Wrist

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Chapter 7

Wrist

The wrist, or carpus, contains eight carpal bones that, as a group, act as a functional “spacer” between the forearm and hand. In addition to numerous small intercarpal joints, the wrist consists of two primary articulations: the radiocarpal and midcarpal joints (Figure 7-1). The radiocarpal joint is located between the distal end of the radius and the proximal row of carpal bones. Just distal to this joint is the midcarpal joint, joining the proximal and distal rows of carpal bones. The two joints allow the wrist to flex and extend and to move from side to side in motions called radial and ulnar deviation. The nearby distal radio-ulnar joint is considered part of the forearm complex rather than the wrist because of its role in pronation and supination (see Chapter 6).

The position of the wrist significantly affects the function of the hand. This is because many muscles that control the digits originate extrinsic to the hand, with their proximal attachments located in the forearm. A painful, unstable, or weak wrist often assumes a position that interferes with the optimal length and passive tension of the extrinsic musculature, thereby reducing the effectiveness of grasp.

Several new terms are introduced here to describe the relative position, or topography, within the wrist and the hand. Palmar and volar are synonymous with anterior; dorsal is synonymous with posterior. These terms are used interchangeably throughout this chapter and the next chapter on the hand.

OSTEOLOGY

Distal Forearm

The dorsal surface of the distal radius has several grooves and raised areas that help guide or stabilize the tendons that course toward the wrist and hand (Figure 7-2). For example, the palpable dorsal (Lister’s) tubercle separates the tendon of the extensor carpi radialis brevis from the tendon of the extensor pollicis longus.

The palmar or volar surface of the distal radius is the location of the proximal attachments of the wrist capsule and the thick palmar radiocarpal ligaments (Figure 7-3, A). The styloid process of the radius projects distally from the lateral side of the radius. The styloid process of the ulna, sharper than its radial counterpart, extends distally from the posterior-medial corner of the distal ulna.

The distal articular surface of the radius is concave in both medial-lateral and anterior-posterior directions (see Figure 6-26, B). Facets are formed in the articular cartilage from indentations made by the scaphoid and lunate bones of the wrist.

Fractures of the distal end of the radius with a dorsal displacement of the distal fragment are very common. A frequent mechanism for this injury is a fall over an outstretched hand. A fracture that heals in an abnormally aligned fashion can significantly alter the congruence, or fit, of both the distal radio-ulnar joint and the radiocarpal joint of the wrist.36,59 Depending on the nature of the incongruence, the joints may become unstable (especially the distal radio-ulnar joint) or may develop degenerative arthritis from altered contact pressure at the articular surfaces.

Abnormal alignment of the distal radius also can change the relationship between the axis of rotation of the forearm and the interosseous membrane. If the misalignment is severe, the interosseous membrane may restrict the full extent of pronation or supination.36

The distal end of the radius has two configurations of biomechanical importance. First, the distal end of the radius angles about 25 degrees toward the ulnar (medial) direction (Figure 7-4, A). This ulnar tilt allows the wrist and hand to rotate farther into ulnar deviation than into radial deviation. As a result of this tilt, radial deviation of the wrist is limited by impingement of the lateral side of the carpus against the styloid process of the radius. Second, the distal articular surface of the radius is angled about 10 degrees in the palmar direction (see Figure 7-4, B). This palmar tilt accounts, in part, for the greater amounts of flexion than extension at the wrist.

Carpal Bones

From a radial (lateral) to ulnar direction, the proximal row of carpal bones includes the scaphoid, lunate, triquetrum, and pisiform. The distal row includes the trapezium, trapezoid, capitate, and hamate (Figures 7-2 and 7-3).

The proximal row of carpal bones is joined in a relatively loose fashion. In contrast, the distal row of carpal bones is bound tightly by strong ligaments, providing a rigid and stable base for articulation with the metacarpal bones.

