Structure and Function of the Wrist

Objectives

• Identify the bones and primary bony features relevant to the wrist complex.

• Describe the supporting structures of the wrist.

• Cite the normal ranges of motion for wrist flexion and extension and radial and ulnar deviation.

• Describe the planes of motion and axes of rotation for the joints of the wrist.

• Cite the proximal and distal attachments and innervation of the primary muscles of the wrist.

• Justify the primary actions of the muscles of the wrist.

• Describe how compressive forces are transferred from the hand through the wrist.

• Explain the function of the wrist extensor muscles when grasping.

• List the structures that travel within the carpal tunnel.

• Explain the synergistic action between the muscles of the wrist when flexion-extension and radial and ulnar deviation are performed.

Key Terms

avascular necrosis

carpal tunnel

carpal tunnel syndrome

dorsal

lateral epicondylitis

palmar

The wrist contains eight small bones that are located between the distal end of the radius and the hand (Figure 6-1). Although slight, the passive movements that occur within the carpal bones help absorb forces that cross between the hand and the forearm, as when crawling on all four limbs, or when bearing weight through the hands when using crutches or a walker.

Figure 6-1 The bones and major articulations of the wrist. Note also the ulnocarpal space, just distal to the ulna. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 7-1.)

The wrist has two major articulations: (1) the radiocarpal joint, and (2) the midcarpal joint. As a functional pair, these joints allow the wrist to adequately position the hand for optimal function.

The wrist can flex and extend and move in a side-to-side fashion known as radial and ulnar deviation. In addition to these important movements, the wrist must serve as a stable platform for the hand. A painful or weak wrist typically cannot provide an adequate base for the muscles to operate the hand. Making a firm grip, for example, is not possible with paralysis of the wrist extensor muscles. As will be presented in this chapter, the kinesiology of the wrist is heavily linked to the kinesiology of the hand.

Several new terms in this chapter describe surfaces of the wrist and hand. Palmar is synonymous with the anterior aspect of the wrist and hand; dorsal refers to the posterior aspect of the wrist or hand. These terms are used interchangeably throughout this chapter and the next chapter on the hand.

Osteology

Ten bones are involved in the kinesiology of the wrist: distal radius, distal ulna, and eight carpal bones.

Distal Radius and Ulna

The distal radius and ulna (Figure 6-2) articulate with the proximal row of carpal bones. The distal forearm is bordered laterally by the radial styloid process and medially by the ulnar styloid process. The radial tubercle, also called Lister’s tubercle, is a small, palpable projection on the dorsal aspect of the distal radius. This ridge of bone helps guide the direction of the tendons of several wrist and thumb extensor muscles.

Figure 6-2 The dorsal aspect of the bones of the right wrist. The muscle’s distal attachments are shown in gray. The dashed lines show the proximal attachment of the dorsal capsule of the wrist. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 7-2.)

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 (see Figures 6-2 and 6-3). The bones within the proximal row are loosely joined. In contrast, strong ligaments tightly bind the bones of the distal row. The natural stability of the distal row provides an important rigid base for articulations with the metacarpal bones.

Figure 6-3 The palmar aspect of the bones of the right wrist. The muscle’s proximal attachments are shown in red, and distal attachments in gray. The dashed lines show the proximal attachment of the palmar capsule of the wrist. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 7-3.)

Consider this…

Carpal Bones: A Few Highlights

Scaphoid

The scaphoid is located in the direct pathway of the forces that naturally cross the wrist. For this reason, fracture of the scaphoid occurs more frequently than fracture of any other carpal bone. Healing is frequently hindered because blood supply to the fractured component of bone is often poor.

Lunate

It is interesting to note that no muscles and only a few ligaments are attached to the lunate. The lunate therefore is loosely articulated and is the most frequently dislocated carpal bone. As with the scaphoid, the blood supply to the lunate is often compromised after trauma, resulting in avascular necrosis.

Triquetrum

The triquetrum is named after its triangular appearance.

