Establishing the safety of the lateral femoral cutaneous nerve when using the Bridging Infix for anterior pelvic fixation

Abstract


Introduction
5][6] Although the prevalence of pelvic ring injuries is lower in comparison to other fractures, these injuries are known to have both high morbidity and mortality rates. 1,2Surgical interventions for anterior pelvic fixation have been well established.Traditional subcutaneous internal fixation techniques have shown reduced wound complications, better quality of life and reduced pain.However, these techniques still have specific indications, contraindications and complications.The most significant known complication for these techniques is injury of the lateral femoral cutaneous nerve (LFCN).
8][9] It exits the lateral border of the psoas major muscle and passes obliquely inferior across the iliacus muscle. 8,9The LFCN courses 20-30 mm inferomedial towards the anterior superior iliac spine (ASIS) where it passes deep to the inguinal ligament. 8,10It pierces through the fascia lata inferior to the inguinal ligament, entering the thigh. 9he course of the LFCN has, however, been reported to vary in at least 25% of patients, 9,11 and up to three sub-branches have been previously described. 12Variations in the origin of the LFCN have also been described, such as contributions from both L1 and L4 nerve roots 13,14 or a single origin from only L2. 14  Reinpold et  al. reported that the LFCN entered the abdomen 5-6 mm laterally (5%), 3-56 mm medially (95%), 10 mm cranially (10%), and 14 mm caudally (90%) to the ASIS. 12However, other publications have reported the LFCN coursing superolateral to the ASIS in 2.9-4% of cases. 15,16ue to its variable course, the LFCN is at a higher risk for damage especially in anterior pelvic surgery. 17,18It can be injured and/or compressed along its entire course, but most injuries occur as the nerve exits the pelvis. 17][19][20] Strydom et al. proposed a modified technique using an internal bridge plate and rod technique. 21This technique combines the benefits of open reduction internal fixation (ORIF) with external fixation methods, with the aim of reducing known complications.It is a modification of both the pelvic bridge and INFIX described by Hiesterman et al. 22 and Vaidya et al., 23 respectively.As the Bridging Infix follows the same course as the pelvic bridge, it is suspected that the incidence of LFCN injury would be similar to that of the pelvic bridge.Since the safety of the LFCN in this novel technique is unknown, this study aims to determine the relationship between the Bridging Infix and the LFCN as well as a safe zone for implantation.

