2. Sciatic Nerve

Question 1

Sciatic

Answer 1

largest peripheral nerve in the body

good analgesia for much lower limb
surgery, and the variety of possible approaches provides an appropriate test of applied
anatomy.

Question 2

The sciatic nerve arises

Answer 2

The sciatic nerve arises from the sacral plexus,
which is formed by the union of the
L4, L5, S1, S2 and S3 nerve roots,

and which lies separated from the anterior sacrum by
the piriformis muscle.

about 2 cm in diameter as it exits the
pelvis posteriorly via the greater sciatic notch.

ischial tuberosity and the greater
trochanter, and then lies behind the femur before dividing in the popliteal fossa into
the common peroneal and the posterior tibial nerves.

Question 3

Supply

Answer 3

lower leg via its main terminal branches (the tibial and common peroneal).

supplies the knee joint (via articular branches) and almost all of the structures
below the knee

extensive, cutaneous area over the medial side of the
knee, lower leg and ankle, and medial side of the foot around the medial malleolus.
This area is supplied by the saphenous nerve (from the femoral).

forefoot, the sole of the foot and the lateral side of the foot and ankle

Question 4

Irritation

Answer 4

Sciatic nerve irritation can result from lumbar disc prolapse, leading to classic
symptoms of sciatica in the distribution of the root that is affected. Impingement of
the nerve can also occur in the pelvis where it crosses beneath or, in 15% of subjects,
through the piriformis muscle leading to the so-called piriformis syndrome.

can be damaged by direct trauma, including surgical trauma, as well as by ill-directed
intramuscular injections in the buttock.

the common
peroneal nerve, is particularly vulnerable as it winds round the fibular head.

Question 5

Sciatic Nerve Block

Posterior (classic approach of Labat)

Answer 5

Posterior (classic approach of Labat)
— The patient lies in the decubitus position with the upper leg flexed to 90at hip
and knee.

— A line is drawn from the greater trochanter to the posterior superior iliac spine.
From the midpoint of this line a perpendicular is dropped 3–5 cm.

— The needle is inserted vertically to the skin, and the nerve is sought at around
6–8 cm.

Alternatively, a line can be drawn from the greater trochanter to the
sacral hiatus and the injection made at its midpoint.

Question 6

Popliteal fossa block

Answer 6

— The sciatic nerve can be blocked in the popliteal fossa before it divides into its
tibial and common peroneal branches.
This is a particularly useful block for providing analgesia for forefoot surgery.

— The patient lies lateral or prone,
and the proximal flexor skin crease of the knee is identified.

— A line is drawn vertically for about 7 cm from the midpoint of the skin crease, and
the injection is made about 1 cm lateral to this point.

— If dorsiflexion is elicited it may be the common peroneal nerve alone that is being
stimulated, and the sciatic nerve may have already branched. Plantar flexion or
inversion of the foot indicates successful location of the posterior tibial nerve.

Ultrasound allows much more precise identification of the level at which the
nerve divides.

— Drug dose: 10–20 ml levobupivacaine 0.5%