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Hand Anatomy and Biomechanics

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

90 citationsUpdated Sep 2026

Illustration: Hand Anatomy and Biomechanics

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Overview¶

Recent advances in hand surgery reflect a shift from static to functional anatomy, recognizing the hand as both an information-gathering and execution organ [3]. Efficient function depends on stable yet mobile proximal joints that keep the hand under visual control, utilizing the shoulder’s mobility, elbow flexion–extension, and wrist positioning to reach various body regions [3]. The hand is an integral part of the upper extremity, with continuous blood and nerve supplies and extrinsic muscles originating in the arm and forearm [3]. Structurally, the open hand forms a balanced oval with a convex dorsal aspect and a concave palmar surface [3]. The five digits converge during closing and diverge during opening, with the thumb positioned proximally and laterally to allow movement inward and outward from the palm [3].

The hand comprises 19 bones, 17 articulations, and 19 intrinsic muscles, along with approximately the same number of tendons activated by forearm muscles [3]. Basic functions of pinch, grasp, and grip require motion, stability, and strength in the digits [20]. Effective pinch relies on a stable, well-positioned thumb, where functional stability is more critical than extremes of movement at the interphalangeal or metacarpophalangeal joints [20]. Power grip requires composite motion of all metacarpophalangeal and proximal interphalangeal joints, particularly those of the ulnar three digits [20]. The thumb carpometacarpal joint defines the thumb’s workspace in all three dimensions, while the metacarpophalangeal joint acts as a force transmitter to the interphalangeal joint, which provides fine motor skills and stability for pinching [67]. Active mobility of the thumb interphalangeal joint is strongly correlated with overall hand function [67].

The osseous skeleton consists of 27 bones, divided into five rays forming polyarticulated chains [19]. The radioulnocarpal articulation permits positioning in any spatial configuration through two axes of movement plus forearm pronation and supination [19]. The radial ray is the shortest and possesses great freedom of movement, with the trapezium angled so the first metacarpal forms a 45-degree angle with the second metacarpal in the sagittal plane [19]. The skeleton presents longitudinal and transverse concavities, creating a cup shape when the thumb is placed next to the index finger [19]. The transverse axis of the palm is oblique, forming an acute angle of approximately 75 degrees with the longitudinal axis [19]. These anatomical features underpin the dynamic gliding mechanisms and vascular supply essential for hand function, with detailed soft tissue and vascular anatomy addressed in subsequent sections.

Osseous Anatomy¶

General Skeletal Architecture¶

The hand comprises 19 bones, 17 articulations, and 19 muscles situated entirely within the hand [3]. The skeleton is organized into five rays, each forming a polyarticulated chain of metacarpals and phalanges [19]. The base of each metacarpal articulates with the distal carpal row, while the carpus connects to the forearm via its proximal row [19]. The radioulnocarpal articulation possesses two axes of movement, supplemented by a third axis of pronation and supination from the forearm [19]. This configuration grants the wrist three axes of movement, allowing the hand to assume any spatial configuration [19].

In the open position, with fingers extended and in contact, the hand forms a balanced, graceful oval along its longitudinal axis [3]. The proximal carpometacarpal half of the hand is flattened, presenting two distinct faces [3]. The posterior or dorsal aspect is convex, whereas the anterior, palmar, or volar aspect is concave [3]. When the thumb spreads to grasp an object, this cup transforms into a gutter whose major oblique axis follows the thumb crease [19]. The transverse axis of the palm, corresponding to the metacarpophalangeal articulations, is not perpendicular to the longitudinal axis [19]. Instead, this transverse palmar axis is oblique, positioned more distally at the index metacarpophalangeal joint and more proximally at the fifth metacarpophalangeal joint [19]. It forms an acute angle of approximately 75 degrees with the longitudinal axis [19]. Epiphyseal plates are located at the proximal ends of the phalanges and the first metacarpal, and at the distal ends of the other metacarpals [19].

Metacarpals and Rays¶

The radial ray, or first ray, is the shortest and consists of only three bones: a metacarpal and two phalanges [19]. It continues the external column of the carpus formed by the scaphoid and trapezium [19]. The trapezium is angled anteriorly out of the carpal plane, causing the first metacarpal to form an angle of about 45 degrees with the second metacarpal in the sagittal plane [19]. The thumb metacarpal is the shortest, while the index metacarpal is by far the longest [19]. The lengths of the remaining metacarpals decrease from the third to the fifth digits [19]. Conversely, the third metacarpal is normally the longest, followed by the second, fourth, and fifth [79]. The proximal and particularly middle phalanges of the middle and ring fingers are longer than those of the index finger, making the long finger, and usually the ring finger, longer than the index finger [19].

The third metacarpal serves as the axis of the hand and a landmark for wrist movements [79]. The styloid process projects from its base, marking the line of the carpometacarpal joints [79]. The two ulnar metacarpals, especially the fifth, exhibit slightly more mobility in flexion–rotation [19]. Metacarpal shaft fractures are inherently stable due to connections at the carpometacarpal joints and intermetacarpal ligaments [7]. In contrast, metacarpal neck fractures are inherently unstable because of volar comminution [7]. Available data provide information regarding metacarpal dimensions, including length, midshaft width, and articular surface areas of the head and base [53].

Phalanges and Digital Joints¶

The bones of the fingers are the proximal (P1), middle (P2), and distal (P3) phalanges [42]. The bones of the thumb are the proximal and distal phalanges [42]. In the fingers, the metacarpophalangeal (MCP) joint is a triaxial condyloid joint [42]. The typical range of motion for the finger MCP joint extends from 15° of hyperextension to 90° of flexion [42]. The trapezoidal shape of the metacarpal head creates a cam effect on the collateral ligaments, rendering them taut during MCP joint flexion and lax during extension [42]. Consequently, the finger MCP joint is stable in flexion and unstable in extension [42]. The internal structure and material properties of the phalanges significantly influence both the magnitude and distribution of stresses in the MCP joint [11].

In the thumb, the metacarpal head is a single broad condyle [42]. Sesamoids are contained within the volar plate of the thumb MCP joint [42]. The proximal interphalangeal (PIP) joint is a hinge joint [42]. The head of the proximal phalanx includes two condyles separated by an intercondylar notch [42]. This intercondylar notch provides inherent joint stability through its articulation with the median ridge at the base of the middle phalanx [42]. The collateral ligaments at the PIP joint remain taut throughout the range of motion [42]. 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 [42].

Thumb Carpometacarpal Joint¶

The carpometacarpal ray of the thumb is anterior to the plane of the other metacarpals and makes an angle of about 47 degrees with the second ray [36]. The distal articular surface of the trapezium is likened to a saddle, with its concave curvature lying in a dorsopalmar direction [36]. The corresponding surface on the base of the first metacarpal presents a grooved surface in the radioulnar direction [36]. These articular surfaces are asymmetrical [36]. The trapeziometacarpal joint possesses two longitudinal axes and two degrees of freedom, best likened to a universal joint [36]. The shape of the articular surface does not provide good stability except in anteposition and pronation, when the two surfaces are congruent [36].

The necessary stability of the trapeziometacarpal joint is provided by a complex ligamentous system that does not limit mobility [36]. The radial side of the joint has a much weaker ligament [36]. There is an intrinsic instability at the level of the trapeziometacarpal joint where pressure is very high [36]. The estimated lengths of principal ligaments stabilizing the carpometacarpal joint change substantially during thumb motions in vivo [15].

Ligaments and Joint Capsule¶

General Principles and Gliding Mechanisms¶

The dorsal integument of the hand must be supple, elastic, and malleable to permit metacarpophalangeal joint flexion [6]. Interphalangeal flexion relies on a specific arrangement of skin folds on the dorsum of each articulation [6]. Vessels and nerves adapt to length differences within loose fibroadipose connective tissue [6]. In unrestricted areas with straight tendon trajectories, tendons are surrounded by paratenon and areolar connective tissue [6]. In narrow, crowded areas, the synovial sheath ensures the gliding mechanism [6]. Fibrous sheaths surrounding synovial sheaths keep tendons close to the skeleton, particularly at articular angles [6]. When a tendon changes direction, the fibrous sheath assumes the role of a pulley [6]. The gliding mechanism represented by synovial sheaths is more developed on the palmar aspect than the dorsal aspect [6]. Superficial and deep flexor tendons of the digits glide over each other [6]. Extensor synovial tendon sheaths are present only at the level of the wrist on the dorsal aspect [6]. Each synovial sheath consists of a visceral and parietal component separated by a potential synovial cavity containing a thin layer of synovial fluid [6]. Alterations of these gliding mechanisms have important functional repercussions [6].

