Forearm — Surgical Anatomy and Approaches
Introduction
The forearm — between the elbow proximally and the wrist distally — is unique among the orthopedic regions in containing two parallel long bones whose precise spatial relationship determines the rotational function of pronation and supination. The surgical anatomy reflects this design: the radius with its characteristic lateral bow, the ulna along its straight axis, the interosseous membrane between them, the proximal and distal radioulnar joints, and the muscular compartments organized around these bones. The orthopedic surgeon’s approaches to the forearm must respect the fundamental requirement to restore the radial bow and the radioulnar joint relationships (the principle articulated in Topic Trauma-16) while protecting the principal neurovascular structures — the radial artery, the median nerve, the ulnar nerve, the posterior interosseous nerve, and the superficial branch of the radial nerve. The two principal approaches to the radius — the Henry (volar) approach and the Thompson (dorsal) approach — together provide access to the entire length of the bone; the direct ulnar approach provides access to the subcutaneous border of the ulna. This chapter draws on Orthopaedic Surgical Approaches, Netter’s Concise Orthopaedic Anatomy, and Gray’s Anatomy.
Bony Anatomy
The radius is the lateral bone of the forearm and the principal weight-bearing bone of the upper extremity. The proximal end consists of the disc-shaped radial head, the narrow radial neck, and the radial tuberosity (insertion of the biceps tendon). The shaft has a characteristic lateral bow with maximum prominence at the junction of the proximal and middle thirds — the radial bow is the geometric feature whose preservation is essential to normal forearm rotation (Topic Trauma-16). The distal radius widens and articulates with the carpus at the wrist (Topic Trauma-17). The ulna is the medial bone, essentially straight along its length, providing the stable axis around which the radius rotates. The proximal end has the olecranon posteriorly and the coronoid process anteriorly, with the trochlear notch between them articulating with the trochlea of the distal humerus. The shaft has a subcutaneous border that is palpable from the olecranon to the wrist. The distal end has the ulnar head with the ulnar styloid (the insertion of the TFCC). The interosseous membrane connects the radius and ulna along their length, with characteristic obliquity (fibers running from the proximal radius to the distal ulna). The membrane transmits axial load from the radius (at the wrist) to the ulna (at the elbow) and provides additional stability to forearm rotation. The proximal radioulnar joint (PRUJ) between the radial head and the radial notch of the ulna is stabilized by the annular ligament, the quadrate ligament, and the interosseous membrane.
The distal radioulnar joint (DRUJ) between the ulnar head and the sigmoid notch of the radius is stabilized by the palmar and dorsal radioulnar ligaments, the triangular fibrocartilage complex (TFCC), the interosseous membrane, the pronator quadratus, and the ulnar collateral ligament.
Muscular Compartments
The forearm has two principal compartments — the volar (flexor-pronator) and dorsal (extensor-supinator) — with the mobile wad of Henry (brachioradialis, ECRL, ECRB) sometimes considered a separate compartment. Volar (Flexor) Compartment The volar compartment is organized in three layers: Superficial layer: From radial to ulnar — pronator teres, flexor carpi radialis (FCR), palmaris longus (PL, absent in approximately 14 percent of population), flexor carpi ulnaris (FCU). These muscles originate from the medial epicondyle of the humerus (the common flexor origin) and pass distally to their insertions. Intermediate layer: Flexor digitorum superficialis (FDS) — originating from the medial epicondyle and proximal radius and ulna, with tendons to the middle phalanges of the fingers. Deep layer: Flexor digitorum profundus (FDP), flexor pollicis longus (FPL), and pronator quadratus. FDP originates from the ulna and interosseous membrane, FPL from the radius and interosseous membrane, and pronator quadratus distally from the ulna to the radius. The median nerve runs in the volar compartment between the FDS and FDP layers along the midline; the anterior interosseous nerve (AIN) is a deep branch running along the interosseous membrane. The ulnar nerve runs ulnar to the FDP, deep to the FCU. The radial artery runs along the radial side, deep to the brachioradialis. The ulnar artery runs along the ulnar side, deep to the FCU. Dorsal (Extensor) Compartment The dorsal compartment is organized into: Superficial layer: From radial to ulnar — extensor digitorum communis (EDC), extensor digiti minimi (EDM), extensor carpi ulnaris (ECU). These muscles originate from the lateral epicondyle (the common extensor origin). Deep layer: Abductor pollicis longus (APL), extensor pollicis brevis (EPB), extensor pollicis longus (EPL), extensor indicis (EI), supinator. These deep muscles arise variously from the radius, ulna, and interosseous membrane. The posterior interosseous nerve (PIN) passes through the supinator (via the arcade of Frohse) and runs along the posterior surface of the interosseous membrane, supplying the dorsal compartment muscles. The posterior interosseous artery runs with the nerve.
