Bones: Two rows
Ligaments: Dorsal, palmar, intercarpal
Membranes: Scapholunate and triquetrolunate
Proximal row moves as a unit
Proximal row is intercalated segment—no tendons attach to it
Ligaments provide stability
Lunate is keystone
The wrist is a unique joint interposed between the distal aspect of the forearm and the proximal aspect of the hand. All three regions have common or shared elements, which integrate form and function to maximize the mechanical effectiveness of the upper extremity. The wrist enables the hand to be placed in an infinite number of positions relative to the forearm and also enables the hand to be essentially locked to the forearm in those positions to transfer the forces generated by the powerful forearm muscles.
Although the wrist is truly a mechanical marvel when it is intact and functioning, loss of mechanical integrity of the wrist inevitably causes substantial dysfunction of the hand and thus the entire upper extremity. It is vital that a thorough understanding of the wrist, including efforts at diagnosis, treatment, and rehabilitation, be acquired by all who treat the wrist. This chapter provides such a foundation by exploring the general architecture of the wrist; the bones; and joints that comprise the wrists and the soft tissues that stabilize, innervate, and perfuse the wrist. In addition, an overview of the mechanics of the wrist, with a discussion of its motions and subparts and the force distribution across the wrist, is provided.
There are eight carpal bones, although many consider the pisiform to be a sesamoid bone within the tendon of the flexor carpi ulnaris (FCU), and thus not behaving as a true carpal bone. The bones are arranged into two rows (proximal and distal carpal row), each containing four bones. All eight carpal bones are interposed between the forearm bones and the metacarpals to form the complex called the wrist joint.
Distal Radius and Ulna
The distal surface of the radius articulates with the proximal carpal row through two articular fossae separated by a fibrocartilaginous prominence oriented in the sagittal plane called the interfossal ridge ( Figs. 2-1 and 2-2 ). The scaphoid fossa is roughly triangular in shape and extends from the interfossal ridge to the tip of the radial styloid process. The lunate fossa is roughly quadrangular in shape and extends from the interfossal ridge to the sigmoid notch. On the dorsal cortex of the distal radius, immediately dorsal and proximal to the interfossal ridge, is a bony prominence called the dorsal tubercle of the radius, or Lister’s tubercle (see Fig. 2-1 ). It serves as a divider between the second and third extensor compartments and functionally behaves as a trochlea for the tendon of extensor pollicis longus. On the medial surface of the distal radius is the sigmoid notch. This concave surface articulates with the ulnar head to form the distal radioulnar joint (DRUJ). It has a variable geometry across a population, both in shape and orientation, but is largely felt to be symmetrical in any given individual.
Under normal circumstances, the ulna does not articulate directly with the carpus. Rather, a fibrocartilaginous wafer called the triangular disk is interposed between the ulnar head and the proximal carpal row (see Fig. 2-2 ). Even the ulnar styloid process is hidden from contact with the carpus by the ulnotriquetral (UT) ligament. The ulnar head is roughly cylindrical in shape, with a distal projection on its posterior border, called the ulnar styloid process. Approximately three fourths of the ulnar head is covered by articular cartilage, with the ulnar styloid process and the posterior one fourth as exposed bone or periosteum. A depression at the base of the ulnar styloid process, called the fovea, is typically not covered in articular cartilage.
Proximal Carpal Row Bones
The proximal row consists of, from radial to ulnar, the scaphoid (navicular), lunate, triquetrum, and pisiform ( Figs. 2-3 and 2-4 ). The scaphoid is shaped somewhat like a kidney bean. The scaphoid anatomy is divided into three regions: the proximal pole, waist, and distal pole. The proximal pole has a convex articular surface that faces the scaphoid fossa and a flat articular surface that faces the lunate. The dorsal surface of the waist is marked by an oblique ridge that serves as an attachment plane for the dorsal joint capsule. The medial surface of the waist and distal surface of the proximal pole is concave and articulates with the capitate. The distal pole also articulates with the capitate medially, but distally it articulates with the trapezium and trapezoid. Otherwise, the distal pole is nearly completely covered with ligament attachments.
