The hand can assume almost countless positions and postures that allow it to perform numerous functions and manipulations.
The muscles of the hand permit it to perform tasks that require both great strength and delicate precision.
The skin of the hand, particularly that of the palm, is richly supplied with a large variety of sensory receptors that allow it to detect minute differences in texture and shape.
The joints and muscles of the hand contain large numbers of proprioceptive receptors that enable it to detect miniscule differences in position and thus perform precise manipulations extremely smoothly.
Osteology of the Hand
The bones of the hand form its framework and are important in maintaining its shape and providing a stable base on which to anchor its various soft tissue structures. The bones are arranged to maximize the functional efficiency of the intrinsic muscles and the tendons of the extrinsic muscles of the hand. The 19 major bones are of only two types: the metacarpals and the phalanges ( Fig. 1-1 ). All of these bones are classified as long bones and have central shafts and expanded proximal and distal ends (epiphyses). Additional small bones, sesamoids, are usually found in the tendons of certain intrinsic thumb muscles.
One metacarpal is associated with each digit, that of the thumb being considerably shorter than the others. These bones form the bony base of the hand, and their integrity is essential for both its natural form and function. Each bone has a dorsally bowed shaft with an expanded base (proximally) and head (distally) ( Fig. 1-2 ). From closely positioned bases, the bones diverge distally to their heads. This arrangement determines the shape of the hand and separates the digits so they can function independently as well as manipulate large objects. The metacarpal of the thumb is anterior to the others and rotated approximately 90 degrees so it is ideally positioned to oppose (see Fig. 1-1 ).
The shaft of each metacarpal is triangular in cross section, with the apex of this triangle directed volarly and composed of more dense bone than the dorsal aspect of the shaft. This concentration of dense bone reflects the significant compressile force on the flexor side of the bone. The overall shape of each metacarpal (along with that of the phalanges) contributes to the longitudinal arch of the hand . The dorsal convexities of the metacarpals along with their triangular cross sections provide significant room for the soft tissue of the palm, the bulk of which consists of the intrinsic interossei muscles and the more volarly positioned long digital flexor tendons and accompanying intrinsic lumbrical muscles. The mechanical advantage of these muscles is also enhanced by the metacarpal shape; their lines of pull are located volar to the flexion–extension axes of the metacarpophalangeal (MCP) joints.
The bases of the four medial metacarpals are irregular in shape and less wide volarly than dorsally, thus contributing to the proximal transverse arch ( Fig. 1-3 ). Articular surface is found on the sides as well as the proximal aspect of the base. The base of the thumb metacarpal is significantly different. The somewhat flattened proximal surface is in the shape of a shallow saddle , all of which is articular surface. The concave surface is oriented from medial to lateral; the convex from anterior to posterior. (Keep in mind that this bone is rotated about 90 degrees relative to the other metacarpals and this description is based on the anatomic position.) The most medial aspect of the base protrudes more proximally than the rest of the base and thus presents a triangular beak .
The heads of all the metacarpals are similar. The articular surface is rounded, both from side to side as well as dorsal to palmar. The side-to-side dimension is considerably shorter than the length from dorsal to palmar, but it is wider on the palmar aspect than it is dorsally. And importantly, the surface extends farther onto the volar aspect of the bone than dorsally. Prominent dorsal tubercles are found dorsally on each side of the head, just proximal to the articular surface.
The shapes of the metacarpals also contribute to the proximal and distal transverse arches of the hand (see Fig. 1-3 ). The proximal arch is at the level of the distal row of carpal bones and the bases of the metacarpals. The bases of the metacarpals as well as the distal row of carpals are wedge-shaped in cross section, and the apex of each wedge is directed volarly. Since the metacarpal bases and distal carpals are positioned very close to one another and are held tightly together, they collectively form a dorsal convexity and thus a side-to-side arch. The distal transverse arch is at the level of the metacarpal heads and is also a dorsal convexity. This arch is larger than the proximal arch and merely reflects the orientation of the metacarpals and the fact that the metacarpal heads are farther apart than their bases.
