Traumatic Instability of the Wrist

The scaphoid is a mechanical link that stabilizes the intercarpal joint during motions of the wrist. Without this stability the proximal carpal row acts as an unsupported intercalated link in a three-link system and zigzag collapse occurs with axial loading. A deformity with dorsiflexion of the lunate within the linkage (dorsiflexion instability) occurs commonly after scaphoid fracture and scapholunate dissociation. When dissociation occurs the scaphoid assumes a vertical position, that is, the angle formed by the longitudinal axes of the scaphoid and the lunate approaches a right angle. Rupture of the distal attachments of the palmar radiocarpal ligament and of the scapholunate ligament appears to induce dissociation. It is therefore concluded that ligamentous injury along with scaphoid fracture is probably necessary if dorsiflexion instability is to develop. Palmar flexion instability characterized by palmar flexion of the lunate within the wrist linkage appears to be associated with ulnar displacement of the carpus as is seen in rheumatoid arthritis or after loss of the distal end of the ulna. This position may be normal in a small percentage of patients. The direction of the intercarpal collapse is related to the location of the pressure of the head of the capitate against the concave surface of the lunate, that is, whether this pressure is dorsal or palmar to the plane of the radiolunate fulcrum on the proximal convex surface of the lunate. The direction of the collapse is also related to the normal oblique path of motion (rotation and sliding) of the capitate during ulnar and radial deviation and is intimately controlled by the geometric configuration of the bones and the resultant of forces on the carpus. These forces are determined by the strength, direction, and leverage of the musculotendinous units, which cross the joints of the wrist complex.