Background: Wrist motion is dependent on the complex articulations
of the scaphoid and lunate at the radiocarpal joint. However, much of what is
known about the radiocarpal joint is limited to the anatomically defined
motions of flexion, extension, radial deviation, and ulnar deviation. The
purpose of the present study was to determine the three-dimensional in vivo
kinematics of the scaphoid and lunate throughout the entire range of wrist
motion, with special focus on the dart thrower's wrist motion, from radial
extension to ulnar flexion.
Methods: The three-dimensional kinematics of the capitate, scaphoid,
and lunate were calculated from serial computed tomography scans of both
wrists of fourteen healthy male subjects (average age, 25.6 years; range,
twenty-two to thirty-four years) and fourteen healthy female subjects (average
age, 23.6 years; range, twenty-one to twenty-eight years), which yielded data
on a total of 504 distinct wrist positions.
Results: The scaphoid and lunate primarily flexed or extended in all
directions of wrist motion, and their rotation varied linearly with the
direction of wrist motion (R2 = 0.90 and 0.82, respectively).
Scaphoid and lunate motion was significantly less along the path of the dart
thrower's motion than in any other direction of wrist motion (p < 0.01 for
both carpal bones). The scaphoid and lunate translated radially (2 to 4 mm)
when extended, but they did not translate appreciably when flexed.
Conclusions: The dart thrower's path defined the transition between
flexion and extension rotation of the scaphoid and lunate, and it identified
wrist positions at which scaphoid and lunate motion approached zero. These
findings indicate that this path of wrist motion confers a unique degree of
radiocarpal stability and suggests that this direction, rather than the
anatomical directions of wrist flexion-extension and radioulnar deviation, is
the primary functional direction of the radiocarpal joint.
Clinical Relevance: The goal of reconstructive surgery for the
treatment of injuries involving the scaphoid, the lunate, or their
interosseous ligaments is to maximize healing, stability, and postoperative
range of motion. Our in vivo findings suggest that specific rehabilitation
protocols employing the dart thrower's motion could be designed to limit
radiocarpal motion while providing the benefits of maintaining wrist