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Hand Anatomy and Biomechanics

Hand anatomy & biomechanics: intrinsic/extrinsic muscle balance, ligamentous constraints, and implications for diagnosing/treating dysfunction.

76 citationsUpdated Oct 2026

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Overview

Recent advances in hand surgery are driven by a shift from static to functional anatomy, recognizing the hand as both an information-gathering and execution organ [2]. The hand operates efficiently only when proximal joints are stable yet mobile, moving within a large spatial volume defined by the shoulder, elbow, and wrist [2]. The shoulder provides the most mobility in the body, while the elbow adjusts distance from the trunk; distally, the wrist and forearm form a physiological unit that positions the hand for grasping [2]. The hand’s blood and nerve supplies are continuous with the limb, and extrinsic muscles arising in the arm and forearm necessitate studying the hand as an integral part of the upper extremity [2].

The skeletal framework consists of 27 bones, including 19 long bones, organized into five rays [17]. The radial ray is the shortest and possesses great freedom of movement, with the trapezium angled to create a 45-degree sagittal plane angle between the first and second metacarpals [17]. The wrist permits three axes of movement, allowing the hand to assume any spatial configuration for grasping [17]. The skeleton presents longitudinal and transverse concavities, forming a cup shape when the thumb is placed next to the index finger, which becomes a gutter with a major oblique axis following the thumb crease during spread [17]. The transverse axis of the palm is oblique, forming an acute angle of approximately 75 degrees with the longitudinal axis [17].

Soft tissue gliding mechanisms are essential for function, with synovial sheaths ensuring tendon movement in narrow areas and fibrous sheaths acting as pulleys at articular angles [7]. No single articulation is an isolated mechanical entity; instead, articulations form functional groups in kinetic chains where the position of each joint depends on the equilibrium of forces from the immediately proximal articulation [7]. Almost all hand movements occur around oblique and variable axes, resulting in combined movements that permit optimal orientation of the phalanges at prehension [7]. Muscles move joints by creating tension and excursion in tendons, a process governed by sarcomere length and cross-sectional area [7].

Osseous Anatomy

General Skeletal Composition

The hand comprises 19 bones, 17 articulations, and 19 muscles situated entirely within the hand [2]. The skeleton is organized into five rays, each forming a polyarticulated chain of metacarpals and phalanges [17]. The base of each metacarpal articulates with the distal carpal row, while the carpus connects to the forearm skeleton via its proximal row [17]. The radioulnocarpal articulation possesses two axes of movement, to which a third axis—pronation and supination from the forearm—is added [17]. Consequently, the wrist exhibits three axes of movement, permitting the hand to assume any spatial configuration [17].

Metacarpals and Rays

The radial (first) ray is the shortest, consisting of only three bones: a metacarpal and two phalanges [17]. The remaining four rays each comprise four skeletal segments: a metacarpal and three phalanges [17]. Among the metacarpals, the thumb metacarpal is the shortest, and the index finger metacarpal is the longest [17]. The lengths of the other metacarpals decrease sequentially from the third to the fifth digits [17]. The first ray continues the external carpal column formed by the scaphoid and trapezium [17]. The thumb carpometacarpal ray lies anterior to the plane of the other metacarpals, forming an angle of approximately 47 degrees with the second ray [26]. The third metacarpal serves as the axis of the hand and a landmark for wrist movements [98]. Its styloid process projects from the base and marks the line of the carpometacarpal joints [98]. The fourth carpometacarpal joint represents the area of greatest variation in osseous morphology at the carpometacarpal level [44].

Phalanges

The finger bones are designated as the proximal (P1), middle (P2), and distal (P3) phalanges [43]. The thumb contains only proximal and distal phalanges [43]. The proximal and particularly the middle phalanges of the middle and ring fingers are longer than those of the index finger [17]. Accordingly, the long finger, and usually the ring finger, are longer than the index finger [17]. When digits are fully extended and touching, their tips almost describe a regular curve, with peripheral digits being the shortest [2]. When fingers are extended and separated, their tips lie on the circumference of a circle centered at the head of the third metacarpal [2]. Epiphyseal plates are located at the proximal ends of the phalanges and the first metacarpal [17]. Conversely, epiphyseal plates are located at the distal ends of the other metacarpals [17].

Palmar Architecture and Axes

The transverse axis of the palm corresponds to the metacarpophalangeal articulations and is not perpendicular to the longitudinal axis [17]. This transverse palmar axis is oblique, positioned more distally at the index metacarpophalangeal joint and more proximally at the fifth metacarpophalangeal joint [17]. It forms an acute angle of approximately 75 degrees with the longitudinal axis [17]. The axes of flexion are arranged such that flexion of all metacarpophalangeal and proximal interphalangeal joints causes the fingers to converge toward the scaphoid [98]. The more ulnar the digit, the more obliquely it must deviate as it approaches the palm [17]. The two ulnar metacarpals, especially the fifth, possess slightly more mobility in flexion–rotation to compensate for their lack of length [17].

Joint Specifics

The finger metacarpophalangeal (MCP) joint is a triaxial condyloid joint [43]. Its typical range of motion extends from 15° of hyperextension to 90° of flexion [43]. The trapezoidal shape of the metacarpal head creates a cam effect on the collateral ligaments, rendering them taut during MCP flexion and lax during extension [43]. Consequently, the MCP joint is stable in flexion and unstable in extension [43]. In the thumb, the metacarpal head presents as a single broad condyle [43]. Sesamoids are contained within the volar plate of the thumb MCP joint [43].

The proximal interphalangeal (PIP) joint is a hinge joint [43]. The head of the proximal phalanx includes two condyles separated by an intercondylar notch [43]. This intercondylar notch provides inherent joint stability through its articulation with the median ridge at the base of the middle phalanx [43]. The distal interphalangeal (DIP) joint is stabilized by the collateral ligaments, the terminal extensor tendon insertion, the flexor digitorum profundus insertion, and the volar plate [43].

The trapeziometacarpal joint possesses two longitudinal axes and two degrees of freedom [26]. It is best likened to a universal joint [26]. The distal articular surface of the trapezium is usually described as a saddle with a concave curvature in the dorsopalmar direction [26]. The corresponding surface on the base of the first metacarpal presents a grooved surface in the radioulnar direction [26]. These articular surfaces are asymmetrical [26]. The longitudinal crest of the trapezium and its transverse groove are curved as if to sit on a “scoliotic” horse [26]. Flexion of the trapeziometacarpal joint is necessarily accompanied by pronation, while extension is accompanied by supination [26]. The shape of the articular surface does not provide good stability except in anteposition and pronation [26].

The thumb interphalangeal articulation is a trochlear type allowing mainly flexion and extension [26]. Flexion of this joint is accompanied by a slight degree of rotation in pronation [26]. The thumb metacarpophalangeal articulation is of a condylar type [26]. Flexion of the thumb metacarpophalangeal joint is always accompanied by radial deviation and pronation [26].

Ligaments and Joint Capsule

General Principles and Kinematics

The articulations of the hand form functional groups arranged in kinetic chains, where the position of each articulation depends on the equilibrium of forces acting at that level [7]. Single articular movements around a fixed perpendicular axis do not exist in the hand; almost all movements occur around oblique and variable axes, resulting in combined movements [7]. Passive factors contributing to articular equilibrium include the restraining action of ligaments and muscular viscoelasticity [7]. The mechanical balance of a hand has nothing to do with having equally strong muscles on opposite sides of each joint [133].

Thumb Carpometacarpal (CMC) Joint

The thumb CMC joint is a biconcave, reciprocating saddle joint with little inherent stability [69]. Sixteen surrounding ligaments impart stability to this joint [69]. The thumb metacarpal is 34% smaller than the distal articular surface of the trapezium [69]. The CMC joint has motion in three principal planes: flexion-extension, abduction-adduction, and pronation-supination [87]. An element of conjunct rotation in pronation results from the asymmetric height of the radial and ulnar condyles of the trapezium [87].

