The treatment of wrist instability.

The concept of carpal instability has evolved considerably over the past few years. Initially, the term ‘instability’ was considered to be synonymous with ‘malalignment’. A wrist was regarded as unstable when there was an alteration of the sagittal and/or anteroposterior alignment of the carpal bones beyond what was judged to be normal. Based on this, four major types of carpal malalignment were recognised: Dorsal intercalated segment instability (DISI). The lunate, regarded as an intercalated segment between the distal row and the forearm bones, is abnormally extended relative to its proximal and distal links. Volar intercalated segment instability (VISI). In the sagittal plane the lunate appears abnormally flexed. Ulnar translocation. The proximal row is abnormally displaced (rotated or translocated) relative to the radius in an ulnar direction. Dorsal translocation. Due to a malunited fracture of the radius, the carpus is subluxed in a dorsal direction. These four types of instability could therefore always be diagnosed by plain radiography. A number of authors including Schernberg and Zdravkovic, Jacob and Sennwald have criticised this concept of instability since not every alteration of carpal alignment is pathological. Wrists showing congenital hyperlaxity often appear to be grossly malaligned and yet are frequently asymptomatic. These patients are able to cope effectively with most activities of daily living and seldom require treatment. It was therefore necessary to reconsider the definition of instability and exclude those patients whose carpal malalignment is simply a variation of the normal. As a result, instability has been redefined as the inability to bear physiological loads with an associated loss of the normal carpal alignment. This definition again was felt to be inadequate since it excludes those patients whose wrist is asymptomatic for most of the time, is well aligned and able to sustain physiological loads, but becomes painful only when performing a specific task, such as opening a jar or lifting heavy objects, with a typical sensation of ‘giving way’. It is apparent that there is dynamic instability, occurring when carpal malalignment appears sporadically under certain loading conditions, and static instability when malalignment is permanent regardless of the amount of load being applied. By admitting differences in the severity of injuries producing carpal instability, some of the problems of definition were solved. Nevertheless, some concerns remained. By defining stability in terms of ‘ability to bear load’, the impression was gained that there was a problem only of load transfer (kinetic dysfunction) rather than of both load and movement (kinetic and kinematic dysfunction). For instance, a wrist with an old carpal collapse and extensive joint degeneration may bear physiological loads despite malalignment. Such cases, however, should be considered as pathologically unstable since their movement cannot be as smooth and well co-ordinated as that of a normally functioning wrist. To fulfil these biomechanical criteria of stability, a normal wrist must be able to maintain a balance between the articulating bones in both kinetic and kinematic terms under physiological loads, through the whole range of movement. The term instability must be interpreted as ‘carpal dysfunction’, which implies that in a normal wrist there is the ability to transfer loads without sudden changes of stress on the articular cartilage (normal kinetics) and the capacity to move throughout the normal range without sudden alterations of intercarpal alignment (normal kinematics).