Scanning Technique

• Position the patient prone and keep the toes off the bed if electrical stimulation will be used to evoke foot movement.
• After skin and transducer preparation, place a linear 38 mm, 7-10 MHz transducer in a transverse plane above the popliteal crease.

	BF = biceps femoris muscle SM = semimembranosus ST = semitendinosus muscle
	BF = biceps femoris muscle SM = semimembranosus ST = semitendinosus muscle

• Optimize machine imaging capability. Select the appropriate depth of field (usually within 5 cm), focus range (usually within 2-3 cm) and gain.
• Obtain a transverse view of the sciatic nerve.
• Scan the region proximally and distally to assess nerve anatomy and the point at which the sciatic nerve branches into its tibial and peroneal components.
• Aim to block the sciatic nerve before it divides.
• The sciatic nerve is commonly hyperechoic in this region and is found lateral to the popliteal artery. It is often necessary to angle the transducer caudally to enhance nerve visibility.

Anatomical Correlation

A Transverse View Using A Low Frequency Transducer (2-5 MHz)

BFM = biceps femoris muscle F = femur G = gracilis muscle PV = popliteal vessels SAR = sartorius muscle SN = sciatic nerve SMM = semimembranosus muscle STM = semitendinosus muscle

Nerve Localization

• Perform a systematic anatomical survey of structures from superficial (skin) to deep and from medial to lateral.
• First identify the femur which is deep and casts a bony shadow.
• Next, identify the pulsatile popliteal artery that is superficial to the femur. If it is not visible, scan distally towards the popliteal crease where the popliteal artery is more superficial.
• The popliteal vein may or may not be visible (collapsed by transducer pressure).
• Note the muscle groups medially (semitendinosus and semimembranosus muscles) and laterally (biceps femoris muscle).
• The hyperechoic sciatic nerve in this location is always superficial to the femur and lateral to the popliteal artery.
• If the sciatic nerve is not easily visible, angle the transducer and aim the beam caudally towards the foot. This will bring the nerve into view once the angle of incidence is approximately 90 degrees to the nerve.
• Scan the region proximally and distally to assess nerve anatomy. Mark the point at which the sciatic nerve branches into its tibial and peroneal components. Position the transducer in a location where the sciatic nerve is clearly visualized as a single nerve before its bifurcation.
• Nerve visualization is significantly improved once local anesthetic is injected due to enhanced contrast between the hyperechoic nerve and the hypoechoic fluid collection.

	Figure A shows the sciatic nerve (arrowheads) in the popliteal fossa before it divides.
	Figure B shows the sciatic nerve has divided into the tibial (TN) and peroneal (PN) components more caudad in the popliteal fossa. PA = popliteal artery

Needle Insertion Approach

• Ultrasound guided sciatic nerve block in the popliteal region is considered a BASIC skill level block because the nerve is easily visualized.
• Both In Plane (IP) and Out of Plane (OOP) approaches are available. The OOP approach is commonly used for single shot and catheter placement.

In Plane Approach

• The in plane approach is performed also with the patient supine whenever possible because of the ease of scanning.
  
• A longer 8 cm block needle is often required as the skin to nerve distance is greater with this approach than with the out of plane approach.
  
• The endpoint for needle insertion and local anesthetic injection is the same for both approaches.
  

Out of Plane Approach

With the patient lying prone, insert a 5-8 cm 22 G insulated needle perpendicular to the ultrasound transducer as seen in figure below.

• Aim to block the sciatic nerve before it divides. Scan proximally towards the apex of the popliteal triangle and follow the course of the nerve before needle insertion.
• As the block needle traverses perpendicular to the ultrasound beam, the monitor only shows tissue movement along the needle path and possibly the transverse view of the needle as a "white" dot.
• Advance the needle until there is needle to nerve contact as indicated by nerve movement.
• Aim to place the needle on either side of the nerve rather than contacting the nerve head on.
• Electrical stimulation of the sciatic nerve before local anesthetic injection is optional (operator preference).

Local Anesthetic Injection

• Once satisfied with nerve stimulation and motor response, inject 20-30 mL of local anesthetic under ultrasound observation.
• Observe the spread of local anesthetic in real time to judge adequacy of spread. Aim to see circumferential spread of hypoechoic local anesthetic solution around the nerve ("donut sign").
• Circumferential spread usually results in a complete block.
• If local anesthetic spread is deemed suboptimal, reposition the needle to place local anesthetic around the region that is spared.
• Nerve swelling, a sensitive sign of intraneural injection, is quite often observed uring local anesthetic injection at this location. One should withdraw the needle incrementally and adjust the needle tip position before further injection.
• Scan along the nerve proximally and distally to check longitudinal local anesthetic spread.

Clinical Pearls

Nerve Localization

1. Transducer Angle Towards the Foot (Caudad)
If the sciatic nerve is not readily visible, angle the transducer and aim the beam caudally towards the foot. The sciatic nerve courses more superficially when it is in the distal popliteal region. Angling the transducer towards the foot will align the beam 90 degrees to the nerve thus bringing the nerve into view.

