THE COEXISTENCE OF ILIACUS MUSCLE VARIATION AND FEMORAL NERVE SPLIT : A CASE REPORT AND POSSIBLE CLINICAL IMPLICATIONS

The iliacus muscle variation and coexisting femoral nerve split were discovered during routine anatomical dissection of iliac fossae in an embalmed cadaver. Careful dissection revealed abnormal branching of the femoral nerve around a muscular slip of iliacus, named iliacus minimus. We reason that the coexistence of a femoral nerve split with aberrant muscular slips increases the chance of nerve compression. The findings reported here support many other cases of unusual femoral nerve branching and highlight the impact on diagnostic testing, imaging and treatment of pathology within the iliac compartment. Understanding and recognising anatomical variation is critical for medical specialists and educators, particularly anatomists, radiologists and surgeons in order to improve medical practice.


Iliacus
Iliacus is a broad muscle lining the iliac compartment of the posterior abdominal wall and is considered the main flexor of the hip joint.It arises from the superior aspect of the inner edge of the iliac crest, anterior sacro-iliac and iliolumbar ligaments, the superior aspect of the lateral surface of the sacrum and capsule of the hip joint.The iliacus converges with psoas major on its inferomedial aspect, before combining to form iliopsoas.It attaches to the lesser trochanter of the femur and to the femur just inferior to the lesser trochanter (Sinnatamby, 2011), and is supplied by a division of the femoral nerve (Drake et al, 2013;Moore et al, 2015).Accessory slips of iliacus have been documented as these may influence current surgical procedures and methods of intervention (Spratt et al, 1996;Jelev et al, 2005).Variable muscle masses will inevitably change the course and structure of surrounding anatomy such as vessels and nerves, causing further implications for surgery in this region.
Reported variations of iliacus show either an accessory or aberrant slip of iliacus, or iliacus minimus (a defined muscle with its own origin and insertion), or both.Examples have been documented in the past whereby both have coexisted within a femoral nerve split, such as presented by Rao et al (2008).In this instance, unusual muscle separation of iliacus was encountered, as well as unusual unilateral splitting of the femoral nerve within the iliac fossa, encircling an identified aberrant slip of iliacus.

The femoral nerve
The femoral nerve is an essential conduit of motor and sensory innervation to the lower limb.It stems from the posterior divisions of ventral rami of the second, third and fourth lumbar nerves (L2-L4), as part of the lumbar plexus.The nerve courses through psoas major, emerging from its inferolateral margin within the iliac fossa of the posterior abdominal wall.Then, it courses obliquely across the iliac fossa superior to iliacus and within the iliac fascia, before continuing under the inguinal ligament lateral to the femoral artery.At the level of the inguinal ligament, it divides into: (1) cutaneous branches which includes intermediate cutaneous nerve of thigh, medial cutaneous nerve of thigh and, saphenous nerve; (2) muscular branches which includes, nerve to Sartorius, nerve to pectineus, nerve to vastus medialis, nerve to vastus lateralis, nerve to vastus intermedius and nerve to rectus femoris; and (3) genicular branches to the hip and knee joints (Drake et al, 2013;Abrahams et al, 2013;Moore et al, 2015;Snell, 2012;Sinnatamby, 2011).Injury of the femoral nerve leads to motor and sensory deficits.The motor effects are paralysis of the quadriceps which leads to loss of ability to extend the knee joint.Paralysis of Sartorius, pectineus and iliacus are not apparent clinically because of compensation by psoas major and adductor muscles (magnus, longus and brevis).The sensory effects include loss of sensation on the anteromedial thigh and the medial side of the leg and foot (Hansen, 2014).The femoral nerve is the largest branch of the lumbar plexus and therefore variation in its course and structure may have consequences manifesting in an array of clinical symptoms.

CASE REPORT
The coexistence of a femoral nerve split and iliacus muscle variation were discovered during anatomical dissection of a previously dissected medical undergraduate teaching specimen.The specimen is from an embalmed (formaldehydefixed) 96 year old female without any visible medical disorders or known history of disease.Further dissection of the posterior abdominal wall uncovered the psoas major and minor muscles and respective tendons, followed by discovery of the trunk of the right femoral nerve where it emerged from the lateral aspect of the psoas major (fig. 1) as expected.Instant splitting of the nerve was evident as more iliac fascia was removed.The split of the femoral nerve encircled an additional slip of iliacus (fig.2), previously named iliacus minimus.The iliacus minimus divided the femoral nerve into an anteromedial branch and a larger posterolateral branch (fig.3).The posterolateral branch emerged posterior to iliacus minimus and fused with the anteromedial branch at its lateral border.Then it was observed to continue on an oblique course through the iliac fossa before passing underneath the inguinal ligament and dividing into its anterior and posterior divisions consisting of the medial cutaneous nerve of the thigh, intermediate cutaneous nerve of the thigh, saphenous nerve, nerve to Sartorius, nerve to pectineus, nerve to vastus medialis, nerve to vastus lateralis, nerve to vastus intermedius and nerve to rectus femoris.

