I. The Nailbed
A. Definitions and anatomic relationships
1. Perionychium—The entire area that includes the nail, nailbed, and surrounding skin.
2. Paronychium—The lateral nail fold.
3. Hyponychium—The skin immediately distal and palmar to the nail, at the junction of the sterile matrix and fingertip skin.
4. Eponychium—The dorsal nail fold, proximal to the nailplate. It adds shine to the nail.
5. Lunula—The white portion of the proximal nail. It is most likely created by retained cellular nuclei.
6. Sterile matrix and germinal matrix—The sterile matrix is the soft tissue deep to and adherent to the nail, distal to the lunula. The germinal matrix is the soft tissue deep to the nail and proximal to the sterile matrix. The germinal matrix is responsible for most nail development.
a. The periosteum of the distal phalanx lies immediately volar to the sterile and germinal matrices.
b. The distance from the extensor tendon insertion to the proximal germinal matrix is approximately 1.2 to 1.4 mm.
II. The Skin/Fascia
A. Definitions and anatomic relationships
1. Palmar fascia
a. The palmar aponeurosis is the terminal extension of the palmaris longus tendon.
b. The pretendinous band lies superficial to the transverse fibers of the palmar aponeurosis. It inserts into the skin at the level of the metacarpophalangeal (MCP) joint and trifurcates into spiral bands (radial and ulnar) and a central band.
c. The superficial transverse metacarpal ligament comprises the transverse fibers of the palmar aponeurosis.
d. The natatory ligament consists of the transverse fibers within the web space; this ligament restricts digital abduction.
e. The vertical septa of Legueu and Juvara are eight vertical septa found on each side of the flexor tendons, creating seven compartments—four containing flexors, and three containing neurovascular bundles and lumbrical muscles. These septa attach to the transverse ligament of the palmar aponeurosis.
f. The palmar plate is the palmar stabilizer of the MCP joint and is connected to the deep transverse metacarpal ligament.
2. Digital fascia—The lateral digital sheet receives contributions from the following:
a. The spiral band, which originates at the pretendinous band, passes dorsal to the neurovascular bundle to insert on the lateral digital sheet.
b. The natatory ligament occupies the interdigital web spaces.
c. The Grayson ligament, which originates in the volar flexor sheath, passes volar to the neurovascular bundles and inserts into the skin. The fibers of the Grayson ligament are oriented perpendicular to the digital axis.
d. The Cleland ligament, which has its origin in the phalanx, passes dorsal to the neurovascular bundles and inserts into the skin. Four Cleland ligaments are on each side of the finger; two are adjacent to the proximal interphalangeal (PIP) joint and two are adjacent to the distal interphalangeal (DIP) joint.
III. Compartments of the Forearm and Hand
A. Overview
1. The osteofascial compartments of the forearm and hand are named for their anatomic locations.
[
Figure 1. Illustrations of the cross-sectional anatomy of the compartments of the forearm (A) and hand (B).]
2. It is imperative for the surgeon to recognize anatomic subcompartments that may require dedicated surgical decompression in the nonphysiologic condition of compartment syndrome. Examples of such subcompartments include the deep flexors (flexor digitorum profundus) or pronator quadratus within the volar compartment of the forearm or an individual muscle ensheathed by its epimysium.
B. Compartments of the forearm (Figure 1, A)
1. Dorsal compartment
2. Volar compartment
3. Mobile wad
C. Compartments of the hand (Figure 1, B)
1. Thenar compartment
2. The adductor pollicis acts as a single compartment.
3. Hypothenar compartment (may have multiple subcompartments)
4. Four dorsal and three volar interosseous muscle compartments
5. Carpal tunnel
IV. Palmar Spaces of the Hand
A. Thenar space
1. The thenar space is located dorsal to the flexor tendons and volar to the interosseous fascia.
2. It is separated from the midpalmar space by the midpalmar septum, a fascial septum extending from the palmar fascia to the long finger metacarpal. The thenar space is radial to the septum.
3. The dorsal boundary of the thenar space is the adductor pollicis fascia; the volar boundary is formed by the index flexor sheath and the palmar fascia.
B. Midpalmar space
1. The midpalmar space is located dorsal to the flexor tendons and volar to the interosseous fascia.
2. It is separated from the thenar space by the midpalmar septum, a fascial septum extending from the palmar fascia to the long finger metacarpal. The midpalmar space is ulnar to the septum.
