Carpal Ligament Instability

In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan.


Carpal instability occurs when the carpus is unable to maintain its normal alignment and motion under the influence of physiologic loads (see image. Muscles and Fascia of the Hand). Carpal instability must be differentiated from carpal misalignment. With carpal misalignment, the carpus may show deviation from normal radiographic alignment, but the joints will remain stable when loaded under physiologic conditions.

The Mayo Clinic Classification is the most commonly used. It divides carpal instability into four types: carpal instability dissociative (CID), carpal instability nondissociative (CIND), carpal instability complex (CIC), and axial type. CID describes carpal dysfunction that occurs between bones within the same carpal row. This includes scapholunate dissociation (SLD) and lunotriquetral dissociation (LTD). CIND occurs when there is instability between the proximal and distal 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 relative to the axis of the radius determines whether DISI or VISI is present. For example, if the lunate is extended (dorsiflexed), there is DISI. It must be noted that DISI or VISI deformities can occur as a consequence of chronic scapholunate or lunotriquetral dissociation, respectively. In this setting, the DISI or VISI is a dissociative malalignment. The third classification, CIC, is the consequence of perilunate or axial dislocations. It has features of both CID and CIND.

It 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 in a resting position.

Due to the breadth of this topic, this article will focus on the two most common types of carpal instability: scapholunate and lunotriquetral dissociations. CIND-DISI and CIND-VISI will be elaborated on in the Complications section.

Wrist Anatomy

The wrist is made up of the distal radius, distal ulna, and eight carpal bones. The radial articular surface contains a triangular scaphoid fossa and the rectangular lunate fossa. It is tilted on the ulnar side by about 23 degrees (range 15 to 35 degrees) and on the volar side by about 11 degrees (range 0 to 20 degrees).

The proximal carpal row consists of the scaphoid, lunate, triquetrum, and pisiform. There is a lack of direct tendon attachments to the proximal row. The distal carpal row is made up of 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 insert into the distal 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. Lastly, the ulnar column is composed of the hamate and triquetrum.

There is a complex organization of a number of ligaments within the wrist. These ligaments are classified as either extrinsic- connect the distal forearm to the carpus, or intrinsic- originate and insert between carpal bones. This article will focus on the ligaments pertinent to carpal instability involving the scapholunate and lunotriquetral joints.

Scapholunate Joint

The scapholunate ligamentous complex stabilizes the scapholunate joint. It consists of the dorsal, palmar, and proximal membranous components. The dorsal component is the thickest (up to 3 mm thick) and structurally 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 palmar radioscaphocapitate, scaphocapitate, and scaphotrapeziotrapezoid ligaments.

The radioscaphocapitate ligament originates off the anterolateral distal radius and inserts onto the palmar capitate. It forms a sling around the waist of the scaphoid. The scaphocapitate ligament originates off 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 off the distal pole of the scaphoid and has volar and dorsal contributions to the trapezium and trapezoid. These latter two 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 an average of 45 degrees of flexion (range 30 to 60 degrees) relative to the radius. As a result, the scaphoid has a tendency to move into further flexion if there is a ligamentous compromise.

Lunotriquetral Joint

The lunotriquetral ligamentous complex primarily stabilizes the lunotriquetral joint. It also has three components: dorsal, palmar, and proximal membranous. The palmar component is the thickest and strongest, while the dorsal assists mainly in rotatory stability. However, the triquetrum has more robust ligamentous insertions compared to the scapholunate ligamentous complex insertions. As discussed below, lunotriquetral dissociation is less common than scapholunate dissociation. This difference in ligamentous insertion on the triquetrum may help to explain why injury to the lunotriquetral ligamentous complex is more stable than what occurs with 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 off the ulnar dorsal rim of the distal radius and inserts on the dorsal triquetrum and lunate. This ligament prevents the lunate from moving into flexion.

Carpal Motion

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 also 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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