Hand
Innervation of Muscles, Skin, and Joints of the Hand
Interaction of the Extrinsic and Intrinsic Muscles of the Fingers
JOINT DEFORMITIES TYPICALLY CAUSED BY RHEUMATOID ARTHRITIS
Similar to the eye, the hand serves as a very important sensory organ for the perception of one’s surroundings (Figure 8-1). The hand is also a primary effector organ for our most complex motor behaviors, and the hand helps to express emotions through gesture, touch, music, and art.
Because of the hand’s enormous biomechanical complexity, its function involves a disproportionately large region of the cortex of the brain (Figure 8-2). Diseases or injuries affecting the hand often create a disproportionate disability. A hand totally incapacitated by rheumatoid arthritis, stroke, or nerve or bone injury, for instance, can dramatically reduce the function of the entire upper limb. This chapter describes the kinesiologic principles behind many of the musculoskeletal impairments of the hand frequently encountered in medical and rehabilitation settings. These principles often serve as the basis for treatment.
TERMINOLOGY
The wrist, or carpus, has eight carpal bones. The hand has five metacarpals, often referred to collectively as the “metacarpus.” Each of the five digits contains a set of phalanges. The digits are designated numerically from one to five, or as the thumb and the index, middle, ring, and small (little) fingers (Figure 8-3, A). A ray describes one metacarpal bone and its associated phalanges.
FIGURE 8-3. A palmar view of the basic anatomy of the hand. A, Major bones and joints. B, External landmarks.
The articulations between the proximal end of the metacarpals and the distal row of carpal bones form the carpometacarpal (CMC) joints (see Figure 8-3, A). The articulations between the metacarpals and the proximal phalanges form the metacarpophalangeal (MCP) joints. Each finger has two interphalangeal joints: a proximal interphalangeal (PIP) and a distal interphalangeal (DIP) joint. The thumb has only two phalanges and therefore only one interphalangeal (IP) joint.
Figure 8-3, B shows several features of the external anatomy of the hand. Note the palmar creases, or lines, that exist in the skin of the palm. They function as dermal “hinges,” marking where the skin folds on itself during movement, and to increase palmar skin adherence for enhancing the security of grasp. The location of the creases also serves as a useful clinical reference for the underlying anatomy. On the palmar (anterior) side of the wrist are the proximal and distal wrist creases. Of clinical interest is the fact that the distal wrist crease marks the location of the proximal margin of the underlying transverse carpal ligament. The thenar crease is formed by the folding of the dermis as the thumb is moved across the palm. The proximal digital creases are located distal to the actual joint line of the MCP joints. The distal and middle digital creases are superficial to the DIP and PIP joints, respectively.
OSTEOLOGY
Each metacarpal has similar anatomic characteristics (Figures 8-4 and 8-5). The first metacarpal (the thumb) is the shortest and stoutest; the second is usually the longest, and the length of the remaining three bones decreases from the radial to ulnar (medial) direction.
Each metacarpal has an elongated shaft with articular surfaces at each end (Figure 8-6). The palmar surface of the shaft is slightly concave longitudinally to accommodate many muscles and tendons in this region. Its proximal end, or base, articulates with one or more of the carpal bones. The bases of the second through the fifth metacarpals possess small facets for articulation with adjacent metacarpal bases.
With the hand at rest in the anatomic position, the thumb’s metacarpal is oriented in a different plane than the other digits. The second through the fifth metacarpals are aligned generally side by side, with their palmar surfaces facing anteriorly. The position of the thumb’s metacarpal, however, is rotated almost 90 degrees medially (i.e., internally), relative to the other digits (see Figure 8-3). Rotation places the very sensitive palmar surface of the thumb toward the midline of the hand. Optimum prehension depends on the thumb flexing in a plane that intersects, versus parallels, the plane of the flexing fingers. In addition, the thumb’s metacarpal is positioned well anterior, or palmar, to the other metacarpals (Figure 7-14). This position of the first metacarpal and trapezium is strongly influenced by the palmar projection of the distal pole of the scaphoid.
The medially rotated thumb requires unique terminology to describe its movement as well as position. In the anatomic position the dorsal surface of the bones of the thumb (i.e., the surface where the thumbnail resides) faces laterally (Figure 8-7). The palmar surface therefore faces medially, the radial surface anteriorly, and the ulnar surface posteriorly. The terminology to describe the surfaces of the carpal bones and all other digital bones is standard: a palmar surface faces anteriorly, a radial surface faces laterally, and so forth.
Phalanges
The hand has 14 phalanges (from the Greek root phalanx, a line of soldiers). The phalanges within each finger are referred to as proximal, middle, and distal (see Figure 8-3, A). The thumb has only a proximal and a distal phalanx.