3. The dorsal boundary of the midpalmar space is the fascia of the second and third volar interosseous muscles; the volar boundary is formed by the flexor sheaths of the long, ring, and little fingers and the palmar aponeurosis.
C. Hypothenar space
1. The hypothenar space is located between the hypothenar septum and the hypothenar musculature.
2. The dorsal boundary of the hypothenar space is the periosteum of the small finger metacarpal and the deep hypothenar fascia; the volar boundary is formed by the palmar fascia and the fascia of the superficial hypothenar muscles.
D. Radial bursa
1. The radial bursa begins at the MCP joint and extends proximally to 1 to 2 cm proximal to the transverse carpal ligament.
2. Typically, the flexor pollicis longus (FPL) sheath is continuous with the radial bursa.
E. Ulnar bursa
1. The ulnar bursa begins at the proximal aspect of the little finger flexor sheath and extends 1 to 2 cm proximal to the transverse carpal ligament.
2. The synovial sheath of the little finger may connect with the ulnar bursa.
F. The Parona space
1. The Parona space is a potential space between the flexor digitorum profundus (FDP) tendon and the pronator quadratus fascia.
2. When present, the Parona space facilitates communication between the radial and ulnar bursae.
V. The Digits
A. Osteology
1. The bones of the finger are the proximal (P1), middle (P2), and distal (P3) phalanges.
2. The bones of the thumb are the proximal and distal phalanges
B. Joints
1. Metacarpophalangeal joint
a. In the fingers, the MCP joint is a triaxial condyloid joint. The typical range of motion is from 15° of hyperextension to 90° of flexion. The trapezoidal shape of the metacarpal head creates a cam effect on the collateral ligaments, causing the ligaments to be taut during MCP joint flexion and lax in extension. Therefore, the MCP joint is stable in flexion and is unstable in extension.
b. In the thumb, unlike in the fingers, the metacarpal head is a single broad condyle. Sesamoids are contained within the volar plate. The MCP joint of the thumb has a wide variation in "normal" range of motion. The ulnar collateral ligament (UCL) includes proper and accessory portions. The proper collateral extends from the proximal phalanx to the metacarpal head, whereas the accessory collateral originates on the metacarpal head and inserts into the volar plate.
2. Proximal interphalangeal joint
a. The PIP joint is a hinge joint. The head of the proximal phalanx includes two condyles separated by an intercondylar notch. This notch provides some inherent joint stability through its articulation with the median ridge at the base of the middle phalanx.
b. The collateral ligaments at the PIP joint are taut throughout range of motion.
i. Proper collateral ligament—The origin is at the pit on the head of the proximal phalanx; the ligament inserts into the lateral tubercle of the base of the middle phalanx.
ii. Accessory collateral ligament—Inserts into the volar plate.
iii. Volar plate—A thick, fibrocartilagenous floor that originates within the A2 pulley and inserts into the "rough area" at the base of P2. It supports the insertion of the accessory collateral ligament and prevents PIP joint hyperextension.
3. Distal interphalangeal joint—The DIP joint is stabilized by the collateral ligaments, the terminal extensor tendon insertion, the FDP insertion, and the volar plate.
C. Dorsum of the finger
1. Extensor apparatus
a. The extensor tendons are divided into anatomic zones. The extrinsic extensor tendon trifurcates at the base of P1, with the central portion inserting into the dorsal base of P2 as the central slip.
b. The lateral slips are joined by contributions from the oblique fibers of the extensor hood (from the interossei and lumbrical muscles) to form the conjoined lateral band. Conjoined lateral bands converge over the middle phalanx to form the terminal tendon, which inserts into the dorsal base of P3.
c. Contributions of the dorsal interosseous muscles and the lumbrical muscles to the dorsal extensor apparatus are listed in
Table 1.
2. Dorsal stabilizing structures
|
a. |
The sagittal bands, which originate at the MCP joint volar plate and the base of P1, stabilize the extrinsic extensor tendon at the dorsal MCP joint. They contribute indirectly to MCP joint extension through a sling-like mechanism. |
|
b. |
The triangular ligament stabilizes the conjoined lateral bands over the base of P2 to prevent volar subluxation. |
|
c. |
The transverse retinacular ligament stabilizes the conjoined lateral bands to prevent dorsal subluxation. The fibers of this ligament are oriented in a dorsal-volar direction at the level of the PIP joint. |
[Table 1. Contributions of Interosseous and Lumbrical Muscles to the Extensor Apparatus]
|
d. |
The oblique retinacular ligament (ORL) links the PIP and DIP joints. It originates at the fibro-osseous gutter at the A2 pulley and middle third of P1 (volar) to insert into the terminal extensor tendon (dorsal). |
D. Volar finger
1. Palmar fascia—This comprises the Grayson and Cleland ligaments, as described above.
2. Flexor tendons
a. Anatomy
i. The flexor tendons are divided into five anatomic zones (
Figure 2). At the level of the A1 pulley, the flexor digitorum superficialis (FDS) flattens and bifurcates to allow the deeper FDP to pass distally to its insertion at the base of P3. Bifurcating limbs of the FDS rotate laterally and dorsally around the FDP and then divide again into medial and lateral slips.
