Hand anatomy
Hand anatomy: thenar, hypothenar, lumbricals, interossei, spaces and infections
Intrinsic hand muscles, their median and ulnar supply, hand deformities, palmar spaces, finger infections and tendon injuries for FCPS Part 1. Predict any deformity from which muscles have lost their nerve, and any infection's spread from the compartment it starts in.
One of the most tested topics in past papers
Intrinsic muscles and their nerves
The median nerve supplies five small hand muscles: the lateral two lumbricals and the three thenar muscles, abductor pollicis brevis, opponens pollicis and the superficial head of flexor pollicis brevis. The thenar three are supplied by its recurrent (motor) branch, which curls round the distal edge of the flexor retinaculum just under the skin, so a shallow cut over the thenar eminence can paralyse opposition. The ulnar nerve supplies all the rest.
| Group | Muscles | Nerve | Action |
|---|---|---|---|
| Thenar | Abductor pollicis brevis, flexor pollicis brevis, opponens pollicis | Median (recurrent branch) | Abduction, flexion, opposition of thumb |
| Adductor pollicis | Oblique and transverse heads | Ulnar (deep branch) | Adducts thumb; grips a card against the index finger |
| Hypothenar | Abductor, flexor and opponens digiti minimi | Ulnar (deep branch) | Little finger movements |
| Palmaris brevis | Subcutaneous, over hypothenar eminence | Ulnar (superficial branch, its only muscle) | Wrinkles skin, aids grip |
| Lumbricals 1–2 | Arise from FDP tendons | Median | Flex MCP, extend IP joints |
| Lumbricals 3–4 | Arise from FDP tendons | Ulnar | Flex MCP, extend IP joints |
| Palmar interossei (4) | Metacarpals | Ulnar | Adduct fingers towards the middle finger |
| Dorsal interossei (4) | Metacarpals | Ulnar | Abduct fingers away from the middle finger |
- Lumbricals and interossei together flex the MCP joints and extend the IP joints (the "intrinsic plus" position used for writing and fine work). This is why the ulnar nerve is the nerve of fine finger movement.
- Card test: hold a card between extended fingers (palmar interossei). Froment sign: to grip paper between thumb and index the patient flexes the thumb IP joint with FPL (median) because adductor pollicis (ulnar) is weak.
- Inability to abduct and adduct the fingers with normal sensation points to the deep branch of the ulnar nerve (purely motor), for example at the hook of hamate.
- Martin–Gruber anastomosis: in some people median fibres cross to the ulnar nerve in the forearm, so ulnar-type muscles such as the first dorsal interosseous keep working after a high ulnar division.
Remember Median in the hand: the lateral two lumbricals and the three thenar muscles. Ulnar: everything else.
Hand deformities and why they happen
| Deformity | Lesion | Mechanism |
|---|---|---|
| Ulnar claw (ring and little fingers) | Ulnar nerve at the wrist | Lumbricals 3–4 and interossei lost; extensor digitorum hyperextends the MCP joints unopposed, FDP and FDS flex the IP joints |
| Less clawing with a high ulnar lesion | Ulnar nerve at the elbow (ulnar paradox) | Medial FDP also lost, so the IP joints are not pulled into flexion |
| Ape hand / flat thenar eminence | Median nerve at the wrist | Thenar muscles lost; thumb lies in the plane of the palm, no opposition |
| Hand of benediction on trying to make a fist | Median nerve above the elbow | FDS and lateral FDP lost; index and middle fingers stay straight |
| Complete claw hand | Lower trunk (C8–T1) or combined median and ulnar | All lumbricals and interossei lost |
| Wrist drop | Radial nerve | Grip is weak too, because the flexors cannot work with the wrist flexed |
Exam trap In ulnar claw the MCP joints are hyperextended by extensor digitorum (radial nerve), not by any ulnar muscle.
