Navigating the anomalous path of the tibial nerve at the ankle – attention to the intricacies!

Dear Editor,

Ankle block is a widely practised regional anaesthesia/analgesia technique for foot and toe surgeries due to its simplicity and reliability. It blocks three superficial (saphenous, superficial peroneal, and sural) and two deep (tibial and deep peroneal) nerves. Among these five nerves, the tibial nerve is to be targeted first, the largest nerve with a higher soak time (~20 min).[1] A successful tibial nerve block is an essential component of ankle block as it contributes significantly to the dermatomal, myotomal, and osteotomy innervation of the ankle and foot. At the ankle level, variations in the distal branching pattern of the tibial nerve have been described in the literature.[2,3,4] We report an incidental finding of an aberrant anatomical course of the tibial nerve and its implications for anaesthesiologists. Consent was obtained for the publication of this report, and visual media.

The tibial nerve travels posterolateral to the posterior tibial vessels and 8–10 mm perpendicularly deep to the skin at the level of the most prominent point of the medial malleolus.[5] In the anatomical landmark-guided technique, the nerve block needle is inserted immediately posterior to the arterial pulsation, which is palpated at the space between the medial malleolus (posterior border) and the Achilles tendon [Figure 1a and b]. With the help of neurostimulation, a tibial nerve block is performed at or just above the level of the medial malleolus before the emergence of the terminal branches (medial plantar, lateral plantar, and medial calcaneal). In an academic session on an adult female volunteer (27 years old, body mass index: 21.4 kg/m²), we were unable to elicit the evoked motor response of tibial nerve (i.e. plantar flexion of left toes) with a transcutaneous nerve mapping technique by using a ballpoint probe (Stimuplex^® Pen, B Braun, Melsungen, Germany), despite several attempts. The high-frequency linear transducer (Sonosite Edge II HFL 38xp/13-6 MHz, Fujifilm SonoSite Inc., Bothell, WA, USA) was placed transversely across the intended location to detect or exclude possible anatomical variations. The tibial nerve was identified as a hyperechoic honeycomb structure anterior to the posterior tibial vessels [Figure 1c]. The tibial nerve was traced proximally and distally to differentiate it from another hyperechoic structure (flexor hallucis longus tendon). The typical echotexture of the nerve was also confirmed in the longitudinal scan. Upon proximal tracing, the tibial nerve was located superficially, anteriorly, and away from the posterior tibial vessels [Figure 1d and e]. The unusual location of the tibial nerve at the medial malleolus level, immediately anterior to the posterior tibial artery pulsation, was reconfirmed using transcutaneous neurostimulation [Figure 1f, Video 1]. The other four nerves of the left ankle were also scanned but had a regular course with no variations. The sonoanatomy of the tibial nerve and other nerves around the right ankle was normal [Figure 1b].

Figure 1.

Normal and aberrant anatomy and sonoanatomy of the tibial nerve (TN) at the ankle: (a) Anatomical landmark for TN block at the level of the medial malleolus (MM, right side); (b) Normal relation between the course of the TN and posterior tibial vessels (PTV, right ankle); (c) Aberrant position of the left TN at the eminence of the MM; (d) Sonoanatomy at 1–2 cm proximal to the tip of MM; (e) Sonoanatomy at 5–7 cm proximal to the tip of MM; (f) Corrected anatomical landmark and transcutaneous neurostimulation of left TN (anterior to the posterior tibial artery) to elicit plantar flexion of toes. PTA = Posterior tibial artery, TP = Tibialis posterior muscle, FDL = Flexor digitorum longus muscle, FHL = flexor hallucis longus muscle, AT = Achilles tendon, brown dotted line = flexor retinaculum, Yellow dotted figure = TN, white dotted figure = flexor hallucis tendon

An austere aberration of tibial nerve position can result in complete block failure, direct injury during needle insertion and manipulation, compression due to tight ankle cast/slab/dressing, and traumatic nerve injury during cannulation of the posterior tibial vessels. By understanding such variations, anaesthesiologists can modify proposed regional anaesthesia/analgesia techniques, improve their success rates, and provide safer and more effective nerve blocks.

Declaration of patient consent

The authors certify that they have obtained all appropriate volunteer consent forms. In the form, the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The volunteers understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.