Hand

Bones of the Hand

The bones of the hand consist of the following:

• Metacarpals
  • There are 5 metacarpal bones corresponding to each of the fingers.
  • The proximal portion is called the base.
  • The middle portion is called the shaft.
  • The distal portion is called the head.
• Phalanges 
  • Digits 2–5 have 3 phalanges: proximal, middle, and distal.
  • The 1st digit or the thumb only has 2 phalanges: proximal and distal.

Joints of the Hand

The joints of the hand and fingers consist of:

• Metacarpal-phalangeal joints, connecting the metacarpals to the fingers
• Interphalangeal joints, the hinge joints between the phalanges of the fingers

Intrinsic Muscles of the Hand

The muscles of the hand are divided into 2 groups based on muscle belly location:

1. Extrinsic: Muscle bellies are in the forearm; they provide strength and grip. (See the following section.)
2. Intrinsic: Muscle bellies are within the hand and are responsible for fine movement of the fingers.
   • Thenar muscles: 
     • Form the thenar eminence
     • Responsible for multiple motions of the thumb
     • All innervated by the median nerve (except for the adductor pollicis, which is innervated by the ulnar nerve)
   • Hypothenar muscles: 
     • Form the hypothenar eminence
     • Responsible for multiple motions of the 5th digit (“little finger”)
     • All innervated by the ulnar nerve
   • Lumbricals: 
     • Link the flexor and extensor tendons of the fingers
     • Flex the MCP joints and contribute to the extension of the interphalangeal joints
     • Lumbricals I and II (lateral side of the hand) are innervated by the median nerve.
     • Lumbricals III and IV (medial side of the hand) are innervated by the ulnar nerve.
   • Interossei: 
     • 4 dorsal interossei abduct the 2nd–5th fingers.
     • 3 palmar interossei adduct the 2nd–5th fingers.
     • Are innervated by the ulnar nerve

Thenar muscles

Hypothenar muscles

Lumbrical muscles

Interossei muscles

Extrinsic Muscles of the Hand

The extrinsic muscles of the hand have their origin and muscle bellies in the forearm and are divided into flexor (anterior compartment) and extensor (posterior compartment) muscles.

Extrinsic extensor muscles: superficial layer

Extrinsic extensor muscles: deep layer

Extrinsic flexor muscles: superficial layer

Extrinsic flexor muscles: deep layer

Movements of the Fingers

Extensor expansion

Also known as the extensor hood, dorsal expansion, dorsal hood, or dorsal aponeurosis of the hand and/or fingers. The extensor expansion comprises the tendons of the extensor muscles of the fingers and how these insert into the phalanges:

• At the distal end of the metacarpal, these tendons expand and merge to form an aponeurosis (“hood”), which covers:
  • Head of the metacarpal (dorsal and lateral sides)
  • Proximal phalanx (dorsal and lateral sides)
• At the proximal phalanx, the tendons divide into bands:
  • Lateral bands (2 per finger): pass on either side of the middle phalanx and insert into the distal phalanx, receiving tendons from the lumbricals, extensor indicis, and dorsal and palmar interossei muscles
  • Central band (1 per finger): passes down the center of the proximal phalanx to insert into the base of the middle phalanx
  • Retinacular bands (2 per finger): pass obliquely on the lateral sides of the middle phalanx to connect the aponeuroses of palmar and dorsal sides of the finger

Flexor pulley system

Comprises the tendons of the flexor muscles of the fingers and how these insert into the phalanges, supported by a system of ligaments:

• Palmar aponeurosis
• 5 annular pulleys: made by transverse retinacular ligaments
• 3 cruciform pulleys: made by oblique cruciate ligaments
• Flexor muscles of the fingers:
  • Flexor digitorum superficialis tendons split at the proximal phalanx and insert laterally into the middle phalanx.
  • Flexor digitorum profundus tendons pass centrally and insert into the distal phalanx.

Vessels of the Hand

Arterial supply

The arterial blood supply of the hand is provided by:

• Radial artery: palpated immediately lateral to the flexor carpi radialis tendon and just proximal to the wrist crease
• Ulnar artery: palpated anterior medially on the wrist and passes into the hand through Guyon’s canal
• Together, the radial and ulnar artery form 2 arterial arches:
  • Superficial palmar arch: ulnar artery + superficial palmar branch of the radial
  • Deep palmar arch: radial artery + deep palmar branch of the ulnar

Venous drainage

The venous drainage of the hand is the origin of the veins of the upper extremity and begins in the dorsal and palmar venous network of the hand.

• Cephalic vein originates from the dorsal venous network in the area of the anatomical snuffbox.
• Basilic vein originates from the ulnar side of the dorsal venous network of the hand.
• Median antebrachial vein originates from the palmar venous network of the hand.

Innervation of the Hand

The innervation of the hand is primarily via the median and ulnar nerves, with the radial nerve only contributing a small area of sensory innervation on the radial dorsal aspect of the hand.

Motor innervation

Sensory innervation

Clinical Relevance

The following are common problems associated with the hand:

• Boxer’s fracture: fracture of the distal portion of the 5th metacarpal bone. Usually due to hitting an object with a closed fist. Symptoms include pain and a depressed knuckle. Diagnosis is suspected based on the clinical exam and confirmed by X-rays.
• Rheumatoid arthritis: a majority of rheumatoid diseases affect the hand and finger joints. The synovial proliferation results in skeleton erosion, destruction of the capsular ligamentous structures, and changes to tendon structure and function. Physical exam of the hand classically presents with ulnar deviation and subluxation of the metacarpal phalangeal (MCP) joints, swan neck deformities of the fingers, and Z deformity of the thumb.
• Swan neck deformity: characterized by flexion of the DIP joint and hyperextension of the PIP joint. Associated with rheumatoid arthritis, trauma, and laceration.
• Boutonniere deformity: characterized by DIP joint extension and PIP joint flexion. Secondary to rupture of the central slip of the extensor mechanism over the PIP joint. Associated with trauma (PIP joint dislocation) and rheumatoid arthritis.
• Osteoarthritis: a degenerative disorder of the articular cartilage, subchondral bones, and other joint structures. Osteoarthritis is the most common type of joint disease and the leading cause of disability in older adults. The main risk factors are family history, female gender, past trauma to the involved joint, aging, and obesity.
  • Heberden’s nodes (DIP): usually secondary to primary osteoarthritis of the DIP joints. Associated with mucous cysts of the DIP joint and nail deformities. May contribute to joint contracture and deformity.
  • Bouchard’s nodes (PIP): hard, bony outgrowths or gelatinous cysts on the PIP joints. Less commonly, Bouchard’s nodes may be seen in rheumatoid arthritis. May contribute to joint contracture and deformity.