Anatomy and basic biomechanics of the wrist

INTRODUCTION — The wrist is a complex joint that serves as the link between the forearm and hand, and it is critical for many upper extremity movements. An understanding of wrist anatomy allows for appreciation of the biomechanics of wrist movement, which helps the clinician to understand injury patterns, perform an efficient history and physical examination, and improve diagnostic accuracy and treatment decisions.

This topic will review the anatomy and biomechanics of the wrist. Approaches to the diagnosis of wrist pain and detailed discussions of specific wrist problems are reviewed separately. (See "Overview of carpal fractures" and "Distal radius fractures in adults" and "Scaphoid fractures".)

ANATOMY

Orientation and terminology — The anatomic position of the wrist defines the palmar or volar surface as anterior and the dorsal surface as posterior (figure 1 and figure 2). The ulna is considered medial and the radius lateral. The wrist is defined as the distal aspect of the radius and ulna, the eight carpal bones, and their articulations with the proximal metacarpals.

Carpal bones — The carpal bones are divided into two rows: proximal and distal (image 1 and figure 3). The proximal carpal row is composed of the scaphoid, lunate, triquetrum, and pisiform. The distal carpal row is comprised of the trapezium, trapezoid, capitate, and hamate.

●The scaphoid, also called the "navicular," is the second largest carpal bone and was so named from the Greek word "scaphe," meaning "dug-out," "trough," or "boat" because of its remote resemblance to a boat. It is shaped somewhat like a kidney bean and traverses the midcarpal joint. It has three named regions, including the proximal pole, the distal pole (tubercle), and the waist, which separates the two poles. Over 80 percent of its surface is covered with articular cartilage.

Blood is supplied to the scaphoid by branches of the radial artery [1]. The dorsal scaphoid branch typically enters the dorsal ridge at the level of the scaphoid waist and supplies the proximal 70 to 80 percent of the bone. The volar (or palmar) scaphoid branch of the radial artery enters the more distal tubercle and supplies the distal 20 to 30 percent of the bone. The blood supply to the proximal pole of the scaphoid can be disrupted by a fracture, which increases the risk of nonunion or delayed union (figure 4). (See "Scaphoid fractures".)

●The lunate, named because of its crescent shape, is located between the scaphoid and the triquetrum. It is divided into a dorsal pole and a palmar pole. The lateral and medial surfaces are small and flat surfaces for articulation with the scaphoid and triquetrum, respectively. Blood supply to the lunate is variable and may arise from palmar and dorsal nutrient vessels (80 percent) or palmar vessels alone (20 percent) [2]. The lunate is held within the proximal row by the scapholunate and lunotriquetral interosseous ligaments as well as the extrinsic radiocarpal ligaments (figure 5). (See "Lunate fractures and perilunate injuries".)

●The triquetrum is a small, pyramid-shaped bone that is largely covered in ligaments. The triquetrum articulates with three bones: the lunate, pisiform, and hamate. The lateral surface of the triquetrum is flat and articulates with the lunate. The distal end faces laterally, forming the sinuous, concave facet that articulates with the hamate bone. The ventromedial facet that articulates with the pisiform bone is oval shaped. The proximal end of the triquetrum forms a smooth facet for articulation with the articular disc of the distal radioulnar joint.

●The pisiform bone, named because of its resemblance to a pea, can be considered a sesamoid bone of the flexor carpi ulnaris tendon. This tendon continues distally as the pisohamate ligament. There is a single, elevated, flat, oval articular facet on the dorsal surface of the pisiform that engages the triquetrum.

●The trapezium, or greater multangular, is situated between the scaphoid and the base of the first metacarpal. It has a slightly concave proximal surface for articulation with the scaphoid, a flat facet medially for articulation with the trapezoid, and a saddle configuration distally for articulation with the first metacarpal.

●The trapezoid, or lesser multangular, is one of the smallest carpal bones, located between the trapezium and the capitate. All of its articular surfaces for the trapezium, base of the second metacarpal, and scaphoid are relatively flat.

