Forearm and carpal tunnel
Forearm compartments, wrist, carpal tunnel and anatomical snuffbox: FCPS anatomy
Forearm flexor and extensor compartments, carpal tunnel, Guyon's canal, anatomical snuffbox and carpal bone injuries for FCPS Part 1. Learn which nerve supplies each muscle group, what passes through each tunnel, and how scaphoid, lunate and wrist fractures present.
Flexor (anterior) compartment
Most flexors arise from the common flexor origin on the medial epicondyle and are supplied by the median nerve. Only one and a half muscles belong to the ulnar nerve.
| Layer | Muscles | Nerve |
|---|---|---|
| Superficial | Pronator teres, flexor carpi radialis, palmaris longus, flexor carpi ulnaris | Median; FCU: ulnar |
| Intermediate | Flexor digitorum superficialis (FDS) | Median |
| Deep | Flexor digitorum profundus (FDP), flexor pollicis longus (FPL), pronator quadratus | Lateral half of FDP, FPL, pronator quadratus: anterior interosseous (median); medial half of FDP: ulnar |
- FDS flexes the proximal interphalangeal (PIP) joint; FDP is the only muscle that flexes the distal interphalangeal (DIP) joint.
- Anterior interosseous nerve palsy: cannot flex the tip of the thumb and index finger, so the "OK" sign becomes a flat pinch; pronation with the elbow flexed is weak; there is no sensory loss (purely motor branch).
- The median nerve enters the forearm between the two heads of pronator teres, where repetitive pronation can compress it (pronator syndrome; see the nerves chapter).
Extensor (posterior) compartment
All extensors are supplied by the radial nerve or its deep branch, the posterior interosseous nerve (PIN), which reaches the back of the forearm by passing through supinator (under its upper edge, the arcade of Frohse). Most arise from the common extensor origin on the lateral epicondyle.
| Group | Muscles | Nerve |
|---|---|---|
| Lateral (mobile wad) | Brachioradialis, extensor carpi radialis longus (ECRL) | Radial nerve proper, above the elbow |
| Superficial | Extensor carpi radialis brevis (ECRB), extensor digitorum, extensor digiti minimi, extensor carpi ulnaris | ECRB: radial or PIN; rest: PIN |
| Deep | Supinator, abductor pollicis longus (APL), extensor pollicis brevis (EPB), extensor pollicis longus (EPL), extensor indicis | PIN |
- PIN palsy (radial head fracture, Monteggia injury, ganglion or lipoma at the elbow): the patient cannot extend the fingers or thumb or abduct the thumb (APL), but can still extend the wrist, which drifts radially, because ECRL is supplied above the division and pulls unopposed by ECU. There is no sensory loss.
- Supination uses biceps (musculocutaneous nerve) and supinator (radial nerve, deep branch). Loss of supination implies both, or the C5–6 roots that feed them.
- Radial deviation (abduction) of the wrist comes from ECRL, ECRB and FCR acting with APL; ulnar deviation from ECU and FCU.
- In the forearm the radial artery is accompanied laterally by the superficial branch of the radial nerve under brachioradialis.
Remember Front of the forearm: median except one and a half (FCU, medial FDP). Back of the forearm: all radial.
Carpal bones
Proximal row from lateral to medial: scaphoid, lunate, triquetrum, pisiform. Distal row: trapezium, trapezoid, capitate, hamate. The pisiform is a sesamoid in the FCU tendon.
| Bone problem | Key facts |
|---|---|
| Scaphoid fracture (commonest carpal fracture) | Fall on outstretched hand; snuffbox tenderness; first X-ray may be normal. Blood enters distally (dorsal carpal branch of radial artery), so a waist fracture risks avascular necrosis of the proximal pole and non-union |
| Lunate dislocation (commonest carpal dislocation) | Lunate tips volarly into the carpal tunnel, acutely compressing the median nerve |
| Kienböck disease | Avascular necrosis of the lunate (middle of the proximal row): wrist pain, stiffness; sclerosis then collapse on X-ray; young adults, often with a short ulna |
| Hook of hamate fracture | Golfers, batsmen; can injure the ulnar nerve in Guyon's canal |
Carpal tunnel
- Roof: the flexor retinaculum, attached laterally to the scaphoid tubercle and trapezium and medially to the pisiform and hook of hamate. The carpal bones form the floor and walls.
