Forearm, wrist, and hand

The wrist is comprised of the carpus and the radiocarpal joint. The carpus is the complex of eight carpal bones (scaphoid, lunate, triquetrum, pisiform, trapezium, trapezoid, capitate, and hamate), while the radiocarpal joint is the region of articulation between the carpus and radius. Distally, the carpus articulates with the metacarpal bones, which, together with the phalanges, make up the bones of the hand. The forearm (lower arm or antebrachium) has an anterior compartment, which consists of the flexor group of muscles and is innervated by the ulnar and median nerve, and a posterior compartment, which consists of the extensor group of muscles and is innervated by the radial nerve. The flexor group of muscles is involved in pronation of the forearm and flexion of the wrist and fingers, while the extensor group of muscles is involved in supination of the forearm and extension of the wrist and fingers. Both groups of muscles are also involved in the abduction and adduction of the wrist. The intrinsic muscles of the hand are responsible for hand and finger movement and consist of the thenar, hypothenar, lumbrical, and interossei muscles. The forearm, the wrist, and the hand are perfused by the radial and ulnar artery and their branches. They are drained by the superficial cephalic and basilic veins and the deep radial and ulnar veins.

Bones of the forearm

• The forearm is the portion of the upper limb between the elbow and wrist joint.
• The bones of the forearm are the radius and ulna.
• They articulate with each other at the proximal and distal radioulnar joints.
• The radioulnar joints are responsible for forearm pronation and supination.
• The proximal ends of the radius and ulna articulate with the distal humerus to form the elbow joint.
• The interosseus membrane connects the shafts of the radius and ulna and stabilizes the radioulnar joints.

Ulna

• Description: the medial bone of the forearm (when in anatomical position)
• Parts of the ulna
  • Proximal ulna
    • Olecranon
      • An anterior-facing, hook-like bony projection that fits into the olecranon fossa on the dorsal aspect of the distal humerus when the elbow is extended
      • Prevents elbow hyperextension (> 180°)
    • Coronoid process: an anterior-facing bony projection that forms the lower lip of the trochlear notch
    • Trochlear notch: a concavity between the olecranon and the coronoid process that articulates with the trochlea of the humerus (humeroulnar joint)
    • Radial notch: a lateral continuation of the trochlear notch that accommodates the head of the radius, forming the proximal radioulnar joint
    • Tuberosity of the ulna: a rough area of bone just distal to the coronoid process into which the brachialis muscle inserts
  • Diaphysis: triangular in shape, with three surfaces (anterior, posterior, and medial) and three borders (anterior, posterior, and interosseous)
  • Distal ulna
    • Ulnar head (articular circumference): articulates with the radius to form the distal radioulnar joint
    • Ulnar styloid: a bony projection posterior and medial to the ulna

Radius (bone)

• Description: the lateral bone of the forearm (when in anatomical position)
• Parts of the radius
  • Proximal radius
    • Head of radius
      • A disk-shaped structure, the medial rim of which articulates with the radial notch of the ulna, forming the proximal radioulnar joint
      • The superior concave surface (articular fovea) articulates with the capitulum of the humerus (humeroradial joint of the elbow).
    • Radial tuberosity
      • Distal to the medial aspect of the head
      • Point of attachment for the biceps brachii tendon
  • Distal radius
    • Radial styloid: insertion site for the brachioradialis muscle and the radial collateral ligament
    • Carpal articular surface of the radius
    • Ulnar notch of the radius
      • Located on the medial surface
      • Articulates with the ulna, forming the distal radioulnar joint
    • Dorsal tubercle of the radius (Lister tubercle)
      • Bony prominence of the distal radius
      • Serves as a pulley for the extensor pollicis longus tendon

Joints of the forearm

Radioulnar joint

• General: the radius and ulna articulate with one another proximally and distally.
• Type of joint: pivot joint, allowing the distal end of the radius to move anterior and medially over the ulna (i.e., forearm pronation and supination)
• Proximal radioulnar joint: allows articulation of the head of the radius and the radial notch on the ulna
• Distal radioulnar joint: allows articulation of the ulnar head and ulnar notch on the radius
• Movement: forearm pronation and supination

Injury to the radial or ulnar diaphysis can disrupt the proximal or distal radioulnar joints.