The following section presents a general anatomic description of each carpal bone. The ability to visualize each bone’s relative position and shape is helpful in an understanding of the ligamentous anatomy and wrist kinematics.

SCAPHOID

The naming of the scaphoid is based on its vague resemblance to a boat (scaphoid from the Greek skaphoeides, like a boat). Most of the “hull” or undersurface of the boat rides on the radius; the cargo area of the “boat” is filled with part of the head of the capitate (see Figure 7-3, A). The scaphoid contacts four carpal bones and the radius.

The scaphoid has two convex surfaces called poles. The proximal pole articulates with the scaphoid facet of the radius (see Figure 6-26, B). The distal pole has a slightly rounded surface, which articulates with the trapezium and trapezoid. The distal pole projects obliquely palmarly, which can be well appreciated from a sagittal plane slice provided by magnetic resonance imaging (MRI) (see Figure 7-3, B). The distal pole has a blunt tubercle, which is palpable at the palmar base of the thenar musculature. Because of its elongated shape, the scaphoid is functionally and anatomically associated with both rows of carpal bones.7

The distal-medial surface of the scaphoid is deeply concave to accept the lateral half of the prominent head of the capitate bone (see Figure 7-3, A). A small facet on the scaphoid’s medial side articulates with the lunate. This articulation, reinforced primarily by the scapholunate ligament, provides an important mechanical link within the proximal row of carpal bones—a point to be revisited later in this chapter.

LUNATE

The lunate (from the Latin luna, moon) is the central bone of the proximal row, wedged between the scaphoid and triquetrum. The lunate is the most inherently unstable of the carpal bones, in part because of its shape, but primarily because of its lack of firm ligamentous attachments to the relatively rigid capitate bone.

Like the scaphoid, the lunate’s proximal surface is convex, fitting into the concave facet on the radius (see Figure 6-26, B). The distal surface of the lunate is deeply concave, giving the bone its crescent moon–shaped appearance (see Figure 7-3, A). This articular surface accepts two convexities: the medial half of the head of the capitate and part of the apex of the hamate.

SPECIAL FOCUS 7-1   imageScaphoid and Lunate: Vulnerability to Injury and Clinical Complications

It is likely that more has been written in the medical literature on the scaphoid and lunate than on all other carpal bones combined. Both bones are lodged between two rigid structures: the distal forearm and the distal row of carpal bones. Like a nut within a nutcracker, the scaphoid and lunate are vulnerable to compression-related injuries. As will be explained, it is not uncommon for either bone to develop avascular necrosis, which interferes with the healing process after fracture.

The Scaphoid Bone and Its Vulnerability to Fracture

The scaphoid is located in the direct path of force transmission through the wrist. For this reason, the scaphoid accounts for 60% to 70% of all carpal fractures.65 A common mechanism for fracturing this bone is to fall on a fully supinated forearm with wrist fully extended and radially deviated. Persons with a fractured scaphoid typically show tenderness within the anatomic “snuffbox” of the wrist. Most fractures occur near or along the scaphoid’s “waist,” midway between the bone’s two poles (Figure 7-6, A). Because most blood vessels enter the scaphoid at and distal to its waist, fractures proximal to the waist may result in a delayed union or nonunion.21,24 If the fracture is untreated, the proximal pole may develop avascular necrosis. Fractures of the proximal pole typically require surgery, followed by immobilization for at least 12 weeks or until there is evidence of radiographic union. Fractures of the distal pole typically do not require surgery, especially if nondisplaced, and generally require only 5 to 6 weeks of immobilization. Actual times of immobilization can vary greatly, based on the specific circumstances of the patient and the fracture.

Often a fractured scaphoid is associated with other injuries along the weight-bearing path of the wrist and hand.44 Associated injuries often involve fracture and/or dislocation of the lunate and fracture of the trapezium and distal radius.