- Contents: median nerve, four FDS tendons, four FDP tendons and the FPL tendon (nine tendons and one nerve).
- Not in the tunnel: ulnar nerve and artery (Guyon's canal), palmaris longus (superficial to the retinaculum), FCR (in its own split of the retinaculum), and the palmar cutaneous branch of the median nerve, which leaves above the wrist and passes superficial to it.
- Carpal tunnel syndrome: pain and tingling in the thumb, index, middle and lateral half of the ring finger, worse at night and relieved by shaking the hand; weakness and wasting of abductor pollicis brevis and opponens (weak opposition); thenar skin sensation spared. Tinel and Phalen tests support it; nerve conduction studies confirm it.
| Cause group | Examples |
|---|---|
| Fluid retention | Pregnancy, hypothyroidism (myxoedema), heart failure |
| Tissue thickening | Acromegaly, rheumatoid synovitis, gout |
| Amyloid deposition | Long-term haemodialysis (β2-microglobulin amyloid), myeloma (AL amyloid) |
| Metabolic and other | Diabetes, obesity, previous wrist fracture |
- Long-term dialysis amyloid is β2-microglobulin, which is poorly cleared by dialysis membranes; it causes bilateral carpal tunnel syndrome, joint pains and bone cysts.
Exam trap Carpal tunnel syndrome spares sensation over the thenar eminence; numb thenar skin means the median nerve is caught higher, above the wrist.
Guyon's canal (ulnar canal)
Between the pisiform and the hook of hamate, roofed by the palmar carpal ligament; contains the ulnar nerve and artery. Compression (cyclists leaning on handlebars, hook of hamate fracture, ganglion) weakens the small hand muscles and numbs the little finger, but the dorsal ulnar hand keeps its sensation because the dorsal cutaneous branch leaves about 5 cm above the wrist. If only the deep branch is caught, there is pure motor loss (weak finger abduction and adduction) with normal sensation.
Anatomical snuffbox
| Feature | Structure |
|---|---|
| Lateral (radial) boundary | Tendons of abductor pollicis longus and extensor pollicis brevis |
| Medial (ulnar) boundary | Tendon of extensor pollicis longus (hooks round Lister's tubercle) |
| Floor | Scaphoid and trapezium; radial styloid proximally |
| Contents | Radial artery; cephalic vein begins in the roof; superficial radial nerve branches cross it |
Bleeding with pain in the snuffbox after scaphoid injury points to the radial artery. Tendons of APL and EPB, which form the lateral boundary, are also those inflamed in de Quervain tenosynovitis (positive Finkelstein test).
Wrist and forearm fractures
| Fracture | Pattern | Note |
|---|---|---|
| Colles | Distal radius within 2.5 cm of the wrist, dorsal displacement and angulation ("dinner fork") | Older women with osteoporosis; fall on outstretched hand |
| Smith | Distal radius, volar displacement ("garden spade") | Fall on a flexed wrist |
| Barton | Intra-articular rim fracture with carpal subluxation | Dorsal or volar |
| Monteggia | Fracture of the proximal ulna with dislocation of the radial head | PIN at risk |
| Galeazzi | Fracture of the radial shaft with dislocation of the distal radioulnar joint | Ulnar head prominent |
| Boxer's fracture | Neck of the fifth metacarpal after punching | Check rotation with the fist clenched |
Exam trap FCU and the medial half of FDP are the only forearm flexors supplied by the ulnar nerve; ECRL is spared in PIN palsy because its branch arises above the elbow.
How FCPS asks about this
- Snuffbox tenderness after a fall: scaphoid fracture; proximal pole avascular necrosis because blood enters distally; bleeding is from the radial artery.
