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Ultrasound Guided Nerve Blocks

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llustration 1: Sensory innervation of the foot, dorsal view.

Illustration 2: Sensory innervation of the foot, plantar view.

Illustration 3: Sensory innervation of the lateral view.

Figure 1:  Probe position for posterior tibial nerve block, needle in plane
(red-artery, blue – vein, yellow – nerve, white – medial malleolus).

Figure 2:  Probe position for posterior tibial nerve block, needle out of plane
(red-artery, blue – vein, white – medial malleolus).

Figure 3:  Still image of “Tom (green), Dick (green), And (red)
Very (blue, 2x) Nervous (yellow)
Harry” (green), from right to left of the screen).

Video 1:  Posterior tibial nerve block with in plane technique.

Video 2:  Posterior tibial nerve block with out of plane technique.

 


Leonard V. Bunting, M.D.

Posterior Tibial Nerve Block - Ultrasound-guided Ankle Blocks

Introduction

There are five nerves that provide sensory innervation to the foot at or below the level of the ankle.  These are four distal branches of the sciatic nerve (posterior tibial, sural, deep and superficial peroneal nerves) and one branch from the femoral nerve (saphenous nerve). Local anesthetic injection can be extremely painful or even impossible in foot injuries with extensive and diffuse cutaneous and subcutaneous trauma or foreign body impaction.  Although ankle blocks are classically a blind technique, ultrasound has been shown to improve both the speed of onset and density of anesthesia.[1,2] Some practitioners mix blind and ultrasound-guided techniques, using ultrasound for the two deeper nerves (posterior tibial, deep peroneal) and landmark-based techniques for the more superficial nerves (saphenous, superficial peroneal and sural).

Like the wrist block, the straightforward anatomy and low risk for complications make ultrasound-guided ankle blocks an excellent choice for novices.  A comprehensive ankle block involving several nerves requires position changes and may not be ideal for uncooperative patients or patients with injuries that make repositioning uncomfortable.  However, not all nerves need to be blocked for every patient, as injuries to the foot often fall under a single nerve's territory.  Documentation of the neurovascular exam findings before application of the block is essential, as this will help prevent missing neurovascular injuries prior to anesthesia.

Innervation areas of the 5 nerve blocks are shown in illustration 1. 
This illustration may also help you choose which nerves to block based on the location of the injury.  As a simplified rule, two nerves innervate the volar aspect of the foot, three nerves innervate the dorsal aspect of the foot.

Volar Foot:
- Posterior tibial nerve
- Sural nerve

Dorsal Foot:
- Saphenous nerve
- Anterior tibial nerve (also called the deep peroneal nerve)
- Superficial peroneal nerve (also called the dorsal cutaneous or musculocutaneous nerve)

             
       
             
  Illustration 1: Sensory innervation of the foot, dorsal view.   Illustration 2: Sensory innervation of the foot, plantar view.   Illustration 3: Sensory innervation of the lateral view.  
             


Posterior Tibial Nerve

The ability of the emergency physician to block the posterior tibial nerve is crucial.  Injuries and procedure in the sole of the foot are common in the emergency department, and cutaneous injection into this region can be extremely painful. Blind blockade can have a reported success rate as low as 22%, and ultrasound dramatically improves the ease and efficacy of this procedure. [1,2]

Anatomy

The sciatic nerve (from sacral plexus L4-S3) divides above the poplitel fossa into the posterior tibial nerve and common peroneal nerve.  The posterior tibial nerve travels in the posterior leg with the posterior tibial artery, in the fascial plane between the superficial and deep muscle groups.  It is a mixed sensory and motor nerve.  Further distally, it passes posterior to the medial malleolus and posterior to the posterior tibial artery at the ankle.  This is the segment used for the ankle block. The anatomical area is often described with the mnemonic "Tom, Dick and very nervous Harry", with


From anterior to posterior, the order of structures is: 
T = tibialis posterior tendon
D = flexor digitorum tendon
A = posterior tibial artery
V = posterior tibial veins (usually more than one)
N = tibial nerve
H = flexor hallucis longus tendon

Near the ankle, the nerve than separates into the medial calcaneal, medial plantar, and lateral plantar branches, innervating much of the plantar surface and parts of the heel of the foot. It does not innervate the lateral heel (sural) or the extreme medial (saphenous) or extreme lateral (sural) aspects of the proximal sole of the foot (Illustration 1).
Motor innervation is provided to the ankle and foot flexors.