The articulations of the hand form functional groups arranged in kinetic chains rather than isolated mechanical entities [6]. The position of each articulation depends on the equilibrium of forces acting at that level and the position of the immediately proximal articulation [6]. The wrist influences the position of the metacarpophalangeal joint, which affects the proximal interphalangeal joint, which in turn affects the distal interphalangeal joint [6]. Passive factors maintaining articular equilibrium include the restraining action of ligaments and muscular viscoelasticity [6]. Single articular movements around a fixed perpendicular axis do not exist in the hand; movements occur around oblique and variable axes [6]. Restraint is provided by the sagittal bands of the common extensor and the check rein ligaments of the volar plates [51]. The fibrous skeleton has zones of condensation kept under tension by a balance of forces exerted in different directions [51]. The fibrous nucleus (force nucleus) at the metacarpophalangeal joint is formed by the convergence of the transverse interglenoid ligament, volar plate, sagittal bands, fibrous flexor tendon sheath, and lateral accessory metacarpoglenoid ligaments [51]. Destruction or displacement of the fibrous complex at the metacarpophalangeal joint disturbs force balance, flattens the transverse arch, and compromises finger function [51]. At the proximal interphalangeal joint, Cleland’s ligaments are attached to the skin and oblique retinacular ligaments cross the joint obliquely [51].

Thumb Carpometacarpal Joint¶

The thumb carpometacarpal joint is a biconcave, reciprocating saddle joint with little inherent stability [57]. Sixteen surrounding ligaments impart stability to this joint [57]. The thumb metacarpal is 34% smaller than the distal articular surface of the trapezium [57]. The primary stabilizer is the deep anterior oblique ligament, also known as the palmar “beak” ligament [57]. This intracapsular ligament emanates from the volar tubercle of the trapezium and inserts on the ulnar volar aspect of the first metacarpal [57]. It tightens with pronation, abduction, and extension, preventing ulnar and dorsal translation of the first metacarpal [57]. The obliquely oriented fibers of the deep anterior oblique ligament create a center of rotation for the carpometacarpal joint [57]. The superficial anterior oblique ligament tightens with pronation and extension of the thumb [57]. The dorsoradial ligament and posterior oblique ligament stabilize and inhibit dorsal translation and ulnar translation of the thumb carpometacarpal joint, respectively [57]. The dorsal intermetacarpal ligament of the thumb prevents radial translation and proximal migration of the thumb metacarpal following trapeziectomy [57].

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 [57]. Mechanoreceptors were identified in the dorsoradial ligament, which was innervated to a greater extent than the anterior oblique ligament [57]. High compressive forces across the thumb carpometacarpal joint during pinch may reach in excess of 12 times the applied load [57]. Compressive forces across the thumb carpometacarpal joint may approach 20 times the applied load during maximum grasp [57]. Shear forces created by cantilever bending are highest at the volar half of the thumb carpometacarpal joint’s articular surface [57]. Flexion of the thumb metacarpophalangeal joint produces unloading of the volar portion of the trapezial metacarpal joint [57]. Degeneration of the volar half of the thumb carpometacarpal joint is associated with diminished integrity of the deep anterior oblique ligament [57]. Dorsal cartilage is relatively spared in thumb carpometacarpal osteoarthritis, even in advanced cases [57]. Articular degeneration involves a greater portion of the trapezium surface area compared with the metacarpal by a ratio of 3:1 [57]. A decrease in the ratio of trapezium to metacarpal degeneration is associated with more advanced disease [57].

The articular surfaces of the thumb carpometacarpal joint resemble two reciprocally opposed saddles with perpendicular transverse axes [70]. The joint has motion in three principal planes: flexion-extension, abduction-adduction, and pronation-supination [70]. Conjunct rotation in pronation results from the asymmetric height of the radial and ulnar condyles of the trapezium [70]. The volar oblique ligament passes from the trapezium to the volar beak of the thumb metacarpal and provides primary restraint to dorsal subluxation force [70]. Recent anatomic work supports the presence of a stout dorsal deltoid ligament complex that is more robust and cellular than the anterior oblique ligament [70]. The estimated lengths of principal ligaments stabilizing the thumb carpometacarpal joint change substantially during thumb motions in vivo [15]. The thumb trapeziometacarpal joint ligaments have an abundance of nerve endings in the dorsal ligaments but little to no innervation in the anterior oblique ligament [98]. Volar beak ligament detachment from the thumb metacarpal is strongly associated with trapeziometacarpal osteoarthritis in elderly cadaveric thumbs [116]. Complete rupture of thumb carpometacarpal joint ligaments permits the thumb metacarpal to dislocate dorsally [68]. Partial rupture of thumb carpometacarpal joint ligaments permits varying degrees of displacement [68]. The volar oblique ligament is a short reflection of the transverse carpal ligament that maintains the thumb metacarpal within the biconcave contour of the trapezium [68].

Metacarpophalangeal Joint¶

The metacarpophalangeal joints are relatively resistant to ligament injury and dislocation due to intrinsic ligamentous structure, surrounding tendons, and protected position [59]. These joints are most vulnerable to injury from forces directed ulnarly and dorsally [59]. 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 [59]. The capsule of the metacarpophalangeal joint extends from the metacarpal neck to the base of the proximal phalanx [59]. The dorsal capsule is composed of areolar tissue and reinforced by the loose insertion of the common extensor tendon [59]. The volar plate is continuous laterally with the deep transverse metacarpal (intervolar plate) ligaments [59]. The volar plate has a thick fibrocartilaginous distal portion and a thin membranous proximal portion [59]. The volar plate does not have strong proximal checkrein ligaments comparable to those of the proximal interphalangeal joint [59]. Collateral and intervolar plate ligaments insert into the lateral margins of the volar plate to produce linked support from one metacarpophalangeal joint to the next [59].

The collateral ligaments are more taut in flexion than in extension due to the cam effect created by the nonspherical shape of the metacarpal head [59]. The metacarpal head has a longer dorsovolar axis than proximodistal axis [59]. Broader and more stable articular contact between the metacarpal head and the base of the proximal phalanx occurs beyond 70 degrees of flexion [59]. The metacarpophalangeal joint is stable laterally in full flexion but allows some abduction and adduction in full extension [59]. The metacarpophalangeal joint must be splinted in at least 50 degrees of flexion to maintain ligament length [59]. Finger metacarpophalangeal joint collateral ligament stability increases with increasing flexion of the joint [115]. There is little to no difference between analogous collateral ligaments on either hand when tested in the same position [115]. 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 [121]. The volar plate of the metacarpophalangeal joint is composed of crisscrossing fibers that have the ability to collapse from the fully extended to the fully flexed position [75]. In flexion, the metacarpophalangeal volar plate collapses from a third to a half of its length in full extension [75]. The volar plate pocket behind the metacarpophalangeal volar plate is smaller than at the proximal interphalangeal joint, resulting in no checkreins [75].

Proximal Interphalangeal Joint¶

The proximal interphalangeal joint volar plate is similar to a unit in a suit of armor, sliding proximally and distally to protect the joint [75]. The proximal interphalangeal joint volar plate is thick, allowing extreme external loading over the joint [75]. The pocket behind the proximal interphalangeal volar plate is large [75]. The excursion of the proximal interphalangeal volar plate between flexion and extension is substantial [75]. Collapse of the proximal interphalangeal volar plate itself between extension and flexion is minimal [75]. Limitation or fixation of the proximal interphalangeal volar plate, such as with the development of checkreins, produces a significant restriction in extension of the proximal interphalangeal joint [75]. Volar elevation of the volar plate seen in active flexion could provide dynamic stresses on adjacent ligaments and contribute to the stability and smooth gliding of the joint [127]. Extension minimizes volar capsular space volume and may be essential for preventing flexion contractures [117]. Although the extensor tendon-reflecting technique revealed the greatest amount of surface, nearly 50% of the proximal phalanx articular surface remained inaccessible as long as the collateral ligaments were intact [46]. The 5 described surgical techniques may provide for equal restraint to proximal interphalangeal joint hyperextension instability in the early postoperative period [130].

