Finger and thumb anatomy

INTRODUCTION — Injuries to the fingers and thumb are a common reason for visits to primary care clinics and emergency departments. To care for such injuries well, clinicians must have a sound grasp of basic hand anatomy.

The basic clinically relevant anatomy of the fingers and thumb is reviewed here. Specific finger injuries and their management are discussed elsewhere. (See "Extensor tendon injury of the distal interphalangeal joint (mallet finger)" and "Flexor tendon injury of the distal interphalangeal joint (jersey finger)".)

NOMENCLATURE AND OVERALL STRUCTURE — Finger function involves a complex interaction among multiple joints, flexor and extensor tendons, and supporting fascia and ligaments [1-6]. Each of the digits, except the thumb, has three phalanges with three hinged joints: distal interphalangeal (DIP), proximal interphalangeal (PIP), and metacarpophalangeal (MCP) (figure 1), with a fingernail at the dorsal end (figure 2 and picture 1 and figure 3). Joint stability is provided by the structure of the phalanges, joint capsule, radial and ulnar collateral ligaments and dorsal and palmar ligaments (figure 4).

Flexion and extension are the primary movements of the fingers. Abduction and adduction can be performed at the MCP joints. The thumb is capable of opposition, abduction, adduction, and retropulsion, in addition to flexion and extension. The thumb is discussed below (see 'Thumb anatomy' below). Tables and diagrams summarizing the movements and innervation of the fingers and thumb are provided (figure 5 and figure 6 and figure 7 and table 1 and table 2 and table 3).

Fingers are referred to by naming and numbering systems, but names appear to cause less confusion among clinicians [7]. Throughout the UpToDate topics dealing with fingers and hands, we use standard names to refer to fingers (ie, thumb, index, middle, ring, and little). It is important to note that while the anatomy described below is considered standard, considerable variation exists among individuals.

VASCULATURE — The arteries of the hand are located mainly on the palmar (or volar) side (figure 8) [3-6]. The ulnar and radial arteries unite in the palm to form a small deep palmar arch and a larger superficial palmar arch (figure 9).

The deep palmar arch is supplied mainly from the radial artery and gives off the princeps pollicis artery to the thumb and the radial digital artery of the index finger. The princeps pollicis artery bifurcates into two palmar digital arteries that run on each side of the thumb.

The superficial palmar arch is supplied mainly by the ulnar artery and gives off three common palmar digital arteries that provide blood to structures in the palm and flow distally toward the fingers. Each common palmar digital vessel bifurcates near the finger web space forming two proper palmar digital arteries that flow distally along both sides of the middle finger and one side the index and little finger. The ulnar side of the little finger is supplied by the palmar digital artery that branches directly off the ulnar artery.

The digital arteries give rise to numerous deep and superficial branches that supply the structures of the fingers (figure 4). The digital arteries run with digital nerves from the ulnar and median nerve. In the palm the arteries lie palmar to the nerve but in the digits this alignment is reversed.

Capillary refill is the test used most often to assess finger perfusion. Normal capillary refill is generally less than 2 seconds but may be slightly longer in the elderly and patients with certain diseases (eg, diabetes) [8].

The hand's venous system is divided into superficial and deep systems. The larger superficial system is located mainly on the dorsal side of the hand. The pattern of the venous system is highly variable.

INNERVATION — The hand is innervated by the median, ulnar and radial nerves (figure 5 and figure 6 and figure 7 and table 1 and table 2 and table 3) [3-6]. Each has motor and sensory components. Of note, there is considerable variation in sensory innervation. The examination of the hand should include sensation of pin and light touch as well as two-point discrimination (figure 8). Although values increase with age and certain disease states (eg, diabetes), normal two-point discrimination is approximately 4 to 5 mm [9,10].

The median nerve supplies sensation to the radial side of the palm and the palmar surface of the thumb, index finger, long finger, and the radial side of the ring finger. It also supplies sensation to the dorsal surfaces of these fingers distal to the distal interphalangeal (DIP) joint. The ulnar nerve supplies sensation to the hypothenar eminence, the palmar and dorsal surfaces of the little finger, and the ulnar side of the ring finger. The radial nerve innervates the dorsal surface of the palm and fingers, from the thumb to the radial side of the ring finger, proximal to the DIP joints.

