In vivo radiocarpal kinematics and the dart thrower's motion.
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.
Crisco
Joseph J
JJ
Bioengineering Laboratory, Department of Orthopaedics, Brown Medical School/Rhode Island Hospital, 1 Hoppin Street, CORO West Suite 404, Providence, RI 02903. joseph_crisco@brown.edu.
Coburn
James C
JC
Bioengineering Laboratory, Department of Orthopaedics, Brown Medical School/Rhode Island Hospital, 1 Hoppin Street, CORO West Suite 404, Providence, RI 02903. joseph_crisco@brown.edu.
Moore
Douglas C
DC
Bioengineering Laboratory, Department of Orthopaedics, Brown Medical School/Rhode Island Hospital, 1 Hoppin Street, CORO West Suite 404, Providence, RI 02903. joseph_crisco@brown.edu.
Akelman
Edward
E
Department of Orthopaedics, Brown Medical School/University Orthopaedics, 2 Dudley Street, Suite 200, Providence, RI 02905.
Weiss
Arnold-Peter C
AC
Department of Orthopaedics, Brown Medical School/University Orthopaedics, 2 Dudley Street, Suite 200, Providence, RI 02905.
Wolfe
Scott W
SW
The Alberto Vilar Center for Hand and Upper Extremity Research, Hospital for Special Surgery, Weill Medical College of Cornell University, 523 East 72nd Street, New York, NY 10021.
D000328:Adult; D001696:Biomechanical Phenomena; D051224:Capitate Bone / Q000000981:diagnostic imaging / Q000502:physiology*; D005260:Female; D006801:Humans; D012667:Lunate Bone / Q000000981:diagnostic imaging / Q000502:physiology*; D008297:Male; D016059:Range of Motion, Articular; D021361:Scaphoid Bone / Q000000981:diagnostic imaging / Q000502:physiology*; D014057:Tomography, X-Ray Computed; D014955:Wrist Joint / Q000000981:diagnostic imaging / Q000502:physiology*