Mobile Wad of Henry The mobile wad consists of the brachioradialis, extensor carpi radialis longus (ECRL), and extensor carpi radialis brevis (ECRB) — three muscles on the radial side of the forearm that can be palpated and mobilized as a single unit. The brachioradialis is innervated by the radial nerve; ECRL is innervated by the radial nerve; ECRB is variably innervated by the radial nerve or the posterior interosseous nerve.
Neurovascular Anatomy
Radial Nerve and Its Branches The radial nerve enters the forearm anterior to the lateral epicondyle, between the brachialis (medial) and brachioradialis (lateral), and divides at the level of the radial head into: Superficial sensory branch: Continues distally beneath the brachioradialis, becomes superficial at the junction of the middle and distal thirds of the forearm, and supplies sensation to the dorsoradial wrist and the dorsum of the radial three and a half digits proximal to the DIP joints. Deep motor branch (posterior interosseous nerve, PIN): Passes through the arcade of Frohse (the fibrous proximal margin of the supinator) and through the supinator muscle to enter the dorsal compartment. The PIN then runs along the posterior surface of the interosseous membrane, supplying the dorsal compartment muscles. The PIN is the principal motor nerve of the extensor compartment. The PIN is at risk in surgical approaches to the proximal radius (particularly the anterior Henry approach and the dorsal Thompson approach), with forearm supination during dissection moving the nerve away from the operative field at the radial neck (anterior approach) or forearm pronation displacing it away in the dorsal approach.
Median Nerve and AIN The median nerve enters the forearm between the two heads of the pronator teres (where it may be compressed in pronator syndrome), then runs deep to the FDS arch, then between FDS (superficial) and FDP (deep) in the midline of the forearm. At the wrist, it passes deep to the flexor retinaculum into the carpal tunnel. The anterior interosseous nerve (AIN) branches from the median nerve approximately 5 to 8 cm distal to the medial epicondyle and runs along the volar surface of the interosseous membrane, supplying the FPL, the radial half of the FDP (to the index and middle fingers), and the pronator quadratus. The AIN is purely motor (no sensory component); AIN palsy is recognized by inability to perform the “OK” sign (loss of FPL and FDP to index). Ulnar Nerve The ulnar nerve enters the forearm through the cubital tunnel between the two heads of the FCU. It runs deep to the FCU, ulnar to the FDP, supplying both these muscles and giving
off a dorsal sensory branch at the junction of the middle and distal thirds (which supplies the dorsoulnar wrist and the dorsum of the ulnar one and a half digits). The main ulnar nerve continues into the Guyon canal at the wrist. Brachial Artery Branches The brachial artery bifurcates at the level of the radial neck into: Radial artery: Runs along the radial side of the forearm, deep to the brachioradialis. The artery is the principal vessel encountered in the Henry approach. Ulnar artery: Runs along the ulnar side, deep to the FCU. The artery is encountered in approaches to the medial forearm. The common interosseous artery branches from the ulnar artery and divides into the anterior interosseous artery (running with the AIN on the volar surface of the interosseous membrane) and the posterior interosseous artery (running with the PIN on the dorsal surface).
