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Forearm

The forearm, also known as the antebrachium, is the anatomical region of the upper limb extending from the elbow to the wrist, serving as a critical link for transmitting forces and enabling precise movements of the hand.^[1] It consists of two parallel long bones—the radius laterally and the ulna medially—that articulate with the humerus at the elbow joint proximally and with the carpal bones at the wrist distally, providing structural support and leverage for upper limb motion.^[2] ^[3] ^[4] The forearm houses an intricate array of soft tissues, including approximately 20 muscles organized into three fascial compartments: the anterior (volar) compartment containing flexor muscles, the posterior (dorsal) compartment with extensor muscles, and the mobile wad comprising the brachioradialis and related extensors.^[5] ^[1] These muscles are classified as intrinsic (originating and inserting within the forearm) or extrinsic (crossing joints to act on the hand), and they collectively facilitate essential actions such as wrist flexion and extension, forearm pronation and supination, finger movements, and elbow stabilization.^[5] ^[2] Enveloping these structures is a rich neurovascular network, with major arteries like the radial and ulnar providing oxygenated blood, veins for drainage, and nerves such as the median, ulnar, and radial innervating sensory and motor functions to ensure coordinated upper limb activity.^[1] ^[6] ^[7] The radius plays a pivotal role in allowing rotational movements like supination (palm up) and pronation (palm down) through its head's articulation with the ulna, while the ulna contributes to elbow stability and wrist support.^[8] ^[9] Overall, the forearm's design supports both gross power grips and fine dexterity, underscoring its importance in daily activities from grasping objects to performing intricate tasks.^[10]

Anatomy

Bones

The forearm skeleton consists of two parallel long bones: the radius laterally and the ulna medially. These bones articulate proximally with the humerus at the elbow and distally with the carpal bones of the wrist, providing structural support and enabling forearm mobility. The radius is slightly shorter than the ulna and exhibits a gentle lateral bow along its shaft, contributing to its role in rotational movements.^[11]^[12]^[13] The radius features a proximal head that is disk-shaped and slightly concave, with a central fovea articulating with the radial notch of the ulna to form the proximal radioulnar joint; this head also articulates with the capitulum of the humerus at the elbow. The shaft of the radius is triangular in cross-section for most of its length, featuring an interosseous border that connects to the ulna via the interosseous membrane and a prominent radial tuberosity just distal to the neck. At the distal end, the radius widens to include a styloid process laterally and articular facets for the scaphoid and lunate carpals, facilitating wrist joint formation.^[14]^[8]^[13]^[14] The ulna, in contrast, has a more robust proximal end characterized by the olecranon process, which forms the bony prominence of the elbow and contributes to the trochlear notch that articulates with the trochlea of the humerus. Adjacent to the olecranon is the coronoid process, which completes the trochlear notch and provides attachment sites for ligaments. The shaft of the ulna displays a prominent subcutaneous border along its posterior aspect and an interosseous crest anteriorly for membrane attachment, maintaining a relatively straight overall alignment compared to the radius. The distal end tapers to a small rounded head that articulates with the triangular fibrocartilage complex and the distal radius, allowing for limited rotation at the distal radioulnar joint.^[15]^[9]^[11]^[16] Ossification of the radius begins with a primary diaphyseal center around the 8th intrauterine week, followed by secondary epiphyseal centers: one for the distal end at approximately 1 year of age, and another for the proximal head and tuberosity at 5-7 years, resulting in three initial centers that fuse by late adolescence. The ulna ossifies from a primary diaphyseal center around the 7th-8th intrauterine week, slightly after the radius, with a secondary distal epiphyseal center emerging around 5-6 years of age; the proximal olecranon epiphysis forms later, around 9-10 years, fusing completely by the early 20s.^[17]^[18]

Joints

The forearm is involved in three primary synovial joints that facilitate its mobility: the proximal radioulnar joint, the distal radioulnar joint, and relevant articulations of the elbow joint with the proximal forearm bones. These joints enable essential movements such as pronation and supination, while providing structural stability through specialized ligaments and articular surfaces.^[19] The proximal radioulnar joint is a pivot-type synovial joint formed by the articulation of the circumferential radial head with the radial notch of the proximal ulna. This joint permits rotational movements of pronation and supination by allowing the radius to pivot around the fixed ulna. It is primarily stabilized by the annular ligament, a strong band that encircles the radial head like a collar, attaching to the anterior and posterior margins of the ulnar radial notch, and the quadrate ligament, a thin fibrous structure that connects the radial neck to the ulnar radial notch, enhancing joint congruity and preventing radial head dislocation.^[20]^[21]^[22] The distal radioulnar joint, also a pivot synovial joint, occurs between the head of the ulna and the ulnar notch of the distal radius, similarly enabling pronation and supination through rotation of the radius around the ulna. Stability is maintained by the dorsal and palmar radioulnar ligaments, which are key components of the triangular fibrocartilage complex (TFCC), a multifaceted structure that extends the articular surface and absorbs compressive forces. The TFCC includes a central articular disk, the meniscus homologue (a proximal fibrocartilaginous extension), ulnocarpal ligaments, and the ulnar collateral ligament, collectively providing both tensile strength against rotational forces and cushioning during load transmission across the ulnocarpal interface.^[23]^[24]^[25] As the proximal boundary of the forearm, the elbow joint incorporates humeroradial and humeroulnar articulations that directly influence forearm positioning. The humeroradial joint forms between the capitulum of the distal humerus and the fovea of the radial head, contributing to flexion-extension and serving as a pivot for forearm rotation. The humeroulnar joint arises from the trochlea of the humerus articulating with the trochlear notch of the proximal ulna, primarily facilitating hinge-like flexion and extension while constraining excessive rotation. These articulations are reinforced by the medial collateral ligament (spanning from the medial epicondyle of the humerus to the ulna) and the lateral collateral ligament complex (including the radial collateral ligament from the lateral epicondyle to the annular ligament), which provide valgus-varus stability and support forearm alignment during motion.^[19]^[26]^[27] The combined congruity of these joints, including the precise curvature of the radial head and ulnar notches, allows for a typical range of motion of 80-90 degrees of pronation and 80-90 degrees of supination from the neutral position, with variations influenced by ligament integrity and osseous geometry.^[28]^[29]