The median nerve innervates the muscles involved in pinching (thumb apposition). In the forearm, the median nerve innervates the flexor digitorum superficialis (FDS), while its anterior interosseus branch innervates the flexor pollicis longus and the flexor digitorum profundus (FDP) of the index and long fingers. As the median nerve passes through the carpal tunnel, it splits into the recurrent motor branch and gives off a number of palmar digital nerves which travel with the digital arteries. The recurrent motor branch innervates the abductor pollicis brevis, opponens pollicis, superficial head of the flexor pollicis brevis, and the lumbrical muscles of the index and long finger. The palmar digital nerves run on the lateral and medial sides of the thumb, index finger, middle finger, and the radial side on the ring finger.

Motor function of the anterior interosseus branch of the median nerve can be tested by having the patient make the “OK” sign (picture 2). Flattening of the “O” or loss of flexion of the IP or DIP joint suggests anterior interosseus nerve dysfunction (picture 3). The remainder of median nerve function can be tested by assessing the strength of index or middle finger proximal interphalangeal (PIP) and DIP flexion, thumb opposition with the small finger, or wrist flexion.

The ulnar nerve innervates the muscles that provide grip strength. In the forearm, the ulnar nerve innervates the FDP of the ring and small fingers. In the hand, the deep motor branch innervates the abductor digiti minimi, opponens digiti minimi, flexor digiti minimi, palmaris brevis, all interossei, the two ulnar lumbricals, adductor pollicis, and the deep head of the flexor pollicis brevis. The superficial branch gives off palmar digital nerves to the little finger and the ulnar side of the ring finger. Motor function of the ulnar nerve can be tested by having the patient abduct the index finger against resistance (picture 4 and picture 5).

The radial nerve innervates the extensor muscles of the hand. The radial nerve divides at the proximal forearm into superficial and deep branches. The deep posterior interosseus branch innervates all the extensor muscles in the hand including the extensor digitorum communis, extensor digiti minimi, extensor indicis proprius, extensor pollicis longus, extensor pollicis brevis, and abductor pollicis longus. Radial nerve motor function can be tested by resisting thumb, MCP, or wrist extension (picture 6 and picture 7).

MAJOR JOINTS — The distal interphalangeal (DIP) joint is a hinge joint with a range of motion of approximately 0 to 80 degrees of flexion (figure 10) [3-6]. The joint is stabilized by the joint capsule, collateral ligaments, and the volar plate. The lateral bands of the extensor tendons cross the DIP joint and form the terminal slip of the extensor tendon as it attaches to the dorsum of the distal phalanx. The terminal slip enables DIP extension. The flexor digitorum profundus attaches to the palmar aspect of the distal phalanx to enable DIP joint flexion (figure 11 and figure 12 and figure 13).

The proximal interphalangeal (PIP) joint is another hinge joint. Its range of motion is approximately 0 to 120 degrees of flexion. The PIP joint is stabilized by the joint capsule, collateral ligaments, and the volar plate. The volar plate consists of thick fibrocartilaginous tissue on the palmar surface and prevents hyperextension. The collateral ligaments prevent radial and ulnar displacement of the PIP joint and remain tight throughout its range of motion.

The dorsal surface of the PIP joint includes the central slip of the extensor digitorum tendon, which attaches to the middle phalanx and enables PIP extension. The lateral bands of the extensor tendon pass around the PIP joint, attaching to the base of the distal phalanx. Injury to the central slip allows the lateral bands to displace volarly, pulling the PIP joint into flexion and leading to extension of the DIP joint (boutonniere deformity). The flexor digitorum superficialis inserts just beyond the PIP joint into the volar aspect of the middle phalanx to enable PIP joint flexion.

The metacarpophalangeal (MCP) joint is capable of 0 to 90 degrees of flexion, 0 to 30 degrees of extension, and 0 to 20 degrees of abduction. A small amount of phalangeal rotation occurs with MCP joint motion. The MCP joints are encompassed by an articular joint capsule, collateral ligaments, and palmar (volar) ligaments. The capsule is continuous with a row of deep transverse metacarpal ligaments connecting the heads of metacarpals. These ligaments support the collateral ligaments and limit rotation and lateral movement. The collateral ligaments that support the MCP are lax in extension and become taut in flexion. This complex structure of ligaments results in more dorsal dislocations but fewer lateral dislocations and collateral ligament injuries.

