Sciatic nerve

Last revised by Yuranga Weerakkody on 17 Jan 2023

The sciatic nerve arises from the sacral plexus from the roots of L4-S3 and runs through the buttock and down the lower limb. It is the longest and widest single nerve in the body.

The nerve forms from the anterior divisions of the L4-S3 roots (which form the tibial component) and posterior divisions of the L4-S2 roots (which form the common peroneal component) of the sacral plexus.

The sciatic nerve enters the lower limb by exiting the pelvis through the greater sciatic foramen, below the piriformis muscle and above the superior gemellus muscle.

It descends midway in between the greater trochanter of the femur and the tuberosity of the ischium and in the posterior compartment of the thigh to the apex of the popliteal fossa, where it divides into two large terminal branches:

  • anteriorly

    • upper part: posterior surface of the ischium, nerve to quadratus femoris, obturator internus, the gemelli

    • lower part: adductor magnus

  • posteriorly

    • upper part: gluteus maximus

    • lower part: long head of biceps femoris (crosses obliquely)

In the upper part of its course, it is accompanied by the posterior femoral cutaneous nerve and the inferior gluteal artery, and is covered by the gluteus maximus muscle.

The nerve gives off articular and muscular branches before dividing into two terminal branches - the tibial nerve and the common peroneal nerve.

The articular branches arise from the upper part of the nerve and supply the hip joint, perforating the posterior part of its capsule; they are sometimes derived from the sacral plexus.

The sciatic nerve supplies the following muscles:

The division of the sciatic nerve into the common peroneal and tibial nerves may take place at any point between the sacral plexus and the lower third of the thigh. When it occurs at the sacral plexus, the common peroneal nerve usually pierces the piriformis muscle.

A range of other variants exist based on the relationship to the piriformis muscle.3

  • division in the pelvis with

    • common peroneal nerve piercing piriformis muscle and tibial nerve exiting below (mentioned above)

    • common peroneal nerve travelling above piriformis muscle and tibial nerve below

    • common peroneal nerve travelling above piriformis muscle and tibial nerve piercing pirformis muscle

    • both common peroneal nerve and tibial nerve travelling below piriformis muscle separately

  • sciatic nerve courses over piriformis muscle

  • sciatic nerve pierces piriformis muscle

  • appears as a hypoechoic round structure, situated deep to piriformis and gluteus maximus muscles 4

Pain and functional symptoms may be caused by a compression or irritation of the sciatic nerve. This may be caused by:

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