Dynamic carpal stability.

The term carpal instability is commonly used, but what carpal stability actually is has not been defined. Much of the mechanically complex wrist's versatility is due to the intercalated three bone proximal carpal row. Landsmeer described the collapse tendency associated with intercalated segments. The factors which provide static stability are the oblique alignment of the scaphoid, the obliquely aligned dorsal and palmar ligamentous complexs, the intrinsic perilunate ligaments, the transiting transcarpal tendons and the negative intraarticular pressure. The proximal carpal row adjusts its posture on the counterbalancing flexion/pronation torque exerted by the scaphoid and the extension/supination torque exerted by the triquetrum. The dynamic factors are the compressive force exerted across the joint acting on the joint surfaces and the effect of the bowstringing force provided by the flexor carpi radialis acting at the scaphoid tuberosity. The proximal carpal row has a tendency to translate ulnarly along the ulnarly sloped radial articular surface while the distal row has a tendency to slide radially on the radially sloped lunatatotriqueteral distal articular surface. This activity produces differential tension in the ligaments attaching to the triquetrum which effects an extension/supination stance of the triquetrum. The force couple acting on the scaphoid effects the flexion tendency of the scaphoid. The bowstringing of the flexor carpi radialis also counteracts scaphoid flexion. Alterations in any of these factors may upset the delicate mechanical balance of the joint.