The hand contains 14 phalanges ; the thumb has only 2, whereas each of the other digits has 3. The proximal and middle phalanges , like the metacarpals, are bowed dorsally along their long axis and thus contribute to the longitudinal arch of the hand. The shafts of the phalanges serve as anchors for the long digital flexor tendons. The volar aspect of the shaft is flat from side to side and rounded dorsally. The junctions of the rounded and flat surfaces are marked by longitudinal ridges that serve as the attachments for the fibrous part of the digital tendon sheath (see Fig. 1-1 ). Each bone has an expanded epiphysis on each end, with the base (proximally) being larger than the head (distally).
The surface of the base of the proximal phalanx is biconcave and consists entirely of articular surface. The bases of both the middle and distal phalanges are concave from dorsal to ventral, with a central ridge oriented in the same direction. This surface is entirely articular surface. The heads of the proximal and middle phalanges are cylindrical from side to side with a central groove oriented perpendicular to the cylinder. This surface is also articular surface. The distal phalanx is shorter than the others. It has no head but rather ends in an expanded and roughened palmar elevation, which supports the pulp of the fingertip as well as the fingernail.
Articulations of the Hand
The carpometacarpal (CMC) joints are the most proximal joints in the hand and connect it to the wrist. Even though they are all synovial joints, the thumb CMC joint is significantly different from those of the four medial digits. The CMC joint of the thumb allows significant and complex motion; those of the other digits allow a small amount to virtually none.
The four medial joints are between the bases of the four medial metacarpals and the distal row of carpal bones: the trapezium, trapezoid, capitate, and hamate. The articular surfaces of both sets of bones are irregular, continue on the medial and lateral aspects of the metacarpal bases and the carpals, but are quite congruent so the bones fit closely together. Each metacarpal base articulates with one, two, or even three carpal bones. Strong ligaments hold all of the bones tightly together, both side to side and across the CMC joint space. A single joint capsule encloses all of these joints so there is a single synovial cavity. This cavity extends not only across the span of the collective joints but also somewhat distally between the metacarpal bases and proximally between the distal carpal bones.
The motion available at these joints is variable and minimal. There is essentially no motion permitted at the CMC joints of the index and middle fingers. These two metacarpals along with the distal carpal row form the rigid and stable central base of the hand. A small amount of motion is permitted at the CMC joints of the ring and small fingers. This motion, primarily a bit of flexion, permits slight cupping of the medial side of the hand and is important in both manipulation and grip ( Fig. 1-4 ).
The first CMC (trapeziometacarpal) joint is between the base of the first metacarpal and the trapezium . Since the thumb articulates with only the trapezium, its location and orientation is the basis for the position of the thumb. The trapezium is obliquely oriented, almost in the sagittal plane, and projects more volarly than the trapezoid or scaphoid with which it articulates.
The articular surfaces ( Fig. 1-5 ) of both the base of the first metacarpal and the distal aspect of the trapezium are shaped like shallow saddles. As a result, each surface has a convex and a concave component, and these elements are perpendicular to one another. The shapes dictate that the major amount of motion occurs in two planes, which also are perpendicular to one another. Motion in the coronal plane , where the thumb moves across the palm, is flexion and extension. These motions occur as the concave surface of the metacarpal base moves on the convex surface of the trapezium. Motion in the sagittal plane , where the thumb moves toward and away from the index finger, is adduction (toward) and abduction (away). This occurs as the convex surface of the metacarpal base moves on the concave surface of the trapezium. Since both saddles are shallow and the soft tissue restraints are somewhat lax, axial rotation is also permitted. This rotation, opposition (pronation), occurs primarily at this first CMC joint and represents an essential ingredient for the usefulness of the thumb. Retroposition (supination) is the opposite of opposition. In reality, certain motions are coupled. Abduction is accompanied by a bit of medial rotation (opposition). This is due to the slightly curved concave surface of the trapezium. Retroposition, then, is a combination of lateral rotation and adduction. Flexion and extension also involve some rotation, albeit less. Flexion is accompanied by a bit of opposition and extension by a bit of retroposition. This is caused by the slightly curved convex surface of the trapezium. Hanes suggested the coupling was due to the tautness of certain of the ligaments of the joint; Zancolli and colleagues considered the coupling was due both to the articular surfaces and the ligaments.
