🦵 NRE Step-1 — Anatomy Notes
Lower Limb Anatomy
High-Yield NRE Notes
Femoral triangle, sciatic nerve injuries, hip and knee anatomy, and compartment syndrome of the leg — with 10 free NRE-style MCQs focused on clinical vignettes.
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Femoral triangle borders
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Sciatic nerve divisions
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Leg compartments
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Free MCQs below
Overview
How Lower Limb Anatomy Is Tested in NRE
Lower limb anatomy is the second most heavily tested anatomy region in NRE Step-1, right after upper limb. The pattern is identical — a clinical scenario is given, and you must identify which nerve is injured, which compartment is involved, or which anatomical landmark explains the finding.
The three most consistently tested areas are sciatic nerve injuries and their divisions, the femoral triangle and its clinical access points, and compartment syndrome of the leg following tibial fractures. Master these three areas and the hip and knee joint anatomy, and you will answer the majority of lower limb questions correctly.
NRE exam pattern: Lower limb questions follow the same formula as upper limb — mechanism of injury or clinical finding → which nerve/structure is injured → what deficit results. “A patient has foot drop after a fibular neck fracture — which nerve?” “A patient cannot rise from a squat — which nerve and muscle?” Always reason from mechanism to anatomy to clinical deficit.
Femoral Triangle — Most Tested
Femoral Triangle — Boundaries and Contents
The femoral triangle is a key clinical landmark in the upper anterior thigh — it provides surgical and procedural access to the femoral vessels and is a classic site for hernia and lymph node examination.
| Boundary / Content | Detail | Key Clinical Point |
|---|---|---|
| Lateral border | Medial border of sartorius | Apex of triangle, continuous with adductor canal NRE |
| Medial border | Medial border of adductor longus | Forms the other side of the apex |
| Superior border (base) | Inguinal ligament | Femoral pulse palpated at mid-inguinal point |
| Floor | Iliopsoas + pectineus + adductor longus | Forms a gutter that continues into adductor canal |
| Contents (lateral to medial) | NAVEL: Nerve, Artery, Vein, Empty space, Lymphatics | Femoral nerve is OUTSIDE the femoral sheath NRE |
⚠️ Femoral sheath — must know for NRE:
The femoral sheath encloses the femoral artery, vein, and the empty space (femoral canal) — but NOT the femoral nerve, which lies outside the sheath, lateral to the artery.
Femoral canal — the medial-most compartment of the sheath. Contains lymphatics (e.g. Cloquet’s node) and allows the femoral vein to expand. It is the site of femoral hernia — a femoral hernia is more likely to strangulate than an inguinal hernia because the canal has rigid, sharp boundaries (lacunar ligament medially).
The femoral sheath encloses the femoral artery, vein, and the empty space (femoral canal) — but NOT the femoral nerve, which lies outside the sheath, lateral to the artery.
Femoral canal — the medial-most compartment of the sheath. Contains lymphatics (e.g. Cloquet’s node) and allows the femoral vein to expand. It is the site of femoral hernia — a femoral hernia is more likely to strangulate than an inguinal hernia because the canal has rigid, sharp boundaries (lacunar ligament medially).
Nerve Injuries — NRE Favourite
Lower Limb Nerve Injuries — Mechanism to Deficit
Every nerve injury question in NRE gives you the mechanism first. Learn the common injury sites for each lower limb nerve and the exact deficit that results.
