Carpal Instability

Extrinsic Ligaments

There are no true “collateral ligaments” that span the radial and ulnar styloid processes and the carpus in the coronal plane, as collateral ligaments would restrict radioulnar deviation. Extrinsic ligaments may be described by three major groups: palmar radiocarpal, palmar ulnocarpal, and dorsal radiocarpal ligaments ( Fig. 13.4 ). There are no true dorsal ligaments between the ulna and the carpus.

Four palmar ligaments connect the radius to the carpus: the radioscaphoid (RS), radioscaphocapitate (RSC), long radiolunate (long RL), and short radiolunate (short RL) ligaments. ^, The first three originate from the anterolateral margin of the distal radius and take an oblique course to insert onto the proximal edge of the scaphoid tuberosity (RS ligament), the palmar aspect of the capitate (RSC ligament), and the palmar surface of the lunate (long RL ligament). The short RL ligament originates from the anteromedial margin of the distal radius and is oriented vertically, inserting into the palmar and proximal aspect of the lunate. The RSC ligament courses around the palmar concavity of the scaphoid and inserts on the capitate, forming a sling over which the scaphoid rotates. The “ligament of Testut” or radioscapholunate (RSL) ligament was long considered an important deep intracapsular ligament. The RSL is no longer considered a true ligament but a bundle of loose connective tissue containing vessels supplying the SL interosseous membrane and adjacent osseous structures.

There are three palmar ulnocarpal ligaments: one superficial (ulnocapitate) and two deep (ulnotriquetral and ulnolunate). ^{, ,} The ulnocapitate ligament arises from a rough surface at the base of the ulnar styloid (i.e., the basi-styloid fovea) and courses distally to attach onto the neck of the capitate. The distal insertions of the ulnocapitate ligament blend with those of the intrinsic triquetrocapitate ligament before inserting onto the capitate. The combined ligaments form the ulnar bundle of the arcuate ligament, while the RSC and SC ligaments form the radial bundle. Deep to the UC ligament, arising from the triangular fibrocartilage, are the ulnotriquetral (UT) and ulnolunate (UL) extrinsic ligaments that run distally and radially toward their insertions into the anterior aspect of the triquetrum and lunate, respectively. These and the superficial UC ligament form the so-called ulnocarpal ligamentous complex. The ulnocarpal and radiolunate ligaments form the inverted “proximal V” of the palmar ligamentous complex. Between the distal and proximal V’s of the palmar ligamentous complex is a relatively weak capsular zone (the space of Poirier) (see Fig. 13.4A ) through which perilunate dislocations frequently occur.

The only dorsal extrinsic ligament is a critical stabilizing structure of the proximal carpal row—the dorsal radiocarpal (DRC) ligament. The DRC is a wide, fan-shaped ligament that connects the dorsal middle third of the distal articular surface of the radius to the dorsal lunate ^{, ,} and to the tubercle of the triquetrum (see Figs. 13.3C and 13.4B ). As it crosses the LT joint, it inserts onto the dorsal distal aspect of the LT interosseous ligament and also receives a strong reinforcement from the retinacular septum dividing the fourth and fifth dorsal extensor compartments. Similarly, the retinacular septum between the third and fourth dorsal compartments fuses with the DRC on its radial margin near the Lister tubercle. The DRC has four variations of its origin, including an accessory origin from the radial dorsal margin of the radius deep to the second dorsal compartment and one from the far dorsoulnar radius. There are no dorsal ligaments between the ulna and the carpus.

Intrinsic Ligaments

Intrinsic carpal ligaments include collections of relatively short fibers that connect neighboring bones of the same carpal row (interosseous ligaments), longer dorsal or palmar ligaments that span bones of the same row, or ligaments that span the midcarpal joint and link the two rows together (intercarpal ligaments).

Interosseous Ligaments

The SL linkage consists of three distinct but continuous portions of a complex C-shaped structure: the two SL ligaments (palmar and dorsal) and the proximal fibrocartilaginous membrane ( Fig. 13.5 ). The dorsal SL ligament (dSLIL) connects the dorsal and distal margins of the scaphoid and lunate. It is formed by a thick collection of fibers, slightly obliquely oriented, and plays a primary role in scaphoid stability. The dSLIL interdigitates dorsally with fibers of the dorsal scaphotriquetral ligament and the dorsal intercarpal ligament (DIC), which continues radially to insert along the dorsal ridge of the scaphoid waist. ^, The anterior counterpart of the dSLIL is the palmar SL ligament, which has longer, more obliquely oriented fibers, and allows substantial flexion and extension of the scaphoid relative to the lunate. The dSLIL has the greatest yield strength, 270 newtons (N) on average, followed by the palmar SL ligament (118 N) and the proximal membrane (63 N). The ligament has considerable torsional flexibility, with a neutral zone (physiologic laxity) of 30 degrees and a 47-degree range of rotation. The proximal membrane is formed by fibrocartilaginous tissue that separates the radiocarpal and midcarpal joints and is often perforated in older or active individuals. Perforation of the proximal SLIL may be recognized by arthrographic leakage of dye between the radiocarpal and midcarpal joints, and does not necessarily indicate instability.

