Background: Tibial and common peroneal nerves can be blocked by the posterior approach to the popliteal fossa. Techniques using fixed measured distances between knee skin crease and puncture point have been described. We report on an approach that is based on manual identification of the apex of the popliteal fossa.

Methods: Five-hundred patients undergoing surgery of ankle or foot were prospectively included. The apex of the popliteal fossa (determined by the crossing point of the biceps femoris and the semitendinosus and semimembranosus muscles) was assessed by manual palpation. The puncture point was 0.5 cm below the apex, on the medial side of the biceps femoris muscle. When indicated for postoperative analgesia, a perineural catheter was placed. We assessed success rate, number of attempts, the distance between knee skin crease and the apex of the popliteal fossa, nerve depth, and acute and late complications.

Results: Block success rate was 94% and 92% when the block was performed through the needle and the catheter, respectively. Inversion was the motor response with the highest success rate. The first attempt was successful in 97.5% of the patients. Mean depth of the nerve was 4.5 cm (range, 2.0 to 7.0 cm) and mean knee skin crease to apex of popliteal fossa distance was 9 cm (range, 7.0 to 12.0 cm). Nine patients (2%) had acute complications. There were no technical problems associated with the perineural nerve catheter. After 12 weeks, no late complications were observed.

Conclusions: The modified posterior anatomical approach for popliteal sciatic nerve block is easy to perform, has a high success rate, and has a low complication rate. The location of the needle insertion point is assessed without any measurement, thus avoiding inaccuracies caused by repeated skin-distance measurements.