Carpal Ligament Instability 

The human wrist joint is a complex arrangement of small bones and ligaments that form a mobile yet stable link from the powerful forearm to the hand. The normally functioning carpus can position the hand precisely relative to the forearm and provides remarkably stable transmission of forces. Motion and stability of the carpus provide the critical foundation for maximum hand function from precise fine motor control to power grip activities.

When the normal mechanics of the wrist are disrupted, the instability of the carpal bones results in weakness, stiffness, chronic pain, and often arthritis if not treated appropriately. Although the early clinical and radiographic findings may be subtle, an understanding of wrist kinematics and instability patterns can facilitate early diagnosis and management. Unfortunately, selecting the optimal treatment remains a difficult judgment in most cases.

Linscheid et al described traumatic carpal instability in 1972.^[1] Since the early reports, anatomic and biomechanical studies have provided a foundation for understanding carpal motion, stresses, and pathologic instability. Building on these studies, various models have been suggested to explain the remarkable strength and mobility of this complex joint and the predictable patterns of failure.

This article presents the current understanding of pathologic carpal instability, the common classification patterns, and early treatment options that may avoid protracted dysfunction. Appropriate hand therapy is essential to maximize recovery but requires an appreciation of the limitations of carpal instability dysfunction and the goals of various treatment options.

Images of the carpal ligament and instability are included below.

Carpal ligament instability is defined as any malalignment of the carpus. This may be evident on plain radiography as a static deformity; alternatively, the situation may be a dynamic one, which becomes evident only when external forces are placed on the wrist.

The malalignment may appear after a single traumatic event or may be secondary to chronic attenuation of supporting ligaments after a traumatic event or secondary to an underlying disease process (eg, rheumatoid arthritis,pseudogout).

Frequency

In 1975, Dobyns et al reviewed their experience and found that 10% of all carpal injuries resulted in instability.^[2]

In 1988, Jones evaluated 100 consecutive patients with wrist sprains by using dynamic radiography (clenched-fist views) and found that 19 had an increased scapholunate gap.^[3]

The incidence of carpal instability that is associated with other specific fractures is relatively high. Reviewing 134 distal radius fractures, Tang in 1992 found radiographic evidence of carpal instability in 30% of the cases.^[4]

Geissler and Freedland prospectively reviewed 60 displaced intra-articular distal radius fractures that were being treated with arthroscopic assisted reduction and internal fixation.^[5] They found 43% had concomitant tears in the fibrocartilage complex, while 32% also had tears in the scapholunate ligament.

Weber reviewed 36 patients with acute scaphoid waist fractures and found that 28% had a dorsal intercalated-segment instability (DISI) deformity.^[6]

Carpal ligament instability results from an injury to one or more ligamentous or bony constraints in the wrist. Depending on the force, rate, and point of impact and on the position of the wrist, a fall on an outstretched wrist can result in a range of injuries. This spectrum includes wrist sprains, distal radius fractures, and fractures to the scaphoid and other carpal bones. This type of trauma can also result in injury to one or more ligamentous structures in the wrist, causing carpal instability. Perilunate instability is described as progressing from the scapholunate and the capitolunate to the lunotriquetral joint.

Using a cadaveric trauma model, Mayfield et al observed progressive injury patterns when the wrist was loaded in extension, ulnar deviation, and carpal supination.^[7] This perilunar instability is divided into 4 stages (see image below). Stage I refers to injury to the scapholunate interosseous ligament (SLIL). Further trauma results in dorsal subluxation of the capitate relative to the lunate, or stage II. As the load increases, the lunotriquetral interosseous ligament (LTIL) is injured, causing a perilunate dislocation in stage III. Finally, stage IV is characterized by dislocation of the lunate from the radiolunate fossa.

However, if the carpus is pronated and the hypothenar area is struck first, an ulnar traumatic pattern may be observed. Specifically, disruption of the ulnotriquetral ligament complex and the LTIL occurs.^[8] As the triquetrum no longer holds the lunate, it falls into a flexed position because of pressure from the capitate and its connection with the scaphoid. With attenuation or injury to the dorsal intercarpal ligament, a volar intercalated-segment instability (VISI) pattern ensues; this can be visualized on lateral radiography. An LTIL tear most commonly results in a VISI deformity.

In addition to a direct loading type of trauma, rotational force to the wrist can also result in ligamentous injuries, eg, the forces that occur when holding a power drill while the drill bit is jammed. This type of trauma can result in injuries to the LTIL and ulnar-triquetral ligament complex and result in the lunotriquetral instability.^[9]

Some instability patterns arise after chronic attrition of supporting ligaments. One traumatic event may result in some subtle ligamentous injury but no clear instability initially. However, over time, continued normal daily loading of the wrist can result in symptomatic instability. An example is seen with scaphoid fractures, where a DISI deformity tends to appear late after the initial traumatic event.

Supporting ligaments can also be important in preventing carpal instability in the presence of other significant ligamentous injury. For example, many cadaveric studies have shown that isolated sectioning of the SLIL does not result in frank radiographic scapholunate gap or dissociation.