The LT joint similarly has two interosseous ligaments (palmar and dorsal) connecting the palmar and dorsal aspects of the two bones and a continuous fibrocartilaginous membrane closing the joint proximally and separating the midcarpal and radiocarpal joints. In contrast to the SL ligaments, the palmar LT ligament is thicker and stronger than its dorsal counterpart (average yield strengths: 301 N and 121 N) with the proximal portion being the weakest (64 N). ^, The fibers of the two interosseous LT ligaments are more taut through all ranges of motion than those of the SL ligaments, making for a closer kinematic relationship. ^,

Functional Kinematics and Dart-Thrower’s Motion

Disparities between different theories of carpal kinematics may also derive from the somewhat arbitrary orthogonal planes of motion represented by standard radiographs and utilized in laboratory cadaveric studies and computational models (posteroanterior [PA] or frontal) and lateral (sagittal). These planes of motion are seldom utilized in occupational activities, activities of daily living, or sporting activities. The most commonly used “functional” plane of motion begins in extension and radial deviation and ends in flexion and ulnar deviation. Fisk described this plane as “when someone is casting a fly or throwing a dart”; thus it is commonly referred to as the “dart-thrower’s” motion (DTM). The obliquity of the optimal functional DT plane of motion is unique to each individual (and perhaps each activity), ranging from 37 to 59 degrees from the sagittal plane, according to various authors. ^{, , ,} This oblique plane of wrist motion, generated by the two powerful radial wrist extensors—extensor carpi radialis longus (ECRL) and extensor carpi radialis brevis (ECRB)—and the muscle with the greatest cross-sectional area in the forearm, the FCU, is the dominant plane of motion in activities of daily living, as well as recreational and occupational activities.

The study of three-dimensional (3D) carpal kinematics using markerless bone registration (MBR) is ideally suited for studying individual carpal bone kinematics in live subjects during wrist motion, especially in combined or “coupled” motion planes. ^, A series of studies of DTM from radial extension to ulnar flexion independently confirmed that proximal row flexion during wrist flexion is balanced by extension forces during ulnar deviation, such that the proximal row remains nearly stationary during this motion path. ^{, ,} The stability of the proximal row in this motion plane underscores the critical importance of the midcarpal joint to functional activities such as hammering or throwing, and has been proposed to have had implications in homo sapien evolution. Further cadaveric research identified that the mechanical axis of the wrist is collinear with the dart-throwing axis and lies oblique to the anatomic axes of the wrist ( Fig. 13.9 ). The unusual behavior of the proximal row during DTM cannot be adequately explained by any of the prevailing theories of carpal kinematics and ushered in an era of “functional kinematic” analysis. This line of research measures the kinematics of normal, injured, and postreconstructive patients in real time during activities, using gait lab technology, biplanar video fluoroscopy (BVR), or electrogoniometry.

The opposite rotation, from extension–ulnar inclination to flexion–radial inclination, has been referred to as the reversed dart-thrower’s motion. It is a rotation mediated by the FCR on the volar side and the extensor carpi ulnaris (ECU) on the dorsal side of the wrist. As opposed to what happens during DT rotation, the contribution of the midcarpal joint to the reversed dart-thrower’s type of rotation is minimal: this motion occurs predominantly through the radiocarpal joint. The very limited range of motion (ROM) attainable in this plane exemplifies the importance of a two-row, two-joint system of normal wrist function that utilizes joint axes situated in two planes angled at 45 degrees to each other. This concept was first advanced by Henle in 1871 as a “universal” or cardan joint, wherein two joint axes are aligned in different planes to transmit torques in multiple directions and permit complex motions ^, ( Fig. 13.10 ).

Sandow utilized a “rules-based” computational model of five human wrists in several different positions of flexion/extension and radioulnar deviation to propose and test a “two-gear, four bar” linkage of the proximal and distal rows. The computational model predicts carpal kinematics based on the isometry of the major capsular stabilizing ligaments. Building on the cardan concept above, the proximal row and distal row axes are oriented approximately 45 degrees to each other in the axial plane, but in this model the relationship varies as the proximal row moves. As this technique involves 3D-computed models of an individual patient’s carpus, a patient-specific kinematic model can be built and utilized for surgical planning and virtual surgery, simultaneously obviating the difficulties encountered with individual anatomic differences and ligament laxity.

Intracarpal Pronosupination

The mechanism of active intracarpal pronosupination of the carpus has recently been elucidated ^, and can be explained as follows. Distal to the extensor retinaculum, the wrist tendons change direction, taking an oblique path to their distal insertion. The ECU tendon, for instance, from a dorsal position at the level of the ulnar head, courses obliquely toward its distal insertion onto the dorsoulnar corner of the fifth metacarpal. The ECRL and the abductor pollicis longus (APL) also have an oblique course but in the opposite direction, toward the lateral border of the wrist.