Interosseous membrane of the forearm

• Definition: fibrous connective tissue that connects the lateral ulna to the medial radius
• Function
  • Connects the radius and ulna without restricting movement
  • Attaches muscles to the radius and ulna
  • Stabilizes the radioulnar joint by preventing proximal migration of the radius

Bones of the wrist (carpal bones)

• Description: Eight carpal bones, arranged in two rows of four bones each, make up the wrist joint.
  • Proximal row (lateral to medial): scaphoid, lunate, triquetrum, and pisiform
  • Distal row (lateral to medial): trapezium, trapezoid, capitate, and hamate

“Stubborn Larry Tried Pills That Triumphantly Cured Him:” Scaphoid, Lunate, Triquetrum, Pisiform, Trapezium, Trapezoid, Capitate, Hamate (carpal bones from lateral to medial and proximal to distal).

Anatomical snuffbox

• Definition: a triangular groove on the radial aspect of the dorsal wrist that becomes prominent when extending and abducting the thumb
• Boundaries
  • Lateral (radial): extensor pollicis brevis, abductor pollicis longus tendons
  • Medial (ulnar): extensor pollicis longus tendon
  • Proximal: distal end of the radius
• Contents
  • A branch of radial artery
  • A branch of radial nerve
  • The proximal end of the cephalic vein
  • Scaphoid and trapezium

Pain and tenderness in the anatomical snuffbox after trauma to the wrist suggest a scaphoid fracture. These fractures can be difficult to see on plain x-ray.

Bones of the hand

• Metacarpal bones
  • There are five metacarpal bones, which are located distal to the carpal bones.
  • Each has a base that articulates with the carpal bones (carpometacarpal joints), a head that articulates with the proximal phalanges (metacarpophalangeal joints), and a shaft in between.
• Phalanges
  • Small, long bones of the digits
  • Each digit has three phalanges, except the thumb, which only has two.

Joints of the wrist and hand

Muscles of the forearm ^[1]

• The anterior (volar) compartment
  • Contains the flexor group of muscles, which are innervated by the median and ulnar nerves
  • Function
    • Flexion of the wrist and fingers
    • Pronation of the hand
• The posterior (dorsal) compartment
  • Contains the extensor group of muscles, which are innervated by the radial nerve
  • Function
    • Extension of the wrist and fingers
    • Supination of the hand
• Most muscles of the forearm move the wrist and hand joints (extrinsic muscles of the hand).

The median nerve innervates all the forearm flexors, with the exception of the flexor carpi ulnaris and the ulnar portion of the flexor digitorum profundus, which are innervated by the ulnar nerve.

De Quervain syndrome involves inflammation of the tendons on the radial side of the wrist, the extensor pollicis brevis, and the abductor pollicis longus.

Supinator syndrome is a relatively rare entrapment syndrome in which the deep branch of the radial nerve is trapped in the supinator tunnel between the heads of the supinator muscle, resulting in weak finger extension. Causes include trauma or overuse of the supinator muscle.

The forearm extensors are innervated by the radial nerve or by its branch, the posterior interosseous nerve.

The superficial flexors originate from the medial epicondyle of the humerus, and the superficial extensors from the lateral epicondyle of the humerus.

Muscles of the hand

• Based on where the muscle belly is located, the muscles of the hand are divided into two groups.
• Extrinsic muscles of the hand: The muscle bellies are located in the forearm (see “Muscles of the forearm” above).
• Intrinsic muscles of the hand: The muscle bellies are located within the hand.
• All hand muscles are supplied by the median nerve or ulnar nerve.
• In the resting position, there is a balance between flexors and extensors (both extrinsic and intrinsic) of the hand.

Thenar muscles

These muscles form the thenar eminence (the muscular prominence on the palmar aspect at the base of the thumb) of the palm and exert their action mainly on the 1^st MCP.

Hypothenar muscles

These form the hypothenar eminence (the muscular prominence located on the palmar aspect at the base of the 5^th finger) and exert their action mainly on the 5^th MCP.

The muscles of the hypothenar eminence are innervated by the ulnar nerve.

Lumbricals and interossei

• There are four lumbricals.
• There are seven interossei muscles: four on the dorsal side and three on the palmar side of the metacarpal bones.
• These muscles exert their actions on the fingers, but not the thumb.

PAD: Palmar interossei ADduct the fingers. DAB: Dorsal interossei ABduct the fingers.

Fascia and retinacula of the wrist and hand ^[1]

The palmar aponeurosis is the structure that hypertrophies and contracts in Dupuytren disease.

Arteries

• The arterial system of the forearm originates from the brachial artery in the cubital fossa, where it bifurcates into the ulnar and radial arteries.
• The radial artery and the ulnar artery are the two major blood vessels that supply blood to the forearm and hand.
• These two arteries anastomose in the hand, forming the superficial palmar arch and the deep palmar arch.
• For further information on the arterial supply of the forearm and hand, see “Neurovasculature of the upper limbs.”