Scanning Technique

The tibial nerve is the largest of the 5 nerves included in the ankle block, and therefore the easiest to visualize. A high-frequency (10 – 15 MHz) linear array probe is applied in a transverse plane posterior to the medial malleolus (. The posterior tibial artery is identified by its pulsations and lack of compressibility.  Anteromedial to the artery lie the flexor digitorum tendons and the tibialis posterior tendon. Flexing of the toes will result in movement of the hyperechoic tendons on the image. The oval tibial nerve is found posteriorlateral to the posterior tibial artery.  The mnemonic “Tom, Dick, And Very Nervous Harry” is used for remembering the structures that pass posterior to the medial malleolus at the level of the ankle.  The tibial nerve can be found on the heel side of the artery.

Figure 1:  Probe position for posterior tibial nerve block, needle in plane
(red-artery, blue – vein, yellow – nerve, white – medial malleolus).



Figure 2:  Probe position for posterior tibial nerve block, needle out of plane
(red-artery, blue – vein, white – medial malleolus).



Figure 3:  Still image of “Tom (green), Dick (green), And (red) Very (blue, 2x)
Nervous (yellow) Harry” (green), from right to left of the screen).

Nerve Block

The 4-5 cm 22 – 25 gauge needle is inserted in either an in-plane or out-of-plane orientation after appropriate skin anesthesia (Figure 1 and2). For the in-plane approach, the needle is inserted from the side of the probe. The needle tip is identified and directed towards the deep border of the nerve. Once movement of the needle causes movement of the nerve, a small amount of local anesthetic is injected. The anesthetic should spread evenly around the nerve. If anesthetic spread is inadequate, the needle may need to be directed towards the anterior surface of the nerve (patient preparation, anesthetic)

For the out-of-plane approach, the needle is inserted from inferior, perpendicular to the probe. Once the needle tip is identified, the needle is guided between the artery and nerve. Once the needle is situated, a small amount of fluid is injected. The anesthetic should spread evenly around the nerve. If anesthetic spread is inadequate, the needle may need to be directed towards the opposite surface of the nerve.

Inject 5 - 8 cc of local anesthetic.  Paresthesias should occur quickly with full block onset in 5 – 15 minutes.

Video 1:  Posterior tibial nerve block with in plane technique.



Video 2:  Posterior tibial nerve block with out of plane technique.

Pearls and Pitfalls

Ensure spread of local anesthetic between the artery and nerve.
The out-of-plane approach can be easiest if the artery and nerve are closely associated.

Placing a rolled towel or pillow underneath the calf may aid in patient positioning for the procedure.

Smaller needles are more difficult to visualize using ultrasound. Novices should consider using 22 gauge needles to start.

Avoid vascular injection and injury by frequently aspirating.  Any injected anesthetic should result in a bolus of fluid into tissue on the ultrasound image.  If no bolus is seen after injection, this may indicate intravascular injection.

For patients with little subcutaneous tissue, the probe may not sit well near the medial malleolus. It is sometimes necessary to perform the procedure more proximal to avoid boney projections.

References:

1.  Redborg KE.
Ultrasound improves the success rate of a tibial nerve block at the ankle. Reg Anesth Pain Med, 2009;34(3):256-60.

2.  Chin KJ, Wong NW, Macfarlane AJ, Chan VW.
Ultrasound-guided versus anatomic landmark-guided ankle blocks: a 6-year retrospective review. Reg Anesth Pain Med. 2011;36(6):611-8.


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