Iatrogenic femoral nerve injuries: Analysis of medico-legal issues through a scoping review approach

Purpose Accidental femoral nerve injury is a well-known iatrogenic complication of orthopaedic, abdominal, and pelvic surgery. Because of the largely transitory nature of the symptoms associated with nerve damage, its true incidence is in all likelihood underestimated. This work aims to illustrate the surgical contexts within which this nerve injury is reported, based on the evidence obtained from a Scoping Review of the literature of the last 20 years, with specific reference to the underlying etiopathogenetic mechanisms and prognostic outcomes, to highlight the evaluation issues of medico-legal interest related to this pathology. Methods We conducted a Scoping Review of iatrogenic femoral nerve injuries reported between 2000 and 2021 by searching the electronic databases Pubmed, Scopus, Ovid Medline, Ovid Emcare, and Web of Science. We conducted the review according to the five-step methodology outlined by Arksey and O'Malley. Results The literature search identified 104 papers, including case reports, case series, and retrospective studies. Surgical contexts within which iatrogenic femoral nerve injuries were reported include orthopaedic, abdominal, gynaecological, urological, vascular, and plastic surgery, as well as locoregional anaesthesiological procedures. The long-term prognosis was generally favourable. Conclusions Because of its frequent iatrogenic genesis, femoral nerve injury is a topic of intense medico-legal interest. From the perspective of estimating the patient's disability, the mostly favourable nature of the prognosis makes the medico-legal assessment, in some respects, complex, thus requiring a precise evaluation methodology.


Background
Iatrogenic femoral nerve damage is a relatively rare complication of orthopaedic, abdominal, and pelvic surgery.
However, femoral nerve injury has an iatrogenic aetiology in as many as 60 % of cases, according to data obtained from large-scale studies [1].
The first case of postoperative femoral palsy was described in 1896 by Gumpertz, who reported a case of post-hysterectomy femoral neuropathy [2].
The six surgical contexts in which iatrogenic lesions of the femoral nerve are most frequently identified are as follows: orthopaedic surgery (mainly hip replacement surgery), abdominal surgery (mainly colorectal surgery, renal transplantation and herniorrhaphy surgery), gynaecological surgery (mainly hysterectomy), urological surgery, endovascular procedures (mainly femoral catheterisation), and anaesthesiological procedures (peripheral nerve blocks).
The following main mechanisms of nerve damage are identified: purely mechanical damage from stretching, traction, compression, or dislocation (in most cases, this is simple neuroapraxic damage); damage from complete nerve transection (neurotmesic damage with a poor prognosis); damage from accidental suturing or entrapment in staples; ischaemic damage (mostly secondary to compression); heat damage (such as in the case of cement extrusion from hip prostheses, where there is an exothermic reaction with heat release during cement polymerisation); and toxic damage.
It is a condition characterised by a widely variable spectrum of severity, with the degree of functional impairment varying depending on the location and the extent of the anatomical damage, as well as the pathogenetic mechanism.
Nerve damage is mostly neuroapraxic or axonotmesic. Neurotmesic damage occurs infrequently.
In the case of neuroapraxic and axonotmesic lesions, functional damage tends to be partial and mostly self-limiting. In contrast, in the case of neurotmesis following a complete nerve section, the anatomicalfunctional impairment accounts for a morbid state with a dramatic impact on the patient's quality of life.
In general terms, the long-term prognosis is good, with scientific evidence of motor and sensory recovery usually occurring within 6-12 months of rehabilitation treatment.
The latest Systematic Review on iatrogenic femoral nerve injury dates back to 2010 and includes 38 scientific papers published between 2000 and 2010 [3].
Given the significant medico-legal implications of the condition, we felt it would be useful to conduct an updated literature review.
In particular, the present work aims to illustrate fully and in an updated way the characteristics of iatrogenic injuries of the femoral nerve by investigating the scientific literature on the subject of the last 20 years.
The surgical context, probable pathogenesis, clinical presentation, prognosis, and the strategies through which the damage could have been avoided were studied for each case found in the literature.
Finally, the collected data were processed to outline the most significant medical-legal implications of nerve injury, which, especially because of the overall favourable prognosis, is of significant interest in terms of healthcare litigation.

