34 The Carpometacarpal Joints

Amy L. Ladd

34 The Carpometacarpal Joints

34.1 Introduction

The structural anatomy reflects the functional demands of the carpometacarpal (CMC) joints: they create an architectural foundation for the digits to position, manipulate, and respond to surfaces and objects that the human hand encounters. The proximal transverse arch of the CMC joints comprises the distal carpal row and the proximal five metacarpals, completed by the transverse carpal ligament; it stabilizes the hand and disperses forces, similar to a masonry arch. J. William Littler likened this to a fixed arch, while the activity at the metacarpal heads creates an adaptive arch (▶Fig. 34.1). Similarly, a fixed longitudinal unit of the second and third metacarpals and the distal carpal row permits the thumb and fingers to create gross and powerful action or, conversely, the intricate fine movement unique to the human hand. ¹ The third CMC (CMC-3) joint serves as the keystone and is the rigid, neutral axis of the hand and forearm.

**Fig. 34.1** (**a, b**) The concept of a fixed unit of the wrist comprises a longitudinal arch of the second and third metacarpal, along with the distal carpal row. The rigid transverse carpal arch of the carpometacarpal joints stabilizes the adaptive arch of the metacarpal heads, and the transverse carpal ligament completes the arch. (After Littler, from Hentz and Chase.) ¹

Emanating in either direction, the CMC articulations become increasingly mobile. The muscles and tendons within the palmar arch enable their dynamic capabilities, acting upon the distal segments to create the range of palmar excursion from a flat hand to the rotation and flexion of palmar cupping. At either end the especially mobile thumb CMC (CMC-1) and the small finger CMC (CMC-5) permit the opening and prehension that affords a powerful grip, extending the vector of the forearm to create a strong powerful lever arm, unique to human function. ² The resourceful thumb also creates a precision pinch; the balance between stability and mobility arguably comes at a price with its vulnerability for osteoarthritis. The proprioception and sensibility within the ligaments, muscles, and the glabrous, tactile skin communicate with the brain to carry out object manipulation. ³

The block like joints have variable curvature and interdigitations (▶Fig. 34.2), stronger dorsal than palmar carpometacarpal ligaments, and intermetacarpal ligaments (▶Fig. 34.3). Five primary ligaments stabilize CMC-1 with a dorsal and volar intermetacarpal ligament attaching to the second metacarpal. ⁴ ^, ⁵ The CMC-2 through CMC-5 joints possess 9 dorsal ligaments and 11 palmar ligaments, an interosseous ligament, and an intra-articular ligament between CMC-3 and CMC-4. Intermetacarpal ligaments exist between each of the metacarpals. ⁶ ^, ⁷

**Fig. 34.2** An early 18th-century engraving demonstrates the osteology of the hand: (a) the dorsal and (b) volar views and (c) the opened view of the carpometacarpal joints. (d) The transverse arch is visualized, demonstrating the metacarpal articulations and dorsal and volar intermetacarpal ligaments with the carpal bones removed. M1, first metacarpal; M5, fifth metacarpal. (a–c with permission, AL Ladd.)

**Fig. 34.3** The dorsal carpometacarpal (CMC) and intermetacarpal ligaments are shown; the block like joint configuration and these stout ligaments confer stability. The most rigid is the third CMC (CMC-3) joint, with increasing mobility toward the small finger and thumb. (a) A dorsal view of a three-dimensional (3D) model of a right wrist. The *solid colored areas* show ligament attachment locations, and the *transparent colored areas* show the paths of the ligaments. 1, dorsal second metacarpal radial base–trapezium ligament; 2, dorsal second metacarpal radial base–trapezoid ligament; 3, dorsal second metacarpal ulnar base–trapezoid ligament; 4, dorsal third metacarpal radial base–trapezoid ligament; 5, dorsal third metacarpal radial base–capitate ligament; 6, dorsal third metacarpal ulnar base–capitate–fourth metacarpal radial base ligament; 7, dorsal fourth metacarpal ulnar base–hamate ligament; 8, dorsal fourth metacarpal ulnar base–hamate–fifth metacarpal radial base ligament. APL, abductor pollicis longus; ECRB, extensor carpi radialis brevis; ECRL, extensor carpi radialis longus. (b) A dorsal view of a 3D model of a right wrist. The *solid colored areas* show ligament attachment locations, and the *transparent colored areas* show the paths of the ligaments. 1, dorsal second metacarpal radial base–trapezium ligament; 2, dorsal second metacarpal radial base–trapezoid ligament; 3, dorsal second metacarpal ulnar base–trapezoid ligament; 4, dorsal third metacarpal radial base–trapezoid ligament; 5, dorsal third metacarpal radial base–capitate ligament; 6, dorsal third metacarpal ulnar base–capitate–fourth metacarpal radial base ligament; 7, dorsal fourth metacarpal ulnar base–hamate ligament; 8, dorsal fourth metacarpal ulnar base–hamate–fifth metacarpal radial base ligament; 9, dorsal fifth metacarpal ulnar base–hamate ligament. ECRB, extensor carpi radialis brevis; ECRL, extensor carpi radialis longus; ECU, extensor carpi ulnaris. (c) A palmar view of a 3D model of a right wrist. The *solid colored areas* show ligament attachment locations, and the *transparent colored areas* show the paths of the ligaments. 1, palmar second metacarpal–trapezium ligament; 2, palmar second metacarpal–trapezoid ligament; 3, palmar third metacarpal radial base–trapezium ligament; 4, palmar third metacarpal radial base–trapezoid ligament; 5, palmar third metacarpal–capitate ligament; 6, palmar third metacarpal ulnar base–hamate ligament; 7, palmar fourth metacarpal ulnar base–hook of hamate ligament; 8, palmar fifth metacarpal–hook of hamate ligament; 9, the third metacarpal, fourth metacarpal, fifth metacarpal, hamate–pisiform ligament; 10, second metacarpal–trapezium interosseous ligament; 11, palmar third metacarpal–fourth metacarpal–fifth metacarpal ligament. APL, abductor pollicis longus; ECU, extensor carpi ulnaris; FCR, flexor carpi radialis; FCU, flexor carpi ulnaris. (Reproduced with permission from Nanno M, Buford WL Jr, et al. Three-dimensional analysis of the ligamentous attachments of the second through fifth carpometacarpal joints. Clin Anat 2007;20(5):530–544.)

