Nerve Stimulation

The nerve stimulator is initially set at 2 mA intensity and plantar flexion of the foot or toes (tibial division of the sciatic nerve) or dorsiflexion/eversion of the foot or toes (peroneal division of the sciatic nerve) are sought. The stimulating current is then reduced in order to obtain this response at 0.5 mA. The mean depth of needle insertion at which the response is obtained is 7 cm.

Upon injection of the first ml of local anesthetic the motor response disappears. The injection is completed with repeated negative blood aspiration tests and verbal contact with the patient is kept throughout. A volume of 15 to 20 mL local anesthetic is sufficient. Multiple stimulation technique is not necessary with this approach because all 3 branches of the sciatic nerve (tibial, common peroneal and posterior cutaneous nerve of the thigh) emerge together above the piriformis muscle.

The catheter is then inserted some 2cm beyond the needle
tip. Deeper insertion of the catheter should be avoided in
order prevent misplacement of the catheter below the pirifor-
mis. Indeed, if the catheter is inserted too far along the scia-
tic nerve, the posterior cutaneous nerve of the thigh may not
be blocked as this nerve leaves the sciatic trunk above the
piriformis muscle.

We prefer to tunnel the catheter below the skin for 4 to 5 cm
in order to move its emergence underneath the skin at a clean area. The catheter is then fixed, an antibacterial filter is connected and a test dose is administered (see figure).