[Figure 2. Anatomic zones for the characterization of flexor tendon injuries.]
ii. Medial slips of the FDS tendon cross dorsal to the FDP, rejoining (chiasma tendinum digitorum manus, or the Camper chiasma) over the distal P1 and PIP joint volar plate. The lateral slip continues distally to insert at the base of P2.
b. Vascular supply—The flexor tendons are nourished by a direct vascular supply and synovial diffusion.
i. Direct vascular supply—This consists of transverse digital arteries ("ladder branches") that arise from digital arteries to supply the vincular system (vinculae are mesotendinous vascular networks on the dorsal surface of the flexor tendons) and a direct arterial supply from intraosseous vessels at tendinous insertions.
ii. Synovial diffusion—The avascular and hypovascular zones of the FDS and FDP within
[
Figure 3. Commonly accepted flexor pulley nomenclature (A1-A5 and C1-C3). Note ulnar bursa (UB) and radial bursa (RB).]
the flexor tendon sheath rely on intratendinous canaliculi for synovial diffusion.
3. Flexor tendon sheath/pulleys—The fibro-osseous digital sheath provides both biomechanical efficiency and a source of nutrition to the flexor tendons. The flexor tendons are enveloped by a layer of visceral paratenon, and each pulley/retinacular system is lined by a layer of parietal paratenon. Condensations of the synovial sheath form at strategic points along the digit to work in conjunction with the transverse carpal ligament and the palmar aponeurosis pulley to maximize efficiency of joint rotation and force transmission. Five annular and three cruciform pulleys are typically described (Figure 3). The A1, A3, and A5 pulleys take their origin from palmar plates of the MCP, PIP, and DIP joints, respectively. The A2 and A4 pulleys originate from the proximal and middle phalanges, respectively. The A2 and A4 pulleys are the most important pulleys biomechanically.
E. Dorsal thumb—The dorsal thumb musculature comprises the extensor apparatus of the thumb. At the MCP joint, the extensor pollicis longus (EPL) tendon is ulnar to the extensor pollicis brevis (EPB) before flattening and continuing to its insertion at the dorsal base of the distal phalanx. The EPL and EPB are stabilized by the sagittal band at the MCP joint. Variable EPB anatomy has been described, including insertion into the extensor hood without insertion into the proximal phalanx.
F. Volar thumb
1. Flexor tendon
a. The FPL emerges from the interval between the adductor pollicis and the thenar musculature to enter the fibro-osseous digital sheath at the level of thumb MCP joint.
b. The FPL flexes the thumb interphalangeal (IP) joint.
2. Flexor tendon sheath/pulleys
a. Similar in function to the pulleys of the finger, the A1, oblique, and A2 pulleys comprise the pulley system of the thumb.
b. The A1 pulley is at the level of the MCP joint; the oblique pulley fibers are oriented in a distal and radial direction at the level of the proximal phalanx; and the A2 pulley originates from the interphalangeal joint volar plate. The A1 and oblique pulleys are most important biomechanically.
c. The radial digital nerve is at particular risk during A1 pulley release.
VI. The Hand
A. Osteology
1. The thumb carpometacarpal (CMC) joint, or trapeziometacarpal (TM) joint, is biconcave-convex, similar to two opposing saddles.
2. The TM joint is stabilized primarily by the dorsal ligamentous complex (dorsoradial ligament plus posterior oblique ligament) and the deep anterior oblique ligament.