Muscles

The forearm muscles are organized into anterior and posterior compartments, separated by the interosseous membrane and deep fascia, with the anterior compartment housing the flexors and the posterior the extensors.^[5] These muscles originate primarily from the humerus, radius, and ulna, and insert onto the carpal bones, metacarpals, and phalanges, facilitating wrist and digit movements.^[30] The anterior compartment is subdivided into superficial, intermediate, and deep layers. The superficial layer includes the pronator teres, originating from the medial epicondyle of the humerus and the coronoid process of the ulna, inserting on the mid-lateral radius; the flexor carpi radialis, originating from the medial epicondyle, inserting on the base of the second and third metacarpals; the palmaris longus (absent in about 14% of individuals), originating from the medial epicondyle, inserting on the palmar aponeurosis and flexor retinaculum; and the flexor carpi ulnaris, originating from the medial epicondyle and the olecranon and posterior ulna, inserting on the pisiform, hamate, and fifth metacarpal.^[30] The intermediate layer consists of the flexor digitorum superficialis, originating from the medial epicondyle, coronoid process, and anterior ulna, inserting on the middle phalanges of digits 2–5.^[30] The deep layer comprises the flexor digitorum profundus, originating from the anterior ulna and interosseous membrane, inserting on the distal phalanges of digits 2–5; the flexor pollicis longus, originating from the anterior radius and interosseous membrane, inserting on the distal phalanx of the thumb; and the pronator quadratus, originating from the distal anterior ulna, inserting on the distal anterior radius.^[30] The posterior compartment is divided into superficial and deep layers. The superficial layer includes the brachioradialis, originating from the lateral supracondylar ridge of the humerus, inserting on the distal radius; the extensor carpi radialis longus, originating from the lateral supracondylar ridge, inserting on the base of the second metacarpal; the extensor carpi radialis brevis, originating from the lateral epicondyle, inserting on the base of the third metacarpal; the extensor digitorum, originating from the lateral epicondyle, inserting on the extensor expansions of digits 2–5; the extensor digiti minimi, originating from the lateral epicondyle, inserting on the extensor expansion of the fifth digit; and the extensor carpi ulnaris, originating from the lateral epicondyle and posterior ulna, inserting on the base of the fifth metacarpal.^[31] The deep layer features the supinator, originating from the lateral epicondyle, radial collateral ligament, and supinator crest of the ulna, inserting on the proximal radius; the abductor pollicis longus, originating from the posterior ulna, radius, and interosseous membrane, inserting on the base of the first metacarpal; the extensor pollicis brevis, originating from the posterior radius and interosseous membrane, inserting on the proximal phalanx of the thumb; the extensor pollicis longus, originating from the posterior ulna and interosseous membrane, inserting on the distal phalanx of the thumb; and the extensor indicis, originating from the posterior ulna and interosseous membrane, inserting on the extensor expansion of the second digit.^[31] Innervation of the anterior compartment is primarily by the median nerve for the superficial and intermediate layers, as well as the lateral half of the deep layer (flexor digitorum profundus and flexor pollicis longus), while the ulnar nerve supplies the flexor carpi ulnaris and medial half of the flexor digitorum profundus; the posterior compartment is innervated by the radial nerve, with its deep branch (posterior interosseous nerve) supplying the deep layer except for the brachioradialis, extensor carpi radialis longus, and extensor carpi radialis brevis, which receive innervation from the superficial radial nerve.^[5]^[30] Key forearm flexors and extensors exhibit characteristic volumes and pennation angles that influence their force generation capacity. For example, the flexor digitorum superficialis has an average volume of approximately 74 cm³ in adults, while the extensor digitorum averages 28.3 cm³ in males and 16.6 cm³ in females; collectively, forearm flexors comprise about 11% and extensors about 5% of total upper limb muscle volume.^[32]^[33] Pennation angles are generally low in these muscles to prioritize excursion over force, ranging from 0° to 15°; the flexor carpi ulnaris, a notable exception among flexors, has an average pennation angle of about 12°.^[34]^[35]

