16 Anatomy of the Forearm (2024)

16.2.1 Volar

Two muscles insert on the proximal volar forearm. The brachialis inserts just distal to the coronoid process of the ulna. The proximal margin of the insertion is about 11 mm from the coronoid tip and extends 2 cm onto the proximal ulna. 9 The biceps inserts on the posteromedial aspect of the bicipital tuberosity of the radius, with the biceps insertion approximately 7 mm wide. The biceps muscle rotates 90° to insert, with the tendinous contribution from the long head inserting distal and the short head contribution inserting proximal on the bicipital tuberosity. The short head also gives rise to the lacertus fibrosus. 3

The remainder of the volar forearm can be conceptualized in layers, either three or four, depending on how the structures are categorized. Any conceptualization of a three-dimensional structure in two dimensions is an oversimplification. For instance, the pronator teres is both a superficial muscle (with a partial superficial origin) and a deep muscle with an insertion onto the radial shaft. However, considering the muscles in layers can aid conceptualization.

When dividing the volar forearm into four layers, the superficial layer can be considered as (from lateral to medial) pronator teres, flexor carpi radialis (FCR), palmaris longus (when present), and flexor carpi ulnaris (FCU). These four muscles arise from the common flexor origin at the medial epicondyle. The pronator teres and the FCU also have deep heads that originate from the proximal ulna. The pronator teres inserts on the midshaft radius, while the other muscles insert distal to the forearm (▶Fig. 16.8).

The pronator teres is bounded by the antecubital fossa laterally (with the pronator teres forming the medial border of the antecubital fossa), the FCR medially, and the flexor digitorum superficialis (FDS) deep (▶Fig. 16.9). The median nerve travels between the superficial and deep heads of the pronator teres, and the ulnar artery passes deep to pronator teres. The FCR, in the proximal forearm, lies medial to the pronator teres, lateral to the palmaris longus or FCU, and superficial to the FDS. In the distal forearm, the FCR is bounded radially by the radial artery and ulnarly by the palmaris longus, with the median nerve lying deep between the FCR and palmaris longus.

The palmaris longus, when present, lies in the proximal forearm between the FCR and FCU. In the proximal forearm, the palmaris longus lies superficial to the FDS. Distally, the palmaris longus lies superficial to the carpal tunnel contents, lying just superomedial to the median nerve, and superficial to the FDS (▶Fig. 16.10, ▶Fig. 16.11, and ▶Fig. 16.12). In the absence of the palmaris longus, the median nerve has been misidentified as the palmaris longus and harvested as a tendon graft. 10 Not only is palmaris longus variably present, but the position of the muscle belly can vary as well. 11

The FCU is the most ulnar muscle of the superficial layer and is bounded laterally by either the palmaris longus or the FCR. The FCU lies superficial and medial to the FDS and flexor digitorum profundus (FDP). The FCU arises from both the common flexor origin and the proximal ulna. At the elbow, the ulnar nerve passes deep to the FCU, between the two heads. In the proximal forearm, the ulnar nerve is joined by the ulnar artery. This neurovascular bundle continues deep to the FCU with the ulnar nerve lying ulnar and dorsal to the artery at the wrist. Unlike most other tendons at the wrist, the FCU is an extrasynovial tendon; while spared from stenosing tenosynovitis, the FCU is susceptible to calcific tendonitis and tendinosis commonly seen in other extrasynovial tendons (▶Fig. 16.13). 12

The second layer of the volar forearm is the FDS. The FDS originates from a broad, obliquely oriented fibrous arcade that includes the common flexor origin, the coronoid process, and the proximal radius. Superficial to the FDS lie the four muscles of the superficial layer. The FDP and flexor pollicis longus (FPL) lie deep to the FDS. The median nerve is found between the second (FDS) and the third layers (FDP and FPL) of the volar forearm. 13

The third layer of the volar forearm can be considered as the FDP and the FPL. The FDP has a broad origin. The FDP originates just distal to the brachialis insertion, on both the IOM and the anterior ulna, including the medial coronoid. The FDP origin extends distally to the pronator quadratus crest on the distal ulna. The FPL has a broad origin, including the medial border of the radius, along the anterior oblique line just distal to the radial head. The FPL origin extends distally along the smooth anterior surface of the radius to just proximal to the pronator quadratus. 13 An accessory origin of the FPL, Gantzer’s muscle, can also be found arising from the medial epicondyle (and sometimes coronoid) in approximately half of limbs. Gantzer’s muscle remains innervated by the anterior interosseous nerve (AIN) and always lies posterior to the median nerve and AIN. 14

