Wrist and Hand

Part 2 · The upper limb

The carpal tunnel, three nerve territories, the arches, and the four grips

Why the hand is different

The hand contains 27 bones, 27 joints, 34 muscles and 3 major nerves in a segment weighing less than half a kilogram. It has the densest sensory innervation and the largest cortical representation of any body part relative to its mass. It is simultaneously the body’s most precise tool and its principal organ of exploration.

Two consequences follow for practice:

• Hand rehabilitation is unforgiving. A degree of stiffness that would be trivial at the shoulder is disabling here, and the window in which it can be prevented is measured in days.
• Sensation matters as much as movement. A strong, mobile, insensate hand is functionally close to useless — which is why nerve injury outcomes are judged on two-point discrimination as much as on power.

Learning outcomes

• Describe the joints of the wrist and hand, their surfaces, ligaments and ranges.
• Explain the arches of the hand and their functional significance.
• Describe the extrinsic and intrinsic muscles with attachments, nerve supply and actions.
• Describe the extensor expansion and explain boutonnière, swan-neck and mallet deformities.
• Describe the flexor tendon sheaths, pulley system and zones of injury.
• Map the three nerve territories precisely and describe the deficits of median, ulnar and radial lesions.
• Describe the carpal tunnel and Guyon’s canal, and the syndromes arising in each.
• Describe the arterial arches and Allen’s test.
• Classify grips and prehension patterns and describe the functional position of the hand.
• State the position of safe immobilisation and explain each element.

The joints

Wrist range and functional range

Ulnar deviation exceeds radial deviation because the radial styloid projects further distally, blocking radial deviation early. The “dart-thrower’s motion” — the oblique arc from radial extension to ulnar flexion — occurs largely at the midcarpal joint with minimal proximal-row motion, which is why it is the preferred early motion after scapholunate and proximal-row injury.

Ligaments

• Extrinsic: palmar radiocarpal (strong — hence the wrist extends less than it flexes at the radiocarpal joint, and hence the pattern of injury on a fall on the outstretched hand), dorsal radiocarpal, radial and ulnar collateral.
• Intrinsic (interosseous): scapholunate and lunotriquetral — the two whose failure produces carpal instability (DISI and VISI respectively).
• TFCC: the articular disc, meniscus homologue, ulnar collateral ligament, dorsal and palmar radio-ulnar ligaments, and the ECU subsheath. It cushions the ulnar carpus (transmitting ~20% of axial load) and is the principal stabiliser of the distal radio-ulnar joint.

Close-packed position of the wrist: full extension with radial deviation. Resting position: neutral with slight ulnar deviation. Capsular pattern: flexion and extension equally limited.

The arches of the hand

Figure 1 · The arches of the hand

Three arches allow the flat hand to cup, conforming to whatever it holds. Their loss is one of the most functionally significant consequences of intrinsic muscle paralysis.

The fixed central pillar (2nd and 3rd CMC joints and metacarpals, plus the distal carpal row) is the stable base against which the mobile units — the thumb, and the ring and little fingers — move. Understanding this explains why the thumb and ulnar two rays carry so much of the hand’s adaptability, and why a fixed, flattened hand is functionally crippled even with normal finger flexion.

The extrinsic muscles at the wrist and hand

Figure 2 · The carpal tunnel and the tunnels at the wrist

The long flexors and extensors are described in Chapter 11. Two structural features at the wrist matter here.

The flexor retinaculum and the carpal tunnel

The flexor retinaculum is a strong fibrous band, ~2–3 cm wide, attached to:

• Laterally: the tubercle of the scaphoid and the tubercle of the trapezium
• Medially: the pisiform and the hook of the hamate

Converting the carpal arch into the carpal tunnel, whose contents are ten structures:

• Flexor digitorum profundus — 4 tendons
• Flexor digitorum superficialis — 4 tendons
• Flexor pollicis longus — 1 tendon
• Median nerve (most superficial, immediately deep to the retinaculum)

Passing superficial to the retinaculum (not in the tunnel): the palmaris longus tendon, the ulnar nerve and artery (in Guyon’s canal), the palmar cutaneous branch of the median nerve, and the palmar cutaneous branch of the ulnar nerve. Also outside: flexor carpi radialis, which runs in its own groove in the trapezium within a split of the retinaculum, and flexor carpi ulnaris, which inserts on the pisiform.