Nerves

The forearm receives its primary neural innervation from the median, ulnar, and radial nerves, which arise from the brachial plexus and provide both motor supply to the intrinsic muscles and sensory innervation to the skin and deeper tissues of the region. These nerves traverse the forearm compartments, branching to support pronation, supination, flexion, extension, and sensory feedback essential for hand and wrist function.^[36] The median nerve, derived from the lateral and medial cords of the brachial plexus (C5-T1 roots), enters the forearm via the cubital fossa, positioned medial to the brachial artery and anterior to the brachialis muscle. In the proximal forearm, it pierces the heads of the pronator teres muscle and descends between the flexor digitorum superficialis and flexor digitorum profundus. A key branch is the anterior interosseous nerve, which arises approximately 5-8 cm distal to the lateral epicondyle and travels along the interosseous membrane to innervate the deep flexor muscles, including the flexor pollicis longus, the lateral half of the flexor digitorum profundus, and the pronator quadratus. The palmar cutaneous branch emerges proximal to the wrist, providing sensory innervation to the skin of the lateral palm without entering the carpal tunnel. Motor innervation from the main median nerve trunk supplies the pronator teres, flexor carpi radialis, palmaris longus, and flexor digitorum superficialis in the anterior compartment, enabling wrist and finger flexion as well as forearm pronation. Sensory distribution includes the palmar aspect of the lateral three and a half digits and the corresponding palm.^[37] The ulnar nerve, originating from the medial cord of the brachial plexus (C8-T1 roots, with occasional C7 contribution), courses posterior to the medial epicondyle of the humerus before entering the forearm through the interval between the humeral and ulnar heads of the flexor carpi ulnaris muscle. It descends along the medial aspect of the forearm, deep to the flexor carpi ulnaris and superficial to the flexor digitorum profundus, accompanied by the ulnar artery. The dorsal cutaneous branch arises about 5-8 cm proximal to the wrist, supplying sensory innervation to the ulnar aspect of the dorsum of the hand and the dorsal surfaces of the little and ring fingers. The palmar cutaneous branch emerges more distally, providing sensation to the medial palm. Motor branches innervate the flexor carpi ulnaris throughout its course in the forearm and the medial half of the flexor digitorum profundus, facilitating wrist adduction and flexion of the ring and little fingers.^[38] The radial nerve, formed from the posterior cord of the brachial plexus (C5-T1 roots), reaches the cubital fossa lateral to the brachioradialis muscle and divides into its superficial and deep branches just proximal to or at the entry of the forearm, near the level of the radial head. The superficial radial nerve continues distally under the brachioradialis tendon, emerging to provide purely sensory innervation to the dorsum of the hand, including the lateral three and a half digits (proximal to the distal interphalangeal joints) and the posterior radial aspect of the forearm skin. The deep branch, known as the posterior interosseous nerve after piercing the supinator muscle (via the arcade of Frohse), winds around the radius and supplies motor innervation to the extensor muscles of the posterior compartment, such as the extensor carpi radialis brevis, supinator, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, abductor pollicis longus, and extensor pollicis longus and brevis, as well as the extensor indicis. This branch also sends articular branches to the elbow, wrist, and intercarpal joints.^[39] The sensory skin of the forearm is organized into dermatomes primarily from spinal levels C6, C7, and C8. The C6 dermatome covers the lateral (radial) aspect of the volar and dorsal forearm, extending to the thumb and index finger; the C7 dermatome occupies the central or posterior forearm, reaching the middle finger; and the C8 dermatome supplies the medial (ulnar) forearm, including the ring and little fingers. These segmental contributions overlap slightly at the boundaries and integrate with peripheral nerve distributions for comprehensive sensory coverage.^[40]

Blood vessels

The blood supply to the forearm primarily arises from the brachial artery, which bifurcates in the cubital fossa into the radial and ulnar arteries, providing oxygenated blood to the muscles, bones, and skin of the region.^[41] These arteries course distally through the forearm, accompanied by deep veins and often in proximity to nerves within neurovascular bundles.^[42] The radial artery originates as the smaller terminal branch of the brachial artery in the cubital fossa and travels distally along the anterior aspect of the radius, initially lying between the brachioradialis and deep head of the pronator teres muscles, then between the brachioradialis and flexor digitorum superficialis.^[42] Its key branches include the radial recurrent artery, which ascends proximally to anastomose with the radial collateral artery from the deep brachial artery, providing collateral flow around the elbow; the palmar carpal branch, which supplies the wrist joint; and the superficial palmar branch, which descends to join the ulnar artery in forming the superficial palmar arch in the hand.^[42] In the distal forearm, the radial artery gives off dorsal carpal branches to the wrist and continues into the hand as the deep palmar arch, anastomosing with the deep branch of the ulnar artery.^[41] The ulnar artery, the larger terminal branch of the brachial artery, arises in the cubital fossa and courses distally along the medial side of the forearm, passing deep to the pronator teres and then under the flexor carpi ulnaris muscle, remaining close to the ulna.^[43] Its major branches comprise the anterior and posterior ulnar recurrent arteries, which extend proximally to form anastomoses around the elbow with the inferior and superior ulnar collateral arteries from the brachial artery; the common interosseous artery, which divides into anterior and posterior interosseous arteries supplying the deep forearm flexors and extensors, respectively; the palmar carpal branch for the wrist; and distal branches including the deep palmar and superficial palmar arteries, the latter completing the superficial palmar arch with the superficial palmar branch of the radial artery.^[43]^[41] Venous drainage of the forearm occurs via superficial and deep systems, with the superficial veins facilitating cutaneous return and the deep veins paralleling the arteries.^[44] The cephalic vein, a prominent superficial vessel, originates from the lateral dorsal venous network of the hand and ascends along the lateral forearm and arm, remaining subcutaneous before piercing the clavipectoral fascia to join the axillary vein.^[44] The basilic vein arises from the medial dorsal venous network, courses along the medial forearm and arm subcutaneously in its proximal portion, then becomes deep by piercing the brachial fascia around mid-arm to unite with the brachial veins forming the axillary vein.^[44] Connecting these is the median cubital vein, which obliquely links the cephalic and basilic veins across the cubital fossa, often receiving tributaries from the palmar venous network.^[44] Deep veins, known as venae comitantes, accompany the radial, ulnar, and interosseous arteries, draining muscular and osseous structures and eventually merging into the brachial veins.^[44] Anastomoses between the brachial, radial, and ulnar arteries and their branches form a robust collateral network, particularly around the elbow, ensuring alternative pathways for blood flow in cases of occlusion.^[41] The radial recurrent artery connects with the radial collateral branch of the deep brachial artery, while the anterior and posterior ulnar recurrent arteries link with the ulnar collateral arteries from the brachial, creating a periarticular arcade that supports the forearm's vascular integrity.^[42]^[43] Additionally, the common interosseous artery and its divisions provide interconnections between the anterior and posterior forearm compartments.^[41]