The fourth layer is the pronator quadratus. The pronator quadratus arises from the pronator ridge of the ulna and inserts on the radius. Superficially, the pronator quadratus is bounded by the FDP and FPL (▶Fig. 16.14). The third and fourth layers are sometimes descriptively grouped together, but this conceptualization limits appreciation of Parona’s space. Between the pronator quadratus and the overlying tendons of the FDP lies this potential space that can allow communication of infected spaces and bursae within the hand, as well as become an abscess.

The common flexor tendon represents the origin of several volar flexors. The common flexor origin can be divided into an anterior and a posterior tendon. The anterior common tendon consists of a robust convergence of the pronator teres, FCR, palmaris longus, and FDS tendons and runs parallel to the anterior bundle of the medial ulnar collateral ligament. The intermuscular fascia between the FDS and FCU also coalesces to form a thinner posterior common tendon located at the inferior end of the medial epicondyle and medial joint capsule. 15

16.2.2 Dorsal

Just as the biceps and brachialis insert just distal to the elbow on the volar forearm, the triceps brachii terminates on the olecranon on the proximal dorsal forearm, providing elbow extension (▶Fig. 16.15). While the vast majority of the triceps muscle is proximal to the forearm, the triceps can have prominent slips at the medial head insertion, which have been suggested as a potential source of ulnar nerve irritation. 16 ^, 17

Excluding the triceps insertion, the dorsal forearm extensor muscles can be conceptualized as superficial and deep groups. The superficial layer can be further subdivided into the three muscles in the lateral group and the three muscles in the superficial long extensor group.

The lateral group includes the brachioradialis, extensor carpi radialis longus (ECRL), and the extensor carpi radialis brevis (ECRB). The brachioradialis arises from the lateral epicondylar ridge of the humerus, starting at approximately the junction of the middle and distal thirds of the humerus, and inserts on the radial styloid. The ECRL arises from the lateral epicondylar ridge of the humerus more distally and inserts on the dorsal base of the second metacarpal. The ECRB arises from the common extensor origin at the lateral epicondyle and inserts on the dorsal base of the third metacarpal.

The brachioradialis forms the lateral border of the antecubital fossa. The brachioradialis is bounded posteriorly by the ECRL. In the proximal forearm, the brachioradialis overlies the supinator. In the proximal and middle forearm, the pronator teres passes deep to the brachioradialis. In the mid-forearm, the brachioradialis overlies the insertion of the pronator teres. In the distal forearm, the brachioradialis passes radial to the FPL and pronator quadratus to insert on the radial styloid. The abductor pollicis longus (APL) and extensor pollicis brevis (EPB) pass obliquely and superficial to the brachioradialis (and also pass superficial to ECRL and ECRB). The radial artery and dorsal sensory branch of the radial nerve (DSBRN) course just deep to the brachioradialis proximally. The DSBRN pierces the antebrachial fascia to become superficial between the brachioradialis and ECRL, at approximately the junction of middle and distal thirds of the forearm. However, even in the mid-forearm, the DSBRN lies barely deep to the brachioradialis. The radial artery continues distally with the brachioradialis, becoming medial to the brachioradialis tendon in the distal forearm (▶Fig. 16.16). The ECRL originates between the brachioradialis and the ECRB on the distal humerus. The ECRB arises from the common extensor tendon at the lateral epicondyle. Both muscles continue distally just superficial to the radius. In the distal forearm, the APL and EPB cross superficial to the ECRL and ECRB. The ECRL and ECRB then continue through the second dorsal compartment of the wrist to insert on the bases of the second and third metacarpals, respectively (▶Fig. 16.17, ▶Fig. 16.18, ▶Fig. 16.19, and ▶Fig. 16.20).