Kienböck’s Disease: Avascular Necrosis of the Lunate

The condition of lunatomalacia (meaning literally “softening of the lunate”) was first described by Kienböck in 1910.61 Kienböck’s disease, as it is called today, is described as a painful orthopedic disorder of unknown cause, characterized by avascular necrosis of the lunate.71 A history of trauma is frequently, but not universally, associated with the onset of the condition. Trauma may be linked with an isolated dislocation or fracture or with repetitive or near-constant lower-magnitude compression forces. It is not understood how the trauma, compression, and avascular necrosis are interrelated in the pathogenesis of the disease.65 What is clear, however, is that as avascular necrosis develops, the lunate often becomes fragmented and shortened, which may alter its relationship with the other adjoining carpal bones (see Figure 7-6, B).2 In severe cases the lunate may totally collapse, disrupting the kinematics and kinetics of the entire wrist. This tends to occur more often in those involved in manual labor, such as pneumatic drill operators.

Treatment of Kienböck’s disease may be conservative or radical, depending on the amount of functional limitation and pain, as well as the progression of the disease. In relatively mild forms of the disease—before the lunate fragments and becomes sclerotic—treatment may involve immobilization by casting or splinting.71 If the disease progresses, the length of the ulna or radius may be surgically altered as a means to reduce the contact stress on the lunate.71 In more advanced cases, treatments may include partial fusion of selected carpal bones, lunate excision, or proximal row carpectomy.9,21

CAPITATE

The capitate is the largest of all carpal bones. This bone occupies a central location within the wrist, making articular contact with seven surrounding bones when considering the metacarpals (see Figure 7-3, A). The word capitate is derived from the Latin root meaning head, which describes the shape of the bone’s prominent proximal surface. The large head articulates with the deep concavity provided by the scaphoid and lunate. The capitate is well stabilized between the hamate and trapezoid by short but strong ligaments.

The capitate’s distal surface is rigidly joined to the base of the third and, to a lesser extent, the second and fourth metacarpal bones. This rigid articulation allows the capitate and the third metacarpal to function as a single column, providing significant longitudinal stability to the entire wrist and hand. The axis of rotation for all wrist motions passes through the capitate.

Carpal Tunnel

As illustrated in Figure 7-5, the palmar side of the carpal bones forms a concavity. Arching over this concavity is a thick fibrous band of connective tissue known as the transverse carpal ligament. This ligament is connected to four raised points on the palmar carpus, namely, the pisiform and the hook of the hamate on the ulnar side, and the tubercles of the scaphoid and the trapezium on the radial side. The transverse carpal ligament serves as a primary attachment site for many muscles located within the hand and the palmaris longus, a wrist flexor muscle.

The transverse carpal ligament converts the palmar concavity made by the carpal bones into a carpal tunnel. The tunnel serves as a passageway for the median nerve and the tendons of extrinsic flexor muscles of the digits. Furthermore, the transverse carpal ligament restrains the enclosed tendons from “bowstringing” anteriorly and out of the carpal tunnel, most notably during grasping actions performed with a partially flexed wrist.

ARTHROLOGY

Joint Structure and Ligaments of the Wrist

JOINT STRUCTURE

As illustrated in Figure 7-1, the two primary articulations within the wrist are the radiocarpal and midcarpal joints. Many other intercarpal joints also exist between adjacent carpal bones (see Figure 7-7). Intercarpal joints contribute to wrist motion through small gliding and rotary motions. Compared with the large range of motion permitted at the radiocarpal and midcarpal joints, motion at the intercarpal joints is relatively small but nevertheless essential for normal wrist motion.

Radiocarpal Joint: The proximal components of the radiocarpal joint are the concave surfaces of the radius and an adjacent articular disc (Figures 7-7 and 7-8). As described in Chapter 6, this articular disc (also called the triangular fibrocartilage) is an integral part of the distal radio-ulnar joint. The distal components of the radiocarpal joint are the convex proximal surfaces of the scaphoid and the lunate. The triquetrum is also considered part of the radiocarpal joint because at full ulnar deviation its medial surface contacts the articular disc.