B. Soft tissue
1. Dorsal hand / extensor tendons
|
a. |
The juncturae tendinae are obliquely oriented interconnections between the extrinsic extensor tendons on the dorsal hand. |
|
b. |
The extensor indicis proprius (EIP) and extensor digiti minimi (EDM) are most commonly located ulnar to the index and small finger extensor digitorum communis (EDC) tendons, respectively. |
|
c. |
The most common extrinsic extensor pattern to the little finger is two EDM tendons and no EDC tendon. |
[
Table 2. The Interosseous Muscles]
[
Table 3. The Lumbrical Muscles]
|
d. |
The extensor digitorum brevis manus, an anomalous muscle belly (incidence ~3%) found between the index and long extensor tendons, is sometimes confused with a soft-tissue tumor. |
2. Intrinsic muscles—The intrinsic muscles of the hand include the interossei (dorsal and volar), lumbrical, thenar, and hypothenar muscles.
|
a. |
The interosseous muscles (Table 2) originate from the metacarpal diaphyses and pass dorsal to the deep transverse intermetacarpal ligament. They are innervated by the ulnar nerve; however, the first interosseous muscle may be innervated by the median nerve through a Martin-Gruber (forearm) or Riche-Cannieu (hand) interconnection. i. Dorsal interossei—There are four dorsal interossei, which are bipennate (arise from both metacarpals of respective intermetacarpal space). Each dorsal interosseous muscle has two muscle bellies, one superficial and one deep. The superficial muscle belly passes under the sagittal hood to become the medial tendon; it inserts into the lateral tubercle of the proximal phalanx. Its action is to abduct the digit. The deep muscle belly passes over the sagittal hood to become the lateral tendon; it inserts into the transverse fibers of the extensor apparatus. Its action is to flex the MCP joint.
ii. Volar interossei—Each of these muscles is unipennate and arises on the metacarpal of the same digit to which it inserts. The volar interossei insert into the extensor apparatus and have no insertion into the proximal phalanx. Their action is to flex the MCP joint and to adduct the index, ring, and little fingers. |
|
b. |
The lumbrical muscles (Table 3) originate from the FDP tendon and pass volar to the deep transverse intermetacarpal ligament on the radial side of each digit. They insert into the radial lateral band of the extensor apparatus. Their action is to extend the PIP and DIP joints through the oblique fibers. A lumbrical muscle is the only muscle able to "relax" its antagonist (as it has its origin on the FDP tendon). |
[
Table 4. The Thenar Muscles]
[
Table 5. The Hypothenar Muscles]
|
|
The first and second lumbricals are innervated by the median nerve and are unipennate. The third and fourth lumbricals are innervated by the ulnar nerve and are bipennate. |
|
c. |
The thenar muscles (Table 4) are the abductor pollicis brevis (APB), flexor pollicis brevis (FPB), opponens pollicis, and adductor pollicis. |
|
d. |
The hypothenar muscles (Table 5) are the abductor digiti minimi, flexor digiti minimi, and opponens digiti minimi. Note that the flexor digiti minimi is absent in 15% to 20% of hands. |
VII. The Wrist
A. Osteology
1. Distal radius and ulna
a. The distal radius articular surface has two concave facets, the scaphoid and lunate facets, separated by the scapholunate, or anterior-posterior, ridge.
b. The sigmoid notch, along the ulnar border of the distal radius, is a shallow concavity for the articulating ulnar head at the distal radioulnar joint (DRUJ).
[
Figure 4. Dissections of a right wrist. A, Dorsal view of a right wrist with the ECRL and ECRB reflected distally. The dorsal extrinsic wrist ligaments are identified. The DRC originates at the dorsal lip of the distal radius, adjacent to the dorsal radial tubercle (the Lister tubercle, designated by #). It traverses the radiocarpal joint obliquely to insert into the lunate and triquetrum. The DIC arises from the triquetrum and inserts into the capitate, the distal scaphoid, and the trapezoid. ERCL = extensor carpi radialis longus, ECRB = extensor carpi radialis brevis, DRC = dorsal radiocarpal ligament, DIC = dorsal intercarpal ligament. B, Dorsal view of a right wrist after resection of the extrinsic ligaments. The dorsal aspect of the TFCC, the dorsal radioulnar ligament, is visualized. The SLIL and LTIL stabilize the proximal carpal row. The capitate and hamate are identified in the distal carpal row. SLIL = scapholunate interosseous ligament, LTIL = lunotriquetral interosseous ligament, Cap = capitate, H = hamate. C, Dorsal view of the DRUJ. The distal ulna articulates with the distal radius at the DRUJ where the distal radial articular surface (sigmoid notch) has a greater radius of curvature than that of the unlar head. The DRUJ is constrained by the dorsal and volar (not shown) distal radioulnar ligaments. The distal radioulnar ligaments are components of the TFCC. The Lister tubercle (designated by #) is identified for reference. UH = ulnar head, US = ulnar styloid, dRUL = dorsal distal radioulnar ligament. D, End-on view of the articular surface of the right distal radius and the ulnocarpal joint with the carpus reflected palmarly and ulnarly. The SF and LF of the distal radius are separated by the scapholunate ridge. The volar extrinsic ligaments originate from the distal radius and include the RC, RSC, and RL ligaments. Components of the TFCC include the central meniscal homologue (TFCC), the dorsal and volar radioulnar ligaments (not shown), the UL, the UT, and the floor of the ECU sheath (not shown). SF = scaphoid facet, LF = lunate facet, RC = radial collateral ligament, RSC = radioscaphocapitate ligament, RL = radiolunate ligament, UL = ulnolunate ligament, UT = ulnotriquetral ligament.]
