What is wrist instability?
Instability of the wrist is a common cause of wrist pain. Instability in general is defined as the inability of the osseous structures to maintain their normal anatomic relationship under physiologic loading forces. Instability of the wrist is caused by injury to ligaments of the wrist either on a traumatic or an attritional (chronic overload) basis and can be classified using many different schemes. One of the most popular schemes, at least for the proximal wrist, is based on the orientation of the lunate bone. Those cases in which the distal surface of the lunate is angled toward the dorsum are identified as dorsal intercalated segmental instability (DISI). Those cases in which the lunate has a volar angulation are referred to as volar intercalated segmental instability (VISI). The DISI pattern of instability is associated with tears of the (intrinsic) ligament between the scaphoid and lunate (scapholunate ligament), and the VISI pattern of instability is associated with tears of the ligament between the lunate and triquetrum (lunotriquetral ligament). Both ligaments are U-shaped structures in sagittal cross-section between the adjacent bones that “open” distally and have strong components on the distal and volar surface and weak components on the proximal surface. The volar or dorsal tilt of the lunate may be evident on radiographs obtained in the lateral projection, in which case it is referred to as “static” instability and is considered to represent a more severe degree of instability. In some cases, this abnormal tilt of the lunate can be demonstrated by the application of provocative maneuvers, either using fluoroscopy or physical examination, and is referred to as “dynamic” instability, which is considered less severe.