2 Structural and Functional Anatomy of the Hand

2.1 Introduction

The human hand is the unique instrument that executes the commands of the brain and expresses the nuances of the mind. In performing these tasks it has to adopt an infinite variety of postures (▶Fig. 2.1). Some of these postures are purely expressive (▶Fig. 2.2), some are associated with varying degrees of touch (▶Fig. 2.3), and others involve manipulating objects to gather information (▶Fig. 2.4)—and then there are those that involve applying forces to an object (▶Fig. 2.5). In order to achieve these myriad of tasks, the hand has to be an adaptable device, a chameleon of all instruments that is able to assume multitudes of postures.

**Fig. 2.1** The hand can adopt a wide variety of postures.

**Fig. 2.2** Expressive posture of the hand.

**Fig. 2.3** Grasp with touch brings the sensate elements of the hand in contact with the object.

**Fig. 2.4** The grasping hand manipulates objects to collect information.

**Fig. 2.5** Grip applying force to an object.

Sir Charles Bell (1833) wrote in The Bridgewater Treatises, “attention to our most common actions will show us, how the division into fingers, by combining motion with the sense of touch adapts the hand to grasp, to feel and to compare.” ²

The shoulder joint functions as the attachment point of the upper extremity to the torso and has a circular arc of motion that keeps the hand in a circular cone within the visual field. It is amazing that the extent of the visual field mimics the limits of the shoulder’s ability to place the hand in space. However, with a combination of movements of all the joints of the upper extremity, the hand can be placed in positions that the eye cannot see. Thus, even in a normal sighted individual, the hand can act as a surrogate of the eye, exploring and gathering tactile information about objects and bodies beyond the visual field. ³

The essential function of the elbow and forearm motion acting in synchrony is to position the hand in space.

2.2 Prehension

In 1956, John Napier defined prehensile movements of the human hand as those motions in which “the object is seized and held within the compass of the hand.” He went on to further classify prehension into (1) power grip and (2) precision grip. ⁴

In power grip, the object is held in a clamp formed by the partly flexed fingers and the palm, with counter pressure applied by the thumb ray in the plane of the palm (▶Fig. 2.6). In precision grip, the object is pinched between the flexor aspects of the fingers and the opposing thumb (▶Fig. 2.7).

The factors that influence the posture of the hand during function include (a) the shape of the object; (b) the size of the object; (c) physical factors such as the weight, temperature, and wetness or dryness of the object; and (d) motivational factors such as fear or hunger.

John Napier in 1956 further outlined prehension as comprising the following:

Power grip: ability to apply forces and resist arbitrary forces that may be applied to the object (▶Fig. 2.8).
Precision handling: small adjustments of posture to control the direction in which the force is being applied (▶Fig. 2.9).

**Fig. 2.8** Combined power grip and precision handling.

Biomechanically, prehension can be defined as the application of functionally effective forces by the hand to an object given numerous constraints. ⁵

Functionally effective forces are able to match the anticipated forces in the task for a stable grasp, are able to impart motion to the object (i.e., manipulate it) or transport the object, and help in gathering sensory information about the state of interaction with the object during the task in order to ensure grasping and manipulative stability.

2.3 Surface Anatomy

Knowledge of surface anatomy is vital for the hand surgeon, as it is very important in the examination of the hand ³ ^, ⁶ ^, ⁷ (▶Fig. 2.10).

**Fig. 2.10** Surface markings of the palm. Kaplan’s cardinal line runs from the first web space in line with the abducted thumb parallel to the proximal palmar crease. The hook of the hamate lies on this line and is easily palpable. The motor branch of the ulnar nerve runs between the pisiform and the hook of the hamate then curves around the hook of the hamate. The superficial palmar arch lies 1 to 1.5 cm distal to the Kaplan’s cardinal line. The deep palmar arch lies just proximal to the Kaplan’s cardinal line. The radial digital nerve to the index finger lies on a line connecting the radial end of the basal crease of the index finger to the Kaplan’s line. The motor branch of the median nerve lies at the junction of a line from the radial border of the middle finger to the Kaplan’s line parallel to the digit. On a line from the ulnar border of the ring finger to the Kaplan’s line parallel to the digit lies the common digital nerve to the ring and small fingers. The ulnar digital nerve to the small finger lies on a line from the ulnar border of the small finger basal digital crease to the Kaplan’s line. The last two lines intersect the Kaplan’s line at the hook of the hamate.