The thick articular surface of the distal radius and the articular disc accept and disperse the forces that cross the wrist. Approximately 20% of the total compression force that crosses the radiocarpal joint passes through the articular disc. The remaining 80% passes directly through the scaphoid and lunate to the radius.58 The contact areas at the radiocarpal joint tend to be greatest when the wrist is partially extended and ulnarly deviated.42 This is also the wrist position at which maximal grip strength is obtained.

Midcarpal Joint: The midcarpal joint is the articulation between the proximal and distal rows of carpal bones (see Figure 7-8). The capsule that surrounds the midcarpal joint is continuous with each of the many intercarpal joints.

The midcarpal joint can be divided descriptively into medial and lateral joint compartments.78 The larger medial compartment is formed by the convex head of the capitate and apex of the hamate, fitting into the concave recess formed by the distal surfaces of the scaphoid, lunate, and triquetrum (see Figure 7-8). The head of the capitate fits into this concave recess much like a ball-and-socket joint.

The lateral compartment of the midcarpal joint is formed by the junction of the slightly convex distal pole of the scaphoid with the slightly concave proximal surfaces of the trapezium and the trapezoid (see Figure 7-8). The lateral compartment lacks the pronounced ovoid shape of the medial compartment. Cineradiography of wrist motion shows less movement at the lateral than the medial compartment.52 For this reason, subsequent arthrokinematic analysis of the midcarpal joint focuses on the medial compartment.

SPECIAL FOCUS 7-2   imageTotal Wrist Arthroplasty

Total wrist arthroplasty (replacement) has not reached the level of success of arthroplasty of other joints in the body, such as the hip or knee.12,74 One obstacle is the small size of the replacement components, which concentrates high stress on the implanted material. Over time, high stress contributes to premature loosening or dislocation. The success rate of total wrist replacement will likely improve with continued advances in surgical technique, preoperative and postoperative management, knowledge of the natural biomechanics, and design of implants.

WRIST LIGAMENTS

Many of the ligaments of the wrist are small and difficult to isolate. Their inconspicuous nature should not, however, minimize their extreme kinesiologic importance. Wrist ligaments are essential to maintaining the natural intercarpal alignment and for transferring forces within and across the carpus. Muscle-produced forces stored in stretched ligaments provide important control to the complex arthrokinematics of the wrist. Ligaments also supply sensory feedback to activated muscles.28 Ligaments damaged through injury and disease leave the wrist vulnerable to weakness, deformity, instability, and degenerative arthritis.

Wrist ligaments are classified as extrinsic or intrinsic (Box 7-1). Extrinsic ligaments have their proximal attachments on the forearm but attach distally within the wrist. As noted in Box 7-1, the triangular fibrocartilage complex (introduced previously in Chapter 6) includes structures associated with the wrist and the distal radio-ulnar joint. Intrinsic ligaments have both their proximal and distal attachments within the wrist. More detailed or alternative descriptions of these ligaments can be found in other sources.6,78

Extrinsic Ligaments: A fibrous capsule surrounds the external surfaces of both the wrist and the distal radio-ulnar joint. Dorsally, the capsule thickens slightly to form the dorsal radiocarpal ligament (Figure 7-9). This ligament is thin and not easily distinguishable from the capsule itself. In general, the dorsal radiocarpal ligament courses distally in an ulnarly direction, attaching primarily between the distal radius and the dorsal surfaces of the lunate and triquetrum.73,77 The dorsal radiocarpal ligament reinforces the posterior side of the radiocarpal joint and helps guide the natural arthrokinematics, especially of the bones in the proximal row.77 The fibers that attach to the lunate provide an especially important restraint against anterior (volar) dislocation of this inherently unstable bone.86

Taleisnik originally described the thickening of the external surface of the lateral-palmer part of the capsule of the wrist as the radial collateral ligament (Figure 7-10).80 More recent anatomic descriptions, however, typically do not include the radial collateral ligament as a distinct anatomic entity.6 This connective tissue, regardless of its name, likely provides little lateral stability to the wrist. Extrinsic muscles, such as the abductor pollicis longus and the extensor pollicis brevis, perform most of this function.