Fascia and other tissues

The deep fascia of the forearm, known as the antebrachial fascia, forms a continuous fibrous sheath that encloses the muscles from the elbow to the wrist, providing structural support and compartmentalization.^[45] This fascia is thicker on the dorsal aspect and distally, attaching proximally to the olecranon process, the posterior border of the ulna, and the medial and lateral epicondyles of the humerus.^[46] Within the forearm, the antebrachial fascia gives rise to intermuscular septa, including the lateral intermuscular septum, which, along with the interosseous membrane, divides the forearm into anterior and posterior compartments to separate flexor and extensor muscle groups.^[47] Additionally, the lacertus fibrosus, or bicipital aponeurosis, is a flattened tendinous expansion arising from the distal biceps brachii tendon that blends with the antebrachial fascia over the proximal flexors, reinforcing the medial aspect of the cubital fossa.^[48] The superficial fascia of the forearm lies immediately beneath the skin and consists of loose areolar connective tissue interspersed with adipose lobules, serving as a cushioning layer that facilitates mobility between the skin and underlying deep fascia.^[49] This subcutaneous tissue contains hair follicles, sebaceous glands, and eccrine sweat glands, which are more densely distributed on the volar (anterior) surface to support thermoregulation and sensory function.^[50] On the volar surface, the skin exhibits dermatoglyphic patterns—raised epidermal ridges that enhance grip and tactile discrimination, extending from the palm proximally into the forearm. Other supportive structures in the forearm include synovial sheaths that envelop the flexor and extensor tendons, reducing friction during movement; the common flexor sheath (ulnar bursa) encloses the tendons of the flexor digitorum superficialis and profundus from the mid-forearm to the palm, while the radial bursa surrounds the flexor pollicis longus tendon proximally.^[51] Individual digital synovial sheaths extend for the flexor tendons into the fingers, and similar sheaths cover extensor tendons in the forearm, often continuous with those at the wrist.^[52] Bursae in the forearm, such as the radial bursa and extensions of the ulnar bursa, act as synovial-lined sacs that lubricate tendon passages, with the radial bursa specifically communicating with the flexor pollicis longus sheath and extending proximally along the forearm.^[53] The lymphatic vessels of the forearm form superficial and deep networks that drain interstitial fluid; superficial vessels parallel the cephalic and basilic veins to the cubital (supratrochlear) lymph nodes in the cubital fossa, while deep vessels accompany arteries and ultimately converge to axillary nodes via cervical pathways.^[54] Fat pads and areolar tissue are distributed throughout the forearm's connective layers, with subcutaneous adipose in the superficial fascia providing insulation and padding, particularly along the volar and dorsal surfaces.^[55] Deeper areolar tissue fills spaces between muscles and around neurovascular structures, while a distinct deep fat pad, the corpus adiposum profundum antebrachii, lies dorsal to the deep flexor tendons in the anterior compartment, aiding in cushioning and compartment integrity.^[56]

Function

Movements and biomechanics

The movements of the forearm primarily involve pronation and supination at the radioulnar joints, as well as flexion and extension primarily at the elbow joint with contributions from the forearm bones. Pronation and supination enable rotational motion of the forearm around its longitudinal axis, allowing the palm to face downward (pronation) or upward (supination). This rotation occurs through a coupled mechanism where the radius pivots around the fixed ulna, facilitated by the proximal and distal radioulnar joints. The axis of rotation approximates a line passing through the center of the radial head proximally and the ulnar head distally, enabling a functional range of approximately 150–180 degrees of total rotation (80–90 degrees pronation and 80–90 degrees supination from neutral).^[57]^[58] Biomechanically, achieving full pronation-supination requires overcoming torsional loads, with torque demands reaching up to 7–12 Nm depending on position and speed, particularly higher in supination due to gravitational and inertial effects. Kinematically, the maximum angular velocity during rapid supination can approach 200 degrees per second, reflecting the dynamic constraints of joint geometry and soft tissue interactions. The moment arms for the rotational forces at these joints vary with elbow flexion angle, typically ranging from 1.5 to 3 cm, which influences the efficiency of torque transmission across the forearm.30770-0/fulltext)^[59]^[60] Flexion and extension of the forearm occur mainly at the humeroulnar joint, where the ulna articulates with the humerus, providing the primary lever for these motions while the radius contributes secondarily through its humeroradial articulation. The typical range includes up to 145 degrees of flexion from full extension and 0–10 degrees of hyperextension, allowing the forearm to approximate the upper arm or extend fully for reaching. These motions involve hinge-like kinematics with minimal translation, governed by the trochlear notch of the ulna sliding along the humeral trochlea.^[61] In load-bearing scenarios, such as weight-bearing postures or impacts from falls, the forearm bones experience significant compressive forces, often 1.5–2 times body weight distributed axially along the radius and ulna. The interosseous membrane plays a critical role in stress distribution by transferring approximately 20–30% of the load from the distal radius to the proximal ulna, particularly in supination where the radius bears about 68% of the axial force and the ulna 32%. This mechanism prevents excessive stress concentration on either bone, maintaining structural integrity during dynamic loading.^[62]^[63]

