The lateral femoral cutaneous nerve (LFCN) of the thigh is normally a derivative of the posterior divisions of the L2 and L3 spinal nerves. The nerve emerges from the lateral border of the psoas major muscle and runs across the iliacus muscle in the pelvis and travels toward the anterior superior iliac spine (ASIS) (▶ Fig. 23.1).
Fig. 23.1 Right posterior abdominal wall illustrating the course of the lateral femoral cutaneous nerve (arrow) exiting lateral to psoas major and traveling toward the anterior superior iliac spine on the iliacus muscle.
The LFCN typically exits the lesser pelvis under the inguinal ligament (IL), medial to the ASIS, where it bifurcates into anterior and posterior divisions along the length of the thigh. In the thigh, the nerve runs through its individual fascial tunnel: the LFCN canal. 15 It provides sensory innervation to the skin of the anterolateral and lateral aspects of the thigh 1 (▶ Fig. 23.2).
Fig. 23.2 Schematic drawing of the exit site of the right lateral femoral cutaneous nerve into the skin of the lateral thigh.
(Reproduced with permission from Gilroy AM, MacPherson BR, Ross LM, Schuenke M, Schulte E, Schumacher U. Atlas of Anatomy. 2nd ed. New York, NY: Thieme Medical Publishers; 2012. Illustration by Karl Wesker.)
23.2 Variant Anatomy
Variations in the LFCN’s anatomy are common; seven different points of exit from the pelvis have been observed. 2 Four of the variations can be classified into four zones relative to the ASIS, through which the LFCN can pass: medial to the ASIS and under the IL, medial to the ASIS and over the IL, directly over the ASIS, and lateral to the ASIS. Three more variations were found in which the nerve passed through another tissue: the ASIS itself, the sartorius muscle, and the IL. Even when the nerve followed the anatomically normal exit pattern—medial to the ASIS under the IL—the distance from the ASIS could vary, creating a zone where the LFCN can be encountered. Furthermore, five branching patterns were observed: the normal single nerve that eventually bifurcates in the area of the thigh, bifurcation within the pelvis, bifurcation near the nerve’s exit from the pelvis, trifurcation, and quadrification. 3
Variations in the anatomy of the LFCN have been known and documented for many years, injury to it having been noted as far back as 1885. 2 Tomaszewski et al assembled all data available on the variations in the anatomy of the LFCN, namely, its points of exit in the pelvis, branching patterns, and distance from other major structures, to provide a better understanding for surgeons operating in its vicinity. 16 Their analysis showed that most nerves follow the pattern of exiting the pelvis medial to the ASIS, under the IL and medial to the sartorius muscle, with an overall prevalence of 86.8%, subgroup analysis showing prevalences above 80% for all groups. When the nerve exited following this pattern, medial to the ASIS and under the IL, it was usually found 1.9 cm medial to the ASIS.
It usually exited as a single nerve, with an overall prevalence of 79.1%. Bifurcation within the pelvis was the second most common pattern with a prevalence of 11.8%. However, it was noted that in studies from South America, there was a much higher prevalence of trifurcation than bifurcation in the pelvis, with prevalences of 24.7 versus 1.2%, respectively.
Several surgeries can cause iatrogenic injury to the LFCN. These include laparoscopic inguinal hernia repair, abdominoplasty, and bone graft harvesting.
Consideration of the variations in the LFCN is especially important in conducting inguinal hernia repairs. Although rare variations of the nerve traveling through the ASIS, the IL, or through the sartorius muscle can actually protect the nerve from injury, the LFCNs of most patients are at a risk of iatrogenic injury. Patients with early bifurcations, including those within the pelvis and in the area of the IL, are at higher risk of iatrogenic injury during surgery as more branches in the area have to be considered than in the normal anatomy. Similarly, trifurcations and quadrifications of the LFCN provide more targets for accidental injury, putting populations from regions of South America, where trifurcations presented with a prevalence of 24.7%, at elevated risk.
Clinical data have shown laparoscopic inguinal hernia repair to be a safer alternative to open repair in terms of incidence of postoperative neuralgias, with a relative risk ratio of 0.66 (95% confidence interval [CI] 0.51–0.87) when compared to open inguinal hernia repair. 4 Regarding a laparoscopic approach, it has been suggested that staples be avoided within 1 cm of the ASIS owing to the proximity of the LFCN. 5 During their subgroup analysis by geographical region, these authors noted very little heterogeneity for the pooled mean distances of the LFCN from the ASIS. Thus, they suspected the cause for heterogeneity in the overall analysis of distance of the LFCN from the ASIS was most likely to be geographical differences. Their analysis revealed that South American populations had LFCNs closest to the ASIS with a mean distance of 0.99 cm (95% CI 0.43–1.55). European and North American populations, in contrast, had LFCNs with mean distances of 2.32 cm (95% CI 1.88–2.81) and 2.31 cm (95% CI 1.54–3.09) from the ASIS, respectively. Asian populations fell between these extremes, with a mean distance of 1.43 cm (95% CI 0.98–1.89). Tomaszewski and Popieluszko suggested that the dangerous zone for staples should be reassessed because their data indicated that the average LFCN passes within 1.9 cm of the ASIS and is highly variable depending on from the patient’s geographical location. With other procedures, such as esthetic abdominoplasties, a zone of 4 cm around the ASIS has been demarcated as a potentially dangerous area, requiring careful dissection and preservation to retain proper LFCN structure and function. 6
Studies have reported that the general rule of thumb used by surgeons is to approximate the LFCN as running two fingerbreadths medial to the ASIS. 7 However, this strategy can grossly miscalculate the location of the nerve depending on the patient and on the surgeon’s anatomical knowledge. Ideally, an imaging approach such as ultrasound would help to determine the precise location of the LFCN and confirm the absence of one of the other common variations. However, if a gross estimate must be made, we would suggest 3 cm as a rule of thumb rather than two fingerbreadths, as finger widths can vary. On the basis of our analysis, we ideally suggest a danger zone for all surgical procedures of about 3 cm around the ASIS, which corresponds to the upper limit of the confidence interval of the subgroup with the highest upper limit in the confidence interval (North America), thus minimizing the risk of iatrogenic injury for most populations.
