’ relationship, which can lead to rotational malalignment. The variation in joint configuration results in a corresponding variation in integrity. This means that some people are more susceptible to SI joint pain because the joints are inherently weaker or more prone to malalignment.