Anatomy and physiology of the femoral nerve
The femoral nerve is the largest branch of the lumbar plexus, originating with three roots from it.
The upper root originates from the anastomotic loop between L2 and L3, the middle root from L3, and the lower root from L4.
It is a mixed sensory and motor nerve responsible for the motor innervation of the anterior muscles of the thigh and the sensory innervation of the skin of the anterior aspect of the thigh and the anteromedial part of the leg, as well as the acetabulofemoral joint and the knee joint.
The nerve trunk emerges at the level of the fifth lumbar vertebra, along the lateral margin of the psoas major muscle, and heads caudally until it crosses the inguinal ligament, which it transits dorsally, immediately lateral to the iliopectineal arch, running along the neuromuscular lacuna together with the iliopsoas muscle [4].
In the abdominal pathway, the nerve provides some collateral branches: the nerve to the psoas major, the nerve to the iliacus muscle, the nerve to the pectineus muscle, and the nerve to the femoral artery, which originates proximal to the inguinal ligament and accompanies the vessel to the mid-thigh.
Shortly after passing the inguinal ligament, it bifurcates into two divisions, one anterior and one posterior, separated by the lateral circumflex femoral artery [5].
The branches of the anterior division are the lateral femoral cutaneous nerve and the medial femoral cutaneous nerve.
Those of the posterior division are the four motor nerve branches destined to the same number of parts of the quadriceps femoris muscle, and a purely sensory nerve, the saphenous nerve, which, at the level of the medial epicondyle of the femur, divides into its two terminal branches, the tibial and the infrapatellar branch [6].
Concerning vascularisation, the nerve has triple blood support: at the pelvic level, it is vascularised by the iliolumbar artery, at the inguinal level by the deep circumflex iliac artery, and in the thigh by the lateral circumflex femoral artery.
It is important to note that the middle femoral nerve, within the pelvic cavity, has a relatively poor blood supply, being a "watershed area" supplied by both the iliolumbar artery (a branch of the internal iliac artery) and the deep circumflex artery (a branch of the external iliac artery) [3].
In a 1987 study of 10 cadavers, Boontje and Haaxma found a disproportion in blood supply between the right and left sides [7]. In particular, the researchers observed that the left femoral nerve was more prone to an ischaemic insult because of the greater vascular supply from the right deep circumflex iliac artery than the contralateral one; the right artery, moreover, was found to have a more significant number of anastomotic connections with the iliolumbar and fourth lumbar arteries.
Finally, in the case of anatomical variants, the only one reported in the literature is the nerve entering the thigh between the artery and the femoral vein [8].

Femoral nerve injury: clinical and diagnostic aspects
Clinically, femoral nerve injury is characterised by both motor and sensory deficits.
As far as motor function is concerned, paresis or paralysis of the quadriceps muscle as a result of nerve damage leads to severe limitation or abolition of knee extension. The patient, who generally has a conspicuous hypotrophy of the quadriceps, has great difficulty climbing stairs or facing a slight slope, and although he can walk with his knee extended, he falls at the slightest flexion of it.
A practical and widely used system for the most objective definition of the residual motor function of the quadriceps muscle after femoral nerve injury is the MRC (Medical Research Council) muscle grading system [9], according to which 0 corresponds to no contraction and 5 indicates normal muscle strength.
However, an even more specific muscular strength assessment system is explicitly designed for estimating the residual strength of the quadriceps after femoral nerve injury. The LSUHSC (Louisiana State University Health Sciences Centre) muscle grading system provides a grading from 0 (motor activity of the muscle completely abolished) to 6 (fully preserved motor activity) [10].
Another typical motor impairment associated with nerve injury is weakness in hip flexion, which is attributable to paralysis of the iliacus and sartorius muscles.
With regard to sensibility, femoral nerve lesions are associated with sensory disturbances (hypo/anaesthesia or paraesthesia) involving the nerve's distribution territory, in other words, the front of the thigh and the inside of the knee, leg, and foot.
The patellar reflex is markedly reduced or completely abolished. The diagnostic workup includes nerve conduction studies (NCS) and needle electromyography (EMG), as well as imaging investigations (CT and MRI) to identify and locate any compression and to define the aetiology of the disorder.

