Femoral triangle
Femoral triangle, femoral sheath, femoral canal and adductor canal anatomy
Femoral triangle, femoral sheath, femoral ring and canal, adductor canal and the femoral artery's branches and anastomoses for FCPS Part 1. Explains the order of the contents, femoral hernia, nerve blocks, and how the thigh survives arterial blockage.
Boundaries of the femoral triangle
A triangular space in the upper anterior thigh through which the main vessels enter the limb.
| Part | Formed by |
|---|---|
| Base (superior) | Inguinal ligament |
| Lateral | Medial border of sartorius |
| Medial | Medial border of adductor longus |
| Apex | Where sartorius crosses adductor longus; continues as the adductor canal |
| Floor (lateral to medial) | Iliopsoas, pectineus, adductor longus (a gutter) |
| Roof | Skin, superficial fascia with superficial inguinal nodes and the great saphenous vein, and the fascia lata pierced by the saphenous opening |
- The saphenous opening is a gap in the fascia lata about 3–4 cm below and lateral to the pubic tubercle, covered by the cribriform fascia. Through it pass the great saphenous vein (to join the femoral vein), small superficial arteries and lymphatics.
Contents and their order
From lateral to medial: femoral nerve, femoral artery, femoral vein, the femoral canal (an empty space), and deep inguinal lymph nodes (the highest is Cloquet's node). From medial to lateral the order simply reverses: canal, vein, artery, nerve.
- The femoral nerve lies outside the femoral sheath, in the iliac compartment behind the iliacus fascia, and breaks into branches within a few centimetres.
- The femoral artery enters at the mid-inguinal point (halfway between the ASIS and the pubic symphysis); its pulse is used for arterial puncture and catheter access. The midpoint of the inguinal ligament (ASIS to pubic tubercle) marks the deep inguinal ring instead.
- The femoral vein lies just medial to the artery; it is used for central access when neck or arm veins cannot be used.
- At the apex the femoral artery lies in front of the femoral vein, with the profunda vessels behind them, so a wound here can injure several large vessels at once.
- A non-tender, expansile, pulsatile swelling below the inguinal ligament is a femoral artery aneurysm (or a pseudoaneurysm after catheterisation). A saphena varix is a soft compressible swelling with a cough thrill that empties on lying flat. A psoas abscess (tuberculosis of the spine) tracks within the psoas sheath and points in the triangle lateral to the vessels, near the lesser trochanter.
Femoral sheath
A funnel-shaped downward extension of abdominal fascia around the upper 3–4 cm of the vessels: transversalis fascia forms its front and iliacus (iliopsoas) fascia its back. It lets the vessels glide during hip movement.
| Compartment | Contents |
|---|---|
| Lateral | Femoral artery, femoral branch of the genitofemoral nerve |
| Intermediate | Femoral vein |
| Medial (femoral canal) | Lymphatics, fat, the lymph node of Cloquet; space allowing the vein to expand |
So the sheath contains the femoral artery, femoral vein and femoral canal, but not the femoral nerve.
Femoral ring, canal and hernia
The femoral ring is the upper opening of the canal, about 1 cm wide.
| Boundary | Structure |
|---|---|
| Anterior | Inguinal ligament |
| Posterior | Pectineal ligament (of Cooper) on the pectineal line, and pectineus |
| Medial | Lacunar ligament |
| Lateral | Femoral vein |
- A femoral hernia passes through the ring and down the canal, then turns forward through the saphenous opening, appearing below and lateral to the pubic tubercle. An inguinal hernia appears above and medial to it.
- Femoral hernias are commoner in women (wider pelvis), but inguinal hernias are still the commonest hernia in women. The ring is narrow and rigid on three sides, so strangulation is common.
- To release a tight neck the surgeon cuts medially, through the lacunar ligament. An abnormal obturator artery, arising from the inferior epigastric artery, may run there (the "corona mortis") and bleed severely.
Adductor (subsartorial, Hunter's) canal
An intermuscular tunnel in the middle third of the thigh from the apex of the femoral triangle to the adductor hiatus in adductor magnus.
| Wall | Structure |
|---|---|
| Anterolateral | Vastus medialis |
| Posterior (floor) | Adductor longus above, adductor magnus below |
| Roof (anteromedial) | Fibrous sheet under sartorius |
- Contents: femoral artery, femoral vein, saphenous nerve, nerve to vastus medialis (in the upper part), and the descending genicular artery arising at its lower end. The femoral nerve trunk and the great saphenous vein are not in the canal.
- The vessels pass through the hiatus to become the popliteal artery and vein. The saphenous nerve does not pass through the hiatus; it emerges between sartorius and gracilis to run with the great saphenous vein down the medial leg.