The Henry Approach to the Radius (Volar Approach)
The Henry approach is the classical volar approach to the radius, providing access for plate fixation of radial shaft fractures, biopsy of the radius, exposure of the radial artery, and anterior approach to forearm tumors. Patient Positioning The patient is supine with the affected arm on a hand table, forearm fully supinated. The full supination is the critical positioning element — it brings the entire radial shaft into the operative field and protects the PIN by displacing it medially (away from the radial neck, which is the most dangerous region for PIN injury).
Surface Landmarks and Incision The biceps tendon proximally, the mid-volar forearm in the middle, and the radial styloid distally mark the line of the incision. The skin incision can extend from the bicipital flexion crease distally over the radial side of the volar forearm to the radial styloid, with the length determined by the desired exposure. Approach The skin and subcutaneous tissue are incised, with attention to superficial branches of the radial nerve (which become subcutaneous at the junction of the middle and distal thirds and run along the radial border of the distal forearm). The fascia is opened in line with the skin incision. The brachioradialis (radial side) and flexor carpi radialis (ulnar side) form the boundaries of the approach. The radial artery runs in the interval between these muscles, slightly toward the ulnar side; it is identified and protected.
The brachioradialis is retracted laterally; the FCR is retracted medially. In the proximal third of the radius, the supinator is identified covering the bone, and the PIN runs through the supinator. The supinator is detached from the radius by subperiosteal elevation, with the forearm fully supinated to protect the PIN. In the middle third, the pronator teres insertion on the radius is identified and detached from its insertion. In the distal third, the pronator quadratus is identified and reflected from the radius. The radius is then exposed subperiosteally, and the desired procedure (plate fixation, biopsy, etc.) is performed. Complications The complications of the Henry approach include PIN injury (the principal concern; minimized by forearm supination and careful subperiosteal elevation of the supinator), superficial radial nerve injury (with painful neuroma if branches are damaged), radial artery injury (rare but can occur with vigorous retraction), and median nerve injury (uncommon, more likely with proximal extension of the dissection).
The Thompson Approach to the Radius (Dorsal Approach)
The Thompson approach provides dorsal access to the radius, used for plate fixation of proximal-third radial fractures (where the volar Henry approach risks the PIN), exposure of the proximal radius in selected pathologies, and dorsal approach to forearm tumors. Patient Positioning The patient is supine with the affected arm on a hand table, forearm pronated. The pronation displaces the PIN to the radial side, away from the operative field (the opposite of supination, which is used in the Henry approach to displace the PIN to the medial side).
Surface Landmarks and Incision The lateral epicondyle proximally and the Lister’s tubercle distally mark the line of the incision. The skin incision extends along the dorsoradial aspect of the forearm.
Approach The skin and subcutaneous tissue are incised. The interval between extensor carpi radialis brevis (ECRB) (radial side, supplied by the radial nerve) and extensor digitorum communis (EDC) (ulnar side, supplied by the PIN) is identified — this is the internervous plane of the Thompson approach. The interval is developed bluntly. The supinator is exposed covering the proximal radius. The PIN runs through the supinator, entering at the arcade of Frohse and exiting after running through approximately 4 to 6 cm of muscle. The PIN must be carefully identified and protected.
For approach to the radial shaft, the supinator is incised over the radial neck (with the forearm pronated to displace the PIN laterally) and the supinator is reflected, exposing the proximal radius. Subperiosteal dissection then exposes the radial shaft. Complications The complications of the Thompson approach include PIN injury (the principal concern; particular risk in the supinator dissection), superficial radial nerve injury, and stiffness of the elbow if the approach is extensive.