Methods
The approach that responds most effectively to the needs of a Scoping Review is that proposed by Arksey and O'Malley [11], who argued that scoping reviews can be implemented to examine the extent, scope, and nature of the literature to identify areas of research where evidence is scarce, to determine the need for a subsequent Systematic Review, to summarize and disseminate current knowledge, or to uncover gaps and direct future research.
This methodological approach requires the study to be carried out in five phases: Step 1: identifying the research questions Step 2: identifying relevant studies Step 3: selecting studies Step 4: charting data Step 5: collating, summarizing, and reporting results In the present work, we used the methodological approach of the Scoping Review to provide a comprehensive overview about current knowledge of iatrogenic femoral nerve injuries.
We conducted this research not only to outline the state of the art on the topic as it emerged from the study of the scientific literature, but also to identify possible injury prevention strategies and to outline an effective medico-legal assessment methodology.
The work was carried out in accordance with the operational methodology outlined in the PRISMA 2020 statement [12], which is an updated guide (replacing the previous 2009 version) aimed at indicating the best strategy to identify, select, evaluate and synthesize studies from systematic reviews and scoping reviews.
The research was registered with the Unique Identification Number "researchregistry7337".
Although the present study is a Scoping Review and not a Systematic Review, we equally verified the adequacy of the methodology conducted through the AMSTAR2 criteria (electively directed at assessing the methodological adequacy of systematic reviews) [13].
The level of compliance was low, in our opinion, due to the particular type of research conducted, namely a Scoping Review, which requires a different methodological approach than that of a Systematic Review.

Identifying the research questions
We developed the research questions in accordance with the population/concept/context (PCC) framework, proposed by the Joanna Briggs Institute [14].
We based the formulation of the review questions on the methodology outlined by the JBI, as the application of the PCC framework is universally recognized for its ability to optimally respond to the characteristic need of a Scoping Review to address the issue through a more general approach than that of a Systematic Review [15].
We formulated four main, wide-ranging questions: We voluntarily omitted to include the databases for the grey literature search in order to avoid contaminating the research with papers with uncertified scientific validity.

Inclusion criteria: the application of the PCC framework
Regarding the subject, we did not use the field "population" as a search criterion, since our research aimed to investigate iatrogenic lesions of the femoral nerve in general terms, without reference to specific classes of patients.
About the field "concept", we used the concept of anatomical injury of the femoral nerve, while with regard to the field "context", we selected the surgical field, since we wanted to specifically investigate iatrogenic nerve injuries, i.e., damages caused during invasive medical practices. Table 1 illustrates the application of the PCC framework to the Scoping Review question.

Search strategy
In accordance with the methodological approach suggested by the Joanna Briggs Institute [16], our first step consisted of a preliminary search within the Ovid Medline database.
For each PCC element, we introduced the relevant keywords, and then we joined the lines related to them to obtain an overall set line for that specific PCC element, combining them with the "OR" Boolean operator.
Finally, we combined all overall set lines with the "AND" operator, in order to find the results that addressed our PCC elements.
We limited the search to the period between January 1, 2000 and March 31, 2021.
We obtained 433 resulting articles. Table 2 shows the search strings used to search the Ovid Medline database.
We then applied the same methodological approachmade the needed adjustments to keywordson the databases Pubmed (2020 resulting articles), Scopus (4760 resulting articles), Ovid Emcare (338 resulting articles), and Web of Science (800 resulting articles).
Overall, we found 8351 articles using the above search terms and databases.
We completed the last search on May 30, 2021.

Selecting studies (screening phase)
Once we completed the bibliographic collection phase, we entered the 8351 articles obtained from the five databases into EndNote software.
The first and preliminary phase consisted of utilising an automatic software tool (and consequent elimination) to identify duplicate articles (n. 2544), articles not written in English (n. 326), and veterinary medicine reports (n. 107).
At the end of the initial skimming procedure, we obtained a library of 5374 articles.
We again used EndNote software for the initial screening phase, eliminating articles that were unrelated to the purpose of the Scoping Review, thus reducing the total number of articles to 545.
Using an automated system based on automatic title and abstract analysis, we discarded 4659 articles as not useful for the Scoping Review.
Thereafter, the fourth and fifth authors independently read the abstracts of the remaining 715 articles and eliminated articles that did not meet the established inclusion criteria (n 611), leaving 104 articles for full-text review.
The following inclusion criteria were adopted at this stage: 1. Case reports, case series, or retrospective studies 2. Papers specifically addressing femoral nerve injuries 3. Articles specifically concerning iatrogenic injuries 4. Articles addressing the probable etiopathogenesis of nerve injury  Table 2 Search strings used to search the Ovid Medline database.

