The mechanical axes of the wrist are oriented obliquely to the anatomical axes.
BACKGROUND
the complex motions of the wrist are described in terms of four anatomical directions that are accomplished through the multiple articulations of the carpus. With minimal tendinous insertions, the carpus is primarily a passive structure. This emphasizes the importance of its mechanical properties, which few studies have examined to date. The purpose of the present study was to determine the mechanical properties of the wrist in twenty-four different directions of wrist motion.


METHODS
the moment-rotation mechanical behavior of six fresh-frozen cadaver wrists was determined in four directions: flexion, extension, ulnar deviation, and radial deviation. Twenty other directions that were a combination of these anatomical directions were also studied. A custom-designed jig was interfaced with a standard materials testing system to apply unconstrained moments. Moments of ± 2 Nm were applied, and the moment-rotation data were recorded and analyzed to determine the neutral zone, range of motion, and stiffness values as well as the orientation of the envelope of these values.


RESULTS
the envelope of wrist range-of-motion values was ellipsoidal in shape and was oriented obliquely (p < 0.001) to the direction of pure flexion-extension by a mean (and standard deviation) of 26.6° ± 4.4°. The largest wrist range of motion was a mean of 111.5° ± 10.2°, in the direction of ulnar flexion, 30° from pure flexion. The largest stiffness (mean, 0.4 Nm/deg) was in the direction of radial flexion, while the smallest stiffness (mean, 0.15 Nm/deg) was in the direction of ulnar flexion.


CONCLUSIONS
the mechanical axes of the wrist are oriented obliquely to the anatomical axes. The primary mechanical direction is one of radial extension and ulnar flexion, a direction along a path of the dart thrower's wrist motion.


CONCLUSIONS
understanding the mechanical function of the wrist can aid clinical treatment decisions, arthroplasty, and implant designs. The findings of this study provide new evidence that the mechanical axes of the wrist are not collinear with the anatomical axes.
Bioengineering Laboratory, Department of Orthopaedics, The Warren Alpert Medical School of Brown University and Rhode Island Hospital, 1 Hoppin Street, CORO West Suite 404, Providence, RI 02903, USA. joseph_crisco@brown.edu
Joseph J
JJ
Crisco
Wendell M R
WM
Heard
Ryan R
RR
Rich
David J
DJ
Paller
Scott W
SW
Wolfe
D000368:Aged; D001696:Biomechanical Phenomena; D002102:Cadaver; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011859:Radiography; D016059:Range of Motion, Articular / Q000502:physiology*; D012399:Rotation; D013314:Stress, Mechanical; D014953:Wrist / Q000033:anatomy & histology / Q000000981:diagnostic imaging / Q000502:physiology; D014955:Wrist Joint / Q000033:anatomy & histology / Q000000981:diagnostic imaging / Q000502:physiology*
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