Sympathetic supply: stellate ganglion block
Sympathetic fibres to the upper limb leave the cord in the upper thoracic segments and relay mainly in the stellate (cervicothoracic) ganglion. A successful block gives ipsilateral Horner syndrome with a warm, dry, pink hand: vasoconstrictor tone is lost (vasodilatation) and sweat glands lose their sympathetic cholinergic drive.
Palmar fascia, spaces and Dupuytren
- The palmar aponeurosis is a triangular sheet continuous with palmaris longus that protects the vessels and nerves beneath it.
- Dupuytren contracture: nodular thickening and shortening of the palmar aponeurosis (myofibroblast proliferation), usually flexing the ring and little fingers at the MCP and PIP joints. Associations: family history (autosomal dominant tendency), male sex, northern European ancestry, alcohol, smoking, diabetes, epilepsy treated with phenytoin, liver disease.
- Deep palmar spaces: the thenar space (lateral, deep to the index flexor tendons) and the midpalmar space (medial, deep to the middle, ring and little tendons), separated by a septum from the aponeurosis to the third metacarpal.
- Synovial sheaths: the little finger sheath continues as the ulnar bursa around the flexor tendons; the FPL sheath continues as the radial bursa. Infection can pass between them (horseshoe abscess) and up into the forearm (Parona's space, in front of pronator quadratus). The index, middle and ring finger sheaths normally end at the MCP joints.
Finger infections
| Infection | Site | Key point |
|---|---|---|
| Paronychia | Nail fold | Commonest hand infection; *Staphylococcus aureus* |
| Felon (pulp space) | Fingertip pulp | Fibrous septa from skin to bone divide the pulp into closed compartments, so pus raises pressure fast: severe throbbing pain, risk of distal phalanx osteomyelitis; the epiphysis is spared because its blood supply enters proximally |
| Acute suppurative flexor tenosynovitis | Flexor tendon sheath, often after a puncture | Kanavel signs: finger held flexed, fusiform swelling, tenderness along the sheath, pain on passive extension; urgent drainage to save the tendon |
| Deep space infection | Thenar or midpalmar space | Loss of palmar concavity, swollen dorsum |
Tendon injuries and fractures
- Mallet finger: extensor tendon avulsed at the DIP joint; the fingertip droops.
- Boutonnière deformity: central slip tear over the PIP joint; lateral bands slip forward, giving PIP flexion and DIP hyperextension.
- Swan neck deformity: PIP hyperextension and DIP flexion, typical of rheumatoid arthritis.
- Jersey finger: the FDP tendon is avulsed from the distal phalanx when a gripping finger, usually the ring finger, is forcibly extended (caught in a shirt). The finger still bends at the PIP joint (FDS), so test DIP flexion while holding the middle phalanx.
- Testing flexors: FDS is tested by holding the other fingers straight (which immobilises the shared FDP) and asking for PIP flexion; FDP by blocking the PIP joint and asking for DIP flexion.
- Flexor zone II ("no man's land"): from the distal palmar crease to the FDS insertion at the middle of the middle phalanx. Both tendons lie in a tight shared fibro-osseous sheath, so repairs form adhesions and glide poorly.
- Metacarpal fractures: boxer's fracture is the neck of the fifth metacarpal. A spiral shaft fracture can rotate, so on making a fist the finger crosses over or under its neighbour (scissoring); check alignment with the fingers flexed and the nail plates aligned.
Blood supply
- Superficial palmar arch: mainly the ulnar artery; at the level of the distal border of the fully extended thumb.
- Deep palmar arch: mainly the radial artery; about 1 cm proximal, at the level of the metacarpal bases, with the deep branch of the ulnar nerve.
- Allen test checks ulnar collateral flow before radial cannulation or radial artery harvest.
How FCPS asks about this
- Muscles of the recurrent branch of the median nerve: abductor pollicis brevis, flexor pollicis brevis, opponens pollicis.
- Cannot abduct or adduct the fingers, sensation intact: deep branch of the ulnar nerve (interossei).