Common Peroneal Nerve (L4–S2)
Foot drop — most recognisable deficit
OriginTerminal branch of sciatic nerve, divides at the popliteal fossa apex
Injury siteNeck of fibula — winds superficially around it, very exposed. Fibular neck fracture, tight plaster cast, prolonged leg crossing
DeficitFoot drop — loss of dorsiflexion (tibialis anterior) and eversion (peroneus longus/brevis). High-stepping gait to clear the foot
SensationLoss over dorsum of foot and anterolateral leg
Key testCannot dorsiflex or evert foot. Foot slaps down on walking (foot drop gait)
Tibial Nerve (L4–S3)
Cannot stand on toes — posterior compartment
OriginTerminal branch of sciatic nerve, runs through popliteal fossa into posterior leg compartment
Injury siteKnee dislocation (posterior), Baker’s cyst compression, tarsal tunnel at ankle
DeficitLoss of plantarflexion (gastrocnemius/soleus) and toe flexion — cannot stand on tiptoes
Tarsal tunnelCompression behind medial malleolus → burning sole pain, sensory loss on plantar foot
Key testCannot plantarflex against resistance. Tinel’s sign positive behind medial malleolus in tarsal tunnel syndrome
Sciatic Nerve (L4–S3)
Posterior hip dislocation — combined deficit
OriginLargest nerve in the body — from sacral plexus, exits via greater sciatic foramen below piriformis
Injury sitePosterior hip dislocation, gluteal IM injection in wrong quadrant, piriformis syndrome
DeficitCombined common peroneal + tibial deficit — foot drop + loss of plantarflexion + sensory loss below knee (except medial leg, saphenous)
Safe IM injectionUpper outer quadrant of buttock — avoids sciatic nerve which runs through lower/medial gluteal region
Key testStraight leg raise test (sciatic stretch) — pain suggests nerve root irritation, e.g. disc prolapse
Femoral Nerve (L2–L4)
Cannot extend knee — quadriceps
OriginLargest branch of lumbar plexus, enters thigh lateral to femoral artery (outside femoral sheath)
Injury siteFemoral triangle trauma, pelvic fracture, psoas haematoma, hip surgery retraction injury
DeficitQuadriceps weakness — cannot extend knee. Loss of patellar (knee jerk) reflex
SensationLoss over anterior and medial thigh, and medial leg/foot via saphenous nerve
Key testWeak knee extension, absent knee jerk reflex (L3–L4)
Superior Gluteal Nerve (L4–S1)
Trendelenburg sign — abductor weakness
OriginSacral plexus, exits via greater sciatic foramen above piriformis
Injury siteHip surgery (posterior approach), IM injection in wrong site, intertrochanteric fracture
DeficitParalysis of gluteus medius + minimus → pelvis drops to the UNAFFECTED side on standing on the affected leg (positive Trendelenburg sign)
GaitTrendelenburg/waddling gait — pelvis dips with each step on the affected side
Key testAsk patient to stand on one leg — contralateral pelvis drops (sign named for the supporting/stance leg)
Obturator Nerve (L2–L4)
Weak adduction — obturator hernia risk
OriginLumbar plexus, exits pelvis via obturator canal into medial thigh
Injury sitePelvic surgery, obturator hernia, anterior hip dislocation
DeficitWeak thigh adduction (adductor group paralysed). Sensory loss over medial thigh
Howship–Romberg signPain along medial thigh on hip extension/rotation — classic sign of obturator hernia compressing the nerve
Key testResisted thigh adduction weakness with medial thigh pain
Fracture — Nerve Correlations
Fractures and Their Nerve Injury Associations
NRE loves pairing a specific fracture or dislocation with its associated nerve injury. This table is one of the most direct recall areas in lower limb anatomy — learn every pair.
| Fracture / Injury | Nerve at Risk | Clinical Deficit |
|---|---|---|
| Neck of fibula fracture | Common peroneal nerve NRE | Foot drop, loss of dorsiflexion + eversion, sensory loss over dorsum of foot |
| Posterior hip dislocation | Sciatic nerve NRE | Combined foot drop + loss of plantarflexion + sensory loss below knee |
| Anterior hip dislocation | Obturator nerve | Weak thigh adduction, medial thigh sensory loss |
| Intertrochanteric fracture / hip surgery | Superior gluteal nerve | Trendelenburg gait, gluteus medius/minimus weakness |
| Posterior knee dislocation | Tibial nerve + popliteal artery NRE | Loss of plantarflexion. Popliteal artery injury risks limb ischaemia — vascular emergency |
| Pelvic fracture / psoas haematoma | Femoral nerve | Quadriceps weakness, absent knee jerk, anteromedial thigh sensory loss |
| Tibial shaft fracture | Compartment syndrome (anterior compartment most common) | Severe pain, pain on passive stretch, pallor, pulselessness (late) — surgical emergency |
| Calcaneal/talar fracture | Tibial nerve branches at tarsal tunnel | Plantar sensory loss, burning sole pain |
Hip and Knee Joint
Hip and Knee — Applied Joint Anatomy
The hip and knee joints are tested both as pure anatomy and as clinical examination findings — ligament tests, dislocation patterns, and blood supply to the femoral head are all recurring NRE themes.
