Location
The
forearm is located
distal to the elbow and
proximal to the wrist. It lies between the
humerus (upper
arm bone) and the
carpal bones of the wrist. The
radius is positioned on the lateral (
thumb) side, while the
ulna lies medially (pinky side) when in the anatomical position.
Anatomy
The
forearm is the region of the upper limb between the
elbow and the
wrist. It plays a structural and functional role in linking the upper arm to the
hand and is involved in
movement and
stability of the wrist and fingers. The forearm contains
two long bones (radius and ulna), a network of
muscles,
nerves,
blood vessels, and soft
tissue structures.
Here is a detailed description of the anatomy of the forearm:
Bones of the Forearm
The forearm contains two long bones, the
radius and the
ulna, which run parallel to each other:
Radius (Lateral Bone):
- Located on the thumb side (lateral side) of the forearm.
- Shorter and slightly thicker than the ulna.
- Proximal End:
- Radial Head: A disc-shaped structure that articulates with the capitulum of the humerus at the elbow joint.
- Radial Neck: Narrow region just below the head.
- Radial Tuberosity: A bony prominence on the medial side for attachment of the biceps brachii tendon.
- Shaft: Slightly curved as it runs distally.
- Distal End:
- Styloid Process: A pointed projection at the lateral end of the radius.
- Ulnar Notch: A medial indentation for articulation with the ulna at the distal radioulnar joint.
- Articulates with the carpal bones of the wrist to form the radiocarpal joint.
Ulna (Medial Bone):
- Located on the pinky side (medial side) of the forearm.
- Longer and thinner than the radius.
- Proximal End:
- Olecranon Process: A large, curved bony projection at the back of the elbow that fits into the olecranon fossa of the humerus.
- Coronoid Process: A small anterior projection that fits into the coronoid fossa of the humerus.
- Trochlear Notch: A large curved notch that articulates with the trochlea of the humerus to form the hinge portion of the elbow joint.[1]
- Radial Notch: A lateral indentation for articulation with the head of the radius at the proximal radioulnar joint.
- Shaft: Runs parallel to the radius.
- Distal End:
- Head of the Ulna: A rounded distal projection that articulates with the ulnar notch of the radius at the distal radioulnar joint.
- Styloid Process: A small, pointed projection at the medial aspect of the wrist.
Joints of the Forearm
The forearm has
two major joints that allow rotational movement of the forearm (pronation and supination):
Proximal Radioulnar Joint
- Type: Pivot joint.
- Articulation: Between the head of the radius and the radial notch of the ulna.
- Supported by the annular ligament that encircles the head of the radius.
Distal Radioulnar Joint
- Type: Pivot joint.
- Articulation: Between the ulnar head and the ulnar notch of the radius.
- Supported by the triangular fibrocartilage complex (TFCC), which stabilizes the joint.
Both joints allow for the
rotation of the radius around the ulna, facilitating pronation and supination of the forearm.
Muscles of the Forearm
The forearm muscles are organized into two compartments, separated by the
interosseous membrane:
Anterior Compartment (Flexors and Pronators)
The anterior compartment muscles are involved in
wrist flexion and
forearm pronation. These muscles are further divided into
superficial,
intermediate, and
deep layers:
- Superficial Layer (originate from the medial epicondyle of the humerus):
- Intermediate Layer:
- Deep Layer:
Posterior Compartment (Extensors and Supinators)
The posterior compartment muscles are involved in
wrist extension and
forearm supination.
[2] These muscles are divided into
superficial and
deep layers:
- Superficial Layer (originate from the lateral epicondyle of the humerus):
- Deep Layer:
Neurovascular Structures
Nerves
The forearm is innervated by three main nerves that originate from the
brachial plexus:
- Median Nerve: Supplies most of the flexor-pronator muscles in the anterior compartment (except flexor carpi ulnaris and part of flexor digitorum profundus).
- Ulnar Nerve: Supplies the flexor carpi ulnaris and the medial half of the flexor digitorum profundus.
- Radial Nerve: Supplies the extensor-supinator muscles in the posterior compartment.
Blood Supply
The forearm’s blood supply is provided by two main arteries:
Both arteries give rise to branches that supply the forearm’s muscles, bones, and
skin.
Veins
Venous drainage occurs through the
cephalic,
basilic, and
median cubital veins in the superficial layer, as well as the
radial and
ulnar veins in the deep layer.
Interosseous Membrane
The
interosseous membrane is a fibrous connective tissue that runs between the
radius and
ulna, connecting the two bones.
- It provides a site for muscle attachment.
- It stabilizes the forearm during movement.
Surface Landmarks of the Forearm
- Olecranon Process: The bony prominence at the elbow.[3]
- Styloid Processes: Palpable at the distal ends of the radius and ulna.
- Radial Head: Felt just distal to the lateral elbow.
- Tendons of Extensors and Flexors: Prominent when the wrist is extended or flexed.
Function
The
forearm is a critical part of the upper limb that connects the
elbow to the
wrist and
hand. Its primary functions include
forearm rotation (pronation and supination),
wrist and finger movements, and the ability to stabilize and position the hand for various activities. The forearm serves as a conduit for muscles, nerves, and blood vessels, facilitating fine and gross motor tasks while also providing
strength,
mobility, and
stability for functional independence.
Below is a detailed explanation of the
functions of the forearm:
Rotation of the Forearm: Pronation and Supination
The forearm allows for two critical rotational movements:
- Pronation: The movement where the palm faces downward (posteriorly in anatomical position).
- Occurs at the proximal and distal radioulnar joints.
- The radius crosses over the ulna during pronation.