The ligaments ( Figs. 1-6 and 1-7 ) of this joint are found on all sides of the joint. Their nomenclature can be confusing because several systems are used to name them and there are differences of opinion relative to how many ligaments there are. The anterior oblique, or beak, ligament is a strong ligament that interconnects the palmar tubercle (beak) of the metacarpal base and the distal part of a ridge on the tubercle of the trapezium. This ligament is generally considered a major stabilizing ligament of the joint and is taut in abduction, extension, and opposition. Bettinger and coworkers described a superficial anterior oblique ligament and a deep anterior oblique ligament, which they considered the beak ligament. The ulnar collateral ligament is on the volar and medial aspects of the joint and extends from the transverse carpal ligament to the palmar-medial aspect of the first metacarpal base. The posterior oblique ligament is on the dorsal aspect of the joint and interconnects the dorsal aspect of the trapezium and the ulnar (medial) base of the metacarpal. An intermetacarpal ligament (or pair of intermetacarpal [anterior and posterior] ligaments) interconnects the bases of the first and second metacarpals. The dorsoradial ligament extends from the dorsolateral aspect of the trapezium to the dorsal base of the first metacarpal. The joint capsule is complete and somewhat loose, which is necessary for axial rotation.
The metacarpophalangeal (MCP) joints ( Fig. 1-8 ) of the four medial digits are formed by the bases of the proximal phalanges and the heads of the metacarpals. The articular surface of the metacarpal head is biconvex, cam-shaped so it extends farther volarly than dorsally, and it is wider volarly than dorsally. The articular surface of the phalangeal base is biconcave, shallow and smaller in area than the articular surface of the metacarpal head. These shapes would appear to permit the phalanx to move in virtually any plane on the metacarpal head. However, due to soft tissue restraints, active motion is limited to flexion and extension and adduction and abduction. Adduction is movement of the digits toward the middle finger; abduction is movement away from the middle finger. The middle finger can be deviated either radially (laterally) or ulnarly (medially). Axial rotation is available only passively.
The joint capsule of the MCP ( Fig. 1-9 ) joint is highly specialized. Like any capsule it encloses the joint space and attaches to the edges of both articular surfaces. It is different in that its volar aspect is formed by a strong plate of fibrocartilage— palmar ligament, or volar plate. The medial and lateral edges of the plate serve as attachments for the fibrous part of the digital tendon sheath, specifically the first annular ligament (A1 pulley). Thus, the plate is important in the stability and positioning of the long digital flexor tendons. The plate is thick and rigid distally and its volar aspect has a thin side-to-side attachment to the volar base of the proximal phalanx. This hingelike attachment allows the plate to move as a unit relative to the proximal phalanx. Proximally the plate thins, is a bit loose and flexible, and attaches to volar base of the metacarpal head. With flexion the volar plate slides proximally ( Fig. 1-10 ); this is possible because the proximal part of the plate can fold.
The collateral ligament (see Fig. 1-10 ) is triangular in shape and consists of two distinct parts, both of which attach proximally to the dorsal tubercle of the metacarpal. From that attachment, the fibers of the ligament diverge as they pass distally. The true, or band, part of the ligament extends more distally and is the strongest part of the ligament. From the dorsal tubercle it passes obliquely volarly and attaches to the volar aspect of the side of the proximal phalangeal base. This true ligament is somewhat loose in extension and thus permits abduction and adduction. As the proximal phalanx is flexed, this part tightens because of the cam shape of the metacarpal head and because the metacarpal head is wider volarly. As a result of the tightness, abduction and adduction are very limited in flexion. The accessory, or fan, part of the ligament is more obliquely oriented and attaches to the volar plate. Since the fibrous tendon sheath also attaches to the volar plate, the accessory collateral ligament plays an important role in stabilizing the long digital flexor tendons. The accessory ligament loosens slightly as flexion occurs.
The MCP joints are reinforced dorsally and laterally by the extensor hood (see Fig. 1-20 , online). This hood consists of a flat layer of fibers that is oriented perpendicular and oblique to the long axis of the digit and sweep around the joint from one edge of the volar plate to the other. The fibers on either side of the joint are in the sagittal plane and called the sagittal bands . The hood blends with the long digital extensor tendon, slides proximally and distally, respectively, with extension and flexion, and is the mechanism through which the proximal phalanx is extended. The hood is also important in centralizing the extensor tendons at the MCP joint.
