Anterior Compartment of the Forearm

Contents

• Muscles
  • Pronator teres
  • Flexor carpi radialis
  • Flexor digitorum superficialis
  • Palmaris longus
  • Flexor carpi ulnaris
  • Flexor digitorum profundus
  • Flexor pollicis longus
  • Pronator quadratus
• Nerves
  • Lateral cutaneus nerve of forearm
  • Medial cutaneus nerve of forearm
  • Radial nerve
  • Median nerve
  • Anterior interosseous nerve
  • Ulnar nerve
• Vessels
  • Brachial artery, branches into radial and ulnar arteries
  • Cephalic vein, basilic vein, and median forearm vein
• Actions
  • Primarily flexors (elbow, wrist, fingers)
  • Pronation

Superficial Muscles

• 5 muscles which share a common origin, the anterior surface of the medial epicondyle of the humerus (common flexor origin)

Pronator teres

• Origin
  • Superficial head arises from common flexor origin and medial supracondylar ridge
  • Deep head arises from medial border of coronoid process
• Insertion
  • Lateral border of mid-radial shaft
• Nerve Supply
  • Median nerve (via first muscular branch)
• Action
  • Pronation of the forearm
  • Flexion of the elbow
• Related structures
  • Median nerve passes between the two heads
  • Ulnar artery passes deep to the deep head
  • Forms the medial border of the cubital fossa

Flexor carpi radialis

• Origin
  • Common flexor origin, distal to pronator teres
• Insertion
  • Base of 2nd and 3rd metacarpals
  • Occasional insertion onto scaphoid
• Nerve Supply
  • Median nerve
• Action
  • Flexion of the wrist
  • Radial abduction of the wrist
  • Pronation of the forearm
  • Flexion of the elbow
• Related structures
  • Radial artery lies lateral to tendon
  • Median nerve and palmaris longus tendon lie medial to tendon

Flexor digitorum superficialis

• Origin
  • Common flexor origin
  • Medial ligament of the elbow joint
  • Medial border of the coronoid process
  • Via fibrous arch, travels to and originates from the length of the anterior oblique line of the radius
• Insertion
  • Middle phalanges of 2nd to 5th fingers
• Nerve Supply
  • Median nerve
• Action
  • Flexor of PIP joints, MCP joints, and wrist joints
  • Flexion of the elbow and wrist
• Related structures
  • Ulnar nerve runs on its deep surface in the forearm

Palmaris longus

• Absent in 13% of arms
• Origin
  • Common flexor origin
• Insertion
  • Flexor retinaculum (adherent to surface) and into the palmar aponeurosis
• Nerve Supply
  • Median nerve
• Action
  • Weak flexor of wrist
  • Minimal MCP joint flexion
• Related structures
  • Overlaps median nerve

Flexor carpi ulnaris

• Origin
  • Common flexor origin (humeral head)
  • Wide aponeurosis arising form the medial border of the olecranon and upper 3/4 of the ulna (ulnar head)
• Insertion
  • Base of 5th metacarpal
• Nerve Supply
  • Ulnar nerve
• Action
  • Flexion of the wrist
  • Ulnar adduction (with extensor carpi ulnaris)
• Related structures
  • Pisiform (sesamoid bone within tendon)
  • Ulnar nerve passes into the forearm between its humeral and ulnar heads

Cubital Fossa

Borders

• Roof: deep fascia; medially, bicipital aponeurosis
• Floor: brachialis
• Medial border: pronator teres
• Lateral border: brachioradialis
• Proximal border: transverse line between humeral epicondyles

Contents

• From medial to lateral:
  • Median nerve
  • Brachial artery
  • Biceps tendon
  • Radial and posterior interosseus nerves (under brachioradialis)

Related structures

• Median basilic vein and medial cutaneous nerve of forearm lie on bicipital aponeurosis (medial roof)
• Median cephalic vein and lateral cutaneous nerve of the forearm lie on lateral roof

Deep Muscles

Flexor digitorum profundus

• Origin
  • Medial surface of the olecranon
  • 3/4 medial and anterior surfaces of hte ulna
  • Interosseous membrane
• Insertion
  • Tip of distal phalanges of 2nd to 3rd digits
• Nerve Supply
  • Anterior interosseous nerve (median nerve)
    • Supplies index and middle finger
  • Ulnar nerve
    • Supplies ring and little finger
  • In 40% of individuals, this shared innervation can be 3:1 or 1:3 in distribution
• Action
  • Flexion of DIP joints, MCP joints, and wrist
• Related structures
  • Lumbricals take origin from the 3 lateral tendons where they separate in the palm
    • Innervation of the lumbricals follows the innervation of the parent tendon

Flexor pollicis longus

• Origin
  • Anterior surface of the radius and the interosseous membrane
  • Some fibres from medial and/or lateral border of the coronoid process
• Insertion
  • Base of the distal phalanx of the thumb
• Nerve Supply
  • Anterior interosseous nerve (median)
• Action
  • Sole flexor of the IP joint of the thumb
  • Flexion of the MCP and CMC joints of the thumb and wrist
• Related structures
  • Passes through carpal tunnel deep to flexor carpi radialis