Muscle roles in motion

The forearm's flexor muscles play a pivotal role in wrist and finger flexion, facilitating essential actions such as grasping and object manipulation. The flexor carpi radialis and flexor carpi ulnaris are the primary contributors to wrist flexion, producing maximum isometric torques ranging from 12 to 32 Nm in healthy adults, with values varying based on forearm position and contraction intensity.^[64]^[65] These muscles generate radial and ulnar deviation alongside flexion, enhancing stability during load-bearing tasks. For finger flexion, the flexor digitorum superficialis and profundus muscles enable flexion at the metacarpophalangeal, proximal interphalangeal, and distal interphalangeal joints, while coordination with the lumbrical muscles ensures balanced grip formation by preventing excessive tension in the extensor hood.^[66]^[67] In contrast, the extensor muscles counteract flexor actions to maintain equilibrium and support extension movements. The extensor carpi radialis longus and brevis drive wrist extension, generating torques of 7 to 17 Nm to balance flexor forces and stabilize the wrist during dynamic activities.^[64] The extensor digitorum extends the fingers, with its tendons interconnected by juncturae tendinum—thin fibrous bands that redistribute forces across digits, ensuring synchronized extension and preventing independent tendon slippage at the metacarpophalangeal joints.^[68] This anatomical linkage allows for efficient force transmission, particularly when extending multiple fingers simultaneously against resistance. Pronation and supination involve specialized muscles that rotate the forearm, enabling hand orientation for diverse tasks. The pronator teres and pronator quadratus initiate and sustain pronation, achieving a typical range of 70 to 90 degrees from neutral, which positions the palm downward for activities like pouring or turning a key.^[69] Conversely, the supinator muscle, assisted by the biceps brachii, facilitates supination over an arc of up to 80 to 90 degrees, contributing to a total forearm rotation range of approximately 160 degrees.^[28] These actions generate torques around 3 to 7 Nm, sufficient for precise rotational control.^[70] Synergistic interactions among forearm muscles enhance overall motion efficiency, particularly in compound movements. For instance, the flexor carpi ulnaris activates concurrently with finger flexors during power grips, providing ulnar stabilization to the wrist and counteracting extension tendencies. This coordination minimizes joint deviation and optimizes force distribution, as seen in tasks requiring sustained hold, such as tool use or weightlifting.

Sensory and vascular support

The sensory functions of the forearm are primarily mediated by the sensory branches of the median, ulnar, and radial nerves, which provide touch and pressure sensation to the skin and underlying tissues. The median nerve's palmar cutaneous branch supplies sensation to the lateral palm and proximal phalanges of the thumb, index, middle, and ring fingers, while its digital branches innervate the corresponding digit pads. The ulnar nerve provides sensory innervation to the medial forearm, hypothenar eminence, and the little finger along with the medial half of the ring finger. The superficial branch of the radial nerve delivers sensation to the dorsal aspect of the forearm, thumb, and the radial sides of the index and middle fingers.^[37]^[38]^[39] Proprioception in the forearm arises from mechanoreceptors within the joint capsules of the elbow, wrist, and radioulnar joints, including Golgi tendon organs that detect tension in ligaments and tendons, and Ruffini endings that respond to sustained stretch and joint position. These receptors contribute to kinesthetic awareness during forearm movements such as pronation and supination, enabling precise coordination without visual input. Pain pathways in the forearm are conveyed by unmyelinated C-fibers, which transmit dull, aching sensations from nociceptors in skin, muscles, and periosteum, integrating into the spinothalamic tract for central processing.^[71]^[72] Vascular structures in the forearm support tissue viability by delivering nutrients and oxygen to muscles and bones, with skeletal muscle extracting approximately 20-40% of arterial oxygen content at rest to meet basal metabolic demands. The radial and ulnar arteries, along with their branches, facilitate this via capillary networks, ensuring efficient gas exchange and waste removal. Thermoregulation occurs through arteriovenous anastomoses (AVAs) in the glabrous skin of the hand and distal forearm, which shunt blood to superficial venous plexuses to dissipate heat during elevated core temperatures, operating effectively within the thermoneutral zone of 26-36°C. Venous return is augmented by the muscle pump mechanism, where forearm contractions compress superficial and deep veins, propelling blood proximally against gravity while one-way valves prevent reflux, thereby enhancing overall circulation during repetitive motions.^[73]^[74] Neurovascular bundles in the forearm, comprising arteries, veins, and nerves encased in fascia, integrate sensory and vascular support to maintain function during motion by protecting conduits from mechanical stress and allowing coordinated responses to activity. Autoregulation of blood flow is achieved via the myogenic response, where vascular smooth muscle in forearm resistance vessels constricts in response to increased transmural pressure and dilates to decreases, stabilizing perfusion across a range of systemic pressures independent of neural input. This mechanism ensures consistent oxygen delivery to active tissues amid fluctuating demands from pronation-supination or flexion-extension.^[75]^[76] Reflex arcs involving forearm sensory nerves contribute to protective responses, such as the withdrawal reflex triggered by noxious stimuli on the dorsal forearm, mediated by the radial nerve's sensory afferents activating spinal interneurons to elicit rapid flexion and retraction of the limb. This polysynaptic pathway enhances survival by minimizing injury exposure, with afferent signals from A-delta and C-fibers converging in the dorsal horn for swift motor output via alpha motor neurons.^[77]