| Structure | Key Facts | Clinical Relevance |
|---|---|---|
| Blood supply to femoral head | Medial circumflex femoral artery (main supply in adults) + obturator artery via ligamentum teres (minor, mainly in children) | Intracapsular (subcapital) femoral neck fracture disrupts retinacular vessels → avascular necrosis of femoral head NRE |
| Anterior cruciate ligament (ACL) | Prevents anterior translation of tibia on femur. Runs from anterior tibia to posterior femur | Anterior drawer test / Lachman’s test positive. Classic non-contact pivot injury in sport |
| Posterior cruciate ligament (PCL) | Prevents posterior translation of tibia. Strongest knee ligament | Posterior drawer test positive. Dashboard injury (knee hits dashboard in RTA) |
| Medial collateral ligament (MCL) | Resists valgus (lateral) force. Attached to medial meniscus | Valgus stress test positive. “Unhappy triad” — ACL + MCL + medial meniscus injury together NRE |
| Menisci | Medial meniscus less mobile, more often torn. Lateral meniscus more mobile, more circular | McMurray’s test for meniscal tears. Locking/clicking knee on movement |
| Popliteal fossa | Diamond-shaped space behind knee. Contents from superficial to deep: tibial nerve, popliteal vein, popliteal artery (NVA from posterior to anterior) | Popliteal artery aneurysm/injury risks limb ischaemia. Baker’s cyst can compress tibial nerve/vein |
Femoral neck fracture — intracapsular vs extracapsular: Intracapsular (subcapital, transcervical) fractures disrupt the retinacular blood supply → high risk of avascular necrosis of the femoral head, especially in displaced fractures. Extracapsular (intertrochanteric, subtrochanteric) fractures spare this blood supply — much lower AVN risk, treated with fixation rather than replacement.
Leg Compartments and Compartment Syndrome
Four Compartments of the Leg — Compartment Syndrome
Compartment syndrome is a surgical emergency tested heavily in both NRE Step-1 anatomy and Step-2 clinical decision-making. The leg has four osteofascial compartments, each enclosed by deep fascia — raised pressure within any one compartment compromises blood flow and nerve function.
| Compartment | Key Muscles | Nerve / Artery |
|---|---|---|
| Anterior | Tibialis anterior, EHL, EDL — dorsiflexors | Deep peroneal nerve, anterior tibial artery — most commonly affected in compartment syndrome NRE |
| Lateral | Peroneus longus + brevis — evertors | Superficial peroneal nerve |
| Superficial posterior | Gastrocnemius, soleus, plantaris — plantarflexors | Sural nerve (cutaneous) |
| Deep posterior | Tibialis posterior, FDL, FHL — toe flexors | Tibial nerve, posterior tibial artery |
Compartment syndrome — the 6 P’s and the most important sign: Pain (out of proportion to injury, worsened by passive stretch — earliest and most reliable sign), Paraesthesia, Pallor, Pulselessness, Paralysis, Poikilothermia. Pulselessness and pallor are LATE signs — do not wait for them. Pain on passive stretch of the toes is the single most sensitive early clinical sign. Diagnosis is clinical ± compartment pressure measurement. Treatment is emergency fasciotomy — delay risks Volkmann’s ischaemic contracture and limb loss.
Rapid Revision
High-Yield Key Points — NRE Checklist
🦵 Lower Limb Anatomy — Must Know for NRE
- Femoral triangle contents (lateral to medial) — NAVEL: Nerve, Artery, Vein, Empty space, Lymphatics. Nerve is OUTSIDE the femoral sheath
- Femoral canal = medial-most part of femoral sheath — site of femoral hernia, high strangulation risk due to rigid boundaries
- Common peroneal nerve at fibular neck → foot drop, loss of dorsiflexion + eversion, high-stepping gait
- Sciatic nerve — posterior hip dislocation → combined foot drop + loss of plantarflexion. Safe IM injection site = upper outer quadrant of buttock
- Superior gluteal nerve → gluteus medius/minimus paralysis → Trendelenburg sign (pelvis drops on contralateral side when standing on affected leg)
- Obturator nerve → weak adduction + medial thigh pain. Howship–Romberg sign = obturator hernia
- Femoral nerve → quadriceps weakness + absent knee jerk + anteromedial thigh/leg sensory loss
- Intracapsular femoral neck fracture → disrupts retinacular vessels → avascular necrosis of femoral head NRE
- Unhappy triad — ACL + MCL + medial meniscus injury together — classic sporting valgus injury with rotation
- Popliteal fossa contents (superficial to deep) — tibial nerve, popliteal vein, popliteal artery
- Posterior knee dislocation → popliteal artery injury → limb-threatening ischaemia — vascular emergency, not just orthopaedic
- Anterior compartment of leg most commonly affected in compartment syndrome — deep peroneal nerve + anterior tibial artery
- Compartment syndrome — pain on passive stretch is the earliest sign. Pulselessness/pallor are LATE. Treatment = emergency fasciotomy
- Tarsal tunnel syndrome — tibial nerve compression behind medial malleolus → burning plantar foot pain
Free Practice MCQs
Test Yourself — 10 NRE-Style Questions
These questions follow the NRE clinical vignette format — mechanism → anatomy → deficit. Use the notes above to work through each scenario.