Ligamentous Anatomy and Function: * Deep Anterior Oblique Ligament: This intracapsular ligament, also known as the palmar “beak” ligament, emanates from the volar tubercle of the trapezium and inserts on the ulnar volar aspect of the first metacarpal [69]. It tightens with pronation, abduction, and extension, preventing ulnar and dorsal translation of the first metacarpal relative to the trapezium [69]. Its obliquely oriented fibers create a center of rotation about which the CMC joint may rotate [69]. The volar oblique ligament passes from the trapezium to the volar beak of the thumb metacarpal and has traditionally been considered to provide the primary restraint to dorsal subluxation force inherent with pinch [87]. It is a short reflection of the transverse carpal ligament that maintains the thumb metacarpal within the biconcave contour of the trapezium [85]. * Superficial Anterior Oblique Ligament: This ligament tightens with pronation and extension of the thumb [69]. * Dorsoradial and Posterior Oblique Ligaments: The dorsoradial ligament stabilizes and inhibits dorsal translation, while the posterior oblique ligament inhibits ulnar translation of the thumb CMC joint [69]. * Dorsal Intermetacarpal Ligament: This ligament prevents radial translation of the thumb metacarpal as well as proximal migration of the thumb metacarpal following trapeziectomy [69].

Biomechanics and Pathology: High compressive forces across the thumb CMC during pinch may reach in excess of 12 times the applied load and may approach 20 times the applied load during maximum grasp [69]. Cantilever bending occurs with applied forces such that shear forces are highest at the volar half of the joint’s articular surface [69]. Flexion of the thumb MCP joint produces unloading of the volar portion of the trapezial metacarpal joint as the CMC joint is relatively unloaded [69]. Degeneration of the volar half of the thumb CMC joint is associated with a diminution in the integrity of the deep anterior oblique ligament [69]. Articular degeneration consistently involved a greater portion of the surface area of the trapezium compared with the metacarpal by a ratio of 3:1 [69]. Volar beak ligament detachment from the thumb metacarpal is strongly associated with trapeziometacarpal osteoarthritis in elderly cadaveric thumbs [127].

Histology and Innervation: Ultrastructural analysis of the deep anterior oblique ligament in patients with osteoarthritis found disorganized connective tissue, with little evidence of collagen fibers and few signs of innervation [69]. Mechanoreceptors were identified in the dorsoradial ligament, which was found to be innervated to a much greater extent than the anterior oblique ligament [69]. The collagen bundles of the dorsoradial ligament were found to be better organized than those of the anterior oblique ligament [69]. The thumb trapeziometacarpal joint ligaments had an abundance of nerve endings in the dorsal ligaments but little to no innervation in the anterior oblique ligament [118]. Recent anatomic work supports the presence of a stout dorsal deltoid ligament complex that appears more robust and cellular than the more hypocellular anterior oblique ligament [87].

Kinematics and Injury: The estimated lengths of principal ligaments stabilizing the CMC joint change substantially during thumb motions in vivo [41]. Complete rupture of the thumb CMC joint ligaments permits the thumb metacarpal to dislocate dorsally [85]. Partial rupture of the thumb CMC joint ligaments permits varying degrees of displacement, with more extensive tears resulting in greater displacement [85].

Metacarpophalangeal (MCP) Joint

The MP joints of the fingers are relatively resistant to ligament injury and dislocation because of their intrinsic ligamentous structure, surrounding tendons, and protected position at the base of the fingers [70]. The MP joints are most vulnerable to injury from forces directed ulnarly and dorsally [70]. The metacarpal head is narrow dorsally with a widened volar flare, resulting in progressively more contact with the base of the proximal phalanx with increasing flexion [70]. The metacarpal head has a longer dorsovolar axis than proximodistal axis [70]. There is broader and more stable articular contact between the metacarpal head and the base of the proximal phalanx beyond 70 degrees of flexion [70]. The MP joint is stable laterally in full flexion but allows some abduction and adduction in full extension [70]. Finger MCP joint collateral ligament stability increases with increasing flexion of the joint [126]. There is little to no difference between analogous ligaments on either hand when tested in the same position [126].

Capsule and Volar Plate: The capsule of the MP joint extends from the metacarpal neck to the base of the proximal phalanx and is composed of areolar tissue dorsally reinforced by the loose insertion of the common extensor tendon [70]. On the volar side, the MP joint is supported by the volar plate, which is continuous laterally with the deep transverse metacarpal (intervolar plate) ligaments [70]. The volar plate has a thick fibrocartilaginous distal portion and a thin membranous proximal portion [70]. The volar plate does not have strong proximal checkrein ligaments comparable to those of the PIP joint [70]. Lateral reinforcement for the volar plate is afforded by the collateral and intervolar plate ligaments, which insert into its lateral margins to produce linked support from one MP joint to the next [70].

Collateral Ligaments and Fibrous Complex: The collateral ligaments are more taut in flexion than in extension because of the cam effect created by the nonspherical shape of the metacarpal head [70]. Three portions of collateral ligaments on both sides of the metacarpophalangeal joint have variable length changes during flexion, which act to stabilize the joint through its flexion arc [130]. The fibrous nucleus (force nucleus of Zancolli) is located on each side of the palmar aspect of the metacarpophalangeal articulation and is formed by the convergence of the transverse interglenoid ligament, the volar plate, the sagittal bands of the extensor communis, the fibrous flexor tendon sheath, and the lateral accessory metacarpoglenoid ligaments [55]. If the fibrous complex at the MP joint is destroyed or displaced, the balance of forces converging at this level will be disturbed, the transverse arch will flatten, and finger function will be compromised [55]. Using anatomical origins and insertions for reconstruction, it is possible to restore MCP flexion and stability seen in an intact ligament [129].

Proximal Interphalangeal (PIP) Joint

The PIP joint volar plate is similar to a unit in a suit of armor, sliding proximally and distally to protect the joint [93]. The PIP joint volar plate is thick, allowing extreme external loading over the joint [93]. The pocket behind the proximal volar plate is large [93]. The excursion of the volar plate between flexion and extension is substantial, and any collapse of the volar plate itself between extension and flexion is minimal [93]. Any limitation or fixation of the proximal volar plate, as with the development of checkreins, produces a significant restriction in extension of the PIP joint [93]. Extension minimizes volar capsular space volume and may be essential for preventing flexion contractures [128].

Distal Interphalangeal (DIP) Joint

The DIP joint is a hinge or ginglymus joint, similar to the PIP joint [93]. The capsule surrounding the DIP joint is reinforced laterally by collateral ligaments that insert into the sides of the head of the middle phalanx and run in a distal and volar direction to insert into the volar lateral tubercle of the distal phalanx [93]. The accessory ligaments of the DIP joint are more volar and extend from the sides of the middle phalanx head to the sides of the volar plate [93]. The volar plate of the DIP joint serves as an accessory insertion point for the flexor digitorum profundus [93]. Proximally, the DIP volar plate extends and is attached to the neck of the middle phalanx but does not have lateral volar extensions [93]. The DIP volar plate does not have checkrein ligaments, allowing the joint to hyperextend [93]. The dorsal aspect of the DIP joint has no reinforcing ligament, but the terminal portion of the extensor mechanism attaches from the dorsal edge of one collateral ligament to the other and blends into the capsular fibers and the periosteum of the distal phalanx [93].