Development

Embryonic formation

The embryonic formation of the forearm begins with the initiation of the upper limb bud during the fourth week of gestation, when lateral plate mesoderm protrudes to form a paddle-like structure covered by ectoderm.^[78] This process is orchestrated by signaling centers, including the apical ectodermal ridge (AER) at the distal tip, which maintains proximal-distal outgrowth through fibroblast growth factor (FGF) secretion, and the zone of polarizing activity (ZPA) at the posterior margin, which patterns the anteroposterior axis via sonic hedgehog (Shh) signaling.^[79] Hox gene clusters, particularly HoxA and HoxD, are expressed in nested domains along the limb bud to specify positional identity, with their regulation influenced by retinoic acid and ensuring proper segmentation of forelimb elements like the radius and ulna precursors.^[80] Chondrogenesis in the forearm follows by the sixth week, as mesenchymal cells within the limb bud condense into cartilaginous models of the radius and ulna, driven by Sox9 and other transcription factors that promote differentiation into chondrocytes.^[2] These precartilaginous anlagen form parallel to the limb's long axis, establishing the basic skeletal framework before endochondral ossification commences around the eighth week, where hypertrophic chondrocytes induce vascular invasion and mineralization at primary ossification centers.^[2] This sequential process ensures the radius develops on the lateral (thumb) side and the ulna on the medial (pinky) side, reflecting the anteroposterior patterning initiated earlier.^[79] Myogenesis in the forearm arises from somitic mesoderm, where cells from the hypaxial myotome migrate into the limb bud starting in the fifth week, guided by chemotactic signals like c-met and scatter factor.^[81] These progenitors proliferate and differentiate into dorsal (extensor) and ventral (flexor) muscle masses by the seventh week, with myogenic regulatory factors such as MyoD and myogenin directing myotube formation specific to forearm compartments.^[81] Concurrently, innervation occurs as precursors of the brachial plexus—formed from ventral rami of C5-T1 spinal nerves—extend axons into the limb bud from the fourth week, establishing motor connections to myoblasts and sensory pathways to ectodermal derivatives.^[82] Vascular ingrowth parallels these events, with angiogenic sprouts from the seventh intersegmental artery penetrating the limb bud core by the fifth to seventh weeks, forming the brachial artery axis that branches into radial and ulnar precursors.^[83] Vascular endothelial growth factor (VEGF), secreted by hypoxic mesenchyme and AER cells, drives this endothelial proliferation and remodeling, ensuring nutrient supply for the expanding forearm structures along the proximo-distal gradient.^[84]

Postnatal growth and changes

Following birth, the forearm undergoes significant postnatal skeletal maturation, particularly in the ossification of its primary bones, the radius and ulna. The distal epiphysis of the radius typically fuses between ages 17 and 19 in males and 17 to 18 in females, while the distal ulna fuses slightly later, between 18 and 20 in males and 17 to 19 in females, marking the completion of longitudinal bone growth.^[85] These growth plates, or physes, remain active sites of chondrocyte proliferation and endochondral ossification until fusion, rendering them particularly vulnerable to injury from trauma, repetitive stress, or overuse during childhood and adolescence, which can lead to growth disturbances if not managed appropriately.^[86] Muscle development in the forearm also progresses markedly during postnatal life, with hypertrophy driven by increases in muscle fiber size and cross-sectional area (CSA). From childhood through adolescence, forearm muscle CSA significantly increases during puberty, with boys exhibiting greater absolute gains than girls due to hormonal influences, particularly surges in testosterone.^[87] This hypertrophy enhances grip strength and overall forearm function, adapting the musculature for finer motor control and load-bearing as body size scales. Neural maturation in the forearm involves the progressive myelination of peripheral sensory and motor nerve pathways, which continues through early childhood, approaching completion by around 4-5 years of age, coinciding with refinements in hand and finger dexterity essential for tasks like grasping and manipulating objects.^[88] This process optimizes conduction velocity along nerves such as the median, ulnar, and radial, supporting the integration of sensory feedback with motor output for coordinated movements. In later life, the forearm experiences age-related remodeling that contributes to functional decline. After age 60, sarcopenia contributes to approximately 10-30% loss in overall muscle mass and strength by age 80, with notable impacts on grip force and disproportionate effects on type II fibers.^[89] Concurrently, age-related arterial stiffening, particularly in central arteries like the aorta, contributes to diminished pulsatile blood flow and endothelial function in the forearm vasculature, potentially exacerbating ischemia during exertion.^[90] Bone density in the forearm also declines, with postmenopausal women losing 1-2% annually in the initial years due to estrogen deficiency accelerating trabecular resorption, increasing fracture risk at sites like the distal radius.^[91]

