Carpal instability occurs when the carpus cannot maintain normal alignment and mechanics under physiologic loads, most commonly resulting from tearing or progressive insufficiency and laxity of key stabilizing intercarpal ligaments (see Image. Muscles and Fascia of the Hand). Instability, however, should be differentiated from carpal misalignment, wherein the carpus may show deviation from normal radiographic alignment, but the joints remain stable when loaded under physiologic conditions.
The Mayo Clinic Classification is commonly used to stratify and characterize carpal instability, dividing carpal instability into 4 categories.
Carpal instability dissociative (CID) describes carpal dysfunction between bones within the same carpal row. This includes scapholunate dissociation and lunotriquetral dissociation.
Carpal instability nondissociative (CIND) results from instability between the proximal and distal carpal row or proximal row and radius. This includes radiocarpal, mid-carpal, volar intercalated segment instability (CIND-VISI), and dorsal intercalated segment instability (CIND-DISI). The direction of the lunate, which is considered the intercalated segment, relative to the axis of the radius determines whether DISI or VISI is present. For example, if the lunate is extended, there is DISI. DISI or VISI deformities can occur due to chronic scapholunate or lunotriquetral dissociation, respectively. In this context, the DISI or VISI represents a dissociative malalignment.
Carpal instability combined (CIC) results from perilunate or axial dislocations. It has features of both CID and CIND. CIC can also be divided into dynamic and static instability. Dynamic instability refers to a deformity that only occurs during motion, while static instability can be seen with the wrist at rest in a neutral position.
Carpal instability adaptive (CIA) is malalignment and instability due to extrinsic deformities such as distal radius malunion, and it may appear similar to mid-carpal CIND.
Due to the many variations of carpal instability, this review focuses on the 2 most common etiologies: scapholunate and lunotriquetral dissociations.
Wrist Anatomy
The osteology of the wrist includes the distal radius, distal ulna/ulnar head, and 8 carpal bones. The distal radial articular surface contains a triangular scaphoid fossa and a rectangular lunate fossa. The articular surface is tilted on the ulnar aspect by approximately 23° (range, 15°-35°) and on the volar aspect by about 11° (range, 0°-20°). The proximal carpal row comprises the scaphoid, lunate, triquetrum, and pisiform. There are no direct tendon attachments to the proximal row. The distal carpal row includes the trapezium, trapezoid, capitate, and hamate. The bones of the distal carpal row have minimal motion between them compared to the proximal carpal row. The flexor and extensor tendons of the forearm are inserted into the distal carpal row.
The carpal bones can also be divided into columns. The radial column consists of the scaphoid, trapezium, and trapezoid. The central column contains the lunate and capitate. The ulnar column comprises the hamate and triquetrum.
The wrist contains a complex organization of ligaments. The ligaments are classified as extrinsic, connecting the distal forearm to the carpus, or intrinsic, originating and inserting between carpal bones. This review focuses on the ligaments of the scapholunate and lunotriquetral joints.
Scapholunate Joint
The scapholunate ligamentous complex stabilizes the scapholunate joint. The scapholunate (intercarpal) ligament is a C-shaped intrinsic ligament consisting of dorsal, palmar, and membranous components. The dorsal component is the thickest (up to 3 mm) and provides the most stability against distraction, torsion, and translation. The thinner palmar component provides restraint against rotational forces. The dorsal component merges with the dorsal intercarpal ligament, which originates on the dorsal ridge of the triquetrum and inserts on the dorsal rim of the scaphoid, trapezium, and trapezoid. The dorsal intercarpal ligament is a secondary stabilizer of the scapholunate joint as it indirectly prevents the proximal pole of the scaphoid from flexing and moving dorsally. Additional secondary stabilizers of the scapholunate joint include the radioscaphocapitate, scaphocapitate, long radiolunate, and scaphotrapeziotrapezoid ligaments, as well as the flexor carpi radialis and ulnaris tendons.
The radioscaphocapitate ligament originates from the anterolateral distal radius and inserts onto the palmar capitate, forming a sling around the scaphoid waist. The scaphocapitate ligament originates from the volar distal scaphoid and inserts onto the waist of the capitate, just distal to the insertion of the radioscaphocapitate ligament. The scaphotrapeziotrapezoid ligament originates from the distal pole of the scaphoid and has volar and dorsal contributions to the trapezium and trapezoid. The latter 2 ligaments stabilize the distal pole of the scaphoid. In the sagittal plane, the lunate is in relative neutral alignment compared to the axis of the forearm. The proximal lunate is narrower dorsally, resulting in its propensity to move into extension if ligamentous stabilization is disrupted. The scaphoid has an oblique orientation in the sagittal plane at a mean of 45° of flexion (range, 30°-60°) relative to the radius. As a result, the scaphoid tends to move further into flexion if there is a ligamentous compromise.
Lunotriquetral Joint
The lunotriquetral ligamentous complex primarily stabilizes the lunotriquetral joint. Like the scapholunate ligament, the lunotriquetral ligament has 3 components: dorsal, palmar, and membranous. The palmar component is the thickest and strongest, while the dorsal component mainly assists in rotatory stability. However, the triquetrum has more robust ligamentous insertions compared to the scapholunate ligamentous complex insertions. Lunotriquetral dissociation is less common than scapholunate dissociation.
This difference in ligamentous insertion on the triquetrum may help explain why injury to the lunotriquetral ligamentous complex is more stable than the scapholunate ligamentous complex injury. Secondary stabilizers of the lunotriquetral joint include the dorsal radiocarpal and scaphotriquetral ligaments. The dorsal radiocarpal ligament is the only extrinsic ligament that connects the dorsal radius to the dorsal carpus. It originates from the ulnar dorsal rim of the distal radius and inserts on the dorsal triquetrum and lunate to prevent the lunate from moving into flexion.
Carpal Mechanics and Motion
The long axis of the scaphoid runs obliquely to that of the radius. Therefore, with radial deviation, the scaphoid is pushed into flexion by the distal carpal row as the trapezium and triquetrum approximate the distal radius. To a lesser extent, the lunate and triquetrum move into flexion through the intact scapholunate and lunotriquetral ligaments. With ulnar deviation, the scaphoid is pulled into extension by the scaphotrapeziotrapezoid ligament. The lunate and triquetrum then follow the scaphoid into extension.
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