Another procedure where the location of the LFCN is of particular interest is bone graft harvesting. The size of the graft and size of incision can greatly influence the risk of injury. The current suggestions are for grafts to be less than 3 cm in size, and the incisions to be made should be at least 3 cm away from the palpable point of the ASIS. 8 This general guideline could potentially injure patients with an LFCN lateral to the ASIS, which was found in 2.6% of the population studied (95% CI 0.0–6.7). Thus, these studies indicate the value of an imaging study such as ultrasound before graft sampling.
A final consideration of the LFCN is for the anterior approach to hip arthroplasty. In a study in 2010, 81% of patients reported new onset of neurapraxia in the area supplied by the LFCN after a hip resurfacing or total hip arthroplasty performed using the anterior approach. 9 The anterior approach offers many advantages over the posterior approach, which has a higher risk of dislocation, or the lateral approach, which puts the adduction function at risk. 10 Because the anterior approach offers the least damage to a patient’s hip function, the loss of sensation provided by the LFCN becomes a larger concern. Current suggestions for minimally invasive anterior approaches involve incisions running parallel to the LFCN. 11 Again, for this approach to be viable and to ensure preservation of the LFCN, the location of the nerve must be strictly determined, not simply estimated, because it is highly variable.
We suggest further analysis of the LFCN and its variations, especially using ultrasound guidance as a quick and effective method, to help surgeons minimize the incidence of meralgia paresthetica due to iatrogenic injury to the LFCN.
The most common pathology associated with the LFCN is meralgia paresthetica, a condition entailing pain, a lack of sensation, or dysesthesia of the skin supplied by this nerve. 12 Meralgia paresthetica can have numerous etiologies including pelvic inflammatory disease, pregnancy, various toxicities, tight clothing, and, importantly, iatrogenic injuries from surgical procedures. 3 Detailed knowledge of variations in the pelvic exits and branching patterns of LFCN is crucial for diagnosing meralgia paresthetica and for avoiding injuries during surgical procedures, especially inguinal hernia repair. The incidence of nerve injury in laparoscopic inguinal hernia repair is about 2%. 13 Although this is a relatively small percentage, current studies estimate that close to 20 million hernia repairs are performed annually worldwide. 14
23.4 Lateral Femoral Cutaneous Nerve Block Anatomical Technique
This block can be performed alone to provide anesthesia of the lateral thigh (e.g., donor area for a skin graft) or to diagnose meralgia paresthetica. It can also be performed along with femoral, obturator, and sciatic blocks to provide anesthesia of the thigh for surgical procedures above the knee and for thigh tourniquet. It is also one of the nerves targeted in a “3-in-1” block, a block of the femoral nerve performed with a higher volume of local anesthetic, with the aim of also blocking the lateral femoral and obturator nerves (not supported by evidence).
23.4.1 Point of Contact with the Nerve
The nerve is approached as it emerges from under the IL, medial, and inferior to the ASIS. The main characteristic of this nerve is a constant relationship to the ASIS, although for several centimeters its trajectory is within the LFCN canal, until it perforates it to become a superficial nerve running on the lateral thigh. 15 It is important to remember this because the fascia lata duplication, forming the anterior and posterior walls of the canal, is thick enough to slow the transfer of local anesthetic to the target nerve.
23.4.2 Patient Position and Landmarks
The patient lies supine. The ASIS is identified by palpation.
The needle entrance point is identified about 1 cm medial and about 2 cm caudal to the ASIS. The needle is advanced perpendicular to the skin until it enters the LFCN canal where a small volume of local anesthetic is injected. A nerve stimulator with pulse duration of 0.3 to 1 ms (300–1,000 µs) can be used to identify the nerve by eliciting a sensory paresthesia on the lateral thigh.
23.4.4 Local Anesthetic and Volume
A volume of around 5 mL of 1% mepivacaine is frequently used. A long-acting agent such as ropivacaine or bupivacaine can be used if longer anesthesia or analgesia is needed.
On occasion, some patients complain of dysesthesia on the lateral thigh, which typically resolves within a few days of the procedure without sequelae.
23.5 Lateral Femoral Cutaneous Nerve Block
23.5.1 Ultrasound Technique
The use of ultrasound facilitates this block. As previously mentioned, the LFCN passes under the IL medial to the ASIS and in its individual canal as shown in ▶ Fig. 23.3.
Fig. 23.3 The lateral femoral cutaneous nerve passes under the inguinal ligament medial to the ASIS. In the thigh, the nerve runs through its individual fascial tunnel (the LFCN canal) as shown by the arrows.