Except for differences in sizes, all phalanges within a particular digit have similar morphology (see Figure 8-5). The proximal and middle phalanges of each finger have a concave base, shaft, and convex head. As in the metacarpals, their palmar surfaces are slightly concave longitudinally. The distal phalanx of each digit has a concave base. At its distal end is a rounded tuberosity that anchors the fleshy pulp of soft tissue to the bony tip of each digit.
Arches of the Hand
Observe the natural concavity of the palmar surface of your relaxed hand. Control of this concavity allows the human hand to securely hold and manipulate objects of many and varied shapes and sizes. This palmar concavity is supported by three integrated arch systems: two transverse and one longitudinal (Figure 8-8). The proximal transverse arch is formed by the distal row of carpal bones. This is a static, rigid arch that forms the carpal tunnel (see Chapter 7). Like most arches in buildings and bridges, the arches of the hand are supported by a central keystone structure. The capitate bone is the keystone of the proximal transverse arch, reinforced by multiple contacts with other bones, and strong intercarpal ligaments.
As depicted in Figure 8-8, all three arches of the hand are mechanically interlinked. Both transverse arches are joined together by a “rigid tie-beam” provided by the second and third metacarpals.27 In the healthy hand, this mechanical linkage reinforces the entire arch system. In the hand with joint disease, however, a structural failure at any arch may weaken another. A classic example is the destruction of the MCP joints from severe rheumatoid arthritis. This topic will be revisited at the end of this chapter.
ARTHROLOGY
Before progressing to the study of the structure and function of the joints, the terminology that describes the movement of the digits must be defined. The following descriptions assume that a particular movement starts from the anatomic position, with the elbow extended, forearm fully supinated, and wrist in a neutral position. Movement of the fingers is described in the standard fashion using the cardinal planes of the body: flexion and extension occur in the sagittal plane, and abduction and adduction occur in the frontal plane (Figure 8-9, A to D). The middle finger is the reference digit for naming abduction and adduction. The side-to-side movement of the middle finger is called radial and ulnar deviation.
Because the entire thumb is rotated almost 90 degrees in relation to the fingers, the terminology used to describe thumb movement is different from that for the fingers (see Figure 8-9, E to I). Flexion is the movement of the palmar surface of the thumb in the frontal plane across the palm. Extension returns the thumb back toward its anatomic position. Abduction is the forward movement of the thumb away from the palm in a near sagittal plane. Adduction returns the thumb to the plane of the hand. (Although not used in this text, other terms frequently used to describe the movements of the thumb include ulnar adduction for flexion, radial abduction for extension, and palmar abduction for abduction.) Opposition is a special term describing the movement of the thumb across the palm, making direct contact with the tip of any of the fingers. Reposition is a movement from full opposition back to the anatomic position. This special terminology used to define the movement of the thumb serves as the basis for the naming of the muscles that act on the thumb (e.g., the opponens pollicis, extensor pollicis longus, and adductor pollicis).
Carpometacarpal Joints
Figure 8-10 shows a mechanical illustration of the relative mobility at the CMC joints. The joints of the second and third digits are rigidly joined to the distal carpus, forming a stable central pillar throughout the hand. In contrast, the more peripheral CMC joints form mobile radial and ulnar borders, which are capable of folding around the hand’s central pillar. The function of the CMC joints allows the concavity of the palm to fit around many objects. This feature is one of the most impressive functions of the human hand. Cylindric objects, for example, can fit snugly into the palm, with the index and middle digits positioned to reinforce grasp. Without this ability, the dexterity of the hand is reduced to a primitive hingelike grasping motion.
SECOND THROUGH FIFTH CARPOMETACARPAL JOINTS
General Features and Ligamentous Support: The second CMC joint is formed through the articulation between the enlarged base of the second metacarpal and the distal surface of the trapezoid, and to a lesser extent the capitate and trapezium (see Figures 8-4 and 8-5). The third CMC joint is formed primarily by the articulation between the base of the third metacarpal and the distal surface of the capitate. The fourth CMC joint consists of the articulation between the base of the fourth metacarpal and the distal surface of the hamate and to lesser extent the capitate.70 The fifth CMC joint consists of the articulation between the base of the fifth metacarpal and the distal surface of the hamate only. (The hamate accepts both the fourth and fifth metacarpals, similar to the manner in which the cuboid bone of the foot accepts both the fourth and fifth metatarsals.) The bases of the second through fifth metacarpals have small facets for attachments to one another through intermetacarpal joints. These joints help stabilize the bases of the second through fifth metacarpals, thereby reinforcing the carpometacarpal joints.
