Introduction

Anatomical structures that exit the pelvis deep to the inguinal ligament are, from lateral to medial, the femoral nerve, the external iliac to the femoral artery, and the external iliac to femoral vein. Distal portions of all three major structures enter what is known as the femoral triangle, found within the anterior proximal portion of the femoral region.[1] Known more commonly as the “thigh,” this area serves as a confluence of all major vasculature, lymphatic and nerve supply to the lower extremity; along with the origin and/or insertion of significant musculature which mediates the full range of motion of the hip.[2] The region’s clinical significance stems from both the anatomical structures that consist of it and the related procedural/surgical implications involved within this area of the body.      

Structure and Function

The main structure of the femoral region relates to the femur and its proximal articulation with the pelvis to form the coxa (hip) joint and its distal articulation with the tibia and patella, and by extension the fibula, to form the knee joint.[3] The femur, considered a long bone in development, from proximal to distal consists of the head, anatomical neck, greater and lesser trochanters, shaft with linea aspera along its posterior surface, and the medial and lateral femoral condyles.[4] Within the hip joint, the femoral head articulates with the acetabular fossa. This articulation has support from the lunate surface (cartilage) lining the acetabular fossa, and the joint is supported by the ligament of the head of the femur (ligamentum teres of the acetabulum). Through this runs the foveal artery to the acetabular labrum and surrounding joint capsule (synovial membrane, iliofemoral ligament, ischiofemoral ligament, pubofemoral ligament).[5] Within the knee joint the medial/lateral femoral condyles articulate with the medial/lateral tibial plateaus. The knee articulation is supported by the medial/lateral menisci and the joint supported by the anterior/posterior cruciate ligaments, tibial (medial)/fibular (lateral) collateral ligaments, and patellar ligament in which sits the patella[6]           

The function relates to the multiple muscle groups contained within this region which support and induce a range of motion in the hip and the knee. The main muscle groups found within this region can be categorized by the relative anatomical locations of each group, which normally correlates with a specific range of motion of the hip that they facilitate when stimulated. The anterior thigh muscles tend to induce flexion and some adduction of the hip and extension of the knee, medial thigh muscles primarily induce adduction with some flexion and internal rotation of the hip, posterior thigh muscles tend to produce extension and some abduction and internal/external rotation of the hip and flexion of the knee.[7] The specific muscles of these compartments are covered below in a later section.

Embryology

During week four of embryologic development, limb buds begin to develop - the upper limbs appear before the lower limbs. On the distally growing end, each limb tip contains apical ectodermal ridge tissue which mediates the proper development of each limb to full differentiation at week 8.[8] Within the base of the lower limb buds, layers of mesoderm organize into what is known as the zone of polarizing activity (ZPA) which produces sonic hedgehog gene product. The function of this protein involves activation of the bone morphogenetic protein (BMP) gene and Hoxd-9 through Hoxd-13 genes; both serving roles which modulate the proper growth and lengthening of the limb.[9] Initially, the lower limb buds, as they begin to develop around week four, grow along a coronal plane. However, by week 6 to 8, the buds start to move and grow in first a horizontal movement growing along a sagittal plane and then rotating internally so that the knee faces anteriorly.[8]

While the limb structure is developing, other tissues within the limb follow suit:

• Vasculature - the external iliac artery and a branch of the umbilical artery known as the axis artery develops in parallel. As the limb continues to lengthen the femoral artery develops from the growing external iliac; the femoral artery also giving rise to the profunda femoris artery. The axis artery, in turn, gives rise to the anterior tibial artery from which develops the dorsalis pedis artery distally, and the posterior tibial artery, which terminates as the medial plantar artery and lateral plantar artery. The axis artery at full development regresses to sections of fully developed arterial structures including the inferior gluteal artery, sciatic artery, the proximal popliteal artery, and the distal peroneal artery.[10]
• Muscle and nervous tissue - the developing musculature arises from mesoderm found in somites L1 to L5, S1 to S2; closely coinciding with developing ventral primary rami of L2 to L5, and S1 to S3. As the extension of the somite mesoderm follows lower limb growth, mesoderm condensations begin to form in the anterior and posterior compartments; along with differentiation of the rami into anterior and posterior divisions. Generally, the anterior condensation goes on to develop the muscles responsible for flexion and adduction, and the posterior condensation develops the extensor and abductor muscles. For the divided rami, the anterior division gives rise to the tibial and obturator nerves, whereas the posterior division gives rise to the superior and inferior gluteal nerves, femoral nerve, and common peroneal nerve.[11]
• Bone formation - the lateral plate mesoderm gives rise to most of the bones of the lower limb; in the femoral region specifically the femur, which undergoes endochondral ossification. Beginning in weeks 5 to 6 of development, the lateral plate mesoderm condenses and chondrifies into hyaline cartilage structures. By weeks 7 to 9, primary ossification centers begin to form in the developing femur and tibia. From week 9 to birth, the diaphysis continues to develop while the epiphysis remains as hyaline cartilage. Newborn years and on, secondary ossifications develop and form the epiphyseal plates separated from the diaphysis by the metaphysis[12].            