Each of these tendons is collinear with the forearm at the radiocarpal level but diverges distally toward their ulnar or radial insertions. If the wrist is in a neutral position, isolated isometric contraction of any of these muscles will generate a pronation or supination moment to the distal row: ECU and FCR muscle contraction will cause distal row pronation, whereas the ECRL, FCU, and APL muscles will induce a supination moment to the distal row. These intracarpal motions have important implications to rehabilitation of the ligament-injured wrist. ^,

The Role of Ligaments in Stabilization of the Proximal Row

The proximal carpal row, suspended between the radius and the distal carpal row with no tendon insertions, is inherently unstable. If not for the capsule, ligaments, and muscles, the three bones would “crumple” as Gilford et al. observed, when compressed by the distal carpal row against the radius. Instead, because of its oblique orientation relative to the long axis of the forearm and to the neighboring trapezium, the loaded scaphoid tends to rotate into flexion and pronation, and the distal carpal row pronates around the nearly spherical scapholunocapitate articulation. Distal row pronation translates the hamate dorsally, creating a counterbalancing extension moment to the triquetrum through its ligament constraints. If both palmar and dorsal SL and LT ligaments are intact, such differences in angular rotation generate increasing torque and intercarpal coaptation of the SL and LT joints, further contributing to the proximal row’s stability. Further, the proximal row is reinforced by an array of radiocarpal and midcarpal-crossing ligaments. Critical proximal row stabilizers are the STT and SC ligaments laterally, the Triquetrohamatecapitate (THC) ligament medially (the so-called ulnar bundle of the arcuate ligament), the long RL ligament palmarly, the DRC (through its insertion on the lunate and triquetrum), and the dorsal intercarpal ligament (through its DST attachments to the dorsum of each proximal row bone and the dSLIL). ^{, , ,} Based on this complex interrelationship, if the SL or LT ligaments are ruptured in isolation, the proximal carpal row will not collapse ( Fig. 13.11 ). This may help explain how so many SLIL injuries are missed in the acute stage.

In theory, assuming the intrinsic and extrinsic ligaments are fully intact, the flexion tendency of the scaphoid and the extension tendency of the triquetrum under heavy or sudden loads would be neutralized, and carpal stability achieved. In practice, these ligaments cannot do this alone, as the tensile forces generated by many hand activities are too high for ligaments to be the only carpal stabilizers. To prevent injury under high load, ligaments need to be protected by neuromuscular stabilization. ^{, ,}

To ensure a timely and protective muscle response, the carpal ligaments play an essential role in coordinating a centralized musculoligamentous response to abnormal wrist loads through mechanoreceptors, which provide the proprioceptive information required by the sensorimotor system to activate that response. In this view, the “helical antipronation ligaments” (HAPL) are coiled around the scapholunocapitate joint in such a way as to provide a first line of defense against instability, sensing application of load, pronation torque of the distal carpal row, and scaphoid flexion. Reflex activation of stabilizing antipronation muscles (ECRL, APL, FCU) provides powerful stabilization.

The Role of Muscles in Stabilization of the Proximal Row

Until recently, muscles were considered to have a negative influence on carpal stability. Instability was assumed to be a ligament insufficiency problem made worse by muscle contraction. Now we know that the assumption is wrong: Muscles play an important stabilizing role ^{, ,} ( Fig. 13.12 ).

The mechanisms of muscle stabilization of the wrist are based on the previously mentioned ability of some muscles to actively pronate or supinate the distal carpal row, thus counteracting or balancing applied loads. ^, When the distal row supinators (ECRL, APL, FCU) contract, the trapezium displaces dorsally, thus tensioning the STT ligament. If the STT ligament is intact, the scaphoid cannot collapse into flexion and pronation. The intracarpal supinators, therefore, protect against excessive flexion-pronation of the scaphoid ( Video 13.1 ). In contrast, when the ECU or FCR contracts, the distal row pronates, tightening the TqH ligament and preventing the proximal row from collapsing into VISI in cases of lunotriquetral insufficiency or dissociation. As a distal row pronator, the role of the FCR in proximal row stability is controversial because distal row pronation stresses the SL relationship ( Video 13.1 ). Recent cadaveric studies of the intact carpus demonstrate that activation of the FCR when the wrist is in neutral or extension actually supinates the scaphoid through a dorsolateral vector at the scaphoid tubercle, thus compressing the SL joint. Thus, strengthening and activation of the distal row and scaphoid supinators may be useful in the nonoperative treatment of scaphoid rotatory instability and the intracarpal pronators in the treatment of LT instabilities.

Assessment of the Symptomatic Wrist

The natural inclination to study radiographs or special imaging studies and reports prior to a thorough history and physical examination should be avoided, as this introduces cognitive bias, which can affect one’s thinking and decision making. Doing so may save time in the short term but may lead to diagnostic and treatment errors. Similarly, physical examination always needs to be preceded by a thorough investigation of the patient’s medical history, with special emphasis on the mechanism of injury and acuity. The patient should be asked details about the location, duration, and characteristics of any pain, including aggravating and relieving factors and previous treatments, if any. With chronic problems, it is also important to inquire about the patient’s jobs and hobbies, and whether there has been exposure to repetitive stress, vibrating tools, or potentially dangerous instruments. Elucidating a history of ligamentous laxity or multiple joint instabilities, especially in younger patients presenting with chronic wrist pain, is also important. Such an evaluation should also include an assessment of their stress coping skills.