All CMC joints of the fingers are surrounded by articular capsules and strengthened by multiple dorsal, palmar, and interosseous ligaments.70 The dorsal ligaments are particularly well developed (Figure 8-11).
Joint Structure and Kinematics: The CMC joints of the second and third digits are difficult to classify, ranging from planar to complex saddle joints (Figure 8-12).101 Their jagged interlocking articular surfaces, coupled with strong ligaments, permit very little movement. As mentioned earlier, stability at these joints forms the central pillar of the hand. The inherent stability of these radial-central metacarpals also provides a very firm attachment for several key muscles, including the extensor carpi radialis longus and brevis, the flexor carpi radialis, and the adductor pollicis.
The slightly convex bases of the fourth and fifth metacarpals articulate with a slightly concave articular surface formed by the hamate. These two ulnar CMC joints contribute a subtle but important element of mobility to the hand. As depicted in Figure 8-10, the fourth and fifth CMC joints allow the ulnar border of the hand to fold toward the center of the hand, thereby deepening the palmar concavity. This mobility—often referred to as a “cupping” motion—occurs primarily by flexion and “internal” rotation of the ulnar metacarpals toward the middle digit. Measurements of maximal passive mobility on cadaver hands have shown that, on average, the fourth CMC joint flexes and extends about 20 degrees and rotates internally about 27 degrees.21 The fifth CMC joint (when tested with the fourth CMC joint firmly constrained) flexes and extends about 28 degrees and rotates internally 22 degrees. The range of flexion and extension of the fifth CMC joint increases to an average of 44 degrees when the closely positioned fourth CMC joint is unconstrained and free to move. This research demonstrates the strong mechanical link between the kinematics of the fourth and fifth CMC joints. This point should be considered when evaluating and treating limitations of motion in this region of the hand.
The greater relative mobility allowed at the ulnar CMC joints is evidenced by the movement of the fourth and fifth metacarpal heads while clenching a fist (Figure 8-13). The increased mobility of the fourth and fifth CMC joints improves the effectiveness of grasp, as well as enhancing the functional interaction with the opposable thumb. The irregular and varied shapes of these CMC joint surfaces prohibit standard roll-and-slide arthrokinematic descriptions.
CARPOMETACARPAL JOINT OF THE THUMB
The CMC joint of the thumb is located at the base of the first ray, between the metacarpal and the trapezium (see Figure 8-7). This joint is by far the most complex of the CMC joints, enabling extensive movements of the thumb. Its unique saddle shape allows the thumb to fully oppose, thereby easily contacting the tips of the other digits. Through this action, the thumb is able to encircle objects held within the palm. Opposition greatly enhances the dexterity of human prehension.
Capsule and Ligaments of the Thumb Carpometacarpal Joint: The capsule at the CMC joint of the thumb is naturally loose to accommodate a large range of motion. The capsule, however, is strengthened by the action of ligaments and by the forces produced by the overriding musculature.
Many names have been used to describe the ligaments at the CMC joint of the thumb.6,20,37,80 The number of named, distinct ligaments reported to cross the base of the thumb ranges from three to perhaps as many as seven.72 This text focuses on five capsular ligaments, each adding an important element of stability to the CMC joint (Figure 8-14). As a set, the ligaments help control the extent and direction of joint motion, maintain joint alignment, and dissipate forces produced by activated muscle.76 Table 8-1 summarizes the major attachments of these ligaments and the motions that pull or wind them taut. In general, extension, abduction, and opposition of the thumb elongate most of the ligaments. Although all five ligaments listed in Table 8-1 are important stabilizers of the thumb’s CMC joint, the anterior oblique ligament warrants distinction.6,44,83 Rupture of this ligament secondary to severe arthritis or trauma often results in a radial dislocation of the joint, forming a characteristic “hump” at the base of the thumb.76
TABLE 8-1.
Ligaments of the Carpometacarpal Joint of the Thumb*
*Ligament names are based on attachment to trapezium surfaces, not the thumb metacarpal.
†Often described as having superficial and deep (“beak”) fibers.
FIGURE 8-14. Palmar and lateral views of the ligaments of the carpometacarpal joint of the right thumb.
SPECIAL FOCUS 8-1 
Osteoarthritis of the Carpometacarpal Joint of the Thumb: a Common and Potentially Disabling Condition
The large functional demand placed on the carpometacarpal (CMC) joint of the thumb is often considering a predisposing factor in the development of local osteoarthritis.76 This common condition receives more surgical attention than any other osteoarthritis-related condition of the upper limb.84 Osteoarthritis may develop at the CMC joint secondary to acute injury or, perhaps more commonly, from the cumulative low-level trauma associated with an arduous occupation or hobby.28,69 An interesting anthropologic study of skeletal remains from the United Arab Emirates (2300 bc) found that 20% of the sample had moderate to severe osteoarthritis of the CMC joint.16 This high incidence of degeneration was attributed to excessive use of the thumbs related to occupation.