The lunate is crescent-shaped in the sagittal plane, such that the proximal surface is convex and the distal surface concave, and it is somewhat wedge-shaped in the transverse plane. With the exception of ligament attachment planes on its dorsal and palmar surfaces, the lunate is otherwise covered with articular cartilage. It articulates with the scaphoid laterally, the radius and triangular fibrocartilage proximally, the triquetrum medially, and the capitate distally. In some individuals, the lunate has a separate fossa for articulation with the hamate, separated from the fossa for capitate articulation by a prominent ridge.
The triquetrum has a complex shape, with a flat articular surface on the palmar surface for articulation with the pisiform; a concave distal articular surface for articulation with the hamate; a flat lateral surface for articulation with the lunate; and three tubercles on the proximal, medial, and dorsal surfaces, respectively. The proximal tubercle is covered in cartilage for contact with the triangular disk, and the medial and dorsal tubercles serve as ligament attachment surfaces.
The pisiform, which means “pea-shaped,” is oval in profile with a flat articular facet covering the distal half of the dorsal surface for articulation with the triquetrum. Otherwise, it is entirely enveloped within the tendon of the FCU and serves as a proximal origin of the flexor digiti minimi muscle.
Distal Carpal Row Bones
The distal carpal row consists of, from radial to ulnar, the trapezium, trapezoid, capitate, and hamate (see Figs. 2-3 and 2-4 ). The trapezium, historically referred to as the greater multangular, has three articular surfaces. The proximal surface is slightly concave and articulates with the distal pole of the scaphoid. The medial articular surface is flat and articulates with the trapezoid. The distal surface is saddle-shaped and articulates with the base of the first metacarpal. The remaining surfaces are nonarticular and serve as attachment areas for ligaments. The anterolateral edge of the trapezium forms an overhang, referred to as the beak, which is part of the fibro-osseous tunnel for the tendon of flexor carpi radialis (FCR).
The trapezoid, referred to historically as the lesser multangular, is a small bone with articular surfaces on the proximal, lateral, medial, and distal surfaces for articulation with the scaphoid, trapezium, capitate, and base of the second metacarpal, respectively. The palmar and dorsal surfaces serve as ligament insertion areas.
The capitate is the largest carpal bone and is divided into head, neck, and body regions. The head is almost entirely covered in articular cartilage and forms a proximally convex surface for articulation with the scaphoid and lunate. The neck is a narrowed region between the body and the head and is exposed to the midcarpal joint without ligament attachment. The body is nearly cuboid in shape with articular surfaces on its medial, lateral, and distal aspects for articulation with the trapezoid, hamate, and base of the third metacarpal, respectively. The large, flat palmar and dorsal surfaces serve as ligament attachment areas.
The hamate has a complex geometry, with a pole, body, and hamulus (hook). The pole is a conical proximally tapering projection that is nearly entirely covered in articular cartilage for articulation with the triquetrum, capitate, and variably the lunate. The body is relatively cuboid, with medial and distal articulations for the capitate and fourth and fifth metacarpal bases, respectively. The dorsal and palmar surfaces serve as ligament attachment areas, except the most medial aspect of the body, where the hamulus arises. The hamulus forms a palmarly directed projection that curves slightly lateral at the palmar margin. This also serves as a broad area for ligament attachment.
Before a discussion of the anatomy of the wrist can be pursued, it is important that a consensus be reached on term definitions. The terms proximal and distal are universally understood, but some confusion may exist regarding terms defining relationships in other planes. Although the terms medial and lateral are anatomically correct, they require a virtual positioning of the upper extremity in the classic anatomic position to be interpretable. Therefore the terms radial and ulnar have been introduced by clinicians to enable an instant understanding of orientation independent of upper extremity positioning, because the reference to these terms (the orientation of the radius and ulna) does not change significantly relative to the wrist. Likewise, the terms anterior, volar, and palmar all describe the front surface of the wrist, whereas dorsal and posterior describe the back surface of the wrist. Some may object to using the term palmar in reference to the wrist, but they should be reminded that the palmar, glabrous skin covers the anterior surface of the carpus; therefore it seems to have an acceptable use in the wrist.