Distal Interphalangeal Joint¶

The distal interphalangeal joint is a hinge or ginglymus joint, similar to the proximal interphalangeal joint [75]. The capsule surrounding the distal interphalangeal joint is reinforced laterally by collateral ligaments that insert into the sides of the head of the middle phalanx and run distally and volarly to insert into the volar lateral tubercle of the distal phalanx [75]. Accessory ligaments at the distal interphalangeal joint are more volar and extend from the sides of the middle phalanx head to the sides of the volar plate [75]. The volar plate serves as an accessory insertion point for the flexor digitorum profundus at the distal interphalangeal joint [75]. Proximally, the distal interphalangeal volar plate extends and is attached to the neck of the middle phalanx [75]. Unlike the proximal interphalangeal joint, the distal interphalangeal volar plate does not have lateral volar extensions [75]. The distal interphalangeal volar plate can hyperextend because it does not have checkrein ligaments [75]. The dorsal aspect of the distal interphalangeal joint has no reinforcing ligament [75]. 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 periosteum of the distal phalanx [75].

General Ligament Properties and Clinical Implications¶

Minimally invasive and percutaneous techniques can effectively release several structures known to cause finger stiffness with minimal damage to surrounding structures [8]. Active and passive intrinsic reconstruction methods improved basic grasp and release kinematics in experimental cadaver hand models [25]. There is some evidence that abnormal joint motion caused by ligamentous and joint laxity may have a deleterious long-term effect clinically [126]. The influence of gender and hormonal environment is under investigation as a factor in evaluation and treatment of musculoskeletal disease related to ligamentous laxity [126]. Using anatomical origins and insertions, reconstruction techniques were able to restore metacarpophalangeal flexion and stability seen in an intact ligament [119]. Generally, the extensibility of the dorsal skin of the hand increased from distal to proximal and from ulnar to radial [128].

Muscles and Tendons¶

Extrinsic Extensor Muscles¶

The extrinsic extensors traverse six distinct fibroosseous retinacular compartments at the wrist [22]. The first compartment houses the abductor pollicis longus and extensor pollicis brevis [22]. The abductor pollicis longus inserts at the base of the thumb metacarpal to radially abduct the thumb, while the extensor pollicis brevis inserts on the dorsum of the proximal phalanx to actively extend the metacarpophalangeal joint [22]. The second compartment contains the extensor carpi radialis longus and brevis [22]. The longus inserts on the index metacarpal, providing dorsiflexion and radial deviation, whereas the brevis inserts on the base of the middle metacarpal to provide balanced wrist dorsiflexion [22].

The third compartment contains the extensor pollicis longus, which turns abruptly radialward around Lister tubercle [22]. This tendon inserts on the distal phalanx, providing forceful extension of the thumb interphalangeal joint [22]. Its oblique course contributes a substantial adduction component to its pull [22]. The fourth compartment contains the extensor indicis proprius, which lies deep to the four tendons of the extensor digitorum communis [22]. The extensor indicis proprius inserts on the index finger ulnar to the extensor digitorum communis and possesses the most distal muscle belly of any digital extensor tendon at the wrist level [22]. The extensor digitorum communis inserts on the index, middle, ring, and, in some cases, little fingers, while the extensor digiti quinti tendon inserts on the little finger ulnar to the extensor digitorum communis [22]. The sixth compartment contains the extensor carpi ulnaris, which inserts at the base of the little finger metacarpal to provide wrist extension and ulnar deviation [22].

The principal bony insertion of the extrinsic digital extensors is on the dorsal proximal aspect of the middle phalanx [22]. Metacarpophalangeal joint extension is driven by extrinsic force transmitted through the sagittal bands, while distal interphalangeal joint extension is achieved via conjoined lateral bands composed of tendinous slips from both extrinsic and intrinsic tendons [22]. The digital extensor tendons are stabilized over the midline of the metacarpophalangeal joint by their attachment to sagittal band fibers, which insert onto the volar proximal phalanx and the lateral borders of the volar plate [22]. These bands form a sling that transmits proximal extrinsic tension to the proximal phalanx, permitting metacarpophalangeal extension without a direct tendinous insertion onto the proximal phalanx [22]. By keeping the extrinsic extensor away from the center of rotation of the metacarpophalangeal joint, the sagittal bands maximize mechanical efficiency [22]. Rupture or attenuation of these fibers allows the extrinsic extensor tendon to sublux to the ulnar side of the metacarpal head, causing ulnar deviation of the finger [22].

The extrinsic extensor apparatus of the fingers is formed by the extensor communis and the extensor proprii 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 have an almost entirely extrasynovial course, which facilitates repair, but they are thin superficial structures that rapidly become adherent to underlying bones and joints when damaged [35]. The excursion of extensor tendons is considerably less than that of flexors [35]. The extensor communis and the two extensor proprius tendons pass under the dorsal retinaculum on the back of the wrist before diverging toward the fingers [35]. On the dorsum of the hand, the extensor communis tendons are interconnected by the juncturae tendinum, which assist extension of adjacent connected fingers [35]. Interruption of an extensor tendon proximally may be masked by the effect of the juncturae tendinum [35]. The extensor communis tendon to the little finger may be absent or replaced by an oblique junctura from the ring finger [35].

The form of the extensor tendons changes at the level of the metacarpophalangeal joints, becoming thin and flat [35]. The extrinsic extensor system has four sites of insertion [35]. The most proximal insertion is at the level of the interglenoid ligament, provided by the sagittal bands on each side of the metacarpophalangeal articulation [35]. The most distal insertion is at the level of the base of the distal phalanx [35]. The insertion of the extensor tendon 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 consist of the flexor digitorum profundus and the flexor digitorum superficialis [22]. The flexor digitorum profundus inserts on the proximal volar aspect of the distal phalanx, flexing the distal interphalangeal joint as well as the proximal interphalangeal and metacarpophalangeal joints [22]. It originates from the proximal ulna and the interosseous membrane [64]. In the forearm, the flexor digitorum profundus divides into two muscle groups: a radial component supplying the index finger and an ulnar component supplying the middle, ring, and little fingers [64]. The flexor digitorum profundus and flexor pollicis longus muscles form the deep compartment of the volar forearm [64]. As these tendons travel through the carpal tunnel, they occupy the floor of the tunnel [64]. The tenosynovial sheath of the flexor pollicis longus is continuous with the radial bursa, while the sheath to the little finger is continuous with the ulnar digital bursa [64]. In some patients, these bursae communicate, allowing a horseshoe abscess to spread between the thumb and little finger if infection occurs in the flexor tendon sheath of either digit [64]. The innervation of the flexor digitorum profundus of the index and middle fingers is through the anterior interosseous branch of the median nerve, while the profundus of the ring and little fingers is innervated by the ulnar nerve [64].

The flexor digitorum superficialis 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 [64]. Each digit has a corresponding independent superficialis muscle [64]. As the superficialis tendons pass through the carpal tunnel, the tendons of the middle and ring fingers are more superficial and central than those of the index and little fingers [64]. In the proximal aspect of the finger, the flexor digitorum superficialis tendon bifurcates around the flexor digitorum profundus at the beginning of the A2 pulley [64]. The 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 of the finger [64]. The flexor digitorum superficialis tendon inserts via radial and ulnar slips into the proximal metaphysis of the middle phalanx [64]. The entire flexor digitorum superficialis muscle receives innervation from the median nerve, and its primary function is digital flexion at the proximal interphalangeal joint [64].

The flexor pollicis longus 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 the medial epicondyle of the humerus [64]. In the palm, the flexor pollicis longus tendon transverses between the abductor pollicis brevis and the flexor pollicis brevis [64]. It inserts into the proximal base of the thumb distal phalanx and is innervated by the anterior interosseous branch of the median nerve [64]. The flexor pollicis longus flexes both the interphalangeal and metacarpophalangeal joints of the thumb [64].