Question 1
A 19-year-old footballer sustains a direct blow to the lateral aspect of his knee, fracturing the neck of the fibula. He now has foot drop and cannot evert his foot. Sensation is reduced over the dorsum of the foot. Which nerve is injured?
A. Tibial nerve
B. Common peroneal nerve ✓
C. Femoral nerve
D. Sciatic nerve
Explanation: The common peroneal nerve winds superficially around the neck of the fibula, making it highly vulnerable to fracture, direct trauma, or compression (tight cast, leg crossing). Injury causes foot drop (loss of tibialis anterior) and loss of eversion (peroneus longus/brevis), with sensory loss over the dorsum of the foot and anterolateral leg. The tibial nerve supplies plantarflexion (intact here). Femoral nerve affects knee extension. Sciatic nerve injury would cause a combined deficit including loss of plantarflexion.
Question 2
A 70-year-old woman falls and sustains a displaced intracapsular fracture of the femoral neck. Six months later, imaging shows collapse of the femoral head. What is the underlying mechanism?
A. Damage to the femoral nerve
B. Disruption of the retinacular blood supply causing avascular necrosis ✓
C. Injury to the obturator artery
D. Compression of the sciatic nerve
Explanation: The femoral head’s main blood supply in adults comes from the medial circumflex femoral artery via retinacular vessels running along the femoral neck within the joint capsule. An intracapsular fracture (subcapital or transcervical) tears these vessels, especially when displaced, cutting off blood supply to the femoral head → avascular necrosis and eventual collapse. Extracapsular fractures (intertrochanteric) spare this supply. This is why displaced intracapsular fractures in the elderly are usually treated with hip replacement rather than fixation.
Question 3
Following a posterior hip dislocation in a road traffic accident, a young man develops foot drop and is unable to plantarflex his foot. Sensation is lost over the entire foot except the medial side. Which nerve is injured?
A. Common peroneal nerve only
B. Femoral nerve
C. Sciatic nerve ✓
D. Obturator nerve
Explanation: Posterior hip dislocation is the classic mechanism for sciatic nerve injury — the femoral head displaces posteriorly and stretches or compresses the nerve as it exits the greater sciatic foramen. Since the sciatic nerve gives rise to both the tibial and common peroneal nerves, injury produces a combined deficit: foot drop (peroneal) AND loss of plantarflexion (tibial). Sensation is preserved only on the medial leg/foot, which is supplied by the saphenous nerve (a branch of the femoral nerve, not the sciatic).
Question 4
A 60-year-old man undergoes a posterior approach total hip replacement. Postoperatively, he has a positive Trendelenburg sign — when standing on the operated leg, his pelvis drops on the opposite side. Which nerve was most likely damaged?
A. Inferior gluteal nerve
B. Superior gluteal nerve ✓
C. Obturator nerve
D. Pudendal nerve
Explanation: The superior gluteal nerve supplies gluteus medius and minimus, the main hip abductors that stabilise the pelvis during single-leg stance. Damage during the posterior approach to hip surgery causes a positive Trendelenburg sign — the pelvis drops on the side OPPOSITE to the leg being stood on, because the abductors on the standing (stance) side fail to hold the pelvis level. Inferior gluteal nerve supplies gluteus maximus (hip extension, rising from sitting), not abduction.
Question 5
A 35-year-old man undergoes open reduction and internal fixation of a tibial shaft fracture. Six hours later, he complains of severe pain in the leg, worse on passive extension of his toes. Pulses are still palpable. What is the most appropriate immediate management?
A. Reassure and wait for pulses to disappear before acting
B. Elevate the limb and give analgesia, review in the morning
C. Urgent fasciotomy — do not wait for pulses to disappear ✓
D. MRI to confirm diagnosis before intervening
Explanation: This is acute compartment syndrome, a surgical emergency. Pain out of proportion to the injury, worsened by passive stretch of the toes, is the earliest and most reliable clinical sign — it precedes pulselessness and pallor, which are LATE findings. Waiting for pulses to disappear risks irreversible muscle and nerve necrosis (Volkmann’s ischaemic contracture) and limb loss. Diagnosis is clinical; if in doubt, measure compartment pressure, but treatment (fasciotomy) should not be delayed for imaging.
