Saphenous Nerve Block

ANATOMY

 

The saphenous nerve is the largest cutaneous branch of the femoral nerve. In the proximal thigh, the saphenous nerve often lies anterior to the femoral artery as this vessel passes beneath the sartorius muscle, and posterior to the aponeurotic covering of the adductor canal. The saphenous nerve descends along the medial side of the knee posterior to the sartorius muscle.

 

 

In the distal thigh, the saphenous nerve pierces the fascia lata between the tendons of the sartorius and gracilis muscles (see picture), and then becomes a subcutaneous nerve. The saphenous nerve may also surface between the sartorius and vastus medialis muscles. Below the knee, the nerve passes along the tibial side of the leg, adjacent to the great saphenous vein subcutaneously. At the ankle, one branch of the nerve is located on the medial side next to the subcutaneous saphenous vein.

 

SCANNING TECHNIQUE

• Position the patient supine with the leg slightly externally rotated.
• Expose the lower part of the thigh, knee and upper leg.
• After skin and transducer preparation, place a linear transducer with the appropriate frequency range (10-12 MHz) starting in the proximal thigh and scan distally to the knee. The saphenous nerve can be blocked reliably in the distal 1/3 of the thigh.
• Optimize machine imaging capability; select appropriate depth of field (usually within 1-3 cm), focus range and gain.

 

Transducer over the medial side of the right thigh.

 

ANATOMICAL CORRELATION

 

Saphenous Nerve in the Distal Thigh Above the Knee

 

AM = adductor magnus muscle   Arrowhead = saphenous nerve   GR = gracilis muscle   SAR = sartorius muscle   SM = Semimembranosus muscle   VM = vastus medialis muscle

 

NERVE LOCALIZATION

• Perform a systematic anatomical survey from proximal thigh to distal thigh. The saphenous nerve is often predominantly hyperechoic. This small nerve is sometimes challenging to visualize in the thigh and leg.
• Identify the femoral artery and the sartorius muscle. The sartorius muscle is noted to cross the femoral artery as the muscle travels from lateral (anterior superior iliac spine) to medial (superior part of medial surface of tibia).
• In the distal thigh, the saphenous nerve is often located deep (posterior) to the sartorius muscle in the subsartorial compartment. The nerve lies next to the femoral artery which eventually descends deep into the adductor canal.
• A motor branch of the femoral nerve supplying the vastus medialis muscle is also found in the subsartorial compartment.
• Electrical stimulation may be used to differentiate the saphenous nerve (sensory stimulation) and the nerve to the vastus medialis muscle (motor stimulation) in the distal thigh.
• More distally in the thigh, the saphenous nerve becomes superficial and can be found in the fascial plane between the vastus medialis and the sartorius muscles.

The saphenous nerve (N) is located deep to the sartorius muscle and adjacent to the femoral artery in this case.   AL = adductor longus muscle   FA = femoral artery   G = gracilis muscle   N = saphenous nerve

 

IN PLANE NEEDLE INSERTION APPROACH

• Ultrasound guided saphenous nerve block is considered a INTERMEDIATE skill level block. It can be challenging to image this small nerve in some individuals.
• Insert a 5-8 cm 22 G needle parallel to and inline with the transducer and the ultrasound beam.

The transducer is over the medial aspect of the right distal thigh; the block needle is inserted in the lateral to medial direction using the in plane approach.

• Aim to place the needle into the fascial plane between the sartorius and vastus medialis muscles.
• Use an insulated needle if electrical stimulation of the nerve (motor branch) to the vastus medialis muscle is intended.

LOCAL ANESTHETIC INJECTION

• If the saphenous nerve is visualized (a predominantly hyperechoic structure), inject 5-10 mL of local anesthetic around the nerve.
• If the nerve is not clearly visualized in the distal thigh, local anesthetic injection is recommended into the fascial plane between the vastus medialis and sartorius muscles. Injection of an additional 5-10 mL deep to the sartorius muscle is also recommended.

LA = local anesthetic   N = saphenous nerve   SAR = sartorius muscle   VM = vastus medialis muscle

Local Anesthetic Injection for Saphenous Nerve Block in the Distal Thigh   The saphenous nerve is located between the sartorius and vastus medialis muscles.   F = femur   FA = femoral artery   N = saphenous nerve

Needle Insertion   The needle is inserted from lateral to medial using the in plane approach.   Arrows = block needle   F = femur   N = saphenous nerve

Post-injection   A hypoechoic local anesthetic (LA) collection is seen surrounding the saphenous nerve (N).   FA = femoral artery

 

CLINICAL PEARLS

 

Nerve Localization

 

Saphenous Nerve in Other Locations

 

The saphenous nerve can be located more distally and subcutaneously in the following locations:

1. between the sartorius and gracilis muscles in the thigh immediately above the knee
2. in the medial side of the leg just below the knee at the level of the tibial tubercle where the saphenous nerve lies next to the saphenous vein subcutaneously
3. in the middle half of the leg where the nerve is adjacent to the subcutaneous saphenous vein
4. at the level of the ankle where the nerve is next to the subcutaneous saphenous vein

Saphenous nerve at the ankle level.   Arrowhead = saphenous nerve   SV = saphenous vein

• It can be challenging to identify the saphenous nerve below the knee since it is small and located in subcutaneous tissue. In this case, it may be useful to place a tourniquet around the leg so that the subcutaneous saphenous vein becomes distended and easily visible. The saphenous nerve often lies immediately adjacent to the vein.

NEEDLE INSERTION AND LOCAL ANESTHETIC INJECTION

 

1. Out of Plane (OOP) Needle Insertion Approach

 

The OOP approach is also commonly used for saphenous nerve block. The needle tip is more difficult to visualize but the needle to nerve distance is shortest using this approach.

 

IMAGE GALLERY

 

2. Saphenous Nerve in the Ankle Region

 

Pre-injection   The saphenous nerve (arrowheads) is next to the saphenous vein (V). Both structures are superficial in the subcutaneous plane.

Post-injection   A wall of local anesthetic (LA) is visualized in the subcutaneous tissue superficial to the saphenous nerve and vein (V).   Arrowheads = saphenous nerve

 

3. Saphenous Nerve in the Distal Thigh

 

Arrowhead = saphenous nerve   SAR = sartorius muscle   VM = vastus medialis muscle

Arrowhead = saphenous nerve   LA = local anesthetic   SAR = sartorius muscle   VM = vastus medialis muscle

 

VIDEO GALLERY

 

Saphenous Nerve Block (In Plane Approach)

 

SELECTED REFERENCES

• Krombach J, Gray AT: Sonography for saphenous nerve block near the adductor canal. Reg Anesth Pain Med 2007; 32: 369-70
• Lundblad M, Kapral S, Marhofer P et al. Ultrasound-guided infrapatellar nerve block in human volunteers: description of a novel technique. Br J Anaesth 2006;97: 710-714.
• Gray A T, Collins A B. Ultrasound-guided saphenous nerve block. Reg Anesth Pain Med 2003;28:148.