Deep and separate from the palmar capsule of the wrist are several stout and extensive ligaments known collectively as the palmar radiocarpal ligaments. Three ligaments are typically described within this set: the radioscaphocapitate, the radiolunate, and, in a deeper plane, the radioscapholunate (see Figure 7-10).78 The palmar radiocarpal ligaments are much stronger and thicker than their dorsal counterparts.80 In general, each ligament arises from a roughened area on the distal radius, travels distally in a generally ulnar direction, and attaches to the palmar surface of several carpal bones. The radioscaphocapitate—the most lateral ligament of this set—often partially blends with the radial collateral ligament.

The palmar radiocarpal ligaments become maximally taut at full wrist extension.44 Passive tension exists in these ligaments even in the relaxed neutral wrist position.88 An example of the role these ligaments play in guiding the arthrokinematics of the wrist will be provided later in this chapter.

Although the ulnocarpal space appears empty on a standard radiograph (Figure 7-11, A), it is actually filled with at least five interconnected tissues, known collectively as the triangular fibrocartilage complex (TFCC) (see Box 7-1). The primary component of the TFCC is the triangular fibrocartilage—the previously described articular disc located within both the distal radio-ulnar and the radiocarpal joints (see Figure 7-11, B).

The primary global function of the TFCC is to securely bind the distal ends of the radius and ulna while simultaneously permitting the radius, with attached carpus, to freely rotate (pronate and supinate) around a fixed ulna. A summary of the more specific functions of the TFCC is included in Box 7-2. Anatomic details of the components of the TFCC are described in the following paragraphs.

The triangular fibrocartilage (TFC) attaches directly or indirectly to all components of the TFCC and therefore forms the structural backbone of the entire complex. The TFC is a biconcave articular disc, composed chiefly of fibrocartilage.78 The name “triangular” refers to the shape of the disc: its base attaches along the ulnar notch of the radius, and its apex attaches near the styloid process of the ulna (see Figure 6-26, A). The sides of the “triangle” are formed by the palmar and dorsal capsular ligaments of the distal radio-ulnar joint.25 The disc’s proximal surface accepts the head of the ulna at the distal radio-ulnar joint, and its distal surface accepts the convex surfaces of part of the lunate and the triquetrum at the radiocarpal joint (see Figures 6-26, 7-7, and 7-8, A). The central 80% of the disc is avascular with little or no healing potential.13

The palmar ulnocarpal ligament originates from the palmar edge of the articular disc and adjacent palmar aspect of the distal radio-ulnar joint capsule (see Figure 7-10).31 From this common proximal attachment the tissue splits into two distinct ligaments: ulnolunate and ulnotriquetral.

The ulnar collateral ligament represents a thickening of the medial aspect of the capsule of the wrist31,80 (see Figure 7-10). (According to the British edition of Gray’s Anatomy, the ulnar collateral ligament and the juxtaposed ulnotriquetral ligament are part of the same structure.78) Along with the flexor and extensor carpi ulnaris muscles, the palmar ulnocarpal and ulnar collateral ligaments reinforce the ulnar side of the wrist. These ligaments must be sufficiently flexible, however, to allow the radius and hand to rotate freely around the fixed ulna during pronation and supination.

The final component yet to be described within the TFCC is a poorly organized and defined connective tissue substance known as the meniscus homologue.31 This tissue likely represents the vestige of a more primitive embryonic connective tissue within the ulnar side of the wrist.78 Referred to as a “cartilaginous filler,”80 the meniscus homologue fills gaps within and immediately medial to the prestyloid recess of the ulnocarpal space (see Figure 7-7). The synovial lining within this recess often becomes distended and painful with rheumatoid arthritis. Tears in the articular disc may permit synovial fluid to spread from the radiocarpal joint to the distal radio-ulnar joint.