Clinical relevance

Injuries and trauma

The forearm is susceptible to a variety of traumatic injuries, ranging from fractures to soft tissue damage, often resulting from falls, direct blows, or high-energy impacts. These injuries can lead to immediate pain, swelling, deformity, and functional impairment, with forearm fractures accounting for approximately 1.5% of all emergency department visits. Incidence peaks in children, primarily due to falls during play or sports, and in the elderly, often linked to osteoporosis-related low-energy falls.^[92]^[93] Fractures represent the most common traumatic injuries to the forearm bones. Distal radius fractures, particularly the Colles' type, involve dorsal angulation, displacement, and comminution of the distal radius metaphysis, typically occurring from a fall on an outstretched hand. These fractures comprise about 18% of all adult fractures and up to 25% of pediatric fractures. Ulna shaft fractures, known as nightstick fractures when isolated, result from a direct blow to the mid-forearm, producing a transverse break without radial involvement. Monteggia fractures combine a proximal third ulna fracture with anterior dislocation of the radial head, often from a fall on the outstretched hand with forearm pronation, and account for 2-5% of proximal forearm fractures.^[94]^[93]^[95]^[96] Soft tissue trauma to the forearm frequently involves contusions or lacerations that compromise vascular supply or tendon integrity. Contusions to the flexor compartment, often from blunt trauma, can elevate intracompartmental pressure, risking acute compartment syndrome and subsequent Volkmann's ischemic contracture, characterized by necrosis of forearm flexor muscles due to ischemia. Tendon lacerations in zones 5-7—encompassing the distal forearm, wrist, and proximal hand—typically affect extensor tendons and result from sharp injuries, leading to impaired finger extension and potential adhesions if untreated.^[97]^[98]^[99] Sprains and dislocations primarily affect the distal radioulnar joint (DRUJ), where ligamentous disruption causes instability. DRUJ sprains or dislocations arise from rotational forces or falls, resulting in painful supination or pronation with dorsal or volar subluxation of the ulna relative to the radius. The Essex-Lopresti injury, a severe variant, features a radial head fracture coupled with DRUJ dislocation and rupture of the interosseous membrane, leading to longitudinal forearm instability and proximal migration of the radius.^[100]^[101]

Disorders and diseases

The forearm is susceptible to several non-traumatic pathological conditions that can impair its function, ranging from acute pressure-related syndromes to chronic degenerative and infectious processes. These disorders often arise from repetitive stress, metabolic factors, or underlying genetic predispositions, leading to pain, weakness, and reduced mobility. Compartment syndrome in the forearm, particularly the exertional form, occurs when increased intracompartmental pressure compromises tissue perfusion without direct injury. In the anterior compartment, pressures exceeding 30 mmHg relative to diastolic blood pressure indicate significant risk, potentially leading to muscle ischemia and rhabdomyolysis if untreated. This condition is often triggered by repetitive exertion, such as in athletes or manual laborers, causing swelling and pain that worsen with activity.^[102]^[103]^[104] Tendinopathies represent common overuse injuries affecting the forearm's extensor and flexor tendons. Lateral epicondylitis, known as tennis elbow, involves degenerative changes primarily in the extensor carpi radialis brevis tendon origin at the lateral epicondyle, resulting from microtears and angiofibroblastic degeneration due to repetitive wrist extension. Symptoms include pain radiating to the forearm and grip weakness. Medial epicondylitis, or golfer's elbow, similarly features tendinosis of the flexor-pronator group at the medial epicondyle, often from forceful wrist flexion and pronation, leading to localized tenderness and reduced forearm strength. Both conditions progress through stages of inflammation to chronic degeneration without acute trauma.^[105]^[106] Neuropathies involving forearm innervation frequently stem from nerve compression at adjacent sites. Carpal tunnel syndrome arises from median nerve entrapment at the wrist within the carpal tunnel, where the nerve and nine flexor tendons pass, causing compression that leads to paresthesia and pain (which may radiate to the forearm) and atrophy in the thenar muscles of the hand in the median nerve distribution.^[107] Cubital tunnel syndrome results from ulnar nerve compression at the elbow's cubital tunnel, exacerbated by flexion, producing medial forearm numbness, tingling, and intrinsic hand muscle weakness due to impaired conduction along the nerve's superficial course. These entrapments disrupt sensory and motor functions extending into the forearm.^[108] Bone disorders of the forearm encompass infectious, metabolic, and congenital pathologies. Osteomyelitis involves bacterial infection of the bone, most commonly caused by Staphylococcus aureus, leading to inflammation, abscess formation, and potential sequestrum development in the radius or ulna; it often spreads from adjacent soft tissues or hematogenously in immunocompromised individuals. Osteoporosis contributes to fragility fractures in the distal forearm, where reduced bone mineral density increases susceptibility to low-energy impacts, such as falls, particularly in postmenopausal women due to accelerated bone loss. Congenital anomalies like radial club hand, or radial dysplasia, feature hypoplasia or absence of the radius, resulting in radial deviation of the hand and shortened forearm; these arise from disruptions in embryonic limb development and may be associated with genetic syndromes involving mutations in genes such as TBX5 in Holt-Oram syndrome.^[109]^[110]^[111]

