Upper and lower limb anatomy for FCPS Part 1
Limb anatomy questions are injury questions. A fracture, a cut or a dislocation damages a nerve, and you are asked what the patient cannot do or feel.
On this page
The pattern behind every limb question
Limb anatomy can look like a list of hundreds of muscles. In the paper, almost every question follows one chain:
injury, then nerve or vessel at risk, then muscles lost, then movement lost or sensory loss, then named sign
If you learn the chain for about 15 classic injuries, most limb questions become recognisable.
Upper limb: injury to nerve
| Injury | Nerve at risk | What you see |
|---|---|---|
| Surgical neck of humerus fracture, shoulder dislocation | Axillary | Weak abduction (15–90°), numb "regimental badge" patch |
| Shaft of humerus fracture | Radial (spiral groove) | Wrist drop; triceps usually spared |
| Supracondylar fracture (child) | Median (anterior interosseous), brachial artery | Cannot make "OK" sign; risk of Volkmann contracture |
| Medial epicondyle fracture | Ulnar | Claw hand of ring and little fingers, Froment sign |
| Slashed wrist | Median and ulnar | Thenar and intrinsic loss |
| Radial head fracture | Posterior interosseous | Finger drop without wrist drop |
| Axillary surgery | Long thoracic | Winged scapula |
The ulnar paradox: a lesion at the elbow gives a less obvious claw than one at the wrist, because flexor digitorum profundus is also lost and cannot pull the fingers into a claw.
Brachial plexus
- Roots C5 to T1, then trunks, divisions, cords and branches.
- Erb palsy (C5–C6): arm hangs by the side, internally rotated, forearm pronated. "Waiter's tip". Typical after shoulder dystocia or a fall on the shoulder.
- Klumpke palsy (C8–T1): claw hand from loss of the small hand muscles, sometimes with Horner syndrome. Typical when the arm is pulled up, or from a Pancoast tumour or cervical rib.
The hand
- Median nerve muscles at the wrist: LOAF (lateral two lumbricals, opponens pollicis, abductor pollicis brevis, flexor pollicis brevis).
- Everything else intrinsic belongs to the ulnar nerve, including adductor pollicis and all interossei.
- Carpal tunnel syndrome spares sensation over the palm, because the palmar cutaneous branch passes above the tunnel. Common in pregnancy, hypothyroidism, diabetes and rheumatoid arthritis.
- Anatomical snuffbox tenderness after a fall means scaphoid fracture until proven otherwise. The radial artery runs across its floor.
Lower limb: injury to nerve
| Injury | Nerve at risk | What you see |
|---|---|---|
| Fibular neck fracture, tight plaster | Common peroneal | Foot drop, loss of eversion, numb dorsum of foot |
| Posterior hip dislocation | Sciatic | Foot drop plus hamstring weakness |
| Wrong-site buttock injection | Sciatic or superior gluteal | Avoid by injecting in the upper outer quadrant |
| Hip surgery (lateral approach) | Superior gluteal | Positive Trendelenburg sign |
| Femoral catheterisation, iliopsoas bleed | Femoral | Weak knee extension, lost knee jerk |
| Tight belt, obesity, pregnancy | Lateral cutaneous nerve of thigh | Burning anterolateral thigh (meralgia paraesthetica) |
To tell common peroneal palsy from an L5 root lesion, test inversion. It is preserved in peroneal palsy (tibial nerve) and weak in L5 radiculopathy.
Femoral triangle and canal
- Boundaries: inguinal ligament above, sartorius laterally, adductor longus medially.
- Contents from lateral to medial: femoral Nerve, Artery, Vein, then the femoral canal (empty space and lymph node). "NAVY".
- The femoral canal is bounded by the inguinal ligament in front, pectineal ligament behind, lacunar ligament medially and femoral vein laterally. Its sharp medial edge is why femoral hernias strangulate.
Knee and leg
- The unhappy triad is anterior cruciate ligament, medial collateral ligament and medial meniscus, after a blow to the outside of the knee.
- The medial meniscus is attached to the medial collateral ligament, so it is injured more often than the lateral.
- Popliteus unlocks the extended knee.
- Compartment syndrome is commonest in the anterior compartment of the leg after tibial fractures. The earliest sign is pain on passive stretch. Pulses are often still present.
Foot and ankle
- Ankle reflex is S1. Knee reflex is L3–L4.
- Great toe extension tests L5.
- The posterior tibial pulse sits behind the medial malleolus. The great saphenous vein lies in front of it, which is the classic cutdown site.
Common traps
- The axillary nerve passes through the quadrangular space, not the triangular space.
- The radial nerve at the spiral groove usually spares triceps.
- Dorsalis pedis is absent in a minority of healthy people, so its absence alone is not diagnostic.
- Avascular necrosis follows scaphoid waist and femoral neck fractures because blood enters these bones from the distal end.
How to revise limb anatomy in a week
- Day 1: brachial plexus and its lesions.
- Day 2: upper limb nerves by fracture site, and the hand.
- Day 3: upper limb vessels, joints and fractures.
- Day 4: lumbosacral plexus, gluteal region and femoral triangle.
- Day 5: knee, leg compartments and foot.
- Days 6 to 7: mixed injury questions. For each wrong answer, write out the full chain from injury to sign.
Practise these chains in the FCPS-1 Prep question bank. Limb questions link to topic reviews covering the whole nerve, from its roots to its sensory area.
Read next: Anatomy high-yield · Neuroanatomy that repeats · Surgery ward survival tips
Frequently asked questions
Is limb anatomy more important for the surgery group?
It appears in Paper 1 for everyone. Surgery & Allied candidates meet more of it again in Paper 2, at an applied level.
Should I learn every muscle's origin and insertion?
No. Learn nerve supply, main action and the classic injuries. Origins and insertions are rarely the point of a question.
What is the fastest way to learn nerve injuries?
Learn them by fracture or injury site rather than by nerve. That is how the questions present them.