Charting data
In order to get a clear view of the research results, we used a form of data graph using the Excel programme.
We decided to extract the following data from the selected individual articles: 1. Bibliographic reference 2. Surgical procedure during which the damage occurred 3. Frequency (in case of retrospective analysis) 4. Laterality of injury (uni/bilateral) 5. Pathogenesis of injury as hypothesised by the authors 6. Clinical presentation 7. Prognosis

Collating, summarizing, and reporting results
We summarised the results of the 104 selected papers by drawing up a table with the items used for the data charting phase.

Results
We identified 104 articles on iatrogenic femoral nerve injury, which we outline in Tables 3-9.
Each table corresponds to each of the seven surgical contexts of the reviewed articles.
Of the 104 papers reviewed, 78 were case reports or case series, while 26 were retrospective analyses.
We used the Preferred Reporting Items for Systematic Reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR) flow diagram guidance [17] to depict the information flow through the several phases of this Scoping Review (Fig. 1).
The main limitations of this Scoping Review are that it did not consider the grey literature and based the search strategy on only 5 databases.
The limitation of the review procedure was the use, in the first phase (initial screening phase), of an automation system, which could reasonably have eliminated some articles suitable for the purposes of the review.

Discussion
This section presents a concise discursive overview that illustrates the main features of femoral nerve injuries in the various surgical branches.
A summary of the clinical and prognostic features of the lesion is then proposed, as well as a review of the main emerging preventive strategies identified in the literature examined.
Finally, the main medico-legal implications related to the topic under discussion are discussed.

Iatrogenic femoral nerve injury in orthopaedic surgery
The frequency of peripheral nerve lesions during total hip arthroplasty ranges from 0.3 % to 3.7 % [122,123].
The most commonly damaged nerve, with a reported frequency of between 0.05 % and 1.9 %, is the sciatic nerve [124].
With particular reference to lesions of the femoral nerve, according to the largest retrospective studies, the incidence of this complication after THA is between 0.01 % and 0.26 % [25,26]. The main mechanisms of damage are the following: excessive lengthening of the limb; nerve compression by self-retaining retractors, haematomas, or pseudotumours; damage due to extruded methylmethacrylate from cemented prostheses (by compression or heat secondary to the exothermic reaction of polymerisation); and laceration from a screw used in the acetabular component. However, only in rare cases is the exact aetiology of the nerve lesion clearly identifiable [125].
The surgical procedure can be performed through three approaches: posterior, direct lateral, and direct anterior.
According to several studies, the direct anterior approach, although characterised by various advantages (shorter rehabilitation, quicker resumption of daily activities, reduced postoperative pain, less obvious surgical scar, and less blood loss), is reported to be accompanied by higher complication rates, particularly during the so-called "learning curve period" for the surgeon.
In a recent retrospective analysis, Hoshino et al. [34] examined 1059 primary THAs (160 bilateral and 739 unilateral) performed between 2007 and 2014 at their institution.
The direct anterior approach was used in 273 cases, the anterolateral approach in 126 cases, and the posterolateral approach in 660 cases.
The reported incidence of iatrogenic femoral nerve injury with direct anterior approach was 1.1 % (3 cases out of 273 operated joints), which is consistent with previous studies investigating the frequency of femoral nerve injury in THA with direct anterior approach [26,33] and not significantly higher than the frequency reported for other approaches.
The damage mechanism that occurs during the procedure is most likely a distractive one related to the prolonged joint traction required for proper visualisation of the innermost regions of the hip socket [43].
In knee surgery, several cases of femoral nerve damage caused by the prolonged use of pneumatic tourniquet, a device that has been widely used for more than 60 years because of its ability to prevent bleeding and technical impediment [48][49][50].
The device is placed at thigh level with an inflation pressure of around 300 mmHg for a maximum recommended 3 h [126].
Excessive inflation pressure (above 350 mmHg) and too long a duration of tourniquet hold were shown to be associated with an increased risk of neuroapraxic compression injury [127].
An approximately three-fold increase in the risk of neurological complications was demonstrated for every 30 min of prolonged device retention [128].
Interestingly, while this is a pure compression injury, in one case an unfavourable evolution was reported, with limited recovery of nerve function at 18 months [49].
In the case of spinal surgery, several cases of femoral nerve injury following lumbar arthrodesis operations are reported. In such operations, the branches of the lumbar plexus are particularly at risk of damage because of anatomical continuity.
Although the role of intraoperative EMG monitoring is indispensable, its diagnostic ability might be overestimated, as plexic lesions are mostly multifactorial [54].
Finally, some cases of femoral nerve injury following iliac crest bone harvesting are reported. In these cases, the cause of the nerve injury was found to be compression due to retroperitoneal haematoma, excessive traction, or diffusion of anaesthetic [56][57][58].

