Midcarpal Instability: Unlocking the Secrets of the Wrist
Keywords: Midcarpal instability; Carpal kinematics; Dorsal radiotriquetral ligament; Wrist.
Instability of the midcarpal joint, like many conditions in medicine, had been observed and was called many different names before anyone realized they were probably describing the same entity. Mouchet & Belot  in 1934 were the first to document asymptomatic subluxation at the midcarpal joint, which they called the "snapping wrist." In 1984, White & Louis  described a patient with wrist instability that they termed capitolunate instability pattern, or CLIP wrist. In 1986, Johnson & Carrera  published a series on patients with chronic capitolunate instability (CCI). These authors attributed this condition to laxity of intrinsic wrist ligaments, but did not attempt to explain the pathomechanics. In retrospect, all these reports probably represented variants of midcarpal instability (MCI).
A major reason for the delayed recognition of MCI is that almost all wrist clicks and clunks were originally thought to be due to scapholunate (SL) or lunotriquetral (LT) instability. This overlap, along with the historically poor understanding of carpal pathomechanics, made the identification and description of MCI challenging. Our experience with MCI over the years, however, has allowed us to gain a better understanding of this unique entity. Along the way, it has revealed several other "secrets" of the wrist as well.
Our first encounter with MCI was in 1976 at the Bethesda Naval hospital. A female surgical technician had undergone operative treatment elsewhere for dorsal dislocation of the ulna. Intraoperatively, no pathology had been identified and, as such, no repair was performed. The patient presented to us with a painful wrist clunk with ulnar deviation that interfered with her job activities. Examination revealed a palmar sag on the ulnar aspect of the wrist in neutral deviation, giving the clinical appearance of a dorsal dislocation of the distal radio ulnar joint (DRUJ). Examination of the DRUJ was unremarkable and stable to stress. Passive and active ulnar deviation with the forearm pronated reproduced the painful clunk at the wrist. Radiographs revealed a mild volar intercalated segment instability (VISI) pattern. These findings were reported in 1981 as part of a small case series .
The initial thought for this patient was SL instability, as this was the cause of wrist noises in the 1970s. Repeated examinations of the wrist revealed some distinctions, however. First, the palmar sag disappeared (with a clunk) as the wrist moved into ulnar deviation. As the wrist moved back to neutral, the palmar sag reappeared. Second, this was not really a click, as heard with SL dissociation, but a distinct clunk, which implied joint subluxation or dislocation (as in the hip). The final distinction was that the location of pain accompanying the clunk was at the ulnar corner of the midcarpal joint, not at the DRUJ or at the anatomic snuffbox.
The patient agreed to a second wrist exploration. Through a dorsal ulnar incision, the proximal carpal row was noted to be normal in appearance. The ulnar stabilizers of the midcarpal joint demonstrated laxity, enough to permit excessive proximal row rotation and dorsal-palmar translation between the proximal and distal rows. This laxity was most pronounced with the wrist in neutral. As the wrist was brought into ulnar deviation, the proximal carpal row was visualized flipping into extension, accompanied by an audible clunk.
There was insufficient capsular tissue to stabilize the triquetrohamate (TH) and capitolunate (CL) joints in this patient, so a formal ligament reconstruction was performed using the distal extensor carpi ulnaris tendon woven in a dorsal to palmar direction to stabilize the ulnar corner of the midcarpal joint. As there was no other term to describe the findings, we named this entity "ulnar midcarpal instability." The patient returned to full duty several months postoperatively. We were pleased to have solved the patient’s problem, but were puzzled by the unique findings and lack of specific information available in the literature.
Soon after, a second patient presented with similar complaints and findings (Figure 1A). This patient, an academic radiologist at our hospital, had difficulty playing tennis due to the painful wrist clunk. A tri-compartmental wrist arthrogram was performed, which was within normal limits. Fluoroscopic videos were taken as he actively recreated his wrist clunk in an attempt to identify any areas of dissociation or abnormal carpal motion. As the wrist moved into ulnar deviation, the entire proximal row flipped from flexion to extension. This sudden rotation was simultaneous with the painful wrist clunk. As the wrist was moved back into neutral, a VISI deformity reappeared. These fluoroscopic findings correlated perfectly with the surgical findings of our previous patient. Clinically, when a dorsally directed force was placed on the pisiform in this patient, not only was the palmar sag corrected (Figure 1B), but the wrist would no longer clunk as it moved from neutral to ulnar deviation. A palmar splint was created to maintain this position with constant 3-point fixation while still permitting active range of motion of the wrist. The patient was able to return to playing tennis using this splint and never required surgical intervention. Based on our findings in these and subsequent patients, we described a midcarpal shift test, which we found diagnostic for midcarpal instability [5-8]. The test is performed with the forearm stabilized in pronation and the wrist in neutral (Figure 2A). A palmarly directed force is applied to the distal capitate, and then the wrist is axially loaded and ulnarly deviated (Figure 2B). A painful clunk that reproduces the patient’s symptoms is a positive test for MCI.
Research into one area often sheds light on related topics and provides unanticipated solutions to others. In this sense, our experience investigating the pathomechanics of MCI has been rewarding but is certainly not unique . It is these unexpected discoveries that make the study of carpal instabilities especially rewarding and we hope will stimulate future investigations into the mysteries that remain.
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