Gliding Mechanisms and Soft Tissue

The dorsal integument must be supple, elastic, and malleable, with skin sliding distally to allow metacarpophalangeal joint flexion [7]. In unrestricted areas with a straight tendon trajectory, tendons are surrounded by paratenon and areolar connective tissue arranged in layers [7]. In narrow crowded areas, the gliding mechanism is assured by the synovial sheath, which allows a considerable amplitude of movement [7]. Synovial sheaths at specific sites are surrounded by fibrous sheaths that keep the tendon close to the skeleton, particularly when crossing the sinus of an articular angle [7]. There are three digitopalmar synovial sheaths for the flexor tendons of the index finger, long finger, and ring finger [7]. Each synovial sheath has a visceral and parietal component separated by a potential synovial cavity containing a very thin layer of synovial fluid [7].

Restraint is provided by the sagittal bands of the common extensor and the check rein ligaments of the volar plates [55]. This complex fibrous apparatus allows each segment of the digit to have great freedom of movement, stability, and high mobility without the need for bulky muscle bellies [55]. During active utilization of the hand, the fibrous skeleton has zones of condensation kept under tension by a balance of forces exerted in different directions [55].

Muscles and Tendons

Extrinsic Extensor Muscles

The extrinsic extensors of the hand traverse six fibroosseous retinacular compartments at the wrist [19]. The extensor pollicis brevis inserts on the dorsum of the proximal aspect of the thumb’s proximal phalanx, actively extending the metacarpophalangeal joint [19]. The third compartment contains the extensor pollicis longus, which turns abruptly radialward about Lister tubercle [19]. The fifth compartment houses the extensor digiti quinti [19]. The extensor carpi ulnaris runs through the sixth compartment, inserting at the base of the little finger metacarpal to provide wrist extension and ulnar deviation [19]. Distal interphalangeal joint extension is achieved via conjoined lateral bands composed of tendinous slips from extrinsic and intrinsic tendons [19].

The digital extensor tendons are stabilized over the midline of the metacarpophalangeal joint by attachment to sagittal band fibers [19]. These fibers insert onto the volar proximal phalanx and the lateral borders of the volar plate, forming a sling that transmits proximal extrinsic extensor tension to the proximal phalanx [19]. This mechanism permits metacarpophalangeal joint extension without a direct tendinous insertion onto the proximal phalanx [19]. By keeping the extrinsic extensor away from the center of rotation, the sagittal bands maximize mechanical efficiency [19]. Rupture or attenuation of these fibers allows the extensor tendon to sublux to the ulnar side of the metacarpal head, causing ulnar deviation of the finger [19].

The extrinsic extensor apparatus of the fingers is formed by the extensor communis and extensor proprius of the index and little fingers [35]. The extensor proprii tendons are situated medially to the corresponding communis tendons and join them at the metacarpophalangeal joint, providing more autonomous movement to the peripheral fingers [35]. Extensor tendons run an almost entirely extrasynovial course, which facilitates repair [35]. However, they are thin superficial structures that rapidly become adherent to underlying bones and joints when damaged [35]. Their excursion is considerably less than that of flexors, making it difficult to compensate for length loss [35].

On the dorsum of the hand, extensor communis tendons are interconnected by juncturae tendinum, which assist extension of adjacent connected fingers [35]. Interruption of an extensor tendon proximally may be masked by the effect of these juncturae [35]. The extensor communis tendon to the little finger may be absent or replaced by an oblique junctura from the ring finger [35]. At the metacarpophalangeal joints, the form of the extensor tendons changes, becoming thin and flat [35]. Halfway down the proximal phalanx, the broad flat extensor tendon splits into an extensor central band and two extensor lateral bands [35].

The extrinsic extensor system has four sites of insertion, ranging from the most proximal at the interglenoid ligament (provided by sagittal bands) to the most distal at the base of the distal phalanx [35]. The most important insertion is that of the central tendon into the base of the middle phalanx [35]. The insertion at the base of the proximal phalanx is inconstant [35]. Under normal physiological conditions, isolated contraction of the long extensors extends only the proximal phalanx, leaving the two distal phalanges flexed in a clawlike position [35].

Extrinsic Flexor Muscles

The extrinsic finger flexors are the flexor digitorum profundus (FDP) and the flexor digitorum superficialis (FDS) [19]. The FDP inserts on the proximal volar aspect of the distal phalanx, flexing the distal interphalangeal, proximal interphalangeal, and metacarpophalangeal joints [19]. The FDS acts as a flexor of the proximal interphalangeal and metacarpophalangeal joints [19]. The FDP originates from the proximal ulna and interosseous membrane [80]. In the forearm, it divides into a radial component supplying the index finger and an ulnar component supplying the middle, ring, and little fingers [80]. The FDP and flexor pollicis longus (FPL) form the deep compartment of the volar forearm [80].

As the FDP and FPL tendons travel through the carpal tunnel, they occupy the floor of the tunnel [80]. The tenosynovial sheath of the FPL is continuous with the radial bursa, while the sheath to the little finger is continuous with the ulnar digital bursa [80]. In some patients, these bursae communicate, allowing a horseshoe abscess to spread between the thumb and little finger if infection occurs in either flexor tendon sheath [80]. The FDP of the index and middle fingers is innervated by the anterior interosseous branch of the median nerve, while the profundus of the ring and little fingers is innervated by the ulnar nerve [80]. The FDP provides digital flexion at both the proximal and distal interphalangeal joints [80].

The FDS has two heads: a radial head originating from the proximal shaft of the radius, and a humeral ulnar head originating from the medial humeral epicondyle and coronoid process of the ulna [80]. Each digit has a corresponding independent superficialis muscle [80]. As the FDS tendons pass through the carpal tunnel, those of the middle and ring fingers are more superficial and central than those of the index and little fingers [80]. In the proximal finger, the FDS tendon bifurcates around the FDP at the beginning of the A2 pulley [80]. The FDS tendon slips reunite distally at the Camper chiasm, with approximately half of the fibers staying on the ipsilateral side and half crossing to the contralateral side [80]. The FDS inserts via radial and ulnar slips into the proximal metaphysis of the middle phalanx [80]. The entire FDS muscle receives innervation from the median nerve, and its primary function is digital flexion at the proximal interphalangeal joint [80].

The FPL originates from two heads: a radial head from the proximal radius and interosseous membrane, and an accessory head from the coronoid process of the ulna and medial epicondyle of the humerus [80]. In the palm, the FPL tendon transverses between the abductor pollicis brevis and flexor pollicis brevis [80]. It inserts into the proximal base of the thumb distal phalanx and is innervated by the anterior interosseous branch of the median nerve [80]. The FPL flexes both the interphalangeal and metacarpophalangeal joints of the thumb [80].

The flexor tendons of the fingers cross five well-defined regions, from proximal to distal: the wrist, the carpal tunnel, the palm extending from the carpal tunnel exit to the fibrous flexor sheath entrance, the osteofibrous canal common to deep and superficial tendons, and the distal segment transmitting only the FDP tendon [99]. The FPL tendon crosses five regions: the wrist, the carpal tunnel, the palmar region (thenar eminence), the digital canal from the proximal pulley opening at the metacarpophalangeal joint volar plate to the exit of the "oblique pulley" halfway down the proximal phalanx, and the distal segment reaching the insertion on the base of the distal phalanx [99]. The digital flexor tendons pass through the carpal tunnel before fanning out in the palm toward their respective digits [99]. The FDS tendon inserts on the middle phalanx, and the FDP tendon inserts on the distal phalanx [99].

In each digit, superficial and deep flexor tendons are surrounded by synovial sheaths for gliding and kept against the phalanges by fibrous sheaths [99]. The fibrous sheath plays an essential mechanical role in preventing tendon divergence from the digit axis in anteroposterior and lateral directions [99]. The digital flexor tendon sheath is formed by five annular pulleys and three cruciform bands [99]. The second and fourth annular pulleys are the most important for function [99]. At the level of the digital sheaths, flexor tendons have a precarious blood supply through a vinculum longum at the chiasma level (common to both tendons) and vincula brevis for the profundus [99]. Lundborg demonstrated several deprived zones, including an "avascular segment" of the FDS just proximal to the chiasma, and two "avascular segments" of the FDP proximal and distal to the vinculum longum [99].

