Upper & Lower Limb

1. What this chapter covers, and how NEET PG actually tests it

Limb anatomy has more named structures than any candidate can hold, and NEET PG does not attempt to test them.

It tests a much narrower thing: what breaks, and what deficit that produces.

Three question archetypes account for almost all of it.

A nerve is compressed or stretched, and the stem describes a hand posture or a gait. A bone fractures at a site where an artery has to cross, and the stem asks about necrosis. A muscle loses its nerve supply, and the stem describes what the patient can no longer do.

In every case the deformity is not arbitrary — it is the unopposed action of whatever still works. That principle turns a memorised list of deformities into something you can reconstruct.

This chapter covers four areas: the brachial plexus, upper limb peripheral nerve lesions, fractures with a blood supply problem, and lower limb nerves and gait.

2. The brachial plexus

2.1 The organising structure

The plexus is formed from the ventral rami of C5 to T1, arranged in the sequence roots, trunks, divisions, cords, branches.

Three trunks form from the roots. Upper trunk from C5 and C6, middle trunk from C7 alone, lower trunk from C8 and T1.

Each trunk splits into anterior and posterior divisions, and these regroup into the lateral, posterior and medial cords, named for their relation to the axillary artery.

All the posterior divisions join to form the posterior cord, which is why the posterior cord supplies extensors — the radial and axillary nerves are its terminal branches.

That single relationship explains why a posterior cord lesion produces weakness of extension throughout the limb, rather than a patchy deficit.

2.2 Erb's palsy — the upper trunk

Erb-Duchenne palsy is an injury to C5 and C6, the upper trunk, at Erb's point.

The mechanism is forcible widening of the angle between the neck and the shoulder — a shoulder dystocia at delivery, or a fall onto the shoulder in an adult.

Now derive the posture rather than memorising it.

C5 and C6 supply the abductors and lateral rotators of the shoulder, the flexors of the elbow, and the supinator.

Losing those leaves the shoulder adducted and medially rotated, the elbow extended, and the forearm pronated.

That is the "waiter's tip" or "policeman's tip" position, and the wrist is typically flexed. Every component is the unopposed action of muscles that remain innervated.

Sensory loss is over the lateral aspect of the arm and forearm, and the biceps and brachioradialis reflexes are lost.

2.3 Klumpke's palsy — the lower trunk

Klumpke's palsy injures C8 and T1, the lower trunk, usually from forcible upward traction of the abducted arm — a breech delivery, or a fall while grasping at a support.

C8 and T1 supply the small muscles of the hand, so the result is a claw hand with wasting of the intrinsics.

T1 also carries the sympathetic outflow to the head and neck, which is why an associated Horner's syndrome — ptosis, miosis and anhidrosis — is a strong clue that the injury is at the lower trunk or root level.

The presence of Horner's therefore distinguishes a proximal lower trunk lesion from a distal ulnar nerve lesion producing a similar claw.

3. Upper limb nerve lesions

3.1 Radial nerve — extensors, so the wrist drops

The radial nerve descends in the spiral (radial) groove of the humerus, which is why a midshaft humeral fracture endangers it.

It supplies the triceps proximally and then all the extensors of the wrist and fingers.

Injury in the groove spares the triceps branch given off higher up, so elbow extension is preserved while the wrist drops.

Wrist drop is the signature, and finger extension at the metacarpophalangeal joints is lost with it.

Two other mechanisms are classically tested. Saturday night palsy results from prolonged compression of the nerve against the humerus by an arm draped over a chair back while unconscious.

Crutch palsy compresses the nerve in the axilla, which is more proximal, so here the triceps is also affected and elbow extension is weak.

Sensory loss is characteristically small — an area over the first dorsal interosseous space on the back of the hand — because of extensive overlap from adjacent nerves.

3.2 Ulnar nerve — the claw, and the paradox

The ulnar nerve is most vulnerable behind the medial epicondyle, and at the wrist in Guyon's canal.

It supplies most intrinsic hand muscles, including the medial two lumbricals.

Lumbricals flex the metacarpophalangeal joints and extend the interphalangeal joints, so losing them leaves the opposite: MCP extension with IP flexion of the ring and little fingers, which is the ulnar claw.

Now the point that generates a favourite question.

The ulnar paradox states that a more proximal lesion produces a less obvious claw.

The reason is that flexor digitorum profundus to the ring and little fingers is also ulnar-supplied. A proximal lesion at the elbow denervates it, so those fingers cannot flex at the distal interphalangeal joints.

A distal lesion at the wrist spares that muscle, so the profundus pulls the fingers into flexion unopposed and the claw looks far worse.

So a nastier-looking hand implies a more distal injury. The counterintuitive direction is exactly why it is asked.

Froment's sign tests the same nerve. Adductor pollicis is paralysed, so gripping paper between thumb and index finger recruits flexor pollicis longus, and the thumb interphalangeal joint visibly flexes.