c. The distal ulna is covered with hyaline cartilage on its dorsal, lateral, palmar, and distal surfaces.
d. The ulnar styloid projects distally; at its base, the fovea is the insertion for the triangular fibrocartilagenous complex (TFCC).
2. Carpus
|
a. |
The carpus comprises eight ossicles. |
|
b. |
The ossicles are traditionally separated into two carpal rows: the proximal row, containing the scaphoid, lunate, triquetrum, and pisiform; |
[
Table 6. Origins and Insertions of the Extrinsic Wrist Ligaments]
|
|
and the distal row, containing the trapezium, trapezoid, capitate, and hamate. |
|
|
i. Scaphoid—The primary vascular supply is a branch of the radial artery at the dorsal ridge. A group of smaller vessels enters the palmar tubercle and supplies the distal 30%. The transverse carpal ligament attaches to the palmar tubercle.
ii. Lunate—Both a dorsal and a palmar vascular supply are found in 80% of wrists; in 20% of wrists only a palmar supply is found. The lunate is broader palmarly than dorsally.
iii. Triquetrum—Articulates with the hamate distally, the lunate radially, and the pisiform volarly. It is stabilized to the fovea of the ulna through the ulnotriqueteral ligament.
iv. Hamate—The hamate consists of the body and the hook (hamulus) of the hamate. The hook of the hamate serves as an attachment for the transverse carpal ligament and for the origins of the flexor digiti minimi and opponens digiti minimi.
v. Capitate—The head (the proximal portion) often relies on a retrograde vascular supply. Two ridges separate the distal articular surface into three facets for articulation with the metacarpals of the index, long, and ring fingers.
vi. Trapezoid—The trapezoid has two distal facets, which articulate with the metacarpal of the index finger.
vii. Trapezium—The trapezium has a saddle-shaped articulation with the base of the thumb metacarpal. It also has a palmar groove for the flexor carpi radialis (FCR), bordered laterally by a palmar tuberosity and the attachment for the transverse carpal ligament.
viii. Pisiform—The pisiform is a sesamoid bone within the flexor carpi ulnaris (FCU) tendon. It is the origin for the abductor digiti minimi. |
B. Ligaments (Figure 4).
1. Extrinsic wrist ligaments—See Table 6.
2. Intrinsic wrist ligaments—The intrinsic wrist ligaments include the scapholunate interosseous ligament and the lunotriquetral interosseous ligament.
[
Figure 5. Cross-sectional anatomy of the proximal third (A), middle third (B), and distal third (C) of the forearm.]
a. The scapholunate interosseous ligament is C-shaped in the sagittal plane. The dorsal one-third of the ligament is the thickest and strongest portion of the ligament.
b. The volar portion of the lunotriquetral ligament is the thickest.
C. Triangular fibrocartilagenous complex (TFCC)
1. The TFCC is formed by the central meniscus homologue, the dorsal and volar radioulnar ligaments, the floor of the extensor carpi ulnaris (ECU) tendon sheath, and the volar ulnocarpal ligaments (Figure 4, C and D).
2. The TFCC arises from the radial border of the distal radius and inserts into the base of the ulnar styloid and distal ulna through the ligamentum subcruatum.
3. The dorsal and volar radioulnar ligaments are the primary stabilizers of the DRUJ.
4. Only the peripheral 10% to 40% of the volar, ulnar, and dorsal TFCC has a vascular supply.
D. Carpal kinematics
1. The proximal carpal row has no muscular or tendinous attachments and is an intercalary segment.
2. With ulnar deviation, the proximal row extends relative to the forearm/distal row. With radial deviation, the proximal row flexes relative to the forearm/distal row.