Other factors besides stress and overuse can cause osteoarthritis at the base of the thumb, including genetics or subtle natural asymmetry of the articular surfaces. Regardless of the specific cause, persons who require medical attention for arthritis of the first CMC joint typically report pain as the foremost presenting symptom, but also experience joint instability.85 A painful and unstable base of the thumb markedly reduces the functional potential of the entire hand, and therefore the entire upper extremity. Persons with advanced osteoarthritis of the base of the thumb often develop a weakened pinch, osteophyte formation, swelling, subluxation or dislocation, and crepitation. The literature strongly suggests that idiopathic osteoarthritis of the CMC joint of the thumb is disproportionally more common in women, typically in their fifth and sixth decades.82,98 This gender-related propensity may be associated with postmenopausal-induced laxity in the joint’s ligaments. Studies also suggest that the trapeziums of women are, in general, less congruent and have a smaller surface area than those of men.48,114 These factors may also contribute to the higher incidence of arthritis in the CMC joints of women.
Common conservative therapeutic interventions for osteoarthritis of the thumb CMC joint include splinting; judicious use of exercise; physical modalities such as cold and heat; nonsteroidal anti-inflammatory drugs (NSAIDs); and corticosteroid injections.76 In addition, patients are instructed in ways to modify their activities of daily living to “protect” the base of the thumb from unnecessarily large forces.67
Surgical intervention is typically used when conservative therapy is unable to retard the progression of pain or instability.76,78,106 Surgery often involves arthroplasty with reconstruction of the damaged ligaments, such as the anterior oblique ligament. The joint is often stabilized by weaving the tendon of the flexor carpi radialis between and through adjacent bones.68 The degenerated trapezium may be left intact or replaced with a “spacer” consisting of a coiled tendon or other material. Although patients typically report significant improvement of function after surgery, long-term success is often hindered by the complex arthrokinematics coupled with the large forces that naturally occur at the CMC joint.78 In certain cases, partial or full fusion (arthrodesis) of the carpometacarpal joint is indicated, especially for young and physically active persons.
Saddle Joint Structure: The CMC joint of the thumb is the classic saddle joint of the body (Figure 8-15). The characteristic feature of a saddle joint is that each articular surface is convex in one dimension and concave in the other. The longitudinal diameter of the articular surface of the trapezium is generally concave from a palmar-to-dorsal direction. This surface is analogous to the front-to-rear contour of a horse’s saddle. The transverse diameter on the articular surface of the trapezium is generally convex in a medial-to-lateral direction—a shape analogous to the side-to-side contour of a horse’s saddle. The contour of the proximal articular surface of the thumb metacarpal has the reciprocal shape of that described for the trapezium (see Figure 8-15). The longitudinal diameter along the articular surface of the metacarpal is convex in a palmar-to-dorsal direction; its transverse diameter is concave in a medial-to-lateral direction.
Kinematics: The motions at the CMC joint occur primarily in two degrees of freedom. Abduction and adduction occur generally in the sagittal plane, and flexion and extension occur generally in the frontal plane. The axis of rotation for each plane of movement passes through the convex member of the articulation.38
Abduction and Adduction at the Thumb Carpometacarpal Joint: In the position of adduction of the CMC joint, the thumb lies within the plane of the hand. Maximum abduction, in contrast, positions the thumb metacarpal about 45 degrees anterior to the plane of the palm. Full abduction opens the web space of the thumb, forming a wide concave curvature useful for grasping large objects.
The arthrokinematics of abduction and adduction are based on the convex articular surface of the thumb metacarpal moving on the fixed concave (longitudinal) diameter of the trapezium (review Figure 8-15). During abduction, the convex articular surface of the metacarpal rolls palmarly and slides dorsally on the concave surface of the trapezium (Figure 8-16). Full abduction at the CMC joint elongates the adductor pollicis muscle and most ligaments at the CMC joint. The arthrokinematics of adduction occur in the reverse order from those described for abduction.
Flexion and Extension at the Thumb Carpometacarpal Joint: Actively performing flexion and extension of the CMC joint of the thumb is associated with varying amounts of axial rotation of the metacarpal. During flexion, the metacarpal rotates medially (i.e., toward the third digit); during extension, the metacarpal rotates laterally (i.e., away from the third digit). The “automatic” axial rotation is apparent by the change in orientation of the nail of the thumb between full extension and full flexion. This rotation is not considered a third degree of freedom because it cannot be executed independently of the other motions.
