the hip, along with the quadratus femoris, gemellus inferior, gemellus superior, obturator externus, and obturator internus. The piriformis laterally rotates the femur with hip extension and abducts the femur with hip flexion. Abduction of the flexed thigh is important in the action of walking because it shifts the body weight to the opposite side of the foot being lifted, which prevents falling. The action of the lateral rotators can be understood by crossing the legs to rest an ankle on the knee of the other leg. This causes the femur to rotate and point the knee laterally. The lateral rotators also oppose medial rotation by the gluteus medius and gluteus minimus. When the hip is flexed to 90 degrees, piriformis abducts the femur at the hip and reverses primary function, internally rotating the hip when the hip is flexed at 90 degrees or more. (Netter's Clinical Anatomy, 2010) Clinical significance Piriformis syndrome occurs when the piriformis irritates the sciatic nerve, which comes into the gluteal region beneath the muscle, causing pain in the buttocks and referred pain along the sciatic nerve. This referred pain is known as sciatica. Seventeen percent of the population has their sciatic nerve coursing through the piriformis muscle. This subgroup of the population is predisposed to developing sciatica. Sciatica can be described by pain, tingling, or numbness deep in the buttocks and along the sciatic nerve. Sitting down, stretching, climbing stairs, and performing squats usually increases pain. Diagnosing the syndrome is usually based on symptoms and on the physical exam. More testing, including MRIs, X-rays, and nerve conduction tests can be administered to exclude other possible diseases. If diagnosed with piriformis syndrome, the first treatment involves progressive stretching exercises, massage therapy (including neuromuscular therapy) and physical treatment. Corticosteroids can be injected into the piriformis muscle if pain continues. Findings suggest the possibility that Botulinum toxin type B may be of potential benefit in the treatment of pain attributed to piriformis syndrome. A more invasive, but sometimes necessary treatment involves surgical exploration; however, the side effects of the surgery could be much worse than alternative treatments such as physical therapy. Surgery should always be a last resort. Landmark The piriformis is a very important landmark in the gluteal region. As it travels through the greater sciatic foramen, it effectively divides it into an inferior and superior part. This determines the name of the vessels and nerves in