3.3 Median nerve — the pointing index and the ape thumb

The median nerve is compressed in the carpal tunnel beneath the flexor retinaculum, and injured at the wrist in penetrating trauma or at the elbow in supracondylar fractures.

A high lesion at the elbow paralyses flexor digitorum superficialis and the radial half of profundus.

On attempting to make a fist, the index and middle fingers cannot flex, so the hand assumes the "pointing index" or hand of benediction posture.

A low lesion at the wrist spares the long flexors but denervates the thenar muscles.

Thenar wasting with loss of opposition gives the ape thumb deformity, where the thumb lies in the plane of the palm.

Carpal tunnel syndrome compresses the same nerve, producing nocturnal paraesthesia over the lateral three and a half digits with sparing of the thenar eminence skin.

That sparing has a precise anatomical cause. The palmar cutaneous branch arises proximal to the flexor retinaculum and passes superficial to it, so it escapes compression.

A patient with numbness over the palm's thenar skin therefore does not have carpal tunnel syndrome — the lesion must be more proximal.

3.4 Axillary nerve — the surgical neck

The axillary nerve winds around the surgical neck of the humerus, so it is injured by fracture there and by anterior shoulder dislocation.

It supplies deltoid and teres minor, so abduction beyond the initial range is lost.

Sensory loss occupies the "regimental badge" area over the lateral shoulder, which should be tested routinely after any shoulder dislocation before and after reduction.

4. Fractures with a blood supply problem

4.1 The shared principle

Some bones receive their arterial supply in a direction that runs from the fracture-prone region towards the fragment at risk.

When the supply must cross the fracture line to reach the distal fragment, that fragment dies. This is retrograde blood supply, and it explains two of the most heavily tested fractures in the exam.

4.2 The scaphoid

The scaphoid receives roughly 80% of its supply through branches of the radial artery entering at the dorsal ridge, distally.

Blood therefore flows retrogradely from distal to proximal to reach the proximal pole.

A fracture at the waist, the commonest site, interrupts that flow. The proximal pole loses its supply and is at high risk of avascular necrosis and non-union.

The clinical presentation is a fall on the outstretched hand with tenderness in the anatomical snuffbox, bounded by extensor pollicis longus medially and abductor pollicis longus with extensor pollicis brevis laterally.

Initial radiographs are frequently normal, and this is itself an examinable point. Clinical suspicion mandates immobilisation and repeat imaging or MRI rather than discharge.

The more proximal the fracture, the higher the necrosis risk, since a smaller proximal fragment is left entirely dependent on vessels crossing the break.

4.3 The femoral neck

The femoral head is supplied chiefly by retinacular branches of the medial circumflex femoral artery, which ascend along the femoral neck beneath the capsule.

The artery of the ligamentum teres contributes little in adults, and the contribution declines with age.

So an intracapsular femoral neck fracture disrupts the ascending retinacular vessels and threatens the head with avascular necrosis — the same geometry as the scaphoid, at a different scale.

This is the anatomical reason a displaced intracapsular fracture in an older patient is usually treated by replacing the head rather than fixing it.

An extracapsular intertrochanteric fracture lies below the level at which those vessels enter, so the head keeps its supply and internal fixation is appropriate.

The limb classically lies shortened and externally rotated after a displaced neck fracture, because the pull of the short external rotators is no longer balanced.

4.4 Two other fracture-nerve associations worth holding

Supracondylar humeral fracture in a child endangers the brachial artery and the median nerve, particularly its anterior interosseous branch.

Untreated ischaemia here produces Volkmann's ischaemic contracture, a flexion deformity from fibrosis of infarcted forearm flexors.

Colles fracture is a distal radial fracture with dorsal displacement, giving the dinner fork deformity after a fall on the outstretched hand.

Smith's fracture is its volar-displaced mirror image, sometimes called a reverse Colles.

5. Lower limb nerves and gait

5.1 The superior gluteal nerve and the Trendelenburg sign

Gluteus medius and minimus do not abduct the leg in normal walking. They stabilise the pelvis when standing on one leg.

Their job is to hold up the opposite, unsupported side of the pelvis during the stance phase.

So a lesion of the superior gluteal nerve makes the pelvis drop on the side opposite the lesion when the patient stands on the affected leg.

That drop is the Trendelenburg sign, and its direction is the detail exams test.

Bilateral involvement produces a waddling gait, since the pelvis dips alternately with each step.

This nerve is at risk during hip surgery and from an incorrectly placed intramuscular gluteal injection, which is why injections are given in the upper outer quadrant of the buttock.

5.2 The common peroneal nerve and foot drop

The common peroneal nerve winds around the neck of the fibula, where it lies subcutaneous against bone.

That position makes it the most commonly injured nerve in the lower limb, vulnerable to a tight plaster cast, prolonged leg crossing, or a fibular neck fracture.