Veins

• The venous system of the forearm and hand consists of a superficial and a deep venous system.
• Superficial venous system
  • Located within the subcutaneous tissue
  • Formed by the cephalic vein, basilic vein, and the median antebrachial vein
• Deep venous system
  • Located below the deep fascia
  • Formed by:
    • Paired veins (venae comitantes), which accompany each artery
      • Radial vein
      • Ulnar vein
    • Brachial vein: formed by the union of the radial and ulnar veins at the elbow
  • The deep veins communicate with superficial veins via perforator veins.
  • Perforating veins run between the deep and superficial veins of the upper limb, connecting the two systems.
• For further information on the venous supply of the forearm and hand, see “Neurovasculature of the upper limbs.”

Lymphatic drainage

• Superficial lymphatics: accompany the superficial veins of the upper limb
  • The lymphatics at the lateral side (radial) of the hand and forearm accompany the cephalic vein and drain mainly into the apical group of axillary lymph nodes.
  • The lymphatics at the medial side (ulnar) of the hand and forearm accompany the basilic vein and drain first into the cubital lymph nodes (epitrochlear nodes), then terminate in the lateral group of axillary lymph nodes.
• Deep lymphatics: follow the deep veins and drain into the axillary group of lymph nodes

The forearm and hand are innervated by branches of the brachial plexus. The median, radial, and ulnar nerves are terminal branches of the brachial plexus that provide motor and sensory innervation.

Motor innervation

• For further information on motor innervation of the forearm and hand, see “Neurovasculature of the upper limbs.”

Sensory innervation

• The sensory innervation of the hand is provided by the median nerve, radial nerve, and ulnar nerve.
• The sensory innervation of the forearm is provided by the lateral cutaneous nerve of the forearm, medial cutaneous nerve of the forearm, and posterior cutaneous nerve of the arm.
• For further information on sensory innervation of the forearm and hand, see “Neurovasculature of the upper limbs.”

Dermatomal distribution of the forearm and hand

• C6: posterolateral forearm, thumb, and lateral side of the index finger
• C7: ventral forearm, middle finger, medial side of the index finger, and lateral side of the ring finger
• C8: distal third of the medial forearm, the little finger, and medial side of the ring finger
• T1: proximal two-thirds of the medial forearm

Bones and joints

• Ulna
  • Ulnar fractures
  • Ulnar styloid fracture (associated with distal radius fractures)
• Radius
  • Distal radius fractures (e.g., Colles fracture, Smith fracture)
  • Radial head subluxation
• Radioulnar joint
  • Monteggia fracture
  • Galeazzi fracture
  • Distal radioulnar joint instability
• Carpal bones
  • Scaphoid fracture
  • Lunate dislocation
• Metacarpal bones: boxer's fracture (metacarpal neck fracture)
• Joints of the wrist and hand
  • Trapeziometacarpal osteoarthritis (rhizarthrosis)
    • The saddle joint of the thumb is susceptible to cartilaginous degeneration, resulting in osteoarthritis.
    • Approximately 8–12% of the population is affected by carpometacarpal osteoarthritis. ^[2]
    • Clinical features: pain and swelling at the base of the thumb
    • Diagnostics ^[2]
      • Thumb grind test: performed by applying axial compression along the plane of the metacarpal bone and rotating the thumb metacarpal base. Pain or crepitus indicates a positive grind test, suggesting carpometacarpal osteoarthritis.
      • Thumb lever test: performed by grasping the first metacarpal just distal to the base and moving it back and forth in the lateral and medial directions. Pain or crepitus indicates a positive lever test, suggesting carpometacarpal osteoarthritis.
      • MCP extension test: performed by placing one finger on the interphalangeal joint and applying resistance against extension while the patient tries to extend the thumb. Pain or crepitus indicates a positive MCP extension test, suggesting carpometacarpal osteoarthritis.
    • Treatment: may involve excision of the trapezium.
  • Metacarpophalangeal joint: 1^st MCP joint is affected early by rheumatoid arthritis
  • Interphalangeal joints
    • Bouchard nodes (in osteoarthritis) and Boutonniere deformity (in rheumatoid arthritis) affect the PIP.
    • Heberden nodes (in osteoarthritis) and Swan neck deformity (in rheumatoid arthritis) affect the DIP.

Muscles and fascia

• De Quervain tenosynovitis
• Dupuytren contracture
• Carpal tunnel syndrome
• Guyon canal syndrome
• Supinator syndrome

Innervation

• Ulnar nerve entrapment
• Median nerve neuropathy
• Radial neuropathies

Other

• Finger injuries
• Volkmann ischemic contracture
• Compartment syndrome