As the flexor tendons pass distal to the metacarpal neck, they enter the fibroosseous tunnel, or digital flexor sheath, which extends distally to the proximal aspect of the distal phalanx [64]. The tendinous sheath consists of annular pulleys, which provide mechanical stability, and cruciate pulleys, which provide flexibility [64]. The first, third, and fifth annular pulleys (A1, A3, and A5) are located over the metacarpophalangeal, proximal interphalangeal, and distal interphalangeal joints, respectively [64]. The second and fourth pulleys (A2 and A4) are situated over the middle portion of the proximal and middle phalanges [64]. The A2 and A4 pulleys are the most essential in maintaining the mechanical advantage of the flexor tendons [64]. The tenosynovium lining the fibroosseous tunnel supplies both nutrition and lubrication to the poorly vascularized flexor tendons [64]. Proximal to the sheath, the tendons are well vascularized by the peritenon [64]. Within the sheath, tendon vascularity is supplied via the vincula system: the vinculum longus and brevis [64].

The flexor tendons of the fingers cross five well-defined regions, from proximal to distal: the wrist, the carpal tunnel, the part of the palm extending from the exit of the carpal tunnel to the entrance of the fibrous flexor sheath suspended from the volar plate of the MP joint, the portion of the osteofibrous canal of the fingers that is common to the deep and superficial flexor tendons and reaches halfway down the middle phalanx, and the distal segment of the same tunnel which transmits only the flexor profundus tendon [80]. The tendon of the flexor pollicis longus crosses five regions: the wrist, the carpal tunnel, the palmar region (thenar eminence), the digital canal extending from the opening of the proximal pulley at the volar plate of the MP joint down to the exit of the “oblique pulley” halfway down the proximal phalanx, and the distal segment reaching the insertion of the tendon on the base of the distal phalanx [80]. The danger of adhesions with fixed structures is greater where the tendon runs within a fibrous sheath [80]. Multiple tendon injuries tend to occur in regions where the tendons are bundled together, specifically the wrist and the carpal tunnel [80]. In each digit, the superficial and deep flexor tendons are surrounded by their synovial sheaths for gliding and kept against the phalanges by their fibrous sheaths [80]. The fibrous sheath plays an essential mechanical role in preventing divergence of the tendons from the axis of the digit in both the anteroposterior and lateral directions [80]. The digital flexor tendon sheath is formed by five annular pulleys and three cruciform bands [80]. The second and fourth annular pulleys are the most important for function [80]. At the level of the digital sheaths, the flexor tendons have a precarious blood supply through a vinculum longum at the chiasma level, common for both tendons superficial and profundus, and vincula brevis for the profundus [80]. Lundborg demonstrated the presence of an “avascular segment” of the flexor superficialis just proximal to the chiasma [80]. Lundborg also demonstrated the presence of two “avascular segments” of the flexor profundus proximal and distal to the vinculum longum [80]. The flexor system of the thumb is less complicated than the flexor system of the fingers, containing one fewer joint and only one tendon (the FPL tendon) [85]. When the FPL tendon is reconstructed, a recovery of 30 to 40 degrees of active IP joint flexion is associated with excellent thumb function [85].

Intrinsic Muscles¶

Control of digital posture requires a complex balance of extrinsic and intrinsic muscle forces [22]. Extrinsic muscles have their origin outside of the hand and their insertion on the hand or carpus, whereas intrinsic muscles have both origin and insertion within the hand [22]. Extrinsic muscles are either flexors or extensors, whereas intrinsic muscles contribute to both digital flexion and extension [22]. The anatomic axis of the hand coincides with the axis of the third metacarpal [28]. The dorsal interossei are abductors and lie to the radial side of the index and middle fingers and the ulnar side of the middle and ring fingers [28]. The little finger is abducted by the abductor digiti quinti [28]. 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 [28].

Each dorsal interosseous muscle, with the exception of the third, has two muscle heads [28]. The superficial head arises most dorsally from the shaft of the contiguous metacarpals and is inserted deeply by a medial tendon onto the lateral tubercle of the base of the proximal phalanx [28]. This head abducts and weakly flexes the proximal phalanx but has no direct effect on the middle or distal phalanges [28]. The deep head of each dorsal interosseous muscle forms a lateral tendon, or lateral band, at the level of the MP joint [28]. The deep head flexes and weakly abducts the proximal phalanx while extending the middle and distal phalanges [28]. At the level of the middle of the proximal phalanx, transverse fibers arch dorsally from each lateral band to join each other over the dorsum of the finger, flexing the proximal phalanx [28]. More distally, oblique fibers (spiral fibers) from the lateral bands sweep over the distal third of the proximal phalanx to insert onto the lateral tubercles at the base of the

Neurovascular Anatomy¶

Arterial Supply and Palmar Arches¶

The ulnar artery is larger than the radial artery and provides the primary arterial contribution to the hand [90]. In most hands, the ulnar artery supplies the superficial palmar arch, which provides the principal blood supply to the common and proper digital arteries [90]. The radial artery enters the hand by passing deep to the tendons of the first dorsal compartment across the anatomic snuffbox, dives palmarward between the bases of the first and second metacarpals, and forms the deep palmar arch [90]. The median artery contributes to the superficial palmar arch in 10% of patients [90]. The superficial palmar arch is located distal to the deep palmar arch [90]. The arterial arch is complete, with total communication between the radial and ulnar arteries in 34% of hands and incomplete communication in 20% [90]. The deep palmar arch runs alongside the motor branch of the ulnar nerve as it travels transversely just palmar to the proximal metacarpal shafts [90]. The princeps pollicis artery is derived from the deep palmar arch in 98% of patients [90]. The deep palmar arch supplies the deep metacarpal arterial branches, which provide secondary blood flow to the digital arteries [90].

A 2018 meta-analysis by Zarzecki et al. reviewed 36 studies of 4481 palmar arches and found that the superficial arch was complete in 81.3% and the deep arch was complete in 95.2% of hands [105]. The ulnar artery flows into the hand on the palmar surface, and its terminal portion becomes the superficial arch [105]. The radial artery courses around the radial volar wrist into the anatomic snuffbox, then enters the palm via the first web space [105]. The terminal branches of the radial artery turn into the princeps pollicis and the deep palmar arch [105]. Acute interruption of flow in either the radial or ulnar artery is extremely unlikely to result in significant ischemia of the hand or fingers [105]. The anterior and posterior interosseous vessels can provide flow via collateral circulation in chronic vascular problems but are generally unable to provide adequate oxygenated blood in the case of acute injury to both the radial and ulnar vessels [105]. The volar forearm’s median artery can provide significant blood flow to the hand, particularly in the case of injury or harvest of the radial artery [105]. A persistent median artery helped maintain blood flow to a nearly amputated hand after complete transection of the radial and ulnar arteries, preserving hand viability [102]. The thumb has significant arterial flow from both palmar and dorsal circulation, which is probably the reason for ischemia of this digit being rare [105].

Thumb Vascular Anatomy¶

The "princeps pollicis" artery, the terminal branch of the radial artery, crosses the first intermetacarpal space, runs along the ulnar side of the first metacarpal bone and 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 [18]. At the metacarpophalangeal joint crease, the princeps pollicis divides into two terminal rami, the collateral palmar arteries of the thumb, which run along the digital tunnel symmetrically and are of equal caliber [18]. In anatomical studies, only 15% of dissections fall into the category of the "typical" classical layout of palmar arteries of the thumb [18]. In the second segment of the thumb (between MCP and IP creases), the main artery is the ulnar collateral artery, which is more often easier to dissect than the radial collateral artery and its size enables a more reliable microanastomosis [18]. In the pulp segment of the thumb, the two arteries are of similar size and run through the thick fatty subcutaneous padding, then cross over and convert into the ends of the digital nerves at the level of the median axis [18]. The most constant artery on the dorsal aspect of the thumb is the ulnar collateral artery [32]. The dorsal arteries of the thumb originate from palmar arteries (princeps, commissural, or anastomoses of the superficial arcade) at the level of the first metacarpal [18]. At the level of the neck of the first phalanx, an anastomosis can be found which originates from the palmar arteries [18].

