Introduction

The muscles of the thigh subdivide into three compartments. The anterior compartment, medial compartment and posterior compartment. The function of the anterior compartment muscles is to extend the lower limb at the knee joint. The anterior compartment has its particular innervation and circulation unique to this compartment. The innervation of the anterior compartment of the thigh is from the femoral nerve (L2 through L4).   

Structure and Function

The function of the anterior compartment of the thigh is to extend the leg at the knee joint. Three major muscles (actually, two muscles and one muscle group) comprise the anterior compartment of the thigh — the pectineus, sartorius and quadriceps femoris. Additionally, the end of the iliopsoas muscle also passes through the anterior compartment. 

The pectineus muscle is responsible for flexion, adduction, and medial rotation of the hip. 

The sartorius is responsible for flexion of the knee and flexion and lateral rotation of the hip joint. 

The quadriceps femoris is a group of 4 muscles: vastus medialis, vastus lateralis, vastus intermedius and the rectus femoris. It is the main extensor of the knee.

The iliopsoas does not cross the knee joint and is responsible for flexion of the thigh and the ability to laterally rotate at the hip.

Embryology

At five weeks of development, the lower limb bud begins to appear. It grows laterally from the L2 through S2 segments. The embryological development of the apical ectoderm ridge gives rise to the cell precursors necessary for anterior thigh muscle development.[1] The cell precursors known as mesenchyme differentiates away from the apical ectoderm ridge and gives rise to muscle that make up the limb bud and eventually the anterior thigh muscles. Once there is a 90-degree rotation of the lower limb medially along the longitudinal axis; the knee moves to an anterior position.

Blood Supply and Lymphatics

The femoral artery supplies blood to the anterior compartment of the thigh. It begins behind the inguinal ligament at the external iliac artery, namely between ASIS and the pubic vertebrae. All the origins of the femoral artery pass through the pelvis. The largest branch of the femoral artery is the deep femoral artery. It branches off into the medial and lateral circumflex femoral arteries, running around the head of the femur and supplying those muscles.

The external iliac lymphatic plexus is the lymphatic system that drains the muscles in the anterior compartment of the thigh, which merges into the common iliac plexus, further draining into the cisterna chyli and then the thoracic duct.[2]

Nerves

The femoral nerve innervates the sartorius, pectineus, and quadriceps femoris, and iliacus muscle of the iliopsoas. It receives nerve supply by the nerve roots L2 through L4, innervating both the hip flexor and quadriceps muscle groups. The femoral nerve is also responsible for anterior thigh and medial leg sensation. The motor division of the femoral nerve further divide into anterior and posterior divisions:

• Anterior motor division: sartorius, pectineus (sometimes the obturator nerve also provides additional innervation), iliacus (from the muscular branches of the femoral nerve L1 through L3)
• Posterior motor division: quadriceps femoris (vastus medialis, vastus lateralis, vastus intermedius, rectus femoris)

Muscles

The pectineus forms the base of the femoral triangle and is flat and quadrangular in shape. The pectineus is considered a transitional muscle between the anterior thigh and medial thigh; this is due to innervation mainly from the femoral nerve and also sometimes from the obturator nerve. It originates at the pectineal line of the pubis and inserts at the posterior aspect of the femur, immediately inferior to the lesser trochanter.[3]

The sartorius is the longest muscle in the body. It is positioned more superficially in the leg than other muscles; the long parallel fibers run anteriorly from lateral to medial to the quadriceps. The sartorius is innervated by the femoral nerve, originating from the ASIS and inserts at the superior, medial surface of the tibia.[3] 

The quadriceps femoris connect to the patella through the quadriceps tendon and receives innervation by the femoral nerve. The vastus muscles are responsible for knee extension and stabilization of the patella. The rectus femoris is responsible for thigh flexion at the hip and knee extension.[3]

1. Vastus Lateralis: largest of the four muscles. Origin is from the greater trochanter and lateral lip of linea aspera. It inserts at the lateral base and border of the patella, forming the lateral patellar retinaculum and the lateral side of the quadriceps femoris tendon.[4]
2. Vastus Medialis: Originates at the inferior portion of the intertrochanteric line and medial lip of the linea aspera. Inserts at the medial base and border of the patella, forming the medial patellar retinaculum and the medial side of the quadriceps femoris tendon.[5]
3. Vastus Intermedius: Originates at the anterior and lateral surfaces of the femoral shaft. It inserts at the lateral border of the patella, forming the deep portion of the quadriceps tendon.[5]
4. Rectus Femoris: Comprised of two proximal heads: The straight head consists of the anterior inferior iliac spine (ASIS) of the ilium. The reflected head consists of the ilium superior to the acetabulum. Inserts at the quadriceps femoris tendon.[1]

The iliopsoas consists of two muscles: the psoas major and iliacus. The psoas major has its origin the lumbar vertebrae and iliacus originates from the iliac fossa of the pelvis. Both muscles insert onto the lesser trochanter of the femur. The psoas major is innervated by the short collateral branches of the lumbar plexus (L1 through L3) while the femoral nerve or (L1 through L4) innervates the iliacus muscle.[6]

Surgical Considerations

It is crucial to understand the anatomy of the thigh in a surgical setting, particularly to avoid neurovascular injury by operating safely through intermuscular planes. Knowing anatomy allows the surgeon to find the area of the pathology, and knowledge of the origins and insertions of the muscles helps to find a tendinous portion appropriate for graft reconstruction. It is also essential to identify the correct nerves when performing peripheral nerve blocks.

One of the most common approaches in orthopedics involving the anterior compartment of the thigh is the lateral approach to the femur. This procedure is useful for open reduction and plate fixation for a variety of fracture types involving the proximal, middle and distal femur. The lateral approach involves marking out the skin at the level of the fracture. Following skin incision, an incision is cut through the subcutaneous tissue and then through a thick band like structure called the tensor fascia lata (TFL). This structure is easy to find, has a glistening white appearance and is repairable at the time of wound closure. Once through the TFL, the vastus lateralis can be split giving access to the femur. Alternatively one can take a subvastus approach by going posterior to the vastus lateralis and elevating it sub-periostially off the bone. 

Clinical Significance

In addition to a complete history and physical exam, it is essential to understand the anatomy of the musculature in the anterior compartment of the thigh to effectively diagnose and formulate a treatment plan for patients. Muscle pain and weakness may accompany a nerve injury. Femoral nerve damage often results in loss of hip flexion and leg extension; this is especially important in making the distinction between focal injuries and systemic conditions.