Blood Supply and Lymphatics

From the external iliac artery that exits the pelvis immediately deep to the inguinal ligament, the segment known as the femoral artery runs through the femoral triangle in parallel with the femoral vein and the deep inguinal lymphatic chain via the femoral canal. Each vessel serves as the proximal origin of the structures supporting the tissues of the femoral region; except for the sciatic nerve (L4 to S3) in the posterior compartment, that comes off the lumbosacral plexus.[1]

The femoral artery, as it exits the femoral triangle, branches into the three major arteries of this region[13]:

• Continuation of the femoral artery - enters the adductor canal along the border of iliopsoas and pectineus muscles and deep to sartorius, initially remaining in the anterior compartment but then diving deep as it exits the adductor hiatus where it becomes the popliteal artery
• Profunda femoris - the deep branch of the femoral artery within the femoral triangle, dives deep to the adductor longus muscle and gives off perforating arteries which wrap around the posterior aspect of the femur
• Lateral circumflex arteries – the lateral branch of the profunda femoris superficial to the hip joint capsule, gives off a transverse branch which wraps posteriorly around the proximal femoral shaft and the descending branch which follows the most lateral thigh muscles to their attachments on the anterior knee

The femoral vein and its branches within the femoral region follow very similar courses to the related arteries; the femoral vein follows the femoral artery, the profunda femoris vein follows the profunda femoris artery, and the lateral circumflex veins follow the lateral circumflex arteries. The only exception is the course of the great saphenous vein, a major superficial branch off the femoral vein as it moves distally through the femoral canal and extends superficial to the fascia lata surrounding the structures of the thigh via the saphenous opening and takes a subcutaneous course along the medial thigh and extended distally past the knee.[14][15]

From this saphenous opening also extends the major lymphatic chains of the region originating from the deep inguinal lymph nodes that move alongside the femoral vessels through the femoral canal. From the deep inguinal chain follows the superficial inguinal lymph chains that extend subcutaneously in a superior directed course towards the landmark of the anterior superior iliac spine of the ilium, as well as extending an inferior division along the medial portion of the proximal thigh.[16]      

Nerves

The main nerve branches of the femoral region include[17][18][19]:

• Lateral cutaneous nerve (L2-3) - exits the pelvis just medial to the ASIS of the ilium and deep to the inguinal ligament and projects superficial to the lateral surface of the anterior and posterior muscle compartments
• Obturator nerve (L2-4) - terminates in the proximal medial thigh
• Sciatic nerve (L4-S3) – projects inferiorly through the posterior muscle compartment
• The posterior cutaneous nerve of the thigh (S1-3) – projects inferior and superficial to the posterior compartment  
• Femoral nerve (L2-4) which exits deep to the inguinal ligament and moves through the femoral triangle lateral to the femoral canal; off the femoral nerve, one of the significant branches is the saphenous nerve (L2-4) which divides off the femoral nerve as it exits the femoral triangle and follows a similar course as the femoral artery and vein through the adductor canal deep to the sartorius muscle  

Motor innervation of these nerves is covered below. Sensory innervation includes both the dermatomal and cutaneous nerve distributions. For dermatomal, the anterior thigh includes coverage by L2-L4, with the margin of L2 indicated just inferior to the inguinal ligament and L4 over most of the anterior/medial knee. Posterior thigh dermatomes include L5 covering the lateral thigh and moving medially by S1-S4, the latter covering a small portion immediately inferior to the medial margin of the gluteal cleft. For cutaneous nerve distribution, the femoral nerve covers most of the anterior medial thigh sharing the most proximal medial portions with the ilioinguinal and genitofemoral nerves and a medial portion just superior to the medial femoral condyle with the obturator nerve. The posterior thigh medial to mid receives coverage from the posterior cutaneous nerve of the thigh and the lateral most portion covered by the lateral cutaneous nerve of the thigh.[20]

Muscles

As stated above, the compartment of the muscles found in the femoral region are generally intrinsic to the function of the compartment The anterior thigh muscles generally induce flexion and adduction of the hip and extension of the knee. Medial thigh muscles as a rule produce adduction, flexion, and internal rotation of the hip. Posterior thigh muscles generally induce extension, abduction, and internal/external rotation of the hip and flexion of the knee.[7] The specific muscles of each compartment and their motor innervation include[21]:       

• Anterior thigh – innervated by the femoral nerve (L2-4); pectineus, iliopsoas (psoas major, iliacus), and quadriceps femoris (rectus femoris, vastus lateralis, vastus intermedius, vastus intermedius), sartorius*, tensor fasciae lata*
• Medial thigh – innervated by the obturator nerve (L2-L4); obturator externus*, adductor longus/brevis/magnus, gracilis
• Posterior thigh – divisions of the sciatic nerve (L4-S2); semitendinosus, semimembranosus, biceps femoris (long and short heads)

