Hand anatomy

Anatomical Structures: Forearm and Wrist Bones

• Anterior Humerus
  • Coronoid Fossa: An indentation on the anterior surface of the humerus.
  • Trochlea: The spool-shaped structure of the humerus that articulates with the ulna.
  • Capitulum: A rounded knob on the humerus that articulates with the radius.
  • Head of Radius: The proximal end of the radius that articulates with the capitulum.
  • Coronoid Process: A projection on the proximal ulna that contributes to the ulnohumeral joint.
  • Neck of Radius: The area just below the head of the radius.
  • Radial Tuberosity: An area on the radius for the attachment of the biceps tendon.
  • Interosseous Membrane: A fibrous membrane connecting the radius and ulna, providing additional support.
  • Radius: The lateral bone of the forearm in anatomical position.
  • Head of the Ulna: The distal end of the ulna that articulates with the radius.
  • Ulnar Notch of the Radius: A concave notch on the radius for the articulation with the head of the ulna.
  • Lunate: A bone in the proximal row of the carpal bones that articulates with the radius.
  • Scaphoid Bone: The largest bone in the proximal row of carpal bones.

Shaft of the Ulna

• Shape: Triangular.
• Borders:
  • Posterior Border: Palpable along the forearm length.
  • Interosseous Border: Attachment site for the interosseous membrane.
  • Anterior Border: Unremarkable.
• Surfaces:
  • Anterior Surface: Attachment for the pronator quadratus muscle.
  • Posterior Surface: Attachment site for various muscles.
  • Medial Surface: Generally unremarkable.
• Distal Movement: Width decreases moving distally.

Shaft of the Radius

• Shape: Triangular, increasing diameter distally.
• Borders:
  • Anterior Border: Unremarkable.
  • Interosseous Border: Similar to ulna's border, for attachment.
  • Posterior Border: Contacts muscles.
• Lateral Surface Feature: Roughening (Pronator Tuberosity) for the attachment of the pronator teres muscle.

Interosseous Membrane

• Function: Connects the radius and ulna, stabilizing the forearm, and serving as a barrier between anterior and posterior compartments. It distributes pressure from the radius to the ulna.
• Passageway: Channels for the anterior interosseous nerve and artery.
• Injury Context: Commonly injured during a fall on an outstretched hand ($FOOSH$).

Distal Radius Features

• Medially: Contains the Ulnar Notch, which articulates with the head of the ulna in the distal radio-ulnar joint.
• Laterally: Features the Radial Styloid Process, which is more distal than the ulnar styloid process.
• Dorsal Tubercle (Lister's Tubercle): Functions as a pulley for thumb extensor muscles (Extensor Pollicis Longus).
• Facets on Distal Surface:
  • For articulation with the Scaphoid and Lunate carpal bones, forming the primary radiocarpal joint.

Distal Ulna

• Structure: Smaller in diameter compared to the proximal end.
• Termination: Ends with a rounded head and the Ulnar Styloid Process.
• Articulation: The head connects with the ulnar notch of the radius; it is separated from the carpal bones by a fibrocartilaginous articular disc.

Carpal Bone Structure

• Proximal Carpal Bones: $8$ irregularly shaped bones arranged in two rows.
• Metacarpals: $5$ bones, each corresponding to a digit, numbered $1-5$ starting from the thumb.
• Phalanges: Bones of the fingers; each finger has $3$ phalanges (proximal, middle, distal) except the thumb (Pollux), which has $2$.

Metacarpals and Phalanges Details

• Metacarpals: Each has an epiphysis (base), diaphysis (shaft), and head (distal). Numbered from $1-5$ (lateral to medial).
• Phalanges Count: Total of $14$ in each hand.
  • Thumb: $2$ phalanges.
  • Digits $2-5$: $3$ phalanges each.
• Palpal Areas: Important landmarks include the pisiform (palpable at the base of the hypothenar eminence) and scaphoid (floor of the anatomical snuffbox).

Carpal Bone Arrangement

• Rows: Comprises two rows of four carpal bones each.
  • Proximal Row: Scaphoid, Lunate, Triquetrum, Pisiform.
  • Distal Row: Trapezium, Trapezoid, Capitate (largest), Hamate (features a hook).