Digital and Metacarpal Vascular Anatomy¶

The blood supply to the fingers comes predominantly from the two palmar collateral digital arteries [32]. The palmar collateral digital arteries give cutaneous branches for the palmar digital skin and also give cutaneous branches for the dorsal skin [32]. Digital anastomoses arcades are present in the pulp and in the nail matrix [32]. Two constant dorsal branches originate from the proper palmar digital artery in the proximal and middle pulp spaces at predictable sites near the proximal interphalangeal joint [44]. The dorsal aspect of the first webspace is supplied by two arteries: the deep dorsal intermetacarpal artery, which runs along the diaphysis of the second metacarpal, and the superficial dorsal intermetacarpal artery, which supplies the skin overlying the proximal phalanx and the metacarpophalangeal joint of the index finger [32]. The deep dorsal intermetacarpal artery vascularizes the base of the second metacarpal [32]. The reliability of pedicled metacarpal bone flaps based on dorsal metacarpal arteries decreases from the second to the fifth metacarpal bone regarding the frequency of presence of these arteries [132]. Perforating branches of the true digital arteries supply the palmar intermetacarpal flap constantly in all three finger intermetacarpal spaces [114]. A nutrient vessel is often the only vessel supplying the head of the proximal phalanx, making this area particularly susceptible to vascular compromise [120].

Venous Drainage¶

Most of the venous drainage of the hand begins on the dorsal fingers and continues on the hand’s dorsal surface [105]. The primary arteries of the hand are each followed by two small venae comitantes, a configuration that continues out to the fingers into the digital arteries [105]. 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 [105]. Problems with venous drainage of the hand are extremely unusual, with the exception of possible venous thrombosis after replantation, due to the many venous channels available [105].

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 and its constant exposure to thermal and postural variation [32]. The general pattern of cutaneous circulation involves muscular arteries running into arterioles forming a plexus superficial to the aponeurosis, a dermal plexus, and a subpapillary plexus [32]. The arterial supply to the skin is classified into three groups: longitudinal cutaneous arteries, septal arteries, and myocutaneous arteries [32]. The blood circuit of the hand contains a system of shunts between arterioles, meta-arterioles, and venules, as well as direct arteriovenous anastomoses known as Sucquet-Hoyer anastomoses [32]. Sucquet-Hoyer anastomoses consist of spiral vessels that run from the arterioles to the venules and are found mostly in the palm, near the roots of the nails, and in the pulps [32]. The rich vascular supply of the skin of the hand has an important thermoregulatory function [32].

Nerve Anatomy and Innervation¶

The ulnar nerve regularly contributes to flexion of the middle finger [118]. The thumb interphalangeal joint is innervated dorsally by two branches of the radial nerve and palmarly by one branch each from the ulnar and radial proper digital nerves [123]. A significant dorsal branch of the ulnar digital nerve of the little finger arises at the level of the MCP joint and remains relatively palmar compared to previous descriptions [124]. 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 [23]. Anatomic knowledge of the course of the superficial radial nerve and its branches is important during open release for avoiding nerve injury [113]. Harvesting a part of the ulnar or median nerve proximal to the third most distal segment of the ulnar nerve and the fourth most distal segment of the median nerve minimizes donor nerve deficits by ensuring the selection of intermingled motor and sensory fascicles [133].

Neurovascular Bundles in Dupuytren's Disease¶

If tissues that form a spiral longitudinal path around a neurovascular bundle tighten into a linear cord, they displace the neurovascular structures into a spiral path or spiral neurovascular bundle [101]. As many as one-half of operated Dupuytren hands have spiral neurovascular bundles [101]. Spiral neurovascular bundles can arise anywhere between the distal palmar crease and the DIP flexion crease [101]. The altered anatomy of a spiral neurovascular bundle places a short length of the bundle superficial to the cord and into harm’s way [101]. Spiral neurovascular bundles are more likely where a fleshy prominence covers a well-defined cord [101]. Spiral bundles are increasingly likely with greater PIP contracture angles and can occur without a soft tissue prominence [101].

Clinical Assessment of Vascular Status¶

The Allen test allows assessment of the extent of connection between the radial and ulnar arteries through the palmar arches [90]. In the Allen test, the examiner compresses both the radial and ulnar arteries at the wrist, asks the patient to flex and extend fingers to blanch the hand, then releases pressure from one artery while maintaining compression on the other to observe reperfusion time [90]. Predominance of the radial artery is common in arterial territories [108]. The digital Allen test is performed in a manner similar to that at the wrist level and may reveal a unilateral occlusion [108]. The presence of an audible Doppler arterial signal in one of the wrist’s vessels is not an indication of adequate nutritional flow to the hand [106]. The digital-brachial index is the ratio of blood pressure measured in the brachial artery and the finger, and any value below 0.7 designates a significant occlusive problem somewhere in the forearm or hand [106]. Cold stress testing evaluates the effect of cold on arterial spasm by measuring the time required for digital rewarming to baseline after exposure to cold water at 5° to 8°C for 5 minutes [106]. A prolonged rewarming response in cold stress testing is often seen in women and can be diagnostic of Raynaud’s, and smokers often have a delayed rewarming response [106].

Biomechanics and Function¶

General Principles and Architecture¶

The hand functions efficiently only if the proximal joints of the limb are stable and yet mobile [3]. It moves within a large volume of space with the shoulder as the apex, utilizing the mobility of the shoulder, elbow, and wrist operating in different planes [3]. Movements of the clavicle amplify those of the shoulder [3]. The combined movements of the wrist and forearm place the hand in a position for grasping [3]. For gripping, the wrist is usually in flexion when close to the trunk and in extension when placed at a distance [3]. Forearm rotation (pronation–supination) plays an important role, particularly for bringing food to the mouth [3]. The hand must be studied as an integral part of the upper extremity because its blood and nerve supplies are continuous with the rest of the limb and some muscles arise in the arm and forearm [3].

The open hand forms a balanced graceful oval in its longitudinal axis [3]. The back of the hand is convex and aesthetically important, while the palmar surface is concave and functional [3]. The digits converge in closing (flexion and adduction) and diverge in opening (extension and abduction) [3]. The hinges of digit movements are located at the thenar crease and the transverse distal palmar crease, not at the bases of the digits [3]. The hand consists of 19 bones, 17 articulations, and 19 muscles situated entirely within the hand, plus about the same number of tendons activated by forearm muscles [3].

The skeleton is divided into five rays, each making up a polyarticulated chain comprising metacarpals and phalanges [19]. The wrist has three axes of movement, permitting the hand to be positioned in any spatial configuration for grasping [19]. The radial ray (first ray) is the shortest, made up of a metacarpal and two phalanges, and possesses great freedom of movement [19]. The lengths of the metacarpals vary, with the thumb metacarpal being the shortest and the index finger metacarpal being the longest [19]. The digital extremes of each ray converge in flexion toward the pulp of the thumb for pinch or toward the base of the thenar eminence for power grip [19]. The more ulnar a digit is, the more obliquely it must deviate as it approaches the palm [19]. The transverse axis of the palm is oblique, forming an acute angle of approximately 75 degrees with the longitudinal axis [19]. The epiphyseal plates are located at the proximal ends of the phalanges and the first metacarpal, and at the distal ends of the other metacarpals [19].

Skeletal Mobility and Arches¶

The metacarpal arch is endowed with adaptability due to the mobility of the peripheral metacarpals [10]. The index metacarpal is the most firmly fixed [10]. The ring metacarpal has about 10 degrees of mobility in flexion and extension [10]. The fifth metacarpal has a range of flexion–extension of approximately 20 degrees [10]. The second to fifth metacarpals are bound together by fibrous structures, including the deep transverse intermetacarpal ligament (interglenoid ligament) [10]. The longitudinal arches are composed of a fixed carpometacarpal portion and a mobile digital portion [10]. The metacarpophalangeal articulations are the keystones of the longitudinal arches, stabilized by thick anterior glenoid capsules (volar plates) that prevent hyperextension [10]. The stability of the metacarpophalangeal joints is essential to the support of both the longitudinal and transverse metacarpal arches [10].

The thumb ray has considerable mobility and independence, while the fifth ray has much less, and the others even less [10]. The index ray has a certain degree of independence at the phalangeal level owing to the arrangement of its flexor and extensor muscles [10]. There is normally no clinically detectable mobility at the second and third carpometacarpal joints [55]. Movement at the fourth carpometacarpal joint is limited to a few degrees in the sagittal plane [55]. The fifth carpometacarpal joint has an appreciable mobility of about 20 degrees, important for adaptation to the transverse arch of the palm [55]. The first metacarpal (trapeziometacarpal) joint is functionally the most important because it allows movement of the whole column of the thumb [55].

