# Functional Anatomy of the Hand
## Overview
The hand is the most complex and functionally demanding structure in the upper extremity. Precise anatomical knowledge of tendons, pulleys, nerves, vessels, and joints is the foundation for diagnosing injuries and planning surgical reconstruction. ---
## Skeletal Anatomy
### Bones
27 bones: 8 carpal bones, 5 metacarpals, 14 phalanges (2 in thumb, 3 in each finger). Carpal bones arranged in proximal row (scaphoid, lunate, triquetrum, pisiform) and distal row (trapezium, trapezoid, capitate, hamate). Mnemonic (proximal to distal, radial to ulnar): "**S**ome **L**overs **T**ry **P**ositions **T**hat **T**hey **C**an't **H**andle".
### Arches of the Hand
**Longitudinal arch**: along each ray; maintained by intrinsic muscles. **Transverse proximal arch** (carpal level): rigid, fixed. **Transverse distal arch** (metacarpal head level): mobile, allows cupping. **Oblique arches**: during opposition of thumb to each digit. Loss of arches (e.g., from intrinsic muscle paralysis) leads to flat hand deformity.
### Joints
MCP joints: condyloid; collateral ligaments taut in flexion (immobilize in flexion to prevent contracture). PIP joints: hinge; most commonly stiff joint in the hand; collateral ligaments taut in extension. DIP joints: hinge; less prone to stiffness. CMC joint of the thumb: saddle (biaxial); allows opposition.
Safe position of immobilization: wrist 20-30 degrees extension, MCP 70-90 degrees flexion, IP joints in extension ("intrinsic plus" position). ---
## Extrinsic Muscles
### Flexors
**Flexor digitorum superficialis (FDS)**: flexes PIP joint; passes through the chiasm of Camper and inserts on the middle phalanx. **Flexor digitorum profundus (FDP)**: flexes DIP joint; passes through the chiasm; inserts on the distal phalanx. **Flexor pollicis longus (FPL)**: flexes thumb IP joint. **Flexor carpi radialis (FCR)** and **flexor carpi ulnaris (FCU)**: wrist flexors.
### Extensors
Six dorsal compartments at the wrist:
| Compartment | Contents | Clinical Note |
|-------------|----------|---------------|
| I | APL, EPB | De Quervain tenosynovitis |
| II | ECRL, ECRB | — |
| III | EPL | Turns around Lister tubercle |
| IV | EDC, EIP | — |
| V | EDM | — |
| VI | ECU | — |
Extensor mechanism: complex hood and lateral band system over the digits.
### Testing Individual Tendons
FDS: hold all other fingers in extension, ask patient to flex the tested finger at the PIP. FDP: hold the PIP joint in extension, ask patient to flex the DIP. FPL: flex the thumb IP joint against resistance. EPL: extend thumb IP joint with hand flat on table (retropulsion). EIP: extend index finger with other fingers in a fist. ---
## Flexor Tendon Pulley System
### Annular Pulleys
| Pulley | Location | Clinical Significance |
|--------|----------|----------------------|
| A1 | Over MCP joint | Trigger finger occurs here |
| A2 | Over proximal phalanx | **Critical pulley** -- must preserve |
| A3 | Over PIP joint | — |
| A4 | Over middle phalanx | **Critical pulley** -- must preserve |
| A5 | Over DIP joint | — |
### Cruciate Pulleys
C1, C2, C3: between the annular pulleys; allow sheath flexibility during flexion.
### Thumb Pulley System
A1 (over MCP), oblique (critical pulley), A2 (over proximal phalanx).
### Clinical Significance
A2 and A4 pulleys are critical for mechanical advantage and preventing bowstringing. Loss of both A2 and A4 results in significant flexion weakness and bowstringing. Pulley reconstruction may use extensor retinaculum, palmaris longus, or remnant tendon.
Palmar view anatomical illustration of a finger showing the flexor tendon sheath and pulley system. The FDP and FDS tendons visible through a windowed sheath, with all five annular pulleys (A1-A5) and three cruciate pulleys (C1-C3) clearly labeled. A cross-section at the level of the proximal phalanx shows the FDS splitting around the FDP (chiasm of Camper), the A2 pulley, the vincula providing blood supply, and the digital nerves and arteries on either side of the sheath.
