The lunate is located in the middle of the wrist, so it is not surprising that the majority of ligamentous injuries are centered on the lunate. Injuries usually result from forceful dorsiflexion of the wrist, most often from a fall on an outstretched hand. The various injuries occur sequentially depending on the degree of force and range from isolated tears to perilunate and lunate dislocations.18,19
Scapholunate Ligament Instability
The scapholunate ligament is the intrinsic ligament that binds the scaphoid and lunate. Because the scaphoid bridges the proximal and distal carpal rows, it is not surprising that the scapholunate ligament has a marked propensity for injury and is the most commonly injured ligament of the wrist. Injury most often is from a fall on an outstretched hand with impact on the thenar eminence.18 Patients complain of pain and swelling on the radial side of the wrist and often a "clicking" sensation with wrist movement. Examination reveals localized tenderness on the dorsum of the wrist in the area immediately distal to Lister's tubercle. Ballottement of the scaphoid may also produce pain in this area.16
This injury is often referred to by the various radiographic appearances it may take. There are three different radiographic signs that may occur separately or in combination with one another (Figure 5A). Scapholunate dissociation is a widening of the scapholunate joint space of >3 mm on the posteroanterior view. If it is not apparent on routine views, a grip compression view or motion study may be necessary to demonstrate the abnormal gap (Figure 5B).19 These maneuvers are particularly helpful in identifying an incomplete tear of the ligament. Rotary subluxation of the scaphoid is another radiographic finding that often accompanies scapholunate dissociation. A torn scapholunate ligament can cause the scaphoid to tilt more palmar and increase the scapholunate angle to >60 degrees on the lateral view. On the posteroanterior view, the scaphoid tilts toward the observer so that it appears shorter as it is viewed more on its end. This causes the circular cortex of the bone to become more prominent and appear as a ring, known as the "cortical ring sign" (Figure 5A). A third radiographic abnormality is a carpal instability pattern known as dorsal intercalated segment instability (Figure 4B). The normal flexed posture of the scaphoid produces a flexion torque on the lunate that is counterbalanced by an extension torque from the triquetrum. When the scapholunate ligament is torn, this balance is disrupted. The lunate tilts dorsal from the unopposed extension torque from the triquetrum, whereas the scaphoid tilts more palmar (rotary subluxation of the scaphoid) because it has lost support from the lunate. The dorsal tilt of the lunate also causes a slight flexion tilt of the capitate. In the lateral view, the normal collinear arrangement of the axes of the capitate, lunate, and radius are replaced by a characteristic zigzag pattern. Both the scapholunate and capitolunate angles are increased. The concept of the proximal carpal row being the middle link or "intercalated segment" in this system, combined with the lunate's pathologic dorsal tilt and zigzag pattern (Figure 5C), is how this abnormality came to be named dorsal intercalated segment instability.
A. Scapholunate dissociation and rotary subluxation of the scaphoid. The scaphoid and lunate are separated by a gap of >3 mm (black arrow), and the scaphoid appears shorter from rotation with a dense ring, the "cortical ring sign" (white arrow). B. Grip compression view showing enhancement of scapholunate dissociation (arrow). C. Dorsal intercalated segment instability. Lateral view exhibiting dorsal intercalated instability with scapholunate dissociation.
Refer to an orthopedist or hand surgeon. ED treatment is with a radial gutter splint or short arm volar posterior mold (see videos "Radial Gutter Splint" and "Short Arm Posterior Mold"). Orthopedic referral is necessary because these injuries require either closed reduction with percutaneous pinning or open reduction and internal repair of the ligament.18 Dorsal intercalated segment instability and subsequent early, severe degenerative arthritis can occur if left untreated.15
Triquetrolunate Ligament Instability
The triquetrolunate ligament binds the triquetrum and lunate on the ulnar aspect of the wrist. Injury to this ligament is the ulnar equivalent of the scapholunate ligament injury. Triquetrolunate ligament injury occurs much less often than scapholunate ligament injury, is more stable, and can be confused with other causes of ulnar-sided wrist pain such as triangular fibrocartilage complex injury or distal radioulnar joint abnormality.15,16 This injury most often results from falls on the outstretched, dorsiflexed hand with impact on the hypothenar eminence. There will be localized tenderness on the ulnar aspect of the wrist just distal to the ulna. Ballottement of the triquetrum may produce a painful clicking sensation.
Subtle injuries may have a normal radiographic appearance.15 Complete disruption of the triquetrolunate ligament removes the ability of the triquetrum to counterbalance the flexion torque from the palmar-flexed scaphoid. The lunate then tilts palmar, and the capitate extends slightly in response. A zigzag pattern in the opposite direction of the scapholunate injury is produced. The capitolunate angle is increased >10 to 20 degrees; however, the scapholunate angle is unaffected because the scapholunate ligament is still intact. The lateral radiograph may reveal the "volar intercalated segment instability" pattern (Figure 4C and Figure 6). The posteroanterior view may reveal a widening of the triquetrolunate joint space and obliteration of the capitolunate joint space and the normal smooth arcs typically seen because of the volar tilt of the lunate.
