USRA - saphenous nerve block (2023)

scanning technology

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

Transducer on the medial side of the right thigh.

anatomical context Saphenous nerve in the distal thigh above the knee

AND= adductor greater MuskellArrow tip= saphenous nerveGR= muskel gracilisRAE= muskel sartoriusSM= muskel semimembranosoVM= respond medial muscle

nerve location

• Perform a systematic anatomical examination of the proximal femur to the distal femur. The saphenous nerve is often predominantly hyperechoic. This small nerve is sometimes difficult to visualize in the thigh and lower leg.
• Identify the femoral artery and the sartorius muscle. Note that the sartorius muscle crosses the femoral artery as the muscle moves from lateral (anterior superior iliac spine) to medial (upper part of the medial surface of the tibia).
• In the distal thigh, the saphenous nerve is usually located deep (posterior) to the sartorius muscle, in the subsartorial space. The nerve lies adjacent to the femoral artery, which eventually descends deep into the adductor canal.
• A motor branch of the femoral nerve that supplies the vastus medialis muscle is also found in the subsartorial space.
• Electrical stimulation can be used to differentiate between the saphenous nerve (sensory stimulation) and the vastus medialis muscle nerve (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= long adductor muscleFA= femoral arteryG= muskel gracilisN= saphenous nerve

Needle insertion method airplane approach

• The ultrasound-guided saphenous nerve block is considered an INTERMEDIATE skill block. Imagining this tiny nerve can be a challenge for some individuals.
• Insert a 5-8 cm 22 G needle parallel and aligned with the transducer and ultrasound beam.

The transducer is on the medial aspect of the right distal thigh; the locking needle is inserted in a lateral to medial direction using the in-plane approach.

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

Out of plan approach

The OOP method is also commonly used for saphenous nerve block. The tip of the needle is more difficult to visualize, but the distance between the needle and the nerve is shorter using this approach.

Local anesthetic injection

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

	O= local anesthetic N= saphenous nerve RAE= muskel sartorius VM= respond medial 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 an in-plane approach. Battery= lock needle F= femur N= saphenous nerve
	After the injection A hypoechoic collection of local anesthetic (LA) is seen around the saphenous nerve (N). FA= femoral artery

clinical pearls Nerve location - Saphenous nerve at other locationsThe saphenous nerve may be located more distally and subcutaneously in the following locations:

1. between the sartorius and gracilis muscles in the thigh just above the knee
2. on 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 ankle level, where the nerve is next to the subcutaneous saphenous vein

Saphenous nerve at ankle level. Arrow tip= saphenous nerve SVsaphenous vein

• Identifying the saphenous nerve below the knee can be challenging because it is small and located in the subcutaneous tissue. In this case, it may be helpful to place a tourniquet around the leg so that the subcutaneous saphenous vein is distended and easily visible. The saphenous nerve is usually immediately adjacent to the vein.

adductor canal blockage

• The newly named adductor canal block refers to saphenous nerve block in the subsartorial space of the proximal thigh. The adductor canal is marked from the apex of the femoral triangle to the adductor hiatus. Anatomically, the apex of the femoral triangle is defined by the intersection of the medial border of the adductor longus muscle and the medial border of the sartorius muscle.


• The adductor canal contains the femoral vessels, the saphenous nerve, and the nerve of the vastus medialis muscle. It also contains the obturator nerve (the posterior division), which usually enters the distal part of the canal and exits through the hiatus of the adductor magnus tendon into the popliteal fossa accompanied by the femoral artery.
• The advantage of the adductor canal block is that it spares the motor fibers of the quadriceps muscles and thus preserves muscle strength after knee surgery and allows earlier ambulation and rehabilitation compared to conventional femoral nerve block. The single injection approach, in-plane or out-of-plane, is the same as for the saphenous nerve block. Authors usually inject 20 mL of 0.5% ropivacaine for total knee replacement surgery.

Needle insertion technique

• During needle advancement, it is important to penetrate 2 layers of fascia/membrane to enter the adductor canal. The first is the fascia surrounding the sartorius muscle and the deepest is the vast adductor membrane, as shown in the figure below.

adductor canal anatomy 1= fascial covering of the sartorius muscle 2= vast adductor membrane RAE= muskel sartorius VM= respond medial muscle

catheter insertion

• A catheter can be inserted for continuous saphenous nerve block, but this is not commonly done.
• Secatheter technologyfor the principles of catheter insertion.
• Note that the catheter exit site and dressing may be in close proximity to the surgical site when inserted prior to total knee replacement surgery. Discuss with the surgeon the optimal time and place for catheter insertion.
• Also note that the catheter exit point is under the mouth of the thigh. Consider the risk of double crush syndrome.

Image gallery saphenous nerve in the ankel region

	Pre-injection The saphenous nerve (arrowheads) is adjacent to the saphenous vein (V). Both structures are superficial in the subcutaneous plane.
	After the injection A wall of local anesthetic (LA) is visualized in the subcutaneous tissue superficial to the saphenous nerve and vein (V). arrowheads= saphenous nerve

Nervo saphenous na coxa distal

	Arrow tip= saphenous nerve RAE= muskel sartorius VM= respond medial muscle
	Arrow tip= saphenous nerve O= local anesthetic RAE= muskel sartorius VM= respond medial muscle

Selected references

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