DISCUSSION
Reporting anatomical variation is crucial for the preservation of quality and efficient medical practice, particularly in the surgical field.Variations of the femoral nerve can have consequences on the normal functioning of anatomical structures such as the femoral nerve itself and surrounding anatomy.Entrapment within the iliac fossa is common (Astik and Dave, 2011;Anloague and Huibregts, 2009); therefore, a clear understanding of anatomic topography in this region is essential.In dissection of 34 cadaveric specimens, Anloague and Huibregts (2009) found a 35.3% (n=12) variation rate of the femoral nerve branching during lumbar plexus investigations.The femoral nerve was observed branching into medial and lateral divisions within the tissue of psoas major.This finding is similar to the results presented by Spratt et al (1996), with 4 out of 68 dissected specimens showing additional slips of iliacus and psoas major muscles splitting the femoral nerve.No involvement of psoas major tissue within a femoral nerve split was observed in this study.These muscular slips may be found during retroperitoneal surgery or through imaging of the iliopsoas compartment and should not be misdiagnosed (Lenchik et al, 1994).Additionally, disparities in the progression of the femoral nerve through the iliac fossa have implications for nerve block administration (Astik and Dave, 2011).Due to the intervening muscular slip and early division of the femoral trunk, it is clear that an attempted femoral nerve block may be incomplete in such cases.As such, a more proximal level of administration of anaesthetic may be required, or an alternative method may be used such as an epidural blockade.There have been many types of iliacus variation documented, usually coinciding with variation of the psoas major as they develop as the same muscular complex -iliopsoas.Surprisingly, our specimen only presented variation with the iliacus and not the psoas major.Accessory muscles are defined as having their own distinct origin and insertion, as well as nervous and blood supply.As defined by Aleksandrova et al ( 2013), our specimen possessed a single large aberrant slip.Involvement of the femoral nerve is rare and only Aleksandrova et al ( 2013) reported a similar finding.Within a 64 year old female cadaver, a small muscular slip of iliacus was identified along with a coexisting split femoral nerve.The femoral nerve was split into two short fibres before they reunited to reform the femoral nerve trunk.Variations of the iliopsoas complex and subsequent nerve and slip formation may be explained by embryonic disruptions.The growing muscular blastema may have been penetrated by the aggressive ingrowths of the developing femoral nerve (Saadeh and Bergman, 1985).Femoral nerve entrapment is rarely caused by muscular compression (Rao et al, 2008), however aberrant slips 'splitting' the femoral nerve present as a potential risk of nerve restriction.In the current study, as the PLfn passes underneath the IMM, there is a potential risk of nerve compression.Any pathology of IM or IMM may disrupt the function of the femoral nerve and subsequently may result in wasting of the quadriceps and paraesthesia in the anterior and middle compartments of the thigh (Aristotle et al, 2013).Few studies (Galzio et al, 1983;Jelev et al, 2005) have reported IM hematoma and IMM abscess: assuming that could make the PLfn more vulnerable to entrapment.Spratt et al (1996) coined the name 'Iliacus minimus' and found that its unusual location in between femoral nerve branches may be the cause of referred hip and knee pain, as well as pain in the L2-L4 dermatome region.Additionally, they proposed IMM to be a potential cause of pain at the hip joint during hyperextension and lateral rotation.The branching pattern of Fn in the current study suggests this could occur due to its unusual position, however the range of movement observed with the AMfn suggests it is a fairly mobile structure and therefore is at less risk of tension during flexion, extension and rotation of the hip.A patient with a shorter AMfn may procure more movement difficulties; Anloague and Huijbregts (2009) proposed that limited mobility of the femoral nerve may lead to 'protective muscular guarding' due to increased 'neural mechano-sensitivity' in order to protect over stretching of the nervous tissue.This would disrupt the patient's locomotive ability.An understanding of anatomical variation can improve diagnostic abilities.Battaglia et al (2012) highlighted that variations in the femoral nerve may devalue results obtained from investigations into lumbar nerve root impingement by nerve traction tests.As such, consideration of these variations can allow for earlier and more targeted testing for suspected lumbar disc herniation or deciphering between entrapment neuropathies and radiculopathy.Variation in femoral nerve branching has been previously documented, suggesting its incidence is of great significance.The involvement of a variety of posterior abdominal wall muscles, namely psoas major, iliacus and an accessory slip of iliacus named iliacus minimus, is widely noted.The posterior branch of the split femoral nerve, named here the posterolateral branch (PLfn), brings cause for concern considering its deep course.We conclude that medical specialists should be aware of this variation, as it may prove useful during imaging and treatment of the posterior abdominal wall and iliac compartment, and reduce the chances of iatrogenic femoral nerve palsy.The PLfn is also at greater risk of entrapment from pathology of surrounding tissue, particularly when considering the regular movement of structures within this anatomical space.

Figure 2 .
Figure 2. Anterior view of right iliac fossa shows IMM as a distinct muscle, separate from IM at its distal end just proximal to the inguinal ligament.The proximal end blended with IM.Both branches of the femoral nerve (AMfn and PLfn) clearly re-join on the lateral margin of IMM, to form the femoral nerve.