Flexor tendons are nourished by direct vascular supply and synovial diffusion [43]. The direct vascular supply consists of transverse digital arteries (ladder branches) arising from digital arteries to supply the vincular system, and a direct arterial supply from intraosseous vessels at tendinous insertions [43]. Avascular and hypovascular zones of the FDS and FDP within the flexor tendon sheath rely on intratendinous canaliculi for synovial diffusion [43]. The fibroosseous digital sheath provides biomechanical efficiency and a source of nutrition to the flexor tendons [43]. The flexor tendons are enveloped by a layer of visceral paratenon, and each pulley/retinacular system is lined by a layer of parietal paratenon [43]. Condensations of the synovial sheath form at strategic points along the digit to work with the transverse carpal ligament and palmar aponeurosis pulley to maximize efficiency of joint rotation and force transmission [43].

Five annular and three cruciform pulleys are typically described in the flexor tendon sheath [43]. The A1, A3, and A5 pulleys originate from the palmar plates of the metacarpophalangeal, proximal interphalangeal, and distal interphalangeal joints, respectively [43]. The A2 and A4 pulleys originate from the proximal and middle phalanges, respectively [43]. The A2 and A4 pulleys are the most important biomechanically [43]. The first, third, and fifth annular pulleys (A1, A3, and A5) are located over the metacarpophalangeal, proximal interphalangeal, and distal interphalangeal joints, respectively [80]. The second and fourth pulleys (A2 and A4) are situated over the middle portion of the proximal and middle phalanges [80]. The A2 and A4 pulleys are the most essential in maintaining the mechanical advantage of the flexor tendons [80].

The tenosynovium lining the fibroosseous tunnel supplies both nutrition and lubrication to the poorly vascularized flexor tendons [80]. Proximal to the sheath, tendons are well vascularized by the peritenon [80]. Within the sheath, tendon vascularity is supplied via the vincula system: the vinculum longus and brevis [80]. Following injury, flexor tendon healing occurs through both extrinsic and intrinsic mechanisms [80]. Extrinsic healing involves cells brought to the repair site by ingrowth of capillaries and fibroblasts, with formation of adhesions at the repair site [80]. Intrinsic healing occurs from tenocytes within the tendon [80]. The goal of flexor tendon repair and postoperative care is to encourage both intrinsic and extrinsic healing without the formation of thick adhesions, which would limit tendon excursion and result in restricted finger motion [80].

Intrinsic Muscles

Extrinsic muscles have their origin outside the hand and insertion on the hand or carpus, whereas intrinsic muscles have both origin and insertion within the hand [19]. Extrinsic muscles are either flexors or extensors, while intrinsic muscles contribute to both digital flexion and extension [19]. There are seven interosseous muscles: four dorsal and three volar [29]. The dorsal interossei are abductors [29]. Because the anatomic axis of the hand coincides with the axis of the third metacarpal, the dorsal interossei lie to the radial side of the index and middle fingers and the ulnar side of the middle and ring fingers [29]. The little finger is abducted by the abductor digiti quinti [29].

The volar interossei are adductors and lie to the ulnar side of the index finger and the radial side of the ring and little fingers [29]. The middle finger has two dorsal interossei (abductors) and no volar interossei (adductors) because the central axis of the hand lies within it [29]. Each dorsal interosseous muscle, with the exception of the third, has two muscle heads [29]. The superficial head of the dorsal interosseous arises most dorsally from the shaft of the contiguous metacarpals and is inserted deeply by a medial tendon onto the lateral tubercle [29].

Neurovascular Anatomy

Arterial Supply

The brachial artery branches in the proximal forearm primarily into the radial and ulnar arteries to supply the hand [122]. The ulnar artery is larger than the radial artery and provides the primary arterial contribution to the hand [110]. In most hands, the ulnar artery supplies the superficial palmar arch, which provides the principal blood supply to the common and proper digital arteries [110]. The radial artery enters the hand by passing deep to the tendons of the first dorsal compartment across the anatomic snuffbox [110]. It then dives palmarward between the bases of the first and second metacarpals to form the deep palmar arch [110]. The superficial palmar arch is located distal to the deep palmar arch [110]. The median artery contributes to the superficial palmar arch in 10% of patients [110].

Arch completeness varies significantly. The arterial arch is complete, with total communication between the radial and ulnar arteries, in 34% of hands [110]. In 20% of hands, communication between the radial and ulnar arteries is incomplete [110]. A 2018 review by Zarzecki et al. of 36 studies looking at 4481 palmar arches found that the superficial arch was complete in 81.3% of hands [122]. The same review found that the deep arch was complete in 95.2% of hands [122]. In 78% of hands studied by Coleman and Anson, there was a "complete" superficial arch [122].

The deep palmar arch runs alongside the motor branch of the ulnar nerve as it travels transversely just palmar to the proximal metacarpal shafts [110]. The princeps pollicis artery is derived from the deep palmar arch in 98% of patients [110]. The deep palmar arch supplies the deep metacarpal arterial branches, which provide secondary blood flow to the digital arteries [110]. Acute interruption of flow in either the radial or ulnar artery is extremely unlikely to result in significant ischemia of the hand or fingers [122]. Ischemia of the thumb is very uncommon as the blood supply to the thumb is particularly rich [122]. The thumb has significant arterial flow from both palmar and dorsal circulation [122].

Collateral circulation plays a critical role in chronic vascular problems. The anterior and posterior interosseous vessels can provide flow via collateral circulation in chronic vascular problems [122]. However, these vessels are much smaller and generally cannot provide adequate oxygenated blood in the case of acute injury to both the radial and ulnar vessels [122]. The volar forearm’s median artery can provide significant blood flow to the hand, particularly in the case of injury or harvest of the first clearly elucidated [122]. Anatomical dissections and radionucleotide flow studies of the ulnar and radial arteries at the wrist failed to demonstrate any difference between the anatomical dimensions of these vessels [90]. The study demonstrated a significant contribution of the anterior interosseous artery to hand vascularity after radial forearm flap surgery [116]. Compensatory increases in blood flow were observed in the anterior interosseous artery compared to the contralateral hand after radial forearm flap surgery [116].

Thumb Vascular Anatomy

The arteries of the thumb vary in both size and number, making surgical reconstruction delicate [16]. The palmar arteries of the thumb can be schematized by dividing the thumb into three segments defined by the metacarpophalangeal and interphalangeal flexion creases [16]. In the classical layout, the "princeps pollicis" artery crosses the first intermetacarpal space and runs along the ulnar side of the first metacarpal bone [16]. It runs along the volar surface of the adductor muscle and emerges onto the subcutaneous palmar tissue at the level of the cutaneous flexion crease of the metacarpophalangeal joint [16]. At this crease, the princeps pollicis divides into two terminal rami, the collateral palmar arteries of the thumb [16]. These arteries run along the digital tunnel symmetrically and are of equal caliber [16]. They head distally to finally unite in the pulp arcade [16]. An arcade located deep in the flexor tendon joins together the two arteries at the level of the distal metaphysis of the first phalanx [16]. Vessels entering the "vincula" and irrigating the flexor tendon originate from this arcade [16]. In anatomical studies, only 15% of dissections fall into the category of the classical description of the palmar arteries of the thumb [16].

Segment 1 (Opposition to MCP Crease): It is rare to find arteries of surgical interest on the volar surface [16].

Segment 2 (MCP to IP Crease): The two arteries run alongside the flexor tendon and behind the collateral nerves [16]. The main artery is the ulnar collateral artery [16]. In cases where the palmar ulnar collateral artery is absent, the dorsal artery takes its place by means of a branch through the subtendinous arcade [16].