Following Hilton’s law, ⁸ the joints are innervated regionally by each of the terminal branches of the median, radial, and ulnar nerves, with CMC-1 ⁹ and likely CMC-5 having the most cross-innervation. Similarly, the terminal carpal and digital branches of the radial and ulnar artery and anterior interosseous artery provide their blood supply. The synovium is typically continuous with occasional disruption between the hamate and fourth and fifth metacarpal. ¹⁰

34.2 Thumb Carpometacarpal Joint

In concert with the human brain, the thumb coordinates elite, precise function with the fingers. Its demands create a paradox between mobility and stability: uniquely poised to grasp a shot put or pen a beautiful script. To oppose its neighbors with stable pinch and grasp, it recessed over millions of years from a three-phalanx digit to the current two phalanges and metacarpal upon a mobile saddle joint. ² ^, ¹¹ ^– ¹³ With the trapezium’s terminal post at the radial carpal arch establishing the thumb’s offset position, its coordinates lie tangential to the plane of the thorax and fingers, respectively (▶Fig. 34.4). ¹⁴ ^– ¹⁷ Composite opposition to hold a pen requires positioning and stabilizing muscles across the CMC joint, imparting abduction, flexion, translation, and pronation; additional contributors are the metacarpophalangeal (MCP) and interphalangeal joints. ¹⁸ Human prehension requires an opposable strong thumb, the small finger’s hypothenar complex, ¹⁹ and wrist positioning in the “dart-thrower’s motion” ²⁰ ^, ²¹ with thumb and forearm in line, extending the lever arm and imparting mechanical advantage to throw a fast ball or swing a golf club. ² ^, ³

**Fig. 34.4** The motion arcs of the metacarpal upon the trapezium are flexion-extension and abduction-adduction. Pronation-supination represents composite rotation and translation of this triaxial joint based on morphology and muscular activity (Fick, Cooney, Zancolli, Haines). The thumb position in relation to the fingers represents a completion of the carpal arch, which places the carpometacarpal joint obliquely volar to the adjacent fingers and proximally oriented on its radial aspect. The arcs of motion thus are out of phase with the fingers. Image redrawn from computed tomography surface rendering of a normal right hand. (Image used with permission from S Hegmann.)

The trapezium’s metacarpal surface is gently comma-shaped (left) or C-shaped (right) with the ulnar–radial axis as a vertical reference to this shape (▶Fig. 34.5). The loose configuration relies upon ligament and muscular stability. The concavo-convex saddle trapezial surface creates “reciprocal reception” (gray) with the metacarpal to permit wide circumduction (▶Fig. 34.6). The larger ulnar surface and eccentric contact provides a “rollback” or “screw-home” mechanism, similar to a knee. ¹⁸ ^, ²² Although largely considered a “saddle” joint, its eccentric morphology permits slide, glide, rotation, and translation circumduction, which includes features reminiscent of a ball- and-socket joint; arguably, the combined motion is somewhere between the two. ²³ The trapezium’s prominences and multiple articulations with the first and second metacarpal, trapezoid, and scaphoid secure its anchoring position as the radial abutment of the transverse arch and its relationship with the proximal carpal row (▶Fig. 34.7).

**Fig. 34.5** (a) The trapezium devoid of articular cartilage. This is a right trapezium with the metacarpal articular surface facing upward. The ulnar side is toward the top, and the flex- or carpi radialis groove is marked with an *asterisk*. (From the Bassett Anatomy collection, Stanford University, with permission.) (b) The metacarpal surface is gently comma-shaped (*left*) or C-shaped (*right*, as indicated here) as positioned in viewing one’s own thumb from above, with the radial side closest to the body. The *asterisk* corresponds to the gentle concavity on the dorsal side. The pin emanates from the ulnar position at the trapezoid articulation; the shape is in reference to the ulnar–volar axis. (With permission from AL Ladd.)