3. With axial loading through the neutral wrist, approximately 80% of forces are transmitted through the distal radius (60% scaphoid facet/40% lunate facet) and 20% through the distal ulna.
4. With wrist flexion, 60% of the motion is midcarpal and 40% is radiocarpal. With wrist extension, 33% of the motion is midcarpal and 66% is radiocarpal.
VIII. The Forearm
A. Figure 5 shows cross sections through the proximal, middle, and distal third of the forearm.
B. Osteology/interosseous membrane
1. The radius and ulna are stabilized at the proximal and distal radioulnar joints and through the interosseous ligament (IOL). The IOL is important in compressive load transfer from the wrist to the elbow.
2. The IOL complex is made up of three parts: the IOL proper (central ligament); the proximal interosseous band; and the accessory bands.
3. The fibers of the IOL are oriented from proximal-radial to distal-ulnar.
C. Dorsal/extensor forearm and wrist musculature
1. This musculature may be divided into three groups: the mobile wad of three, the superficial extensor muscles, and the deep extensor muscles (
Table 7).
2. At the wrist, there are six synovial, dorsal fibroosseous compartments beneath the extensor retinaculum.
|
a. |
First extensor compartment (abductor pollicis longus [APL] and extensor pollicis brevis [EPB]) |
[Table 7. Musculature of the Dorsal Forearm and Wrist]
[
Table 8. Musculature of the Volar/Flexor Forearm and Wrist]
|
|
i. APL—Palmar and radial to the EPB, the APL may have one or multiple slips. In approximately 70% of wrists, 2 slips are present. The APL has multiple insertions into the opponens, abductor pollicis brevis, and trapezium.
ii. EPB—Always one slip; may be within its own subcompartment (found in approximately 30% of wrists). The first dorsal extensor tendons cross those of the second compartment approximately 7 cm proximal to the wrist crease. |
|
b. |
Second extensor compartment—extensor carpi radialis longus and brevis (ECRL and ECRB) |
|
c. |
Third extensor compartment (EPL)—The EPL extends and adducts the thumb. |
|
d. |
Fourth extensor compartment (EDC and EIP) |
|
|
i. The EIP origin is deep to the EDC; its muscle belly extends most distally. The EIP is located ulnar to the index EDC.
ii. Most commonly (in about 55% of wrists), there are 2 EDM tendons and no EDC to the little finger. Next most common is one EDM and one EDC to the little finger. |
|
e. |
Fifth extensor compartment (EDM)—This fibrous compartment overlies the DRUJ. |
|
f. |
Sixth extensor compartment (ECU)—Stabilized by the extensor retinaculum and a subcompartment, the ligament jugatum. Failure may lead to ECU subluxation. |
D. Volar/flexor forearm and wrist musculature
1. This musculature may be divided into two groups: the superficial layer and the deep layer.
2. Table 8 lists the muscles of the volar forearm and wrist.
IX. Vascular Anatomy of the Hand, Wrist, and Forearm
A. The vascular anatomy of the hand, wrist, and forearm is shown in
Figure 6.
[Figure 6. Illustrations of the primary arterial anatomy of the upper extremity. A, Volar forearm, wrist, and hand. B, Dorsal forearm, wrist, and hand.]
B. Arteries
1. Brachial artery
a. The brachial artery lies lateral to the median nerve in the antecubital fossa.
b. The origins of the radial, ulnar, and recurrent radial arteries are variable.
2. Radial artery—Located in the brachioradialis and FCR interval in the distal forearm, the radial artery divides into a palmar branch and a dorsal branch.
a. The palmar branch courses over the volar surface of the FCR tendon and penetrates the thenar musculature to contribute to the superficial palmar arch.
b. The dorsal branch passes deep to the APL and EPB tendons at the anatomic snuffbox, splits the two heads of the first dorsal interosseous muscle, and divides into the princeps pollicis artery and a branch to the deep palmar arch.
3. Ulnar artery
a. The ulnar artery passes deep to the pronator teres in the proximal forearm. It remains radial, or lateral, to the ulnar nerve in the forearm and at the distal ulnar tunnel, or the Guyon canal.
b. The ulnar artery splits into a volar carpal branch and a branch to the superficial palmar arch.
C. Arches
1. Superficial palmar arch
a. Approximating the level of the Kaplan cardinal line in the palm, the superficial palmar arch lies superficial to the median nerve.
[
Figure 7. Illustration of the brachial plexus (right).]
b. A complete superficial palmar arch is found in 84% of hands. It is the origin for the proper digital artery to the ulnar little finger and the common palmar digital arteries, which split into the proper digital arteries and continue along the radial or ulnar side of their respective digit.