- Boundaries of the snuffbox: APL and EPB laterally, EPL medially, scaphoid and trapezium in the floor.
- Carpal tunnel syndrome: median nerve, thenar wasting and weak opposition, thenar skin spared; causes include pregnancy, hypothyroidism, acromegaly, dialysis (β2-microglobulin amyloid).
- Cannot extend fingers or thumb but wrist extends with radial deviation: PIN palsy; ECRL supplied by the radial nerve above the division.
- Cannot supinate: musculocutaneous (biceps) and radial (supinator) nerves.
- Commonest carpal fracture and dislocation: scaphoid; lunate (into the carpal tunnel). Sclerosis and collapse of the lunate is Kienböck disease.
- Fifth metacarpal neck fracture after punching: boxer's fracture.
Test yourself on this topic
5 of the 42 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A young man has wrist pain and stiffness. X-ray shows sclerosis and collapse of the bone in the middle of the proximal carpal row. What is the diagnosis?
Answer: Kienböck disease of the lunate
Kienböck disease is avascular necrosis of the lunate, the central bone of the proximal carpal row. It typically affects young adults who do manual work and is linked to a short ulna (negative ulnar variance), which loads the lunate more. Progressive sclerosis, fragmentation and collapse cause pain and stiffness.
Kienböck disease
- AVN of the lunate
- Young manual workers
- Linked to short ulna
- X-ray: sclerosis, collapse
Remember The moon bone loses its blood.
Exam trap Lunate dislocation, a different injury, can compress the median nerve acutely.
Why the other options are wrong
- Scaphoid non-union The scaphoid is lateral, in the snuffbox.
- Perthes disease Avascular necrosis of the femoral head.
- De Quervain tenosynovitis Tendon problem, normal X-ray.
- Hamate hook fracture Medial carpal bone, distal row.
A 30-year-old man falls on his outstretched hand. X-ray shows a fracture of the radius at the junction of its middle and distal thirds, and the ulnar head is prominent on the back of the wrist. Which of the following associated injuries is present?
Answer: Distal radioulnar joint dislocation
This is a Galeazzi fracture: a fracture of the distal radial shaft with disruption of the distal radioulnar joint. The radius and ulna form a ring held by the joints at each end and the interosseous membrane, so a displaced, shortened fracture of one bone usually means a dislocation at one of the joints. The distal fragment is pulled by pronator quadratus, brachioradialis and the thumb muscles, so these fractures are unstable and need open reduction and fixation of the radius with a check of the radioulnar joint. The reverse pattern, an ulnar fracture with radial head dislocation, is the Monteggia injury.
Galeazzi fracture
- Distal radial shaft fracture
- Plus distal radioulnar joint dislocation
- Forearm bones form a ring
- Needs open fixation
Remember Galeazzi: radius broken, ulna dislocated at the wrist.
Exam trap Always X-ray the elbow and wrist with any forearm shaft fracture; the joint injury is the part most often missed.
Why the other options are wrong
- Dislocation of the radial head This is part of the Monteggia injury, with an ulnar fracture.
- Fracture of the scaphoid waist Tender in the snuffbox; does not make the ulnar head prominent.
- Perilunate dislocation of the carpus A carpal injury, not linked to a radial shaft fracture.
- Tear of the annular ligament Seen with radial head dislocation at the elbow, not the wrist.
A golfer strikes the ground hard with his club and has hypothenar pain. CT shows a fracture of the hook of hamate. He has weak finger abduction and adduction, but sensation in the little finger is normal. Which nerve is injured?
Answer: Deep (motor) branch of the ulnar nerve
In Guyon's canal the ulnar nerve divides into a superficial branch, which supplies palmaris brevis and the skin of the little finger and the medial half of the ring finger, and a deep motor branch. The deep branch curves round the hook of hamate to supply the interossei, the medial two lumbricals and adductor pollicis, after giving branches to the hypothenar muscles. A hook fracture can therefore injure the deep branch alone, giving weakness of finger abduction and adduction with normal sensation. A lesion above the wrist would also cause sensory loss.