Radiologic examination

Routine radiograph views

The initial radiographic examination of a patient with a suspected carpal injury should include at least four images of the wrist including a PA, lateral, scaphoid (PA in ulnar deviation and supination), and a 45-degree pronated oblique view. Contralateral wrist radiographs are an essential part of the workup for suspected ligament injury.

Posteroanterior projection

The PA view should be obtained with the hand placed flat over the radiograph film, the shoulder abducted 90 degrees, the elbow flexed 90 degrees, and the forearm in neutral rotation. One can determine whether it is a true PA radiograph by checking that the ECU groove is seen radial to the ulnar styloid. In the PA view of a normal wrist, the proximal and distal outlines of the proximal row, as well as the proximal outline of the distal row (i.e., “Gilula’s lines”), are smooth, without breaks in their continuity ( Fig. 13.15 ). Any step-off of these lines indicates abnormality at the site where the arc is broken. In addition, the ulnar styloid should form the ulnar border of the ulna.

The articulating carpal bones have parallel opposing surfaces separated by 2 mm or less. Any overlap between well-profiled carpal bones identifies an intercarpal abnormality and should prompt further imaging ( Fig. 13.16 ). Joint diastasis should not be interpreted as a sign of joint disruption unless it is compared with the uninjured, contralateral side.

In the PA view of a wrist in neutral, the normal lunate has a trapezoidal configuration. It is possible to differentiate a flexed from an extended lunate based on the shape of its contour (see Fig. 13.16 ). To do so, it is important to remember that, in the transverse plane, the lunate is wider palmarly than dorsally. Therefore, when abnormally extended, the lunate has an ovoid configuration due to the distal displacement of the wider anterior horn. Conversely, when abnormally flexed the lunate is triangular shaped, often with a half moon–like configuration, with its concavity facing toward the scaphoid. ^,

Lateral projection

The lateral view must be taken with the arm adducted to the patient’s side, and with both the forearm and the wrist in neutral rotation. For a lateral projection to be correct, the axis of the third metacarpal needs to be parallel to that of the radius, and the palmar outline of the pisiform needs to be located between (and equidistant to) the palmar surfaces of the scaphoid tuberosity and the capitate head.

Scaphoid projection

The best projection to confirm a scaphoid fracture is a PA view centered on it, with the wrist in ulnar inclination, slightly supinated, and the fingers fully flexed.

Pronated (oblique) projection

A lateral radiograph with 45 degrees of pronation profiles the anterolateral and posteromedial corners of the carpus and is useful to identify fractures of the dorsal ridge of the triquetrum and of the scaphoid tuberosity.

Additional radiograph views

When routine radiographs do not confirm the suspected diagnosis, obtaining additional views may be needed. The following are the most commonly used projections for the wrist.

• Anteroposterior (AP) (palm up) view with a clenched fist: If an SL ligament injury is suspected, obtaining a fully supinated clenched fist view may accentuate the gap that often appears with this injury by axial-directed force to the carpus through the capitate ( Fig. 13.17A ). It is preferable to obtain this view in a neutral wrist posture. Correct positioning can be objectively evaluated by looking at the third CMC joint. When the wrist is in neutral, the joint’s surfaces are parallel. Contralateral stress views of the wrist may be of benefit when a suspicion of dynamic instability is suspected.

  

• PA (palm down) view with 10 degrees of tube angulation from ulna toward radius: This view is ideal to assess the SL interval. Measurement of its separation (SL gap) is made at the midportion of the joint where its anatomy is more consistent. The spacing should be compared with the opposite wrist and with the surrounding carpal articulations (see Fig. 13.17B ).

• Clenched-pencil PA view: This is one of the best ways to demonstrate SLD because it optimally profiles the joint while it is being separated with some grip force. It consists of a PA view of both wrists while the two hands are gripping a pencil tightly, with the index fingers apposed and the thumb metacarpals lying flat on the radiographic cassette (see Fig. 13.17C and D). The technique provides comparison views of both wrists. ^,

• Oblique view at 20 degrees of pronation from the lateral position: This view is used to visualize the dorsum of the triquetrum, where avulsion fractures frequently occur, and to evaluate the distal tuberosity and waist of the scaphoid. It is ideal for fracture-subluxations of the fifth CMC joint.

• Oblique view at 30 degrees of supination from the lateral position: The pisotriquetral relationship is best seen with this view.

• Lateral view with the wrist radially deviated: The palmar outline of the hook of the hamate can be seen on a lateral view with the wrist radially deviated and the thumb in maximal palmar abduction. It places the hook of the hamate between the bases of the first and second metacarpals.

• Carpal tunnel view: By profiling the carpal concavity of the wrist, a clearer view of the hook of the hamate, the pisiform, and the palmar ridge of the triquetrum can be obtained. In patients with acute injuries or wrist stiffness, however, pain produced by extending the wrist may preclude this exam.