2. Deep palmar arch
a. The deep palmar arch is found proximal to the superficial arch.
b. The deep palmar arch is the origin for the proper digital artery to the radial index finger and usually the thumb (princeps pollicis artery).
X. Nerves of the Hand, Wrist, and Forearm
A. The nerves of the hand, wrist, and forearm originate as terminal branches from the brachial plexus (Figure 7).
B. Radial nerve (
Figure 8)
1. The radial nerve arises from the posterior cord of the brachial plexus (C5-C8; +/-T1).
2. It passes anterior to the lateral epicondyle, deep to the brachioradialis (BR) and extensor carpi radialis (ECR) muscles, and divides into the superficial branch of the radial nerve (SBRN) and posterior interosseous nerve (PIN).
3. The SBRN descends in the forearm deep to the BR muscle and then pierces the deep fascia ulnar to the BR tendon approximately 7 cm proximal to the radial styloid to provide sensation to the dorsal-radial hand.
4. The ECRB may be innervated by the radial nerve, the SBRN, or the PIN.
5. The PIN courses perpendicularly through the supinator to lie between the APL/ECU (deep) and EDM/EDC (superficially).
6. The PIN arborizes into three branches to the ECU, EDM, and EDC and into two longer branches to the APL, EPL, EPB, and EIP.
[Figure 8. Illustration of the anatomic course and branches of the radial nerve in the right upper extremity. BR = brachioradialis, ECRL = extensor carpi radialis longus, ECRB = extensor carpi radialis brevis, ECU = extensor carpi ulnaris, EDC = extensor digitorum communis, EDM = extensor digitorum communis, APL = abductor pollicis longus, EPL = extensor pollicis longus, EPB = extensor pollicis brevis, EIP = extensor indicis proprius, PIN = posterior interosseous nerve.]
7. The classic order of innervation for the PIN is ECU, EDC, EDM, APL, EPL, EPB, EIP.
8. The arcade of Frohse is the fibrous, proximal leading edge of the supinator and may be a site of nerve compression.
9. The PIN innervates the extensor muscles of the forearm and courses distally deep to the fourth dorsal compartment with the posterior interosseous artery (PIA) to terminate as an afferent nerve to the dorsal wrist capsule.
C. The ulnar nerve (
Figure 9)
1. The ulnar nerve arises from the medial cord of the brachial plexus (C8-T1, +/-C7).
2. It passes from the posterior compartment of the brachium to the anterior compartment of the forearm via the cubital tunnel.
3. The fibro-osseous cubital tunnel is bordered by the ulnar groove of the medial epicondyle, a fascial arcade (spanning from the medial epicondyle to the olecranon; connects the humeral and ulnar heads of the FCU origin), and the FCU muscle bellies. The nerve courses over the ulnar collateral ligament.
[Figure 9. Illustration of the anatomic course and branches of the ulnar nerve in the right upper extremity. FCU = flexor carpi ulnaris, FDP = flexor digitorum profundus, FPB = flexor pollicis brevis, ADM = abductor digiti minimi, AdP = adductor pollicis, FDM = flexor digiti minimi, ODM = opponens digiti minimi.]
4. Muscular branches to FCU may arise 4 cm proximal to the elbow joint and from within the cubital tunnel.
5. The ulnar nerve runs between the FCU and FDS in the forearm before emerging more superficially, radial to the FCU tendon.
6. The ulnar nerve lies ulnar (medial) and volar to the ulnar artery.
7. At the wrist, the ulnar nerve is 45% motor and 55% sensory.
8. The distal ulnar tunnel, or Guyon canal, comprises three zones:
a. Zone 1—Begins at the proximal edge of the volar carpal ligament and ends at the nerve bifurcation, approximately 1 cm distal to the
[
Figure 10. Illustration of the anatomic course and branches of the median nerve in the right upper extremity. PT = pronator teres, FDS = flexor digitorum superficialis, FCR = flexor carpi radialis, FDP = flexor digitorum profundus, AIN = anterior interosseous nerve, PQ = pronator quadratus, FPB = flexor pollicis brevis, APB = abductor pollicis brevis, OP = opponens pollicis.]
pisiform. It lies dorsal to the volar carpal ligament and volar to the transverse carpal ligament. The ulnar artery bifurcates distal to the ulnar nerve.
b. Zone 2—The roof comprises the palmaris brevis; the floor comprises the pisohamate and pisometacarpal ligaments. The deep motor branch passes around the hook of the hamate and between the abductor digiti minimi and flexor digiti minimi muscles, innervating these muscles. It then pierces the opponens digiti minimi to follow the deep palmar arch and to (typically) innervate the interosseous, third and fourth lumbrical, adductor pollicis, and flexor pollicis brevus muscles.
c. Zone 3—Includes the sensory branch, which remains superficial. It innervates the palmaris brevis muscle and is sensory to the little finger and (typically) to the ulnar ring finger.
