Introduction
The lateral femoral cutaneous nerve (LFCN) of the thigh is normally a derivative of the posterior divisions of the L2 and L3 spinal nerves that travels through the pelvis heading towards the anterior superior iliac spine (ASIS). It then usually exits the lesser pelvis under the inguinal ligament (IL), anterior to the ASIS, bifurcates into an anterior and posterior division along the length of the thigh, and provides sensory innervation to the skin of the anterolateral and lateral aspects of the thigh [
1].
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 the LFCN [
2]. Meralgia paresthetica may 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 in diagnosing meralgia paresthetica, as well as avoiding injuries during surgical procedures, especially inguinal hernia repair. The incidence of nerve injury in laparoscopic inguinal hernia repair is about 2 % [
4]. Although relatively a small percentage, current studies estimate that close to 20 million hernia repairs are performed annually worldwide [
5].
Variations in the LFCN’s anatomy are common, with seven different points of exit from the pelvis having been observed [
6]. Four of the variations can be classified into four zones relative to the ASIS, through which the LFCN may pass. These include medial to the ASIS and under the IL, medial to the ASIS and over the IL, directly over the ASIS, or lateral to the ASIS. Three more variations were found where the nerve passed through another tissue: the ASIS itself, the sartorius muscle, or the IL. Even when the nerve followed the anatomically normal exit pattern of medial to the ASIS and under the IL, the distance from the ASIS can vary, creating a zone where the LFCN can be encountered. Furthermore, five branching patterns were observed. These included the normal single nerve that would eventually bifurcate in the area of the thigh, bifurcation within the pelvis, bifurcation near the nerve’s exit from the pelvis, trifurcation, and quadrification [
3].
Though many original studies have been performed on the LFCN, to our knowledge there has yet to be a comprehensive meta-analysis of all the data reported from anatomical studies around the world. With a lack of such a study and the possible clinical implications of the anatomy of the LFCN in surgery in the area of the hip or pelvis, the aim of our study was to perform a comprehensive meta-analysis on the prevalence of the reported variations in the LFCN to provide evidence-based foundation of anatomical knowledge for surgeons preparing for interventions in the area of the pelvis.
Discussion
The variations in the anatomy of the LFCN have been known and documented for many years, with the injury to the LFCN noted as far back as 1885 [
6]. The goal of our study was to collect 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.
Our analysis showed that a majority of nerves follow the pattern of exiting the pelvis anterior to the ASIS, under the IL and medial to the sartorius muscle, with an overall prevalence of 86.8 %, with 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.
The nerve 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 %. It was noted, however, that in studies from South America, there was a much higher prevalence of trifurcation than bifurcation in the pelvis, with a prevalence of 24.7 versus 1.2 %, respectively.
The most common pathology described pertaining to the LFCN is meralgia paresthetica, or pain and/or dysesthesia in the area of the lateral thigh that the nerve supplies [
2]. The etiology of this pathology can be entrapment of the nerve caused by everything from physiological changes in the inguinal area, to the clothes a person wears. It is important to note that iatrogenic injury during surgery is also a common cause. Therefore, proper knowledge of the possible variations in the anatomy of the LFCN is important in the planning and execution of surgery in the vicinity of the LFCN.
Consideration of the variations in the LFCN are especially important in conducting inguinal hernia repairs. Though rare variations of the nerve travelling through the ASIS, the IL, or through the sartorius muscle may be better protected from superficial injury, the majority of patients’ LFCNs are at a risk of iatrogenic injury. Patients with early bifurcations, including those within the pelvis and in the area of the IL would be at higher risk of iatrogenic injury during surgery as there are more branches to keep track of in the area compared to 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 an 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 % CI 0.51–0.87) when compared to open inguinal hernia repair [
33]. When evaluating a laparoscopic approach, it has been suggested that staples be avoided within 1 cm of the ASIS due to the proximity of the LFCN [
34]. In our subgroup analysis by geographical region, we noticed that there was very little heterogeneity for the pooled mean distances of the LFCN from the ASIS. Thus, we suspect the cause for heterogeneity in our overall analysis of distance of the LFCN from the ASIS was most likely due to the geographical differences. Our 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, on the other hand, had LFCN’s 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 in between with a mean distance of 1.43 cm (95 % CI 0.98–1.89). We would like to suggest that the dangerous zone for staples should be re-evaluated due to the fact that our data suggests that the average LFCN will pass within 1.9 cm of the ASIS and is highly variable depending on where the patient is from. With other procedures, such as aesthetic 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 [
31].
Studies have reported that the general rule of thumb used by surgeons is approximating the LFCN as running two fingerbreadths medial to the ASIS [
30]. Such a strategy however, can grossly miscalculate the location of the nerve depending on the patient as well as the surgeon’s anatomical knowledge. Ideally an imaging approach like ultrasound would help to determine the precise location of the LFCN and confirm that one of the other common variations is not present. However, if a gross estimate must be made, we would suggest 3 cm as a rule of thumb, rather than simply two fingerbreadths, as finger width can vary among the population. Based on 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), and thus minimizing the risk of iatrogenic injury for the majority of the population.
Another procedure where the location of the LFCN is of particular interest is bone graft harvesting. Size of the graft, and size of incision can greatly influence the risk of injury. The current suggestions are that the grafts should be <3 cm in size and that the incisions being made should be at least 3 cm or more away from the palpable point of the ASIS [
29]. This general guideline could potentially injure patients with an LFCN that is lateral to the ASIS, which was found in 2.6 % of the population studied (95 % CI 0.0–6.7). Thus we would recommend an imaging study like 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 [
35]. The anterior approach offers many advantages over the posterior approach, which has a higher risk of dislocation, and the lateral approach, which puts the adduction function at risk [
36]. With the anterior approach offering 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 suggest incisions running parallel to the LFCN [
37]. Again, in order for this approach to be viable and the LFCN preserved, the location of the LFCN must be strictly determined, not simply estimated due to the high variability.
Our meta-analysis was limited by the variety of ways individual studies assess the anatomy of the LFCN. Though most studies follow a general pattern, new imaging studies and three-dimensional imaging may lead to some interpretation problems when comparing to older studies’ measured values. Additionally, high heterogeneity between studies, and a lack of assessment of publication bias due to a lack of statistical measure for multi-categorical prevalence, were limiting factors. According to our analysis, the clearest source of heterogeneity for measurements of the LFCN from the ASIS was geographical distribution. Whenever possible, authors were contacted for clarification and a consensus was reached with the research team to minimize bias in the collection and analysis.
We suggest further analysis of the LFCN and its variations, especially with the use of USG as a quick and effective method, to help surgeons minimize the incidence of meralgia paresthetica due to iatrogenic injury to the LFCN.