Background: Iatrogenic injury to the femoral neurovascular bundle is not uncommon during primary and revision total hip replacement (THR) and can result in permanent weakness, pain and poor function. Prevention of injury to these structures relies on a sound knowledge of their relationships to the hip joint. Methods: We studied 115 consecutive hip magnetic resonance imaging (MRI) results in order to identify objective relationships between these structures and the hip joint that can be used intraoperatively. Results: We determined that the shortest mean distances of the femoral nerve, artery and vein from the hip joint are 23.62 (standard deviation, SD = 5.44), 19.62 (SD = 4.17) and 17.47 (SD = 4.41) mm, respectively. The femoral nerve was lateral to the hip joint in 30 (55.5%) left- and 37 (60.7%) right-sided hip joints. The femoral artery was located medial to the hip joint in 28 (51.9%) left- and 34 (55.7%) right-sided hips. The femoral vein was medial to the hip joint in 52 (96.3%) left- and 58 (95.1%) right-sided hips. Conclusion: We have identified objective relationships between the hip joint and femoral neurovascular bundle that can be used with ease intraoperatively during THR. Our data show that patients with a low body weight and the elderly may be at a higher risk of iatrogenic injury due to increased proximity of the neurovascular structures to the hip. Application of this knowledge may serve to reduce the risk of iatrogenic injury to these structures and thereby improve patient satisfaction and outcomes. Keywords: Total hip arthroplasty, Iatrogenic injury, Femoral neurovascular structures Background the femur . Riouallon et al. have identified cases of Iatrogenic injury to the femoral neurovascular bundle is iatrogenic femoral artery injury and proposed risk factors a rare but well documented complication of total hip for injury that include presence of a vascular history replacement (THR). A recent systematic review found (tobacco use, arteriopathy, bypass surgery), acetabular that up to 60% of femoral nerve injuries are iatrogenic. dysplasia, protrusion, rheumatoid arthritis, Paget’sdisease The incidence of injury to the femoral nerve varies in and previous pelvic or acetabular fractures . Injury to the literature between 0.2–2.4% of cases in primary THR these structures results in poor functional outcome with the and 1.4 to 3.8% in the revision setting [1–5]. The aetiology majority of patients having ongoing weakness, paraesthesia of injury includes direct trauma, haemorrhage/haematoma, or neuropathic pain [11, 12]. anatomical variances, retraction and leg lengthening [6–8]. Prevention of iatrogenic femoral neurovascular injury Injury to the femoral vessels has also been reported requires a sound knowledge of the anatomy of the native during internal fixation of intertrochanteric fractures of hip joint and constant vigilance intraoperatively to pre- vent inadvertent damage. There is however significant anatomic variation in the location and course of the * Correspondence: email@example.com nerve and vessels . Westmead Hospital, corner of Hawkesbury and Darcy Roads, Westmead, Sydney, New South Wales 2145, Australia © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Mehta et al. Journal of Orthopaedic Surgery and Research (2018) 13:135 Page 2 of 7 In this study, we identified objective relationships be- The reference point defined as the hip joint, from which tween the femoral neurovascular bundle to the hip joint all measurements were made, corresponded to the ante- based on magnetic resonance imaging (MRI) results. These riormost point of the anterior acetabular wall. From this relationships may prevent iatrogenic injury to the femoral point, three measurements were made with respect to neurovascular bundle adjacent to the hip joint. each neurovascular structure in question: (femoral nerve, artery and vein) anteroposterior (AP), mediolateral (ML) Methods and the shortest direct distance (d). The distance from the The anatomic relationships of the femoral neurovascular nerve and vessels was measured from their closest identifi- bundle to the hip joint during total hip replacement was able margins on the selected axial image (Figs. 2 and 3). studied by reviewing 115 consecutive hip and pelvis MRI The vein was identified by its medial relation, a larger results. MRIs performed at our institution within the last lumen and thinner wall. Conversely, the artery was identi- year were reviewed for inclusion, regardless of their indica- fied as having a smaller lumen and thicker wall. For the tion or the patient’s underlying pathology. The majority of purpose of generating a gross three-dimensional map