• Static deviation views: For patients suspected of having carpal instability, a routine “motion” series may be needed to determine the mobility and reducibility of the carpus, and in particular, the lunate. It should include PA and AP views in radial and ulnar inclination, in addition to lateral views in extension and flexion. Actual motion assessment can be easily performed in these planes with fluoroscopy of the injured and uninjured wrist (see Fluoroscopy, page 505).

Measurement of carpal bone alignment

Carpal malalignment has traditionally been determined by measuring specific distances and angles on PA or lateral radiographs. ^, When interpreting these data, however, one must be aware that the normal ranges for these parameters are quite diverse. Also, reproducibility is low and small errors in rotational positioning of the hand at the time of radiograph exposure may result in substantial variation in angle determination. Larsen et al. demonstrated the highest intraobserver and interobserver reliability using a standardized technique, incorporating tangent lines of the scaphoid, lunate, and third metacarpal to determine interosseous angles ( Fig. 13.18 ).

• Radiolunate (RL) angle: This gives objective evidence of the dorsal or palmar tilt of the lunate. The normal RL angle should be between 20 degrees volar and 15 degrees dorsal. ^{, ,} This measurement is the best evaluation of DISI and VISI posture, but its utility can be compromised by radiographs without true neutral sagittal alignment and should always be checked against the opposite wrist.

• SL angle: This angle, formed by a line tangential to the proximal and distal convexities of the scaphoid’s palmar aspect and that of the lunate, has been quoted in the literature as one of the major determinants of SLD. Normal values range from 30 to 70 degrees (average, 47 degrees). ^{, ,} Although angles greater than 80 degrees indicate rotatory subluxation of the scaphoid, lower readings do not rule out SL disruption. Values less than 30 degrees are not unusual for patients with STT joint osteoarthritis or VISI.

• RS angle: This angle is formed between a line along the longitudinal axis of the radius and a line from the proximal palmar and distal margins of the scaphoid. Normal values range from 35 to 65 degrees, with an average of 52 degrees.

• Lunocapitate angle (LC): In theory, the long axes of the radius, lunate, capitate, and third metacarpal are colinear. In practice, this occurs in less than 11% of normal subjects. Even so, every attempt should be made to standardize the lateral film by ensuring that the long axis of the third metacarpal and radius are as close to parallel as possible and that neutral rotation has been achieved by aligning the pisiform and scaphoid tubercle. The axis of the lunate is a line perpendicular to a line connecting the palmar and dorsal lunate rims. The capitate axis is identified by connecting a point in the center of the convexity of its head to a point at the center of the third CMC joint. Because of the difficulty in identifying the true axis of the capitate, Larsen et al. defined a substitute method as the tangent to the dorsum of the third metacarpal. A normal CL angle should be 0 ± 15 degrees with the wrist in neutral.

• Ulnar variance: Ulnar variance is usually measured on true (zero rotation) PA radiographs, obtained with the shoulder 90 degrees abducted, the elbow 90 degrees flexed, the forearm and wrist in the neutral position, and the central x-ray beam centered directly over the wrist. The usual way to assess ulnar variance is to measure the distance between two lines perpendicular to the longitudinal axis of the radius. One is tangential to the most distal point of the ulnar dome, and the other is tangential to the distal articular surface of the lunate fossa. When the ulna is shorter than the radius, the ulnar variance is negative, and when longer, it is positive. The radioulnar length relationship can also be assessed with a pronated grip film in cases of suspected ulnar impaction, which should increase ulnar variance, and should be done on each wrist for comparison.

• Carpal height ratio: This measurement can be used to determine carpal collapse. The term carpal height refers to the distance between the base of the third metacarpal and the distal articular surface of the radius measured along the proximal projection of the axis of the third metacarpal. The ratio (carpal height ÷ length of third metacarpal) was found to be 0.54 ± 0.03 in normal wrists. Given that some wrist radiographs do not often include the entire third metacarpal, some authors have proposed using the length of the capitate instead (i.e., carpal height ÷ capitate length), with a normal range of 1.57 ± 0.05 ( Fig. 13.19A ). This method has greater accuracy than the original method and can be calculated on standard wrist films.

  

• Ulnar translocation ratio: This measures the coronal displacement of the carpus and in particular the lunate. As described by Bouman and colleagues, a line measuring the length of the articular surface of the radius is divided by the distance from the radial styloid to the proximal ulnar corner of the lunate (see Fig. 13.19B ). Normal ratio is 0.87 ± 0.04.

Distraction views

In patients with acute fracture-dislocations, the four routine views described earlier may be difficult to interpret because of overlapping and displaced carpal bones. In such cases, obtaining AP and lateral radiographs with the hand suspended in finger traps is recommended. Distraction views may reveal articular fracture fragments or an articular step-off that cannot be identified on routine films. Caution should be used when interpreting Gilula’s lines in patients with ligamentous laxity, and the contralateral wrist should be imaged for comparison.