A. Volar intercalated instability. Note widened capitolunate angle. B. Dorsal intercalated instability.
Refer to an orthopedist or hand surgeon. ED treatment is an ulnar gutter splint or short arm posterior mold (see videos "Ulnar Gutter Splint" and "Short Arm Posterior Mold") and referral to an orthopedist. Immobilization in a cast for 6 to 8 weeks, followed by a protective splint, is sufficient in most cases. Open reduction and internal fixation are generally reserved for chronic injuries.15 Unrecognized injuries can cause early degenerative arthritis and chronic wrist pain.
Perilunate and Lunate Dislocations
Perilunate and lunate dislocations represent the final stages of midcarpal ligament disruption and are thought to account for 10% of all carpal injuries.20 These injuries are the result of forceful dorsiflexion and impact on the outstretched hand, but usually with great force, such as a fall from height, impact from a motor vehicle collision, or a sporting event.21
Perilunate dislocation is the posterior dislocation of carpal bones while the lunate maintains its position with respect to the distal radius. This is a very rare dislocation. Lunate dislocation produces posterior dislocation of carpal bones with the concavity of the lunate facing anteriorly.
The injury can begin on either side of the lunate, but typically begins on the radial aspect, with either a tear of the scapholunate ligament or a fracture of the scaphoid. Injury progresses around the lunate in a semicircular fashion, tearing the volar ligament arcade at the radiocapitate ligament. Remember that the extrinsic ligaments form two strong volar arcades with an inherently weak area between them that widens with dorsiflexion of the wrist (Figure 2). The space of Poirier lies at the junction of the lunate and capitate. This space opens further as heavy loading disrupts the lunatotriquetral ligament. Besides ligament disruption, any number of carpal bones may fracture along an arc around the lunate (Figure 7). If sufficient force is applied, the ligaments and carpal bones around the lunate are stripped away. The capitate is displaced dorsal to the lunate, producing a perilunate dislocation. If the capitate rebounds with sufficient force, it can push the lunate off the radius and into the palm, creating a lunate dislocation. These injuries are all part of a continuous spectrum of ligament disruption (Figure 7).22
Four stages of perilunate instability. The first stage (I) is disruption of the scapholunate articulation (scapholunate dissociation). The second (II) and third (III) stages are separation of the capitolunate and triquetrolunate joints (perilunate dislocation). The fourth (IV) stage is a lunate dislocation.
On clinical examination, there is generalized swelling, pain, and tenderness of the wrist. However, a gross deformity, typical of many joint dislocations, is often absent. Radiographic interpretation is the key to diagnosis. The perilunate dislocation is best appreciated on the lateral view. The linear arrangement of the three C's sign is disrupted with the capitate, represented by the third C, displaced dorsal to the lunate. The lunate retains its contact with the radius. The scapholunate and capitolunate angles are increased. On the posteroanterior view, the three smooth arcs are disrupted, and the capitolunate joint space is obliterated as the bones overlap one another. The scapholunate and triquetrolunate joint space may either be increased because of torn ligaments or obliterated by rotation of the fractured carpal fragments. The scaphoid will appear shortened from rotary subluxation or fracture (Figure 8). A perilunate dislocation may also overshadow any associated carpal bone fracture. The scaphoid and capitate are most often involved, so carefully inspect these bones for fractures. Such fractures are designated by adding the prefix "trans-" to the carpal bone name (e.g., transscaphoid perilunate dislocation) (Figure 9).
Perilunate dislocation. A. Posteroanterior view shows obliteration of the three smooth arcs as bones overlap one another (white hash marks). B. Lateral view shows capitate dorsal to lunate, disrupting the "three C's" (arrow).
A and B. Transscaphoid perilunate dislocation. [Photos contributed by: Brooke Beckett, MD, Department of Radiology, Oregon Health & Science University, Portland, OR.]
A lunate dislocation has many similar and several distinct radiographic features when compared with a perilunate dislocation. On the posteroanterior view, the lunate has a triangular shape ("piece-of-pie" sign) that is suggestive of lunate dislocation (Figure 10A). On the lateral view, it also disrupts the three C's sign. The lunate (represented by the middle C) is pushed off the radius into the palm. This has been called the "spilled teacup" sign because it resembles a cup spilling in the direction of the palm (Figure 10B). The capitate may rebound back and even rest on the radius. The signs of ligament disruption and the associated carpal bone fractures described with perilunate injuries may also be present.
Lunate dislocation. A. Posteroanterior view demonstrates pathognomonic triangular shape of the lunate (piece-of-pie sign; circle). B. Lateral view exhibits the lunate tilting into the palm (spilled teacup sign; circle) and the capitate positioned dorsal to the lunate (arrow).
Perilunate or lunate dislocations require emergency orthopedic/hand consultation.22 Treatment is determined by the extent of the injury. Closed reduction and long arm splint immobilization (see video "Long Arm Posterior Splint") is appropriate for reducible dislocations.23 Open, unstable, and irreducible dislocations require open reduction and internal fixation, with repair of the ligaments and fractures.
Some orthopedists operate on all perilunate and lunate dislocations.15The complications include development of carpal instability patterns that lead to early degenerative arthritis, delayed union, malunion, nonunion, avascular necrosis, and, occasionally, median nerve compression from the volar dislocation of the lunate into the carpal tunnel.21