Segment 3 (Pulp): The two arteries are of similar size and run through the thick fatty subcutaneous padding [16]. The arteries cross over and convert into the ends of the digital nerves at the level of the median axis [16].

The posterior area of the thumb is vascularized by two arteries which originate from the palmar arteries at the level of the first metacarpal [16]. At the level of the neck of the first phalanx, an anastomosis can be found which originates from the palmar arteries for dorsal supply [16]. The ulnar dorso-collateral artery generally stems from the "princeps pollicis" onto the medial border of the neck of the first metacarpal [16]. The most constant artery on the dorsal aspect of the thumb is the ulnar collateral artery [33].

Digital and Dorsal Hand Vascular Anatomy

In the fingers, the blood supply comes predominantly from the two palmar collateral digital arteries [33]. These arteries give cutaneous branches for the palmar digital skin and also give cutaneous branches for the dorsal skin [33]. They present digital anastomoses arcades in the pulp and in the nail matrix [33]. Additionally, they present anastomoses in the digital part of the proximal and middle phalanx, under the flexor tendons [33]. The arterial supply to the perionychium is by two dorsal terminal branches of the proper digital artery [82].

The dorsal aspect of the first webspace is supplied by two arteries [33]. The deep dorsal intermetacarpal artery runs along the diaphysis of the second metacarpal and vascularizes the base of this bone [33]. The superficial dorsal intermetacarpal artery supplies the skin overlying the proximal phalanx and the metacarpophalangeal joint of the index finger [33]. The deep and superficial dorsal intermetacarpal arteries have anastomoses at the metacarpal head [33]. These anastomoses provide reversed flow vascularization to a bone graft taken on the second metacarpal [33]. Deep anastomoses between the other dorsal intermetacarpal arteries and the proximal arterial system allow dorsal intermetacarpal flaps and dorsal webspace flaps [33].

Venous Drainage

Most of the venous drainage begins on the dorsal fingers and continues on the hand’s dorsal surface [122]. The primary arteries of the hand are each followed by two small venae comitantes [122]. The configuration of venae comitantes continues out to the fingers into the digital arteries [122]. The primary drainage of the hand is via the dorsal system, which eventually drains into the cephalic and basilic veins of the forearm and upper arm [122]. Problems with venous drainage of the hand are extremely unusual, with the exception of possible venous thrombosis after replantation [122].

Cutaneous Circulation and Thermoregulation

The cutaneous circulation of the hand has special anatomical and physiological features related to its distal situation far from the cardiac impulse [33]. It also has special features related to its constant exposure to thermal and postural variation [33]. The general pattern of cutaneous circulation includes muscular arteries running into arterioles forming a plexus superficial to the aponeurosis [33]. It includes a dermal plexus and a subpapillary plexus [33]. Return occurs through venous loops [33].

Arterial supply to skin is classified into three types: Longitudinal cutaneous arteries: Run part of their course through the subcutaneous tissues [33]. Septal arteries: Arise from the main artery and pass through the deep fascia to form a subdermal vascular plexus [33]. Myocutaneous arteries: Arise from intramuscular arteries and emerge from the muscle belly to supply an area of overlying skin [33].

Axial pattern flaps are based on longitudinal cutaneous arteries [33]. Fascio-cutaneous flaps are based on septal arteries [33]. Myocutaneous flaps are based on the vascular pedicle of the muscle [33]. The rich vascular supply of the skin of the hand has an important thermoregulatory function [33]. The blood circuit contains a system of shunts between arterioles, meta-arterioles, and venules [33]. Direct arteriovenous anastomoses known as Sucquet-Hoyer anastomoses are found mostly in the palm, near the roots of the nails, and in the pulps [33]. Sucquet-Hoyer anastomoses consist of spiral vessels that run from arterioles to venules [33].

Nerve Anatomy and Innervation

Two classification systems based on detailed dorsal hand cutaneous innervation patterns can be used to specify the placement of a safe dorsal skin incision away from major nerve branches [27]. Neural loop penetrations by digital arteries were found in 47 of 51 dissected palms (92%) [28]. Nerve dysfunction associated with compression syndromes results from an interruption or localized interference of microvascular function due to structural changes in the nerves or surrounding tissues [106]. The frequent presence of an anastomotic nerve branch in the palm was observed in 95% of cases [115]. The lumbrical muscle flap has a wide arc of rotation and a favorable shape, enabling coverage of the median nerve and its branches in the palm [109]. The anatomy of the first dorsal interosseous muscle, with particular reference to its vascular and nerve supply, was studied in ten fresh cadaveric hands [56].

General Neurovascular Context

Recent developments in hand surgery have come about as a result of a better understanding of the dynamic anatomy and function of the hand [2]. The concept of functional rather than static anatomy is the subject of the section on hand architecture and functions [2]. The hand is both an organ designed to obtain information and an organ of execution [2]. The hand functions efficiently only if the proximal joints of the limb are stable and yet mobile [2]. The hand moves within a large volume of space, the shoulder being the apex [2]. Distal to the elbow, there is in effect only one physiological unit [2]. The combined movements of the wrist and forearm place the hand in a position for grasping [2]. The presence of two ulnae in mirror hand means there are no radio-ulnar joints [20]. The presence of two ulnae in mirror hand results in very limited rotation of the forearm [20]. The presence of two ulnae in mirror hand results in restricted movements of the wrist and elbow [20].

Biomechanics and Function

General Principles and Kinematics

The hand operates within a large spatial volume with the shoulder as the apex, facilitated by the mobility of the shoulder, elbow, and wrist [2]. Clavicular movements amplify those of the shoulder [2]. The hand comprises 19 bones, 17 articulations, and 19 intrinsic muscles, alongside a comparable number of tendons activated by forearm muscles [2]. Anatomically, the dorsal aspect is convex while the palmar aspect is concave [2]. Digits converge during closing (flexion and adduction) and diverge during opening (extension and abduction) [2]. The radioulno-carpal articulation possesses two axes of movement, supplemented by a third axis of pronation and supination from the forearm, allowing the hand to assume any spatial configuration [17].

Specific geometric relationships define hand orientation. The thumb metacarpal forms an angle of approximately 45 degrees with the second metacarpal in the sagittal plane [17]. The transverse axis of the palm forms an acute angle of approximately 75 degrees with the longitudinal axis [17]. Epiphyseal plates are located at the proximal ends of the phalanges and the first metacarpal, whereas they reside at the distal ends of the other metacarpals [17]. The hand presents longitudinal and transverse concavities, creating a cup shape with palmar concavity when the thumb is placed next to the index finger [17].

No single articulation in the hand functions as an isolated mechanical entity; instead, they form functional groups arranged in kinetic chains [7]. Single articular movements around a fixed perpendicular axis do not exist in the hand [7]. Almost all movements occur around oblique and variable axes, resulting in combined movements that permit optimal orientation of the phalanges during prehension [7]. Articular equilibrium relies on active and passive factors. The active factor is the dynamic balance between antagonist muscles [7]. Passive factors include the restraining action of ligaments and muscular “viscoelasticity” [7].

Tendon Mechanics and Gliding

Dorsal integument must be supple, elastic, and malleable to allow metacarpophalangeal joint flexion [7]. Interphalangeal flexion is accomplished by a special arrangement of skin folds on the dorsum of each articulation [7]. Tendon gliding mechanisms vary by location. In unrestricted areas with a straight tendon trajectory, the tendon is surrounded by paratenon and areolar connective tissue [7]. In narrow crowded areas, the gliding mechanism is assured by the synovial sheath [7]. Fibrous sheaths surround synovial sheaths at specific sites to keep the tendon close to the skeleton, particularly when crossing an articular angle [7].