Its deep branch supplies the dorsiflexors and its superficial branch the evertors.

Injury therefore causes foot drop with loss of eversion, and a high-stepping gait as the patient lifts the knee to clear the toes.

Sensory loss covers the dorsum of the foot and the lateral leg.

The eversion component is the discriminator. An L5 radiculopathy can also cause foot drop, but inversion, supplied by tibial-innervated tibialis posterior, remains a distinguishing test alongside the pattern of sensory loss.

5.3 The tibial and sciatic nerves

The tibial nerve supplies the posterior compartment, so its injury abolishes plantarflexion and inversion, giving a calcaneovalgus foot and loss of the ankle jerk.

It also supplies the sole's intrinsic muscles, and passes behind the medial malleolus in the tarsal tunnel.

The sciatic nerve carries both divisions, and is injured by posterior hip dislocation or by an intramuscular injection placed in the wrong quadrant.

The common peroneal division is more susceptible than the tibial, being more laterally placed and more tightly tethered, so a partial sciatic injury often presents as an isolated foot drop.

5.4 The femoral nerve

The femoral nerve supplies the quadriceps, so injury causes weak knee extension with a lost knee jerk, and the patient may lock the knee in extension to walk.

Sensory loss follows the anterior thigh, and continues along the medial leg through its saphenous branch.

The femoral triangle's contents run lateral to medial as nerve, artery, vein, with the femoral canal medial to the vein — which is where a femoral hernia descends.

7. Common exam traps

• Reversing the Trendelenburg sign's side. The pelvis drops on the side opposite the affected gluteal nerve, because the abductors hold up the unsupported side during stance.
• Reversing the ulnar paradox. A more distal lesion gives a more obvious claw, because flexor digitorum profundus is spared.
• Expecting large sensory loss with radial nerve injury. The autonomous zone is small, over the first dorsal interosseous space, because of overlap from adjacent nerves.
• Missing thenar skin sparing in carpal tunnel syndrome. The palmar cutaneous branch passes superficial to the retinaculum, so palmar thenar numbness points to a more proximal median lesion.
• Treating a normal radiograph as excluding a scaphoid fracture. Early films are frequently normal, and discharge is the error that leads to non-union.
• Confusing intracapsular and extracapsular hip fractures. Only the intracapsular fracture disrupts the ascending retinacular vessels and threatens the head, which is why the management differs.
• Forgetting to check the regimental badge area after shoulder dislocation. Axillary nerve status should be documented both before and after reduction.
• Attributing all foot drop to the common peroneal nerve. L5 radiculopathy also causes it; the state of inversion and the sensory pattern separate them.

Summary

• The brachial plexus runs roots, trunks, divisions, cords, branches from C5 to T1, and all posterior divisions form the posterior cord, which is why it supplies the extensors through the radial and axillary nerves.
• Erb's palsy is a C5 and C6 upper trunk injury giving the waiter's tip posture, each element being the unopposed action of surviving muscles.
• Klumpke's palsy is a C8 and T1 lower trunk injury giving a claw hand, and an accompanying Horner's syndrome localises the lesion proximally because T1 carries sympathetic fibres.
• Radial nerve injury in the spiral groove causes wrist drop with preserved elbow extension; an axillary lesion such as crutch palsy also weakens the triceps.
• The ulnar claw is loss of the medial two lumbricals; the ulnar paradox means a distal lesion looks worse because flexor digitorum profundus is spared.
• A high median lesion gives the pointing index on attempting a fist; a low lesion gives thenar wasting and the ape thumb.
• Carpal tunnel syndrome spares thenar skin sensation because the palmar cutaneous branch passes superficial to the flexor retinaculum.
• Axillary nerve injury follows surgical neck fracture or shoulder dislocation, causing deltoid weakness and regimental badge sensory loss.
• The scaphoid's supply enters distally and flows retrogradely, so a waist fracture puts the proximal pole at risk of avascular necrosis, and initial radiographs are often normal.
• The femoral head depends on ascending retinacular branches of the medial circumflex femoral artery, so intracapsular fractures risk avascular necrosis while extracapsular ones do not.
• Supracondylar humeral fracture threatens the brachial artery and can cause Volkmann's ischaemic contracture; Colles fracture gives the dinner fork deformity.
• Superior gluteal nerve injury produces a Trendelenburg sign with the pelvis dropping on the contralateral side, and a waddling gait if bilateral.
• The common peroneal nerve at the fibular neck is the most commonly injured lower limb nerve, giving foot drop with loss of eversion and a high-stepping gait.
• Tibial nerve injury abolishes plantarflexion and inversion with loss of the ankle jerk, and the peroneal division of the sciatic nerve is the more vulnerable of the two.
• Femoral nerve injury weakens knee extension and abolishes the knee jerk; the femoral triangle runs nerve, artery, vein from lateral to medial, with the canal medial to the vein.