The tendon of the flexor carpi ulnaris (FCU) can be readily palpated on the distal portion of the forearm on the ulnar side. As one goes distally palpating the FCU, one reaches a bony prominence that is the pisiform. The ulnar neurovascular bundle is located just on the radial side of the FCU tendon, the ulnar nerve being closer to the tendon and the ulnar artery located more radially. The hook of the hamate is located a fingerbreadth distal and radial to the pisiform and corresponds to the intersection of a line drawn from the center of the ring finger base to the distal wrist crease and a second line from the center of the base of the index finger to the pisiform. The deep motor branch of the ulnar nerve passes between the pisiform and the hamate hook and quickly runs deep to supply the interosseous muscles. The digital nerve to the small finger closely follows a line drawn from the hook of the hamate to the small finger palmar digital crease.

On the radial side one can easily palpate the tendon of the flexor carpi radialis (FCR). As one follows the tendon of the FCR, the first bony point that is palpable at the level of the wrist crease is the tubercle of the scaphoid. Distal to the tubercle of the scaphoid, the trapezial ridge can be palpated as can the mobile first carpometacarpal joint. The distal edge of the transverse carpal ligament spans the palm between the trapezial ridge and the hook of the hamate. The distal wrist crease marks the proximal border of the pisiform and hence the start of the transverse carpal ligament and the carpal tunnel. Kaplan’s cardinal line is a line extended on the palm along the ulnar border of the fully abducted thumb. This marks the distal aspect of the tunnel. The superficial palmar arch is located 1 cm distal to this line. The radial digital nerve to the index finger lies deep to a line connecting the trapezial ridge to the radial border of the proximal digital crease of the index finger.

The surface markings of the index, long, and ring finger metacarpophalangeal joints are along the distal palmar crease, whereas the index metacarpal head lies deep to the proximal palmar crease.

On the dorsum of the wrist, the Lister’s tubercle is a readily palpable bony marking. There is a soft spot just distal to the Lister’s tubercle. This marks the gap between the third and fourth dorsal compartments and is the site for the 3/4 arthroscopic portal. This area also marks the point that corresponds to the scapholunate joint.

On the ulnar side of the wrist, the ulnar head and the ulnar styloid are easily palpable, as is the extensor carpi ulnaris (ECU) tendon. The fourth and the fifth carpometacarpal joints are quite mobile and are easily palpable, as are the more stable second and third carpometacarpal joints.

2.4 Structure

The skeletal structure of the hand consists of 27 bones, of which 19 are long bones (▶Figs. 2.11a and 2.11b), 17 articulations, and 19 muscles. ³ ^, ⁷

**Fig. 2.11** (a) Dorsal and (b) palmar views of the hand skeleton.

The hand skeleton can be studied in the following manner:

2.4.1 The Rays of the Hand

The thumb is a highly mobile component of the hand. Special structures like the saddle-shaped trapeziometacarpal joint and the special arrangements of the intrinsic and extrinsic muscles of the thumb allow a great deal of freedom to this ray and the ability to place the thumb in a plane at 45° to the digits and move across them. Together with the index and middle fingers, the thumb is useful in the precision manipulation of objects by the hand.

The ulnar digits are mainly used in power grasp. All the four digits of the hand from index to small fingers converge toward the tubercle of the scaphoid in flexion. In extension, generally the middle finger is the longest, whereas in flexion the ring finger becomes the longest, thus exposing it to injuries. The ring finger flexor digitorum profundus (FDP) is commonly ruptured in “jersey finger” injuries due to this anatomical peculiarity.

