Perilunate Injury Imaging 

Preferred examination

The preferred method of imaging of perilunate injuries is conventional radiography with posteroanterior (PA) and lateral radiographs, but additional views may be necessary to appreciate subtle carpal fractures.

Bone scintigraphy has high sensitivity for bone injury, but its specificity is low, as soft tissue injuries and other osseous inflammatory conditions can show enhanced isotope uptake. Bone scintigraphy has an important role in evaluating occult carpal fractures, but this is generally not indicated for assessing perilunate injuries after diagnosis with conventional radiography.^{[1, 2, 3, 4, 5]}

Adequately exposed conventional radiographs should not have a false-negative rate in the diagnosis of perilunate dislocations. However, subtle associated carpal fractures can be missed.

Perilunate injuries are displayed in the radiographic images below.

CT and MRI have been used with increasing frequency in both initial and follow-up evaluations of carpal fractures and its complications.^[5] MRI is particularly useful when neurologic complications occur and in the assessment of tendon and capsular injuries. MRI is also useful in assessing ligamentous stability and is an invaluable tool in depicting bone bruises rather than fracture as the source of pain. It has been used in the evaluation of complications, particularly osteonecrosis.

CT is excellent in the initial evaluation of perilunate dislocations and associated fractures, particularly in athletes in whom initial radiographic findings are normal in terms of associated fractures. Also, CT can demonstrate healing, which is sometimes misleadingly shown on radiographs, particularly with hardware in place.^[5]

MRI can supplement or be used instead of CT when radiographs are negative. MRI typically is not useful in the evaluation of healing. Depictions of osteonecrosis with MRI should be interpreted with caution because some ischemia is expected in the proximal pole of the scaphoid after fractures of the waist and proximal pole occur.

Overview

The proximal row of carpal bones is tightly bound or held to the radius by the radiocarpal ligaments, particularly the radiolunate, radioscaphoid, and radiotriquetral, on the volar aspect of the wrist. A potential space between the volar radiotriquetral and radiocapitate components of the volar ligaments is called the space of Poirier; this area accentuates the stresses at the midcarpal joint, resulting in dislocation between the capitate and the lunate.^[6]

In the normal wrist (see the image below), the lunate bone is part of the proximal row. This bone articulates with the scaphoid (navicular) laterally, the triquetral bone (triangular) medially, and the capitate and the hamate distally. The scapholunate, lunotriquetral, and lunocapitate ligaments maintain stability of the aforementioned articulations.

The sites of fracture that commonly accompany perilunate dislocation are determined by the relationship of the carpal bones to the position and attachment of the principal volar carpal alignments.^{[7, 8]}

Perilunate injuries of the wrist describes the radius-to-ulnar disruption injuries through the scaphoid and capitate bones and ligaments at some distance from the lunate (see the images below).^[9] These injuries usually occur along the greater arc of the carpus, which is projected across the midscaphoid, capitate head or neck, and lunate articulation of the triquetrum with or without fracture of the proximal pole of the hamate.

The perilunate is an uncommon type of carpal dislocation and is commonly associated with carpal and other wrist fractures. Each carpal bone involved is included in the designation of a fracture-dislocation. The direction of displacement precedes the designation of fractures, which in turn precedes the term dislocation.^{[10, 11, 12, 13, 14, 15, 16]} Therefore, pure perilunate dislocation associated with a fracture of the scaphoid and capitate is designated as a volar or dorsal transscaphoid, transcapitate perilunate fracture-dislocation.

Carpal injuries can therefore be classified into perilunate fracture-dislocations and perilunate dislocations. These injuries are often missed at initial presentation, and knowledge of the clinicopathologic features is essential for accurate diagnosis. Perilunate injuries are severe disruptions of the carpus and present formidable challenges to the treating physician. Accurate recognition of the pattern of injury is not always straightforward. The injury can propagate through ligaments and/or bone, creating multiple variations of a basic injury pattern.^[17]

Types of injuries

Approximately 95% of perilunate dislocations are dorsal because the most common mechanism of injury is acute dorsiflexion of the wrist. Most injuries are the result of a fall on an outstretched hand or a blow to the palm or hand that results in acute dorsiflexion and ulnar deviation of the hand and wrist. As the injury occurs, a torsional force is created by pronation of the forearm and supination of the hand and carpus. These forces are focused at the midcarpal joint and often referred to as intercarpal supination.

Less commonly, a blow to the back of the wrist or a fall on the dorsum of the wrist results in acute volar flexion of the hand and wrist and volar displacement of the distal row of carpal bones. Therefore, these injuries are designated volar perilunate dislocations.

Presentation

Most patients present acutely, with wrist pain and swelling after a fall from a height onto a hyperextended hand. Patients with perilunate dislocations and perilunate fracture-dislocations may have an obvious clinical deformity, with marked restriction of movement, particularly flexion and extension. Alternatively, they may present with an innocuous presentation of a sprained wrist.