Composed of eight carpal bones as the wrist proper, the wrist should be functionally considered as having a total of 15 bones. This is because of the proximal articulations with the radius and ulna and the distal articulations with the bases of the first through fifth metacarpals. The geometry of the wrist is complex, demonstrating a transverse arch created by the scaphoid and triquetrum/pisiform column proximally and the trapezium and hamate distally. In addition, the proximal carpal row demonstrates a substantial arch in the frontal plane.
The distal radioulnar joint (DRUJ) is mechanically linked to the wrist and provides two additional degrees of motion to the wrist/forearm joint. The DRUJ is the distal of two components of the forearm joint (with the proximal radioulnar joint, or PRUJ). The motion exhibited through the DRUJ is a combination of translation and rotation, created as a pivot of the radius about the ulna through an obliquely oriented axis of rotation passing between the radial head proximally and the ulnar head distally.
From an anatomic standpoint, the carpal bones are divided into proximal and distal carpal rows, each consisting of four bones. This effectively divides the wrist joint spaces into radiocarpal and midcarpal spaces. Although mechanically linked to the distal radioulnar joint (DRUJ), the wrist is normally biologically separated from the DRUJ joint space by the triangular fibrocartilage complex (TFCC).
The radiocarpal joint is formed by the articulation of confluent surfaces of the concave distal articular surface of the radius and the triangular fibrocartilage, with the convex proximal articular surfaces of the proximal carpal row bones.
The midcarpal joint is formed by the mutually articulating surfaces of the proximal and distal carpal rows. Communications are found between the midcarpal joint and the interosseous joint clefts of the proximal and distal row bones, as well as to the second through fifth carpometacarpal joints. Under normal circumstances, the midcarpal joint is isolated from the pisotriquetral, radiocarpal, and first carpometacarpal joints by intervening membranes and ligaments. The geometry of the midcarpal joint is complex. Radially, the scaphotrapezial trapezoidal (STT) joint is composed of the slightly convex distal pole of the scaphoid articulating with the reciprocally concave proximal surfaces of the trapezium and trapezoid. Forming an analog to a “ball-and-socket joint” are the convex head of the capitate and the combined concave contiguous distal articulating surfaces of the scaphoid and the lunate. In 65% of normal adults, it has been found that the hamate articulates with a medial articular facet at the distal ulnar margin of the lunate, which is associated with a higher rate of cartilage eburnation of the proximal surface of the hamate. The triquetrohamate region of the midcarpal joint is particularly complex, with the mutual articular surfaces having both concave and convex regions forming a helicoid-shaped articulation.
Interosseous Joints: Proximal Row
The interosseous joints of the proximal row are relatively small and planar, allowing motion primarily in the flexion–extension plane between mutually articulating bones. The scapholunate (SL) joint has a smaller surface area than the lunatotriquetral (LT) joint. Often, a fibrocartilaginous meniscus extending from the membranous region of the SL or LT interosseous ligaments is interposed into the respective joint clefts.
Interosseous Joints: Distal Row
The interosseous joints of the distal row are more complex geometrically and allow substantially less interosseous motion than those of the proximal row. The capitohamate joint is relatively planar, but the mutually articulating surfaces are only partially covered by articular cartilage. The distal and palmar region of the joint space is devoid of articular cartilage, being occupied by the deep capitohamate interosseous ligament. Similarly, the central region of the trapeziocapitate joint surface is interrupted by the deep trapeziocapitate interosseous ligament. The trapezium-trapezoid joint presents a small planar surface area with continuous articular surfaces.