In the anatomic position the CMC joint can be extended an additional 10 to 15 degrees.15 From full extension the thumb metacarpal flexes across the palm about 45 to 50 degrees.
The arthrokinematics of flexion and extension at the CMC joint are based on the concave articular surface of the metacarpal moving across the convex (transverse) diameter on the trapezium (review Figure 8-15). During flexion, the concave surface of the metacarpal rolls and slides in an ulnar (medial) direction (Figure 8-17, A).38 A shallow groove in the transverse diameter of the trapezium helps guide the slight medial rotation of the metacarpal. Full flexion elongates tissues such as the radial collateral ligament.115
During extension of the CMC joint, the concave metacarpal rolls and slides in a lateral (radial) direction across the transverse diameter of the joint (see Figure 8-17, B). The groove on the articular surface of the trapezium guides the metacarpal into slight lateral rotation.15,51 Full extension stretches ligaments situated on the ulnar side of the joint, such as the anterior oblique ligament. Table 8-2 shows a summary of the kinematics for flexion-extension and abduction-adduction at the CMC joint of the thumb.
Opposition of the Thumb Carpometacarpal Joint: The ability to deliberately and precisely oppose the thumb to the tips of the other fingers is perhaps the ultimate expression of functional health of this digit—and arguably of the entire hand. This complex motion is a composite of the other primary motions already described for the CMC joint.57
For ease of discussion, Figure 8-18, A shows the full arc of opposition divided into two phases. In phase one, the thumb metacarpal abducts. In phase two, the abducted metacarpal flexes and medially rotates across the palm toward the small finger. Figure 8-18, B shows the detail of the kinematics of this complex movement. During abduction, the base of the thumb metacarpal takes a path in a palmar direction across the surface of the trapezium. During flexion–medial rotation, the base of this metacarpal turns slightly medially, led by the groove on the surface of the trapezium.115 Muscle force, especially from the opponens pollicis, helps guide and rotate the metacarpal to the medial side of the articular surface of the trapezium. The partially abducted CMC joint increases passive tension in most connective tissues associated with the CMC joint. Increased tension in the stretched posterior oblique ligament, for instance, promotes the medial rotation (spin) of the thumb metacarpal.115
As evidenced by the change in orientation of the thumbnail, full opposition incorporates 45 to 60 degrees of medial rotation of the thumb.13 The CMC joint of the thumb accounts for most but not all of this rotation. Lesser amounts of axial rotation occur in the form of accessory motions at the MCP and IP joints. The trapezium also medially rotates slightly against the scaphoid and the trapezoid, thereby amplifying the final magnitude of the metacarpal rotation.75 The small finger contributes indirectly to opposition through a cupping motion at the fifth CMC joint. This motion allows the tip of the thumb to more easily contact the tip of the small finger.
Full opposition is often considered the CMC joint’s close-packed position.63,101 This position is stabilized not only by a twisting of several ligaments, but by activation of muscle. Although maximum in full opposition, only about half of the surface area within the joint makes articular contact. Considering the large and frequent forces that cross this joint, the relatively small contact area may naturally predispose the joint to large and potentially damaging pressures.
Metacarpophalangeal Joints
General Features and Ligaments: The metacarpophalangeal (MCP) joints of the fingers are relatively large, ovoid articulations formed between the convex heads of the metacarpals and the shallow concave proximal surfaces of the proximal phalanges (Figure 8-19). Motion at the MCP joint occurs predominantly in two planes: flexion and extension in the sagittal plane, and abduction and adduction in the frontal plane.
FIGURE 8-19. The joints of the index finger.
Mechanical stability at the MCP joint is critical to the overall biomechanics of the hand. As discussed earlier, the MCP joints serve as keystones that support the mobile arches of the hand. In the healthy hand, stability at the MCP joints is achieved by an elaborate set of interconnecting connective tissues. Embedded within the capsule of each MCP joint are a pair of radial and ulnar collateral ligaments and one palmar plate (Figure 8-20). Each collateral ligament has its proximal attachment on the posterior tubercle of the metacarpal head. Crossing the MCP joint in an oblique palmar direction, the ligament forms two distinct parts. The more dorsal cord part of the ligament is thick and strong, attaching distally to the palmar aspect of the proximal end of the phalanx. The accessory part consists of fan-shaped fibers, which attach distally along the edge of the palmar plate.
