Carpal Instability: Clarification of the Most Common Etiologies and Imaging Findings

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Intrinsic ligament injuries of the wrist are common with varying degrees of scapholunate ligament tears occurring in more than one-third of involved wrists.1 A fundamental understanding of wrist anatomy and common patterns of injury help increase early detection and proper management of carpal instability.

Plain film radiographs offer an adequate cursory evaluation of carpal instability and exclude other entities that may mimic instability such as fracture. Additionally, radiographs display the alignment of the wrist and carpal bones (Figure 1). Ensuring proper positioning of the wrist (ie, frontal and lateral projections) allows for appropriate assessment of the joint spaces and carpal arcs2 as well as initial screening for acute fracture or dislocation.

MRI provides a more thorough evaluation for underlying pathology and associated sequelae3,4 (Figures 2). Incorporating an appropriate protocol for MRI of the wrist (Table 1 and 2) offers exquisite detail of the bones, cartilage, ligaments, tendons and nerves. At the authors’ institution, this includes axial proton density-weighted sequences with and without fat saturation, coronal T1-weighted images, coronal proton density-weighted images with fat saturation, and coronal 3D double echo steady state as well as sagittal proton density-weighted images with fat saturation.

Carpal Instability Classification

The first step toward accurate wrist radiography is to ensure proper anatomic positioning. In the frontal projection, the carpal bones should be parallel with undisrupted arches, normal in shape (implying normal tilt and axis), and equally spaced. The shape of the lunate, capitate, and scaphoid requires close attention as they are the most common carpal bones to be malaligned.5 The lateral projection is also critical, particularly in determining the alignment and/or angulation of the capitate, lunate, scaphoid and radius, and is critical in evaluating intercalated segment instability.6

Overall classification of carpal instability is separated into 4 large groups (Table 3).3 Within this classification scheme, the most common etiologies include dorsal intercarpal (DIC) and dorsal perilunate (DPL) dislocations. These common clinical entities have specific radiographic and MRI characteristics.

Carpal Instability Dissociative

These injuries include scapholunate dissociation, scapholunate advanced collapse, scaphoid nonunion advanced collapse, and the less common lunotriquetral dislocation, which can range from incomplete tears to complete dissociation.7 These entities can be adequately evaluated and diagnosed with radiography; however, specific ligamentous injuries and extent of degenerative disease is better appreciated with MRI.

Dorsal intercalated segment instability (DISI) and volar intercalated segment instability (VISI) are the most common patterns of carpal instability and are associated with scapholunate and lunotriquetral ligament injuries, respectively.8 They can be suggested on radiographic evaluation with typical findings and abnormal angulation of the carpal bones, but are not always evident even in the setting of ligamentous injury, which is better seen by MRI.9 Dorsal perilunate dislocations can be separated into 4 stages of injury with progressive perilunate instability occurring secondary to ligamentous injuries. These stages of instability include scapholunate dissociation, perilunate dislocation, midcarpal dislocation, and lunate dislocation. Each stage has a differing radiographic appearance allowing accurate diagnosis with specific ligamentous injury, well visualized by MRI.10

Carpal instability dissociative (CID) involves injury within or between bones of the same carpal row. Most commonly, this instability pattern occurs in the proximal carpal row as a result of scapholunate or lunotriquetral ligament injury and the different specific type of injuries include scapholunate dissociation, dorsal intercalated segment instability, scapholunate advanced collapse, scaphoid nonunion advanced collapse, and lunotriquetral dislocation.11

Scapholunate Dissociation

Scapholunate dissociation (SLD) is disruption of the ligamentous connection between the scaphoid and lunate (Figures 3). This is seen on radiography as diastasis of the scapholunate interval with a gap of > 3mm (“Terry Thomas” or “David Letterman” sign).12 This is the most frequent carpal instability pattern and can be isolated or associated with scaphoid fractures.

Dorsal Intercalated Segment Instability

Dorsal intercalated segment instability (DISI) involves injury to the scapholunate ligament and concomitant failure of the scaphoid stabilizers, which often results in permanent carpal malalignment. In this pattern, the lunate is dorsiflexed and the scaphoid is tilted volarly with a scapholunate angle > 60˚ (normal range is 30˚ to 60˚) and a capitolunate angle > 30̊, as measured on a lateral radiograph (Figure 4).13

Scapholunate Advanced Collapse

Scapholunate advanced collapse, commonly abbreviated as “SLAC wrist,” occurs with degenerative joint disease centered at the radioscaphoid joint from chronic SLD. There are 3 progressive stages of SLAC wrist: stage I, which involves radial styloid and scaphoid degeneration; stage II (Figure 5), which involves degeneration between the scaphoid and the entire scaphoid facet of the radius; and stage III (Figure 6), which involves degeneration between the capitate and lunate.13 The hallmark of a SLAC wrist is scapholunate ligament tear and progressive scapholunate interval widening.