●The capitate is the largest carpal bone and is divided into three parts: the head, neck, and body. The head is hemispherical and entirely covered by articular cartilage with no ligamentous insertions. The neck is covered in periosteum and represents a narrowed transition between the head and the body. The body is large and covered completely on both the palmar and dorsal sides by ligament insertions.

●The hamate is named for the hook "hamulus." It is a large bone and forms the ulnar border of the distal carpal row. The hamate is divided into proximal pole, hook, and body regions. The proximal pole is completely covered with articular cartilage, and the hook is completely covered with ligament attachments. The hook of the hamate projects from the volar surface approximately 1 to 2 cm distal and is radial to the pisiform. The body has two relatively flat distal facets that engage the bases of the fourth and fifth metacarpals, as well as a panhandled lateral surface that engages the capitate.

Metacarpals and carpometacarpal joints — The proximal metacarpals and the carpometacarpal joints are considered part of the wrist. The metacarpals are numbered by convention, with the thumb metacarpal being the first, the index finger metacarpal being the second, and so on. The carpometacarpal joints are the articulations between the distal row of carpals and the bases of the first through fifth metacarpals. Both the first and fifth carpometacarpal joints are saddle joints allowing primarily two degrees of freedom of movement. The second through fourth carpometacarpal joints are plane synovial joints allowing one degree of freedom of movement.

Distal radius and ulna and major wrist articulations — Major articulations of the wrist include the distal radioulnar joint, the interface between the distal radius and the proximal row of carpal bones, and the interface between the proximal and distal rows of carpal bones.

The distal radioulnar joint is an L-shaped pivot or trochoid joint that allows for forearm and hand pronation and supination. The joint is at risk for both acute and chronic injury, including dorsal and volar dislocations, chronic instability, and degenerative joint disease. Distal radioulnar joint function depends upon the integrity of the triangular fibrocartilage complex (TFCC; (figure 6)), extensor carpi ulnaris, interosseous ligament, pronator quadratus, and associated forearm muscles.

The articular surface of the distal radius is convex and forms an ellipsoidal joint with the proximal row of carpals, specifically the scaphoid, lunate, and triquetrum. The distal ulna, or ulnar head, articulates with a fibrocartilaginous disc that separates it from the medial carpals. Both the distal radius, which normally extends beyond the end of the ulna, and the distal ulna have palpable styloid processes, which are clinically important for the wrist examination and for evaluating wrist injury.

Medially, the proximal carpal row consists of the triquetrum and pisiform. The triquetrum articulates with the TFCC proximally and the hamate distally. The distal ulna does not articulate directly with the carpal bones but serves as an attachment for a number of the stabilizing ligaments that comprise the ulnar portion of the wrist. The TFCC interposes between the distal ulna and carpus, serving as both a force-transmitting and a stabilizing structure [3].

The distal carpal row articulates through the midcarpal joint with the proximal carpal row and distally with the metacarpal bases. The trapezium articulates with the distal pole of the scaphoid and the base of the first metacarpal. It plays an important role in thumb function and is vulnerable to trapeziometacarpal (ie, first carpometacarpal joint) osteoarthritis through repetitive-use injury [4]. The capitate articulates proximally with the lunate and distally with the base of the third metacarpal and sometimes the fourth metacarpal. The hamate articulates with the ulnar side of the capitate as well as the fourth and fifth metacarpal bases distally and the triquetrum proximally.

Wrist ligaments — The ligaments of the wrist form a complex network of collagen fascicles that almost completely cover the carpal bones (figure 7 and figure 5). The majority of carpal ligaments are capsular. This means that the ligament resides within the layers of the joint capsule. Ligaments found entirely within the joint are defined as intra-articular ligaments. These ligaments have a composition similar to capsular ligaments but are surrounded entirely by a synovial lamina. Individual ligaments may show some variation in tissue composition. As an example, some include fibrocartilage [5].

The wrist ligaments can be grouped into distal radioulnar, palmar radiocarpal, dorsal radiocarpal, ulnocarpal, palmar midcarpal, dorsal midcarpal, and interosseous, depending upon the principal location of the fibers [6]. Generally, the ligament is named for its most prominent bony connections.