Located palmar to each MCP joint are ligamentous-like structures called palmar (or volar) plates (see Figure 8-20). The term plate describes a composition of dense, thick fibrocartilage. The distal end of each plate attaches to the base of each proximal phalanx. At this region the plates are relatively thick and stiff. The thinner and more elastic proximal end attaches to the metacarpal bone, just proximal to the head. Fibrous digital sheaths, which form tunnels or pulleys for the extrinsic finger flexors, are anchored on the palmar (anterior) surface of the palmar plates. The primary function of the palmar plates is to strengthen the structure of the MCP joints, and limit the extremes of extension.
Figure 8-21 illustrates several anatomic aspects of the MCP joints. The concave component of an MCP joint is formed by the articular surface of the proximal phalanx, the collateral ligaments, and the dorsal surface of the palmar plate. These tissues form a three-sided receptacle aptly suited to accept the large metacarpal head. This structure adds to the stability of the joint while also increasing the area of articular contact. Attaching between the palmar plates of each MCP joint are three deep transverse metacarpal ligaments. The three ligaments merge into a wide, flat structure that interconnects and loosely binds the second through the fifth metacarpals.
Osteokinematics: In addition to the volitional motions of flexion-extension and abduction-adduction at the MCP joints, substantial accessory motions are possible. With the MCP joint relaxed and nearly extended, the ample passive mobility of the proximal phalanx relative to the head of the metacarpal can be appreciated. The joint can be distracted-compressed, translated in anterior-to-posterior and side-to-side directions, and axially rotated. The extent of passive axial rotation is particularly remarkable. These ample accessory motions at the MCP joints permit the fingers to better conform to the shapes of held objects, thereby increasing control of grasp (Figure 8-22). The range of this passive axial rotation at the MCP joints is greatest at the ring and small fingers, with average rotations of about 30 to 40 degrees.50
The overall range of flexion and extension at the MCP joints increases gradually from the second to the fifth digit: the second (index) flexes to about 90 degrees, and the fifth to about 110 to 115 degrees.4 The greater mobility allowed at the more ulnar MCP joints is similar to that expressed at the CMC joints. The MCP joints can be passively extended beyond the neutral (0-degree) position for a considerable range of 30 to 45 degrees. Abduction and adduction at the MCP joints occur to about 20 degrees on both sides of the midline reference formed by the third metacarpal.
Arthrokinematics: The head of each metacarpal has a slightly different shape but in general is rounded at the apex and nearly flat on the palmar surface (see Figure 8-6). Articular cartilage covers the entire head and most of the palmar surface. The convex-concave relationship of the joint surfaces is readily apparent (Figure 8-23). The longitudinal diameter of the joint follows the sagittal plane; the shorter transverse diameter follows the frontal plane.
The arthrokinematics at the MCP joint are based on the concave articular surface of the phalanx moving against the convex metacarpal head. Figure 8-24, A shows the arthrokinematics of active flexion, driven by one of the extrinsic flexor muscles: the flexor digitorum profundus. Flexion stretches and therefore increases the passive tension in both the dorsal capsule and the collateral ligaments. In the healthy state this passive tension helps guide the joint’s natural arthrokinematics.7 For example, as depicted in Figure 8-24, A, the increased tension in the stretched dorsal capsule (depicted by the thin elongated arrow) prevents the joint from unnaturally “hinging” outward on its dorsal side. The tension helps maintain firm contact between the articular surfaces as the proximal phalanx slides and rolls in a palmar direction. The increased tension in the dorsal capsule and collateral ligaments stabilizes the joint in flexion, which is useful during grasp.
Figure 8-24, B illustrates active extension of the MCP joint, driven through a coordinated coactivation of the extensor digitorum and one of the intrinsic muscles (to be further described later in this chapter). The arthrokinematics of extension are similar to those illustrated for flexion except that the roll and slide of the proximal phalanx occur in a dorsal direction. By 0 degrees of extension, the collateral ligaments have slackened while the palmar plate has elongated and unfolded to support the head of the metacarpal. The relative slackness created in the collateral ligaments accounts, in part, for the increased passive mobility (“play”) within the joint in the extended position. Extension beyond the 0-degree position is normally blocked by contraction of an intrinsic muscle, such as a lumbrical.
The arthrokinematics of abduction and adduction of the MCP joints are similar to those described for flexion and extension. During abduction of the index MCP joint, for instance, the proximal phalanx rolls and slides in a radial direction (Figure 8-25). The first dorsal interosseus muscle not only directs the arthrokinematics of abduction, but stabilizes the joint radially as the radial collateral ligament progressively slackens.7
The extent of active abduction and adduction at the MCP joints is significantly less when the motions are performed in full flexion compared with full extension. (This can be readily verified on your own hand.) Two factors can account for this difference. First, the collateral ligaments are taut near full flexion. Stored passive tension in these ligaments theoretically increases the compression force between the joint surfaces, thereby reducing available motion. Second, in the position of about 70 degrees of flexion, the articular surface of the proximal phalanges contacts the flattened palmar part of the metacarpal heads (see Figure 8-24, A). This relatively flat surface blocks the natural arthrokinematics required for maximal abduction and adduction range of motion.