Iatrogenic femoral nerve injury in abdominal surgery
In the case of colorectal surgery, in most reported cases, the nerve damage is compressive and related to the use of self-retaining retractors, which may result in constriction of the nerve against the abdominal wall (on its way into the psoas muscle) or the lateral pelvic wall.
The self-retaining retractors most likely to compress the nerve trunk are the so-called "ring retractors", which are often used because they allow multidirectional exposure of the surgical site [67].
Indirect compressive damage is also possible, in other words, primarily ischaemic damage secondary to compression of the iliac vessels.
In a reported case, the compressive effect was not exerted by a surgical retractor but by the direct pressure exerted by the elbow of one of the surgeons during a complex abdominoperineal resection operation [66].
In inguinal hernia repair surgery, the most common mechanisms of purely surgical nerve damage (excluding anaesthetic procedures) are accidental suturing of the nerve, entrapment within scar tissue, and compression by the action of direct intraoperative pressure or postoperative oedematous tissue [71,72]. This complication occurs most frequently in operations for recurrent hernias, evacuation of haematomas, or debridement of surgical infections [129].
Iatrogenic femoral nerve injury has also been described during laparoscopic inguinal hernia repair [130].
Three pathogenetic mechanisms are noted: physical compression, traction or impact damage, and ischaemic injury In the case of compressive etiopathogenesis, an anatomical feature that makes the femoral nerve vulnerable to compression damage by ND: Not described in the paper.  surgical retractors during renal transplantation is the exceptionally shallow position it occupies within a groove between the iliacus and psoas major muscles. However, the compressive effect may be secondary to the formation of a perineural haematoma intraoperatively or postoperatively, which is particularly common in individuals on anticoagulant/antiplatelet therapy or with haemocoagulation disorders.
Furthermore, ischaemic damage may occur because of a temporary ischaemic period of the nerve secondary as a result of clamping the iliac artery. This ischaemic period, which is usually relatively short, may, however, result in considerable ischaemic damage in patients with circulatory disorders (e.g., diabetes mellitus) and because of the relatively poor vascularisation of the femoral nerve at the pelvic level, especially on the left side [7,75].
Moreover, in renal surgery, cases of nerve damage are reported during nephrectomy (due to excessive nerve stretching) [78] and percutaneous sclerotherapy of renal cysts (due to toxic damage from ethanol leakage) [79].
Finally, a single case of transection of the L3 spinal root during sympathectomy is reported, with neurotmesic damage treated by nerve grafting with good recovery of function at 2 years [10].

Iatrogenic femoral nerve injury in gynaecological surgery
According to the two most authoritative retrospective studies on the subject, the incidence of iatrogenic femoral nerve injury during major gynaecological oncological procedures is between 1.1 % and 1.9 % [80,133]. However, at present, the incidence of this nerve injury in the context of routine gynaecological surgery has not been precisely defined.
The gynaecological operation most frequently associated with the complication appears to be hysterectomy, with a reported post-surgical incidence of between 7.45 % and 11.6 % [134,135].
Two pathophysiological mechanisms are most commonly responsible for nerve damage during hysterectomy: stretching and compression of the nerve against the inguinal ligament secondary to the lithotomy position (in this position, the hip is flexed, adducted, and externally rotated, thus giving the nerve a particularly sharp angle) in the case of transvaginal procedure, and compression of the nerve by retractors in the case of abdominal hysterectomy.
Femoral nerve injury during ovariectomy is also reported and is generally secondary to the improper use of retractors [10,86,87]; ]; a case is also reported in which nerve damage was caused by an incorrect trocar insertion procedure during laparoscopic surgery [85 Nevertheless, in gynaecological surgery, bilateral nerve damage is not uncommon [136].

Iatrogenic femoral nerve injury in urologic surgery
The pathogenetic mechanism most commonly associated with femoral nerve injury during urological procedures is compression of the nerve by self-locking retractors, which are typically held in the pelvis for prolonged periods during complex procedures such as radical cystectomies or prostatectomies.
Less frequently, three further damage mechanisms are involved: direct damage due to accidental cutting or coagulation, nerve ischaemia due to interruption of the intrapelvic blood flow, and nerve traction in a lithotomy position with hip hyperabduction [137]. In urological surgery, as in gynaecological surgery, bilateral involvement of the femoral nerve is also possible.