- A stab or an abscess in the canal threatens the femoral vessels and the saphenous nerve (numb medial leg and foot). A collection here can compress the thin-walled femoral vein.
Nerve blocks: femoral versus adductor canal
| Block | Nerves reached | Motor effect | Use |
|---|---|---|---|
| Femoral nerve block | Whole femoral nerve, below the inguinal ligament lateral to the artery | Weak quadriceps; risk of falls | Hip fracture analgesia |
| Adductor canal block | Mainly the saphenous nerve (sensory) plus small articular twigs | Spares most quadriceps motor fibres | Knee replacement, allowing early walking |
The adductor canal block covers only the front and medial side of the knee, so the back of the knee still needs other analgesia.
Branches of the femoral artery
- Superficial branches: superficial epigastric, superficial circumflex iliac, superficial and deep external pudendal.
- Profunda femoris: arises from the lateral side about 4 cm below the inguinal ligament, runs deep behind adductor longus, and is the main supply of the thigh. It gives the medial circumflex femoral (the key supply of the femoral head), the lateral circumflex femoral and four perforating arteries.
- Descending genicular artery in the adductor canal, joining the anastomosis around the knee.
Anastomoses around the hip
| Anastomosis | Site | Arteries | Importance |
|---|---|---|---|
| Trochanteric | Near the trochanteric fossa | Superior gluteal, inferior gluteal, medial and lateral circumflex femoral | Supplies the femoral head through retinacular branches |
| Cruciate | Behind the upper femur at the level of the lesser trochanter | Inferior gluteal (internal iliac), medial and lateral circumflex femoral, first perforating (profunda) | Bypass route between the internal iliac and the femoral system |
- In occlusion of the external iliac and common femoral arteries, blood flows from the internal iliac through the inferior gluteal artery into the cruciate anastomosis and fills the profunda femoris, keeping the thigh alive.
- The cruciate anastomosis links the internal iliac and femoral arteries, not the femoral and external iliac.
Remember Cruciate for bypass, trochanteric for the femoral head.
Femoral neck angles
- Neck–shaft (inclination) angle: about 125° in adults (larger in children). Less is coxa vara, more is coxa valga.
- Anteversion angle: the femoral neck points forwards relative to the transverse axis of the femoral condyles by about 10–15° in adults (larger in infants). Excess anteversion causes in-toeing.
Exam trap The femoral nerve is not inside the femoral sheath; the sheath holds the artery, vein and canal.
Exam trap The femoral pulse is at the mid-inguinal point, not the midpoint of the inguinal ligament.
How FCPS asks about this
- Order from medial to lateral in the femoral triangle: canal (lymphatics), vein, artery, nerve.
- Contents of the femoral sheath: artery, vein and canal; the nerve lies outside.
- Boundaries of the femoral ring: inguinal ligament in front, pectineal ligament behind, lacunar ligament medially, femoral vein laterally.
- Contents of the adductor canal: femoral vessels, saphenous nerve, nerve to vastus medialis; not the femoral nerve trunk or great saphenous vein.
- Why an adductor canal block helps early walking after knee replacement: it spares most quadriceps motor fibres.
- Internal iliac contributor to the cruciate anastomosis: inferior gluteal artery; it keeps the thigh alive when the femoral inflow is blocked.
- Normal adult femoral anteversion: about 10–15°; neck–shaft angle about 125°.
Test yourself on this topic
5 of the 21 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
During CPR, femoral venous access is needed and the femoral pulse is felt with each compression. Where should the needle go?
Answer: About 1 cm medial to the pulse
Just below the inguinal ligament, the femoral vein lies immediately medial to the femoral artery within the femoral sheath, with the nerve lateral to the artery outside the sheath. Inserting the needle about 1 cm medial to the palpable arterial pulse, below the ligament, gives the best chance of entering the vein while avoiding the artery and nerve. Ultrasound guidance is preferred when available.
Femoral vein access
- NAVY: nerve, artery, vein, Y-fronts
- Vein medial to artery
- Insert 1 cm medial to pulse
Remember Vein is medial: find the pulse, then move in.
Exam trap Puncture above the inguinal ligament risks retroperitoneal bleeding.
Why the other options are wrong
- About 1 cm lateral to the pulse This is the femoral nerve.
- Directly over the pulse This is the artery.
- At the pubic tubercle Too medial; the femoral canal lies here.
- Two finger-breadths below the ASIS Too lateral; well away from the vessels.
For a femoral arterial puncture for blood gases, where is the artery felt most reliably?