The lumbricals are four small muscles that arise from the flexor digitorum profundus after passing through the flexor retinaculum. The four lumbrical tendons run along the radial side of all digits other than the thumb and insert at the level of the proximal phalanx. They are primarily involved in extension of the interphalangeal joints and also contribute to flexion of the MCP joints. The lumbricals of the index and middle fingers are innervated by the median nerve, while the ulnar nerve innervates the lumbricals of the ring and little fingers.

FINGER EXTENSION — Extension of the fingers is controlled by the extensor digitorum, extensor indicis proprius, and extensor digiti minimi (figure 11 and figure 13) [3-6]. The tendons of these muscles are tethered by a dorsal hood over the metacarpophalangeal (MCP) joint, formed by an extension of the joint capsule. Just after the extensor tendon passes over the MCP, it divides into three slips, the central slip and the lateral bands, which are described above. (See 'Major joints' above.)

The lateral slips diverge just past the MCP joint, where they receive contributions from the lumbrical muscle tendon on the radial side and interosseous muscle tendons on both sides of the digit. The lateral slips are connected distally by the triangular ligament and form the terminal slip just before their insertion onto the dorsum of the distal phalanx. The central slip attaches to the middle phalanx just beyond the proximal interphalangeal (PIP) joint. The extensor mechanism is innervated by the posterior interosseous nerve and deep branches of the radial nerve.

FINGER FLEXION AND PULLEY SYSTEM — Finger flexion is controlled by the flexor digitorum superficialis and flexor digitorum profundus muscles (figure 11 and figure 12) [3-6]. The two tendons of these muscles run in pairs across the palm. The tendon of the flexor digitorum superficialis inserts on the palmar shaft of the middle phalanx of the four fingers. This allows for flexion of the proximal interphalangeal (PIP) joint. The flexor digitorum profundus passes distally through a split in the superficialis tendon to insert at the base of the distal phalanges of the four fingers. The flexor digitorum profundus is responsible for flexion of the distal interphalangeal (DIP) joint.

The digital flexor system consists of membranous and retinacular tissues. The membranous tissues include the flexor digitorum profundus and flexor digitorum superficialis tendons. The retinacular portion of the sheath consists of cruciform, annular, and transverse tissues that overlie the membranous tissues. These retinacular tissues facilitate smooth gliding of the tendons and form a pulley system that enables effective finger flexion without displacement of the tendons (ie, "bowstringing").

The retinacular pulley system is comprised of the palmar aponeurosis (PA) pulley, five annular (A) pulleys, and three cruciform (C) pulleys (figure 14) [2-6]. The PA pulley consists of transverse fascicular bands arising from the palmar aponeurosis that attach to the deep transverse metacarpal ligament. The PA pulley is less directly involved in finger flexion than the annular and cruciform pulleys.

The annular pulleys are larger and are located over and between the finger joints. The A1 pulley arises from the palmar plate and proximal portion of the proximal phalanx. It overlies the membranous sheath at the level of the metacarpophalangeal (MCP) joint. It is released during surgical treatment of trigger finger. The A2 pulley consists of oblique fibers that overlie annular fibers and is located at the level of the proximal phalanx. The A3 pulley overlies the PIP joint. The A4 pulley also consists of oblique fibers overlying annular fibers and is located at the middle phalanx. The A5 pulley is located just proximal to the DIP joint and just proximal to the termination of the flexor sheath.

The cruciform pulleys are thin and narrow and located near the finger joints. They are able to accommodate flexion more readily than the larger annular pulleys. The C1 pulley lies distal to the A2 pulley and just proximal to the PIP joint. The C2 pulley lies between the A3 and A4 pulleys and just distal to the PIP joint. The C3 pulley lies distal to the A4 pulley and proximal to the DIP joint.

The area between the A1 pulley and the insertion of the flexor digitorum profundus (zone II) is often described as "no man's land." This area is prone to adhesions and complications following surgery because of the complex pulley system.