Tendon sheaths: the ulnar bursa (common flexor sheath) surrounds the FDS and FDP tendons; the radial bursa surrounds FPL. The ulnar bursa communicates with the little finger’s digital sheath, and the radial bursa with the thumb’s — which is why infection in the little finger can track proximally into the palm and, across the potential communication between the bursae, into the thumb (a “horseshoe abscess”).

The extensor retinaculum and its six compartments

The flexor pulley system

Fibrous bands hold the flexor tendons against the phalanges, preventing bowstringing and preserving the moment arm.

• Five anular pulleys (A1–A5) and three cruciate (C1–C3) per finger.
• A2 (over the proximal phalanx) and A4 (over the middle phalanx) are the most important biomechanically — losing them causes bowstringing and loss of full flexion.
• A1 pulley is the site of trigger finger (stenosing tenosynovitis): a nodular thickening of the tendon catches at the pulley, producing triggering, locking and pain, most often affecting the ring and middle fingers, and strongly associated with diabetes.
• The thumb has A1, oblique and A2 pulleys, the oblique pulley being the critical one.

Flexor tendon zones (Verdan) — zone II, from the distal palmar crease to the middle of the middle phalanx, is where FDS and FDP lie together within the sheath, and was historically called “no man’s land” because repairs there adhere so readily. Modern early controlled mobilisation protocols (Chapter 7) have transformed the outcome, but the zone still dictates the protocol.

The intrinsic muscles

Nineteen muscles wholly within the hand, in four groups. All are supplied by the ulnar nerve except the “LOAF” muscles, which are median.

Mnemonic: LOAF — Lateral two Lumbricals, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis (superficial head) — the median intrinsics. Everything else in the hand is ulnar.

The lumbricals and interossei: the hand’s most elegant mechanism

They pass anterior to the MCP axis but posterior to the IP axes. One contraction therefore flexes the MCP joints while extending the IP joints — the position needed to flatten the hand around a large object, to hold a book, or to write.

Lose them (ulnar nerve palsy) and the long extensors act unopposed at the MCP joints while FDP acts unopposed at the IP joints — producing clawing (intrinsic-minus hand): MCP hyperextension with IP flexion. Lose the opposite (spasticity, Volkmann’s contracture) and you get the intrinsic-plus hand: MCP flexion with IP extension.

This is the reason the position of safe immobilisation (§12.10) puts the MCPs in flexion and the IPs in extension — it is the position in which the collateral ligaments are at maximum length and cannot shorten.

The extensor mechanism and finger deformities

Over the dorsum of each finger, the extensor digitorum tendon flattens into the extensor expansion (dorsal hood), joined on each side by the lumbrical and interossei. It divides into:

• A central slip, inserting into the base of the middle phalanx (extends the PIP joint)
• Two lateral bands, converging to insert into the base of the distal phalanx (extends the DIP joint)

Held in place by the sagittal bands at the MCP joint, the triangular ligament dorsally and the transverse retinacular ligaments laterally.

The three nerve territories

Figure 3 · Nerve supply of the hand

Sensory territories — learn the autonomous zones

The autonomous zones are the small areas supplied by one nerve only, without overlap. Test there. Testing in overlap zones is the commonest reason a clinical examination fails to detect a nerve lesion.

Motor: the three hands

Carpal tunnel syndrome

The commonest peripheral entrapment neuropathy: prevalence ~3–5%, peak 40–60 years, more common in women.

• Symptoms: nocturnal paraesthesia and pain in the median distribution, relieved by shaking the hand (the “flick sign”); pain may radiate proximally to the forearm and shoulder; later, clumsiness, dropping objects and thenar wasting.
• Sparing of the thenar eminence skin because the palmar cutaneous branch passes superficial to the retinaculum — a point that distinguishes it from a proximal median lesion.
• Tests: Tinel’s (percussion), Phalen’s (sustained wrist flexion), carpal compression (Durkan’s) — all with modest and variable sensitivity and specificity; the hand diagram and nocturnal symptom pattern are as diagnostically useful as any provocative test, and nerve conduction studies are the reference standard.
• Associations: pregnancy, hypothyroidism, diabetes, rheumatoid arthritis, obesity, acromegaly, amyloidosis, and repetitive forceful/vibratory work (the occupational association is real but weaker than commonly assumed).
• Management: night splinting in neutral (not extension — tunnel pressure is lowest at neutral), activity modification, corticosteroid injection (good short- to medium-term evidence), nerve and tendon gliding exercises, and surgical release for severe or refractory cases, which has good long-term outcomes.

