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CHAPTER 42 – Midcarpal Instability

Carlos Heras-Palou, MD

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 colleagues[2] 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 description[3] 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.

Most patients with MCI respond to nonoperative treatment. Combinations of immobilization, splints, anti-inflammatories, activity modification, and exercise have been prescribed with diverse success. Surgical treatment suggested for MCI includes soft tissue stabilizations, limited carpal arthrodesis, corrective osteotomies, and arthroscopic thermal capsulorrhaphy. The role of proprioception in carpal instability is starting to be recognized, but is not yet fully understood.

TABLE 42-1   -- Summary Table for Midcarpal Instability (MCI)*
 Ligaments InvolvedStatic AlignmentFluoroscopyConservative TreatmentOperative Treatment
Palmar MCI (ulnar sided)Dorsal rad-triqVISI“Clunk” seenSplintECRL tenodesis
 Volar triq-cap  Strengthen FCU and hypothenar 
 Volar triq-ham    
Palmar MCI (radial sided)Volar rad-capVISI“Clunk” seenStrengthen FCRFCR tenodesis
 Dorsal scaph-triq STT opens Modified Brunelli
 Volar scaph-cap    
Dorsal MCIVolar radiocarpal ligamentsNormalOften clunk, dorsal displacementActivity modification and strengtheningTighten space of Poirier
Combined MCIVolar and dorsal extrinsic ligamentsVISIClunk and dorsal displacementStrengthen FCR, FCU, and hypothenarRadiolunate arthrodesis
   Radiocarpal opens  
Extrinsic MCIMalunion radiusDISIOften clunk, dorsal displacementSplintCorrective osteotomy of distal radius

DISI, dorsal intercalated segment instability; ECRL, extensor carpi radialis longus; FCR, flexor carpi radialis; FCU, flexor carpi ulnaris; STT, scaphotrapeziotrapezoid; VISI, volar intercalated segment instability.

*In each of the groups there is a wide spectrum of severity. The treatment of midcarpal instability is controversial. The treatments in this table are the author's recommendation.

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).

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FIGURE 42-1  A, In a normal wrist, the proximal carpal row moves smoothly from flexion to extension, as the wrist moves from radial to ulnar deviation. B, In palmar midcarpal instability, the proximal row remains flexed until nearly the extreme of ulnar deviation, when it suddenly jumps into extension, causing a catch-up clunk.

Classification of Midcarpal Instability

Lichtman and Wroten[4] 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 coworkers[5] 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).

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FIGURE 42-2  A, The ligaments in the wrist form two slings that go around the carpus in opposite directions, providing stability in all positions of the range of movement. B, The antisupination sling is formed by the dorsal radiotriquetral ligament (1) and the volar triquetrohamate and triquetrocapitate (2). C, The antipronation sling is formed by the volar radiocapitate ligament (1) and its connections to the triquetrum, dorsal triquetroscaphoid (2), and volar scaphoid capitate (3).
(Courtesy of Dr. Marc Garcia-Elias.)

On the volar aspect of the wrist, the ligaments are organized in the shape of a proximal “V” and a distal “V.” The proximal “V” is formed by the ligaments from the radius to the lunate and from the volar band of the triangular fibrocartilage complex to the lunate. The distal “V,” also called the arcuate ligament, is formed mainly by the radiocapitate and scaphocapitate ligaments on the radial side, and the ulnotriquetral, ulnocapitate, triquetrohamate, and triquetrocapitate ligaments, which are also known as the triquetrohamate capitate ligament, on the ulnar side. Between the two “Vs” there is the loose “space of Poirier.”

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 studies[2] 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]

Assessment of Palmar Midcarpal Instability

The symptoms of a patient presenting with a clunking wrist range from none at all to severe. Looking at the wrist from the ulnar side, a palmar sag can be seen, which reduces with ulnar deviation of the wrist (Fig. 42-3). The midcarpal shift test, described by Lichtman,[2] reproduces the patient's wrist instability. This test is done by placing the patient's wrist in neutral with the forearm pronated. A palmar force is applied to the hand at the level of the distal capitate. The wrist is simultaneously loaded axially and deviated ulnarly. The test is positive if a painful clunk occurs that reproduces the patient's symptoms.

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FIGURE 42-3  A palmar sag can be seen, when the carpus is in a volar intercalated segment instability alignment, in patients with midcarpal instability.

Plain radiographs generally show a VISI pattern, but no dissociation between the bones of the proximal carpal row (Fig. 42-4). If there is a dissociation between lunate and triquetrum, this is a type of carpal instability dissociative and not MCI (Fig. 42-5).

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FIGURE 42-4  A and B, Plain radiographs show a volar intercalated segment instability alignment of the carpus. There is no dissociation. This is typical of palmar or combined midcarpal instability.

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FIGURE 42-5  A and B, In this case, there is a volar intercalated segment instability alignment, but also a dissociation between the lunate and triquetrum. This is not midcarpal instability; it is a static lunotriquetral instability.