The Direct Ulnar Approach
The direct ulnar approach uses the subcutaneous border of the ulna, providing direct access to the bone without the need for an intermuscular plane. Patient Positioning The patient is supine with the affected arm on a hand table, forearm in neutral rotation or slight supination. Surface Landmarks and Incision The subcutaneous border of the ulna is palpable from the olecranon to the ulnar styloid. The skin incision is made directly over this subcutaneous border for the desired length. Approach The skin and subcutaneous tissue are incised. The fascia is opened directly over the subcutaneous border. The flexor carpi ulnaris (FCU) is reflected ulnarly (along with the ulnar nerve, which is protected throughout); the extensor carpi ulnaris (ECU) is reflected radially. Subperiosteal dissection then exposes the ulna. The approach is straightforward and produces no specific neurological risk because there are no major nerves crossing the subcutaneous border of the ulna; however, the dorsal cutaneous branch of the ulnar nerve runs along the ulnar side of the forearm at the junction of the middle and distal thirds and may be encountered. The approach is used for plate fixation of ulnar shaft fractures, exposure of the ulna for biopsy or tumor resection, and ulnar shortening osteotomy (a procedure for ulnar impaction syndrome).
Approach to the Distal Radius
The standard volar approach to the distal radius for volar plate fixation of distal radius fractures uses a modified Henry approach with longitudinal incision over the FCR tendon. The FCR sheath is opened, the tendon is retracted ulnarly along with the median nerve and tendon contents, and the pronator quadratus is released from the radius. The distal radius is then exposed for plate placement. The watershed line (the transverse ridge marking the volar prominence of the distal radius) is identified, and the plate is positioned
at or proximal to this landmark to avoid flexor tendon irritation. Topic Trauma-17 details the specific considerations. The dorsal approach to the distal radius is used for selected indications (dorsal Barton fracture, dorsal die-punch fragments, specific dorsal plating). The approach is between the third extensor compartment (extensor pollicis longus, EPL) and the fourth extensor compartment (EDC), with the EPL routed dorsal to its sheath at closure.
Approach to the Radial Artery (Forearm Segment)
Direct surgical access to the radial artery uses a longitudinal incision over the artery, with the interval between brachioradialis (lateral) and FCR (medial), exposing the artery deep to these muscles. The approach is used for vascular injury repair, radial artery harvest for coronary artery bypass grafting, and arteriovenous fistula creation for dialysis access.
Summary and Take-Home Points
The forearm contains two parallel long bones (radius and ulna) connected by the interosseous membrane and articulated at the proximal and distal radioulnar joints, with the rotational function of pronation and supination depending on preservation of the radial bow and the radioulnar joint relationships. The principal surgical approaches must respect this anatomy while protecting the posterior interosseous nerve (the principal neurological structure at risk in radial approaches), the superficial radial nerve (subcutaneous at the radial border of the distal forearm), the median and anterior interosseous nerves (volar compartment), the ulnar nerve and its dorsal branch (medial compartment), and the radial and ulnar arteries. The Henry approach (volar) uses the interval between brachioradialis and FCR, with forearm supination to displace the PIN medially during dissection of the supinator and proximal radius. The approach provides access to the entire length of the radius and is the standard for radial shaft fixation. The Thompson approach (dorsal) uses the internervous interval between ECRB and EDC, with forearm pronation to displace the PIN radially during dissection. The approach is used for proximal radial fractures and selected indications where the volar approach is contraindicated. The direct ulnar approach uses the subcutaneous border of the ulna and provides straightforward access without specific neurological risk; the approach is used for ulnar shaft fixation and ulnar shortening osteotomy. The distal radius approaches (volar Henry-modification and dorsal) were addressed in Topic Trauma-17, with the watershed line being the critical anatomical landmark for volar plate positioning. The principles of anatomical reduction of the radius with preservation of the radial bow, avoidance of synostosis (heterotopic bone formation between radius and ulna) by careful operative technique, and protection of the PIN by appropriate forearm
positioning are central to forearm surgery. The chapter that follows turns to the hand, completing the upper extremity anatomical sequence.