Answer: Mid-inguinal point
The femoral artery passes under the inguinal ligament at the mid-inguinal point, halfway between the anterior superior iliac spine and the pubic symphysis. The midpoint of the inguinal ligament (halfway between ASIS and pubic tubercle) marks the deep inguinal ring instead. In the femoral sheath the order from lateral to medial is nerve (outside the sheath), artery, vein, then the femoral canal.
Femoral artery landmark
- Mid-inguinal point
- Halfway ASIS to pubic symphysis
- Lateral to medial: NAVY
- Deep ring: midpoint of ligament
Remember Artery at the mid-inguinal point; ring at the midpoint of the ligament.
Exam trap Puncture too high (above the ligament) risks retroperitoneal bleeding.
Why the other options are wrong
- Midpoint of the inguinal ligament That marks the deep ring.
- Pubic tubercle Landmark for inguinal and femoral hernias.
- Anterior superior iliac spine Too lateral.
- Medial to the femoral vein The vein lies medial to the artery.
After knee replacement, why is an adductor canal block preferred to a femoral nerve block for early walking?
Answer: It spares most quadriceps motor fibres
The adductor canal, under sartorius in the middle third of the thigh, contains the femoral artery and vein, the saphenous nerve and the nerve to vastus medialis. A block here numbs the medial knee and leg through the saphenous nerve but avoids the main motor branches of the femoral nerve to the rest of the quadriceps, which leave higher in the femoral triangle. Quadriceps strength is largely preserved, so falls are less likely and early walking is possible.
Adductor canal block
- Canal under sartorius
- Contains saphenous nerve, nerve to vastus medialis
- Spares most quadriceps power
Remember Adductor canal block: pain relief with legs that still work.
Exam trap The posterior knee still needs other analgesia because the block covers only the front and medial side.
Why the other options are wrong
- It blocks the sciatic nerve as well The sciatic nerve is in the posterior thigh.
- It also blocks the obturator nerve The obturator nerve is not in the canal.
- It lasts several days longer Duration depends on the drug, not the site.
- It avoids blocking the saphenous nerve The saphenous nerve is the main target of the block.
For a femoral nerve block, why is the anaesthetic placed lateral to the artery rather than inside the femoral sheath?
Answer: Femoral nerve lies outside it
The femoral sheath is a funnel of extraperitoneal fascia (transversalis fascia in front, iliacus fascia behind) carried down below the inguinal ligament. It encloses the femoral artery laterally, the femoral vein in the middle and the femoral canal medially. The femoral nerve lies lateral to the sheath, deep to the fascia iliaca, so local anaesthetic is placed lateral to the artery and under the fascia iliaca rather than in the sheath.
Femoral sheath
- Lateral: artery
- Middle: vein
- Medial: canal
- Nerve outside, under fascia iliaca
Remember NAVEL: the N is outside the sheath.
Exam trap Injecting into the sheath risks intravascular injection without blocking the nerve.
Why the other options are wrong
- The sheath contains only the vein It contains the artery, the vein and the canal.
- The nerve lies within the canal The canal holds lymphatics and fat, not the nerve.
- The sheath ends above the ligament It extends 3 to 4 cm below the ligament.
- The sheath is pierced by the nerve The nerve stays outside the sheath.
A 65-year-old smoker has cramping calf pain after walking 200 metres, relieved by rest. The femoral pulse is strong but the popliteal and foot pulses are absent. Angiography shows a short occlusion with good collaterals. Which of the following is the most likely site of the occlusion?
Answer: Distal superficial femoral artery
Calf claudication with a normal femoral pulse and absent popliteal pulse localises disease to the superficial femoral artery. The commonest site of atherosclerotic occlusion in the leg is the distal superficial femoral artery in the adductor (Hunter's) canal, where the artery is fixed by the tendon of adductor magnus at the hiatus. Collaterals from the profunda femoris to the genicular arteries often keep the limb viable.
SFA occlusion
- Commonest leg occlusion: distal SFA
- At adductor canal (Hunter's)
- Femoral pulse present, popliteal absent
- Collaterals from profunda
Remember Calf claudication, femoral pulse present: SFA in Hunter's canal.
Exam trap Pain is felt one level below the arterial disease.
Why the other options are wrong
- Common iliac artery Would weaken the femoral pulse and cause buttock or thigh pain.
- Profunda femoris artery Rarely occluded; it provides the collaterals.
- Anterior tibial artery origin Would not remove the popliteal pulse.
- Dorsalis pedis artery Would not remove the popliteal pulse.
Where this sits in the CPSP syllabus
- Paper 1: Lower limb, Femoral triangle, femoral sheath, adductor canal
- Surgery & Allied, Paper 2: Lower limb, Femoral triangle, femoral sheath, adductor canal