Fluoroscopy is diagnostic, and it is useful to record it so that radiation exposure can be minimized. Recording also allows the examiner to watch the video as many times as it takes to understand the mechanical problem, and it provides a permanent record of the most important diagnostic test.

On a mini-C-arm, a lateral view of radioulnar deviation of the wrist shows the typical jump of the proximal row from flexion to extension. The examiner should perform a “passive palmar displacement” and a “passive dorsal displacement” of the wrist, and observe if there is displacement and opening at either joint. This maneuver establishes if the laxity is at the midcarpal joint or radiocarpal or both. On the posteroanterior view, radioulnar deviation needs to be observed, to rule out any scapholunate or lunotriquetral dissociation, and to look for any opening of the scaphotrapeziotrapezoid (STT) joint, which would indicate laxity of the ligaments on the radial side of the wrist.

The main role of arthroscopy in the assessment of MCI is to exclude other pathology, in particular lunotriquetral dissociation, and to assess the joint surfaces before reconstructive surgery. It is easy to perform arthroscopy of a wrist with MCI because there is a lot of space owing to the laxity of the joint, and sometimes there is synovitis. The synovitis is most often found on the dorsal aspect of the radiocarpal and the midcarpal joints. No particular ligaments seem to be injured on inspection. In long-standing cases, there may be some degenerative changes of the proximal lunate or the proximal hamate.

Treatment of Palmar Midcarpal Instability

Conservative treatment is often successful, particularly in milder cases of palmar MCI. Training of the flexor carpi ulnaris and the hypothenar muscles with isometric contractures can improve symptoms. These exercises generate a dorsally directed force toward the triquetrum, through the pisiform, which may support the ulnar side of the wrist and decrease instability. Attempts to train proprioception and gain strength using hand-held gyroscopes (Powerball, NSD, South Africa) have helped in some patients.

Arthroscopic thermal capsulorrhaphy has been used for MCI with promising early results. Standard portals for the radiocarpal and midcarpal joints and a 2.3-mm VAPR probe (DePuy Mitek, Leeds, UK) have been used. The objective is to shrink the relevant ligaments, followed by immobilization of the wrist in a cast for 6 weeks. All the volar extrinsic wrist ligaments should be treated in an attempt to tighten the arcuate ligament. There are no published series on this form of treatment as yet. At present, it should still be considered an experimental treatment.

For palmar MCI, published studies support the use of limited carpal arthrodesis over soft tissue stabilizations.[9,10] Triquetrohamate,[11] four-corner arthrodesis,[12] and STT[5] fusions have been proposed. The downside of these procedures is that they block the midcarpal joint, preventing the useful movement in the “dart throwing” arc, from dorsoradial to palmar-ulnar.

If a particular ligament is found to be the cause of the instability, augmentation seems to be the most sensible option. In palmar MCI in which the pathology is ulnar-sided, the author's limited experience with the procedure proposed by Garcia-Elias and Geissler[13] has produced excellent results. This technique uses half the extensor carpi radialis brevis tendon to recreate the ulnar limb of the arcuate ligament and the dorsal radiotriquetral ligament.

If the cause of palmar MCI is on the radial side, conservative treatment including exercises to strengthen the flexor carpi radialis should be tried. Failing that, a modified Brunelli reconstruction using half of the flexor carpi radialis tendon would recreate the ligaments across the STT joint and the dorsal scapholunate ligament, providing a good alternative to an arthrodesis.[13]

Dorsal Midcarpal Instability

Patients with dorsal MCI present with clicking and pain in the wrist, related to dorsal subluxation of the midcarpal joint. The pathomechanics differ from palmar MCI. In dorsal MCI, the capitate is reduced when the wrist is in neutral radioulnar deviation and subluxates dorsally in ulnar deviation. It is not a “catch-up” clunk, but more of a subluxation click. When the wrist moves from extreme ulnar deviation toward neutral, the capitate relocates, and this may be accompanied by a click or a clunk.

There are two subgroups in this type of MCI. The first subgroup manifests typically without a history of trauma, with pain and clicking of the wrist, precipitated by tight grasping, particularly with the forearm in supination; this has been named the capitolunate instability pattern.[14] In a reported series of 11 patients, plain radiographs seemed normal, but doing a dynamic dorsal displacement under fluoroscopy revealed the diagnosis in all patients. To perform the test, the examiner applies dorsally directed force to the scaphoid tuberosity, with longitudinal traction and flexion of the wrist. X-rays show a dorsal subluxation of the proximal row with characteristic dorsal subluxation of the capitate from the lunate. This test reproduces the patient's symptoms. Ten of 11 patients became asymptomatic with conservative treatment. This condition was attributed to laxity of the radiolunate ligaments and extrinsic ligaments of the scaphoid.