Guyon’s canal (ulnar tunnel)

Boundaries: floor = flexor retinaculum and hypothenar muscles; roof = palmar carpal ligament and palmaris brevis; medial wall = pisiform and FCU; lateral wall = hook of the hamate. Contents: the ulnar nerve and ulnar artery (no tendons).

Causes: ganglion (the commonest), hook of hamate fracture, ulnar artery thrombosis (hypothenar hammer syndrome), and prolonged handlebar pressure (cyclist’s palsy).

The localising sign: the dorsal cutaneous branch of the ulnar nerve leaves the nerve 5–7 cm proximal to the wrist, so dorsal ulnar hand sensation is preserved in a Guyon’s canal lesion but lost in a cubital tunnel lesion. That one finding separates the two, and it is worth having at your fingertips.

Blood supply

Allen’s test: compress both radial and ulnar arteries, have the patient clench and open the hand until it blanches, then release one artery and time the return of colour (normally <5–7 seconds). It tests whether one artery alone can perfuse the hand — essential before radial artery harvest, cannulation, or in suspected hypothenar hammer syndrome.

Digital arteries run on the palmar-lateral aspect of each digit with the digital nerves, which is why digital blocks are placed there and why lacerations on the sides of a finger threaten both.

Grips and prehension

Figure 4 · The four grips

Napier’s classification remains the standard framework, with common clinical elaborations.

Two things that are worth internalising:

• Power grip requires wrist extension. The wrist extensors are the obligatory synergists (Chapter 6), which is why radial nerve palsy destroys grip and why a wrist splint restores it.
• The thumb accounts for roughly 40–50% of hand function. Opposition is what distinguishes the human hand, and its loss — through median nerve injury, first CMC arthritis, or thumb amputation — is functionally catastrophic in a way that loss of a finger is not.

Positions of the hand

Why the safe position is the safe position

• MCP joints at 70–90° flexion: the MCP collateral ligaments are cam-shaped, so they are taut in flexion and lax in extension. Immobilising the MCPs in extension lets the collaterals shorten, producing an MCP joint that will never flex again.
• IP joints in full extension: their collaterals are taut in extension; and the volar plate contracts if the joint is held flexed, producing a fixed flexion contracture that is extremely difficult to reverse.
• Wrist in extension: preserves the tenodesis relationship and grip.
• Thumb abducted and opposed: preserves the first web space, whose contracture is a functional disaster.

Splinting a hand flat, which looks intuitively correct, produces exactly the wrong contractures at every joint. This is one of the highest-value pieces of applied anatomy in the whole subject.

Oedema management deserves a note of the same weight: hand oedema organises into fibrous tissue quickly, and once the dorsal skin and extensor mechanism are involved the hand stiffens into the intrinsic-minus posture. Elevation, active movement, compression and early mobilisation in the first days matter more than anything done later.

Where students consistently go wrong

• Putting the ulnar nerve in the carpal tunnel. It runs in Guyon’s canal, superficial to the retinaculum.
• Counting nine or eleven carpal tunnel contents. It is ten: 4 FDS + 4 FDP + FPL + median nerve.
• Forgetting palmaris longus is superficial to the retinaculum and absent in 10–15% of people.
• Getting the lumbrical nerve supply wrong. Lateral two median, medial two ulnar.
• Reversing the interossei. Dorsal ABduct, Palmar ADduct.
• Testing sensation in overlap zones. Use the autonomous zones: index pulp, little finger pulp, dorsal first web space.
• Missing the dorsal cutaneous branch sign. It separates cubital tunnel from Guyon’s canal.
• Splinting the hand flat. MCPs must be flexed 70–90° and IPs extended.
• Letting a mallet finger flex during splint changes. Six to eight weeks of uninterrupted extension is the whole treatment.
• Missing a central slip injury before the boutonnière develops. Use Elson’s test.

Check yourself

15 questions on this chapter. Tap one to see the answer and the reasoning.

Quick review

Further reading

Chapter 12 of 24 · Human Anatomy · Physiotherapist India End of Part 2 — The upper limb. Next: Chapter 13 — Bones of the Lower Limb, opening Part 3.

Reviewed by the Physiotherapist India Team. · Human Anatomy contents