Effect of Nerve Stimulation Use on the Success Rate of Ultrasound-Guided Subsartorial Saphenous Nerve Block: A Randomized Controlled Trial.

BACKGROUND AND OBJECTIVES: Ultrasound-guided subsartorial saphenous nerve block is commonly used to provide complete surgical anesthesia of the foot and ankle in combination with a popliteal sciatic nerve block. However, in part owing to its small caliber and absence of a prominent vascular landmark in the subsartorial plane distal to the adductor canal, the saphenous nerve is more difficult to reliably block than the sciatic nerve in the popliteal fossa. Although the saphenous nerve is a sensory nerve only, neurostimulation can be used to elicit a "tapping" sensation on the anteromedial aspect of the lower leg extending toward the medial malleolus. Our objective was to test the hypothesis that the addition of nerve stimulation use to an ultrasound (US)-guided technique will increase the success rate of subsartorial saphenous nerve block.

METHODS: With institutional human ethics board approval and participants' written informed consent, we enrolled 80 patients undergoing foot and ankle surgery in a randomized, single-blinded, parallel-group clinical trial. Patients were randomly assigned to receive US-guided subsartorial saphenous nerve block either alone (US group) or with the use of additional nerve stimulation (NS group; time limit, 5 minutes). For saphenous nerve blockade, all patients received 10 mL of 0.5% ropivacaine. The primary end point was complete absence of sensation to pinprick at 30 minutes at two different anatomic areas in the distribution of the saphenous nerve (2 cm proximal to the medial malleolus and 10 cm distal to the medial tibial condyle). Secondary end points included decreased sensation at 30 minutes and block failure (normal sensation) at 30 minutes. This trial was registered at ClinicalTrials.gov: NCT02382744.

RESULTS: All 80 patients completed the trial (40 patients in each group). Twenty-two patients (55%) in the NS group versus 18 (45%) in the US group had complete absence of sensation to pinprick at 30 minutes at both anatomic areas of assessment (Fisher exact test, P = 0.25 [one sided]; 95% confidence interval of difference in proportions, -11.9% to 31.9%). The percentages of patients with any evidence of block (decreased or complete absence of sensation) at both areas at 30 minutes were 92.5% (NS) and 97.5% (US), respectively (P = 0.62 [two sided]); corresponding failure rates (normal sensation) were 7.5% (NS) and 2.5% (US). In the NS group, no response in the saphenous nerve distribution was elicited within 5 minutes of stimulation time limit in 20% of patients (n = 8). All of the patients in the NS group with normal sensation at 30 minutes (n = 3) were among this subcohort.

CONCLUSIONS: The addition of the use of nerve stimulation did not improve the success rate of US-guided subsartorial saphenous nerve block. However, in the NS group, an inability to elicit a "tapping" sensation in the saphenous nerve distribution was associated with block failure.