Computed tomography

Computed tomography (CT) scans are usually taken with 0.625 to 2.0 mm slice thickness at contiguous or overlapping increments along the axial, sagittal, and coronal planes. The recent introduction of ultrafast CT scans has substantially reduced image-acquisition time to a fraction of a second, and radiation doses accordingly. High-resolution CT scans can be digitally reformatted in any plane for optimal visualization (e.g., along the true sagittal and coronal axes of the scaphoid for assessing alignment of fracture fragments), provided the slices are ultrathin (0.625 mm) and contiguous. CT is also useful in evaluating the union of fractures or arthrodesis, although in many instances the image can be altered by the presence of retained hardware.

CT has the added advantage of allowing computer manipulation to obtain 3D images of the carpal bones, which help visualize the structure to be analyzed. When surgery is planned on a fractured or malunited scaphoid or a complex carpal dislocation, a 3D reconstruction provides visual information about the amount and direction of the displacement ( Fig. 13.20 ). However, it is important to note that all the information provided by such a reconstruction is already present in the original CT image.

4D CT (3D CT and time) scanning requires continuous data acquisition at a fixed table position using dual source scan mode. It allows for high spatial and resolution of images of the wrist in motion and as such has been used to study carpal instabilities. The ability to do this noninvasively may allow for the detection and location of ligament injuries and help guide selective surgical treatments ( Video 13.2 ). Although promising, this technology needs to be used with caution owing to the higher dose of radiation involved.

Magnetic resonance imaging

Historically, MRI without dedicated wrist coils demonstrated reduced sensitivity and specificity in the diagnosis of SL ligament injury compared with arthroscopy. With state-of-the-art technology and scanning algorithms, MRI’s exceptional soft tissue contrast and direct multiplanar acquisition have enabled an accurate, noninvasive, and reproducible means to evaluate the articular cartilage, injury to the intrinsic and extrinsic ligaments of the wrist, and the TFCC without ionizing radiation ( Fig. 13.21A and B). Properly executed high-resolution MRI using 1 mm contiguous slices, cartilage-sensitive pulse sequence parameters, and dedicated wrist coils may provide complementary information on interosseous and extrinsic ligament injury to wrist arthroscopy. Gadolinium enhancement does not improve diagnostic accuracy for cartilage and ligament assessment and converts noninvasive imaging to a more invasive procedure. While good accuracy has been described with appropriate protocols at 1.5 T systems, 3T studies provide higher signal-to-noise ratios and allow for potentially shorter acquisition times. Ultrahigh field (7T) MRI may provide increased spatial resolution of the wrist, but is currently limited by the lack of available wrist coils and greater artifact generation.

Together with 4D CT imaging of the wrist, real-time dynamic 3D MRI has been investigated for the assessment of dynamic carpal instabilities. Rather than replacing high spatial resolution static scans, it may be used as an adjunct given its lower in-plane spatial resolution.

Pathomechanics of SL Dissociation

A fall on the outstretched hand with the wrist in extension, ulnar deviation, and intercarpal supination may cause a wide spectrum of injuries, from minor SL sprains to complete perilunate dislocations. Although differing in severity and prognosis, these clinical conditions are stages of the same progressive perilunate destabilization process described earlier.

The kinematic and kinetic consequences of SL ligament injury have been investigated by many authors. ^, In cadaveric studies, if only the palmar SL ligament and the proximal membrane are ruptured, minor kinematic alterations may occur in scaphoid flexion or lunate extension during wrist motion. ^, These partial injuries are theorized to result in “dynamic” or “occult” instability. There is no diastasis of the SL interval, no resting postural changes of the carpus, but patients may be intermittently symptomatic under heavy mechanical load.

Complete sectioning of the SL (palmar, membranous, and dorsal) ligaments results in more substantial alterations of wrist kinematics and force transmission parameters, but acutely does not result in SL diastasis or carpal malalignment. ^, Immediate carpal postural changes occur only if there is concomitant rupture of one or more of the critical ligament stabilizers of the proximal carpal row; that is, the radiopalmar STT ligament complex, one or both components of the dorsal intercarpal ligament, or the long RL ligament. ^, Even when not acutely ruptured, it is theorized that these same ligaments may gradually attenuate with loading over time, allowing secondary rotatory and dorsal subluxation of the scaphoid, dorsal tilt of the intercalated segment (DISI), and, ultimately, carpal collapse. Kauer believed that the unconstrained lunate has a natural tendency toward displacing into extension owing to its shape, but also due to the fact that it articulates with a radial surface that is volarly inclined. Without its proximal tether to the lunate, and under the flexion loads of the radial column, the scaphoid flexes around the RSC ligament, while pronating and translocating dorsally and radially. ^, Without the joint reaction force of the proximal scaphoid, and with slackening or attenuation of the long RL or the DIC or both, the triquetrum and lunate rotate dorsally under the extension moment of the hamate’s articular surface. As a result of the scaphoid collapsing into flexion and pronation and the lunate and triquetrum into extension, the distal row translates dorsally and rotates into pronation (see Fig. 13.14 ).