Once the diagnosis is established, early intervention is necessary for optimal results. Initial closed reduction with sedation and traction are performed to restore overall carpal alignment. However, subsequent closed or open reduction is necessary to restore anatomic alignment of the injuries. The outcome of perilunate injuries is correlated with adequacy of reduction. Complications such as chondrolysis, carpal instability, and traumatic arthritis can occur despite satisfactory treatment.

A careful and thorough trauma survey with an assessment for associated injuries of the head, thorax, and extremities is imperative because of the high-energy nature of perilunate fracture-dislocations. Damage to the median nerve is the most commonly associated injury in lunate and perilunate dislocations of the wrist. In certain situations, volar skin lacerations can represent an open dislocation or fracture-dislocation.

Symptoms from associated injuries of such fractures of the distal radius and ulna may dominate. Additionally, the volar skin can become ischemic because of pressure from the volar radius occurring as a result of a dorsally dislocated hand. With long-standing perilunate dislocations, patients may present with arterial compromise or established compartment syndromes.

In the dorsal perilunate dislocation or one of its transosseous variants, the carpus is dislocated dorsally, and the radius is prominent volarly. In pure lunate dislocation, the lunate alone is prominent volarly. The injuries are diagnosed late in up to 25% of cases.

Conventional radiography is the most important imaging modality for the assessment of wrist injuries (see the images below). Significant wrist injuries, such as perilunate and scapholunate dissociations, may occur without carpal bone fractures.^[18]

Meticulous care should be taken to obtain good-quality radiographs. The radiologist should recognize these ligamentous wrist injuries by noting abnormalities of the shapes, joint spaces, and alignments of the carpal bones. It is important that the radiologist be familiar with detailed wrist anatomy so as not to miss these injuries. Early diagnosis allows for prompt referral and optimal outcomes.

Perilunate dislocations are frequently missed on initial radiographs. Reasons for overlooking this injury are inadequate posteroanterior (PA) and lateral and oblique imaging. Other reasons for a false-negative diagnosis are unfamiliarity with the anatomy of the carpus and obscuration by splints. Therefore, when trauma to the carpus is suspected, radiographs should be obtained without splints and other bandages.

When lateral radiographs are obtained, excessive ulnar deviation of the wrist should be avoided, as this gives the erroneous impression that the lunate is in the extended position. With the lunate in the extended position, dorsal intercalated instability can be incorrectly diagnosed.

Sochart and associates described 5 cases of perilunate fracture-dislocation in which the radiological appearances were typical but the diagnoses were initially missed.^[19]

Smith and Murray reported 5 examples of avulsion fractures of the volar aspect of triquetral bone, which they described as subtle sign of carpal ligament injury. These fractures from the volar aspect of the triquetral bone are important because they are easily missed with conventional wrist radiographs; yet, these are associated with significant ligament injuries and carpal instability. The authors recommend that the patients should undergo further evaluation for associated ligament injury and carpal instability when this fracture is identified.^[20]

Posteroanterior radiographs

Standard PA and lateral radiographs should initially be obtained to assess carpal injury, with oblique radiographs centered over the carpus. The PA radiograph is obtained with the patient seated, the shoulder abducted 90°, and the hands palms down on the x-ray plate. A PA distraction view of the carpus can be helpful in the acute setting to better define the injury anatomy (eg, small fractures or dislocations of the carpal bones).

Gilula described a series of lines that can be traced along the proximal margins of the scaphoid, lunate, and triquetrum, as well as the proximal poles of the capitate and the hamate. These lines should be smooth and uninterrupted, they can be quickly mapped, and they should provide a reference in the assessment of perilunate dislocation or perilunate fracture-dislocation.^[21]

Lateral view

A lateral radiograph should also be obtained with the patient sitting. The patient's shoulder is adducted with the hand and wrist at the side, enabling the ulnar border of the hand to be placed on the x-ray plate. An accurate lateral radiograph of the wrist should be superimposed on the lunate, proximal scaphoid pole, and triquetrum.

The lateral radiograph should be inspected for the wide carpus sign, where the capitate is overriding the dorsal aspect of the lunate in the dorsal perilunate dislocation. In the volar lunate dislocation, the lunate is clearly volar to the radius. Although perilunate dislocations are rarer, they may occur volarly, with the lunate dislocation present dorsally.

In addition, the lateral radiograph can be used for measurement of the lateral scapholunate angle and the capitolunate angle. The lateral scapholunate angle is formed by the intersection of the longitudinal axes of the lunate and the scaphoid. Normally, this angle is 30-60°. The capitolunate angle normally is 0-15°. These angles are disrupted in scapholunate dislocations.

General findings

Scapholunate dissociation, the most common form of wrist instability, is suggested by the Terry Thomas or David Letterman sign, in which the scapholunate distance is wider than 2 mm.^[22] Palmar tilting of the scaphoid, which appears foreshortened, indicates disruption of the scapholunate ligament.