Thumb Biomechanics¶

The wide range of motion of the thumb column allows opposition to the palm and other digits [36]. The thumb interphalangeal articulation is a trochlear type allowing mainly flexion and extension, with flexion accompanied by slight 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 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, likened to a universal joint [36]. Longitudinal rotation at the trapeziometacarpal joint changes the orientation of the thumb pulp, which is in pronation during opposition and supination during retroposition [36]. The trapeziometacarpal joint has intrinsic instability at the level of high pressure, which is proportional to grip force and lever arm length [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]. The tendon of the abductor pollicis longus provides dynamic stability to the trapeziometacarpal joint only when the first metacarpal is abducted [36]. In adduction, the abductor pollicis longus increases the risk of subluxation of the trapeziometacarpal joint [36]. Total joint arthroplasty can restore thumb function but cannot fully replicate the kinematics of the healthy trapeziometacarpal joint [107].

Tendon Gliding and Excursion¶

The dorsal integument slides distally to allow metacarpophalangeal joint flexion [6]. In narrow crowded areas, the gliding mechanism is assured by synovial sheaths surrounded by fibrous sheaths that act as pulleys [6]. The gliding mechanism represented by synovial sheaths is much more developed on the palmar aspect [6]. The amplitude of tendon gliding depends on muscular contraction, which is about one-third the length of the muscle’s resting fleshy belly [33].

Extensor Digitorum: Amplitude of 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 [33]. Maximal excursion is 4.5 cm [33]. Flexor Digitorum Profundus: Amplitude of gliding of 5 mm at the distal interphalangeal joint, 17 mm at the proximal interphalangeal joint, 23 mm at the metacarpophalangeal joint, and 38 mm at the carpal canal [33]. Maximal excursion is 7.0 cm [33]. Flexor Digitorum Superficialis: Amplitude of gliding of 16 mm at the proximal interphalangeal joint, 26 mm at the metacarpophalangeal joint, and 46 mm at the carpal canal [33]. Maximal excursion is 6.4 cm [33].

Movements are determined by the modulation of forces between antagonists, a concept known as "synergistic antagonism" [33].

Muscle Mechanics and Joint Kinematics¶

To maintain equilibrium around a joint, the sum of the moments (torque) acting around it must be equal [33]. Mechanical advantage is defined as the moment arm of one force acting at a joint divided by the moment arm of a second force acting in the same plane around that joint [33]. The internal structure and material properties of the phalanges play a significant role in both the magnitude and distribution of stresses in the metacarpophalangeal joint [11]. From a mechanical perspective, the metacarpophalangeal joint represents a joint with 5 kinematic degrees of freedom [54].

No single articulation in the hand is an isolated mechanical entity; they form functional groups arranged in kinetic chains [6]. Single articular movements around a fixed perpendicular axis do not exist in the hand; almost all movements are around oblique and variable axes [6]. The active factor in articular equilibrium is the dynamic balance between antagonist muscles, while passive factors include ligaments and muscular viscoelasticity [6]. In power grip, extrinsic muscles provide the major gripping force while interossei act as phalangeal rotators and metacarpophalangeal flexors [104].

For effective pinch, a stable, well-positioned thumb is crucial, and extremes of movement of the thumb interphalangeal or metacarpophalangeal joint are less important than stability in a functional position [20]. Meeting the thumb at the tip requires metacarpophalangeal motion of the index and long fingers, with less motion needed at their proximal interphalangeal joints [20]. Power grip requires the composite motion of all metacarpophalangeal and proximal interphalangeal joints, especially those of the ulnar three digits [20]. The highest priority of function combined with the highest probability of success is seen with attempts to improve flexion at the metacarpophalangeal and proximal interphalangeal joints following trauma [20]. Pianists can adapt so that little proximal interphalangeal motion is needed, as long as the joint is pain-free [20]. Musicians who play string instruments need both strength and mobility, especially of the fingering or fretting hand, including the distal interphalangeal joint [20].

Load Distribution and Grip Dynamics¶

There is no universal or typical load distribution pattern of the hand, but only an individual pattern [40]. To analyze a hand's load-distribution pattern, the opposite hand can be used as a reference [4]. Measurement of individual finger forces can provide more accurate biomechanical models of the hand and determine the effect of disease on hand functions [5]. The dynamic interaction of finger joints during cylinder grip shows specific patterns, with distal interphalangeal joints consistently initiating flexion last [112]. Synchronization of finger joint flexion during cylinder grip increases significantly by the end of motion compared to the beginning [112]. At increasing amounts of finger flexion, progressive metacarpal shortening produces proportionally greater loss of fingertip flexion force [29]. Impairment in the kinematics of precision pinch is associated with index finger proximal interphalangeal joint fusion [111].

Fracture Biomechanics and Deformity¶

Most metacarpal diaphyseal fractures have apex dorsal angulation because of the pull of the interossei, which results in flexion of the distal fragment [7]. The hand can accommodate dorsal angulation by compensating with metacarpophalangeal hyperextension and carpometacarpal motion [7]. Carpometacarpal motion is greatest at the little finger (30°), followed by the ring finger (20°) [7]. The long and index finger carpometacarpal joints are fixed and thus can tolerate less angular deformity [7]. Rotational deformity of more than 5° can lead to overlapping of the fingers in flexion (scissoring) [7]. Angular deformity at the metacarpal neck/metaphysis influences tendon balance less than at the diaphyseal level [7].

Cadaver experiments estimate 7° of extensor lag per 2 mm of metacarpal shortening, but compensation is variable [7]. Metacarpophalangeal hyperextension usually can compensate for up to 4 mm of metacarpal shortening [7]. Metacarpal shortening or angulation greater than 30° can result in shortening of the intrinsics, with the potential for extensor lag at the proximal interphalangeal joint [7]. Metacarpal shaft fractures are inherently stable because of the connections at the carpometacarpal joints and the intermetacarpal ligaments [7]. Deformity at the little finger metacarpal neck level does not affect tendon balance, motion, or strength [7].

Most transverse proximal phalanx fractures are apex palmar [31]. The central extensor tendon pulls the distal fragment dorsal, and the interossei insertion flexes the proximal fragment in proximal phalanx fractures [31]. Proximal phalanx fractures have less stability than metacarpal fractures because they are not supported by adjacent bones and multiple tendon forces act on the fragments [31]. 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 [31]. Proximal middle phalanx fractures are apex dorsal because of the pull of the central slip [31]. Distal middle phalanx fractures displace palmarly as a result of the pull of the superficialis insertion [31]. Shortening of the middle phalanx following fracture may result in distal interphalangeal joint extension lag [31]. The angulation of metacarpal diaphysis fractures is usually apex volar as a result of the pull of the central slip and lateral bands [31].

Acceptable Angulation: * Metacarpal Diaphysis: 5° to 10° at the index and long fingers, and 10° to 40° at the ring and little fingers [7]. * Thumb Metacarpal: Up to 30° of angulation is acceptable for extra-articular thumb metacarpal fractures because of the mobility of the saddle joint [7].

Common Sites of Injury¶

Epidemiology and Mechanisms¶

Hand injuries, including fractures and dislocations, account for 1.5% of all US emergency room visits [97]. Phalangeal fractures occur at a rate of 12.5 per 10,000 persons annually [97], while metacarpal fractures occur at a rate of 8.4 per 10,000 persons annually [97]. Patients in their second to fourth decade of life have an incidence of 16.1 metacarpal fractures and 12.5 phalangeal fractures per 10,000 persons [97]. Males are more likely to sustain metacarpal and phalangeal fractures, while females are more prone to distal forearm fractures [97].

Mechanisms of injury vary significantly by age and activity. Sporting injuries account for most phalangeal fractures in patients in their second and third decades of life [97]. Accidental falls are the leading cause of hand fractures in patients over the age of 70 [97]. Industrial trauma accounts for the most injuries in patients aged between 40 and 69 years [97]. Axial mechanisms often result in articular fractures with a shearing component [97]. Rotational force or torque applied to a digit often results in a spiral-type fracture requiring surgical attention due to malrotation [97].

