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AN20.1-10 | General Features, Joints, radiographs & surface marking (Lower Limb) — Part 3

Important Bony Landmarks of the Lower Limb

Palpate these on yourself as you read (AN20.7):

Figure: Important Bony Landmarks of the Lower Limb

Pelvis/Hip level:
- Highest point of iliac crest: L4 vertebral level (used for lumbar puncture landmark — iliac crest line)
- Posterior superior iliac spine (PSIS): Posterior dimples of Venus — at the level of S2 vertebra and the posterior end of the sacroiliac joint
- Iliac tubercle: 5 cm posterior to ASIS, at L5 level
- Pubic tubercle: Medial to the inguinal ligament origin; at the medial end. Spermatic cord in males passes superior to pubic tubercle. Femoral hernia passes inferior and lateral to it.
- Ischial tuberosity: The 'sitting bone' — palpable in the gluteal fold with hip flexed
- Adductor tubercle: Superior to medial femoral condyle — origin of adductor magnus

Figure: Pelvis/Hip level

Knee level:
- Tibial tuberosity: Attachment of patellar ligament (L4 reflex — patellar reflex)
- Head of fibula: Lateral side below knee joint. Common peroneal nerve winds around its neck — vulnerable to injury (foot drop)
- Medial and lateral condyles of femur and tibia: Define the knee joint line

Ankle/Foot level:
- Medial malleolus: Distal end of tibia — tip is 1 cm higher than lateral malleolus
- Lateral malleolus: Distal end of fibula — 1 cm lower and more posterior than medial malleolus
- Sustentaculum tali: Shelf of calcaneus on its medial surface, 2.5 cm below medial malleolus — supports talus; FHL passes beneath it in a groove
- Tuberosity of 5th metatarsal: Base of 5th MT on lateral border of foot — peroneus brevis inserts here; avulsion fracture site in ankle sprains
- Tuberosity of navicular: Medial mid-foot — tibialis posterior inserts here; accessory navicular is a common variant
- Calcaneal tuberosity: Heel — plantar fascia and intrinsic foot muscles originate here

Figure: Ankle/Foot level

Surface Projection of Lower Limb Arteries

AN20.8 — Arterial Pulse Palpation (practice on your own leg)

Figure: Surface Projection of Lower Limb Arteries

1. Femoral Artery
- Surface marking: Mid-inguinal point (midpoint between ASIS and pubic symphysis) — the femoral artery enters the thigh here under the inguinal ligament
- Pulse palpation: Press firmly just below the inguinal ligament at the mid-inguinal point
- Clinical: Femoral artery cannulation for angiography, IABP insertion, cardiac catheterisation, CPR in PEA
- Note: Femoral vein is just medial to the artery; femoral nerve is just lateral

2. Popliteal Artery
- Surface marking: Deep in the popliteal fossa — runs from the adductor hiatus (junction of middle and lower thirds of femur) to the lower border of popliteus
- Pulse palpation: Knee slightly flexed, press fingers into the popliteal fossa and feel for the deep pulsation
- Hardest pulse to palpate; absent in Baker's cyst, popliteal artery entrapment syndrome

3. Anterior Tibial Artery
- Passes through the interosseous membrane near the proximal fibula to enter the anterior compartment
- Pulse palpation: On the dorsum of the foot as the dorsalis pedis artery — lateral to extensor hallucis longus tendon, between the 1st and 2nd metatarsals

4. Posterior Tibial Artery
- Pulse palpation: In the groove between medial malleolus and the Achilles tendon (posterior to medial malleolus)

5. Dorsalis Pedis
- Continuation of anterior tibial artery on the dorsum of the foot
- Palpation: Lateral to extensor hallucis longus tendon between 1st and 2nd metatarsals
- Absent in ~10% normal population (not always pathological)
- Critical for diabetic foot assessment: Compare both feet; absent dorsalis pedis + absent posterior tibial = critical ischaemia → risk of amputation

Surface Projection of Lower Limb Nerves & Veins

AN20.9 — Surface Projections

Femoral Nerve:
- Lateral to femoral artery at the mid-inguinal point
- Divides just below the inguinal ligament into multiple branches
- Saphenous nerve is its longest cutaneous branch — accompanies GSV, supplies medial leg/foot (L4)
- Femoral nerve block: Inject lateral to the femoral artery at the inguinal ligament — anaesthetises anterior thigh and knee

Figure: AN20.9 — Surface Projections

Sciatic Nerve:
- Exits the greater sciatic foramen below piriformis, midway between ischial tuberosity and greater trochanter
- Runs down the posterior thigh to the popliteal fossa where it bifurcates
- Sciatic nerve injection danger zone: Intramuscular injections in the gluteal region must be in the upper outer quadrant to avoid the sciatic nerve

Common Peroneal Nerve:
- Winds around the neck of the fibula (most superficial major nerve in the body at this point)
- Injury here → foot drop (loss of dorsiflexion, eversion; loss of sensation on dorsum of foot)
- Causes: Tight cast, prolonged crossing of legs, fibula neck fracture

Figure: Common Peroneal Nerve

Great Saphenous Vein (GSV):
- Anterior to medial malleolus → medial leg → posterior to medial femoral condyle → saphenous opening → femoral vein
- GSV cutdown: Classic emergency access site — anterior to medial malleolus (always present, predictable anatomy)

Figure: Great Saphenous Vein (GSV)

Small Saphenous Vein (SSV):
- Posterior to lateral malleolus → midline posterior leg → popliteal fossa → popliteal vein

Figure: Small Saphenous Vein (SSV)

Saphenous Opening (Fossa Ovalis):
- Oval deficiency in deep fascia, 4 cm below and lateral to pubic tubercle
- GSV drains into femoral vein here. Femoral hernia protrudes through the femoral canal (medial to femoral vein), covered by cribriform fascia, emerges below and lateral to pubic tubercle.

Figure: Saphenous Opening (Fossa Ovalis)