The Posterior Tibial Nerve
The recording electrode is routinely placed over the abductor hallucis muscle, located one fingerbreadth behind and below the navicular bone. You can also place it over the abductor digiti quinti muscle, the latter placement being useful for comparing the medial and lateral plantar nerve functions. Place the reference electrode over the base of either the great or little toe.
Stimulate the nerve distally posterior to the medial malleolus at the ankle. The proximal stimulation point is in the popliteal space, about a fingerbreadth lateral to midline.
At times, surface stimulation of the nerve in the popliteal space may be difficult, especially in obese patients. Often you can get a response if the stimulus voltage and duration are raised to uncomfortable levels. Often, too, this response has an initial positive deflection and its amplitude drops by more than 2 mv. It should however remain within 4 mv of the distal response. You may need to position the patient in several ways before a response is obtained, often the best one being with the patient prone.
The posterior tibial nerve may be involved as part of a sciatic nerve injury; at the popliteal fossa; in the tarsal tunnel following ankle injury; and rarely at an anterior opening of the abductor hallucis muscle.