The upper extremity is the most common anatomic area burned, being involved in up to 89% of burns [78]. The dorsum of the hand is predominantly involved in flame or explosion injuries [78]. Injuries to the palm are more frequently found with chemical exposure, friction burns, or high-voltage injuries [78].

Anatomical Vulnerability¶

The dorsal skin is thin and very flexible, lying on a thin subcutaneous layer of fatty tissue that provides little mechanical protection [78]. Blood vessels, tendons, and joints are situated just beneath the skin surface over the dorsum of the fingers, making these structures extremely susceptible to thermal energy [83]. The skin is especially thin over the PIP joints, where extensor tendons are at risk [78]. Attenuation or rupture of the central slip with PIP joint exposure is one of the most frequently encountered complications after deep dorsal burns [78].

The cylindrical form of the fingers, with ligaments limiting swelling, can lead to dermal compartment syndromes in the event of deep dermal circumferential burn injury [83]. Palmar skin can withstand greater thermal energy than dorsal skin [83]. Persistent interstitial edema in the hand that has not resolved after 72 hours may lead to subcutaneous fibrosis with subsequent stiffness of the joints [83]. Edema in the intrinsic musculature of the hand may lead to a compartment syndrome requiring fasciotomy [83].

Metacarpal Fracture Biomechanics¶

The hand can accommodate dorsal angulation by compensating with metacarpophalangeal (MCP) hyperextension and carpometacarpal (CMC) motion [7]. CMC motion is greatest at the little finger (30°), followed by the ring finger (20°) [7]. The long and index fingers CMCs are fixed and thus can tolerate less angular deformity [7]. Cadaver experiments estimate 7° of extensor lag per 2 mm of metacarpal shortening [7]. MCP hyperextension usually can compensate for up to 4 mm of metacarpal shortening [7]. Metacarpal shortening or angulation >30° can result in shortening of the intrinsics, with the potential for extensor lag at the proximal interphalangeal joint [7].

The only way to prevent recurrence of deformity after closed reduction of metacarpal neck fractures is surgical fixation with percutaneous K-wires, intramedullary fixation, or other methods [7]. Patients with little finger metacarpal neck deformity may have a “lump in palm” sensation or painful callosities with gripping because of the volar prominence of the metacarpal head [7]. Acceptable angulation for metacarpal diaphyseal fractures is 5° to 10° at the index and long fingers [7]. Acceptable angulation for metacarpal diaphyseal fractures is 10° to 40° at the ring and little fingers [7].

Thumb Fracture Pathoanatomy¶

In Bennett fractures, the volar oblique ligament is attached to the volar ulnar fragment of the base [7]. In Bennett fractures, the abductor pollicis longus displaces the distal metacarpal proximally [7]. In Bennett fractures, the adductor pollicis displaces the metacarpal into adduction [7]. In Bennett fractures, the metacarpal base is displaced dorsally and rotated into supination [7]. Fracture-dislocations of the CMC joint of the little finger and the metacarpal diaphysis are displaced proximally and ulnarly as a result of the pull of the extensor carpi ulnaris tendon [7]. Thumb carpometacarpal joint osteoarthritis reflects the joint's teleology, where pathomechanics drive inflammatory markers and injury to soft tissue structures rather than simple wear and tear [41].

Neurovascular and Soft Tissue Considerations¶

The Cleland ligament is dorsal to the neurovascular bundle and provides stability in the finger [78]. The Grayson ligament is palmar to the neurovascular bundle and provides stability in the finger [78]. Knowledge of the thumb sagittal band anatomy supports repair of radial sagittal band injuries to prevent tendon instability [62].

Surgical Anatomy¶

Digital and Thumb Soft Tissue Anatomy¶

The dorsal aponeurosis of the thumb receives contributions from both intrinsic and extrinsic muscles [2]. In the palm, volar sensory 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 [71]. Palmar cutaneous branches of the proper digital nerves are a constant anatomical feature that can be mistaken for proper digital nerves, potentially leading to inadvertent injury during Dupuytren’s surgery [23]. The existence of constant dorsal branches from the proper palmar digital artery supports the feasibility of using flaps based on these vessels [44].

Surgical planning benefits from specific cutaneous landmarks and anatomical variations. The distance between the digital-palmar and proximal interphalangeal creases may be used as a cutaneous landmark for the exact location of the proximal edge of the A1 pulley in the palm [94]. Using this distance as a landmark ensures greater safety in surgical procedures such as percutaneous release of trigger finger [94]. The anatomical variation of the digital pulley system is beneficial for hand surgeons performing open or percutaneous trigger finger releases and for radiologists examining the area sonographically [43]. Minimally invasive and percutaneous techniques can effectively release structures known to cause finger stiffness with minimal damage to surrounding structures [8].

Operative decisions require attention to specific anatomical constraints. The choice between ulnar and radial superficialis slip resection is not arbitrary, and findings should be considered to preserve function and strength in the injured hand [16]. Suture anchors should be used with caution in the little finger due to the dimensions of the distal phalanges, and patients should be forewarned about potential complications [96].

Extensor Mechanism and Nerve Variations¶

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 [63]. Identification and protection of just a single branch of the dorsal cutaneous branch of the ulnar nerve is unlikely to ensure safe dissection during exposure of the dorsal and ulnar areas of the wrist because multiple branches are normally present [77]. 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 [24].

Vascular Anatomy¶

Vascular patterns of the palmar type of median artery as a source of the superficial palmar arch are important to hand surgeons [13].

Bony and Joint Anatomy¶

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 [99]. The ring finger and small metacarpal bases share a common articulation with the hamate [99]. The base of the fifth metacarpal can slide radially if the entire fourth metacarpal is excised, a possibility enhanced by the lack of tendinous attachments to the fourth metacarpal base [99]. The carpal bones are intraarticular, so the onset of symptoms for isolated intraosseous carpal lesions is generally associated with synovitis and joint invasion [91].