SPECIAL FOCUS 8-2 
Clinical Relevance of the Flexed Position of the Metacarpophalangeal Joints of the Fingers
It has long been recognized that the flexed MCP joint is more stable and exhibits less passive, accessory movement than an extended joint.26 Accordingly, flexion is considered the MCP joint’s close-packed position.101 As described in Chapter 1, the close-packed position of most joints is that unique position at which accessory movements (joint “play”) are minimal and congruency within the joint is greatest. The close-packed position of the MCP joint is associated with increased tension in many of the surrounding ligaments. The increased tension in the collateral ligaments provides added stability to the base of the fingers during activities such as gripping or pinching, or the use of a key—activities that are typically performed in about 60 to 70 degrees of flexion.36
Although specific research is lacking on this topic, it is likely that the increased stability associated with flexion of the MCP joint is a result of, at least in large part, the elongation and subsequent stretch placed on the collateral ligaments. The stretch is caused by the eccentric or “out-of-round” cam shape of the metacarpal head. Because of this shape, flexion increases the distance between the proximal and distal attachments of the ligament (Figure 8-26).18
In some cases, however immobilizing the MCP joints in full flexion is contraindicated.7 For example, after surgical reconstruction of the dorsal capsule or implantation of a total joint arthroplasty, the MCP joints must be immobilized in an extended (near 0-degree) position. This position reduces the strain on the healing tissues located on the dorsal aspect of the joint.
THUMB
General Features and Ligaments: The MCP joint of the thumb consists of the articulation between the convex head of the first metacarpal and the concave proximal surface of the proximal phalanx of the thumb (Figure 8-27). The basic structure and arthrokinematics of the MCP joint of the thumb are similar to those of the fingers. Marked differences exist, however, in osteokinematics. Active and passive motions at the MCP joint of the thumb are significantly less than those at the MCP joints of the fingers. For all practical purposes, the MCP joint of the thumb allows only one degree of freedom: flexion and extension within the frontal plane.94 Unlike the MCP joints of the fingers, extension of the thumb MCP joint is usually limited to just a few degrees. The arthrokinematics of active flexion at the metacarpophalangeal joint of the thumb is illustrated in Figure 8-28. From full extension, the proximal phalanx of the thumb can actively flex about 60 degrees across the palm toward the middle digit.41
Although the limited abduction and adduction at the MCP joint provide some natural stability to thumb, the normally taut collateral ligaments at the joint are particularly vulnerable to injury from excessively large external torques. This is well exemplified by the relatively common “skier’s injury” in which the handle and strap of the ski pole of a falling skier create a large abduction torque against the MCP joint, damaging the joint’s ulnar collateral ligament. The rupture point of this ligament occurs at about 45 degrees of abduction.25 Furthermore, the ligament is most vulnerable to rupture when the abduction torque is applied with the MCP joint flexed to about 30 degrees, a scenario likely present at the time of the skiing accident.
Interphalangeal Joints
Distal to the MCP joints are the proximal and distal interphalangeal joints of the fingers (see Figure 8-27). Each joint allows only one degree of freedom: flexion and extension. From both structural and functional perspectives, these joints are simpler than the MCP joints.
General Features and Ligaments: The proximal interphalangeal (PIP) joints are formed by the articulation between the heads of the proximal phalanges and the bases of the middle phalanges. The articular surface of the joint appears as a tongue-in-groove articulation similar to that used in carpentry to join planks of wood (Figure 8-29). The head of the proximal phalanx has two rounded condyles separated by a shallow central groove. The opposing surface of the middle phalanx has two shallow concave facets separated by a central ridge. The tongue-in-groove articulation helps guide the motion of flexion and extension as it restricts axial rotation.
Each PIP joint is surrounded by a capsule that is reinforced by radial and ulnar collateral ligaments.101 The cord portion of the collateral ligament at the PIP joint significantly limits abduction and adduction motion. As with the MCP joint, the accessory portion of the collateral ligament blends with and reinforces the palmar plate (see Figure 8-29). The anatomic connections between the palmar plate and collateral ligaments form a secure seat for the head of the proximal phalanx.53 The palmar plate is the primary structure that limits hyperextension of the PIP joint.113 In addition, the palmar surface of the plate serves as the attachment for the base of the fibrous digital sheath—the structure that houses the tendons of the extrinsic finger flexor muscles (see index and small fingers, Figure 8-21).