Pisiform

Strictly speaking, the pisiform is not a true carpal bone. Rather, it is a sesamoid bone that develops within the tendon of the flexor carpi ulnaris. Technically, therefore, the wrist has seven carpal bones; this matches the arrangement of seven tarsal bones of the ankle.

Trapezium

The distal, saddle-shaped surface of the trapezium articulates with the base of the first metacarpal. The resulting carpometacarpal joint is a highly specialized articulation allowing a wide range of motion of the thumb.

Trapezoid

This bone is tightly wedged between the trapezium and the capitate, serving as a stable base for the second metacarpal.

Capitate

The capitate is the largest of all carpal bones, occupying a central location within the wrist. The axis of rotation for all wrist motion passes through this bone.

Hamate

The hamate (from Latin, meaning “hook”) is named after its prominent hook-like process on its palmar surface.

Carpal Tunnel

The transverse carpal ligament bridges the palmar side of the carpal bones, helping to form the carpal tunnel (Figure 6-4). The carpel tunnel serves as a passageway that helps protect the median nerve and the tendons of the extrinsic flexor muscles of the digits.

Clinical insight

Carpal Tunnel Syndrome

All the tendons that flex the digits travel with the median nerve and pass through the tightly packed carpal tunnel (see Figure 6-4). Also traveling within the carpal tunnel are several synovial membranes that help reduce friction between tendons and surrounding structures. Hand activities that require prolonged and often extreme wrist positions can irritate these tendons and synovial sheaths. Because of the small size of the carpal tunnel, swelling of the synovial membranes can increase pressure on the median nerve. Carpal tunnel syndrome, which is characterized by pain or paresthesia (tingling), or both, over the sensory distribution of the median nerve, may result. In more extreme cases, muscular weakness and atrophy may occur in the intrinsic muscles around the thumb.

Arthrology

Joint Structure

As is illustrated in Figure 6-1, the wrist is a double-jointed system, consisting of the radiocarpal and midcarpal joints. Many smaller intercarpal joints also exist between carpal bones. Compared with the large ranges of motion permitted at the radiocarpal and midcarpal joints, motion at the many intercarpal joints is relatively small.

Major Joints of the Wrist

• Radiocarpal joint

• Midcarpal joint

Radiocarpal Joint

The proximal part of the radiocarpal joint consists of the concave surface of the radius and the adjacent articular disc (Figure 6-5). The distal part of the joint consists primarily of the convex articular surfaces of the scaphoid and the lunate. Approximately 80% of the force that crosses the wrist passes between the scaphoid and the lunate, and then to the radius. The large, expanded distal end of the radius is well designed to accept this force. Unfortunately, however, for many persons, a fall onto an outstretched hand fractures the distal end of the radius, as well as the scaphoid. Persons with weakened bones due to osteoporosis are particularly susceptible to these fractures.

Figure 6-5 A frontal plane cross section through the right wrist and distal forearm showing the shape of the bones and connective tissues. The margins of the radiocarpal and midcarpal joints are highlighted in red. (Modified from Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 7-7.)

The ulnar-located carpal bones and the distal ulna are less likely to fracture from such a fall because they are not in the direct path of weight bearing. Furthermore, a relatively wide space exists between the distal ulna and the ulnar carpal bones. This space, formally known as the ulnocarpal space (see Figure 6-1), helps buffer the forces that cross the wrist.

Midcarpal Joint

The midcarpal joint separates the proximal and distal rows of carpal bones (see Figure 6-5). Although this joint involves several articulations, the most prominent is formed between the head of the capitate and the socket formed by the distal surfaces of the scaphoid and lunate. Note that the scaphoid and the lunate bones are important members of the main two articulations of the wrist.

