Ultrasound-guided assessment of the site of branching of the tibial nerve about the medial malleolus - A volunteer-based observational study

Dear Editor,

Ankle blocks are effective regional anaesthesia techniques with fewer complications than combined femoral and sciatic nerve blocks. The overall success rate of traditional landmark-guided techniques is around 80%, but studies on cadavers suggest tibial nerve coverage to be as low as 40%.[1] This emphasises the need for improved methods to target the tibial nerve, especially when anatomical landmarks are unreliable due to factors like oedema and poor arterial pulses. The objectives of the study were to determine the site and ease of identifying tibial nerve branching, its relationship to the medial malleolus, factors influencing the branching site, and to locate the medial calcaneal nerve, if possible. We hypothesised that ultrasonography would enhance the accuracy of locating the tibial nerve and its branches, thereby improving patient outcomes.

This prospective observational study was conducted from March 2023 to February 2024 at a tertiary care medical institute, following approval from the Institutional Ethics Committee (MGMCRI/Res/01/2021/5/IHEC/52). The study was registered with the Clinical Trials Registry-India (CTRI/2023/02/050082 dated 25/02/2023; accessible at https://ctri.nic.in/Clinicaltrials/login.php). The study included 200 healthy volunteers of both genders without anatomical deformities in the ankle. The exclusion criteria were prior injuries, infections, surgical scars, and conditions that could affect visualisation. All participants gave their written informed consent.

Volunteers were placed in a prone position, and a high-frequency linear-array transducer (HFL 50, 13-6 MHz) of the X-porte ultrasound system (SonoSite, Inc., USA) was used to identify key vascular and neural structures, beginning from the popliteal fossa and extending down to the bifurcation of the sciatic nerve into the tibial and common peroneal nerves. The branching of the tibial nerve into medial and lateral plantar nerves was identified and marked on the skin, with distances from the medial malleolus recorded [Figures 1 and 2]. Results were stratified according to the bifurcation point as either proximal to the medial malleolus or distal. We scored ease of identification on a simple scale: easy, difficult, or impossible, recording anatomical variations. Volunteers for whom sono-anatomy could not be clearly visualised were excluded to preserve the integrity of the data collected.

Figure 1.

(a) Figure showing the undivided tibial nerve; (b) Probe position. PTA = posterior tibial artery

Figure 2.

(a) Figure showing branching of the tibial nerve; (b) Probe position and scale. PTA = posterior tibial artery

All 200 volunteers completed the study, with 135 males and 65 females. The median age was 32 [interquartile (IQR): 23–45] years. The median height was 166 cm (IQR: 158–172 cm). The median weight was 65.5 (IQR: 58–77) kg. The median body mass index was 24.25 (IQR: 22.15–26.51). In 95.5% of cases, bifurcation occurred above the tip of the medial malleolus. Concerning the distance, the median in cm was 3 (IQR: 3–4). The branching was at the tip in 4.5% of subjects, with no cases below it. The tibial nerve was easily identifiable in 86% of cases; none were deemed impossible to locate. A significant positive correlation was found between the height of volunteers and the distance of tibial nerve branching (which occurred further proximally), with a Pearson correlation of 0.727 (P = 0.0001). In contrast, no correlation existed with age (P = 0.860). The medial calcaneal branch was found in 25% of the participants. It was found at a distance of median 4 (IQR: 3.8–4.54) cm above the medial malleolus. In 8%, it was easy to identify; otherwise, it was impossible to trace in 75% of the cases.

This study emphasises the need for preoperative or intraoperative ultrasound assessment to avoid failures due to a considerable anatomical variation in the placement of the tibial nerve.[2] Rezvani et al.[3] demonstrated higher success using ultrasound in such blocks in emergency care. We suggest that during ankle blocks, it is preferable to scan proximally to identify the division and deposit before it happens. Scanning at the ankle alone at the level of the medial malleolus is likely to miss a branch of the tibial nerve. The study identifies the bifurcation of the tibial nerve at a distance of 3.33 cm above the medial malleolus. USG-guided blocks at the medial malleolus level may need larger volumes for effective nerve blockage. A significant correlation with height suggests the possibility of increased failure rates with low-level ankle blocks in taller individuals. The medial calcaneal branch is given off above the medial malleolus, indicating a need to trace the nerve proximally to identify this nerve to administer blocks with lesser volumes successfully. Many studies[4] reveal that the point of needle entry should be at the level of the medial malleolus, and we state that we should proceed proximally at an average of 3.3 cm above the medial malleolus.

Our study confirms significant anatomical variation in the tibial nerve at the ankle level, with predominant bifurcation occurring above the medial malleolus at a median (IQR). The use of ultrasound may optimise the site of drug administration for more precise and successful low-volume tibial nerve blocks, especially in patients with varying heights and altered anatomy.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patients consented to their images and other clinical information to be reported in the journal. The patient understands that their name and initials will not be published and that due efforts will be made to conceal their identity; however, anonymity cannot be guaranteed.

Author contributions

SPS: Design, concept, write-up, supervision, KJ: Data collection, NP: Data collection, write up and statistics.

Disclosure of use of artificial intelligence (AI)-assistive or generative tools

The AI tools or language models (LLM) have not been utilised in the manuscript, except that software has been used for grammar corrections and references.

Conflicts of interest

Srinivasan Parthasarathy, one of the authors, serves on the editorial board of the journal and was not involved in the editorial decision-making process for this manuscript.

Funding Statement

Nil.