Muscle contraction amplitude is approximately one third the length of the muscle’s resting fleshy belly [34]. Tendon excursion amplitude increases if muscle fibers are longer and if the angle they make with the tendon is acute [34]. Specific tendon excursions are quantified as follows: * Extensor digitorum: Approximately 4 mm at the distal interphalangeal joint, 8 mm at the proximal interphalangeal joint, 15 mm at the metacarpophalangeal joint, and 45 mm at the wrist [34]. * Flexor digitorum profundus: 5 mm at the distal interphalangeal joint, 17 mm at the proximal interphalangeal joint, 23 mm at the metacarpophalangeal joint, 38 mm at the carpal canal, and 85 mm in the distal forearm [34].

Maximal tendon excursions are defined for key tendons: * Flexor digitorum profundus: 7.0 cm [34]. * Flexor digitorum superficialis: 6.4 cm [34]. * Extensor digitorum: 4.5 cm [34]. * Flexor pollicis longus: 5.0 cm [34]. * Extensor pollicis longus: 5.0 cm [34].

Movements are determined by the modulation of forces between antagonists, a concept known as “synergistic antagonism” [34]. All movements result from a “displacement of dynamic equilibrium” between two or more muscles or groups of muscles [34].

Thumb Biomechanics

The wide range of motion of the thumb column allows opposition to the palm and to the other digits [36]. The thumb carpometacarpal ray is anterior to the plane of the other metacarpals and makes an angle of about 47 degrees with the second ray [36]. The thumb interphalangeal articulation is a trochlear type allowing mainly flexion and extension, with flexion accompanied by a slight degree of rotation in pronation [36]. A lack of extension of greater than 15 degrees at the thumb interphalangeal joint is functionally more disabling than lack of flexion [36]. The thumb metacarpophalangeal articulation is of a condylar type and is capable of small lateral movements, especially to the radial side [36]. Flexion at the thumb metacarpophalangeal joint is always accompanied by radial deviation and pronation [36].

The trapeziometacarpal joint has two longitudinal axes and two degrees of freedom and is likened to a universal joint [36]. Longitudinal rotation at this joint is largely responsible for changing the orientation of the thumb pulp [36]. The thumb pulp is in pronation when the thumb is in opposition and in supination when the thumb is in retroposition [36]. The shape of the trapeziometacarpal articular surface does not provide good stability except in anteposition and pronation [36]. Stability is provided by a complex ligamentous system that does not limit mobility [36]. The oblique ligaments of the trapeziometacarpal joint tighten and become efficient stabilizers in complete pronation of the thumb [36]. The radial side of the trapeziometacarpal joint has a much weaker ligament, creating intrinsic instability at the level where pressure is very high [36].

Joint compression forces during simple pinch averaged 3.0 kg at the interphalangeal joint, 5.4 kg at the metacarpophalangeal joint, and 12.0 kg at the carpometacarpal joint [36]. Compression forces of up to 120 kg may occur at the carpometacarpal joint during strong grasp [36]. Stability at the trapeziometacarpal joint is dynamically provided by the tendon of the abductor pollicis longus, which has stabilizing function only when the first metacarpal is abducted [36]. In adduction, the abductor pollicis longus increases the risk of subluxation of the trapeziometacarpal joint [36]. The estimated lengths of principal ligaments stabilizing the thumb carpometacarpal joint change substantially during thumb motions in vivo [41]. Higher thumb mobility is associated with more trapezium tilt under load [68]. Abduction of the thumb in the plane of the palm can be measured using a specific set of discs [21].

Finger and Metacarpal Biomechanics

The long and index fingers' carpometacarpal joints are fixed and thus can tolerate less angular deformity [8]. Cadaver experiments estimate 7° of extensor lag per 2 mm of metacarpal shortening [8]. Deformity at the metacarpal neck level does not affect tendon balance, motion, or strength [8]. Acceptable angulation for metacarpal diaphyseal fractures varies by digit: * Index and long fingers: 5° to 10° [8]. * Ring and little fingers: 10° to 40° [8].

Most transverse proximal phalanx fractures are apex palmar [32]. The central extensor tendon pulls the distal fragment dorsal, and the interossei insertion flexes the proximal fragment in proximal phalanx fractures [32]. Proximal phalanx fractures have less stability than metacarpal fractures because they are not supported by adjacent bones [32]. Shortening of the proximal phalanx produces an extensor lag at the proximal interphalangeal joint, with each millimeter of bone loss equaling 12° of extensor lag [32]. Proximal middle phalanx fractures are apex dorsal because of the pull of the central slip [32]. Distal middle phalanx fractures displace palmarly as a result of the pull of the superficialis insertion [32]. Shortening of the middle phalanx following fracture may result in distal interphalangeal joint extension lag [32]. The angulation of proximal and middle phalanx diaphyseal fractures is usually apex volar as a result of the pull of the central slip and lateral bands [32].

The metacarpophalangeal joint represents a joint with 5 kinematic degrees of freedom from a mechanical perspective [59]. At increasing amounts of finger flexion, progressive metacarpal shortening produces proportionally greater loss of fingertip flexion force [78]. Wrist flexion resulted in a decrease of total force production during forceful grip [81]. The finger moved from the rest position toward the intrinsic position with loads of less than 5 N during lumbrical contraction [74]. Active and passive intrinsic reconstruction methods improved basic grasp and release kinematics in experimental cadaver hand models [76].

Muscle Mechanics and Extensor Mechanism

Extrinsic muscles have their origin outside of the hand and their insertion on the hand or carpus [19]. Intrinsic muscles have both origin and insertion within the hand [19]. The extensor pollicis longus provides forceful extension of the thumb interphalangeal joint [19].

Common Sites of Injury

Epidemiology and Mechanisms

Hand injuries, including fractures and dislocations, account for 1.5% of all US emergency room visits [117]. Phalangeal fractures occur at a rate of 12.5 per 10,000 persons annually, while metacarpal fractures occur at a rate of 8.4 per 10,000 persons annually [117]. In patients in their second to fourth decade of life, the incidence is 16.1 metacarpal fractures per 10,000 persons and 12.5 phalangeal fractures per 10,000 persons [117].

Mechanisms of injury vary significantly by age group: * Sporting injuries: Account for most phalangeal fractures in patients in their second and third decades of life [117]. * Accidental falls: Are the leading cause of hand fractures in patients over the age of 70 [117]. * Industrial trauma: Accounts for the most injuries in patients aged between 40 and 69 years [117].

Burns involve the upper extremity in up to 89% of cases [97]. The dorsum of the hand is predominantly involved in flame or explosion injuries, whereas injuries to the palm are more frequently found with chemical exposure, friction burns, or high-voltage injuries [97].

Anatomical Vulnerability

The dorsal skin is thin, very flexible, and contains no sweat glands, lying on a rather thin subcutaneous layer of fatty tissue [97]. Blood vessels, tendons, and joints are situated just beneath the skin surface over the dorsum of the fingers [101]. In contrast, the palmar skin has a thick, subcutaneous fatty layer with a honeycomb-like structure that provides shock-absorbing properties and can withstand greater thermal energy than the dorsal skin [97, 101].

The cylindrical form of the fingers, with ligaments limiting swelling, can lead to dermal compartment syndromes after deep dermal circumferential burn injury [101]. Persistent interstitial edema in the hand that has not resolved after 72 hours may lead to subcutaneous fibrosis with subsequent stiffness of the joints [101]. Attenuation or rupture of the central slip with PIP joint exposure is one of the most frequently encountered complications after deep dorsal burns [97].

Metacarpal Fracture Biomechanics

Most metacarpal diaphyseal fractures have apex dorsal angulation because of the pull of the interossei [8]. The hand can accommodate dorsal angulation by compensating with metacarpophalangeal (MCP) hyperextension and carpometacarpal (CMC) motion [8]. CMC motion is greatest at the little finger (30°), followed by the ring finger (20°), while the long and index fingers CMCs are fixed and thus can tolerate less angular deformity [8].

