Forearm

The forearm is the region of the upper limb between the elbow and the wrist. The term “forearm” is used in anatomy to distinguish this area from the arm, a term that is commonly used to describe the entire upper limb. The forearm consists of 2 long bones (the radius and the ulna), the interosseous membrane, and multiple arteries, nerves, and muscles. The muscles are grouped into 2 compartments: anterior and posterior. The function of these muscles is flexion and extension of the wrist and fingers, while also contributing to flexion of the elbow.

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Bones and Articulations of the Forearm

Bones

The forearm contains 2 long bones: 

  • Radius
    • Located on the lateral side of the forearm
    • Articulates proximally with the humerus and ulna
    • Articulates distally with the ulna, scaphoid, and lunate bones
  • Ulna
    • Located on the medial side of the forearm
    • Tapers gradually from proximal to distal
    • Articulates proximally with the humerus and radius
    • Articulates distally with the radius and the articular disc of the wrist (triangular fibrocartilage complex (TFCC))

The bones of the forearm, the radius laterally and ulna medially, are held together by the fibrous interosseous membrane.

Joints

  • Proximal radioulnar joint
    • Pivot joint
    • Between the head of the radius and the ring formed by the radial notch of the ulna and the annular ligament
  • Distal radioulnar joint
    • Pivot joint
    • Between the head of the ulna and the ulnar notch of the distal radius
    • Uniaxial, allows for the movements of pronation-supination
    • Supported by the TFCC, a major ligamentous stabilizer of the distal radioulnar joint and ulnar carpus that cushions the forces transmitted through the ulnar side of the wrist. Consists of:
      • Central articular disc
      • Meniscus homolog
      • Ulnar collateral ligament
      • Dorsal and volar radioulnar ligaments
      • Extensor carpi ulnaris subsheath
  • Interosseous membrane
    • Fibrous tissue that creates the radioulnar syndesmosis
    • Spans the space between the radius and ulna and transfers forces during pronation and supination
Inferior view of the distal radio-ulnar joint

Distal radioulnar joint

Image by Lecturio.

Muscles of the Anterior Compartment of the Forearm

The muscles of the anterior compartment of the forearm are often separated into superficial, intermediate, and deep layers.

Superficial layer

MuscleOriginInsertionInnervationFunction
Pronator teres
  • Ulnar head: coronoid process of the ulna
  • Humeral head: medial epicondyle
Middle of lateral surface of radiusMedian nerve (C7)Pronates and flexes forearm
Flexor carpi radialisMedial epicondyle of humerusBase of 2nd metacarpalFlexes and abducts wrist
Palmaris longusFlexor retinaculum and palmar aponeurosisFlexes wrist and tenses palmar aponeurosis
Flexor carpi ulnarisOlecranon and posterior ulnaPisiform, hook of hamate, 5th metacarpalUlnar nerve (C8)Flexes and adducts wrist
Anterior view of the right forearm, featuring the muscles of the superficial layer of the anterior compartment

Anterior view of the right forearm, featuring the muscles of the superficial layer of the anterior compartment

Image by BioDigital, edited by Lecturio.

Intermediate layer

MuscleOriginInsertionInnervationFunction
Flexor digitorum superficialis
  • Humero-ulnar head: medial epicondyle (humerus) and coronoid process (ulna)
  • Radial head: shaft
Middle phalanges of medial 4 digitsMedian nerve (C7, C8, T1)
  • Flexes wrist
  • Flexes proximal interphalangeal joint
Anterior view of the right forearm, featuring the only muscle of the intermediate layer of the anterior compartment

Anterior view of the right forearm, featuring the muscles of the deep layer of the anterior compartment

Image by BioDigital, edited by Lecturio.