The remaining muscles in the superficial group, the superficial long extensors, arise from the lateral epicondyle and include the extensor digitorum communis (EDC), the extensor digit minimi (EDM), also called the extensor digiti quinti, and the extensor carpi ulnaris (ECU). The EDC and EDM, along with the ECRB, arise from a common extensor origin on the superior aspect of the lateral epicondyle. The ECU arises from the posteroinferior lateral epicondyle. 18

The EDC lies medial to the ECRB and lateral to the EDM. In the proximal forearm, the EDC lies superficial to the supinator. More distally, the EDC passes superficial to the APL, the extensor pollicis longus (EPL), the EPB, and the extensor indicis proprius (EIP), also known as the extensor indices. The EDM lies between the EDC and the ECU. Similar to the EDC, the EDM passes superficial to the supinator, then to the APL, the EPL, and the EIP. The ECU lies medial to the EDM and overlies the supinator, APL, EPL, and EIP. The origin of the EPB is not as ulnar as the other muscles of the deep layer (▶Fig. 16.21, ▶Fig. 16.22, ▶Fig. 16.23, ▶Fig. 16.24, ▶Fig. 16.25, ▶Fig. 16.26, and ▶Fig. 16.27).

The deep layer, from proximal to distal, lying directly on bone and/or IOM, includes the anconeus (although the anconeus is also superficial since it has no overlying muscles), the supinator, the APL, the EPL, the EPB, and the EIP. The APL, EPL, EPB, and EIP all have oblique origins, from the ulna, IOM, and medial radius, with the muscles angling from ulnar proximal to radial distal. The APL is larger than the other deep extensor muscles that insert distal to the wrist. The EIP is classically considered the most distal muscle to be innervated or reinnervated and is used to monitor posterior interosseous nerve recovery, although the EPB is innervated at approximately the same level as the EIP. Patients who have undergone index extensor tendon transfer to replace EPL function after tendon rupture may retain some independence of both the reconstructed thumb and donor index finger, suggesting that independent index extension may not always require a functional EIP. 20

The first dorsal compartment contains the tendons of the APL (which often has multiple slips—up to 14 have been reported) and the EPB (which is dorsal and often much smaller, although variable in size). The APL and EPB usually lie together within the first compartment. However, the EPB may lie within its own subcompartment with a septum between the two tendons, a variant found in 20% of limbs in one cadaver study (▶Fig. 16.28 and ▶Fig. 16.29). 21

The second dorsal compartment contains the tendons of the ECRL and ECRB. The dorsal radius, between the first and second compartments, can be harvested as a vascularized bone graft based on a pedicle from the radial artery, using the 1,2 intercompartmental supraretinacular artery. 22 ^, 23 The third dorsal compartment contains the EPL tendon. The fourth dorsal compartment contains the tendons of the EDC along with the tendon of the EIP (which is oblique and deep to the tendons of the EDC). The fifth dorsal compartment contains the EDM (usually a split tendon distally) (▶Fig. 16.30, ▶Fig. 16.31, and ▶Fig. 16.32). The sixth compartment, which lies in a groove in the ulna, does not rotate with the radius and contains the ECU (▶Fig. 16.33).

A summary of the extensor tendon and their insertions is shown (▶Fig. 16.34 and ▶Fig. 16.35). Anatomic cross-sections showing the muscular relationships at the elbow, midproximal third, junction of proximal and middle thirds, mid-forearm, junction of middle and distal thirds, mid-distal third, and wrist, are shown (▶Fig. 16.36, ▶Fig. 16.37, ▶Fig. 16.38, ▶Fig. 16.39, ▶Fig. 16.40, ▶Fig. 16.41, and ▶Fig. 16.42). The seven cross-sections are courtesy of the National Library of Medicine Visible Human Project^TM. Of note, in these images, the forearm is not positioned in “anatomic position” (with the shoulder in neutral and the forearm fully supinated). Instead, the shoulder is internally rotated and the forearm rotation is near neutral instead of supinated. In addition, the elbow is flexed mildly, creating oblique sections through the forearm, rather than true transverse sections, with the obliquity more apparent at the wrist, where the cross-section is just proximal to the articular surface of the radius, nearly parallel with the articular surface, but cuts obliquely through the ulnar neck.

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Tags: Orthopaedics and Trauma Surgery, Plastic, Reconstructive and Cosmetic Surgery, Section IV Regional Anatomy and Function, The Grasping Hand