Don’t confuse what we are calling SI pain with other types of back pain, because, in most cases, the explanations and suggestions for back pain caused by the SI can't are not the same for people with other types of pain.
The cardinal symptom of SI pain is an ache on or around the posterior superior iliac spine (PSIS), on one side of the body only.
The PSIS is the rear-most point of bone on the pelvis. In most students you can palpate it by pressing your fingers into the back of the pelvis above the main mass of the buttock, about two or three inches to the side of the center line of the upper sacrum.
Most students with SI problems will tell you that long periods of sitting and most types of forward bends aggravate their pain, but this is also true for students with sciatica and other back problems.
Those with SI pain are often particularly aggravated by wide-legged (abducted) poses, such as Baddha Konasana (Bound Angle Pose), Upavistha Konasana (Wide-Angle Seated Forward Bend), Prasarita Padottanasana (Wide-Legged Forward Bend), Utthita Trikonasana (Extended Triangle Pose), Virabhadrasana II (Warrior II Pose), and Utthita Parsvakonasana (Extended Side Angle Pose). They also have trouble with twists, such as Marichyasana III (Pose Dedicated to the Sage Marichi III), and side-bends, such as Parivrtta Janu Sirsasana (Revolved Head-to-Knee Pose). For many, the worst pose is a combination of twisting, abduction, and forward bending, namely Janu Sirsasana (Head-to-Knee Pose).
A joint is where two bones come together. The sacroiliac joint is where the sacrum bone and the ilium bone join one another.
The sacrum and the two ilium bones often merge into a single bone but this is not always the case. Many health professionals who have worked with yogis believe that the cause of their sacroiliac pain is excessive movement of the joint, leading to misalignment, ligament strain, and, possibly, eventual deterioration of cartilage and bone on the auricular surfaces. Another hypothesis is that the source of SI pain is sprained or torn ligaments, rather than injury to the joint surfaces themselves.
Backbends can be good or bad for the SI joints, while forward bends usually spell trouble. Postures that spread the thighs wide apart (into abduction), like Baddha Konasana, Upavistha Konasana, and Virabhadrasana II are also big time troublemakers.