THUMB ANATOMY — The thumb consists of only two phalanges but is capable of greater motion than the fingers, which have three [3-6]. The greatest motion occurs at the carpometacarpal (CMC) joint, where the thumb can flex, extend, abduct, adduct, oppose, and retropulse (picture 8). At the metacarpophalangeal (MCP) joint, the thumb can flex, extend, abduct, and adduct. At the interphalangeal (IP) joint, the thumb can only flex and extend.

Flexion of the thumb occurs in the plane parallel to the palm; the thumb moves into the palm. Abduction occurs in the plane perpendicular to the palm; the thumb moves away from the palm. Opposition is a combination of abduction, rotation, and flexion that occurs as the thumb tip is brought to the tip of the little finger. Retropulsion is movement of the thumb dorsally beyond its natural resting position.

The thumb is capable of 60 degrees of abduction. Approximately 10 to 15 degrees of hyperextension can occur at the IP and MCP joints. The IP joint is capable of 80 degrees of flexion, while the MCP and CMC joints are capable of 55 and 35 degrees, respectively.

Thumb motion is controlled by extrinsic flexors, extensors, and abductors and by intrinsic muscles (figure 5 and table 4 and figure 15 and figure 16 and table 1). The flexor pollicis longus (FPL) inserts on the distal phalanx and is primarily responsible for IP flexion. The abductor pollicis longus (APL) inserts at the base of the first metacarpal and is responsible for CMC abduction and extension. The extensor pollicis brevis (EPB) inserts on the proximal phalanx and assists with MCP and CMC extension. The APL and EPB tendons run along the radial aspect of the anatomic snuffbox and are involved in de Quervain's tenosynovitis. The extensor pollicis longus (EPL) runs along the ulnar side of the snuffbox. It inserts on the distal phalanx and contributes to MCP and IP extension.

The intrinsic muscles of the thumb consist of the thenar muscles and the adductor pollicis. The adductor pollicis inserts at the base of the proximal phalanx on the radial side and is responsible for adduction and opposition. The abductor pollicis brevis (APB), flexor pollicis brevis (FPB), and opponens pollicis comprise the thenar group and are responsible for flexion, abduction, and opposition of the thumb.

Most thumb muscles are innervated by the median nerve. The ulnar nerve innervates the adductor pollicis and the deep head of the flexor pollicis brevis. The princeps pollicis artery comes from the deep palmar arch to supply the structures of the thumb. (See 'Innervation' above and 'Vasculature' above.)

SUMMARY

●Overall function with graphic aids – Finger function involves a complex interaction among multiple joints, flexor and extensor tendons, and supporting fascia and ligaments. Tables and diagrams summarizing the movements and innervation of the fingers and thumb are provided (figure 5 and figure 6 and figure 7 and table 1 and table 2 and table 3). (See 'Nomenclature and overall structure' above.)

●Finger joints – Each of the digits, except the thumb, has three phalanges with three hinged joints: distal interphalangeal (DIP), proximal interphalangeal (PIP), and metacarpophalangeal (MCP) (figure 1). Joint stability is provided by the structure of the phalanges, joint capsule, radial and ulnar collateral ligaments, and dorsal and palmar ligaments.

●Finger movements – Flexion and extension are the primary movements of the fingers. The extensor digitorum muscle and central slip are primarily involved in PIP extension, while the terminal slip is involved in DIP extension. Finger flexion involves a complex pulley system, which is described in the text. The flexor digitorum superficialis performs flexion of the PIP joint, while the flexor digitorum profundus performs DIP flexion. Abduction and adduction can be performed at the MCP joints. (See 'Finger extension' above and 'Finger flexion and pulley system' above.)

●Vascular anatomy – The arteries of the hand are located mainly on the palmar (or volar) side (figure 8). The ulnar and radial arteries unite in the palm to form a small deep palmar arch and a larger superficial palmar arch. (See 'Vasculature' above.)

●Nerve anatomy – The hand is innervated by the median, ulnar and radial nerves. Each has motor and sensory components. Of note, there is considerable variation in sensory innervation. (See 'Innervation' above.)

●Thumb anatomy – The thumb consists of only two phalanges but is capable of opposition, abduction, adduction, and retropulsion, in addition to flexion and extension. (See 'Thumb anatomy' above.)