MCP hyperextension usually can compensate for up to 4 mm of metacarpal shortening [8]. However, metacarpal shortening or angulation >30° can result in shortening of the intrinsics, with the potential for extensor lag at the proximal interphalangeal joint [8]. Metacarpal shaft fractures are inherently stable because of the connections at the CMC joints and the intermetacarpal ligaments [8].

Thumb Fracture Biomechanics

In a Bennett fracture, specific muscle forces dictate displacement patterns: * Abductor pollicis longus: Displaces the distal metacarpal proximally [8]. * Adductor pollicis: Displaces the metacarpal into adduction [8]. * Base displacement: The metacarpal base is displaced dorsally and rotated into supination [8].

The volar oblique ligament is attached to the volar ulnar fragment of the base in a Bennett fracture [8].

Vascular and Neural Anatomy

In specific injuries or surgery around the PIP joint, at least one distal and one proximal vascular system must be present on either the radial or ulnar side of the joint to ensure full vascularity [62]. Palmar cutaneous branches of the digital nerves are a constant anatomical feature that can be mistaken for proper digital nerves, potentially leading to inadvertent injury [57]. The histologic configurations of the ulnar nerve in the hand explain the difficulties and setbacks encountered in the repair of injuries to the ulnar nerve [65].

Within the finger, the Cleland ligament is located dorsal to the neurovascular bundle, while the Grayson ligament is located palmar to the neurovascular bundle [97].

Surgical Anatomy

General Principles and Simulation

Advances in hand biomechanics simulation emphasize the underlying mathematical models representing the functional behavior of the hand and its constituent sub-structures [1]. This work also emphasizes the anatomy of the hand and methods of interacting with the models [1]. The text Surgical Anatomy of the Hand & Upper Extremity is described as a comprehensive, highly detailed, and readable resource providing user-friendly, portable, and clinically relevant anatomy for the upper extremity [4].

Ligaments and Pulleys

Recent studies indicate that the anatomy of the collateral ligaments of the proximal interphalangeal joint differs significantly from that shown in most anatomic and hand surgery literature, particularly in line drawings [3]. Volar cutaneous sensory branches originate at the proximal margin of, or alongside, the A1 pulley in most digits [88]. These branches are vulnerable to damage during operations for release of the A1 pulley [88]. Variations of the thumb pulley system must be accounted for in current hand surgery to avoid misconceptions in thumb anatomy [51].

Nerves and Innervation

During exposure of the dorsal and ulnar areas of the wrist, identification and protection of just a single branch of the dorsal cutaneous branch of the ulnar nerve are unlikely to ensure safe dissection because multiple branches normally are present [96].

Muscles and Tendons

The choice between ulnar superficialis slip resection and radial superficialis slip resection is not arbitrary; hand surgeons should consider biomechanical findings when deciding which slip to sacrifice to preserve function and strength [15]. Awareness of anomalies in the extensor pollicis longus and extensor indicis muscles is crucial for surgeons to avoid complications during procedures in the forearm, wrist, and hand [79]. The dorsal aponeurosis of the thumb receives both intrinsic and extrinsic contributions [42]. Minimally invasive and percutaneous techniques can effectively release several structures known to cause finger stiffness with minimal damage to surrounding structures [9]. Hand surgery and hand therapy practice interventions, including use of relative motion flexion orthoses for management of non-surgical and surgical extensor mechanism injuries, may benefit from an in-depth look at the extensor mechanism zone III and IV anatomy and biomechanics [66].

Bony Anatomy and Joint Mechanics

In the ring finger, there is 15 to 30 degrees of mobility between the ring and small finger metacarpal bases and the hamate at the carpometacarpal joint [119]. The ring finger and small metacarpal bases share a common articulation with the hamate [119]. The base of the fifth metacarpal can slide radially if the entire fourth metacarpal is excised [119]. The possibility of radial sliding of the fifth metacarpal base is enhanced by the lack of tendinous attachments to the fourth metacarpal base [119]. Transmetacarpal ray resection of the ring finger can leave an open space between the long finger and small finger as the remaining metacarpal base cannot converge [119]. Complete deletion of the ring finger ray can lead to a loss of small finger abduction and possible scissoring [119].

Surgical Considerations and Reconstruction

The pre-operative assessment and intraoperative findings of all thumb elements should be considered in surgical decision-making to define the methods of reconstruction for the hypoplastic thumb [49]. Malignancies of the proximal palmar surface of the hand and volar aspect of the wrist often require amputation [111]. Tumors that arise on the dorsum may allow preservation of the hand if staging studies show that the lesion has not penetrated into the palm and the excision margin verifies a safe plane of normal tissue [111]. Isolated intraosseous carpal lesions that have not invaded soft tissue are rare but can be excised locally [111]. The carpal bones are intraarticular, so the onset of symptoms is generally associated with synovitis and joint invasion [111]. Below-elbow amputation or complete en bloc excision of the entire radiocarpal articulation and carpus is likely to be necessary for carpal tumors that have invaded soft tissue [111]. Growths on the volar aspect of the distal part of the forearm must be widely excised with negative margins [111]. If a tumor arises in a location that does not specifically involve the ulnar nerve and artery, it may be possible to save a portion of the hand and wrist along with the neurovascular bundle and to consider later reconstruction after longitudinal hemiamputation [111]. Tumors on the extreme ulnar side of the wrist can be handled in a way that mirrors those on the radial aspect [111]. Lesions in the extreme end of the ulna may be amenable to wide excision of the distal ulna [111]. Intracompartmental lesions within the distal radius or ulna can be treated by wide excision of the bone and arthrodesis or autograft replacement [111]. Transosseous excision for intracompartmental lesions must include biopsy of the medullary canal from the retained segment or segments [111]. When tumors have invaded tissues or crossed compartments extensively, above- or below-elbow amputation may be required [111].