In 1986, Johnson and Carrera described a midcarpal instability in which the capitate dorsally subluxes out of the cup of the lunate during a fluoroscopic dorsal-displacement stress test.^[10] This is associated with a painful snap or click that reproduces the patient's symptoms. They attributed the cause of this instability to attenuation of the radioscaphocapitate ligament after prior trauma.

Column theory of carpal kinematics

Over the past several decades, many theoretical models have been described to explain the complexities of carpal motion.

The column theory, as Navarro first proposed in 1935,^[11] recognized some motion between the bones of the proximal row and divided the wrist into 3 columns: the radial column, which consists of the scaphoid, trapezium, and trapezoid; the central column, which includes the lunate and capitate; and the ulnar column, which consists of the triquetrum and hamate. Although this theory does not explain the coupled motions that occur within the proximal and distal rows, it does help explain the load patterns seen through the wrist.

Row theory of carpal kinematics

The row theory is based on the fact that the proximal and distal rows work as 2 separate functional units. Gilford et al expanded on this row theory by noting that flexion-extension motions of the wrist are accomplished by relatively equal contributions from the radiolunate and lunocapitate joints and proposed that each row rotates around a single center of rotation near its proximal articular surface (see top Image below).^[12] They also emphasized the instability of such a 2-link system under load and the tendency for the system to crumple without a stabilizing mechanism. They believed that the scaphoid should be considered part of both rows and underscored its importance as a bridge, or tie rod, to stabilize an otherwise unstable arrangement (see bottom Image below).

Because no tendons insert on the scaphoid, lunate, and triquetrum, in 1972 Linscheid et al considered the scaphoid, lunate, and capitate to be an intercalated segment interposed between the articular surfaces of the radius and ulna and the rigidly bound distal carpal row.^[1] Muscle contractions impart rotational moments to the proximal row through the distal row, and carpal motion is governed by a combination of ligamentous and articular constraints. The strong interosseous ligaments between the 3 proximal carpal bones enable them to move in a synchronized fashion during wrist motion. The scaphoid, lunate, and triquetrum rotate in the same primary direction, albeit to different magnitudes, during any motion of the hand.

A specific example of this interaction is during radial-ulnar deviation. As the wrist ulnarly deviates, the entire proximal row extends. Conversely, the entire proximal row flexes as the wrist radially deviates. Although the mechanism by which this occurs is not entirely clear, most authors believe that this motion is a result of a combination of ligamentous constraints and carpal articular geometry between the proximal intercalated row and the distal row.

A theory that Linscheid and Dobyns proposed in 1989 is that the distal pole of the scaphoid flexes because of pressure by the trapezium and trapezoid during radial deviation.^[13] The rest of the proximal row then flexes because of the strong interosseous ligaments connecting the lunate to the scaphoid and the triquetrum to the lunate.

In another theory, Weber proposed that the unique helicoidal shape of the triquetrohamate articulation forces the distal row to translate dorsally and the triquetrum to tilt into extension as the wrist ulnarly deviates.^[14] Dorsal translation of the distal row contributes to an extension moment on the proximal row. The opposite occurs during radial deviation with palmarly directed force on the proximal row, causing flexion.

Combined column and row theory of carpal kinematics

Some have theorized that an individual's carpal kinematic behavior can be explained by some combination of both the columnar and row theories.

Craigen and Stanley analyzed radiographs of 52 normal wrists and found that, from ulnar to radial deviation, the amount of scaphoid shortening and ulnar translation of the scaphoid varies in a normal distribution.^[15] If the scaphoid shortens more, it translates less. By their interpretation, a column-type wrist shows greater shortening. They also found females were more likely to have greater scaphoid shortening and less translation.

This individual variation in kinematic behavior was also supported by Garcia-Elias et al who attributed it to the individual variation of laxity.^[16] They examined 60 healthy volunteers and found that physiologic differences in wrist ligamentous laxity affected carpal kinematics. In radial-ulnar deviation, the scaphoid of very lax wrists moved preferentially in the sagittal plane (flexion-extension), whereas in the more rigid wrists, the scaphoid moved preferentially in the frontal plane (radio-ulnar deviation).

Oval-ring theory of carpal kinematics

The oval-ring theory functionally depicts the carpus as a transverse ring formed by proximal and distal rows and joined by 2 physiologic links.^[17] The radial link is the mobile scaphotrapezial joint, while the ulnar link is the rotatory triquetrohamate joint.

The diagnosis of carpal instability in patients with obvious fracture and carpal instability patterns on radiography is sometimes relatively easy. Making the diagnosis in patients with subtle carpal instability can be more difficult. These patients often present with a history of a traumatic event. Noting the position of the wrist at the time of injury and determining the resultant force vector is extremely valuable.

Patients may have pain; if so, its location can be important when making the diagnosis. They may also have weakness and feelings of giving away. They may have clicking or snapping sensations on certain motions or upon loading the wrist.

As in many situations, physical examination starts with palpation. Nearly every critical ligament on the wrist can be palpated. Point tenderness over specific carpal ligaments such as the SLIL or LTIL may represent injuries to those ligaments. Pain at the extremes of motions may be present. Many dynamic maneuvers have been described to diagnose specific carpal instabilities.