Question 6
A 17-year-old footballer twists his knee during a tackle, with his foot planted and the knee forced into valgus with rotation. He reports a popping sensation and his knee gives way. Examination reveals positive anterior drawer test, valgus instability, and joint line tenderness medially. What is this injury pattern called?
A. Posterior cruciate ligament tear alone
B. Unhappy triad — ACL + MCL + medial meniscus ✓
C. Lateral meniscus tear alone
D. Patellar tendon rupture
Explanation: The “unhappy triad” (O’Donoghue’s triad) results from a valgus force combined with rotation on a planted foot — a classic contact sports injury. It involves the ACL (positive anterior drawer/Lachman’s), the MCL (valgus instability), and the medial meniscus (which is anchored to the MCL and torn alongside it). The combination of a popping sensation, instability, and medial joint line tenderness after a valgus-rotation mechanism is the classic NRE description of this triad.
Question 7
A 45-year-old woman complains of burning pain and tingling on the sole of her foot, worse at night, with a positive Tinel’s sign on percussion behind the medial malleolus. What is the diagnosis and which nerve is involved?
A. Morton’s neuroma — digital nerve
B. Tarsal tunnel syndrome — tibial nerve ✓
C. Common peroneal nerve compression
D. Sural nerve entrapment
Explanation: Tarsal tunnel syndrome is the lower limb equivalent of carpal tunnel syndrome — the tibial nerve is compressed as it passes behind the medial malleolus, beneath the flexor retinaculum. It causes burning pain and paraesthesia over the sole of the foot (plantar surface), worse at night, with a positive Tinel’s sign on percussion over the tunnel. Morton’s neuroma causes pain between the metatarsal heads, not behind the malleolus.
Question 8
A 50-year-old man presents with a swelling in the groin that increases on coughing. On examination, the swelling is below and lateral to the pubic tubercle. He develops sudden severe pain and the swelling becomes tender and irreducible. What is the diagnosis and the major clinical concern?
A. Inguinal hernia — risk of testicular atrophy
B. Femoral hernia — high risk of strangulation ✓
C. Saphena varix — risk of venous thrombosis
D. Psoas abscess — risk of sepsis
Explanation: A femoral hernia protrudes through the femoral canal, which lies below and lateral to the pubic tubercle (distinguishing it from an inguinal hernia, which is above and medial to the pubic tubercle). Femoral hernias have a high risk of strangulation because the femoral canal has rigid boundaries — the lacunar ligament medially, the femoral vein laterally, and the inguinal ligament anteriorly — that do not allow the contents to reduce easily, especially as the neck of the sac narrows further with the femoral canal’s natural shape.
Question 9
A 28-year-old man sustains a posterior knee dislocation in a motorbike accident. On examination, the foot is cold, pale, and pulseless distally. What is the most urgent concern and which structure is most likely injured?
A. Common peroneal nerve — foot drop only, not urgent
B. Popliteal artery — limb-threatening ischaemia, vascular emergency ✓
C. Tibial nerve — sensory loss only
D. Long saphenous vein — minor bleeding risk
Explanation: The popliteal artery is tethered proximally and distally as it passes through the popliteal fossa, making it vulnerable to traction injury during knee dislocation, even when the dislocation has spontaneously reduced. A cold, pale, pulseless foot after knee trauma is a vascular emergency — popliteal artery injury risks limb loss within hours if not promptly diagnosed (CT angiography) and revascularised. This must always be actively excluded after any knee dislocation, even if the knee looks reduced.
Question 10
A patient is unable to rise from a deep squat and has weakness extending the knee. The knee jerk reflex is absent. Sensation is reduced over the anterior and medial thigh and the medial leg. Which nerve is responsible and which reflex arc is affected?
A. Sciatic nerve — ankle jerk (S1–S2)
B. Femoral nerve — knee jerk (L3–L4) ✓
C. Obturator nerve — no reflex arc
D. Common peroneal nerve — no reflex arc
Explanation: The femoral nerve (L2–L4) supplies the quadriceps femoris, the main knee extensor, and carries the afferent and efferent limbs of the knee jerk (patellar) reflex (L3–L4). Injury causes weak knee extension (difficulty rising from a squat or climbing stairs), an absent knee jerk, and sensory loss over the anterior/medial thigh and medial leg/foot (via the saphenous nerve, its terminal cutaneous branch). The ankle jerk tests S1–S2 via the tibial nerve, not the femoral nerve.
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