The transverse axis of the palm corresponds to the metacarpophalangeal joints and is at 75° to the long axis of the palm (▶Fig. 2.12). This anatomical fact has design implications for glove design as well as during cast applications, allowing for appropriate movement of the metacarpophalangeal joints when the wrist is immobilized in a cast.

**Fig. 2.12** The transverse axis of the hand is at 75° to the long axis of the hand.

2.4.2 The Fixed and Mobile Elements

Littler in 1960 described the fixed and mobile elements of the hand ⁸ (▶Fig. 2.13). The fixed elements of the hand consist of the distal carpal row and the second and the third metacarpals. The carpometacarpal joints of the index and long fingers have very thick and strong ligaments, allowing little movement between the components.

The thumb is the most mobile component in the hand, as it is able to assume a wide variety of positions and can be made to touch each digit due to its peculiar anatomic arrangement.

The ulnar two metacarpal are very mobile due to rather lax carpometacarpal joints. The fourth carpometacarpal joint can move 20° while the fifth carpometacarpal joint can move about 44° ⁷ ^, ⁹ ^, ¹⁰ (▶Fig. 2.14).

**Fig. 2.14** The 4th and 5th metacarpals can move to cup the hand.

2.4.3 The Arches of the Hand (▶Fig. 2.15)

**Fig. 2.15** Arches of the hand. (Copyright Kleinert Institute, Louisville, KY.)

The arches of the hand consists of multiple longitudinal (one for each digit) (▶Fig. 2.16, ▶Fig. 2.17) and two transverse arches ³ ^, ⁷ (▶Fig. 2.18, ▶Fig. 2.19).

**Fig. 2.16** Each digit represents a longitudinal arch.

**Fig. 2.17** Longitudinal arches of the hand.

**Fig. 2.18** The carpal arch at the CMC joint.

**Fig. 2.19** The transverse metacarpal arch at the MC heads.

The metacarpophalangeal joint is the main joint of the longitudinal arch of the digit. It has very important ligaments that span across the joint and control its stability. The anatomy of this key joint is discussed in great detail in Chapter 35.

The carpal arch (▶Fig. 2.18) has a deep palmar concavity and is formed by the osseous masses of the proximal and distal carpal rows bounded on the dorsal and palmar side by thick ligaments. The spanning transverse carpal ligaments also maintain the arch. The capitate forms the keystone of this arch.

The transverse metacarpal arch is formed by the metacarpal heads, which are arranged in a curved configuration maintained by the arrangement of the interpalmar plate ligaments (IPPL) and the fibrous structures (▶Fig. 2.19). The second and the third metacarpal heads are very stable, whereas the fourth and the fifth metacarpals can move and increase the concavity of this arch by virtue of their mobility at the carpometacarpal joints. ³ ^, ⁷ ^, ⁹

2.4.4 The Fibrous Skeleton

The fibrous skeleton of the hand is very important for function and provides the following:

Stability
Mobility
Fixation of the skin
Containment
Partition
Protection and padding
Connection
Nourishment
Tendon guidance
Restraint

The shape of the joints (the PIP joints have bicondylar shape with inherent stability; the MP joints have great lateral stability in flexion as the joint surfaces are wider at the anterior aspect), the eccentric arrangement of the collateral ligaments, and the volar plates and joint capsule all provide stability to the finger joints.

Amazingly, the same components also provide mobility to the joints. The metacarpophalangeal joints are so shaped that on extension the narrow portion of the metacarpal head is in contact with the base of the proximal phalanx, thus providing great lateral mobility in extension. The situation changes when the joint flexes and the wider part of the metacarpal head comes in contact with the base of the proximal phalanx, locking the joint and making it more stable and less mobile. Similarly, the eccentric arrangement of the collateral ligaments results in these ligaments’ being tighter in flexion of the joint, enhancing stability, but lax in extension, providing mobility. The volar plates are thick cartilaginous structures that increase the joint surface yet are able to get out of the way in flexion of the joint. ³ ^, ⁷

The carpometacarpal joint of the thumb is structurally unstable and very mobile due to the lax arrangement of the ligaments and the unique saddle shape of the joint surfaces. The detailed anatomy of this joint is discussed in Chapter 34.

