Imaging description

On a lateral radiograph of the wrist, axes can be drawn and carpal relationships can be inferred based on the angle of these axes. A line parallel to the center of the radial shaft is its axis. The lunate axis is a line drawn perpendicular to the anterior and posterior distal lunate poles. To determine the scaphoid axis, a line is drawn connecting the proximal and distal ventral convexities of the scaphoid. This line is parallel to or is only a few degrees off the central axis of the scaphoid, and can easily be drawn on any adequate lateral wrist view. In the normal wrists, the longitudinal axes of the third metacarpal, capitate, lunate, and radius should all fall on the same plane.

The scapholunate angle normally ranges from 30 to 60°, average 47°. When the lunate is extended dorsally, it is called dorsal intercalated segmental instability (DISI) (Figure 35.1) and when it is tipped volarly, it is called volar intercalated segmental instability (VISI) (Figure 35.2). The scapholunate angle increases when the concave distal articular surface of the lunate faces or tilts dorsally, and the scaphoid stays in its normal position or tilts volarly. A scapholunate angle greater than 70° or 80° indicates DISI. VISI exists when the distal articular surface of the lunate faces volarly. In this situation, the lunate may tilt volarly more than the scaphoid, so that the scapholunate angle is less than 30°.