Diagnostic and surgical aspects

Diagnosis of forearm conditions relies on a combination of imaging and electrophysiological studies tailored to the suspected pathology. Plain radiographs, including anteroposterior (AP) and lateral views, serve as the initial and primary modality for detecting fractures of the radius and ulna, offering high sensitivity approaching 95% for bony injuries.^[112] For soft tissue evaluation, magnetic resonance imaging (MRI) provides detailed visualization of tendons, ligaments, and nerves, identifying injuries such as tears or entrapments that may not be apparent on plain films.^[113] Electromyography (EMG) combined with nerve conduction studies is essential for assessing neuropathies, where reduced nerve conduction velocities below 50 m/s indicate abnormal function in the median or ulnar nerves traversing the forearm.^[114] Doppler ultrasound complements these by non-invasively assessing vascular flow, detecting occlusions or stenoses in the radial and ulnar arteries.^[115] Surgical interventions for forearm pathologies focus on restoring alignment, stability, and function through precise techniques. Open reduction and internal fixation (ORIF) with plates and screws is the standard for displaced radius and ulna fractures, achieving union rates exceeding 95% in simple cases and up to 88% in comminuted fractures.^[116] Flexor tendon repairs in zone 5, which spans the forearm proximal to the wrist, commonly employ the modified Kessler core suture technique, often reinforced with a peripheral running suture to optimize tensile strength and minimize gapping.^[117] Nerve decompressions address compressive neuropathies; for instance, carpal tunnel release involves transecting the transverse carpal ligament to relieve median nerve pressure, with extensions into the forearm for proximal entrapments if needed.^[118] Postoperative rehabilitation protocols prioritize early mobilization to promote healing while preventing stiffness. Following ORIF for forearm fractures, gentle active range-of-motion exercises for the wrist and elbow are typically initiated within 2-4 weeks in stable constructs, progressing to full mobilization by 6-8 weeks, with union confirmed radiographically.^[119] Tendon injury rehabilitation involves protective splinting in a position of function for 3-4 weeks, followed by controlled motion protocols to restore gliding without rupture.^[116] Complications from forearm surgeries, though relatively low, require vigilant monitoring. Non-union occurs in 2-10% of cases overall, rising to 5-10% or higher in smokers due to impaired vascularity and healing.^[120] Postoperative infection rates hover around 2%, with smokers facing a 2.1-fold increased risk compared to non-smokers.^[121]

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Acute Exertional Compartment Syndrome with Rhabdomyolysis - NIH
Feb 8, 2018 · Acute exertional compartment syndrome (AECS) is a rare cause of leg pain often associated with a delay in diagnosis and potentially leading to irreversible ...
[105]
Lateral Epicondylitis (Tennis Elbow) - StatPearls - NCBI Bookshelf
This condition is primarily a degenerative overuse process of the extensor carpi radialis brevis and the common extensor tendon. Aside from degenerative ...
[106]
Medial Epicondylitis - StatPearls - NCBI Bookshelf - NIH
May 2, 2024 · Medial epicondylitis, commonly called golfer's or thrower's elbow, manifests as chronic tendinosis of the wrist flexors and pronators attaching to the medial ...
[107]
Carpal Tunnel Syndrome - StatPearls - NCBI Bookshelf - NIH
Oct 29, 2023 · The carpal tunnel accommodates 9 flexor tendons and the median nerve, which traverses through it. CTS develops due to mechanical trauma, ...
[108]
Cubital Tunnel Syndrome - StatPearls - NCBI Bookshelf - NIH
Aug 14, 2023 · Cubital tunnel syndrome is neuropathy of the ulnar nerve causing symptoms of numbness and shooting pain along the medial aspect of the forearm.
[109]
Osteomyelitis - StatPearls - NCBI Bookshelf - NIH
May 31, 2023 · [1] The most common pathogens in osteomyelitis depend on the patient's age. Staphylococcus aureus is the most common cause of acute and chronic ...
[110]
Osteoporosis and the Nature of Fragility Fracture: An Overview - NCBI
Jun 16, 2018 · This chapter will provide an overview of how osteoporosis and fragility fractures are linked, with a focus on fracture prevention.
[111]
Congenital Differences of the Upper Extremity - PubMed Central - NIH
Descriptive classification is based on findings of deformity, such as radial clubhand, which describes a radially deviated hand looking like a golf club, or ...
[112]
Forearm Fractures - StatPearls - NCBI Bookshelf - NIH
Aug 8, 2023 · The forearm fractures are routinely diagnosed with the help of plain radiographs, including anteroposterior and lateral views of the forearm.Missing: AP | Show results with:AP
[113]
Magnetic Resonance Imaging (MRI) of the Bones, Joints, and Soft ...
An MRI may be used to examine bones, joints, and soft tissues such as cartilage, muscles, and tendons for injuries or the presence of structural abnormalities.
[114]
Nerve Conduction Studies - Hand - Orthobullets
May 23, 2021 · comprises nerve conduction velocity (NCV) studies and electromyography (EMG) ... velocity of < 52 m/sec is abnormal. Findings on NCV.Missing: forearm | Show results with:forearm
[115]
Forearm blood flow by Doppler ultrasound during test and exercise
Doppler ultrasound measures of arterial MBV and diameter during both rest and exercise were reproducible across different test days.
[116]
Radius and Ulnar Shaft Fractures - Trauma - Orthobullets
Aug 2, 2025 · Treatment is generally surgical open reduction and internal fixation with compression plating of both the ulna and radius fractures.
[117]
Functional outcome of flexor tendon repair of the hand at Zone 5 and ...
The aim of this study was to assess the results of flexor tendon repair in this zone using modified Kessler method reinforced by peripheral running suture and a ...
[118]
Carpal Tunnel Release | Johns Hopkins Medicine
Carpal tunnel release is a surgery used to treat and potentially heal the painful condition known as carpal tunnel syndrome.
[119]
In adults, early mobilization may be beneficial for distal radius ...
Nov 24, 2021 · In adults, early mobilization may be beneficial for distal radius fractures treated with open reduction and internal fixation: a systematic ...
[120]
Forearm Fracture Nonunion with and without Bone Loss - NIH
Jul 15, 2022 · Nonunion occurs in 2–10% of all forearm fractures, with or without infection and bone loss. A peak incidence has been observed in the age range ...
[121]
Fracture, nonunion and postoperative infection risk in the smoking ...
Nov 19, 2021 · More specifically, the risk of smokers developing infections after fracture surgery was 2.1 times higher than for non-smokers, which is higher ...