The retinacular ligaments of the hand anchor the glabrous skin of the hand to the hand skeleton. The superficial layer of the palmar aponeurosis is directly responsible for the tethering of the hand skin (▶Fig. 2.20). The skin creases represent attachment of the palmar aponeurosis with their vertical fibers to the skin without the intervention of adipose tissue. The septae of Legueu and Juvara are fibrous bands that anchor the palmar skin to the palmar plate of the metacarpophalangeal joint in the sagittal plane. ¹¹ They also provide containment and partition for the flexor tendons, as there are two septae on either side of each digital flexor tendon. (▶Fig. 2.21).

**Fig. 2.20** The superficial layer of the palmar aponeurosis.

**Fig. 2.21** Septa of Legueu and Juvara.

The natatory ligament part of the palmar fascia holds the web spaces in their fascinating three-dimensional configuration (▶Fig. 2.22). Further distally in the finger, on the lateral sides, Grayson’s ligaments on the palmar side and Cleland’s ligaments on the dorsal side of the neurovascular bundles anchor the skin to the digital skeleton ¹² (▶Fig. 2.23 and ▶Fig. 2.24).

**Fig. 2.23** Grayson’s and Cleland’s ligaments.

**Fig. 2.24** Natatory, Grayson’s, and Cleland’s ligaments. (Copyright Kleinert Institute, Louisville, KY.)

The dorsal hand skin is very lax due to loose areolar tissue and the lack of any firm attachment to the underlying skeleton. The mobility of the dorsal skin in both longitudinal and transverse directions is essential for full digital flexion. Since there is a lot of loose areolar tissue in this region, edema fluid can easily collect here, hyperextending the metacarpophalangeal joints and secondarily flexing the proximal interphalangeal joints. One of the ways to minimize this effect is to keep the metacarpophalangeal joints flexed in a splint. This stretches the dorsal skin and decreases any dead space. The mobility, space and compliance of the dorsal skin must be addressed if the surgeon wants good movement of the digits following correction of hand stiffness.

The compartments of the hand contain the structures ¹² . There are six hand compartments:

Thenar (flexor pollicis brevis, abductor pollicis brevis, and opponens pollicis)
Hypothenar (abductor digiti minimi, opponens digiti minimi, and flexor digiti minimi)
Adductor (adductor pollicis)
Interosseous: a. dorsal; b. palmar
Carpal tunnel
Digital

The flexor tendon pulley system ¹³ also acts as containment structure and is discussed in detail in Chapter 28.

Dorsally, around the extensor tendons, there are infratendinous and supratendinous fascia that form a sharp falx between each of the metacarpal heads proximal to the interdigital webs.

The very special glabrous skin of the palm provides protection and padding and is stabilized by deep fibrous connections of the palmar fascia especially in the mid palmar area. Fibrous septae contain the special fat in the thenar and hypothenar areas, over the metacarpophalangeal joints and over each phalanx. ³ This structural arrangement allows the hand to apply considerable grip forces to the object without the direct contact of the skeletal structures and the object. The palmaris brevis, palmaris longus, flexor carpi ulnaris, and abductor pollicis brevis inserting onto the skin also assist in this function.

The dorsal retinaculum and transverse carpal ligament in the wrist region, and the palmar pulleys in the digits allow connection, nourishment, tendon guidance, and restraint while at the same time providing mechanical advantage and a fulcrum to change direction.

In the wrist area, the dorsal retinaculum is a thick fibrous layer divided into compartments (▶Fig. 2.25) that contain the dorsal tendons, termination of the posterior interosseous nerve, and vascular bundles either inside the compartments or on its surface (intercompartmental supraretinacular vessels). From anatomy books that are based on diagrams, one gets the impression that the dorsal retinaculum is a flat and two-dimensional structure. On the contrary, careful and detailed dissection has shown that the dorsal retinaculum is a beautiful three-dimensional layered structure (▶Fig. 2.26). The ulnar attachment of the dorsal retinaculum is to the pisiform to which the flexor retinaculum is also attached (▶Fig. 2.27). The dorsal and palmar ligaments thus form a tight “watchband”-like structure around the wrist, with the pisiform being the anchor point (▶Fig. 2.28). The dorsal compartments guide the extensor tendons and keep them restrained.