Pronator quadratus

• Origin
  • Distal 1/4 of anteromedial ulna
• Insertion
  • Anterolateral border of the distal radius
  • Interosseous membrane above ulnar notch
• Nerve Supply
  • Anterior interosseous nerve (median)
• Action
  • Pronation of the forearm
  • Holds the radius and ulna together during weight-bearing
• Related structures
  • Space of Parona is superficial to pronator quadratus and deep to the flexor tendons, containing the proximal extensions of the flexor synovial sheaths

Nerves

Lateral cutaneous nerve of forearm

• Course
  • Continuation of the musculocutaneous nerve
  • Pierces deep fascia lateral to biceps tendon in the elbow
• Innervation
  • Supplies lateral border of the forearm via anterior and posterior branches, down to the thenar eminence

Medial cutaneous nerve of forearm

• Course
  • Arises from medial cord in the brachial plexus
• Innervation
  • Supplies the medial border of the forearm via anterior and posterior branches, down to the wrist

Superficial radial nerve

• Course
  • Cutaneous branch of radial nerve
  • Travels deep to brachioradialis, superficial to supinator, pronator teres, and flexor digitorum superficialis
    • Lies radial to the radial artery during this course
  • Passes backwards under tendon of brachioradialis near the radial styloid and exits the flexor compartment, branching in the dorsum of the hand
• Innervation
  • Cutaneous innervation to radial 2/3 of dorsum of hand, proximal dorsal thumb, index, middle, and radial half of the ring fingers

Median nerve

• Course
  • Gives off branch to pronator teres then passes between its two heads, travelling in the midline of the forearm deep to flexor digitorum superficialis
  • Passes beneath flexor retinaculum into the hand
• Innervation
  • Pronator teres
  • Flexor carpi radialis
  • Palmaris longus
  • Flexor digitorum superficialis
• Branches
  • Anterior interosseous nerve
    • Runs with anterior interosseous artery
    • Supplies flexor digitorum profundus to index and middle fingers, flexor pollicis longus, and pronator quadratus; the interosseous membrane and periosteum of the radius and ulna; and terminally the capsule of the wrist and carpal joints (no cutaneous branches)
  • Palmar cutaneous
    • Given off above flexor retinaculum
    • Supplies skin over thenar muscles

Ulnar nerve

• Course
  • Enters from the extensor compartment by passing between the two heads of flexor carpi ulnaris
  • Runs deep to flexor carpi ulnaris on the ulnar side of the ulnar artery, superficial to flexor digitorum profundus
• Innervation
  • Flexor carpi ulnaris
  • Ulnar half of the flexor digitorum profundus
• Branches
  • Dorsal branch
    • Supplies the ulnar one and a half fingers (proximal to nailbeds) and dorsum of the hand
  • Palmar cutaneous branch
    • Runs along ulnar artery and pierces fascia proximal to the flexor retinaculum
    • Supplies skin of the hypothenar eminence

Vessels

Arteries

• Course
  • Brachial artery enters via cubital fossa and divides into the radial and ulnar arteries:
    • Radial artery
      • Passes medial to biceps tendon, crosses supinator and pronator teres, radial flexor digitorum supeficialis, flexor pollicis longus origin, pronator quadratus insertion and the distal radius; travels under brachioradialis
      • In the mid forearm, lies medial to the radial nerve
      • Traverses anatomical snuffbox deep to tendons of abductor pollicis longus and extensor pollicis brevis (1st extensor compartment)
    • Ulnar artery
      • Exits cubital fossa deep to deep head of pronator teres and the fibrous arch of flexor digitorum superficials, superficial to flexor digitorum profundus with the ulnar nerve to its ulnar side
      • Continues into palm as the superficial palmar arch
      • Branches into common interosseous nerve which further branches into anterior and posterior interosseous branches
        • Anterior interosseous artery lies on top of the interosseous membrane, deep to flexor digitorum profundus and flexor pollicis longus and sending perforating branches through the membrane to the deep extensor compartment
    • Both give off anterior and posterior carpal branches forming the anterior and posterior carpal arches
      • Anterior carpal arch anastamoses with the deep palmar arch
      • Posterior carpal arch sends branches into each metacarpal space and supplies the fingers via anastamoses with the palmar digital and metacarpal branches of the palmar arches

Veins

• Course
  • Blood from the deep palm drains into a dorsal venous arch
    • Radial side drains into the cephalic vein (in anatomical snuffbox) which continues into the arm in the deltopectoral groove, perforating the clavipectoral fascia and draining into the axillary vein
    • Ulnar side drains into the basilic vein which travels medially up the arm joining the brachial veins midway between the elbow and axilla to form the axillary vein
  • Blood from subcutaneous tissues of the wrist and hand drains into the median forearm vein
    • Branches into median cephalic and median basilic veins at the elbow which drain into the cephalic and basilic veins respectively

References

Last, R. and McMinn, R., 1994. Last’s Anatomy. 9th ed. Edinburgh: Churchill Livingstone.
Netter, F., 2018. Atlas Of Human Anatomy. 7th ed. Elsevier.

Author Contributions

Karen Bourns