The sciatic nerve arises from the spinal nerves of L4 through S3. The sciatic nerve runs down the posterior thigh with its two terminal branches, the common peroneal and tibial nerves. The sciatic nerve emerges from the pelvis below the piriformis muscle and enters the thigh between the ischial tuberosity and the greater trochanter. In about 10% of patients, the sciatic nerve is separated in the greater sciatic foramen. The sciatic nerve usually separates into its two main branches in the upper part of the popliteal fossa. The common peroneal nerve is the one that is most commonly affected by traction injury, especially during total hip replacement if the extremity is lengthened or from traction such as posterior approach to the hip. You want to keep the knee flexed and the hip extended during traction to avoid injury to the sciatic nerve. If the injury occurs, it usually affects the common peroneal nerve. The patient will probably get foot drop or weakness of foot dorsiflexion. It is also important to know that innervation of the short head of the biceps femoris muscle comes from the common peroneal nerve in the middle of the thigh. The long head of the biceps femoris muscle is innervated by the tibial nerve.