Carpal Tunnel Anatomy and Syndrome

• Description: An osseofibrous gateway formed by carpal bones (deep) and the flexor retinaculum (superficial).
• Contents: Includes the median nerve and $9$ tendons: $4$ flexor digitorum superficialis, $4$ flexor digitorum profundus, and $1$ flexor pollicis longus.
• Syndrome: Carpal Tunnel Syndrome (CTS) occurs via median nerve compression. Symptoms include paresthesia, numbness in the lateral $3.5$ digits, and thenar muscle atrophy.

Fracture Types

• Scaphoid Fracture: Most common carpal fracture. High risk of avascular necrosis and non-union due to retrograde blood supply.
• Colles' Fracture: Transverse fracture of the distal radius with dorsal displacement of the distal fragment. Common in osteoporosis; presents with a "dinner fork" deformity.
• Smith's Fracture: Reversed Colles'; distal radius fracture with volar (anterior) displacement.

Wrist Movements

• Flexion: Primarily by flexor carpi ulnaris ($FCU$) and flexor carpi radialis ($FCR$).
• Extension: Primarily by extensor carpi radialis longus ($ECRL$), brevis ($ECRB$), and extensor carpi ulnaris ($ECU$).
• Adduction (Ulnar Deviation): Produced by $FCU$ and $ECU$.
• Abduction (Radial Deviation): Produced by $FCR$, $ECRL$, $ECRB$, and abductor pollicis longus ($APL$).

Ligaments of the Wrist

• Palmar Radiocarpal Ligaments: Strong bands ensuring the hand follows the radius during supination.
• Dorsal Radiocarpal Ligaments: Ensure hand follows the radius during pronation.
• Collateral Ligaments: Radial collateral (radius to scaphoid) and ulnar collateral (ulna to triquetrum/pisiform).

Joints of the Hand

• Intercarpal Joints: Plane synovial joints facilitating sliding movements.
• Metacarpophalangeal (MCP) Joints: Condyloid joints; allow flexion, extension, abduction, and adduction.
• Interphalangeal (IP) Joints: Hinge joints; allow only flexion and extension (proximal $PIP$ and distal $DIP$).

Muscles of the Forearm: Overview

• Anterior Compartment: Generally flexors and pronators. Mostly supplied by the median nerve.
• Posterior Compartment: Generally extensors and supinators. Supplied by the radial nerve.

Anterior Compartment Muscles:

• Superficial Layer:
  • Pronator teres: Pronation and weak elbow flexion.
  • Flexor carpi radialis ($FCR$): Flexion and abduction of wrist.
  • Palmaris longus: Weak wrist flexor (absent in approximately $14\%$ of the population).
  • Flexor carpi ulnaris ($FCU$): Flexion and adduction of wrist (Innervated by the ulnar nerve).
• Intermediate Layer:
  • Flexor digitorum superficialis ($FDS$): Flexes middle phalanges at $PIP$ joints.
• Deep Layer:
  • Flexor digitorum profundus ($FDP$): Flexes distal phalanges at $DIP$ joints (Lateral half: Median nerve; Medial half: Ulnar nerve).
  • Flexor pollicis longus ($FPL$): Flexes thumb.
  • Pronator quadratus: Prime mover for pronation.

Posterior Compartment Muscles:

• Brachioradialis: Flexes forearm at the elbow; active during rapid movement or resisted flexion in mid-prone position.
• Superficial Extensors:
  • Extensor carpi radialis longus/brevis: Extend and abduct wrist.
  • Extensor digitorum: Primary extender of medial four digits.
  • Extensor digiti minimi: Extends the little finger.
  • Extensor carpi ulnaris: Extends and adducts wrist.
• Deep Extensors:
  • Supinator: Supinates forearm.
  • Abductor pollicis longus ($APL$): Abducts thumb.
  • Extensor pollicis brevis ($EPB$): Extends proximal phalanx of thumb.
  • Extensor pollicis longus ($EPL$): Extends distal phalanx of thumb (wraps around Lister's tubercle).
  • Extensor indicis: Extends index finger independently.