Scaphoid Nonunion Advanced Collapse

Scaphoid nonunion advanced collapse, commonly abbreviated as a “SNAC wrist,” occurs with a scaphoid fracture (particularly nonunion fractures) with distal scaphoid fracture segment flexion and results in abnormal radioscaphoid articulation and degeneration (Figure 7). The hallmark of SNAC wrist is post-traumatic arthritis and carpal collapse following a nonunion scaphoid fracture.14

Lunotriquetral Dissociation

Lunotriquetral dissociation can occur following trauma or ulnocarpal abutment associated with triangular fibrocartilage complex pathology.7 Injury to the lunotriquetral ligament results in volar intercalated segment instability (VISI), in which the lunate is flexed volarly secondary to the scaphoid flexion. In this pattern, the radiographic findings demonstrate a capitolunate angle > 30˚ and a scapholunate angle > 30˚.13

Carpal Instability Nondissociative

Carpal instability nondissociative (CIND) refers to dysfunction between the radius and first carpal row (radiocarpal) or between the first carpal row and the second carpal row (midcarpal). This dysfunction can involve either the intrinsic or extrinsic ligaments of the wrist; however, there is no disruption between carpal bones in the same row, as in CID.15 In this pattern, the individual carpal bones in each carpal row maintain their normal anatomic relationship with each other. As a result, the carpal rows and arcs maintain their intrinsic morphology and positioning.

Ulnar translocation occurs the extrinsic ligaments of the wrist are torn; resulting in ulnar shift of the proximal carpal row. Type I ulnar translocation involves tearing of the radioscaphoid and radioscaphocapitate extrinsic ligaments with resultant widening of the radioscaphoid interval and ulnar shift of the entire proximal carpal row (Figure 8). In type II ulnar translocation, the radioscaphoid joint is maintained with ulnar shift of the remaining proximal carpal row.16

Carpal Instability Complex

Carpal instability complex (CIC) refers to carpal derangement involving an altered relationship between bones in the same carpal row and between the proximal and distal carpal rows (ie, both CID and CIND injuries).17 There are 5 subgroups of CIC (Table 4), with the more common groups 1 and 2 reviewed below.

Dorsal perilunate dislocations are ligamentous lesser arc injuries within the carpal instability complex class of injuries. There are 4 stages of progressive perilunate instability involving ligamentous injuries surrounding the lunate:18

Stage I: Scapholunate dissociation, which is defined by disruption of the dorsal scapholunate ligament when torque on the scapholunate ligament reaches threshold. Ligamentous injury is well visualized by MRI.

Stage II: Perilunate dislocation, which is when the scaphoid-capitate complex dislocates dorsal to the lunate. The extent of dorsal translation is determined by laxity of the radioscaphocapitate extrinsic ligament. Radiographic findings include dorsal displacement of the capitate in relation to the lunate while alignment of the lunate with the distal radius is maintained (Figure 9).

Stage III: Midcarpal dislocation involves progressive carpal hyperextension, which pulls the triquetrum into abnormal extension. This leads to tearing of the lunotriquetral ligament with possible avulsion injury of the triquetrum, leaving only the short radiolunate and volar ulnolunate ligaments as stabilizers. This injury is best evaluated by MRI. Radiographic findings include abnormal alignment of the lunate and radius (Figure 10).

Stage IV: Lunate dislocation, in which the capitate is pulled proximal and volar by the intact radioscaphocapitate extrinsic ligament causing the capitate to push the lunate volarly. Radiographic findings demonstrate maintained alignment of the capitate with the radius and volar tilting and displacement of the lunate in relation to the radius. There is increased volar tilt of the lunate compared to stage III (Figure 11).

Dorsal perilunate fracture-dislocation involves perilunate dislocation secondary to carpal bone fracture (eg, scaphoid, capitate, hamate, or triquetrum). The most common subtype is the trans-scaphoid perilunate dislocation (Figure 12).

Carpal Instability Adaptive

Carpal instability adaptive (CIA) occurs when the carpal rows adapt and change their angle in response to pathology or abnormal anatomy near the wrist.5 CIA results most commonly from abnormal tilt of the radius (ie, Madelung’s deformity or fracture malunion). Intrinsic ligament injury should be excluded with MRI of the wrist.

Conclusion

Carpal instability is a significant source of chronic pain and disability. The wrist is a highly organized group of ligaments and bones that normally allow for stable transition of strength, dexterity, and fine-motor control from the forearm to the hand—functions that are progressively limited as carpal instability worsens. Therefore, discerning the most common etiologies of instability and their imaging findings is important to avoid increased morbidity and degenerative disease that can result from misdiagnosis. While carpal instability is recognizable on plain-film radiography, MRI offers superior visualization of the extent of carpal instability, specific ligamentous injuries, and its long-term sequelae.

References

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Treiman ML, Strle NA, Matthews CR, von Borstel D.  Carpal Instability: Clarification of the Most Common Etiologies and Imaging Findings.  J Am Osteopath Coll Radiol.  2019;8(4):5-10.

September 27, 2019
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