Bony landmark · Pelvis

Posterior superior iliac spine

Spina iliaca posterior superior

The PSIS is the bony point at the back corner of the pelvis. The pair of them lie directly under the two dimples in the lower back, which makes them the standard surface landmark for locating the sacroiliac joints. The thoracolumbar fascia and part of the gluteus maximus attach around it and the sacrotuberous ligament anchors nearby, so this point is the crossover between the low back and the buttock.