- Card test or Froment sign: ulnar nerve; palmar interossei and adductor pollicis.
- MCP hyperextension in ulnar claw: extensor digitorum unopposed; clawing is worse in low lesions (ulnar paradox).
- Swollen flexed finger with pain on passive extension after a prick: acute suppurative flexor tenosynovitis (Kanavel signs).
- Why a pulp abscess is so painful: fibrous septa create closed compartments.
- Cannot flex the fingertip after a jersey grab, or zone II repair failure: FDP avulsion; shared tight sheath causes adhesions.
Test yourself on this topic
5 of the 56 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
Which muscles abduct the fingers away from the middle finger, and which nerve supplies them?
Answer: Dorsal interossei; ulnar nerve
PAD and DAB: Palmar interossei ADduct (towards the middle finger axis), Dorsal interossei ABduct (away from it). There are four dorsal and three (or four) palmar interossei, all supplied by the deep branch of the ulnar nerve. Interossei and lumbricals also flex the MCP joints and extend the IP joints (the 'lumbrical position'). Card test (holding paper between fingers) tests palmar interossei; spreading fingers against resistance tests dorsal interossei. Ulnar nerve lesions cause wasting of the first dorsal interosseous.
Interossei
- PAD: palmar adduct
- DAB: dorsal abduct
- All ulnar (deep branch)
- Also flex MCP, extend IP
Remember PAD and DAB.
Why the other options are wrong
- Palmar interossei; ulnar nerve Adduct fingers.
- Lumbricals; median nerve Flex MCP and extend IP joints.
- Dorsal interossei; median nerve Wrong nerve.
- Extensor digitorum; radial nerve Extends fingers, does not abduct them significantly.
A man with an ulnar nerve injury at the wrist has his ring and little fingers hyperextended at the MCP joints and flexed at the IP joints. Which of the following muscles causes the MCP hyperextension?
Answer: Extensor digitorum
The medial two lumbricals and the interossei, all supplied by the ulnar nerve, normally flex the MCP joints and extend the IP joints through the extensor expansion. When they are paralysed, extensor digitorum, acting mainly on the proximal phalanx, hyperextends the MCP joints without opposition, while FDS and FDP, intact in a wrist lesion, flex the IP joints: the claw. The index and middle fingers are spared because their lumbricals are supplied by the median nerve. If MCP hyperextension is blocked with a splint, extensor digitorum can then extend the IP joints, which is the basis of claw-correcting splints.
Mechanism of the ulnar claw
- Paralysed lumbricals and interossei
- Unopposed extensor digitorum: MCP hyperextension
- Long flexors: IP flexion
- Index and middle spared (median lumbricals)
Remember Claw: extensor digitorum lifts the MCP, long flexors curl the IP joints.
Exam trap Clawing is worse in a low ulnar lesion because the long flexors are still working.
Why the other options are wrong
- Extensor indicis Acts only on the index finger.
- Flexor digitorum profundus Flexes the IP joints; does not extend the MCP joints.
- Palmar interossei Paralysed in this lesion; they normally flex the MCP joints.
- Third and fourth lumbricals Paralysed; they normally flex the MCP joints.
A boxer has an intra-articular fracture of the base of the first metacarpal. A small palmar fragment stays attached to the trapezium, but the shaft is pulled proximally and radially. Which muscle mainly displaces the shaft?
Answer: Abductor pollicis longus
This is a Bennett fracture–dislocation of the thumb carpometacarpal joint. The small palmar-ulnar fragment is held in place by the strong anterior oblique (beak) ligament to the trapezium, while abductor pollicis longus, which inserts into the base of the first metacarpal, pulls the shaft proximally and radially, subluxing the joint. Adductor pollicis adds adduction of the shaft. Because these pulls redisplace the fracture, it usually needs reduction and wire or screw fixation to protect this important saddle joint from arthritis.
