Piriformis syndrome is compression of the sciatic nerve through the piriformis muscle. The piriformis muscle originates on the anterior surface of the sacrum near the sacroiliac joint and attaches to the greater trochanter of the femur. The piriformis covers the greater sciatic foramen. In the majority of people, the sciatic nerve exits the greater sciatic foramen under surface of the piriformis. For some people, the sciatic nerve actually pierces through or splits the piriformis muscle, predisposing them to piriformis syndrome.
The piriformis muscle can be irritated with spasms or local inflammation. This muscle acts as an external rotator, weak abductor and hip flexor. It provides stability when walking and standing. If the sciatic nerve is compressed or pinched, nerve irritation can cause pain along the sciatic nerve. Pain may be felt in the buttocks down to the lower thigh and leg.