Intrinsic Ligaments: The primary intrinsic ligaments of the wrist can be classified into three sets: short, intermediate, or long (see Box 7-1).80 Short ligaments connect the bones of the distal row by their palmar, dorsal, or interosseous surfaces (see Figures 7-9 and 7-10). The short ligaments firmly stabilize and unite the distal row of bones, permitting them to function essentially as a single mechanical unit.

Three intermediate ligaments exist within the wrist. The lunotriquetral ligament is a fibrous continuation of the palmar radiolunate ligament (see Figure 7-10). The scapholunate ligament is a broad collection of fibers that forms the primary bond between the scaphoid and the lunate (see Figure 7-8, A).76 Several scaphotrapezial ligaments reinforce the articulation between the scaphoid and the trapezium (see Figure 7-9).

Two relatively long ligaments are present within the wrist. The palmar intercarpal ligament firmly attaches to the palmar surface of the capitate bone (see Figure 7-10). From this common attachment the ligament bifurcates proximally, forming two discrete fiber groups that resemble the shape of an inverted V. The lateral leg of the inverted V attaches to the scaphoid, and the medial leg to the triquetrum. These ligaments help guide the arthrokinematics of the wrist.

Lastly, a thin dorsal intercarpal ligament provides transverse stability to the wrist by interconnecting the trapezium, scaphoid, lunate, and triquetrum (see Figure 7-9).46,85

Kinematics of Wrist Motion

OSTEOKINEMATICS

The osteokinematics of the wrist are defined for 2 degrees of freedom: flexion-extension and ulnar-radial deviation (Figure 7-12). Wrist circumduction—a full circular motion made by the wrist—is a combination of the aforementioned movements, not a distinct third degree of freedom.

Most natural dynamic movements of the wrist combine elements of both frontal and sagittal planes: extension tends to occur with radial deviation, and flexion with ulnar deviation.41 The resulting natural path of motion for the wrist follows a slightly oblique path, similar to a dart thrower’s motion.50 This natural combination of movements occurs with other functions, such as tying shoelaces or combing hair. These natural kinematics should be considered during rehabilitation of the wrist after injury.

SPECIAL FOCUS 7-3   imagePassive Axial Rotation at the Wrist: How Much and Why?

In addition to flexion-extension and radial-ulnar deviation, the wrist possesses some passive axial rotation between the carpal bones and forearm. This accessory motion (or joint “play”) can be appreciated by firmly grasping your right fist with your left hand. While securely holding your right hand from moving, strongly attempt to actively pronate and supinate the right forearm. The passive axial rotation at the right wrist is demonstrated by the rotation of the distal radius relative to the base of the hand. Gupta and Moosawi have measured an average of 34 degrees of total passive axial rotation in 20 asymptomatic wrists; the midcarpal joint permitted on average three times more passive axial rotation than the radiocarpal joint.27

The axial rotation at the wrist is limited by the shapes of the joints, especially the elliptic fit of the radiocarpal joint, and the tension in the obliquely oriented radiocarpal ligaments.64 The relatively limited axial rotation permitted at the radiocarpal joint has important kinesiologic implications. With the wrist’s potential third degree of freedom mostly restricted, the hand ultimately must follow the pronating and supinating radius; and furthermore, the restriction allows the pronator and supinator muscles to transfer their torques across the wrist to the working hand.

Accessory motions within the wrist—as in all synovial joints—enhance the overall function of the joint. For instance, axial rotation at the wrist amplifies the total extent of functional pronation and supination of the hand relative to the forearm, as well as dampening the impact of reaching these end-range movements. These functions are useful for activities such as wringing out clothes or turning doorknobs.

The axis of rotation for wrist movements is reported to pass through the head of the capitate (Figure 7-13).94

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