---
## Intrinsic Muscles
### Thenar Muscles (Median Nerve, Recurrent Motor Branch)
**Abductor pollicis brevis (APB)**: most important thenar muscle; tested by palmar abduction against resistance. **Opponens pollicis**: pronates and flexes the thumb metacarpal. **Flexor pollicis brevis**: superficial head (median nerve), deep head (ulnar nerve).
### Hypothenar Muscles (Ulnar Nerve)
Abductor digiti minimi, flexor digiti minimi, opponens digiti minimi.
### Lumbricals
4 muscles; origin from FDP tendons. Insert into lateral bands of the extensor mechanism. Function: MCP flexion with IP extension. Lumbricals 1 and 2: median nerve; lumbricals 3 and 4: ulnar nerve.
"Lumbrical plus" deformity: paradoxical IP extension during attempted flexion (occurs with distal FDP laceration or lumbrical origin advancement).
### Interossei
**4 dorsal interossei**: abduct fingers (DAB - Dorsals ABduct); bipennate. **3 palmar (volar) interossei**: adduct fingers (PAD - Palmars ADduct); unipennate. All innervated by the ulnar nerve (deep branch). Insert into the lateral bands of the extensor mechanism. Contribute to MCP flexion and IP extension.
### Intrinsic Muscle Balance
Loss of intrinsic function (ulnar nerve palsy): claw hand deformity (MCP hyperextension, IP flexion) -- worse in ring and small fingers. Paradox of ulnar nerve palsy: clawing worse with low (distal) ulnar nerve lesion because FDP to ring and small remains intact. ---
## Extensor Mechanism
### Components
**Central slip**: extends PIP joint; inserts on base of middle phalanx. **Lateral bands**: formed by contributions from interossei and lumbricals; converge into the terminal tendon inserting on the distal phalanx. **Triangular ligament**: prevents volar subluxation of lateral bands at the PIP level. **Transverse retinacular ligament**: connects the volar plate to the lateral bands; prevents dorsal subluxation during extension.
**Oblique retinacular ligament (Landsmeer)**: from the flexor sheath/proximal phalanx to the terminal tendon; coordinates DIP extension with PIP extension.
### Deformities from Extensor Mechanism Disruption
**Mallet finger**: disruption of terminal tendon at DIP -- DIP flexion deformity. **Boutonniere deformity**: central slip disruption at PIP -- PIP flexion with DIP hyperextension (lateral bands sublux volar). **Swan neck deformity**: PIP hyperextension with DIP flexion (volar plate laxity, FDS insufficiency, or intrinsic tightness). ---
## Neurovascular Anatomy
### Radial Artery
Enters the hand through the anatomical snuffbox. Passes between the two heads of the first dorsal interosseous to form the deep palmar arch. Deep palmar arch: 2 cm proximal to superficial arch.
### Ulnar Artery
Enters through Guyon canal (with ulnar nerve). Forms the superficial palmar arch. Superficial palmar arch: at the level of the distal border of the extended thumb (Kaplan cardinal line). Superficial arch is the dominant blood supply to the fingers in most hands.
### Allen Test
Assesses adequacy of dual arterial supply to the hand. Compress both arteries, have patient pump fist, release one artery at a time. Refill within 5-7 seconds is normal. Essential before radial artery harvest (for radial forearm free flap or coronary bypass).
### Digital Arteries and Nerves
Proper digital arteries and nerves run on the volar-lateral aspect of each digit. Two digital arteries and two digital nerves per finger. Nerve is more superficial (volar) to the artery in the finger. Digital arteries dominant: ulnar digital artery in the thumb, index, middle; radial digital artery in ring, small.
### Median Nerve in the Hand
Passes through the carpal tunnel (deep to transverse carpal ligament). Recurrent motor branch: innervates thenar muscles (APB, opponens pollicis, superficial head of FPB). Palmar cutaneous branch: branches proximal to the carpal tunnel; provides sensation to the thenar eminence. Digital sensory branches: sensation to thumb, index, middle, and radial half of ring finger.
