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AN15.1-5 | Front & Medial side of thigh — Part 1
CLINICAL SCENARIO
A 45-year-old woman with pulmonary tuberculosis is evaluated for a painless, fluctuant swelling in the upper thigh that appears below the inguinal ligament and lateral to the femoral artery. She has no local bone tenderness. Her spine X-ray shows erosion of L3–L4 vertebral bodies.
How did pus from a lumbar vertebral infection reach the thigh? Why does it track below the inguinal ligament into the femoral triangle? Why is it painless?
Meanwhile, her husband presents to the same clinic with a strangulated femoral hernia — he has had a groin swelling for three months that suddenly became tender and irreducible.
Both conditions hinge on the anatomy of the femoral triangle and the structures passing beneath the inguinal ligament. Let us understand this region precisely.
WHY THIS MATTERS
The anterior thigh is one of the highest-yield regions in clinical anatomy because:
- Femoral hernia — a surgical emergency with high strangulation risk; distinguishing it from inguinal hernia requires knowing the femoral ring anatomy
- Psoas abscess — tuberculosis of lumbar spine; pus tracks through the psoas sheath into the femoral triangle; common in India
- Femoral artery — primary access site for cardiac catheterisation, angiography, and IABP; you will cannulate it in surgery and cardiology
- Femoral nerve block — used in pain management for hip and femur fractures
- Saphenous nerve — the terminal branch of the femoral nerve; medial leg sensation; damaged in varicose vein surgery
RECALL
From your pelvis study and lower limb bones:
- The inguinal ligament runs from ASIS to the pubic tubercle
- The femur has a lesser trochanter (iliopsoas attachment) and a linea aspera (adductor attachments)
- The femoral nerve is from the lumbar plexus (L2, L3, L4)
Now you will trace those structures as they enter the thigh and understand their clinical relationships.
Muscles of the Anterior Thigh
The anterior thigh is dominated by the quadriceps femoris group and the iliopsoas.
Iliopsoas (composite muscle):
- Iliacus: origin — iliac fossa; nerve — femoral nerve (L2, L3)
- Psoas major: origin — T12–L5 vertebral bodies and intervertebral discs; nerve — direct branches L1–L3
- Combined insertion: lesser trochanter of femur
- Action: primary hip flexor; also flexes the trunk when the leg is fixed (sitting up from supine)
- Clinical: psoas abscess — TB of lumbar spine → pus tracks through psoas sheath → presents as fluctuant swelling in femoral triangle (cold abscess, no fever)
Quadriceps femoris (four heads, one insertion via patellar ligament onto tibial tuberosity):
- Rectus femoris: anterior inferior iliac spine (AIIS) + acetabular rim; only bi-articular head — both hip flexion and knee extension
- Vastus medialis: medial lip of linea aspera; its oblique fibres (VMO) prevent lateral patellar subluxation
- Vastus lateralis: lateral lip of linea aspera and greater trochanter
- Vastus intermedius: anterior shaft of femur
- Nerve supply: femoral nerve (L2, L3, L4) for all four heads
- Action: knee extension (all); hip flexion (rectus femoris only)
- Clinical: quadriceps wasting is the earliest sign of disuse after knee injury; vastus medialis wasting → patellar subluxation
Sartorius:
- Longest muscle in the body
- Origin: ASIS → oblique course medially → insertion: upper medial tibia (as part of pes anserinus with gracilis and semitendinosus)
- Nerve: femoral nerve (L2, L3)
- Action: flexion, abduction, lateral rotation of hip; knee flexion
- Forms the lateral boundary of the femoral triangle
Muscles of the Medial Thigh (Adductors)
The medial compartment is the adductor group — all supplied mainly by the obturator nerve (L2, L3, L4) except pectineus (femoral nerve).
| Muscle | Origin | Insertion | Nerve |
|---|---|---|---|
| Pectineus | Pecten pubis | Pectineal line of femur | Femoral nerve (+ obturator) |
| Adductor longus | Body of pubis | Middle linea aspera | Obturator nerve |
| Adductor brevis | Body and inf. ramus of pubis | Upper linea aspera | Obturator nerve |
| Adductor magnus | Ischiopubic ramus + ischial tuberosity | Linea aspera + adductor tubercle | Obturator n. + tibial n. (ischial part) |
| Gracilis | Body and inf. ramus of pubis | Upper medial tibia (pes anserinus) | Obturator nerve |
All primary adductors: adduction of the hip; some assist flexion/extension.
Adductor magnus is the largest and has a special adductor hiatus (opening) near the adductor tubercle — this allows the femoral vessels to pass through into the popliteal fossa, transitioning from the adductor canal to become popliteal vessels.
Obturator nerve (L2, L3, L4):
- Enters the thigh through the obturator canal (obturator foramen)
- Divides into anterior and posterior divisions around adductor brevis
- Anterior division: supplies adductor longus, adductor brevis, gracilis, pectineus (shared); sensory — medial thigh
- Posterior division: supplies obturator externus, adductor magnus (adductor part)
- Clinical: hip joint pain can be referred to the medial knee via the obturator nerve (Hilton's law application)
The Femoral Triangle
The femoral triangle is a triangular depression in the upper medial thigh, immediately below the inguinal ligament.
Boundaries:
- Superiorly (base): inguinal ligament
- Laterally: medial border of sartorius
- Medially: medial border of adductor longus
Floor (from lateral to medial): iliopsoas → pectineus → (adductor longus)
Roof: fascia lata (deep fascia of thigh) + cribriform fascia (over the saphenous opening)
Contents (from lateral to medial — mnemonic: NAVEL):
- N — femoral Nerve (L2–L4; most lateral)
- A — femoral Artery (central)
- V — femoral Vein (medial to artery)
- E — Empty space (femoral canal)
- L — Lymphatics (deep inguinal lymph nodes in femoral canal)
Note: the femoral nerve is outside the femoral sheath; the femoral sheath is a fascial sleeve that encloses only the femoral artery, vein, and canal (lymphatics + fat).
Femoral sheath and canal:
The femoral sheath is a funnel-shaped fascial investment (derived from extraperitoneal fascia), containing:
- Lateral compartment: femoral artery
- Middle compartment: femoral vein
- Medial compartment: femoral canal (lymphatics, fat, occasional Cloquet's node)
The femoral canal is ~1.25 cm long; its opening (femoral ring) is bounded by: inguinal ligament (anteriorly), femoral vein (laterally), lacunar ligament (medially), pectineal ligament (posteriorly).
Femoral hernia: abdominal contents push through the femoral ring into the femoral canal → exits at the saphenous opening. It appears below and lateral to the pubic tubercle (vs inguinal hernia which is above and medial). Femoral hernias have a higher strangulation risk due to the rigid boundaries (especially the lacunar ligament).