Deep layer

MuscleOriginInsertionInnervationFunction
Flexor digitorum profundusProximal surfaces of medial and anterior surfaces of ulna and interosseous membraneMiddle phalanges of medial 4 digits
  • Digits 2–3: median nerve (C7, C8, T1)
  • Digits 4–5: ulnar nerve (T1)
  • Flexes wrist
  • Flexes distal interphalangeal joint
Flexor pollicis longusAnterior surface of radius and interosseous membraneDistal phalanx of 1st digitAnterior interosseous nerve, branch of median (C8)
  • Flexes wrist
  • Flexes interphalangeal and metacarpo- phalangeal joints of 1st digit
Pronator quadratusDistal quarter of ulnaDistal quarter of radiusPronates forearm
Anterior view of the right forearm, featuring the muscles of the deep layer of the anterior compartment

Anterior view of the right forearm, featuring the muscles of the deep layer of the anterior compartment

Image by BioDigital, edited by Lecturio.

Muscles of the Posterior Compartment of the Forearm

The muscles of the posterior compartment of the forearm are separated into superficial and deep layers.

Superficial layer

MuscleOriginInsertionInnervationFunction
Brachio-radialisProximal ⅔ of lateral supracondylar ridgeLateral surface of distal radius and pre-styloid processRadial nerve (C6)Weak flexor of elbow, strong flexor when midpronated
Extensor carpi radialis longusLateral supracondylar ridgeDorsal aspect of 2nd metacarpalRadial nerve (C6, C7)Extend and abduct wrist
Extensor carpi radialis brevisLateral epicondyle (common extensor origin)Dorsal aspect of 3rd metacarpalDeep branch of radial nerve (C7)
Extensor digitorumLateral epicondyle (common extensor origin)Extensor expansion of medial 4 digitsPosterior interosseous nerve (C7; from deep radial nerve)
  • Extend wrist
  • Extend medial 4 digits at metacarpophalangeal and interphalangeal joints (5th digit for EDM)
Extensor digiti minimiExtensor expansion of 5th digit
Extensor carpi ulnarisLateral epicondyle and posterior surface of ulnaDorsal aspect of 5th metacarpalExtends and adducts wrist
AnconeusInserts on the posterior aspect of lateral epicondyleLateral surface of the olecranonRadial nerve (C7, C8)Extends the elbow

Deep layer

MuscleOriginInsertionInnervationFunction
Abductor pollicis longusPosterior surface of radius and ulna and interosseous membraneMetacarpal of 1st digitPosterior interosseous nerve (C8) from deep radial nerve
  • Extends wrist
  • Abducts thumb and extends it at carpometacarpal joint
Extensor pollicis longusPosterior surface of ulna and interosseous membraneDorsal surface of distal phalanx of 1st digit
  • Extends wrist
  • Extends distal (extensor pollicis longus (EPL))/proximal (extensor pollicis brevis (EPB)) phalanx of thumb at interphalangeal joint
  • Extends metacarpophalangeal and carpometacarpal joints
Extensor pollicis brevisPosterior surface of radius and interosseous membraneDorsal surface of proximal phalanx of 1st digit
SupinatorLateral epicondyle of humerus, supinator fossa, and proximal ulnaPosterior/lateral/anterior surfaces of proximal radiusDeep branch of radial nerve (C8)Supination of forearm
Extensor indicisPosterior surface of ulna and interosseous membraneExtensor expansion of 2nd digitPosterior interosseous nerve (C8) from deep radial nerve
  • Extends wrist
  • Extends 2nd digit (independent)

Vessels of the Forearm

Arteries

The 2 main arteries of the forearm, the radial and ulnar arteries, are branches of the brachial artery. These 2 arteries join in the hand, forming an anastomosis, the superficial and deep palmar arteries.