The MCP joint of the thumb is both similar to and different from the other MCP joints. The articular surfaces and collateral ligaments are quite similar. In general, the joint capsule is similar but part of it, the volar plate, varies. The volar plate contains two sesamoids bones, which form a trough for the tendon of the flexor pollicis longus muscle. The sesamoids are also partial insertions for the adductor pollicis muscle on the ulnar side and the flexor pollicis brevis muscle on the radial side. The more superficial layer of fibrous support is a somewhat modified extensor hood . The ulnar side of the hood is stronger and heavier than the radial side and formed by the tendon and aponeurosis of the adductor pollicis muscle. It extends dorsally to blend with the tendons of the extensor pollicis brevis and extensor pollicis longus muscles. The radial side of the hood is formed by the tendons of the abductor pollicis brevis and flexor pollicis brevis, which also blend with the extensor pollicis brevis and extensor pollicis longus tendons dorsally. The aponeurosis on the ulnar side forms a strong restraint against abduction forces. However, since the thumb is in a different plane than the other digits it is more vulnerable to adduction and abduction forces.
The motion available at the thumb MCP is similar in direction to the other MCP joints but more limited because of the stability of the joint. Flexion and extension are less free, and adduction and abduction are significantly more limited. However, motion varies considerably from person to person so possible limitation should be compared with motion on the opposite side.
The proximal interphalangeal (PIP) joint ( Fig. 1-11 ) is formed by the head of the proximal phalanx, which is shaped like a short transverse cylinder, and the base of the middle phalanx, which is concave from dorsal to ventral and thus conforms to the cylindrical head. In addition, the phalangeal head has a sagittally oriented groove and the phalangeal base has a sagittally oriented ridge. These surfaces enhance the stability of the joint and ensure that the motion is limited to one degree of freedom, which is in the sagittal plane (flexion and extension).
The joint capsule is similar to that of the MCP joint. It is reinforced by the volar plate palmarly, the collateral and retinacular ligaments and the lateral bands on both sides, and the triangular membrane and central band dorsally. These structures blend with the capsule to different degrees and thus move (glide) differently relative to the capsule and to each other.
The volar plate ( Fig. 1-12 ) is similar to that of the MCP joint and moves in the same way during flexion and extension. The sides of the proximal attachment are longer than the central part and are referred to as the “ check-rein ligaments .” These ligaments tighten as the middle phalanx is extended and thus limit hyperextension at the PIP joint. The volar plate is also the attachment for the third annular ligament (A3 pulley) of the fibrous flexor digital tendon sheath. This pulley attaches along the sides of the plate and ensures the flexor tendons stay in place as they cross the joint. The stability of this plate is therefore essential for proper flexor tendon position and function.
The collateral ligaments (see Figs. 1-11 and Fig. 1-12 ) are similar to those of the MCP joints, are triangular in shape, and consist of true (band) and accessory (fan) parts. From their attachment to the dorsal tubercle of the proximal phalanx, the two parts diverge as they cross the joint—the true part attaching to the side of the base of the middle phalanx and the accessory part attaching to the volar plate. The true part is taut throughout the range of motion and thus stabilizes the joint in all positions; the accessory part stabilizes the volar plate.
Like the MCP joints, the PIP joints are reinforced to some degree by components of the extensor mechanism. The central band and triangular membrane are positioned dorsally, and the lateral band and retinacular ligament located on the sides. The tendons of both the flexor digitorum profundus and flexor digitorum superficialis pass volar to the joint.
The distal interphalangeal (DIP) joint is quite similar to the PIP joint. The architecture of the articular surfaces is similar, so the motion is limited to only the sagittal plane and that is flexion and extension. The joint capsule, volar plate, and collateral ligaments are also similar, so the motion of each and the support they provide are very much the same as the PIP joints. The volar plate provides an attachment for the fibrous part of the flexor digital tendon sheath; in this case it is the fifth annular ligament (A5 pulley).