With SLD, the forces crossing the wrist are not distributed normally. An increased compressive and shear stress appears on the dorsal and lateral aspect of the RS articulation as the proximal pole of the scaphoid subluxates dorsoradially with wrist flexion and during manual load, ultimately leading to cartilage loss and osteoarthritis. The lunate extends but its kinetics remain stable because the lunate and the lunate facet of the radius are perfectly congruent. This explains why the RL articular cartilage is spared until very late in the degenerative process. There is considerable variability in the progression of SLD and degeneration following complete SLIL rupture. O’Meeghan et al. reported on the natural history of arthroscopically proven SL rupture without either SL widening or DISI on x-ray and found that at an average of 7 years, all 11 patients had functional sequelae and wrist pain, but none went on to develop SLAC changes.

Watson and associates coined the term scaphoid-lunate advanced collapse to define a three-stage progression of degenerative changes secondary to SLD. In the earliest stage, only the distal radial styloid-scaphoid articular surface degenerates, with characteristic breaking of the styloid tip. In stage II, progressive loss of the proximal scaphoid articular cartilage and the dorsal scaphoid fossa ensues under the shear forces described above. In stage III, the midcarpal joint degenerates. Whether the RL joint undergoes progressive degeneration is controversial, though often the dorsal lip of the RL fossa narrows appreciably.

Diagnosis of SL Dissociation

A history of a fall on an outstretched hand should warn the clinician about the potential for a SL injury, sometimes masked by a fracture of the scaphoid or distal radius. SLD is frequently missed at presentation, especially in isolated partial lesions, or when it is masked by other, more obvious injuries. When SLD is static or results from a perilunate dislocation, the diagnosis is generally more obvious. According to Geissler, up to 30% of distal radial fractures are associated with variable degrees of carpal ligament disruption. Aside from wrist trauma, iatrogenic SL injury can occur with excessive capsular excision that disrupts dorsal ligaments when removing ganglions or after ligament attenuation from inflammatory, crystalline, or septic arthritis. Although SLD is rare in children, diagnosis is more problematic because it is not easy to clearly identify bony alignment in an immature skeleton, and performing a clinical examination is difficult.

In both children and adults, a high index of suspicion is needed to avoid missing this injury. The symptoms of SLD vary markedly depending on the magnitude of the ligament rupture, the extent of associated injuries, and elapsed time since injury. Poor grip strength, limited mobility, dorsoradial swelling, and point tenderness over the dorsal aspect of the SL interval are the most common findings. ^, Pain is common and is usually aggravated by intense use of the hand. The patient may report a clunking sensation during wrist movement due to dislocation or reduction of a dorsally subluxated proximal scaphoid.

Physical Examination

On inspection, there may be minimal or no changes in the appearance of the wrist with SL instability. Even in the acute phase, swelling and ecchymosis may be moderate. Palpation at the areas of maximal tenderness is useful in the diagnosis of wrist pathology, especially for patients with chronic SL instability. By flexing the wrist and palpating the dorsum of the capsule distal to the Lister tubercle, one can obtain important information about the SL joint. If sharp pain is elicited by pressing this area, the probability of either a recent injury or a chronic localized synovitis is high. Most of these patients also have tenderness in the anatomic snuffbox and over the palmar scaphoid tuberosity. In acute cases, ROM is usually limited by pain, whereas it may be normal in chronic cases.

Scaphoid shift test

Passive mobilization of an unstable SL joint is valuable not only in determining the presence of abnormal scaphoid subluxation but also in reproducing the patient’s pain. A positive scaphoid shift test, as described by Watson and colleagues (painful reproduction of the patient’s scaphoid subluxation), is said to be diagnostic of SLD. The examiner places four fingers behind the radius, and the thumb on the scaphoid tuberosity (distal pole). The other hand is used to move the wrist passively from ulnar to radial deviation. In ulnar deviation, the scaphoid is extended and assumes a position more in line with the forearm. In radial deviation, the scaphoid is flexed. Pressure on the tuberosity while the wrist is moved from ulnar to radial inclination prevents the scaphoid from flexing. If the SL ligaments are completely ruptured or elongated, the proximal pole subluxates dorsally out of the radius, inducing pain in the dorsoradial aspect of the wrist ( Fig. 13.22 ). When pressure is released, a typical clunk may occur, indicating reduction of the scaphoid into the scaphoid fossa of the radius.

When performing the scaphoid shift test, one should be aware of its low specificity. If the SL ligaments are intact, but there are other pathologies inducing a local synovitis (e.g., occult ganglion or dorsal RS impingement), this test may also provoke sharp pain, making it difficult to discern whether there is an abnormally unstable proximal scaphoid. Additionally, patients with generalized laxity may exhibit painless clunks during this maneuver, which most likely emanate from the LC joint. Comparison of the two sides is important, although sometimes the opposite “asymptomatic” wrist has a painful scaphoid shift test as well. ^, Experience with this test is necessary before it can be interpreted with confidence, and repeating this test under fluoroscopy in equivocal cases can be revealing.

Radiographic Examination

On standard radiographs, SLIL injury is suspected in the presence of a widened SL gap, dorsal tilt of the lunate, or increased scaphoid flexion. Dynamic instabilities require special projections or loading conditions for these features to be observed. Radiographic findings should always be compared with the asymptomatic side.

