CHAPTER 42 Midcarpal Instability
The first description of a “snapping wrist,” diagnosed as anterior midcarpal subluxation,1 was recorded in 1934, although “dorsal luxation of the capitate” had been presented in a congress in Paris in 1919. The article by Lichtman and colleagues2 in 1981 brought this particular condition to the attention of orthopaedists. Of all the forms of carpal instability, midcarpal instability (MCI) has been the most confusing. The two main reasons for this are that MCI is a mixed bag of conditions, and that their pathophysiology is not well understood. The management of MCI remains controversial. Table 42-1 provides a summary of MCI.
The term MCI covers a range of conditions characterized by a painful clunk, usually felt in ulnar deviation of the wrist. It has been suggested that the term “instability of the proximal carpal row” would be a more accurate description3 because the mechanical problem is a carpal instability nondissociative, affecting the radiocarpal or the midcarpal joints or both. The scaphoid, lunate, and triquetrum move like one unit, but not in a predictable, smooth manner.
What Causes the Clunk?
Clunking of the wrist can be caused by congenital laxity of the wrist ligaments, bone or joint dysplasia, lunotriquetral injury, distal radius malunion, or insufficiency of the extrinsic ligament affecting the radiocarpal or the midcarpal joint or both. In a normal wrist, during radial deviation, the proximal carpal row goes into flexion, and during ulnar deviation it extends, in a smooth transition (Fig. 42-1A). In a wrist with palmar MCI, there is palmar subluxation of the capitate head with the wrist in radial deviation in the midcarpal joint, and the proximal carpal row remains flexed until terminal ulnar deviation, at which time it is suddenly forced into extension. This sudden extension causes the so-called catch-up clunk felt by the patient and often clearly seen and heard by observers (Fig. 42-1B).
Classification of Midcarpal Instability
Lichtman and Wroten4 proposed a classification of MCI that includes intrinsic MCI, which is caused by ligamentous insufficiency, and extrinsic MCI, secondary to distal radius fractures. Intrinsic MCI is subdivided further into palmar, dorsal, and combined, depending on the direction of displacement. This classification is helpful in understanding the spectrum of conditions that constitute MCI, and in defining the boundaries between them.
Caputo and coworkers5 believe that only conditions with a static or dynamic volar intercalated segment instability (VISI) of the proximal carpal row constitute MCI. Their definition of MCI is more restrictive than generally accepted. These authors have classified MCI depending on whether the cause of the instability lies on the ulnar side or on the radial side of the wrist. Ulnar-sided MCI corresponds to palmar MCI without significant symptoms (type I) or with significant symptoms (type II). Radial-sided MCI manifests with a rotary subluxation of the scaphoid, with either an intact scapholunate ligament (type III) or a scapholunate ligament disruption (type IV).
Palmar Midcarpal Instability
Patients with palmar MCI present with a painful clunk of the wrist, often with a history of trivial injury or no trauma at all. Sometimes they can demonstrate the clunk voluntarily “to the amusement of friends, and consternation of doctors.”6 In the author’s experience, this is the most common form of MCI.
Pathomechanics of Palmar Midcarpal Instability
Palmar MCI is considered to be caused by insufficiency of some of the extrinsic wrist ligaments. Ligaments do not work in isolation, but in a synergistic manner, where groups of ligaments resist certain forces. The extrinsic ligaments in the wrist form an antisupination sling and an antipronation sling, both running around the carpus in opposite directions (Fig. 42-2A).
On the ulnar side of the wrist, the dorsal radiotriquetral, which is also known as the dorsal radiocarpal ligament, and the volar triquetrohamate capitate ligaments prevent the proximal carpal row from falling into a VISI alignment. The ligaments act as a sling from the dorsum of the radius, around the triquetrum, and onto the volar aspect of the capitate. This has an antisupination action on the wrist (Fig. 42-2B). If these ligaments fail, the carpus has a tendency to fall into a VISI alignment and into intercarpal supination. The proximal row loses its normal transition from flexion to extension during ulnar deviation of the wrist, causing a clunk.
The distal scaphoid has ligaments to the capitate and to the trapezium. If these ligaments fail, there is a tendency for the scaphoid to flex and to take with it the lunate and the triquetrum, causing a VISI alignment. There is a ligamentous sling, which starts with the volar radiocapitate ligament, follows around the triquetrum, connects to the dorsal scaphotriquetral ligament, and goes around the scaphoid ending with the volar scaphocapitate ligament (Fig. 42-2C). This sling goes round the wrist causing an antipronation effect. Failure of this mechanism may cause flexion of the proximal row, starting on the radial side, and a tendency toward intercarpal pronation. This would cause radial-sided palmar midcarpal instability.
Several studies have attempted to reproduce carpal instability in cadaver models. Biomechanical studies2 reported that sectioning the dorsal radiotriquetral and the triquetrohamate portion of the triquetrohamate capitate ligament did not produce frank instability or a clunk. Further division of the triquetrocapitate portion of the triquetrohamate capitate ligament produced a clunk similar, but not identical, to clunks observed in patients. Other studies have shown that sectioning either the ulnar arm of the distal volar “V” ligaments or the dorsal radiotriquetral ligament can produce a VISI and mechanical changes typical of palmar MCI.7,8
