Anterior Compartment of the Thigh - Full Exam Answer

1. Overview and Compartmental Organisation

2. Muscles of the Anterior Compartment

Group function summary:

• Psoas major + iliacus (iliopsoas) - act on the hip joint (flex thigh)
• Sartorius + rectus femoris - act on both hip and knee joints
• Vastus muscles (3) - act only on the knee joint (extend leg)

Detailed Muscle Table

Key point: Sartorius is the longest muscle in the body. It is the "tailor's muscle" - it produces the cross-legged sitting posture (hip flexion, abduction, lateral rotation + knee flexion).

Key point: The rectus femoris is the only quadriceps muscle that crosses the hip joint - it is therefore the only one of the four that can flex the hip.

Key point: The patellar tendon reflex (knee jerk, L3-L4) tests the quadriceps. A tap on the patellar ligament stretches the quadriceps, mainly testing spinal cord levels L3 and L4.

3. Iliopsoas - Special Note

• Psoas major: innervated by direct branches of anterior rami L1-L3 (not the femoral nerve)
• Iliacus: innervated by the femoral nerve (in the abdomen, before it passes under the inguinal ligament)

4. Quadriceps Femoris - Key Details

1. Converge on the quadriceps femoris tendon, which attaches to the base of the patella
2. Continue below as the patellar ligament (patellar tendon) to the tibial tuberosity
3. The vastus muscles stabilise the patella during knee movement by attaching to its margins

5. The Femoral Triangle

Contents of the Femoral Triangle (lateral to medial - mnemonic: NAVEL)

1. N - Femoral Nerve (outside the femoral sheath)
2. A - Femoral Artery
3. V - Femoral Vein
4. E - Empty space (femoral canal)
5. L - Lymphatics (in the femoral canal)

Clinical point: The femoral artery can be palpated just inferior to the inguinal ligament, midway between the ASIS and the pubic symphysis. This is the surface marking for femoral pulse and for catheterisation (coronary angiography, angioplasty, vascular procedures).

Femoral Sheath

6. Adductor Canal (Subsartorial Canal / Hunter's Canal)

• A fascial canal that runs down the medial side of the thigh, deep to sartorius
• Extends from the apex of the femoral triangle to the adductor hiatus in adductor magnus
• Contents: femoral artery, femoral vein, saphenous nerve, nerve to vastus medialis
• At the adductor hiatus, the femoral vessels pass posterior to become the popliteal vessels behind the knee
• The saphenous nerve does NOT pass through the adductor hiatus - it exits through connective tissue near the end of the canal to emerge between sartorius and gracilis on the medial side of the knee

7. Nerve Supply - Femoral Nerve (L2, L3, L4)

• Originates from the lumbar plexus (L2-L4) on the posterior abdominal wall
• Enters the femoral triangle by passing under the inguinal ligament, lateral to the femoral artery
• Lies outside the femoral sheath (lateral to the artery)
• Before entering the thigh, gives branches to iliacus and pectineus
• Immediately after entering the thigh, divides into anterior and posterior branches

Branches of the femoral nerve:

Clinical note - femoral nerve injury: Loss results in weakness of knee extension (quadriceps paralysis), loss of the knee jerk reflex (L3-L4), and sensory loss over the anterior thigh and medial leg/foot (saphenous distribution).

8. Blood Supply

• Femoral artery - the main artery of the anterior compartment; continuation of the external iliac artery below the inguinal ligament
• Profunda femoris (deep femoral artery) - the largest branch of the femoral artery; given off in the femoral triangle; supplies most of the thigh muscles via perforating branches
• The femoral artery continues through the adductor canal and exits via the adductor hiatus to become the popliteal artery

9. Clinical Correlations (Exam Scoring Points)

Summary Mnemonic

• Iliopsoas (psoas + iliacus)
• Rectus femoris
• Sartorius
• Vastus medialis
• Vastus intermedius
• Vastus lateralis