Fig. 27.1
Surface anatomy and landmarks for posterior tibial nerve block
Fig. 27.2
Surface anatomy and landmarks for deep peroneal nerve block
27.3 Nerve Stimulation Technique
The posterior tibial nerve and the peroneal nerve are the only motor nerves at the ankle. Table 14.5 summarizes motor responses associated with nerve stimulation.
27.3.1 Needle Insertion
The nerves are all superficially located, and a short (20–30 mm) 25G–27G needle with a short bevel (or a hypodermic needle) should be used for the blocks.
Superficial nerves: Insert the needle to deposit local anesthetic subcutaneously in a ringlike fashion along the line drawn between the malleoli.
Isolated saphenous nerve block: Infiltrate the area overlying and anterior to the medial malleolus.
Posterior tibial nerve: Insert the needle perpendicular to the skin between the medial malleolus and the medial border of the Achilles tendon, directed toward the pulsation of the posterior tibial artery. The nerve is located behind the posterior tibial artery.
Aspiration will be important to rule out intravascular placement.
Deep peroneal nerve: Insert the needle perpendicular to the skin and with a slight anterior tilt either lateral to the extensor hallucis longus tendon or the anterior tibial artery (whichever is used) and inject into the deep planes below the fascia.
Sural nerve: Insert the needle perpendicular to the skin between the lateral malleolus and the calcaneus.
27.3.2 Current Application and Appropriate Responses
Applying an initial current of 0.8 mA (2 Hz, 0.1–0.3 ms) is sufficient for nerve stimulation. After obtaining the appropriate motor response, the current is reduced to aim for a threshold current of 0.4 mA (0.1–0.2 ms) before injection.
Twitches of the first (medial plantar branch) and fifth (lateral plantar branch) toes are typical responses to nerve stimulation.
For the deep peroneal nerve, toe extension is seen.
27.3.3 Modifications to Inappropriate Responses
In general, alternate motor responses from direct muscle stimulation will not be elicited since this region is mainly composed of tendons rather than muscle fibers.
27.4 Ultrasound-Guided Technique
Ultrasound imaging may be suitable for visualizing the deep nerves (posterior tibial and deep peroneal). Imaging at the medial aspect of the ankle will enable the block to be localized to the posterior tibial nerve prior to its division into the medial and lateral plantar nerves. Imaging for the deep peroneal block may be helpful in children since the voluntary extension of the toe, to localize the extensor hallucis tendon, will not be possible in anesthetized patients.
27.4.1 Scanning Technique
High-frequency, short footprint probes (e.g., SLA 6–13 MHz, 25 mm footprint hockey stick probe, MicroMaxx, SonoSite, Bothell, WA) are used for these blocks.
Posterior tibial nerve: The probe is positioned in transverse (short) axis to the nerve just posterior and inferior to the medial malleolus. Alternatively, the nerve can be identified in the distal quarter of the lower leg above the medial malleolus (Fig. 27.3).
Fig. 27.3
(a) VHVS and MRI images of major anatomical structures surrounding the tibial nerve. (b) Ultrasound image of major anatomical structures surrounding the tibial nerve
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Color Doppler is helpful to localize the nerve at the above locations since in each one the nerve lies posterior and deep to the posterior tibial artery (Fig. 27.4). The nerve should be localized before it branches into the medial and lateral plantar nerves.
Fig. 27.4
Transverse section at the ankle showing spatial relationship of subcutaneous structuresRelated posts:
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