Anatomy of hand

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Anatomy of the Hand

Surface Anatomy and Orientation

The hand is described using palmar (volar) for the anterior surface and dorsal for the posterior surface. Rather than medial/lateral (which change with rotation), the terms radial (thumb side) and ulnar (little finger side) are used. Abduction and adduction of the fingers refer to movement away from and toward the middle finger, respectively.

Surface anatomy of the dorsal (A) and volar (B) aspects of the hand, showing key landmarks including metacarpal heads, palmar creases, thenar and hypothenar eminences, hook of hamate, and pisiform.

1. Bones

The hand contains 19 bones arranged in 5 rays (numbered 1-5, thumb to little finger). Each ray runs from the metacarpal base to the fingertip.

Bony anatomy of the hand showing carpal bones, metacarpals, and phalanges with labeled parts.

Carpal Bones (8 bones, 2 rows)

Proximal Row	Distal Row
Scaphoid	Trapezium
Lunate	Trapezoid
Triquetrum	Capitate
Pisiform	Hamate (with hook)

Mnemonic: Some Lovers Try Positions That They Can't Handle

Metacarpals (5 bones)

Form the visible palm
Each has a base (proximal), body/shaft, and head (distal - forming the "knuckles")

Phalanges (14 bones)

Thumb: 2 phalanges (proximal + distal only - no middle phalanx)
Fingers: 3 phalanges each (proximal, middle, distal)

Joints

Metacarpophalangeal (MP) joint: 90° of flexion, slight hyperextension; fingers also actively abduct/adduct here
Proximal interphalangeal (PIP) joint: hinge joint, ~110° flexion
Distal interphalangeal (DIP) joint: hinge joint, ~80° flexion
Thumb carpometacarpal (CMC) joint: saddle joint allowing opposition

2. Muscles

Muscles are divided into extrinsic (originating in the forearm, inserting in the hand) and intrinsic (originating and inserting within the hand itself).

Extrinsic Muscles

Flexor (volar) compartment - 3 layers:

Layer	Muscles
Superficial	Pronator teres, Flexor carpi radialis (FCR), Palmaris longus*, Flexor carpi ulnaris (FCU)
Intermediate	Flexor digitorum superficialis (FDS) - flexes PIP joints
Deep	Flexor digitorum profundus (FDP) - flexes DIP joints; Flexor pollicis longus (FPL) - flexes thumb IP; Pronator quadratus

*Palmaris longus is absent in ~10-15% of individuals.

Extensor (dorsal) compartment:

Subgroup	Muscles
Mobile wad (radial)	Brachioradialis, ECRL, ECRB
Superficial	EDC (extends fingers), EDM/EDQ (extends little finger), ECU (ulnar deviation + wrist extension)
Deep	APL, EPB, EPL (thumb function), EIP (index finger extension)

Dorsal forearm and hand showing the extensor muscles and their arrangement.

Intrinsic Muscles

Thenar muscles (thumb - all median nerve except deep FPB head):

Abductor pollicis brevis (APB) - abducts thumb
Flexor pollicis brevis (FPB) - flexes thumb MP
Opponens pollicis (OP) - opposition (most important)
Adductor pollicis - adducts thumb (ulnar nerve)

Hypothenar muscles (little finger - all ulnar nerve):

Abductor digiti quinti (ADQ) - abducts little finger
Opponens digiti quinti (ODQ) - brings little finger across palm
Flexor digiti quinti (FDQ) - flexes little finger metacarpal

Lumbricals (4):

Unique: originate from FDP tendons in the palm
Pass along the radial aspect of each finger
Action: flex MP joints + extend IP joints
Lumbricals 1 & 2 (index, middle) - median nerve
Lumbricals 3 & 4 (ring, little) - ulnar nerve

Interosseous muscles:

3 palmar interossei - ADduct fingers (PAD mnemonic)
4 dorsal interossei - ABduct fingers (DAB mnemonic)
All: flex MP joints, extend IP joints
All innervated by ulnar nerve

3. Tendons and Pulleys

Flexor Tendon Pulleys

Each finger has 5 annular (A1-A5) and 3 cruciate (C1-C3) pulleys. The A2 and A4 pulleys are the most critical - they prevent bowstringing of the tendon. The thumb has its own pulley system (A1, oblique, A2).