The extra-articular structures that cross the joint are quite different. Only the tendon of the flexor digitorum profundus crosses its volar aspect. Dorsally, only the central band blends with the joint capsule as it crosses the joint.
Skin, Retinacular System, and Compartmentation of the Hand
The skin on the dorsum of the hand is different from that on the palmar aspect. The dorsal skin is thin, loose, and quite mobile. This mobility is due to a very thin subcutaneous tissue (superficial fascia) that is loosely attached to the deep fascia. The palmar skin is thicker and less mobile. The subcutaneous tissue of the thenar and hypothenar eminences is thick and fatty and thus forms considerable pads. Centrally the palmar skin is firmly attached to the palmar aponeurosis by multiple septa and is thus almost immobile. This arrangement greatly enhances grasp.
The entire upper limb is enclosed in a sleeve of connective tissue called the investing fascia . In the arm and forearm this layer is connected medially and laterally to the bones by intermuscular septa with resulting anterior and posterior compartments. This same layer continues into the hand, where it becomes a complex system of fibrous layers and septa that form multiple compartments. Structures of similar function are isolated and confined to individual compartments. Since a retinaculum is a structure (usually composed of connective tissue) that retains other anatomic structures, this is called the retinacular system .
At the wrist the investing fascia is reinforced by circumferential bands of fibers both dorsally (extensor retinaculum) and volarly (flexor retinaculum). Both of these retinacula stabilize tendons that enter the hand from the forearm. The flexor retinaculum has a more proximal superficial part , the superficial part of the flexor retinaculum or the volar carpal ligament, and a deeper distal part called the deep part of the flexor retinaculum or the transverse carpal ligament. The deep part forms the volar boundary of the carpal tunnel and is significantly thicker and stronger.
In the hand the investing fascia attaches to both the first and fifth metacarpals ( Fig. 1-13 ). Dorsally it is thin, attaches to the other metacarpals, and is called the dorsal interosseous fascia . In the palm it is thin over the thenar (thenar fascia) and hypothenar (hypothenar fascia) eminences. Centrally it is greatly thickened to form the palmar aponeurosis .
This palmar aponeurosis (palmar fascia) is a strong fibrous structure composed of fibers that are oriented from proximal to distal. It is narrow proximally where it is continuous with the tendon of the palmaris longus muscle and blends with the transverse carpal ligament. It widens as it is followed distally, and just proximal to the MCP joints it separates into four digital slips, which contribute to the formation of the fibrous digital tendon sheaths. The digital slips are interconnected by transverse fasciculi proximally and the transversely oriented superficial transverse metacarpal ligament at the level of the MCP joints. The palmar aponeurosis is firmly attached to the skin by multiple septa and to the metacarpals by several septa.
Additional fibrous layers separate various structures in the palm and define four definitive compartments. The thenar septum extends from the junction of the thenar fascia and the palmar aponeurosis to the first metacarpal and with the thenar fascia forms the thenar compartment . Similarly, on the ulnar side of the hand, the hypothenar septum extends from the junction of the hypothenar fascia and the palmar aponeurosis to the fifth metacarpal and with the hypothenar fascia forms the hypothenar compartment . A deep layer crosses the palm, attaching to the first, third, fourth, and fifth metacarpals. This adductor–interosseous fascia , together with a dorsal interosseous fascia that interconnects all of the metacarpals dorsally, forms the adductor –interosseous compartment, which more or less is between the metacarpals. The central area of the palm, the central compartment , is deep to the palmar aponeurosis, bounded medially and laterally by the hypothenar and thenar septa, respectively, and limited deeply by the adductor–interosseous fascia. Like the compartments in the arm and forearm, these compartments contain muscles that have similar function and are innervated by one or two nerves. The contents of the compartments are listed in Table 1-1 (online).
|Thenar||Flexor pollicis brevis, abductor pollicis brevis, opponens pollicis, tendon of flexor pollicis longus, radial bursa|
|Hypothenar||Flexor digiti minimi, abductor digiti minimi, opponens digiti minimi|
|Adductor–interosseous||All (4) dorsal interossei, all (3) palmar interossei, adductor pollicis|
|Central||All (4) lumbricals; tendons of flexor digitorum superficialis and profundus, ulnar bursa; superficial palmar arterial arch|