Iatrogenic femoral nerve injury in endovascular procedures and vascular surgery
Another typical context in which iatrogenic lesions of the femoral nerve can occur is femoral vessel catheterisation procedures (for blood sampling, angiographic procedures, intra-aortic balloon pump placement, leadless pacemaker insertion, and extracorporeal membrane oxygenation).
According to a recent retrospective study conducted by El-Ghanem et al. on 15,894,201 percutaneous catheterisation procedures, the incidence of this complication is 3.8 events per 100,000 operations [100].
Mechanisms of damage include the following: nerve compression from haematoma, pseudoaneurysms or sandbags applied to the injection site; compression and stretching secondary to the use of large-bore delivery sheaths; penetrating damage from direct puncture of the nerve trunk; accumulation of local anaesthetic injected before the procedure around the femoral artery or directly into the nerve myelin sheath.
A few cases of nerve injury secondary to vascular surgery are also reported: one case following crossectomy and stripping of the great saphenous vein [102], one case secondary to percutaneous transluminal angioplasty of the lower limbs [103] and a series of 8 cases following aortofemoral bypass [10].
The pathogenesis of the lesion was identified as a multiple stab avulsion procedure performed on completion of saphenous stripping, nerve compression secondary to bladder distension and nerve compression secondary to haematoma or pseudoaneurysm, respectively.

Iatrogenic femoral nerve injury in anaesthesiological procedures
Regional anaesthesia by femoral nerve block, administered as a single injection or as a continuous infusion through a catheter, is a commonly used procedure to relieve postoperative pain related to hip and knee surgery.
The larger study on the occurrence of femoral nerve injury following single-shot nerve block identified a frequency of 0.03 % (3 cases out of 10,309 procedures) [105].
Iatrogenic femoral nerve injuries following continuous femoral block are reported with a frequency of 0.4 %-0.8 % [110,111,113] The mechanism of injury is usually the penetration of the needle into the nerve, often in combination with nerve toxicity due to the high doses of anaesthetic introduced.
Iatrogenic lesion of the femoral nerve as a result of ilioinguinal nerve block during inguinal hernia repair occurs frequently. In these circumstances, the anaesthetic agent diffuses from a ventral fascial plane (between the transversus abdominis and the transversalis fascia) to a dorsal plane (between the iliacus and the overlying fascia, where the femoral nerve is located) [116].

Iatrogenic femoral nerve injury in plastic surgery
We found only two reports of iatrogenic femoral nerve injuries Motor function grade 4 according to LSUHSC muscle grading system 2 years after nerve grafting surgery ND: Not described in the paper.
occurring in plastic surgery. One was a medial thigh lift in a formerly obese woman who had lost weight, in which the development of a post-surgical haematoma resulted in a transient femoral neuropathy that resolved within 24 h [120].
The other case was an abdominoplasty complicated by postoperative femoral neuropathy of uncertain origin, probably caused by intraoperative compression, suture ligation, or a side effect of anaesthetic infiltrating the surgical site.

Management and prognosis
In the vast majority of cases, a proper course of physical therapy and rehabilitation can restore normal nerve function within a few weeks.
In some cases, such conservative treatment must be preceded by a surgical approach to resolve the anatomical issue underlying the nerve injury (e.g., removal of pseudotumoral masses, evacuation of haematomas, and removal of extruded cement from hip prostheses).
In a small percentage of cases, surgical procedures such as neurolysis and nerve grafting are required.
In general terms, the prognosis for iatrogenic femoral nerve injury is good, with almost complete recovery of nerve function within a few weeks to a few months.
However, ischaemic lesions generally have longer recovery times than compression lesions, due to the concomitant axonal injury [138].
A review of 44 cases quantified the recovery time as one week in 25 % of cases, one month in another 25 % of cases, less than five months in 35 % of cases, and less than one year in the remaining 15 % [139].
The main prognostic factor is, in fact, represented by the estimate of axonal loss, carried out by electroneurophysiology investigations.
Albrecht et al. [50], suggest a possible way to standardise the extent of axonal loss, which involves assessing CMAP (compound muscle action potential) before and after an operation that is particularly at risk of iatrogenic femoral nerve injury. The amount of axonal loss, if any, is calculated using the following formula: 100 × [PreCMAP -PostCMAP] ÷ PreCMAP, establishing 20 % as the axonal deficit cut-off for the definition of femoral neuropathy.