Increased SL joint space

The “Terry Thomas sign,” named by Frankel after the English film comedian’s dental diastema, ^, is considered positive when the space between the scaphoid and lunate appears abnormally wide compared with the contralateral side ( Fig. 13.23 ). The SL gap should be measured in the middle of the flat ulnar facet of the scaphoid. A unilateral gap greater than 5 mm is diagnostic of SLD. If there is no history of a specific traumatic episode, and yet there is an obvious SL diastasis, one must consider either a constitutionally increased SL gap, which is usually bilateral, with or without hyperlaxity. Other less common causes of bilateral SL diastasis include rheumatoid arthritis, gout, and calcium pyrophosphate deposition disease. To rule out the presence of an asymptomatic SL ligament injury, dynamic stress views or advanced imaging is critical.

Scaphoid ring sign

When the scaphoid has rotated into flexion and pronation, it has a foreshortened appearance in the PA view. In such circumstances, the scaphoid tuberosity is shown in the PA projection in the form of a radiodense circle or ring over the distal two-thirds of the scaphoid ( Fig. 13.24 ). This is the “ring sign” and is present in all cases where the scaphoid is abnormally flexed, regardless of the cause. The presence of this sign does not always indicate SLD, as this can also occur in a dissociative or nondissociative VISI deformity.

Increased SL angle

In the lateral view, if the scaphoid lies more perpendicular to the long axis of the radius than the contralateral side (>60 degrees) and the lunate appears normally aligned or extended, SLD should be suspected ( Fig. 13.25A ). This angle will progressively increase as the lunate extends to accommodate the loss of height of the radial column, causing a dorsal subluxation of the capitate.

Advanced Imaging

While history, clinical examination, and radiographs are often diagnostic, other imaging modalities may be useful to better understand chronicity, kinematics, cartilage health, and potential concomitant injury.

Magnetic resonance imaging

A properly performed, high-resolution MRI aids in the evaluation of ligament injuries of the wrist. A static magnetic field strength of at least 1.5 T using a dedicated wrist coil is recommended for analyzing the interosseous, intrinsic, and extrinsic ligament insertions ( Fig. 13.26A–C ). Higher magnetic field strengths result in a higher signal-to-noise ratio with shorter scan times. The volar extrinsic, SL interosseous, dorsal intercarpal, and LT ligaments are best visualized using 1 mm slices (with no interslice gap) in the coronal plane. The DRC and intercarpal ligaments are best viewed on both the coronal and sagittal images (see Fig. 13.26D ). Oblique axial views along the longitudinal axes of these ligaments allow further analysis, especially when an injury is suspected. Concomitant cartilage-sensitive imaging is imperative to complement the assessment of ligaments, as the cartilage integrity will influence the clinical/surgical management, especially in the setting of SLAC wrist (see Fig. 13.21 ). Real-time MRI has been used to investigate dynamic instabilities, although its routine use in clinical practice is yet to be further determined.

Treatment of SL Dissociation

Given the myriad of factors that need to be considered in the treatment of SL ligament injury, the outcome is not always predictable. ^{, , ,} When the initial injury is a partial SL ligament disruption, radiographs are usually normal, and the injury is frequently missed at presentation. Even if diagnosed early, the ligament remnants may be short and difficult to repair. Given that the SL ligament is exposed to considerable tension and torsion, it is not unusual for repairs to deteriorate with time. There is thus no guarantee that a repaired SL ligament will retain good functional strength and adequate stabilizing capability, even with early diagnosis and proper treatment. Importantly, the critical extrinsic and intrinsic ligament stabilizers may have been damaged at the time of injury, and while structurally present, may not be functionally competent.

More commonly, SLD is discovered in the subacute or chronic phase, when carpal malalignment is evident on plain radiographs. Even if not injured initially, abnormal SL kinematics induced by SLIL disruption may gradually attenuate the critical ligamentous stabilizers under load. Once carpal postural changes are evident on static films, the underlying pathology no longer involves a single structure but consists of a complex, multilevel ligament injury. Some ligament remnants are disrupted, whereas others are attenuated and/or insufficient to stabilize the joint. Furthermore, if substantial time has elapsed since the initial injury, cartilage degeneration may have set in, making an acceptable outcome from ligament repair less likely. ^{, ,}

For these reasons, treating the injury in the acute phase, when the healing potential is greatest, is generally more successful than trying to reverse chronic or degenerative injuries. It is important to carefully consider the patient’s age, occupation, recreational demands, and level of symptoms prior to embarking on a treatment plan. To facilitate patient selection and appropriate treatment, Garcia-Elias ^{, ,} developed an algorithm for the treatment of SLD (see Critical Points).

The answers to the Critical Points questions enable categorization of the SLD into seven stages ( Table 13.4 ). An increased number of negative answers indicates progression from a minor problem (stage 1) to a global dysfunction (stage 7). Treatment must be tailored to the specifics of each case, and importantly, to the patient’s needs. The following subsections describe all the stages in turn, including the most commonly used treatment for each.