### Ulnar Nerve in the Hand
Passes through Guyon canal (superficial to transverse carpal ligament). Deep motor branch: innervates hypothenar muscles, all interossei, lumbricals 3 and 4, adductor pollicis, deep head of FPB. Superficial sensory branch: sensation to small finger and ulnar half of ring finger.
Palmar view anatomical illustration of the hand showing the vascular anatomy: radial and ulnar arteries entering the hand, the superficial palmar arch (from ulnar artery) and deep palmar arch (from radial artery), common and proper digital arteries, and digital nerves. The relationship of the ulnar nerve and artery within Guyon canal and the median nerve within the carpal tunnel are shown in cross-sectional insets. The recurrent motor branch of the median nerve to the thenar muscles is highlighted.
---
## Spaces and Compartments
### Thenar Space
Bounded by the thenar muscles, adductor pollicis, and third metacarpal. Deep space infections present with a swollen, flexed thumb.
### Midpalmar Space
Between the palmar aponeurosis and metacarpals 3-5. Infections cause loss of palmar concavity.
### Hypothenar Space
Beneath the hypothenar muscles.
### Parona Space
Proximal forearm space where radial and ulnar bursae communicate. "Horseshoe abscess": infection tracking from the small finger (ulnar bursa) to the thumb (radial bursa) through Parona space.
### Dorsal Subaponeurotic Space
Between the extensor tendons and metacarpals. Dorsal hand swelling from palmar space infections occurs here (dependent drainage). ---
## Fascial Structures
### Palmar Aponeurosis (Palmar Fascia)
Continuous with the palmaris longus tendon (when present). Longitudinal fibers: pretendinous bands (become cords in Dupuytren disease). Transverse fibers: superficial transverse metacarpal ligament, natatory ligaments. Vertical fibers: Grayson ligaments (volar to neurovascular bundle) and Cleland ligaments (dorsal to neurovascular bundle).
In Dupuytren disease, the digital nerve is displaced by the spiral cord -- understanding the relationship is critical to avoid nerve injury during fasciectomy.
### Carpal Tunnel Boundaries
Floor: carpal bones (concave arch). Roof: transverse carpal ligament (flexor retinaculum). Contents: 9 flexor tendons (4 FDS, 4 FDP, 1 FPL) + median nerve. The ulnar nerve does NOT pass through the carpal tunnel. ---
## Clinical Pearls
The safe position of immobilization (intrinsic plus) is MCP flexion 70-90 degrees and IP extension -- this maintains collateral ligament length at the MCP joints and prevents contracture. A2 and A4 are the critical pulleys; their loss results in bowstringing and significant grip weakness -- preserve them during flexor tendon surgery. The ulnar nerve claw deformity paradox: low (distal) lesions produce worse clawing because the FDP to ring and small fingers remains intact, creating unopposed IP flexion.
Always test FDS and FDP independently in hand trauma -- FDS is tested by blocking the other fingers in extension; FDP is tested by stabilizing the PIP and asking the patient to flex the DIP. The recurrent motor branch of the median nerve has variable anatomy (extra-ligamentous, sub-ligamentous, or trans-ligamentous course) -- awareness prevents injury during carpal tunnel release. The digital nerve lies volar to the digital artery in the finger -- during digital nerve block, inject dorsal to the neurovascular bundle. "Horseshoe abscess" occurs when infection travels from the small finger flexor sheath (ulnar bursa) through Parona space to the thumb flexor sheath (radial bursa) -- requires drainage of all three compartments. ---.
## References
- Doyle JR. Palmar and digital flexor tendon pulleys. *Clin Orthop Relat Res*. 2001;(383):84-96.
- Zancolli EA. *Structural and Dynamic Bases of Hand Surgery*. 2nd ed. Lippincott; 1979.
- Green DP, Hotchkiss RN, Pederson WC, Wolfe SW, eds. *Green's Operative Hand Surgery*. 8th ed. Elsevier; 2022.
- Kaplan EB. Functional and Surgical Anatomy of the Hand. *JB Lippincott*; 1965.
- Taleisnik J. The palmar cutaneous branch of the median nerve and the approach to the carpal tunnel. *J Bone Joint Surg Am*. 1973;55(6):1212-1217.
- Lanz U. Anatomical variations of the median nerve in the carpal tunnel. *J Hand Surg Am*. 1977;2(1):44-53.