Sciatic Nerve

Original Editor - George Prudden

Top Contributors - George Prudden, Manisha Shrestha, Kim Jackson, Lucinda hampton, Candace Goh and Joao Costa

Description

The sciatic nerve is the largest nerve in the body, and consists of the medially placed tibial nerve and the laterally placed common peroneal nerve. It is formed from the ventral rami of the fourth lumbar to third sacral spinal nerves and is a continuation of the upper band of the sacral plexus.

The sciatic nerve is the largest nerve in the body, and consists of the medially placed tibial nerve and the laterally placed common peroneal nerve. It is formed from the ventral rami of the fourth lumbar to third sacral spinal nerves and is a continuation of the upper band of the sacral plexus.

It leaves the pelvis through the greater sciatic foramen, below the piriformis muscle, and descends between the greater trochanter of the femur and the ischial tuberosity. Initially deep to piriformis, it runs inferiorly and laterally posterior to the ischium, crossing over the nerve to quadratus femoris. Inferior to piriformis; it lies deep to gluteus maximus. It passes inferiorly crossing obturator internus, the gemelli and quadratus femoris. The posterior cutaneous nerve of thigh and the inferior gluteal artery lie on its medial side. Descending vertically, it enters the thigh at the lower border of gluteus maximus, where it lies on the posterior surface of adductor magnus. It gives off nerves to the hamstring muscles. The nerve is crossed obliquely on its superficial aspect by the long head of biceps femoris. The nerve ends at the upper aspect of the popliteal fossa by dividing into the tibial and common perineal nerves.

The nerve can be represented on the back of the thigh by a line drawn from just medial to the midpoint of the line from the ischial tuberosity to the apex of greater trochanter down to the apex of popliteal fossa.

It supplies articular branches to the hip joint, with muscular branches to biceps femoris, semitendinosus and semimembranosus and the ischial head of adductor magnus. The nerve to the short head of biceps is from the common peroneal division, with the other muscular branches emerging from the tibial division.

Branches

• Tibial nerve
• Common peroneal nerve

Clinical relevance

A great deal of variability exists in relationship of the sciatic nerve to the piriformis muscle and short external rotators. In approximately 85% of cases the sciatic nerve exits the pelvis deep to the muscle belly of the piriformis. It is usually superficial (posterior to the other external rotators). In 11% of individuals a portion of the piriformis muscle splits the common peroneal nerve and tibial nerve. These anatomic variations are important in the interpretation of intra-operative findings.

Piriformis Syndrome

Youngman described 'Piriformis Syndrome' in 1928 as an evolving compression of the sciatic nerve by the piriformis muscle. This is associated with acute trauma to the buttock and occurs when the sciatic nerve exits posterior to the piriformis. The patient finds sitting difficult and participation in activities where hip flexion or internal rotation is required, almost impossible. The pain is in the sciatic nerve distribution.

Physical examination reveals tenderness directly over the piriformis tendinous or in the gluteal area, and the pain can be listed by forced internal rotation of the extended thigh – this is sometimes called 'Pace's sign'. There is sometimes weak abduction against resistance or external rotation against resistance, and the pain may also be reproduced by rectal or vaginal examination.

Treatment involves rest and oral anti-inflammatory drugs. The diagnosis can also be confirmed by the injection of local anesthetic under fluoroscopy into the area of injury. Steroid injection may occasionally be necessary. In refractory cases, surgical exploration of the piriformis and/or division of the piriformis muscle and/or mobilization of the sciatic nerve may be necessary.

The piriformis syndrome is thought to be due to irritation of the sciatic nerve as it passes over the piriformis tendon. This causes buttock pain and sciatica. The pain can be reproduced by applying pressure to the piriformis fossa on the posterior aspect of the greater trochanter and by stressing the piriformis muscle. Injections can once again be diagnostic and therapeutic. Some authors have reported good results by sectioning the piriformis to relieve the pain.

Hamstring Syndrome

This pathology commonly affects athletes who present with localised and radiating pain near the ischial tuberosity. The pathophysiology is thought to be that of an insertional tendopathy at the ischium but there may also be involvement of sciatic nerve compression. The pain in hamstring syndrome radiates down the posterior thigh or popliteal region and is exacerbated when the hamstrings are on tension. This is often seen in sprinters or hurdlers. On examination there is exquisite tenderness over the ischial tuberosity and percussion in that region may reproduce the sciatic distribution of pain. Treatment involves rest, anti-inflammatory agents and steroid injections. [2]