of pelvic MRI scans performed at our institution were indi- the relationships, the measurements were also completed cated for monitoring of patients with cancer. This explains at a point 8 mm superior and 8 mm inferior to the afore- the high incidence of cancer rather than severe arthritic hip mentioned reference point. pathology within our cohort. Only minor hip pathologies Measurements were carried out and tabulated inde- were included due to their non-deforming nature. pendently by three orthopaedic registrars. In the case of Exclusion criteria included patients younger than 18 years disagreement of more than 2 mm between measurements, of age, a history of prior surgery to the hip and/or the image in question was reviewed by all three registrars surrounding soft tissues, studies that revealed infiltration and a consensus reached based on a majority vote. (neoplastic or otherwise) of the bony pelvis, severe The mean of the measurements was used for statistical deforming arthropathy, subluxation or dislocation of analysis. Correlations between the measurements were the hip joint, motion artefact and studies in which the analysed using Pearson’s correlation coefficient (r). Statistical femoral nerve and vessels were difficult to identify. analysis was performed using SPSS version 22.0 software Images were acquired using either a 3T (Siemens (IBM-SPSS, Armonk, New York). A value of p <0.05 was Magnetom Trio) or 1.5T (General Electric) MRI system. taken as statistically significant. The study had institutional Axial proton density images were examined at the level review board approval. corresponding to the widest diameter of the acetabular floor. This was routinely the axial image two slices above Results the teardrop on the coronal sections. This is a point that The study included a total of 115 hips in 64 patients can be reproduced intraoperatively, with a slice thickness with an average age of 56.8 (SD = 16.8) years. There were of 4 mm, making the reference point 8 mm superior to 61 right and 54 left hips. The average patient weight at the the inferomedial aspect of the acetabulum (Fig. 1). time of MRI was 75.5 kilogrammes (kg) (SD = 18.4 kg). Fig. 1 Anterior acetabular retractor in the 9 o’clock position of the left hip 8 mm superior (from the red line) to the approximate level of the acetabular teardrop (black circle) Mehta et al. Journal of Orthopaedic Surgery and Research (2018) 13:135 Page 3 of 7 Fig. 2 a Identification of structures on MRI. N femoral nerve, A femoral artery, V femoral vein. b Yellow line indicates the level of the axial slice used The most common pathology found in the studied patients The mean AP distance of the femoral artery was was rectal cancer (Table 1). 17.61 mm (SD = 7.28), the ML distance was 3.55 mm The mean AP distance of the femoral nerve from the (SD = 4.98) medial to the hip joint and the shortest distance hip joint was 21.89 (SD = 5.27) millimetres (mm). The ML from the artery to the hip joint was 19.62 mm (SD = 4.17). distance was 4.84 mm (SD = 7.27) lateral with respect to It was located medial to the hip joint in 28 (51.9%) left- and the joint, and the shortest distance from the hip joint to 34 (55.7%) right-sided hips. It was directly anterior (i.e. the femoral nerve was 23.62 mm (SD = 5.44). It was lateral above) to the hip joint in 22 (40.7%) left- and 22 (36.1%) to the hip joint in 30 (55.5%) left- and 37 (60.7%) right-sided hips. right-sided hip joints (Fig. 4). The mean AP distance of the femoral vein was 12.53 mm (SD = 4.17) from the hip joint and 11.05 mm Table 1 Pathology by number of hips Pathology Number of hips Normal 22 Rectal cancer 22 Vaginal/cervical cancer 17 Labral tear 11 Anal fistula/mass 8 Osteoarthritis 6 Uterine fibroid/fistula 4 Sacroiliitis 4 Gluteal pathology 3 Pelvic mass 2 Crohn’s disease 2 Spermatic cord pathology 2 Tumour of vastus lateralis 2 Ovarian cancer 2 Prostate cancer 2 Benign prostatic hyperplasia 2 Femoro-acetabular impingement 2 Fig. 3 Measurement of femoral artery (red star) to the most anterior Bursitis 1 point of the hip joint at a point two slices above the teardrop (yellow star). ML mediolateral, AP anteroposterior, d shortest direct distance Hip effusion 1 Mehta et al. Journal of Orthopaedic Surgery and Research (2018) 13:135 Page 4 of 7 Fig. 4 Mean distance (±standard deviation) of the femoral nerve, artery and vein from the hip joint. AP anteroposterior, ML mediolateral, d shortest direct distance (SD = 5.43) medial and the shortest distance from the distance between the nerve and the hip joint (r = − 0.253, artery to the hip joint was 17.47 mm (SD = 4.41). It was p = 0.006, df = 114). medial to the hip joint in 52 (96.3%) left- and 