The ligaments of the wrist have been described in a number of ways, leading to substantial confusion in the literature regarding various features of the carpal ligaments. Several general principles have been identified to help simplify the ligamentous architecture of the wrist. No ligaments of the wrist are truly extracapsular. Most can be anatomically classified as capsular ligaments with collagen fascicles clearly within the lamina of the joint capsule. The ligaments that are not entirely capsular, such as the interosseous ligaments between the bones within the carpal row, are intra-articular. This implies that they are not ensheathed in part by a fibrous capsular lamina. The wrist ligaments carry consistent histologic features, which are, to a degree, ligament-specific. The majority of capsular ligaments are made up of longitudinally oriented laminated collagen fascicles surrounded by loosely organized perifascicular tissue, which are in turn surrounded by the epiligamentous sheath. This sheath is generally composed of the fibrous and synovial capsular lamina. The perifascicular tissue has numerous blood vessels and nerves aligned longitudinally with the collagen fascicles. The function of these nerves is currently not well understood. It has been hypothesized that these nerves are an integral part of a proprioceptive network, following the principals of Hilton’s law of segmental innervation. The palmar capsular ligaments are more numerous than the dorsal, forming almost the entire palmar joint capsules of the radiocarpal and midcarpal joints. The palmar ligaments tend to converge toward the midline as they travel distally and have been described as forming an apex-distal V . The interosseous ligaments between the individual bones within a carpal row are generally short and transversely oriented and, with specific exceptions, cover the dorsal and palmar joint margins. Specific ligament groups are briefly described in the following sections and are divided into capsular and interosseous groups.
Distal Radioulnar Ligaments
Although a description of the DRUJ is beyond the scope of this chapter, a brief description of the anatomy of the palmar and dorsal radioulnar ligaments is required to understand the origin of the ulnocarpal ligaments. The dorsal and palmar DRUJ ligaments are believed to be major stabilizers of the DRUJ. These ligaments are found deep (proximal) in the TFCC and form the dorsal and palmar margins of the TFCC in the region between the sigmoid notch of the radius and the styloid process of the ulna (see Fig. 2-2 ). Attaching radially at the dorsal and palmar corners of the sigmoid notch, the ligaments converge ulnarly and attach near the base of the styloid process, in the region called the fovea. The palmar ligament has substantial connections to the carpus through the ulnolunate (UL), UT, and ulnocapitate (UC) ligaments. The dorsal ligament integrates with the sheath of extensor carpi ulnaris (ECU). The concavity of the TFCC is deepened by more superficial fibers of the distal radioulnar ligament complex, which attaches to the styloid process.
Palmar Radiocarpal Ligaments
The palmar radiocarpal ligaments arise from the palmar margin of the distal radius and course distally and ulnarly toward the scaphoid, lunate, and capitate ( Figs. 2-3 and 2-5 ). Although the course of the fibers can be defined from an anterior view, the separate divisions of the palmar radiocarpal ligament are best appreciated from a dorsal view through the radiocarpal joint (see Fig. 2-5 ). The palmar radiocarpal ligament can be divided into four distinct regions. Beginning radially, the radioscaphocapitate (RSC) ligament originates from the radial styloid process, forms the radial wall of the radiocarpal joint, attaches to the scaphoid waist and distal pole, and passes palmar to the head of the capitate to interdigitate with fibers from the UC ligament. Very few fibers from the RSC ligament attach to the capitate. Just ulnar to the RSC ligament, the long radiolunate (LRL) ligament arises to pass palmar to the proximal pole of the scaphoid and the SL interosseous ligament to attach to the radial margin of the palmar horn of the lunate. The interligamentous sulcus separates the RSC and LRL ligaments throughout their courses. The LRL ligament has been called the radiolunatotriquetral ligament historically, but the paucity of fibers continuing toward the triquetrum across the palmar horn of the lunate renders this name misleading. Ulnar to the origin of the LRL ligament, the radioscapholunate (RSL) “ligament” emerges into the radiocarpal joint space through the palmar capsule and merges with the SL interosseous ligament and the interfossal ridge of the distal radius. This structure resembles more of a “mesocapsule” than a true ligament, because it is made up of small-caliber blood vessels and nerves from the radial artery and anterior interosseous neurovascular bundle. Very little organized collagen is identified within this structure. The mechanical stabilizing effects of this structure have recently been shown to be minimal. The final palmar radiocarpal ligament, the short radiolunate (SRL) ligament, arises as a flat sheet of fibers from the palmar rim of the lunate fossa, just ulnar to the RSL ligament. It courses immediately distally to attach to the proximal and palmar margin of the lunate.