Extensor Retinaculum (6 compartments, dorsal wrist)

Compartment	Tendons
1st	APL, EPB (De Quervain's territory)
2nd	ECRL, ECRB
3rd	EPL
4th	EIP, EDC
5th	EDM/EDQ
6th	ECU

Carpal Tunnel (Flexor Retinaculum)

Attaches from the scaphoid tubercle and trapezium (radially) to the hook of hamate and pisiform (ulnarly). Contents:

FDS tendons (4)
FDP tendons (4)
FPL tendon (1)
Median nerve

Extensor Hood (Dorsal Digital Apparatus)

At the MP joint, the long extensor tendon broadens into the extensor hood. The proximal portion at MCPJ is the sagittal band. The intrinsic muscles (interossei and lumbricals) enter as lateral bands, inserting distally to the axis of the PIP joint - this is why intrinsics flex the MP but extend the IP joints. The extensor inserts as the central slip (to base of middle phalanx) and terminal slip (to base of distal phalanx).

4. Blood Supply

Arterial Arches

Two main arteries supply the hand:

Radial artery:

Passes superficially at wrist (radial to FCR tendon) - the pulse palpation site
At wrist, splits: superficial branch enters palm; larger branch passes dorsally through anatomic snuffbox (between EPL and APB/EPB), then through first web space to form the deep palmar arch

Ulnar artery:

Travels through Guyon's canal (between hamate and pisiform, superficial to flexor retinaculum)
Larger superficial branch forms the superficial palmar arch

Key fact: In 97% of patients, at least one arch is intact, so the entire hand can survive on either the radial or ulnar artery alone.

Digital arteries:

Each digit receives radial and ulnar proper digital arteries
Run palmar to the midaxis of the digit, but dorsal to the digital nerves

5. Nerve Supply

Three nerves innervate the hand:

Median Nerve

Passes through the carpal tunnel and gives off the recurrent motor branch immediately distal to it.

Motor (LOAF muscles): Lumbricals 1 & 2, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis (superficial head)
Sensory: Palmar surface of thumb, index, middle finger, and radial half of ring finger; dorsal tips of same fingers

Ulnar Nerve

Enters the hand through Guyon's canal (superficial to flexor retinaculum).

Motor: All interossei (3 palmar + 4 dorsal), lumbricals 3 & 4, hypothenar muscles, adductor pollicis, deep head of FPB
Sensory: Little finger + ulnar half of ring finger (palmar and dorsal); dorsal ulnar hand via dorsal branch (arises proximal to wrist)
Responsible for the power grasp function of the hand

Radial Nerve

Motor: All extrinsic wrist and finger extensors (in forearm, via posterior interosseous nerve)
Sensory only in hand: Superficial branch supplies dorsal radial hand, dorsum of thumb, index, long finger, and radial half of ring finger proximal to the DIP joints
No intrinsic motor function in the hand itself

Summary Innervation Table

Structure	Nerve
Thenar muscles (APB, OP, FPB superficial)	Median
Adductor pollicis, FPB deep head	Ulnar
Lumbricals 1 & 2	Median
Lumbricals 3 & 4	Ulnar
All interossei	Ulnar
All hypothenar muscles	Ulnar
Wrist/finger extensors (extrinsic)	Radial (PIN)

6. Skin and Fascia

The palmar skin is thick, hairless, and tethered to underlying fascia by vertical fibrous septa - forming 8 compartments for flexor tendons and neurovascular bundles
The palmar fascia (palmar aponeurosis) is the condensation of the deep fascia of the palm, continuous with the palmaris longus tendon proximally - central thickening is important in Dupuytren's contracture
The dorsal skin is thin and loose, allowing for the large excursion required with finger flexion

Sources: Schwartz's Principles of Surgery, 11th Ed., Chapter 44; Sabiston Textbook of Surgery, Chapter 119

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