Scaphoid shift test

One of the most common tests is the scaphoid shift test, as Watson described in 1997, shown below.^[18] In this test, the examiner's thumb is placed on the scaphoid tuberosity of the volar aspect of the wrist. Pressure is applied to the tuberosity as the wrist is passively brought from ulnar to radial deviation. This pressure attempts to block normal scaphoid flexion. In theory, if the SLIL is torn and scapholunate instability is present,^[19] the proximal scaphoid subluxates dorsally over the rim of the radius. A positive result is when a painful "clunk" is elicited as the scaphoid reduces back into the radial scaphoid fossa as the thumb pressure is released.

Easterling and Wolfe have shown that results of this test may be positive in a significant number of asymptomatic healthy wrists.^[20] Therefore, examination of the contralateral uninjured wrist is critical. In addition to the classic definition of a positive result, some surgeons believe that just pain and no subluxation with this maneuver may define a lesser scapholunate instability, such as a partial tear of the SLIL.

Maneuvers to diagnose lunotriquetral instability

A few maneuvers have been described that can help diagnose lunotriquetral instability. Distinguish lunotriquetral instability from a tear in the triangular fibrocartilage.

The Kleinman shear test, shown below, is performed with the wrist in neutral position.^[21] The examiner's contralateral thumb is placed over the dorsal lunate while the ipsilateral thumb loads the pisotriquetral joint with a dorsally directed force. A shear force is created across the lunotriquetral joint. A positive result is when this maneuver produces pain.

The Reagan shuck test, shown below, is similar, except the examiner's thumb and index finger grasps the whole pisotriquetral unit.^[22] The contralateral thumb and index finger hold the lunate. The lunotriquetral joint is stressed by applying dorsally directed force with one hand and volarly directed force with the other hand. This force is switched in the opposite directions in both hands. This creates a shear stress at the lunotriquetral joint, and if painful, the result is positive.

Linscheid described a compression test, shown below, in which the examiner uses his thumb to apply a load in the radial direction at the ulnar border of the triquetrum.^[23] This loading results in a compression force across the lunotriquetral joint. If this maneuver produces pain, the result is considered positive.

Lichtman et al described a pivot shift test for midcarpal instability.^[24] This maneuver is a combination ulnar deviation, axial compression, and pronation of the wrist. A positive result is when this maneuver results in a painful wrist click.

Another test for midcarpal instability (as described above) is a dorsal-displacement stress test.^[10] Under fluoroscopic control, a positive result is when the capitate subluxates dorsally compared with the lunate and when the patient experiences a painful snap or click.

Carpal bones

The wrist contains 8 carpal bones. Anatomically and functionally, these 8 carpal bones are divided into proximal and distal rows. The proximal row is formed by the scaphoid, the lunate, and the triquetrum. Although the pisiform is anatomically located on the palmar surface of the proximal row, it is a sesamoid bone within the flexor carpi ulnaris tendon. The pisiform does not contribute to the kinematics of the proximal row. The trapezium, the trapezoid, the capitate, and the hamate form the distal row.

Carpal ligaments

Multiple ligaments help stabilize the wrist to the forearm and hand. Extrinsic ligaments span the radiocarpal joint, while intrinsic ligaments connect between individual carpal bones. An important extrinsic ligament on the dorsal aspect of the wrist is the dorsal radiocarpal ligament (see top image below). This ligament originates on the radius and has minor attachments to the lunate while the bulk of the attachment is on the triquetrum. There are many more extrinsic ligaments on the volar aspect of the wrist. From radial to ulnar, they include the radioscaphocapitate, radioscapholunate, short radiolunate, long radiolunate, ulnolunate, and ulnotriquetrum ligaments (see bottom image below).

The intrinsic ligaments consist of stout ligaments that originate and insert within the carpus. The 2 most important intrinsic ligaments include the SLIL and the LTIL. The SLIL, which joins the scaphoid and the lunate, is probably one of the most important ligaments in the wrist. Injury of the SLIL can result in one of most common causes of carpal instability: scapholunate dissociation.

The SLIL is a C-shaped ligament that is divided into 3 separate components^[25] :

• The proximal component is made up of fibrocartilage and has minimal mechanical strength.
• The dorsal SLIL (dSLIL) and palmar SLIL (pSLIL) components have true ligament characteristics; the dSLIL is stouter than the pSLIL.
• The LTIL connects the lunate and triquetrum.

Similar to the SLIL, the LTIL is C-shaped and has 3 separate components. In contrast to the SLIL, its palmar component is stouter than the volar component.

Two intrinsic ligaments that cross from the proximal to the distal carpal row are the scaphocapitate and the dorsal intercarpal ligaments. The scaphocapitate ligament crosses the volar midcarpal joint and attaches from the distal pole of the scaphoid to the body of the capitate. Across the dorsal midcarpal joint, the dorsal intercarpal ligament originates from the triquetrum, attaches to the scaphoid dorsal ridge, and then inserts into the dorsal distal third of the scaphoid and to the scaphoid-trapezium ligament.