58 (95.1%) There was no statistically significant difference between right-sided hips (Table 2). left- and right-sided measurements (correlation coefficient The distances between the neurovascular bundle and >0.7, p < 0.05) (Fig. 5). the hip were also measured at a point 8 mm superior and 8 mm inferior (i.e. at the level of the teardrop) to Discussion our described reference point. The mean (±SD) differ- Our study has examined a large sample size of patients ences in distance for the measured relationships both in order to provide objective, anatomic relationships that 8 mm superior and 8 mm inferior to our reference point can be used with ease intraoperatively in order to avoid are given in (Table 3). iatrogenic neurovascular injury during THR. There is a statistically significant correlation between In this study, we assessed the AP, ML and shortest dis- patient weight and the shortest distance of the artery tances of the femoral nerve, artery and vein from the hip and nerve to the hip joint. The strength of correlation is joint on 115 proton density MRI studies. We determined weak-moderately positive for both the distance of the that the shortest mean distances of the femoral nerve, artery (r = 0.388, p < 0.001, df = 67) and distance of the artery and vein from the hip joint are 23.62 (SD = 5.44), nerve (r = 0.445, p < 0.0001, df = 67). 19.62 (SD = 4.17) and 17.47 (SD = 4.41) mm, respectively. There are very strong, statistically significant negative There was no statistically significant difference in the AP, correlations between the age of the subject and the shortest ML or shortest distance measurements when comparing distances for the vein (r = − 0.371, p < 0.001, df = 114) and the left- and right-sided hips. Shubert et al. have previously artery (r = − 0.380, p < 0.001, df = 114). Similarly, there is a described the relationship of the sciatic nerve to retractors weak negative correlation that is strongly statistically during THR in 48 patients and found that women are at a significant between the age of the subject and the shortest higher risk of suffering iatrogenic neurovascular injury Table 2 Number (and percentage) of location of the nerve, artery and vein relative to the hip joint Left hip Right hip N (%) medial N (%) above N (%) lateral N (%) medial N (%) above N (%) lateral Nerve 6 (11.1) 18 (44.4) 30 (55.5) 6 (9.8) 18 (29.5) 37 (60.7) Artery 28 (51.9) 22 (40.7) 4 (7.4) 34 (55.7) 22 (36.1) 5 (8.2) Vein 52 (96.3) 2 (3.7) 0 (0) 58 (95.1) 3 (4.9) 0 (0) N number Mehta et al. Journal of Orthopaedic Surgery and Research (2018) 13:135 Page 5 of 7 Table 3 Distances between neurovascular bundle 8 mm superior and inferior to the described reference point (mm ± SD) V (AP) V (ML) V (d) A (AP) A (ML) A (d) N (AP) N (ML) N (d) Above 0.75 (1.5) 2 (1.83) 2.25 (0.96) 1.75 (0.96) 0.5 (0.58) 3.25 (1.71) 1.25 (1.89) 4 (3.56) 4 (3.27) Below 1.75 (0.5) 2.75 (0.96) 1.25 (0.5) 0.75 (0.5) 1 (1.4) 0.75 (0.5) 2.75 (3.59) 2.25 (2.87) 2.5 (3.3) V vein, A artery, N nerve, AP anteroposterior, ML mediolateral, d shortest distance . Similarly, Wang et al. identified distances between anterior acetabular retractor positions during total hip bony landmarks and the femoral nerve in hip MRIs and replacement. The anterior wall retractor is placed at the identified that shorter patients will be at higher risk for 3o’clock position for the right hip or 9 o’clock for the neural injury intraoperatively . Our study design left. This position was estimated to lie at a point 8 mm involved the retrospective analysis of MRI images and proximal to the acetabular teardrop as illustrated in patient data that were collected for purposes other than Fig. 1. completing our study. In our institution, it is not routine Using the above relationships allows us to establish to record patient height prior to conducting an MRI and safe zones that may be used intraoperatively to prevent therefore this information was unavailable to us for inclu- iatrogenic injury. For example, based on our findings sion in our analysis. Hence, we cannot comment on any that the femoral nerve has a mean ML distance of 4.81 relationship that may or may not exist between height and (SD = 7.27) mm lateral to the hip joint, it can be seen the distance of the femoral neurovascular structures to that pericapsular local anaesthetic injection should be the hip joint. This should be a focus of future research. done within 11 mm of the hip joint in order to prevent The femoral nerve was found to lie lateral to the hip in inadvertent injection into the femoral nerve. Similarly, 55–60% of cases, the femoral artery was medial to the hip when performing a capsular release, if one uses