Prevention strategies
One of the most significant features of iatrogenic femoral nerve injury, which is also of intense medico-legal interest, is its preventability, as it can be avoided in most cases by adopting simple preventive measures.
In the case of interventions using self-locking retractors, the appropriate size should be selected and positioned so that they retract only the rectus abdominis without compressing the psoas muscle.
Another useful practice is to periodically check the correct positioning of the valves, given the possibility of small movements of the retractor during surgery.
The shortest blades possible should also be used, especially in a patient with a thin abdominal wall, poorly developed rectus abdominis muscles, and a narrow pelvis [141].
Furthermore, it is important to test the femoral artery pulse after retractor placement, bearing in mind, however, that the finding of a normosfigmic pulse does not rule out the possibility of ischaemic damage.
In the case of surgery on a patient in a lithotomic position, the surgeon must limit flexion, external rotation, and adduction of the hip as much as possible to avoid possible compression of the nerve against the inguinal ligament [64].
With regard to total hip replacement surgery involving the placement of acetabular screws, the screws must be placed in the posterosuperior area of the acetabular cavity to minimise the risk of contact with the vascular-nervous structures [142].
In cases of THA-related nerve injury secondary to compression due to foreign body reactions to wear debris, no specific preventive measures are reported, but close postoperative monitoring is strongly recommended since it can very easily detect by pelvic CT and electromyography any nerve dysfunction and then quickly resolve it before the nerve damage becomes irreversible.
In knee surgery with pneumatic tourniquet, the tourniquet should be applied in the proximal region of the thigh, where the circumference is greater, as this is where the muscle mass is more robust and, therefore, able to offer greater protection to the nerve structure [49].

Table 6
Reports of iatrogenic femoral nerve injury in urologic surgery  In major gynaecological surgery, due to the anatomical complexity of the pelvis, a thorough and detailed knowledge of the neuroanatomy of the region is probably the most effective preventive measure.
Apart from this, special attention should be paid to positioning the patient in a lithotomic position, positioning the retractor blades, and avoiding extreme lateral extension of the transverse lower abdominal incisions.
Incorporation of the internal oblique muscle during fascial repair of such wounds should also be avoided.
In the case of nerve blocks, ultrasound-guided technique rather than "blind" blocking is strongly recommended, not only because it allows better topographic localisation of the block, but also because it allows faster absorption of the anaesthetic (requiring a lower dose of drug to be injected) [115]. ND: Not described in the paper.  Almost complete recovery at 10 months ND: Not described in the paper.