Methods
The study sample consisted of 50 (n = 50) formalin-fixed cadavers and two (n = 2) fresh frozen specimens.The samples were obtained and handled in accordance with the National Health Act no.61 of 2003 (ethical clearance: 182/2021).Cadavers with evidence of previous abdominopelvic surgeries, pathology, or damage in the abdominopelvic region were excluded.The proximity of surrounding anatomical structures to the Bridging Infix was investigated through superficial dissections of the anterior abdominopelvic wall.
Prior to implanting the Bridging Infix, each plate-rod from the Occipito-Cervical Fusion System manufactured by DePuy Synthes (Massachusetts, USA) was externally contoured, by an orthopaedic surgeon, according to the curvature of each cadaver's pelvis.Direct visualisation was used to determine if the construct was adequately contoured as no imaging was available.
Both samples followed similar dissection procedures.With the cadaver in a supine position, the skin and subcutaneous tissue was reflected laterally and removed.A vertical incision was made ± 10 mm inferior to the ASIS, into the tensor fascia lata to expose the LFCN as it emerged from the inguinal ligament.Parts of the tensor fascia lata were removed to fully expose the LFCN course.
In cases where the nerve emerged as more than one branch, the most lateral branch was measured.
In the two different samples, the Bridging Infix implantation procedures differed.Due to the rigidity of the formalin-fixed cadavers, it was deemed unfeasible to easily create a subcutaneous tunnel as required.Therefore, the implantation procedure was a modified version published by Strydom et al. 21In the fresh frozen samples, the surgical technique was strictly adhered to as the tissue elasticity was identical to a live patient undergoing the surgical procedure.
In the fresh frozen specimens, the surgical implantation procedure was followed before dissection.Three surgical incisions were made: two incisions for the lateral windows, extending from the ASIS, 40 mm along the iliac crest, and a horizontal incision for the medial window ± 10 mm superior to the pubic symphysis.A subcutaneous tunnel was created between the medial and lateral windows.Kocher forceps were used to pull the plate-rod through the tunnel.
In both samples, the external oblique fascia was lifted off the iliac crest and the periosteum was cleaned off the iliac crest to create a bare area for fixation.The plate was placed on the bare area and secured with three cortical screws into the ala.The plate-rods were connected to a connecting rod with rod-to-rod clamps (Figure 1).
Following the dissection and implantation procedures, the following distances were measured using a sliding mechanical calliper of 0.1 mm accuracy (Figure 2).First, the distance from the midpoint of the most medial screw head to the LFCN emergence at the inguinal ligament was measured.Next, the distance between the ASIS and the LFCN emergence at the inguinal ligament was determined.Thirdly, a measurement was taken from the midpoint Figure 1.Bridging Infix implanted between the iliac crests and pubic symphysis on the right of the implant rod-to-rod connector single screw to the LFCN as it emerges from the inguinal ligament.Finally, the distance originating from the most prominent point of pubic tubercle to the LFCN emergence was measured.In addition, the distance between the ASIS and pubic tubercle was measured.All measurements were taken bilaterally and recorded for data analysis.
In the fresh frozen samples (n = 2), additional measurements were recorded to determine if flexion of the hips (> 45°) influenced the measurements taken.In order to accurately evaluate the impact flexion had on the measured distances, only one hip was flexed at a time.The distance from the 40 mm cortical screw head and the distance from the midpoint of the implant rod-to-rod connector single screw to the emergence of the LFCN from the inguinal ligament was re-measured.These measurements were compared to the same measurements taken with the hips extended.
Statistical analysis was conducted using SPSS IBM Statistics version 27.The data analysis included descriptive statistics and statistical tests.The samples were tested for normality, followed by a paired t-test or Wilcoxon signed rank test to test for differences between the left-and right-side measurements.To test for differences between the sexes, a t-test was conducted for normally distributed data or a Mann-Whitney U test for skewed data.For testing difference in the BMI groups, a Kruskal-Wallis test was performed.Furthermore, Mann-Whitney U tests were conducted for comparisons between the flexed and straight hips of the fresh frozen sample.A p-value ≤ 0.05 was considered statistically significant.