Triangular fibrocartilage complex — The TFCC is formed by the triangular fibrocartilage discus, the radioulnar ligaments, and the ulnocarpal ligaments (figure 6). The triangular fibrocartilage discus is an articular discus with a triangular shape and a biconcave body that lies on the pole of the distal ulna. The central portion of the triangular fibrocartilage discus is thin and consists of chondroid fibrocartilage; this type of tissue is often seen in structures that bear compressive loads.

The radioulnar ligaments are the principal stabilizers of the distal radioulnar joint. There are two radioulnar ligaments, the palmar and dorsal radioulnar ligaments. These ligaments arise from the medial border of the distal radius and insert on the ulna at two separate and distinct sites: the ulna styloid and the fovea.

The ulnocarpal ligaments consist of the ulnolunate and the ulnotriquetral ligaments. They originate from the ulnar styloid and insert into the carpal bones of the wrist; the ulnolunate ligament inserts into the lunate and the ulnotriquetral ligament into the triquetrum. These ligaments prevent dorsal migration of the distal ulna. They become tauter during supination as the ulnar styloid moves away from the carpal bones' volar side [7].

The TFCC performs several important functions in the wrist. Its most important function is to stabilize the distal radioulnar joint. In addition, the TFCC assists with distributing forces exerted upon the ulnar aspect of the wrist, while the triangular fibrocartilage discus in particular helps to absorb compressive forces.

Ulnar variance refers to the relative distance between the articular surfaces of the ulna and the radius. This distance influences the proportion of a load that is transmitted through the distal ulna. In neutral ulnar variance (articular surfaces are at the same level), approximately 20 percent of the load is transmitted through the ulna. With negative ulnar variance (ulnar articular surface is proximal to that of the radius), the load across the ulna and the TFCC diminishes. This decrease in force transmission also occurs during supination because the radius moves distally on the ulna during this movement. The load placed upon the ulna and the TFCC increases with positive ulnar variance and during wrist pronation [8].

The TFCC is at substantial risk for injury and degeneration because of its anatomic complexity and multiple functions. Chronic and excessive loading cause degenerative TFCC tears. Most acute traumatic TFCC injuries occur during a fall onto an outstretched hand. These injuries are caused by the substantial axial load placed upon the wrist while it is extended and pronated. A powerful dorsal rotation or distraction force exerted upon the wrist can also injure the TFCC.

Wrist tendons — There are no intrinsic muscles of the wrist. All have their origins proximal to the wrist joint and insert at the hand, predominantly onto the metacarpals. An exception is the flexor carpi ulnaris, which inserts at the pisiform and fifth metacarpal base.

Wrist compartments — The extensor tendons are held in place by the extensor retinaculum (figure 8). As these tendons move over the posterior aspect of the wrist, they are protected within synovial tendon sheaths. These sheaths reduce friction as the extensor tendons traverse the osseous-fibrous tunnels of the wrist.

The extensor tendons that traverse the wrist are contained within six compartments:

●Compartment 1 – Contains the abductor pollicis longus and the extensor pollicis brevis tendons (image 2). These tendons lie within the radial border of the anatomical snuff box.

●Compartment 2 – Contains the extensor carpi radialis longus (ECRL) and extensor carpi radialis brevis (ECRB) tendons (image 3).

●Compartment 3 – Contains the extensor pollicis longus tendon, which passes medial to the dorsal tubercle (Lister's tubercle) of the radius (image 4). The extensor pollicis longus tendon attaches to the base of the distal phalanx of the thumb, forming the ulnar border of the anatomical snuff box (picture 1 and picture 2).

●Compartment 4 – Contains the extensor digitorum and extensor indicis tendons (image 5). Proximally, the four tendons of the extensor digitorum join the tendon of the extensor indicis to pass deep to the extensor retinaculum through the tendinous sheath of the extensor digitorum and extensor indicis. Then, on the dorsum of the hand, the tendons fan out as they run towards individual fingers.

●Compartment 5 – The extensor digiti minimi tendon travels through this compartment, posterior to the distal radioulnar joint (image 6).

