The advantage of throwing the first stone: how understanding the evolutionary demands of Homo sapiens is helping us understand carpal motion

Abstract

Unlike any other diarthrodial joint in the human body, the "wrist joint" is composed of numerous articulations between eight carpal bones, the distal radius, the distal ulna, and five metacarpal bones. The carpal bones articulate with each other as well as with the distal radius, distal ulna, and the metacarpal bases. Multiple theories explaining intercarpal motion have been proposed; however, controversy exists concerning the degree and direction of motion of the individual carpal bones within the two carpal rows during different planes of motion. Recent investigations have suggested that traditional explanations of carpal bone motion may not entirely account for carpal motion in all planes. Better understanding of the complexities of carpal motion through the use of advanced imaging techniques and simultaneous appreciation of human anatomic and functional evolution have led to the hypothesis that the "dart thrower's motion" of the wrist is uniquely human. Carpal kinematic research and current developments in both orthopaedic surgery and anthropology underscore the importance of the dart thrower's motion in human functional activities and the clinical implications of these concepts for orthopaedic surgery and rehabilitation.

Figure 1

Illustration of the “column theory” of wrist kinematics, initially described by Navarro. The carpal bones are clustered in radial (red), central (blue), and ulnar (orange) columns.

Figure 2

Illustration of the carpus as described by Destot. Two distinct rows of carpal bones are evident: the proximal carpal row (fuschia), comprising the triquetrum and the lunate, and the distal carpal row (blue), comprising the trapezium, trapezoid, capitate, and hamate. The scaphoid (red) was initially described as a critical and independent link between the proximal and distal rows.

Figure 3

AP (A) and lateral (B) illustration of the “intercalated segment,” the term coined by Landsmeer to describe the proximal row with respect to the forearm and the distal row. The intercalated segment (red) describes the proximal carpal bones (ie, scaphoid, lunate, and triquetrum) that have no tendon insertions and are balanced between the articular surface of the distal forearm (beige) and the bones of the distal row (blue). Their motion is guided by mechanical signals from the distal row and is constrained by a complex system of intrinsic and extrinsic ligaments.

Figure 4

Three-dimensional images demonstrating the unique pattern of proximal row kinematics during the dart thrower’s motion, as revealed by in vivo studies of carpal kinematics. The top row is a volar view and the bottom row a radial view of the capitate, scaphoid, and lunate in the neutral position (gray bones) during wrist flexion (purple)-extension (yellow) (A), wrist ulnar deviation (purple)-radial deviation (yellow) (B), and dart thrower’s motion of wrist radial extension (yellow)-ulnar flexion (purple) (C). The total range of wrist motion, as illustrated by the positions of the capitate, is approximately the same for each direction of wrist motion in each panel. Despite the nearly identical amount of wrist motion, the motion of the proximal row is substantially and significantly reduced as the wrist moves along the dart thrower’s path, as seen in panel C. This difference is most readily appreciated by focusing on the distance the scaphoid tubercle has traveled in each panel of the figure. The motion of the capitate, scaphoid, and lunate are visualized relative to the radius that was mathematically fixed by its coordinate system (red, blue, and green vectors).

Figure 5

Photograph of the three-jaw grip, which was refined during primate evolution by morphologic adaptations in carpometacarpal mobility, finger-to-thumb-length ratio, and hypothenar pad development, allowing for precision handling of tools and weapons. (Reproduced with permission from Wolfe SW, Crisco JJ, Orr CM, Marzke MW: The dart-throwing motion of the wrist: Is it unique to humans? J Hand Surg Am 2006;31 : 1429–1437.)

Figure 6

Photograph demonstrating the relatively recent “squeeze” modification of the power grip. (Reproduced with permission from Wolfe SW, Crisco JJ, Orr CM, Marzke MW: The dart-throwing motion of the wrist: Is it unique to humans? J Hand Surg Am 2006;31 : 1429–1437.)

Figure 7

Photographs of kinematic studies of the dart thrower’s plane of motion using clusters of reflective skin markers and motion-analysis technology. Dart throwing (A) and hammering (B and C) involve a similar coupling of wrist flexion-extension and ulnar-radial deviation. This is exemplified by the wrist extension posture of the cocking phase of hammering and the follow-through of ulnar flexion.