**Fig. 34.6** The reciprocal articulating surfaces of a right first metacarpal upon the trapezium. (With permission from AL Ladd.)

**Fig. 34.7** The osteology of the trapezium; articular cartilage intact. (a) The flexor carpi radialis (FCR) groove runs volar and in an oblique direction (*arrow*); since the FCR runs relatively longitudinally in the palm, this orientation indicates the offset and oblique position of the trapezium relative to the rest of the fingers. (b) The scaphoid (S) and trapezoid (Td) facets of the trapezium are visualized. (c) As viewed from the dorsal side, the S, Td, second metacarpal (M2), and first metacarpal (M1) articular facets of the trapezial are seen. (d) The M2–Td articulation viewed from the volar side, with articulation trapezial facets visualized; the trapezium is removed. Note the dorsally positioned trapezial facet of the M2 (*), which illustrates the volar position of the body of trapezium relative to the remainder of the transverse fixed arch. A, dorsal branch radial artery; M1, articular facet of first metacarpal; S, scaphoid; Td, trapezoid. (Images a–c courtesy AL Ladd.)

The CMC joint’s investing ligaments are stout dorsally and thin volarly, both as measured in thickness and cellular content. ²⁴ ^– ²⁶ The structural difference has been verified from outside-in gross dissection, inside-out arthroscopically, and radiographic position on a total of 47 cadavers as follows (▶Fig. 34.4, ▶Fig. 34.8, ▶Fig. 34.9, and ▶Fig. 34.10). The dorsal deltoid complex emanates from the dorsal tubercle and distally fans across the dorsum of the metacarpal (▶Fig. 34.8). The stout collagen is organized and cellular and possesses proprioceptive mechanoreceptors known as Ruffini endings near ligamentous attachments. ⁴ ^, ⁸ ^, ⁹ Conversely, the volar ligaments are thin, capsular structures, variable in location, with thenar muscles intimal to their presence (▶Fig. 34.9) that variably take origin upon the trapezium. Absence of organization and proprioceptive nerve endings support the concept that muscular activity contributes to volar stability. Muscle is rich in proprioceptive muscle spindles, and their presence in thenar muscles has been demonstrated with conditioning experiments in both central and peripheral evoked potential testing. ²⁷ ^, ²⁸ These findings support thenar strengthening in functional CMC rehabilitation with favorable MCP flexed positioning, preventing hyperextension. ²⁹ ^, ³⁰ The anterior oblique “beak” ligament (AOL) has been implicated for its importance in the presence and creation of arthritis, and reconstructing it surgically is important. ³¹ ^– ³⁴ Although current evidence for the AOL’s being an attenuated, thin structure challenges its historic importance, the emphasis on both restoring the palmar stability and strengthening with surgical reconstruction, along with postoperative rehabilitation to maximize functional return, complements both current and historical emphasis on thenar muscular coupling as integral to palmar support.

**Fig. 34.8** The dorsal ligamentous anatomy presented from gross dissections, immunohistochemical staining, and radiographic marking. (a) The stout dorsal ligaments form a deltoid complex emanating from the dorsal tubercle (*), representing the dorsal radial ligament (DRL), dorsal central ligament (DCL), and posterior oblique ligament (POL) in a left hand. (**b, c**) Dorsal ligament immunofluorescent PGP9. 5 and 4′,6-diamidino-2-phenylindole (DAPI) staining demonstrating a Ruffini ending (b) and nucleated collagen (c). These were essentially absent in all volar ligaments examined. (d) The radiographic representation of the dorsal ligaments with anteroposterior, lateral, and Robert’s views. ¹ ^– ³ DRL (*green*), DCL (*orange*), POL (*magenta*), and APL (*red*). (Image d courtesy AL Ladd.)

**Fig. 34.9** The volar ligamentous anatomy from gross dissections and radiographic marking. (a) The volar ligaments in passive extension: anterior oblique ligament (AOL) and ulnar collateral ligament (UCL). The window between the thin ligaments is commonly found intimal to the thenar muscles. (b) The radiographic representation of the volar ligaments with anteroposterior, lateral, and Robert’s views ¹ ^, ² ^, ³ . AOL (*blue*), UCL (*yellow*), APL (*red*). FCRg, the obliquely oriented flexor carpi radialis groove. (Images a and b courtesy AL Ladd, E Hagert.)

**Fig. 34.10** The ulnar checkrein complex: the ulnar collateral ligament, or more appropriately, the volar trapeziometacarpal ligament and the dorsal trapeziometacarpal ligament span from their more central locations proximally to a conjoined attachment directly ulnarly on the first metacarpal (M1). ⁴ ^, ⁵ ^, ¹⁸ (Image courtesy AL Ladd.)

The ulnar complex creates a checkrein effect: the ulnar collateral ligament or, more appropriately, the volar trapeziometacarpal ligament and the dorsal trapeziometacarpal ligament span from their more central locations proximally to a conjoined attachment directly ulnarly. ⁴ ^, ⁵ ^, ¹⁸ , The stout ulnar complex may be critical to volar concentration of forces (▶Fig. 34.10).