●Compartment 6 – Contains the extensor carpi ulnaris tendon (image 7). It runs in a groove between the ulnar head and its styloid process.

Carpal tunnel — A total of nine flexor tendons pass through the carpal tunnel (figure 9 and figure 10 and image 8):

●Flexor digitorum profundus (four tendons)

●Flexor digitorum superficialis (four tendons)

●Flexor pollicis longus (one tendon)

The median nerve courses between tendons of flexor digitorum profundus and flexor digitorum superficialis. There are multiple naturally occurring anomalies of the median nerve. The recurrent branch of the median nerve innervates the thenar muscles. Bifurcation of the median nerve typically occurs after the nerve exits the carpal tunnel. However, in approximately 5 to 10 percent of individuals, the median nerve bifurcates more proximally in the carpal tunnel, wrist, or forearm [9].

The carpal bones form an arch that is convex on the dorsal side of the hand and concave on the palmar side (image 8). The groove on the palmar side is called the sulcus carpi and is covered by a sheath of tough connective tissue called the flexor retinaculum. This is what forms the carpal tunnel. The flexor retinaculum is attached radially to the scaphoid tubercle and the ridge of trapezium and, on the ulnar side, to the pisiform and hook of hamate [10].

Guyon's canal — Also known as the "ulnar canal" or "ulnar tunnel," Guyon's canal is the space between the pisiform bone and the hamate bone through which the ulnar artery and the ulnar nerve travel into the hand (figure 11). Entrapment of the ulnar nerve at the ulnar canal can result in ulnar neuropathy [11].

Neurovascular anatomy — The innervation of structures within the wrist joint corresponds to the innervation of the overlying skin. Each superficial nerve sends articular branches to the wrist joint capsule that enter the ligamentous or capsular tissue. The nerves of the wrist joint capsule are the superficial radial nerve, the dorsal sensory branch of the ulnar nerve, the deep branch of the ulnar nerve, the lateral antebrachial cutaneous nerve, the anterior interosseous nerve, and the posterior interosseous nerve (figure 11 and figure 12 and figure 13) [12].

The blood supply of the wrist is provided by branches of the radial artery, ulnar artery, and the interosseous arterial system (figure 14) [13]. There are many anastomotic pathways, including the longitudinal and transverse and the palmar and dorsal [14]. Typically, three dorsal arches and three palmar arches traverse the region of the carpals.

Of note, the scaphoid and the lunate have vulnerable blood supplies [15]. The scaphoid gets its blood supply from nutrient vessels entering at the distal pole and the waist of the scaphoid. This means that the proximal pole is dependent on continuity with the waist of the scaphoid for a blood supply. Therefore, the proximal pole of the scaphoid is at risk of osteonecrosis following fractures proximal to the waist of the scaphoid (figure 4). The lunate generally receives nutrient vessels from the dorsal and palmar ligamentous attachments, but these vessels have variable intraosseous anastomotic patterns. When the vascular supply to the lunate is compromised, such as following a traumatic injury, the bone may develop avascular necrosis (Kienböck disease), especially the proximal pole. (See "Scaphoid fractures" and "Lunate fractures and perilunate injuries".)

BASIC BIOMECHANICS

Wrist structure and positioning — Structurally, the hand is a linked system of bony segments arranged in a series of longitudinal and transverse arches. An arch better withstands external forces than most other structures and helps distribute force across the entire structure rather than to any one carpal bone.

There are basically two transverse arches: the proximal transverse arch, formed by the carpal bones; and the distal arch, formed by the metacarpal heads. The longitudinal arches consist of the bones of the five digital rays. The proximal part of the longitudinal arches and the proximal transverse arch converge at the carpal bones. The carpal bones stabilize the longitudinal arches and their related structures, and they are central to the function of the hand.

The dominant wrist position during daily activities is dorsiflexion with radial deviation, which is seen in writing, typing, lifting, and holding. The ability to maintain the carpals in this position relies on the bony contour of the distal radius and radial carpus. The broad distal portion of the radius causes the overlying tendons to protrude outwards, increasing the distance between these tendons and their centers of motion, thereby creating greater force moments. These larger force moments allow the wrist to be more easily maintained or moved into the position of function.