Key Evidence¶

[L5] This investigation provides a detailed anatomic study of the dorsal aponeurosis of the thumb with observation of both intrinsic and extrinsic contributions. [2] (10.1016/j.jhsa.2017.11.009)
[L4] To analyze a hand's load-distribution pattern, the opposite hand can be used as a reference. [4] (10.1016/j.jhsa.2018.02.016)
[L4] Measurement of individual finger forces can provide more accurate biomechanical models of the hand and determine the effect of disease on hand functions. [5] (10.1016/j.jht.2020.04.002)
[L5] Minimally invasive and percutaneous techniques can effectively release several structures known to cause finger stiffness with minimal damage to surrounding structures. [8] (10.1016/j.jhsa.2019.01.006)
[L5] The internal structure and material properties of the phalanges were found to play a significant role in both the magnitude and distribution of stresses. [11] (10.1007/s11552-012-9430-4)
[L4] These vascular patterns are important to hand surgeons. [13] (10.1007/s11552-009-9197-4)
[L4] The estimated lengths of principal ligaments stabilizing the CMC joint change substantially during thumb motions in vivo. [15] (10.1016/j.jhsa.2010.11.007)
[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. [16] (10.1177/15589447211060416)
[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. [23] (10.1007/978-3-642-22697-7_33)
[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. [24] (10.1016/j.jht.2023.01.002)
[L5] Active and passive intrinsic reconstruction methods improved basic grasp and release kinematics in experimental cadaver hand models. [25] (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. [29] (10.1177/1753193412461589)
[L2] There is no universal or typical load distribution pattern of the hand but only an individual pattern. [40] (10.1016/j.jht.2016.10.009)
[L5] The article reviews current evidence suggesting that thumb carpometacarpal joint osteoarthritis reflects the joint's teleology, where pathomechanics drive inflammatory markers and injury to soft tissue structures rather than simple wear and tear. [41] (10.1016/j.jhsa.2018.01.002)
[L5] A greater understanding of the anatomical variation of the pulley system is beneficial for hand surgeons performing open or percutaneous trigger finger releases and for radiologists examining the area sonographically. [43] (10.1177/17531934221101716)
[L5] The study confirms the existence of two constant dorsal branches originating from the proper palmar digital artery in the proximal and middle pulp spaces at predictable sites near the proximal interphalangeal joint, supporting the feasibility of using flaps based on these vessels. [44] (10.1054/jhsb.2002.0830)
[L5] The dorsal fibrocartilage of the MCP joint is a constant anatomical structure that appears to complement the structural support for the metacarpal head and extensor tendon. [45] (10.1016/j.jhsa.2015.03.019)
[L5] Although the extensor tendon-reflecting technique revealed the greatest amount of surface, nearly 50% of the proximal phalanx articular surface remained inaccessible as long as the collateral ligaments were intact. [46] (10.1016/j.jhsa.2014.02.004)
[L5] The data reveal further information regarding metacarpal dimensions of length, midshaft width, and articular surface areas of the head and base. [53] (10.1177/1558944719880026)
[L5] From a mechanical perspective, the metacarpophalangeal joint represents a joint with 5 kinematic degrees of freedom. [54] (10.1016/j.jhsa.2008.10.004)
[L5] Knowledge of this anatomy supports repair of radial SB injuries to prevent tendon instability. [62] (10.1016/j.jhsa.2008.01.039)
[L4] The authors emphasize that awareness of these variations is crucial for surgeons to avoid complications during procedures in the forearm, wrist, and hand. [63] (10.1007/s11552-013-9512-y)
[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. [71] (10.1177/1753193416642658)
[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. [77] (10.1016/j.jhsa.2013.03.016)
[L5] The distance between the digital-palmar and proximal interphalangeal creases may be used as a cutaneous landmark on the palmar surface for the exact location of the proximal edge of the A1 pulley in the palm of the hand, thereby ensuring greater safety in surgical procedures such as percutaneous release of trigger finger. [94] (10.1016/j.jhsa.2010.11.045)
[L5] Suture anchors should be used with caution in the little finger and patients forewarned about potential complications. [96] (10.1177/1753193411419595)
[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. [98] (10.1016/j.jhsa.2011.12.038)
[Case_report] A persistent median artery helped maintain blood flow to a nearly amputated hand after complete transection of the radial and ulnar arteries, preserving hand viability. [102] (10.1016/j.jhsa.2011.01.020)
[L4] In power grip, extrinsics provide the major gripping force while interossei act as phalangeal rotators and metacarpophalangeal flexors. [104] (10.2106/00004623-197052050-00001)
[L4] We also showed that, whereas total joint arthroplasty is able to restore thumb function, it cannot fully replicate the kinematics of the healthy TMC joint. [107] (10.1016/j.jhsa.2017.10.011)
[L1] This study reports impairment in the kinematics of precision pinch associated with index finger PIP joint fusion. [111] (10.1016/j.jhsa.2011.09.010)
[L4] The dynamic interaction of finger joints during cylinder grip shows specific patterns, with DIP joints consistently initiating flexion last and synchronization increasing significantly by the end of motion compared to the beginning. [112] (10.1177/1753193412444399)
[L5] Anatomic knowledge of the course of the superficial radial nerve and its branches is important during open release for avoiding nerve injury. [113] (10.1016/j.jhsa.2013.12.004)
[L5] This anatomical study demonstrated that perforating branches of the true digital arteries supply the palmar intermetacarpal flap constantly in all three finger intermetacarpal spaces. [114] (10.1177/1753193408095353)
[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. [115] (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. [116] (10.1016/j.jhsa.2025.10.015)
[L5] Extension minimizes volar capsular space volume and may be essential for preventing flexion contractures. [117] (10.1016/j.jht.2024.12.015)
[L4] Our clinical and neurophysiologic observations confirm that the ulnar nerve regularly contributes to flexion of the middle finger. [118] (10.1177/1753193413505581)
[L5] Using these anatomical origins and insertions, the study was able to restore the MCP flexion and stability seen in an intact ligament. [119] (10.1016/j.jhsa.2012.09.015)
[L5] This nutrient vessel is often the only vessel supplying the head of the proximal phalanx, making this area particularly susceptible to vascular compromise. [120] (10.1016/j.jhsa.2022.09.014)
[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. [121] (10.1177/1753193417692708)
[L5] The study demonstrates a consistent pattern of innervation of the thumb interphalangeal joint, provided dorsally by two branches of the radial nerve and palmarly by one branch each from the ulnar and radial proper digital nerves, which may provide the anatomical basis for effective and safe denervation. [123] (10.1177/1753193418771311)
[L4] The study confirms the presence of a significant dorsal branch of the ulnar digital nerve of the little finger arising at the level of the MCP joint, which remains relatively palmar compared to previous descriptions. [124] (10.1177/1753193408101468)
[L5] There is some evidence that abnormal joint motion caused by ligamentous and joint laxity may have a deleterious long-term effect clinically, and the influence of gender and hormonal environment is under investigation as a factor in evaluation and treatment of musculoskeletal disease. [126] (10.1016/j.jhsa.2008.09.012)
[L4] Volar elevation of the volar plate seen in active flexion could provide dynamic stresses on the adjacent ligaments and contribute to the stability and smooth gliding of the joint. [127] (10.1016/j.jhsa.2012.03.004)
[L4] Generally, the extensibility increased from distal to proximal and from ulnar to radial. [128] (10.1177/1753193419864881)
[L5] The 5 described techniques may provide for equal restraint to PIP joint hyperextension instability in the early postoperative period. [130] (10.1016/j.jhsa.2015.05.011)
[L5] The reliability of such flaps decreases from the second to the fifth metacarpal bone regarding the frequency of presence of the dorsal metacarpal arteries. [132] (10.1177/1753193411412872)
[L5] Harvesting a part of the ulnar or median nerve proximal to the third most distal segment of the ulnar nerve and the fourth most distal segment of the median nerve minimizes donor nerve deficits by ensuring the selection of intermingled motor and sensory fascicles. [133] (10.1177/1753193420901441)

References¶

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[118] The ulnar nerve consistently drives flexion of the middle finger. Journal of Hand Surgery (European Volume). 2013. DOI: 10.1177/1753193413505581

[119] Mechanics of an Anatomical Reconstruction for the Thumb Metacarpophalangeal Collateral Ligaments. The Journal of Hand Surgery. 2013. DOI: 10.1016/j.jhsa.2012.09.015

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[124] The anatomy of the ulnar digital nerve of the little finger: a cadaveric study. Journal of Hand Surgery (European Volume). 2009. DOI: 10.1177/1753193408101468

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[133] Safe level for harvesting for ulnar and median nerve transfers: a microanatomical and histological study. Journal of Hand Surgery (European Volume). 2020. DOI: 10.1177/1753193420901441

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Hand Anatomy and Biomechanics

Overview¶

Osseous Anatomy¶

General Skeletal Architecture¶

Metacarpals and Rays¶

Phalanges and Digital Joints¶

Thumb Carpometacarpal Joint¶

Ligaments and Joint Capsule¶

General Principles and Gliding Mechanisms¶

Thumb Carpometacarpal Joint¶

Metacarpophalangeal Joint¶

Proximal Interphalangeal Joint¶

Distal Interphalangeal Joint¶

General Ligament Properties and Clinical Implications¶

Muscles and Tendons¶

Extrinsic Extensor Muscles¶

Extrinsic Flexor Muscles¶

Intrinsic Muscles¶

Neurovascular Anatomy¶

Arterial Supply and Palmar Arches¶

Thumb Vascular Anatomy¶

Digital and Metacarpal Vascular Anatomy¶

Venous Drainage¶

Cutaneous Circulation and Thermoregulation¶

Nerve Anatomy and Innervation¶

Neurovascular Bundles in Dupuytren's Disease¶

Clinical Assessment of Vascular Status¶

Biomechanics and Function¶

General Principles and Architecture¶

Skeletal Mobility and Arches¶

Thumb Biomechanics¶

Tendon Gliding and Excursion¶

Muscle Mechanics and Joint Kinematics¶

Load Distribution and Grip Dynamics¶

Fracture Biomechanics and Deformity¶

Common Sites of Injury¶

Epidemiology and Mechanisms¶

Anatomical Vulnerability¶

Metacarpal Fracture Biomechanics¶

Thumb Fracture Pathoanatomy¶

Neurovascular and Soft Tissue Considerations¶

Surgical Anatomy¶

Digital and Thumb Soft Tissue Anatomy¶

Extensor Mechanism and Nerve Variations¶

Vascular Anatomy¶

Bony and Joint Anatomy¶

Key Evidence¶

See Also¶

References¶