Load transmission through the wrist in the extended position.
OBJECTIVE
The wrist is subjected to extremely high compressive loads in the extended position, but pathoanatomy of this region remains unclear. The purpose of this study was to analyze force transmission in the maximum extended position to clarify the pathomechanics of wrist injury.


METHODS
Two sets of computed tomography images of wrist joints were obtained for 7 normal subjects: one set in neutral position and the other set in maximum extension. A three-dimensional rigid body spring model was used to analyze stress distributions through the wrist joint. The wrist joint was constructed from computed tomography images. External force was applied to the 5 metacarpals in neutral position and to the palm in extended position. Force transmissions through the carpus and ligament tension in extended position were compared with those in neutral position, and force distributions were compared in each position.


RESULTS
Force transmission ratio on the scaphoid fossa significantly increased from 52% in neutral to 62% in extension (p< .05), whereas the ratio through the lunate fossa decreased from 42% to 36%. In the midcarpal joint, force to the scaphoid significantly increased from 60% to 69% (p< .05). Force distributions of the radiocarpal joint in the extended position moved on the center of the lunate fossa and interfossal ridge of the scaphoid fossa. The dorsal ridge of the radial articular surface appeared as the new contact area. Tension in 3 palmar intrinsic ligaments and the flexor retinaculum greatly increased in the extended position.


CONCLUSIONS
Force transmission in the extended position shifted radially, concentrating at the scaphoid. We could show how bending force causes scaphoid fracture and concentration of force on the radius surface might cause intra-articular fracture coinciding with the fracture pattern introduced by Melone. The palmar intrinsic ligaments appear key to maintaining the carpal arch in push-up position. Our theoretical analysis could well explain several patterns of wrist injuries.
Majima
Masataka
M
Department of Hand Surgery, Nagoya University School of Medicine, Showaku, Nagoya, Japan. masamotomiu1062mmm@yahoo.co.jp
Horii
Emiko
E
Matsuki
Hiroshi
H
Hirata
Hitoshi
H
Genda
Eiichi
E
D000328:Adult; D001696:Biomechanical Phenomena; D003198:Computer Simulation; D006801:Humans; D021621:Imaging, Three-Dimensional; D008023:Ligaments, Articular / Q000502:physiology; D008297:Male; D008954:Models, Biological; D009068:Movement / Q000502:physiology; D014057:Tomography, X-Ray Computed; D014955:Wrist Joint / Q000000981:diagnostic imaging / Q000502:physiology*