The proximal lateral regions of each palmar plate at the PIP joints thicken longitudinally, forming a fibrous tissue referred to as check-rein ligaments (see Figure 8-29).101,113 These tissues reinforce the proximal attachments of the palmar plate, as well as assist in limiting hyperextension of the joint. When enlarged, check-rein ligaments are often considered a pathologic tissue and as such are often excised during surgical release of a flexion contracture at the PIP joint.
The distal interphalangeal (DIP) joints are formed through the articulation between the heads of the middle phalanges and the bases of the distal phalanges (see Figure 8-29). The structure of the DIP joint and the surrounding connective tissue are similar to those of the PIP joint, except for the absence of the check-rein ligaments.
Kinematics: The PIP joints flex to about 100 to 120 degrees. The DIP joints allow less flexion, to about 70 to 90 degrees. As with the MCP joints, flexion at the PIP and DIP joints is greater in the more ulnar digits. Minimal hyperextension is usually allowed at the PIP joints. The DIP joints, however, normally allow up to about 30 degrees of extension beyond the neutral (0 degree) position.
Similarities in joint structure cause similar arthrokinematics at the PIP and DIP joints. During active flexion at the PIP joint, for instance, the concave base of the middle phalanx rolls and slides in a palmar direction by the pull of the extrinsic finger flexors (Figure 8-30). During flexion, the passive tension created in the dorsal capsule helps guide and stabilize the roll-and-slide arthrokinematics.
In contrast to the MCP joints, passive tension in the collateral ligaments at the IP joints remains relatively constant throughout the range of motion.62 Perhaps the more spheric shape of the heads of the phalanges prevents a significant change in length in these collateral ligaments (see Figure 8-26).18 The close-packed position of the PIP and DIP joints is considered to be full extension,101 most likely because of the stretch placed on the palmar plates. During periods of immobilization of the hand, the PIP and DIP joints are often splinted in near extension. This position places a stretch on the palmar plates, reducing the likelihood of a flexion contracture developing in these joints.
THUMB
The structure and function of the interphalangeal (IP) joint of the thumb are similar to those of the IP joints of the fingers. Motion is limited primarily to one degree of freedom, allowing active flexion to about 70 degrees (see Figure 8-28).41 The IP joint of the thumb can be passively extended beyond neutral to about 20 degrees. This motion is often employed to apply a force between the pad of the thumb and an object, such as pushing a thumbtack into a board. The amount of passive hyperextension often increases throughout life owing to years of stretch placed on palmar structures, including the palmar plate.
SPECIAL FOCUS 8-3 
“Position of Function” of the Wrist and Hand
Some medical conditions, such as a traumatic head injury, stroke, or high-level quadriplegia, can result in a permanent deformity of the wrist and hand. The deformity is caused by a combination of long-term paralysis, disuse, or abnormal tone in the muscles. Clinicians therefore, often use splints that favor a position of the wrist and hand that maximally preserves functional potential. This splinted position, often called the position of function, is shown in Figure 8-31. This position provides a slightly opened and cupped hand, with the wrist in position to maintain optimal length of the finger flexor muscles.
MUSCLE AND JOINT INTERACTION
Innervation of Muscles, Skin, and Joints of the Hand
The highly complex and coordinated functions of the hand require a rich source of motor and sensory innervation of the region’s muscles, skin, and joints. Consider, for instance, the very precise and delicate movements of the digits performed by a concert violinist. One fact allowing such precision is that a single axon traveling to an intrinsic muscle of the hand, such as the thumb, may innervate as few as 100 muscle fibers.74 In this case, one axon would simultaneously activate all 100 muscle fibers. By contrast, a single axon traveling to the medial head of the gastrocnemius muscle—a muscle not involved with fine movements—may innervate about 2000 muscle fibers.24 The smaller fiber-per-axon ratio typical of most intrinsic muscles of the hand allows for a more precise gradation between levels of force, ultimately permitting finer control of movement.
MUSCLE AND SKIN INNERVATION
Innervation to the muscles and the skin of the hand is illustrated in Figure 6-32. The radial nerve innervates the extrinsic extensor muscles of the digits. These muscles, located on the dorsal aspect of the forearm, are the extensor digitorum, extensor digiti minimi, extensor indicis, extensor pollicis longus, extensor pollicis brevis, and abductor pollicis longus. The radial nerve is responsible for the sensation on the dorsal aspect of the wrist and hand, especially around the dorsal region of the thenar web space.
As a reference, the primary nerve roots that supply the muscles of the upper extremity are listed in Appendix II, Part A. In addition, Appendix II, Parts B to D include additional reference items to help guide the clinical assessment of the functional status of the C5-T1 nerve roots and several major peripheral nerves of the upper limb.