- Muscles involved:
- Pronator Teres: Initiates pronation at the proximal radioulnar joint.
- Pronator Quadratus: Stabilizes and completes pronation at the distal radioulnar joint.
- Supination: The movement where the palm faces upward (anteriorly in anatomical position).
- The radius and ulna are parallel during supination.
- Muscles involved:
- Supinator Muscle: Acts when the forearm is extended.
- Biceps Brachii: Acts as a powerful supinator, particularly when the elbow is flexed.
Significance: These movements enable the hand to perform tasks such as turning keys, twisting doorknobs, rotating tools, and manipulating objects with precision.
Flexion and Extension of the Wrist
The forearm controls
wrist flexion and
extension, allowing the wrist to move up and down. This movement is achieved through muscles in the forearm that attach to the hand via tendons.
- Wrist Flexion: Bending the wrist so the palm moves toward the forearm.
- Muscles involved (anterior compartment):
- Flexor Carpi Radialis: Flexes and abducts the wrist.[4]
- Flexor Carpi Ulnaris: Flexes and adducts the wrist.
- Palmaris Longus: Weak flexor that assists with wrist movement.
- Wrist Extension: Moving the wrist upward (backward).
- Muscles involved (posterior compartment):
- Extensor Carpi Radialis Longus and Brevis: Extend and abduct the wrist.
- Extensor Carpi Ulnaris: Extends and adducts the wrist.
Significance: These movements enable tasks such as typing, holding objects, pushing, pulling, and supporting weight on the hands.
Movements of the Fingers and Thumb
The forearm houses muscles that control the
fingers and
thumb.
[5] These muscles are categorized into
flexors (anterior compartment) and
extensors (posterior compartment):
- Finger Flexion: Bending the fingers toward the palm.
- Muscles involved:
- Flexor Digitorum Superficialis: Flexes the middle phalanges.
- Flexor Digitorum Profundus: Flexes the distal phalanges.
- Finger Extension: Straightening the fingers.
- Muscles involved:
- Extensor Digitorum: Extends the fingers.
- Extensor Indicis: Extends the index finger.
- Extensor Digiti Minimi: Extends the little finger.
- Thumb Movements: The thumb performs a variety of complex movements, including flexion, extension, abduction, adduction, and opposition.
- Muscles involved:
- Flexor Pollicis Longus: Flexes the thumb.
- Extensor Pollicis Longus and Brevis: Extend the thumb.
- Abductor Pollicis Longus: Abducts the thumb.
Significance: Finger and thumb movements are critical for activities like gripping, pinching, writing, using tools, and performing precise tasks.
Stabilization of the Wrist and Hand
The forearm plays a key role in stabilizing the wrist and hand during movements:
- Static Stabilization: Provided by the interosseous membrane connecting the radius and ulna, which transmits forces between the two bones.
- Dynamic Stabilization: Achieved by the coordinated activity of forearm flexor and extensor muscles to maintain wrist position during tasks like writing, gripping, or lifting objects.
Significance: Stabilization ensures strength and control, allowing efficient movement without injury.
Support for Gripping and Weight-Bearing
The forearm muscles generate strength and stability for
gripping and
weight-bearing tasks:
- Flexor Muscles: Provide strong grip strength by flexing the wrist and fingers.[6]
- Extensor Muscles: Stabilize the wrist to counterbalance the flexor muscles during gripping.
Examples: Gripping a dumbbell, holding a suitcase, or supporting body weight during push-ups or planks.
Force Transmission
The forearm transmits mechanical forces between the
hand and the
elbow:
- Forces applied to the hand (e.g., during impact or weight-bearing) are transmitted through the radius and ulna to the elbow joint.
- The interosseous membrane helps distribute these forces evenly, preventing localized stress and injury.
Positioning of the Hand
The forearm adjusts the
position and
orientation of the hand through the combined action of
flexion, extension, pronation, and supination.
These adjustments are necessary for tasks that require hand positioning, such as reaching for objects, using tools, or performing fine motor activities.
[7]
Fine and Gross Motor Control
The forearm muscles enable both
fine motor control (precision tasks) and
gross motor control (powerful movements):
- Fine Motor Control: Small, precise movements such as writing, drawing, or threading a needle.
- Gross Motor Control: Large, forceful movements like lifting, swinging, or throwing.
The coordination of these movements allows the forearm to act as a versatile tool for both strength and precision.
Clinical Significance
The
forearm is a vital region of the upper limb, housing bones, muscles, nerves, and blood vessels essential for mobility and function. It is clinically significant due to its susceptibility to trauma, overuse injuries, and nerve compression.
[8]
Fractures
Radius and Ulna Fractures: Common due to falls, direct trauma, or accidents. Fractures like
Colles’ fracture (distal radius fracture) and
Monteggia fracture (ulna fracture with radial head dislocation) are common clinical concerns.
Compartment Syndrome
Increased pressure within the forearm’s compartments (from trauma or fractures) can compress nerves and blood vessels, leading to pain and tissue damage, requiring immediate surgical intervention.
Nerve Injuries
- Median Nerve Compression: Leads to pronator syndrome or carpal tunnel syndrome (distal compression).
- Ulnar Nerve Compression: Affects hand strength and sensation.
- Radial Nerve Injuries: May result in wrist drop due to impaired wrist extension.
Tendinopathies and Overuse Injuries
Repetitive activities can cause
tennis elbow (lateral epicondylitis) or
golfer’s elbow (medial epicondylitis).
Soft Tissue Injuries
Strains or tears in the flexor or extensor muscles affect wrist and finger function.