The lateral femoral cutaneous nerve (LFCN) is a sensory nerve derived from the second and third lumbar nerve roots. The nerve is a branch of the lumbar plexus that provides cutaneous sensation from the lateral aspect of the thigh. The nerve emerges from the lumbar plexus to travel across the iliacus muscle and rise up toward the anterior-superior iliac spine (ASIS). The nerve usually enters the anterior thigh medial to the ASIS and then crosses over the sartorius muscle from medial to lateral.

Anatomic variation of the LFCN is common. The nerve may consist of as many as four branches where it exits the pelvis. The nerve passes medial to the ASIS by a variable distance and may cross the pelvic bone to reach the anterior thigh. Rather than passing over the sartorius, in some patients the LFCN can pass through the sartorius muscle. The estimated incidence of this ranges between 3% and 22%. Although the LFCN is referred to as a discrete nerve, this designation can indicate a collective set of nerves. Like other cutaneous nerves, the LFCN branches extensively when it enters the subcutaneous tissue. In rare cases (≈7%), the LFCN is absent, and its territory is covered by the ilioinguinal and femoral nerves.

High-resolution ultrasound imaging can identify the LFCN superficial to the sartorius muscle in the proximal thigh.^1,2 The nerve has a characteristic medial to lateral course over this muscle. The nerve and muscle are best visualized just medial and distal to the ASIS. As with other small nerves, it is necessary to scan along the length of the nerve to confirm nerve identity. The best imaging technique is to slide the transducer along the known course of the nerve with the nerve viewed in short axis. LFCNs that pass through the sartorius are easier to image because the nerve is surrounded by hypoechoic muscle that provides better acoustic contrast than echobright subcutaneous tissue. Although ultrasound imaging of the LFCN has been reported, the nerve is normally small (1-3 mm in diameter), and sonographic visualization can be difficult within the echobright subcutaneous tissue of the anterior thigh.

LFCN block can be used alone or in conjunction with other lower extremity blocks. It is useful for skin graft harvests and surgical procedures with lateral incisions of the thigh. It is one of the few lower extremity blocks for weight-bearing patients (this group also includes ankle block and saphenous block). LFCN block can improve thigh tourniquet tolerance when combined with other lower extremity blocks.³

If adequate volume is administered, separate LFCN block is not normally necessary after ultrasound-guided femoral block. However, as volume reduction for femoral blocks becomes more feasible because of more targeted injections, sparing of the other nerves such as the LFCN will occur.

Ultrasound imaging may be useful for the diagnosis and treatment of meralgia paresthetica (from Greek meros for “thigh,” and algos for “pain”). Meralgia can result from mechanical stretch or compression injury of the LFCN. Abnormal nerve morphology has been described in patients with meralgia paresthetica. Fusiform enlargement of the LFCN, loss of fascicular discrimination, and hyperemia can all occur in this condition.^4–6

Suggested Technique

LFCN blocks are performed in the supine position. The sartorius muscle is imaged in short-axis view near its insertion on the ASIS. By sliding the ultrasound transducer back and forth between proximal and distal locations, the LFCN is identified by its characteristic lateral course over the sartorius muscle. The in-plane approach can be used from the lateral aspect of the thigh. Acoustic standoff may be advantageous in thin patients to image the LFCN close to the ASIS, so that the conforming gel pad forms around the bone.

It may not be critical to directly visualize the LFCN for block success. When the nerve is not sonographically visible, our recommendation for performing LFCN block is to image the sartorius muscle in short-axis view near the ASIS. The sartorius muscle has a triangular or circular shape near its origin on the ASIS. The fascial planes that lie over the anterior border of the sartorius muscle (in particular, the fascia lata) can be separated by infiltrating local anesthetic between these layers.

Key Points

Lateral Femoral Cutaneous Nerve Block	The Essentials
Anatomy	The LFCN usually crosses over the Sa.
	The LFCN lies in the space between the Sa and TFL.
	The Sa is usually triangularly shaped near its ASIS insertion.
	The LFCN is about 2.5 mm in diameter.
Positioning	Supine
Operator	Standing on side of patient
Display	Across the table
Transducer	High-frequency linear, 23- to 38-mm footprint
Initial depth setting	20 mm
Needle	25 gauge, 38 mm in length
Anatomic location	Proximal lateral thigh, 1 to 2 cm distal to ASIS
	Begin by sliding transducer back and forth (proximal and distal).
	Look for LFCN crossing over the Sa
Approach	SAX view of LFCN, in-plane from lateral to medial
	Place the needle tip between the LFCN and Sa.
	Alternatively, place needle tip in space between Sa and TFL.
Sonographic assessment	The injection should track distally along the LFCN.
Anatomic variation	The LFCN can travel through the Sa.
	The LFCN can divide proximally and be difficult to visualize.

ASIS, Anterior superior iliac spine; LFCN, lateral femoral cutaneous nerve; Sa, sartorius; SAX, short axis; TFL, tensor fascia lata.

Clinical Pearls

• The LFCN can be imaged between the fascia iliaca and the fascia lata. If light probe pressure is applied, the nerve can be seen within the tissue between these two fascial layers for needle tip placement.⁷

• LFCN block is assessed by cutaneous anesthesia midway between the ASIS and the lateral knee joint line.

• In some patients the crossing of the LFCN with the deep circumflex iliac artery can be identified on sagittal oblique sonograms.⁸ This view is parallel to the course of the nerve and perpendicular to the course of the artery.

FIGURE 38-1 External photograph showing in-plane approach to lateral femoral cutaneous nerve block from the lateral aspect of the thigh.

FIGURE 38-2 Short-axis view of the lateral femoral cutaneous nerve (LFCN) over the sartorius muscle near its proximal insertion on the ASIS. This large bifascicular nerve divides more distally in the thigh.

FIGURE 38-3 Needle tip in place for injection of local anesthetic for in-plane lateral femoral cutaneous nerve (LFCN) block. In this example the nerve is pushed against the sartorius muscle.

FIGURE 38-4 An in-plane approach demonstrates local anesthetic distributed around a fascicular lateral femoral cutaneous nerve (LFCN).

FIGURE 38-5 After injection, local anesthetic is seen to surround the lateral femoral cutaneous nerve (LFCN).

FIGURE 38-6 Pass-through lateral femoral cutaneous nerve (LFCN). In this anatomic variation the LFCN passes through the sartorius muscle rather than over it. Although relatively uncommon, this variant is easily recognized. The nerve is shown before (A) and after (B) injection of local anesthetic.