Hook of hamate fracture
- Deep ulnar branch curves round the hook
- Supplies interossei, medial lumbricals, adductor pollicis
- Purely motor: sensation normal
- Golf, racquet and bat injuries
Remember Hamate hook fracture, weak interossei, normal skin: deep ulnar branch.
Exam trap Hook of hamate fractures are often missed on routine wrist X-rays; a carpal tunnel view or CT is needed.
Why the other options are wrong
- Superficial branch of the ulnar nerve Mainly sensory; would numb the little finger.
- Dorsal cutaneous branch of ulnar nerve Purely sensory to the back of the medial hand.
- Recurrent branch of the median nerve Supplies the thenar muscles, not the interossei.
- Ulnar nerve above the wrist Would also cause sensory loss in the little finger.
Eight hours after a displaced fracture of both forearm bones is placed in plaster, a 16-year-old has severe forearm pain despite morphine, much worse when his fingers are passively straightened. Which compartment is most at risk?
Answer: Deep flexor compartment
The forearm has a volar (flexor) compartment with superficial and deep parts, a dorsal (extensor) compartment and the mobile wad. The deep flexor compartment, containing flexor digitorum profundus, flexor pollicis longus and pronator quadratus lying on the bones and interosseous membrane, is the one most often and most severely affected in compartment syndrome, because it is tightly bound and closest to the fracture. Passive finger extension stretches FDP and FPL and causes severe pain, the most useful early sign. Untreated, muscle necrosis leads to Volkmann ischaemic contracture, so the cast is split and fasciotomy done urgently.
Forearm compartment syndrome
- Worst in the deep flexor compartment
- FDP, FPL, pronator quadratus
- Pain on passive finger extension
- Untreated: Volkmann contracture
Remember Pain on passive finger stretch after a forearm fracture: split the cast now.
Exam trap Pulses are usually present in compartment syndrome; waiting for a pulseless hand means waiting too long.
Why the other options are wrong
- Superficial flexor compartment Affected, but less severely than the deep muscles beside the bone.
- Mobile wad (brachioradialis group) Rarely involved; contains brachioradialis and the radial wrist extensors.
- Dorsal extensor compartment Passive finger flexion, not extension, would hurt most.
- Thenar compartment of the hand A hand compartment, not stretched by finger extension.
A man on haemodialysis for 12 years develops bilateral carpal tunnel syndrome and joint pains. Which amyloid protein is responsible?
Answer: β2-microglobulin
β2-microglobulin, the light chain of MHC class I molecules, is normally filtered and catabolised by the kidney. In long-term dialysis it accumulates (poorly removed by older membranes) and forms amyloid in joints, synovium, tendons and bone: carpal tunnel syndrome, destructive arthropathy, bone cysts and spondyloarthropathy. High-flux dialysis and transplantation reduce it. Localised amyloids: amyloid β (Alzheimer disease), calcitonin (medullary thyroid carcinoma), amylin (islets in type 2 diabetes), atrial natriuretic peptide (isolated atrial amyloid).
Localised and dialysis amyloid
- Dialysis: β2-microglobulin → carpal tunnel
- Aβ: Alzheimer
- Calcitonin: medullary thyroid Ca
- Amylin: type 2 diabetes islets
Remember Years of dialysis, tight wrists: β2-microglobulin.
Why the other options are wrong
- Serum amyloid A Reactive (AA) amyloid.
- Immunoglobulin light chain AL amyloid.
- Amyloid β Alzheimer disease.
- Calcitonin Medullary carcinoma of thyroid.
Where this sits in the CPSP syllabus
- Paper 1: Upper limb, Forearm compartments, wrist, carpal tunnel and anatomical snuffbox
- Surgery & Allied, Paper 2: Upper limb, Forearm compartments, wrist, carpal tunnel and anatomical snuffbox