Identifying the Superficial Peroneal (Fibular) Nerve for Ultrasound-Guided Ankle Block: A Divisive Issue

Abstract

Background and Aims: Chin et al showed a 16% failure rate for ultrasound-guided (USG) ankle block [1]. We find the region supplied by the superficial peroneal nerve (SPN) to have the highest frequency of incomplete anaesthesia. We sought to determine whether ultrasound can reliably identify where the SPN pierces deep fascia of the leg, and correlate this to cadaveric findings, to instruct USG SPN block. Methods: Ethical approval was obtained to undertake a sonographic assessment of 26 adult volunteers. The point the SPN pierced deep fascia was measured as a proportion of the distance from the lateral malleolus (LM) to the fibula head (HF). The distance anterior/posterior to the LM-HF straight line was measured. 16 formalin-fixed cadavers were dissected to reveal the SPN, and the same assessment performed. Results: Sonographically, the SPN was identified piercing deep fascia 0.31 of the total LM-HF distance (95% CI 0.29 - 0.32). The main SPN trunk pierced at 0.30 in the cadaveric group (0.28 - 0.32, t = -0.391, p = 0.697). However, the SPN divided before piercing deep fascia in 46.9% of cadaveric limbs (0% on ultrasound; t = 5.230, p<0.000). The SPN pierced deep fascia anterior to the LM-HF line in all but one limb. Conclusions: The point the SPN pierced deep fascia, as seen sonographically, matched the main trunk of the SPN when dissected. The cadaveric group demonstrated multiple branches piercing deep fascia, which was not identified on ultrasound. This unidentified anatomical variation may contribute to USG SPN block failure.