CHAPTER 39 Chronic Volar-Flexed Intercalated Segment Instability
The definition of instability of the wrist has been described by the International Wrist Investigators’ Workshop as “the inability of the carpus to maintain its normal anatomical relationships under physiological loading.” Dobyns and colleagues,1 and more recently the International Wrist Investigators’ Workshop, have suggested a number of terms that describe the patterns of instability:
It is now accepted that the stability of the carpus is wholly dependent on the integrity of the interosseous and capsular ligaments of the wrist joint and not on any inherent stability conferred by bone shape or position.2
One might look at the effects of failure of these ligaments by using an analogy of the coiled spring. This concept, proposed by Garcia-Elias, identifies that the scaphoid, the lunate, and the triquetrum are intimately connected and, as Kapandji has described,3 there is a variable geometry of the proximal row of the carpus as the loaded wrist is moved. Thus, the scaphoid flexes, extends, pronates, supinates, and translates. The lunate flexes, extends, and translates but does not pronate or supinate. Finally, the triquetrum flexes and extends much less than the lunate but does not pronate or supinate and although it does translate, it does not articulate with the distal radius and barely articulates with the triangular fibrocartilage complex (TFCC). The scaphoid has a long lever arm by comparison to the lunate and triquetrum and therefore has a profound effect on the biomechanics of the wrist. Using the coiled spring analogy, axial loading of the intact wrist requires the rotational moment of the scaphoid to be balanced by the opposite rotational moment of the triquetrum, with the lunate acting as the torque converter in its position as the intercalated segment of the proximal row. Failure of the scapholunate ligament gives rise to the potential for the scaphoid to flex under the axial loading but, of course, the triquetral/lunate complex rotational moment is unopposed and the lunate and triquetrum extend, giving rise to the radiographic appearance of dorsiflexed intercalated segment instability (DISI). If, however, the lunate is separated from the triquetrum, the rotational moment of the scaphoid takes the lunate into flexion, thus creating the radiographic appearances of volar-flexed intercalated segment instability (VISI) (Fig. 39-1).4–7
The work of Mayfield and Johnson8 has shown us that there are patterns of ligamentous injury, through a radial-sided applied force, in which, with the wrist loaded in extension and ulnar deviation, axial loading resulted in sequential ligament disruption. In effect, these injuries occurred as a result of applying what might be described as a proximal row supination force to the cadaver specimens. This is translated into the in vivo injury when, as a result of falling on the outstretched hand, the thenar eminence (Fig. 39-2) contacts the ground first and thereafter the protective pronation reflex of the forearm forces the hand, and therefore the carpus, into supination, resulting in either a pronator quadratus fracture in the child, a Colles’ type fracture in the elderly, a scaphoid fracture in the young male, and a scapholunate, perilunate lesser arc, type of soft tissue injury in most adults. The sequence of ligament failure in this pattern of injury is well recognized and described elsewhere.
The so-called reverse Mayfield sequence, or the proximal row pronation injury, occurs as a result of the forces acting on the hand as the hypothenar eminence strikes the ground first (Fig. 39-3). The patterns of injury are complex in that there are many more ligament attachments to the triquetrum than to the scaphoid. The act of falling on the outstretched hand when the heel of the hand strikes the ground concentrates the forces that now act on the ulnar side of the carpus and therefore through the pisiform to the triquetrum. The position of the pisiform on the volar aspect of the triquetrum results in this pair of bones decelerating rapidly, but the momentum of the individual suffering the injury drives the ulna toward the ground, which results in the forces being transmitted to the structures on the ulnar side of the wrist. There is a tendency to thus force the hand into a pronation attitude with reference to the distal radius and ulna. The ulnar head continues to approach the ground, which leads to a dorsal shearing force that may detach the posterior aspect of the TFCC from the dorsal ulnar carpal ligaments, giving rise to a separation of these two structures. This does not involve a true tear of the TFCC.
Any disruption of connections between the scaphoid and the lunate or the lunate and triquetrum can prevent the wrist from supporting physiological loading without collapsing.5 The proximal row variable geometry system becomes dyskinetic, that is, it is not in the appropriate configuration in most positions of the loaded wrist. It is accepted from anatomical studies that the most important part of the scapholunate interosseous ligament lies dorsally and the most important part of the triquetrolunate interosseous ligament lies anteriorly.7 This allows each of the bones to have its own axis or rotation different from the others and also for each bone to move within its limits set down by these two ligaments.