Bennett fracture
- Base of first metacarpal, intra-articular
- Fragment held by anterior oblique ligament
- Shaft pulled by abductor pollicis longus
- Usually needs fixation
Remember Bennett: APL pulls the shaft away from its fragment.
Exam trap Do not call every thumb base fracture a Bennett; a comminuted Y or T fracture is a Rolando fracture.
Why the other options are wrong
- Adductor pollicis Adds adduction of the shaft but is not the main proximal pull.
- Extensor pollicis brevis Inserts on the proximal phalanx, not the metacarpal base.
- Flexor pollicis longus Inserts on the distal phalanx.
- Opponens pollicis Inserts along the metacarpal shaft and rotates it, without this pull.
A small glass cut over the thenar eminence leaves a woman unable to oppose her thumb, with normal skin sensation. Which nerve is injured?
Answer: Recurrent (motor) branch of median nerve
The recurrent branch of the median nerve arises just distal to the flexor retinaculum and curls back into the thenar muscles, supplying abductor pollicis brevis, opponens pollicis and the superficial head of flexor pollicis brevis. It lies just under the skin and fascia of the thenar eminence, so even a small cut can divide it; because it is purely motor, skin sensation stays normal. Loss of palmar abduction and opposition follows. The palmar cutaneous branch is sensory, the deep ulnar branch supplies adductor pollicis, and the anterior interosseous nerve supplies forearm muscles.
Recurrent median branch
- Arises distal to the flexor retinaculum
- Curls back into the thenar muscles
- Purely motor; lies superficially
- Loss: opposition and abduction
Remember Thenar cut, weak thumb, normal skin: recurrent branch.
Exam trap Normal sensation does not exclude a median nerve injury in the hand; test thumb abduction separately.
Why the other options are wrong
- Palmar cutaneous branch of median nerve Purely sensory, to the skin over the thenar eminence.
- Deep branch of the ulnar nerve Supplies adductor pollicis and the interossei, not abduction or opposition.
- Anterior interosseous nerve Supplies FPL, lateral FDP and pronator quadratus in the forearm.
- Superficial branch of the radial nerve Purely sensory to the back of the hand.
A 20-year-old man has a spiral fracture of the shaft of the ring finger metacarpal. When he makes a loose fist, the ring finger crosses under the little finger. Which of the following does this indicate?
Answer: Rotational malalignment of the fracture
When the fingers are flexed, all the fingertips normally point towards the scaphoid tubercle and do not overlap. Spiral and oblique fractures of the metacarpal or phalangeal shafts tend to rotate, and even a small rotation at the metacarpal produces a large overlap (scissoring) at the fingertip. Rotation does not remodel and badly disturbs grip, so it needs reduction and usually fixation, whereas some angulation of a fourth or fifth metacarpal neck fracture is well tolerated. Shortening mainly causes a lost knuckle and extensor lag, not crossing.
Finger scissoring
- Flexed fingertips point to the scaphoid tubercle
- Crossing = rotational malalignment
- Common with spiral shaft fractures
- Rotation does not remodel
Remember Check a fist: crossing fingers mean rotation.
Exam trap Rotation is easily missed with the fingers straight; always examine the flexed fingers and the nail plates.
Why the other options are wrong
- Shortening of the metacarpal Causes a dropped knuckle and extensor lag, not crossing.
- Flexor tendon rupture The finger would not flex at all at the affected joints.
- Volar angulation of the metacarpal neck Well tolerated in the ring and little fingers and causes no scissoring.
- Ulnar nerve palsy Would cause clawing and numbness, not crossing.
Where this sits in the CPSP syllabus
- Paper 1: Upper limb, Hand: thenar and hypothenar muscles, lumbricals, interossei, palmar spaces, Dupuytren
- Surgery & Allied, Paper 2: Upper limb, Hand: thenar and hypothenar muscles, lumbricals, interossei, palmar spaces, Dupuytren
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