*Ssome exceptions to the general rules: sartorius flexes/abducts/externally rotates hip, tensor fasciae lata flexes/abducts hip, obturator externus externally rotates the hip.[21] 

Physiologic Variants

Significant variants that can be present within the femoral region involve angle of inclination of the femoral head and neck, the origin of the profunda femoris and its lateral circumflex branches of the femoral artery, and the locations of vessels within the femoral canal. For the angle of inclination, an average measurement for an adult male is between 115 -140 degrees, adult females tending to run more acutely in comparison.[22] Pathologically, whether due to a genetic defect in ossification or process that weakens the femoral neck (i.e., osteomalacia), a decreased angle is known as coxa vara, and an increased angle is termed coxa valga.[23] As for the origin of the profunda femoris and circumflex branches, there are discrepancies on distance from the inguinal ligament as reported by various cadaveric studies[24]; the most common origin pattern from the femoral artery has been found to be posterolateral, then followed by the next common being the posterior, and then the posteromedial.[25] Not an extremely common variation though an important one due to its clinical implications in central femoral venous access, ultrasound-guided evaluations of several cases have reported a transposition of the femoral artery and vein in the femoral canal. The normal orientation being the femoral artery lateral and the femoral vein medial, several cases warn that without ultrasound guidance complications can arise due to this anatomical variation.[26]     

Surgical Considerations

Surgical interventions commonly indicated in the femoral region involve the repair and fixation of femur fractures. Performed by orthopedic surgeons, several important notes merit consideration within the perioperative setting which involves the complex neuro-vasculature present around the proximal femur as outlined above.[27] Interventions for repair and modes of fixation are dependent on the type of fracture; which in turn determines the specific anatomical considerations when performing these procedures:

• Femoral neck fractures – in most cases, the femoral head loses its only blood supply from the foveal artery found within the ligamentum teres of the acetabulum, which gets damaged as the femoral head becomes displaced within the joint capsule and undergoes avascular osteonecrosis. This injury sometimes can result in a hematoma that remains contained within the joint capsule and so imposes the lowest risk of damage to surrounding structures. Repair and fixation are done via hemiarthroplasty, performed through a posterior approach which minimizes blood loss and fixes a prosthesis that can articulate with the native acetabulum.[28] In less frequently encountered cases, nondisplaced or impacted fractures may result in intact blood supply to the femoral head, and so a similar posterior approach is performed with internal fixation via intramedullary nails.[29]    
• Intertrochanteric fractures of the hip – have a high risk of blood loss due to the proximity of the lateral circumflex arteries and, depending on extent and severity of the mechanism of action of the fracture, the proximal perforating branches of the profunda femoris. Taking a lateral approach for repair and internal fixation via a sliding hip screw, one must consider the locations of above-mentioned arteries and lateral cutaneous nerve of the thigh, and the location of the sciatic nerve while dissecting through tissues involved.[30]
• Subtrochanteric/Femoral shaft fractures of the hip – heavy blood loss can occur with these cases, the most commonly damaged vessel in femur shaft fractures specifically being the profunda femoris, which can eventually result in a severe neurovascular emergency known as compartment syndrome.[31] Repair and internal fixation involve anterograde or retrograde intramedullary nailing or open reduction internal fixation (ORIF) with a plate.[32]  

Clinical Significance

Within the femoral region, two clinically significant anatomical associations are noted, which are heavily utilized by anesthesiologists for perioperative pain management in lower extremity procedures and intensivists and ED physicians in accessing central venous circulation via the femoral vein.  Known as the femoral triangle and adductor canal, these anatomic associations serve as landmarks for important neuro-vasculature connected to tissues of the femoral region - providing motor/sensory innervation and blood supply.[26] The femoral triangle is bordered superiorly by the inguinal ligament, medially by the adductor longus, and laterally by the sartorius. Running through this location, the femoral vein is the most medial and the femoral artery immediately lateral as they course through the femoral canal as formed by the fascia lata – both separated from the femoral nerve which runs immediately lateral to the canal[33] The two blood vessels and a branch of the femoral nerve known as the saphenous nerve then dive deep to the sartorius muscle and enter the adductor canal. This association is bordered superficial medially by the sartorius muscle, deeply by the adductor longus muscle, and superficial laterally by the vastus medialis muscle. Contained within this canal are the structures mentioned above, the saphenous nerve being the most superficial, and the femoral vein medial and femoral artery lateral.[34]

For perioperative pain management of the lower extremity, a nerve block can be administered to the femoral triangle, or in many cases both the femoral and adductor canals. Using ultrasound-guided needle insertion, space around the femoral and saphenous nerves within these neuro-vascular bundles can be infiltrated with analgesic agents.[1][2] Common agents used in these blocks include lidocaine or bupivacaine, among other up and coming agents; the choice of agent used in the block based on the provider’s judgment and the reason for use.[35]

Due to the typical location of the femoral vein within the femoral triangle and canal, ease of access allows providers to insert central IV lines if indicated. Though not the primary site for central catheter placement, this being the internal jugular vein, central venous access via femoral vein has been proven as an efficient alternative.[36]