**Fig. 2.25** The dorsal retinaculum, showing II, III, IV, V and VI compartment tendons.

**Fig. 2.26** The beautiful 3D layered structure of the dorsal retinaculum.

**Fig. 2.27** Palmar carpal ligament and the carpal tunnel.

**Fig. 2.28** The extensor and flexor retinaculum are both attached to the pisiform.

In the digits, the palmar pulleys perform a similar function for the finger tendons ¹³ (▶Fig. 2.29). They guide the digital tendons to their osseous attachment, keep them restrained, provide mechanical advantage for the flexors, and also provide nourishment by virtue of their collagen structure. ¹⁴ The dorsal third of the tendons are nourished by direct blood supply through the vincula longum and vincula breve (the long and short vincula) (▶Fig. 2.30). The palmar part, being mostly avascular, is nourished by diffusion of synovial fluid from the flexor tendon sheath. The collagen fibers of the flexor tendons are longitudinally orientated, while those of the dorsal surface of the annular pulleys are transversely oriented. On movement of the tendons in the pulley system, a latticework of longitudinal and transverse fibers traps and pumps the synovial fluid into the palmar third of the flexor tendons (the synovial pump) (▶Fig. 2.31).

**Fig. 2.29** The Palmar pulley system in the digit. (Copyright Kleinert Institute, Louisville, KY.)

**Fig. 2.30** Vincula longum and vincula breve.

**Fig. 2.31** The longitudinal collagen fibers of the flexor tendons and the transverse collagen fibers of the deep surface of the pulleys form a latticework and trap synovial fluid droplets that are then forced onto the palmar surface of the flexor tendons on movement of the digit.

2.5 Muscles and Tendons

2.5.1 Extensor Tendons

After passing through the 4th compartment, the four-extensor digitorum communis (EDC) tendons travel toward each digit (▶Fig. 2.32). The extensor indices (EI) tendon is located deeper and ulnar to the extensor communis to the index finger. The extensor digiti minimi (EDM) tendon travels in a separate (5th) compartment, as does the extensor pollicis longus (EPL) tendon (3rd compartment). ¹⁷ ^, ¹⁸ ^, ¹⁹

**Fig. 2.32** Anatomical arrangement of the extensors in the dorsum of the wrist, hand, and digits.

Proximal to the metacarpal head, strong intertendinous connections (juncturae tendinum) originate from the ring finger EDC and join the middle finger EDC and small finger EDC distally (▶Fig. 2.33). These juncturae limit the independent extension of the ulnar three digits. ²⁰ On flexing the metacarpophalangeal (MP) joint of the middle finger and/or small finger, the ring finger cannot be actively extended, as the individual extensor tendons pull the ring finger extensor distally through the juncturae, making it lax and thereby preventing active extension of that digit.

**Fig. 2.33** Anatomy of the juncturae tendinum.

With a completely lacerated EDC to the middle or small finger proximal to the juncturae tendinum, the patient may still be able to actively extend the digit with the help of the juncturae from the ring finger EDC.

Over the MP joints, the EDCs to the middle and ring finger are located in the midline, while in the index finger the EDC is on the radial side and the EI is on the ulnar side. In the small finger, the EDC is on the radial side and the EDM is on the ulnar side.