the in 51–56% of cases and the femoral vein was medial to mean AP distance of the nerve from the hip joint, it can the hip in 95–96% of cases. These findings support the be seen that this release should not be performed to a previously described relationships of the neurovascular depth greater than 27 mm from the aforementioned refer- structures anterior to the hip joint . The reference ence point. Using this method, safe zones for the artery point used in our study is located 8 mm superior to the and nerve can also be calculated. All of the measurements acetabular teardrop. During acetabular exposure, three both above and below the reference point were found to four retractors are placed around the acetabulum in to fall within the standard deviations of our previously order to increase visualization. The anterior retractor is described relationships. It can therefore be seen that described as being passed over the anterior rim of the even if the reference point for measurement is altered, acetabulum . There are three commonly described these relationships are still valid. In spite of this, for the Fig. 5 Comparative distances between left and right femoral neurovascular structures from the hip joint. AP anteroposterior, ML mediolateral, d shortest direct distance Mehta et al. Journal of Orthopaedic Surgery and Research (2018) 13:135 Page 6 of 7 reasons outline above, the reference point chosen in used with ease intraoperatively during THR. Our data this study is the most applicable and pragmatic for use show that patients with a low body weight and the elderly intraoperatively. have a greater proximity of the neurovascular structures Making measurements above and below the reference to the hip joint and therefore may be at a higher risk of point gives rough three-dimensional description of the iatrogenic injury. Application of this knowledge will serve neurovascular bundle’s relationship to the multiple refer- to reduce the risk of iatrogenic injury to these structures ence points. However, the relationships are similar at all and thereby improve patient satisfaction and outcomes. three levels, so the need for a three-dimensional descrip- Abbreviations tion is less. More sophisticated software could be used AP: Anteroposterior; d: Distance; ML: Mediolateral; MRI: Magnetic resonance in future studies to confirm this relationship. imaging; p: p value; r: Pearson’s correlation coefficient; SD: Standard deviation; THR: Total hip replacement There was a statistically significant positive correlation between the weight of the patient and the shortest Availability of data and materials distance of the femoral artery and nerve to the hip joint. The dataset supporting the conclusions of this article is included within the Similarly, there was a negative correlation between age of article. the subject and the shortest distances for the vein and Authors’ contributions artery. That is, the neurovascular structures were found to CRM, AC and AK collected data and were involved in writing the paper. MF be closer to the hip joint in elderly individuals and those performed the statistical analysis. MS, EG and AK were involved in the conception of the study design, review and editing of the paper. All authors with lower body weight; thus, these patients may be at a read and approved the final manuscript. higher risk of iatrogenic injury during THR . Clinical applications of these results include when per- Ethics approval and consent to participate Ethics approval was sought and the study was approved by the institutional forming anterior capsular releases during the posterior review board at Westmead Hospital. approach to the hip. Additionally, with the increasing use of periarticular local anaesthetic infiltration during total Competing interests hip replacement, these relationships can be helpful to The authors declare that they have no competing interests. undertake safe injection, especially around the femoral nerve as aspiration of blood from the artery or vein is not Publisher’sNote Springer Nature remains neutral with regard to jurisdictional claims in possible when injecting close to the nerve . published maps and institutional affiliations. Given the study population, the findings of this study are only relevant in cases where the anatomy is not distorted. Received: 8 March 2018 Accepted: 28 May 2018 The majority of patients included in the study did not have hip pathology and are not necessarily reflective of patients References undergoing THR. They are not relevant in situations of 1. Kretschmer T, Heinen CW, Antoniadis G, Richter H, König RW. 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Published: Jun 4, 2018