The protection imparted by the radial articular surface causes perilunate dislocations to occur 3 times more often than lunate dislocations. Radiographs demonstrate normal radiolunate alignment with dorsal dislocation of the other carpal bones. Commonly associated with scaphoid fracture, this injury is then called a transscaphoid perilunate fracture-dislocation. The nature of the dislocation is more apparent on the lateral radiograph of the wrist. Normally, the capitate is aligned with the lunate, which is aligned with the radius. A perilunate dislocation is manifested by disruption of the capitolunate joint. In a dorsal perilunate dislocation, the carpal bone lies dorsal to the lunate bone, and in volar perilunate dislocation, the capitate lies volar to the lunate.

Lunate dislocation occurs least commonly. In lunate dislocation, the proximal articular margin and central axis of the capitate remain aligned with the distal articular surface and axis of the distal radius. The lunate is usually displaced volarly, disrupting both the capitolunate and lunatoradial joints. Both the distal and proximal sets of articular arcs are disrupted.

The carpus typically is dislocated volarly and remains in the normal position, so that the capitate appears to be aligned with the distal portion of the radius. The proximal and distal rows of the carpus are overlapped, giving rise to the crowded-carpus appearance.

An associated fracture of the waist of the scaphoid is the most common fracture. Fractures involving the radial styloid and capitate are next in frequency, and associated fractures of the triquetrum, hamate, and ulnar styloid are least common.

Generally, CT sections of 1-2 mm in thickness suffice. Sections 1 mm thick provide data for excellent reformatted image reconstructions. Reformatted images along the oblique sagittal plane through the long axis of the scaphoid may be the preferred plane of orientation.^[5]

Masmejean and associates reported an uncommon palmar translunate, transhamate carpal fracture-dislocation. CT scans with 3-dimensional reconstruction were instrumental in the assessment of the injury, which was treated operatively through a palmar approach. The lunate and hamate fractures were fixed by using mini-screws, and the radial styloid fracture and the scaphoid were reduced and stabilized with K-wires.^[23]

CT scanning or conventional tomography is not usually needed to diagnose perilunate dislocations. However, CT defines the anatomy of greater arc fractures (eg, scaphoid fractures, capitate fractures, radial styloid fractures, triquetral fractures) better than other methods. Generally, 1-mm sections in the sagittal and coronal planes of the capitate are helpful.

CT permits accurate anatomic assessment of carpal fractures. Bone contusions are not evaluated with CT, but true fractures can be excluded. CT allows volumetric analysis for determining the size of graft needed to correct angular deformities.

Pseudofractures are a plain radiographic concept and are not depicted on CT scans. However, an entering vessel may occasionally cause an apparent breach in the superficial cortex and thus mimic a fracture. This depiction usually poses no great problem, and the breach can be distinguished on contiguous images. As the vessel enters the bone, the walls have a thin, sclerotic rim not found about a fracture line.

When the mechanism of injury is considered, optimal display of the volar and dorsal cortices is preferred. Incomplete fractures can be missed on oblique coronal images. Axial imaging with reformatted images can be obtained, provided the reformatted images are in the planes of the scaphoid and not in the anatomic planes. There is some loss of edge detail and some blurring inherent to spiral techniques, although the images provided are generally adequate.

Persistent pain in the wrist after trauma may be the result of osseous or soft tissue injury, which may be radiologically occult. MRI is useful in the detection of occult fractures, bone bruises, nerve and ligamentous injuries, and carpal instability.^[24]

The characteristic finding of a fracture on an MRI is a linear focus of decreased signal intensity on T1-weighted images and increased signal on T2-weighted images. The fracture line may be more difficult to see on T2-weighted images. Short-tau inversion recovery (STIR) or fat-suppressed T2-weighted sequences are sensitive to bone marrow edema. Although they are more sensitive for edema than the T1-weighted images, STIR T2-weignted MRIs may cause fractures to be overdiagnosed.

A localized or diffuse region of decreased signal intensity without a discrete fracture line is consistent with the microtrauma associated with impaction of bone trabeculae of bone bruises and contusions. Ligamentous injuries and avascular necrosis (AVN) are well shown on MRI, although no large series of these findings has been reported.

MRI can lead to the overdiagnosis of carpal fractures because bone marrow edema due to bone bruises or contusions may be diagnosed as fractures. However the sensitivity of MRI is high for the detection of fractures. MRI is the only modality that noninvasively depicts capsular and ligamentous injuries.

In a study by Imaedo et al, an oblique image obtained through the long axis of the scaphoid allowed visualization of 11 of 11 fracture lines. The fracture line was visible in 10 of 11 fractures in the coronal plane. High signal intensity, seen in the distal fragment on T2-weighted images, was characteristic of recent fractures.^[23]