Medico-legal implications
Iatrogenic lesions of the femoral nerve are of great interest from a medico-legal perspective because of the preventability of the damage, on the one hand, and the particularly favourable characteristics of the dynamics of functional recovery, on the other hand.
Concerning the first aspect, for almost every damage mechanism there are specific preventive measures, which, if properly implemented, can effectively prevent injury.
Compression and traction injuries, which are particularly common in abdominal and orthopaedic surgery, can be easily prevented by careful positioning of retractors and periodic checking of retractor placement, as well as by prudent use of the lithotomy position, which avoids maximal joint angles.
In the case of neurotmesic damages from complete transection, which are very rare and usually associated with pelvic or spinal surgery, the only effective preventive strategy is a thorough anatomical knowledge combined with caution in the surgical act.
A lower coefficient of preventability is associated with compression by haematomas of the iliac and iliopsoas muscles since the occurrence of a muscular haematoma is a subsequent circumstance and not concomitant to surgery. Therefore, the only possibility of effectively combating nerve damage remains scrupulous postoperative monitoring, which must be even more careful in predisposed subjects (e.g., subjects with coagulation disorders or undergoing treatment with anti-platelet/ anticoagulants).
Similar considerations apply to ischaemic-type damage, which is essentially compressive damage, from crushing the iliac-femoral vessels against the inguinal ligament. A special case is knee surgery with pneumatic tourniquet, where the compression is directly exerted by the inflation device, and whose potential for neurological damage can be drastically reduced by a prudent choice of filling pressure and dwell time.
From a medico-legal perspective, therefore, the wide margins of preventability of the damage make the iatrogenic lesion of the femoral nerve a complication that can be classified as "preventable and foreseeable" and, therefore, constitutes an essentially objective liability of the healthcare professional and facility, whose only defence should be represented by the demonstration of the intervention of an exceptional and unforeseeable event that played a decisive role in causing the injury.
Regarding the medico-legal assessment of the damage, it is particularly interesting to note that in the vast majority of cases, as can be deduced from the results of this Scoping Review, the damage is reversible, resulting in almost complete functional recovery within 6-12 months, at most, with minor residual deficits.
As our study of the literature demonstrated, the initial damage (close to the surgical insult) is often significantly more severe than the final impairment.
It is, therefore, clear that the medico-legal assessment of permanent damage must be calibrated in relation to a precise temporal dynamic to avoid definitively evaluating impairments that are still evolving.
Electrophysiological investigations are essentially useful in four temporal contexts: intraoperatively (monitoring of action potentials distal to the possible site of injury); in an initial post iatrogenic damage phase, 7-10 days after surgery to distinguish between a simple conduction block and an axonotmesis picture; about one month after surgery in which the damage was determined, when it is possible to characterise the damage more precisely; and 3-4 months after a possible nerve repair operation, while carefully monitoring the evolution of the reinnervation process [143,144].
The timing of detection of abnormalities such as fibrillation potentials and sharp positive waves on EMG is strongly influenced by the length of the nerve tract distal to the site of injury. While such findings are usually detectable about two weeks after injury in the case of short "stumps", in the case of longer stumps, it can take up to 30 days before electromyographic abnormalities are detectable.
In nerve conduction studies, neuroapraxic lesions are characterised by normal conduction distal to the lesion.
Motor conduction studies on nerves that have undergone axonotmesis and neurotmesis are initially completely superimposable until Wallerian degeneration occurs (usually nine days after injury), which makes it possible to distinguish between the two types of lesions.
Following Wallerian degeneration, there are absent motor responses both proximal and distal to the lesion.
Based on these findings, it is evident that a synergistic use of electromyography and nerve conduction studies allows the precise definition of the degree and probable prognosis of the nerve lesion [145,146].
Determining the time interval that must elapse before a femoral nerve injury can be considered definitively stabilised is a complex matter.
The prognosis of nerve injuries, in general, is strongly influenced by the length of the nerve tract that must undergo regeneration, which is a relatively slow process.
The rate of axonal regrowth of 1 mm per day is only theoretical and can be significantly reduced by several factors, including the formation of scar tissue between stumps.
According to consolidated literature data, the complete regeneration of an injured nerve can take between 3 and 6 years [147].
The possible intervention of atrophy and fibrosis of the denervated muscle fibres must also be considered, which occur within a few weeks and about 1-2 years after the initial injury, respectively.
The absence of nerve potentials 2-3 months after the trauma is considered a negative prognostic sign, indicating the poor possibility of nerve function recovery.

Conclusion
Iatrogenic femoral nerve injury is a well-known and documented complication of many surgical procedures A review of the cases reported in the literature over the last 20 years allowed us to precisely define the mechanisms and aetiology of the damage, which in almost all cases can be traced to preoperative or intraoperative conduct characterised by imprudence or inexperience.
We also noted that the lesion is characterised by a decidedly favourable prognosis, with a satisfactory motor and sensory recovery achievable in most cases in 6-12 months and with only rehabilitation treatment, neurolysis, and nerve grafting being reserved for exceptional cases.
This can be explained by the nature of the damage, which in most cases, is neuroapraxic or axonotmesic.
This prognostic behaviour, combined with a complex interpretation of neurophysiological data, makes the medico-legal assessment of impairment sequelae particularly challenging, requiring particular sensitivity and medico-legal expertise.
The results of this research can have a potential impact both on clinical practice, having clearly highlighted how in many cases the injury is preventable and having discussed the methods of prevention, and on the medico-legal methodology, having provided useful elements for the formulation of a more accurate prognostic judgment.
The results of this scoping review may represent the basis for a systematic review.

Funding statement
The authors received no financial support for the research, authorship, and/or publication of this article.

Provenance and peer review
Not commissioned, externally peer-reviewed.

Declaration of competing interest
The authors declare that there is no conflict of interest regarding the publication of this paper.

Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi. org/10.1016/j.amsu.2021.103055.

Data availability
The data used to support the findings of this study are included within the article.