Results
The LFCN was identified in all specimens on both the left and right sides.All measurements were determined to be normally distributed using a paired t-test.In order to determine if a difference exists between equivalent measurements taken on the left and right side of the cadavers, a paired t-test was performed in the formalin-fixed samples only.A significant difference (p < 0.05) was only found between the measurements taken on the left and right side for the distance between the midpoint of the single rod-to-rod connecting screw and the LFCN emergence at the inguinal ligament.All the other measurements were determined to be analogous and therefore the measurements were pooled for further testing.The results are indicated in Table I.The minimum distance in the formalin-fixed samples from the midpoint of the 40 mm cortical screw head to the LFCN was 18.00 mm, while being 26.06 mm in the fresh frozen samples.
The minimum distance from the LFCN to the single rod-torod connecting screw was 67.28 mm on the left and 74.07 mm on the right for the formalin-fixed cadavers, while 84.07 mm was the minimum distance in the fresh frozen samples.The average distance between the ASIS and LFCN was 28.77 mm for the formalin-fixed sample, while the minimum distance was 17.68 mm in the fresh frozen samples.The measurements in the fresh specimens differed widely, but no inference could be made due to the small sample and should just be noted.An independent sample t-test was performed to determine if sex had an influence on the measurements taken in the formalin-fixed cadaver sample.A significant difference between the sexes was only determined for the measurements originating from the implant rod-to-rod connector single screw to LFCN emergence on the left (p = 0.012) and the pubic tubercle to the LFCN emergence pooled (p = 0.002).In both cases, males were found to have larger values in comparison to females and can be explained by the sexual dimorphic differences between males and females.
The samples were divided into the three BMI categories: underweight (BMI < 18.5 kg/m 2 ; n = 19), healthy (18.5 kg/m 2 ≥ BMI < 25 kg/m 2 ; n = 17) and overweight/obese (BMI ≥ 25 kg/m 2 ; n = 8).BMI values were unavailable for six of the cadaver specimens which were therefore excluded from this statistical analysis.A Kruskal-Wallis test determined that there was a significant difference for the distance from the rod-to-rod connecting single screw to the LFCN emergence on the left side only and BMI (p = 0.005), with overweight cadavers having greater measurements.The pooled distance between the pubic tubercle and LFCN was also significantly influenced by BMI, with individuals with BMI values greater than 25 kg/m 2 having larger measurements (p = 0.009).
In the fresh frozen samples, selected measurements were taken with the cadavers' hips in both a straight and flexed position.A Mann-Whitney U test was conducted to determine if flexion > 45° would influence the measurements taken, but no significant difference was found.Although the sample is small, it is possible to assume that flexion does not seem to influence the measurements taken.However, observations noted during the measurement process indicate that the LFCN was seen having slightly moved from the straight position to the flexed position.
Interclass correlation coefficients (ICC) were determined to ensure the reliability of results.ICCs were determined for each measurement.It was found that all ICCs were greater than 0.9, with a minimum of 0.914 and a maximum of 0.999.One distance, from the 40 mm cortical screw head to the LFCN on the right, was an exception with an ICC value of 0.204.Therefore, an acceptable intra-and interobserver reliability was concluded.

