Posterior approach to the sciatic nerve in adults: Is euclidean geometry still necessary?

Background: Every approach to the sciatic nerve in the buttocks currently requires the identification of pelvic bone structures. The large size of the nerve and its constant trajectory suggest that a simplified approach is possible. Methods. During the first stage, 24 sciatic nerves in adult cadavers were dissected. The most significant findings were, (1) caudal to the piriformis muscle the sciatic nerve runs parallel to the midline (intergluteal sulcus); (2) the distance between the nerve and the intergluteal sulcus during this course is approximately 10 cm in adults, regardless of their gender, size, or body habits. In the clinical stage 20 blocks were performed at 10 cm from the midline of the buttocks using a nerve stimulator and insulated needles. Results. All blocks were accomplished in less than 8.5 min. The technique proved easy to teach. Residents performed most of the blocks. Incision time, measured from the time of the injection, was less than 29 min in all cases. There were 18 successful blocks and 2 failures. Conclusions: Because of the intimate relationship of the sciatic nerve to the bony pelvis, the position of this nerve in the buttocks is constant. Caudal to the piriformis muscle the nerve runs vertically between the ischium and the greater trochanter. The location of this narrow passage, not the buttocks' size, determines the position of the nerve. While the size of the buttocks is variable among different individuals and in the same individual at different stages of adult life, the relationship of the sciatic nerve to the pelvis is constant throughout life. Using this relationship to our advantage, a sciatic block in adults can be accomplished at 10 cm lateral to the intergluteal sulcus without a need for identification of buried structures or line tracings.