The second group of patients with dorsal MCI, called chronic capitolunate instability, present with post-traumatic chronic wrist pain, weakness, and wrist clicking, and a history of an extension wrist injury. In the report of a series of 12 patients, radiographs were normal except in 1 patient with a dorsal intercalated segment instability pattern.[15] Under fluoroscopy, dorsal subluxation of the capitate on the lunate could be shown by pushing the capitate dorsally. This caused apprehension, and the patients recognized the click caused by the sudden movement of the lunate dorsal and ulnar. Of the 12 patients, 11 underwent surgery to suture the volar radiocapitate ligament to the radiotriquetral ligament, in an attempt to close the space of Poirier. Surgery was generally successful at stabilizing the wrist, although some extension was lost. This condition has been attributed to traumatic attenuation of the palmar radiocapitate ligament. Dorsal MCI seems to be related to insufficiency of the ligaments in the antipronation sling.

Treatment of Dorsal Midcarpal Instability

The two subgroups of dorsal MCI probably represent different stages of the same spectrum. Patients presenting with a short duration of symptoms and no history of trauma tend to improve with conservative treatment.

In the rare cases when surgery is required, the author recommends the method described by Johnson and Carrera.[15] Through a volar approach, the space of Poirier is obliterated using a strong nonabsorbable suture, accepting certain loss of extension of the wrist.

Combined or Palmar-Dorsal Midcarpal Instability

Some patients present with a combination of palmar MCI and a positive passive dorsal displacement test, usually in a wrist with marked laxity. In a series of 14 such patients presenting with wrist pain and weakness, half of them had a history of trauma.[16] Radiographs showed a VISI deformity. Doing a dorsal displacement test showed subluxation of the capitolunate joint and in some cases subluxation of the radiolunate also. These patients did not improve with nonoperative treatment, and they underwent surgery to obliterate the space of Poirier. This was done by sutureing the radiocapitate ligament to the long radiolunate on the radial side, and the capitotriquetral to the lunotriquetral on the ulnar side. The authors reported excellent outcome in eight cases, good in five, and fair in one.

Treatment of Combined Midcarpal Instability

Patients with combined dorsal and palmar MCI usually have marked laxity of the wrists, and they should be treated nonoperatively as far as possible. The reported result of surgery in multidirectional instabilities is unsatisfactory.[3] This coincides with the author's experience that patients with very lax wrists do not do well with soft tissue surgery.

If all conservative measures fail, and the patient is very symptomatic, the best option is a radiolunate arthrodesis as originally proposed by Halikis and colleagues.[17] This procedure orientates the lunate appropriately and abolishes the “proximal row instability,” providing an excellent range of movement in these patients.

Extrinsic Midcarpal Instability

The cause of extrinsic MCI is not primarily in the ligaments, but a bony abnormality with secondary insufficiency of the ligaments. The common reason is the malalignment of a distal radius malunion, but rarely STT osteoarthritis can be a cause.

Carpal malalignment secondary to malunion of distal radial fractures was recognized in the 1970s.[6] In 1984, Taleisnik and Watson[18] reported 13 patients with symptoms of instability and wrist pain, 6 of them with recurrent voluntary midcarpal subluxation. Nine patients were treated by corrective osteotomy of the radius, which resulted in symptom relief and stabilization of the wrists. The one patient whose wrist was stabilized by ligament reconstruction presented with a recurrence of the voluntary midcarpal subluxation.

Caputo and coworkers[5] no longer consider this condition to be MCI. They prefer the term “dorsal radius angulation carpal overload,” considering that the cause of the instability is the repetitive overload of the midcarpal joint as a result of the reversal of the normal palmar tilt of the distal radius. This cause is supported by the fact that the instability often manifests months after the injury. The ligaments in the wrist are developed to resist volar and ulnar directed forces, but are inadequate to resist dorsal or radial directed forces such as the ones present after a distal radius malunion.

An alternative explanation for this instability is insufficiency of the volar extrinsic wrist ligaments resulting from the reversal of the volar tilt of the radius.[4] The secondary malalignment of the carpus shortens the distance these ligaments span, causing relative laxity and creating a situation similar to dorsal MCI. The instability immediately disappears when the carpal bones realign after a corrective osteotomy. Perhaps another contributing factor is that the mechanism of injury in a distal radius fracture is the same one that can cause an injury of the volar extrinsic wrist ligaments.

There also is a relationship between STT osteoarthritis and dorsal MCI.[19] It seems that shortening of the scaphoid can cause extension of the proximal carpal row into a dorsal intercalated segment instability, which can be made worse with certain treatments, such as excision of the distal pole of the scaphoid (Fig. 42-6).

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FIGURE 42-6  A and B, This patient underwent excision of the distal one fourth of the scaphoid as treatment for scaphotrapeziotrapezoid osteoarthritis. This procedure was successful in eliminating the pain. A few weeks after the surgery, the patient developed clicking when gripping and pain localized to the dorsum of the wrist. Doing a dynamic dorsal displacement test shows dorsal subluxation of the distal row, indicative of dorsal midcarpal instability.

Treatment of Extrinsic Midcarpal Instability

For extrinsic MCI with significant symptoms, surgery should address the cause of the problem. In the case of distal radius malunion, the bone anatomy usually can be restored by a corrective osteotomy of the radius. The results are predictably good.


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