Key Evidence

• [Paper] The real emphasis of this work is on the underlying mathematical (biomechanical) models representing the functional behavior of the hand and its constituent sub-structures, the anatomy of the hand, and methods of interacting with the models. [1] (10.1016/s0894-1130(89)80052-3)
• [L5] The anatomy shown by this study is quite different from that shown in most of the anatomic and hand surgery literature, particularly in line drawings. [3] (10.1016/j.jhsa.2005.05.015)
• [Paper] The book 'Surgical Anatomy of the Hand & Upper Extremity' is described as a comprehensive, highly detailed, and readable text that provides user-friendly, portable, and clinically relevant anatomy for the upper extremity. [4] (10.1197/j.jht.2003.10.013)
• [Paper] An understanding of the nature of thumb motions is essential in the evaluation of and treatment for thumb pathology. [6] (10.1016/s0894-1130(12)80307-3)
• [L5] Minimally invasive and percutaneous techniques can effectively release several structures known to cause finger stiffness with minimal damage to surrounding structures. [9] (10.1016/j.jhsa.2019.01.006)
• [L5] Our findings show this is not an arbitrary choice; hand surgeons should keep our findings in mind when deciding which slip to sacrifice, in effort to preserve function and strength in the injured hand. [15] (10.1177/15589447211060416)
• [L4] The presence of two ulnae means there are no radio-ulnar joints, resulting in very limited rotation of the forearm and restricted movements of the wrist and elbow. [20] (10.1016/0266-7681_86_90151-8)
• [Paper] Abduction of the thumb in the plane of the palm can also be measured in a similar manner using a different set of discs. [21] (10.1016/s0363-5023(81)80137-2)
• [L4] The application of the Fibonacci sequence to the anatomy of the human hand, although previously accepted, is a relationship that is not supported mathematically. [22] (10.1053/jhsu.2003.50000)
• [L4] These vascular patterns are important to hand surgeons. [26] (10.1007/s11552-009-9197-4)
• [L5] Two classification systems based on detailed dorsal hand cutaneous innervation patterns can be used to specify the placement of a safe dorsal skin incision away from major nerve branches. [27] (10.1016/j.jhsa.2005.12.021)
• [L5] Neural loop penetrations by digital arteries were found in 47 of 51 dissected palms (92%). [28] (10.1016/s0266-7681(99)90019-0)
• [L4] The estimated lengths of principal ligaments stabilizing the CMC joint change substantially during thumb motions in vivo. [41] (10.1016/j.jhsa.2010.11.007)
• [L5] This investigation provides a detailed anatomic study of the dorsal aponeurosis of the thumb with observation of both intrinsic and extrinsic contributions. [42] (10.1016/j.jhsa.2017.11.009)
• [L5] The fourth carpometacarpal joint is the area of greatest variation in the osseous morphology at the carpometacarpal joint level in the human hand. [44] (10.1016/0363-5023(91)90026-8)
• [L5] The pre-operative assessment and intraoperative findings of all thumb elements should be considered in surgical decision-making to define the methods of reconstruction. [49] (10.1177/1753193418793579)
• [L5] This fact must be accounted for in current hand surgery to avoid misconceptions in thumb anatomy. [51] (10.1177/1558944716660555fb)
• [L5] The anatomy of the first dorsal interosseous muscle, with particular reference to its vascular and nerve supply, was studied in ten fresh cadaveric hands. [56] (10.1016/s0363-5023(86)80027-2)
• [Textbook] Palmar cutaneous branches of the digital nerves are a constant anatomical feature that can be mistaken for proper digital nerves, potentially leading to inadvertent injury. [57] (10.1007/978-3-642-22697-7_33)
• [L5] From a mechanical perspective, the metacarpophalangeal joint represents a joint with 5 kinematic degrees of freedom. [59] (10.1016/j.jhsa.2008.10.004)
• [L5] In specific injuries or in surgery around the PIP joint, it is necessary that at least one distal and one proximal system be present on either the radial or ulnar side of the joint to ensure full vascularity. [62] (10.1016/s0363-5023(85)80161-1)
• [L5] These configurations explain the difficulties and setbacks encountered in the repair of injuries to the ulnar nerve. [65] (10.1016/s0363-5023(85)80119-2)
• [L5] Hand surgery and hand therapy practice interventions, including use of RMF orthoses for management of non-surgical and surgical EM injuries may benefit from an in-depth look at the EM zone III and IV anatomy and biomechanics. [66] (10.1016/j.jht.2023.01.002)
• [L4] However, it has been found that the higher the thumb mobility, the more the trapezium tilts under load. [68] (10.1016/j.main.2011.04.002)
• [L5] The finger moved from the rest position toward the intrinsic position with loads of less than 5 N. [74] (10.1016/s0363-5023(87)80210-1)
• [L5] Active and passive intrinsic reconstruction methods improved basic grasp and release kinematics in experimental cadaver hand models. [76] (10.1016/j.jhsa.2014.09.031)
• [L5] At increasing amounts of finger flexion, progressive metacarpal shortening produces proportionally greater loss of fingertip flexion force. [78] (10.1177/1753193412461589)
• [L4] The authors emphasize that awareness of these variations is crucial for surgeons to avoid complications during procedures in the forearm, wrist, and hand. [79] (10.1007/s11552-013-9512-y)
• [L4] Wrist flexion resulted in a decrease of total force production. [81] (10.1016/s0894-1130(03)89028-2)
• [Paper] The arterial supply is by two dorsal terminal branches of the proper digital artery and the nerve innervation by dorsal terminal branches of the proper digital nerve. [82] (10.1016/s0363-5023(80)80100-6)
• [L5] Volar branches originate at the proximal margin of, or alongside, the A1 pulley in most digits, rendering them vulnerable to damage during operations for release of the A1 pulley. [88] (10.1177/1753193416642658)
• [L4] Anatomical dissections and radionucleotide flow studies of the ulnar and radial arteries at the wrist failed to demonstrate any difference between the anatomical dimensions of these vessels. [90] (10.1016/s0266-7681(05)80060-9)
• [L5] In 75% of the 50 cadaver hands studied, there was a single end-arterial blood supply to the thumb originating from the deep radial artery. [94] (10.1016/s0363-5023(78)80044-6)
• [L5] During exposure of the dorsal and ulnar areas of the wrist, identification and protection of just a single branch of the DCBUN are unlikely to ensure safe dissection because multiple branches normally are present. [96] (10.1016/j.jhsa.2013.03.016)
• [L5] Nerve dysfunction associated with compression syndromes results from an interruption or localized interference of microvascular function due to structural changes in the nerves or surrounding tissues. [106] (10.1197/j.jht.2005.02.001)
• [L4] These flaps have a wide arc of rotation and a favorable shape, enabling coverage of the median nerve and its branches in the palm. [109] (10.1016/s0363-5023(98)80098-1)
• [L5] We have observed the frequent presence of an anastomotic nerve branch in the palm (95%). [115] (10.1016/0266-7681(91)90105-w)
• [L3] The study demonstrated a significant contribution of the anterior interosseous artery to hand vascularity after radial forearm flap surgery, with compensatory increases in blood flow observed in this vessel compared to the contralateral hand. [116] (10.1054/jhsb.2000.0523)
• [L5] The thumb trapeziometacarpal joint ligaments had an abundance of nerve endings in the dorsal ligaments but little to no innervation in the anterior oblique ligament, inferring a proprioceptive function of these ligaments in addition to their biomechanical importance. [118] (10.1016/j.jhsa.2011.12.038)
• [L2] Finger MCP joint collateral ligament stability increases with increasing flexion of the joint, and there is little to no difference between analogous ligaments on either hand when tested in the same position. [126] (10.1016/j.jhsa.2014.02.033)
• [L4] Volar beak ligament detachment from the thumb metacarpal, not dorsal capsular degeneration, is strongly associated with trapeziometacarpal osteoarthritis in elderly cadaveric thumbs. [127] (10.1016/j.jhsa.2025.10.015)
• [L5] Extension minimizes volar capsular space volume and may be essential for preventing flexion contractures. [128] (10.1016/j.jht.2024.12.015)
• [L5] Using these anatomical origins and insertions, the study was able to restore the MCP flexion and stability seen in an intact ligament. [129] (10.1016/j.jhsa.2012.09.015)
• [L4] Three portions of collateral ligaments on both sides of the metacarpophalangeal joint have variable length changes during flexion, which act to stabilize the joint through its flexion arc. [130] (10.1177/1753193417692708)
• [L5] The mechanical balance of a hand has nothing to do with having equally strong muscles on opposite sides of each joint. [133] (10.1016/s0894-1130(12)80324-3)

See Also

• Trapeziectomy
• Bones and Joints
• Flexor tendon repair
• Dislocations
• Tumors

References

[1] Advances in hand biomechanics simulation. Journal of Hand Therapy. 1989. DOI: 10.1016/s0894-1130(89)80052-3

[2] Exam Of The Hand Wrist 2Ed. INTRODUCTION.

[3] Anatomy of the Collateral Ligaments of the Proximal Interphalangeal Joint. The Journal of Hand Surgery. 2005. DOI: 10.1016/j.jhsa.2005.05.015

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[127] Revisiting the Critical Pathology of Primary Thumb Trapeziometacarpal Osteoarthritis: Volar Beak Ligament Degeneration. The Journal of Hand Surgery. 2026. DOI: 10.1016/j.jhsa.2025.10.015

[128] Prioritize proximal interphalangeal joint extension, a cadaver study clarifying PIPJ neutral position and joint capsular behavior. Journal of Hand Therapy. 2025. DOI: 10.1016/j.jht.2024.12.015

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