Ligaments of the Wrist

The joints of the wrist are enclosed within a fibrous capsule. The capsule is thickened by extrinsic and intrinsic ligaments. Extrinsic ligaments have their proximal attachments outside the carpal bones but attach distally within the carpal bones. Intrinsic ligaments, in contrast, have both their proximal and distal attachments located within the carpal bones. Table 6-1 lists the main attachments and primary functions of the four primary extrinsic ligaments: radial collateral, ulnar collateral, dorsal radiocarpal, and palmar radiocarpal. Three of the four primary extrinsic ligaments are indicated by red dots in Figure 6-6, A and B, and are summarized along with their individual functions in Table 6-1. The detailed anatomy of the intrinsic ligaments is beyond the scope of the text. As a group, however, the intrinsic ligaments (1) interconnect various carpal bones; (2) help transfer forces between the hand and the forearm; and (3) maintain the natural shapes of radiocarpal and midcarpal joints, thereby minimizing joint stress during movement.

Table 6-1

Ligaments of the Wrist

Ligament	Function	Comments
Dorsal radiocarpal ligament	Resists extremes of flexion	Attaches between the radius and the dorsal side of the carpal bones
Radial collateral ligament	Resists extremes of ulnar deviation	Strengthened by muscles such as the abductor pollicis longus and the extensor pollicis brevis
Palmar radiocarpal ligament	Resists extremes of wrist extension	Thickest ligament of the wrist; consists of three parts
Ulnar collateral ligament	Resists extremes of radial deviation	Part of the ulnocarpal complex; helps stabilize the distal radioulnar joint

Consider this…

Ulnocarpal Complex

A complex set of connective tissues, known as the ulnocarpal complex, exists near the ulnar border of the wrist (see Figure 6-6, B). (This group of tissues is often referred to as the triangular fibrocartilage complex, or TFCC). The ulnocarpal complex includes the articular disc (described in Chapter 5 as an important component of the distal radioulnar joint), the ulnar collateral ligament, and the palmar ulnocarpal ligament. This set of tissues fills most of the ulnocarpal space between the distal ulna and the carpal bones (see Figure 6-1). The ulnocarpal space allows the carpal bones to follow the pivoting radius during pronation and supination of the forearm, without interference from the distal end of the ulna. Tears in the articular disc, the central component of the ulnocarpal complex, may result in instability and pain of the wrist and the distal radioulnar joint.

Wrist Instability

Compression forces naturally cross the wrist every time an overlying muscle contracts or weight is placed through the hand. Normally, the wrist remains stable when compressed, even under substantial forces. Resistance from healthy ligaments, muscles, and tendons and the fit of the articulations add an important element of stability to the wrist. However, damage from a large force such as a fall or, in more extreme cases, degeneration associated with rheumatoid arthritis can significantly destabilize this region.

Consider that the loosely articulated proximal row of carpal bones is located between two rigid structures: the radius and the distal row of carpal bones. Ligaments weakened by injury or disease often lead to instability of the wrist and even collapse. When compressed strongly from both ends (e.g., from a fall), the proximal row of carpal bones is prone to collapse in a zigzag fashion, much like derailed cars of a freight train (Figure 6-7). An unstable wrist can become painful and is often disabling.

Figure 6-7 A highly diagrammatic depiction of a “zigzag” collapse of the wrist secondary to a large compression force after a fall. Note that only selected bones representing the major joints of the wrist are shown. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 7-18.)

Even a moderately unstable wrist can disrupt the natural arthrokinematics, eventually leading to severe pain and overall weakening caused by atrophy of the surrounding muscles. A painful and weak wrist typically fails to provide a stable platform for the hand. In severe cases, surgery is required, often combined with physical therapy. Components of physical therapy typically include strengthening, efforts to relieve pain, education on ways to protect the wrist, and splinting.

Kinematics

Osteokinematics

Osteokinematics of the wrist involves flexion and extension and ulnar and radial deviation. Except for minimal accessory motions, the wrist does not spin in a circular motion relative to a fixed radius. The bony fit and ligaments of the radiocarpal joint naturally block this twisting motion. As studied in Chapter 5, pronation and supination involve rotation of the forearm, with the hand and wrist “following” the path of the radius.

The axis of rotation for wrist movement pierces the head of the capitate (Figure 6-8). The axis runs in a medial-lateral direction for flexion and extension, and in an anterior-posterior direction for radial and ulnar deviation. The firm articulation between the capitate and the base of the third metacarpal bone causes rotation of the capitate to direct the overall path of the entire hand.