  • Radial artery
    • Supplies the posterior lateral forearm
    • Gives off important branches and contributions:
      • Radial recurrent artery: anastomoses around the elbow
      • Palmar carpal branch: supplies the carpal bones
      • Contributes to the superficial and deep palmar arches of the hand 
  • Ulnar artery
    • Supplies the anterior medial forearm and interosseous membrane
    • Gives off important branches and contributions:
      • Ulnar recurrent artery: anastomoses around the elbow
      • Common interosseous artery: gives off anterior and posterior interosseous arteries
      • Dorsal and palmar carpal branches: supply the carpal bones
      • Contributes to the superficial and deep palmar arches of the hand

Veins

The veins of the forearm are divided into superficial and deep:

  • Superficial veins
    • Cephalic: lateral forearm, connected to basilic via the median cubital vein
    • Basilic: medial forearm, connects with the median cubital vein and median antebrachial vein
    • Median antebrachial vein: within subcutaneous tissue, most commonly terminates in the median cubital vein
  • Deep veins
    • Radial and ulnar: communicate with superficial veins via perforator veins, joined by transverse branches

Innervation of the Forearm

The median, ulnar, and radial nerves pass through the forearm and innervate the muscles of both the anterior and posterior compartments.

The 3 main motor nerves of the forearm innervate the following muscles:

  • Median: all muscles of the anterior or flexor compartment of the forearm (except for the 2 muscles supplied by the ulnar nerve)
  • Ulnar: flexor carpi ulnaris and medial half of the flexor digitorum profundus in the anterior or flexor compartment
  • Radial: all muscles of the posterior or extensor compartment of the forearm

There are 3 main sensory nerves of the forearm:

  • Lateral antebrachial cutaneous nerve (from the musculocutaneous nerve): innervates the skin over the lateral arm
  • Medial antebrachial cutaneous nerve (from the medial cord of the brachial plexus): innervates the skin over the medial forearm
  • Inferior or posterior lateral cutaneous nerve (from the radial nerve): innervates the skin over the posterior arm

Clinical Relevance

The following are common clinical conditions related to the forearm:

  • Distal radius fractures: common fractures of the arm, with a bimodal distribution. May be caused by low-energy falls or high-energy trauma in younger patients. More common in women over the age of 50 with osteoporosis.
  • Triangular fibrocartilage complex injuries: The TFCC is the primary ligamentous stabilizer of the distal radioulnar joint. The TFCC can be injured acutely with a fall on an outstretched hand or chronically secondary to overuse injuries.
  • Peripheral nerve injuries in the upper extremity:
    • Median nerve neuropathy in the forearm: The most common location of median nerve mononeuropathy is the carpal tunnel. Two less common median nerve entrapments found in the proximal forearm include pronator syndrome and anterior interosseous syndrome. 
      • Carpal tunnel syndrome (CTS): The most common compressive neuropathy is often precipitated by repetitive vibration and motions and is associated with diabetes, thyroid disease, rheumatoid arthritis, and pregnancy.
    • Radial nerve neuropathy: The radial nerve can become injured in the forearm through its branch, the posterior interosseous nerve (PIN), causing supinator syndrome and radial tunnel syndrome. In addition, the radial superficial nerve (RSN) can be injured at the distal forearm, and is called cheiralgia paresthetica or “handcuff neuropathy.”
    • Ulnar nerve neuropathy: The ulnar nerve may be injured at the posterior medial elbow secondary to direct trauma to the medial elbow, lacerations, fractures of the distal humerus, or entrapment of the nerve. 
      • Cubital tunnel syndrome: compressive neuropathy of the ulnar nerve at the elbow; the 2nd-most common compressive neuropathy. Cubital tunnel syndrome can present with interosseous muscle atrophy and a “claw hand.”  
      • Guyon’s canal neuropathy: compression of the ulnar nerve at the distal forearm/wrist at Guyon’s canal. The condition is sometimes called “handlebar palsy,” as it is seen in cyclists.

References

  1. Drake, R.L., Vogl, A.W., & Mitchell, A.W.M. (2014). Gray’s Anatomy for Students (3rd ed.). Philadelphia, PA:  Churchill Livingstone.

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