Sciatic Nerve Block - Popliteal Region

ANATOMY

The sciatic nerve in the popliteal fossa is bordered superolaterally by the long head of the biceps femoris muscle and superomedially by the semimembranosus and semitendinosus muscles. The sciatic nerve branches into the common peroneal nerve and the tibial nerve at variable location along its course in the thigh. Popliteal sciatic nerve block is indicated for procedures in the foot and ankle.

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.

OUT OF PLANE NEEDLE INSERTION 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.
• 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.   Arrowhead = sciatic nerve   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.