A. The sciatic nerve is not well visualized when the transducer is pointing perpendicular to the skin due to a poor angle of incidence.
B. The sciatic nerve (arrowheads) is now clearly visualized when the transducer is pointing caudad. This brings the angle of incidence to approximately 90 degrees to the nerve. Arrowheads = sciatic nerve

2. Visualization of the Popliteal Vein

Identification of the popliteal vein and its location is important to prevent unintentional intravascular injection. This is achieved by reducing the transducer pressure. Arrowheads = sciatic nerves PA = popliteal artery PV = popliteal vein

3. See Saw Sign

If nerve visualization is difficult, ask the patient to plantar flex and dorsiflex the foot. One may see the "seesaw" sign as the tibial and peroneal components slide up and down during foot movement (Schafhalter-Zoppoth I, Anesthesiology 2004; 101: 808-9).

	A. Baseline Transverse Scan
	B. Plantar flexion The peroneal nerve (PN) component is elevated (arrow); that is, it moves towards the posterior surface.
	C. Dorsiflexion The tibial nerve (TN) component is elevated (arrow); that is the nerve moves towards the posterior surface.

Needle Insertion [Link to top]

Alternate Body Positions For Needle Insertion

	The supine position is suitable for the in plane (IP) needle approach.
	The lateral decubitus position is suitable for both the IP and out of plane approaches.

Catheter Insertion

• Continuous popliteal sciatic nerve block (CPSNB) is indicated for foot and ankle analgesia (see Catheter Technique).
• The out of plane needle insertion approach is generally recommended for CPSNB in a patient lying prone.
• The goal is to place the needle and the catheter proximal to the site where the sciatic nerve divides so that both the peroneal and tibial nerves will be anesthetized.
• Injection of local anesthetic or D5W solution (if nerve stimulation is desired) through the needle to distend the perineural space is recommended to facilitate the ease of catheter advancement.
• Aim to advance the catheter 3-5 cm into the perineural space with or without nerve stimulation guidance.
• It is often difficult to capture the transverse view of the catheter as it is advanced with the Out of Plane approach.
• Local anesthetic spread can be observed in real time during catheter injection.
• Suboptimal catheter position may be corrected by withdrawing the catheter a short distance before further local anesthetic is injected.

A. Patient preparation, sterile draping and pre-block scanning	B. Out of plane needle advancement under ultrasound guidance
C. Catheter insertion with the help of an assistant	D. Catheter exits in the distal 1/3 part of the thigh
E. Catheter is secured with dermabond at the exit site	F. Catheter is further secured with a transparent dressing

	A. Pre-block transverse scan Arrowheads = sciatic nerve PA = popliteal artery
	B. Hydro dissection distends the perineural space Arrow = block needle in cross section
	C. Arrow = catheter track
	D. Circumferential local anesthetic (LA) spread is visualized around the sciatic nerve
	E. A longitudinal scan shows local anesthetic (LA) on both sides of the nerve (arrowheads) Arrow = catheter
	F. Another longitudinal scan shows the catheter (arrows) and its tip (CT)

Image Gallery

1. Improper Local Anesthetic Spread Outside the Fascial Sheath

Arrowhead = sciatic nerve Asterisk = improper local anesthetic spread outside the fascial sheath

2. Proper Circumferential Local Anesthetic Spread Inside the Fascial Sheath

	Arrowhead = sciatic nerve
	Arrowhead = sciatic nerve Asterisks = local anesthetic spread

3. Visualization of the Popliteal Vein

	Arrowhead = sciatic nerve PA = popliteal artery
	Arrowhead = sciatic nerve PA = popliteal artery PV = popliteal vein

Video Gallery

Selected References

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• Lee J-H, Lee B-N, Lee MY, An X, Han S-H. The significance of tibial and common peroneal nerves in nerve blocks. Surg Radiol Anat 2013;35:211-5.
• Ehlers L, Jensen JM, Bendtsen TF. Cost-effectiveness of ultrasound vs nerve stimulation guidance for continuous sciatic nerve block. Br J Anaesth 2012;109:804-8.
• Missair A, Weisman RS, Suarez MR, Yang R, Gebhard RE. A 3-dimensional ultrasound study of local anesthetic spread during lateral popliteal nerve block: what is the ideal end point for needle tip position? Reg Anesth Pain Med 2012;37:627-32.
• Andersen HL, Andersen SL, Tranum-Jensen J. Injection inside the paraneural sheath of the sciatic nerve: direct comparison among ultrasound imaging, macroscopic anatomy, and histologic analysis. Reg Anesth Pain Med 2012;37:410-4.
• Franco CD. Connective tissues associated with peripheral nerves. Reg Anesth Pain Med 2012;37:363-5.
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• Karmakar M, Li X, Li J, Sala-Blanch X, Hadzi? A, Gin T. Three-dimensional/ four-dimensional volumetric ultrasound imaging of the sciatic nerve. Reg Anesth Pain Med 2012;37:60-6.
• Germain G, Levesque S, Dion N, Nadeau M-J, Cote D, Nicole PC, Turgeon AF. A Comparison of an Injection Cephalad or Caudad to the Division of the Sciatic Nerve for Ultrasound-Guided Popliteal Block: A Prospective Randomized Study. Anesth Analg 2012;114:233-5.
• Tran DQH, Dugani S, Pham K, Al-Shaafi A, Finlayson RJ. A randomized comparison between subepineural and conventional ultrasound-guided popliteal sciatic nerve block. Reg Anesth Pain Med 2011;36:548-52.
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