Pressure distribution in the wrist joint.

We performed a study to determine pressure distribution properties of the normal radio-carpal joint. A system was developed for measurement of the contact pressure within the wrist joint surfaces. The transducer was based on Fuji pressure-sensitive paper, which was inserted into the joint space through a dorsal capsular incision. The hand was then positioned using a jig that permitted free axial loading of the joint. Each of five specimens was tested in 36 positions combining flexion/extension with radio/ulnar deviation and supination/pronation. The transducers were analyzed for contact area, scapho-lunate contact area ratio, pressure, and centroid locations using a microcomputer-based video-imaging system. The scaphoid and lunate contact areas on the radius and triangular fibrocartilage were separate and distinct in all wrist positions. Together these contact areas accounted for a relatively small fraction of the total joint surface area (average contact area/total joint area = 0.206, SD = 0.0495). For an applied 103 Newton compressive load, the high pressure averaged 3.17 MPa (SD = 0.83 MPa). Overall, the scaphoid contact area was 1.47 times that of the lunate, although variations occurred with position, as in flexion, in which the scaphoid/lunate area ratio was 0.83. The high-pressure centroids of both scaphoid and lunate contact areas shifted palmarly from 20 degrees of flexion to 20 degrees of extension and then dorsally with further extension. The scaphoid-lunate intercentroid distance averaged 14.91 mm with a range of 10-20 mm.