Ever tried injecting local anesthetic into the bottom of someone’s foot? If you have, you’ll understand the incredible pain and difficulty in trying to anesthetize this region. Fortunately, there’s a better way to anesthetize the plantar foot, and avoid getting kicked in the face or causing a needle-stick injury: performing an ultrasound-guided tibial nerve block.1
Anatomy
The plantar foot has five primary cutaneous nerves. The main three – the medial plantar nerve (MPN), lateral plantar nerve (LPN), and calcaneal nerve (CN) – all arise from the tibial nerve.2
The tibial neurovascular bundle is organized similarly to the hip. Remember “VAN Out” – vein, artery, nerve, each becoming more lateral. This is illustrated below in the tarsal tunnel.
Equipment
- Linear ultrasound probe (L12-4)
- 22-Gauge 1.5′ needle or larger – note that anything smaller becomes difficult to visualize in-plane
- 3-5 mL of 2% lidocaine
- Chlorhexidine or iodine to clean the skin
- Pillow
Procedure
1. Positioning: Place a pillow under the patient’s calf and then have them rotate their foot externally. If this is too challenging, and the patient is able to lie on their side – position them with the affected foot closest to the bed
2. Ultrasound Settings: Use the nerve or needle-visualization preset, which optimizes image brightness (aka: gain), and set a shallow depth (often 2-3 cm) to maximize your screen real estate.
3. Probe placement: Place the probe on the medial tibia a few cm proximal to the malleolus. be mindful that if you’re too distal, you risk missing anatomical variant nerve branches, leading to incomplete anesthesia
4. Identify the Anatomy: The nerve should appear bright (hyperechoic) and have a ‘honeycomb’ like structure, running lateral to the tibial vessels and closest to the Achilles tendon. Colour Doppler can help differentiate the vessels. Occasionally, a tendon might be mistaken for the nerve; to confirm it is the nerve, follow it up and down the tibia to ensure it doesn’t expand into a muscle belly. Also, note that a small percentage of patients may have a variant tibial nerve running anterior to the vessels. 2
5. Skin prep: Clean the skin with chlorhexidine or iodine
6. Needle insertion: Inject posteriorly, in-plane, just proximal and anterior to the Achilles tendon. Surround the nerve with local anesthetic, being careful not to inject directly into the nerve or surrounding vasculature.3
7. LAST(ly): Although the low volume makes this very low risk for Local Anesthetic System Toxicity, be aware of how to recognize and treat this rare complication. ie: Do you know where lipid emulsions can be found in your ED?
Congratulations – you’ve just completed an ultrasound-guided tibial nerve block!
References
- Franco, C.D., and Williams, J.M. “Ultrasound-Guided Ankle Block: The Tibial Nerve.” Regional Anesthesia and Pain Medicine, vol. 31, no. 1, 2006, pp. 68-71.
- Stone, M.L., et al. “Anatomy and Variations of the Tibial Nerve.” Clinical Anatomy, 2012.
- Sites, B.D., et al. “Ultrasound-Guided Nerve Blocks of the Lower Extremity.” Regional Anesthesia and Pain Medicine, vol. 34, no. 6, 2009, pp. 547-558.
For images:
- https://teachmeanatomy.info/lower-limb/nerves/tibial-nerve/
- https://www.kgmu.org/digital_lectures/medical/anatomy/back_of_leg_I.pdf
- https://www.nature.com/articles/srep46351
- https://www.researchgate.net/figure/Ultrasound-guided-tibial-nerve-block_fig1_349552835
- https://coreultrasound.com/uotw-47-answer/