At the metacarpophalangeal (MP) joint level, a sheet of tissue surrounds each extensor tendon (▶Fig. 2.34a and ▶Fig. 2.34b). This sheet of tissue, the sagittal band, passes palmarly to attach to the palmar periosteum of the proximal phalanx and the volar plate of the MP joint. As it goes palmarly, it runs between the deep and the superficial parts of the intrinsic tendons (the deep part inserting to the base of the proximal phalanx and the superficial portion inserting into the dorsal expansion). The sagittal band or the dorsal hood thus forms a sling that helps lift the base of the proximal phalanx and thereby extends the MP joint. The sagittal band also stabilizes the extensor tendon over the MP joint, keeping the tendon centrally located. ¹

**Fig. 2.34** (a) Sagittal band, dorsal digital expansion and interosseous contribution at the MP Joint. (b) Detailed anatomy at the MCP joint with the MC head removed, showing the sagittal band passing between the superficial and deep interossei to insert into the palmar plate of the MP joint. (Copyright Kleinert Institute, Louisville, KY.)

The fibers of the sagittal band lie perpendicular to the long axis of the proximal phalanx. In hyperextension of the MP joint, the band migrates proximally, and in flexion of the MP joint, it translates distally (▶Fig. 2.35 and ▶Fig. 2.59).

**Fig. 2.35** The sagittal band and the interosseous hood.

The extrinsic extensor continues over the proximal phalanx, dividing and reuniting, receiving contributions from the interossei and lumbricals, forming a latticework of transverse and oblique fibers over the proximal phalanx, called the dorsal digital expansion or the interosseous hood. The continuous system of interlaced fascia of the sagittal bands and the dorsal digital expansion is also known as metacarpophalangeal extensor hood.

If the MP joint is hyperextended, the distal extensor is lax and is no longer able to extend the PIP joint. In the absence of a PIP joint extender like the interosseous or the lumbrical, and especially if both the flexors (FDS and FDP) are functioning (as happens in low ulnar nerve palsy), the finger assumes a significant claw posture. In these situations, if clinical examination demonstrates that the patient can actively extend the PIP joint on passive flexion of the MP joint (Bouvier positive), the clawing can be mitigated by a procedure that can keep the MP joint in flexion (MP volar capsulodesis or Zancolli lasso procedure).

Distal to the MP joint, the extensor tendon divides into three, one central slip and two lateral bands (▶Fig. 2.36). The central slip continues distally to insert onto the base of the middle phalanx (▶Fig. 2.37). The two lateral bands receive contributions from the interossei and on the radial side from the lumbrical (▶Fig. 2.38). The lateral bands unite to form the terminal extensor tendon inserting onto the base of the distal phalanx at the dorsal lip (▶Fig. 2.39). ²¹

**Fig. 2.36** Anatomy of the extensor mechanism in the finger. (Copyright Kleinert Institute, Louisville, KY.)

**Fig. 2.37** Detailed dissection of the extensor mechanism in the digit.

**Fig. 2.38** The interosseous contribution to the extensor.

**Fig. 2.39** The terminal extensor tendon of the finger.

The triangular ligament is bounded proximally by the distal insertion of the central slip, laterally by the lateral bands, and distally by the terminal extensor tendon. The triangular ligament holds the lateral bands in place, restraining them from translating volarly.

Additionally, there are two retinacular ligaments described by Landsmeer ²² that add to the functionality of the extensor mechanism. The oblique retinacular ligament of Landsmeer, arising from the distal third of the proximal phalanx and the A2 pulley, travels distally to join the lateral band (▶Fig. 2.40). It acts as a dynamic tenodesis helping coordinate the motion of the proximal and distal interphalangeal joints. Extension of the proximal interphalangeal joint places the ligament in tension and extends the distal interphalangeal joint. It is possible to fully flex the distal interphalangeal joint actively only when the proximal interphalangeal joint is flexed. The transverse retinacular ligament of Landsmeer originates from the edge of the flexor sheath at the PIP joint and attaches to the lateral extensor band. The transverse retinacular ligament prevents excessive dorsal translation of the lateral bands on PIP extension and pulls the lateral bands over the PIP joint with PIP flexion.

**Fig. 2.40** (a) The oblique and (b) transverse retinacular ligaments of Landsmeer.

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2 Structural and Functional Anatomy of the Hand