Discussion
The current study demonstrated that the use of the Bridging Infix for anterior pelvic fixation did not pose a significant risk to the LFCN.][29][30][31][32][33][34] Additional and more pertinent distances were needed in order to establish the safety of the Bridging Infix technique, as structures in close relation to the ASIS, iliac crest and pubic tubercle would potentially be at risk during dissection and implant placement.
The cause of meralgia paresthetica can be either impingement or iatrogenic injury to the LFCN, 20,33 with both conservative and surgical treatment being described with varying success rates. 31his highlights the importance of knowledge of the LFCN anatomy and its variations for surgeons performing anterior pelvic surgical procedures.
The mean distance from the LFCN to the medial screw was 37.96 ± 12.20 mm (range: 18.00-68.68mm).The pedicle screws of the INFIX may cause potential nerve injury as it was reported that there was no safety margin in 90.9% of cadavers. 25This is the first anatomical study of the Bridging Infix, thus measurements can only be compared against other similar techniques.
In all our samples the LFCN was observed emerging deep to the inguinal ligament 10,34 and medial to the ASIS, 29,31,34 which is comparable to existing literature.A recent meta-analysis by Tomaszewski et al. found the LFCN crossing over the ASIS in only 1.9% of the cases or through the ASIS in 2.4%. 20Only in these circumstances would the LFCN be at risk with the Bridging Infix technique.
The ASIS is an easily palpable surgical landmark to determine the position of the LFCN. 34A general guideline to estimate the LFCN course is two finger breadths medial to the ASIS. 33We found that 30% of the measurements lay within a range of 30-40 mm medial to the ASIS.Since finger breadths vary, a 30 mm distance is recommended by some authors. 20We propose approximately 40 mm or three finger breadths medial to the ASIS may be a better approximation, taking the current study's measurement proportions into consideration.
Our findings regarding the distance between the ASIS and LFCN's emergence at the inguinal ligament are comparable to previous publications (Table II). 30,32,34We found the mean distance between the ASIS and LFCN was 28.77 ± 11.88 mm, while other studies reported smaller distances (Table II).The differences in measurements may be attributed to the fact that not all studies directly defined the exact position on the ASIS from where their measurements were taken.
Previous publications concluded that a danger zone for LFCN injury exists extending along the lateral border of the sartorius muscle and along the inguinal ligament for the common trunk of the LFCN, reaching as far as 73 mm. 19,31Our data is comparable with a maximum value of 60.89 mm.
Tomaszewski et al. 20 reported the mean distance from the ASIS to the LFCN as 19.0 mm and further analysis revealed that the North American and European groups had similar mean distances of 23.2 mm, while South Americans had a shorter distance of 9.9 mm.Our sample had the largest average ASIS-LFCN distance of 28.77 mm.Various differences, including geographical, BMI or population specificity, may result in measurement differences of the LFCN-ASIS.To our knowledge, no published studies have been conducted on the South African population regarding the LFCN location.
We found the average distance from ASIS to PT was 135.35 ± 12.69 mm, which is similar to previous studies. 33,34Bjurlin et al. related the distance from the ASIS to the LFCN emergence at the inguinal ligament as a percentage of the distance between the ASIS and PT as 19 ± 14%. 30Similarly we found the LFCN-ASIS distance was 21.24 ± 8.57% of the ASIS-PT distance.Üzel et al. also calculated the ratio for LFCN-ASIS/ASIS-PT to assist surgeons with patients of different body types and found the ratio to be 0.22 ± 0.16 (0.24 ± 0.20 for the left and 0.20 ± 0.12 for the right) which corresponds to a ratio of 0.21 ± 0.09 in our study. 34Thus, the LFCN can be found medial to the ASIS approximately one-fifth of the distance between the ASIS and PT along the inguinal ligament.Doklamyai et al. measured the distance between the LFCN and PT and reported the distance as approximately 110 mm in males and 100 mm in females (range: 83-127 mm). 10 This is equivalent to the 115.97 mm average documented in our study.
In the fresh frozen samples, selected measurements were taken with the cadavers' hips in both extension and flexion.When comparing hip flexion (± 70°) and extension positions, flexion did not significantly influence the measurements taken between the Bridging Infix and surrounding structures.However, observations noted during the measurement process indicate that the LFCN was seen having slightly moved from the extended to the flexed position.Although the sample is small, it is possible to assume that flexion does not seem to significantly influence the measurements taken.In contrast, Osterhoff et al. investigated 90° hip flexion using the INFIX and indicated that the LFCN was compressed in up to 80% of the samples. 24The INFIX is, however, fastened to the anterior inferior iliac spine and does not align with the inguinal ligament. 26It can be hypothesised that the Bridging Infix does not compress the LFCN due to its alignment with the inguinal ligament.
Limitations of the study include the small sample size of the fresh frozen samples.Majkrzak et al. reported that fresh and formalin-fixed specimen data sets were statistically equivalent in their study. 33This analysis could not be conducted in the present study due to the sample size difference.Significant variations in the measurements originating from the rod-to-rod connector were seen as this is not a stable point on the implant.
Complications can be avoided by strict surgical technique and advanced knowledge of the known variation prevalence.Only if the surgeon employs layer-by-layer dissection down to the level of the periosteum of the iliac crest, can a possible LFCN variation be appreciated and preserved.The Bridging Infix technique already employs this and therefore it is believed to reduce the risk of damage due to anatomical variations.

Conclusion
Variations of the LFCN are of importance, as various surgical approaches relating to anterior pelvis fixation would put the LFCN at risk.The LFCN emerges approximately 28.77 mm medial to the most prominent point of the ASIS along the inguinal ligament.This is equivalent to approximately one-fifth the distance between the ASIS and PT.Relating specifically to the Bridging Infix procedure, the LFCN can be considered to be a safe distance from the cortical screws when they are directly inserted into the iliac crest.

Figure 2 .
Figure 2. Cadaver images indicating the LFCN measurements of the right proximal thigh

Table I :
Test for difference between lateral femoral cutaneous nerve on the left and right n: sample size; SD: standard deviation; CI: confidence interval; ASIS: anterior superior iliac spine; PT: pubic tubercle; L: left; R: right (bold indicates statistically significant values p < 0.05)

Table II :
Measurements for anterior superior iliac spine -lateral femoral cutaneous nerve emergence compared between studies n: sample size; SD: standard deviation (bold indicates measurements ± 10 mm from current study)