Anatomy

ANATOMY

The femoral nerve is a branch of the lumbosacral plexus. It originates at the posterior divisions of the ventral rami of L2, L3 and L4 nerves.
It enters the thigh posterior to the inguinal ligament lateral to the femoral artery before dividing into anterior and posterior divisions. The anterior division supplies sensation to the anterior thigh and gives motor branches to pectineus and sartorius muscles. The posterior division supplies motor branches to the quadriceps muscles, gives rise to the saphenous nerve and gives articular branches to the knee joint.

Landmarks

LANDMARKS

Inguinal ligament, femoral artery, femoral vein, iliacus muscle and fascia, sartorius muscle.

Positioning

POSITIONING

Patient lying supine. Slightly abducting the leg can sometimes help to optimise the ultrasound image. Place the transducer transversely across the upper thigh over the femoral artery just below the inguinal ligament.

Figure 1: Patient, transducer and needle positioning for a) in-plane and b) out-of-plane femoral nerve block.

Preliminary Scan

PRELIMINARY SCAN

Identify the femoral artery and the femoral vein medially. If you are too inferior to the inguinal ligament you may find the femoral artery has already divided into its superficial and deep (profunda femoris) branches – in this case move the transducer closer to the inguinal ligament. Scan laterally to identify the iliacus muscle. There are two fascial lines superficial to the iliacus muscle – the iliacus fascia which is adherent to the iliacus muscle, and the fascia lata which is slightly more superficial. Alter the transducer tilt angle to highlight the iliacus muscle fascia. Trace this fascia back towards the femoral artery and identify the femoral nerve lateral to the artery, underneath the iliacus fascia. The fascia iliaca is adherent to the super cial aspect of the femoral nerve and continues as the deep portion of the femoral sheath. It may be necessary to rotate and tilt the transducer significantly to optimise your view of the nerve.

Figure 2: a) ultrasound image of the femoral nerve and surrounding structures b) annotated image (FN=femoral nerve; FA=femoral artery; FV=femoral vein; super cial dashed line=fascia lata; deep dashed line=fascia iliaca).

Because the femoral nerve divides into multiple branches immediately after entering the thigh it may not always appear as a distinct structure. Two distinct fascial layers overly the nerve, the fascia lata and the deeper iliacus fascia, which is closely applied to the iliacus muscle. The femoral nerve always lies beneath the ilacus fascia and the local anaesthetic must be placed deep to the fascia to achieve a reliable block.

Technique

Position the needle lateral to the nerve, beneath the iliacus fascia. Inject 2 ml of local anaesthetic, observing spread below the iliacus fascia and spreading around nerve. If the spread is satisfactory the needle can be re-positioned and multiple deposits of local anaesthetic made to encircle the nerve.

Tips

TIPS

1. The femoral nerve begins to divide soon after passing under the inguinal ligament. It may be more visible as a discrete structure closer to the inguinal ligament.
2. If using an out-of-plane technique it is best to approach the nerve from the lateral aspect as medially the nerve may be in the shadow of the lateral wall of the artery making neural puncture more likely.
3. The lateral circumflex femoral artery can often be seen passing through the branches of the femoral nerve. Use Doppler to help avoid puncturing it.
4. The femoral nerve can be difficult to visualise clearly due to anisotropy. Methods to try and improve the view of the nerve include: caudad and cranial tilt of the transducer to optimise the angle of insonation and hydrolocation with a small volume of local anaesthetic deep to the iliacus fascia.
5. The use of peripheral nerve stimulation can be used to help identify the position of the nerve
6. If the femoral nerve is still difficult to visualise then a fascia iliaca block (deposit local anaesthetic deep to the iliacus fascia) can be reliably performed to block the femoral nerve instead.

Other Resources

Plan B/C/D blocks for the lower limb-hip include: Fascia Iliaca Block and Lumbar Plexus Block

 More information about the femoral block can be found at the excellent  article here

Videos

https://www.ra-uk.org/index.php/plan-a-blocks-home/plan-a-block-videos/video/femoral-nerve-block

Authors