The dominant pattern of wrist motion is from the position of dorsiflexion with radial deviation into palmar flexion with ulnar deviation, the so-called "dart-throwing" motion. Both the bony contours and the relationship between the extensor and flexor tendons and the carpal bones make this motion possible [16].

Wrist motion

●Flexion – The primary wrist flexors are the flexor carpi radialis and the flexor carpi ulnaris, with assistance from the palmaris longus and abductor pollicis longus. The digital flexors are involved only when the fingers are held in extension [17]. The flexor carpi radialis and flexor carpi ulnaris originate from the medial epicondyle of the humerus at the elbow and insert at the volar base of the second, third, and fifth metacarpals, respectively.

●Extension – Wrist extension occurs when the extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), and extensor carpi ulnaris contract. The ECRL originates at the lateral supracondylar ridge of the humerus, and the ERCB and extensor carpi ulnaris muscles originate at the lateral epicondyle of the humerus. The ECRL, ECRB, and extensor carpi ulnaris insert at the dorsal base of the second, third, and fifth metacarpals, respectively. The thumb and digital extensors can also participate in extension when the fist is clenched.

●Ulnar deviation – Ulnar deviation is performed through contraction of the flexor and extensor ulnaris muscles.

●Radial deviation – Radial deviation is performed through contraction of the ECRL and flexor carpi radialis muscles. The ECRB and thumb extensors and abductors may contribute as well.

●Pronation and supination – These movements do not actually take place at the wrist joint but occur in the forearm. Pronation is performed by contraction of the pronator teres and pronator quadratus muscles, with some contribution from the flexor carpi radialis when the wrist is flexed. Supination is performed by contraction of the supinator and biceps brachii muscles.

●Circumduction – Circumduction is the joint action that produces a circular, conical movement of the limb extending from that joint. The wrist can produce this motion by combining palmar flexion and dorsiflexion with radial and ulnar deviation.

SUMMARY AND RECOMMENDATIONS

●Terminology – The anatomic position of the wrist defines the palmar or volar surface as anterior and the dorsal surface as posterior (figure 1 and figure 2). The ulna is considered medial and the radius lateral. The wrist is defined as the distal aspect of the radius and ulna, the eight carpal bones, and their articulations with the proximal metacarpals.

●Carpal bones – The carpal bones are divided into two rows: proximal and distal (image 1 and figure 3). The proximal carpal row is composed of the scaphoid, lunate, triquetrum, and pisiform. The distal carpal row is comprised of the trapezium, trapezoid, capitate, and hamate. (See 'Carpal bones' above.)

●Articulations – Major articulations of the wrist include the distal radioulnar joint, the interface between the distal radius and the proximal row of carpal bones, the interface between the proximal and distal rows of carpal bones, and the interface between the distal row of carpal bones and the metacarpals. (See 'Metacarpals and carpometacarpal joints' above and 'Distal radius and ulna and major wrist articulations' above.)

●Ligaments and triangular fibrocartilage complex (TFCC) – The ligaments of the wrist form a complex network of collagen fascicles that almost completely cover the carpal bones (figure 5). The TFCC performs several important functions in the wrist, most importantly stabilizing the distal radioulnar joint. (See 'Wrist ligaments' above.)

●Tendons – There are no intrinsic muscles of the wrist. The extensor tendons that traverse the wrist are held in place by the extensor retinaculum. Nine flexor tendons pass through the carpal tunnel, along with the median nerve (figure 9 and figure 10 and image 8). (See 'Wrist tendons' above.)

●Neurovascular structures – The innervation of structures within the wrist joint corresponds to the innervation of the overlying skin. The blood supply of the wrist is provided by branches of the radial artery, ulnar artery, and the interosseous arterial system (figure 14). (See 'Neurovascular anatomy' above.)

●Biomechanics – The basic structure and motion of the wrist is reviewed in the text. (See 'Basic biomechanics' above.)