D. Median nerve (Figure 10)
1. This nerve arises from the medial and lateral cords of the brachial plexus (C5-T1).
2. It enters the forearm between the ulnar and humeral heads of the pronator teres and runs distally between the FDS and FDP muscles, although it may course within the FDS muscle substance.
3. The nerve is separated from the ulnar artery (deep) by the deep head of the pronator teres in the proximal forearm.
4. The anterior interosseous nerve (AIN) typically branches immediately distal to the FDS arch and innervates the FDP (index and long fingers), FPL, and pronator quadratus (PQ).
5. The AIN passes dorsal to the PQ muscle with the anterior interosseous artery and provides afferent innervation of the volar wrist capsule.
6. The palmar cutaneous branch of the median nerve (PCBMN) arises approximately 5 cm proximal to the volar wrist crease and runs in the median nerve epineurium for 2 cm. The PCBMN runs in the palmaris longus-FCR interval until it passes superficially to the transverse carpal ligament to supply sensory innervation to the thenar eminence.
7. At the carpal tunnel, the median nerve is 94% sensory and 6% motor.
8. Variations of the recurrent motor branch have been described; however, it typically innervates the APB, FPB, and opponens.
9. The median nerve branches into radial and ulnar divisions.
10. The radial division branches into the common digital nerve to the thumb (divides to proper digital nerves of the thumb) and the proper digital nerve to the radial index finger.
11. The ulnar division divides into the common digital nerves to the second and third web spaces.
12. The first and second lumbrical muscles are innervated by branches of the common digital nerves.
13. Common digital nerves are dorsal to the superficial palmar arch and volar to the flexor tendons in the palm.
14. The proper digital nerves become volar to the digital arteries at the level of the metacarpal neck.
E. Other structures
1. Carpal tunnel—The carpal tunnel is defined by the following structures:
a. Floor: carpal bones and the palmar radiocarpal ligaments
b. Roof: transverse carpal ligament, which spans from the pisiform and hook of hamate to the scaphoid tubercle and trapezial ridge.
c. Radial border: scaphoid and trapezium
d. Ulnar border: triquetrum and hamate
2. Martin-Gruber connection
a. Connection between the median and ulnar nerves in the proximal third of the forearm proximal to AIN branch, or distally within the FDP muscle (between ulnar nerve and AIN)
b. Occurs in approximately 15% of individuals
3. Riche-Cannieu connection
a. Connection between the median and ulnar nerves within the substance of the flexor pollicis brevis (FPB)
b. Occurs in 50% to 77% of individuals
Top Testing Facts
1. The conjoined lateral bands are stabilized by the triangular ligament, which prevents volar subluxation, and the transverse retinacular ligament, which prevents dorsal subluxation.
2. The A2 and A4 pulleys are the most important for the biomechanical efficiency of the digital flexor retinacular pulley system.
3. The radial nerve of the thumb is at risk during trigger thumb release in both adults and children because of its oblique orientation overlying the A1 pulley.
4. The lumbricals course palmar to the deep transverse intermetacarpal ligament, whereas the interossei pass dorsal to the ligament.
5. The EIP and EDM/EDQ tendons typically are located ulnar to the EDC tendons to the index and little fingers, respectively. The EIP is usually identified at the wrist as having the most distal muscle belly relative to the EDC tendons within the fourth dorsal compartment.
6. The ulnar nerve is medial and volar to the ulnar artery at the level of the wrist.
7. The classic order of innervation for the PIN, from proximal to distal, is ECU, EDC, EDM, APL, EPL, EPB, EIP.
8. The deep head of the pronator teres separates the ulnar artery (deep) and the median nerve (superficial).
9. The interosseous membrane runs from proximal-radial to distal-ulnar. (Think: Essex-Lopresti lesion.)
10. Inadequate surgical decompression of the volar compartment of the forearm is often the result of incomplete decompression of the FDP and/or pronator quadratus, which may be anatomic "subcompartments" within the volar forearm.
Bibliography
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