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. It branches medially to the Tibial Nerve and laterally to the 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 part of the sacral plexus.

It leaves the Pelvis through the greater sciatic foramen, inferior to 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. Below the 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. Nerves branch off 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 where it branches to distal 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.

Root

• L4, 5, S1, 2, 3

Branches

• Tibial nerve
• Common peroneal nerve

Function

Motor

• Hamstrings
  • Biceps femoris
  • Semitendinosus
  • Semimembranosus
• Adductor magnus
• Indirectly innervates (via common peroneal and tibial nerves) the muscles of the leg and foot.

Sensory

• Indirectly innervates (via common peroneal and tibial nerves) the skin of the lateral leg, heel, and both the dorsal and plantar surfaces of the foot.

Clinical relevance

There is some variability between the relationship of the sciatic nerve, piriformis muscle and short external rotators. In approximately 85% of cases the sciatic nerve exists as described above. In 11% of individuals a portion of the piriformis muscle splits the common peroneal nerve and tibial nerve.

Piriformis Syndrome

Youngman described 'Piriformis Syndrome' in 1928 as 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's may report that sitting and activities that require hip flexion or internal rotation as aggravate the pain. The pain radiates in a sciatic nerve distribution.

Objective examination reveals tenderness directly over the piriformis or in the gluteal area, and the pain can be replicated by internal rotation of the extended thigh. This finding is called the 'Pace's sign'. There is sometimes weak abduction or external rotation against resistance. The pain may also be reproduced by rectal or vaginal examination.

Initial medical management is oral anti-inflammatory medication. Steroid injection and surgical exploration may be considered for chronic cases.

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.

Hip Dislocation

Hip dislocations are most commonly associated high-energy trauma and therefore are often associated with multi-system injuries. It is important to examine for sciatic nerve injuries. The common peroneal division of the sciatic nerve is most commonly involved. Care is needed durring assessment as a femoral fracture can mask otherwise obvious hip dislocation.

Surgical Approach

Kocher-Langenbeck Approach (Posterior Approach)

Standard posterior approach to the hip is the 'Kocher-Langenbeck' approach. This is typically performed in a lateral decubitous position without use of a traction table. After splitting the gluteus maximus muscle, the sciatic nerve is identified and examined for contusion, hemorrhage or partial or complete laceration. The gluteus maximus must not be split too proximally as this can lead to denervation secondary to injury of the inferior gluteal nerve. After identifying the sciatic nerve, the tendinous insertions of the piriformis muscle and the obturator internus are identified and tagged with heavy absorbable sutures. If torn, they are detached and retracted. Care must be taken not to injure the acetabulum labrum when performing capsulotomies. Quite frequently, however, capsulotomies have already been created by the injury. The joint can then be assessed, any fracture fragments removed or stabilized and the entire area thoroughly washed out.

Anterior Approach

The anterior approach to the hip for injuries can be performed through the 'Smith Peterson' or 'Watson Jones' approaches. These can be performed with the patient in the semi-lateral or lateral dicubitous position. The latter has particular advantages if a simultaneous or posterior approach is also to be considered.

Intramuscular Injections

The path of the sciatic nerve must be considered when administering intramuscular injections into the gluteal region. The region can be divided into quadrants using 2 lines, marked by bony landmarks:

• One line descends vertically from the highest point on the iliac crest.
• The other horizontal line passes through the vertical line half way between the highest point on the iliac crest and ischial tuberosity.

The sciatic nerve passes through the lower medial quadrant. To avoid damaging the sciatic nerve therefore, intramuscular injections are given only in the upper lateral quadrant of the gluteal region.