Workup
Laboratory Studies
Laboratory studies typically are not necessary in the evaluation of wrist dislocations if the history includes acute injury.
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Imaging Studies
Plain radiographs are helpful.
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Obtain posteroanterior (PA) and lateral radiographs in all patients who present with a history of acute wrist trauma.
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PA and lateral radiographs should also be obtained with 10- to 15-lb traction on the upper arm.
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The normal PA view should show 2 rows of carpal bones in a normal anatomic position with uniform joint spaces of no more than 1-2 mm. No overlap should be seen between the carpal bones or between the distal ulna and the radius.
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On a normal lateral radiograph, the 4 C s should be easily visualized. The 4 C s are the convexity of the distal radius, the convexity and the concavity of the lunate, and the convexity of the capitate (see the image below).
On a normal lateral radiograph, the 4 Cs should be easily visualized. The 4 Cs are the convexity of the distal radius, the convexity and the concavity of the lunate, and the convexity of the capitate. A longitudinal axis aligns the radius, the lunate, the capitate, and the third metacarpal bone. The scapholunate angle is normally 30-60 degrees.
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A longitudinal axis aligns the radius, the lunate, the capitate, and the third metacarpal bone. The scapholunate angle is normally 30-60° as depicted below. [5]
On a normal lateral radiograph, the 4 Cs should be easily visualized. The 4 Cs are the convexity of the distal radius, the convexity and the concavity of the lunate, and the convexity of the capitate. A longitudinal axis aligns the radius, the lunate, the capitate, and the third metacarpal bone. The scapholunate angle is normally 30-60 degrees.
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One order of obtaining radiographs is as follows:
- Anteroposterior (AP) and/or PA views, with the lateral view as a screening test
- Navicular series
- Possibly, a clenched-fist view with radial and ulnar deviation. (This forces the capitate head into the scapholunate joint and widens it if laxity is present.)
- Possibly, traction views
- Possibly, comparison views, especially in patients with nonfused growth plates
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Radiographic findings for the various types of dislocation are as follows:
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Lunate dislocation: On the usual PA image, the lunate has a trapezoidal shape that changes with flexion and extension. In this type of dislocation, the lunate is displaced volarly and rotated with the capitate. The rest of the carpal bones are in a normal anatomic position in relation to the radius. On the lateral radiograph, the lunate has the classic "spilled-teacup" sign from the disruption of the 4 C s. On the PA image, the lunate has a triangular or pie shape as shown below.
Lunate dislocation. Posteroanterior projection of the wrist showing the pie shape of the lunate.
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Perilunate dislocation: The lunate is in a normal anatomic position with respect to the radius, and the rest of the carpal bones are displaced dorsally. On the PA radiograph, crowding is evident between the proximal and distal carpal bones as depicted in the images below.
Perilunate dislocation. On the posteroanterior radiograph, crowding is evident between the proximal and distal carpal bones.
Perilunate dislocation. The lunate is in a normal anatomic position with respect to the radius. The rest of the carpal bones are displaced dorsally.
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Scapholunate dislocation (rotary subluxation of the scaphoid)
- On a PA radiograph, the scapholunate space is usually greater than 4 mm, a scenario also known as the Terry-Thomas sign, named after the British comic with frontal dental diastema. [13]
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On the clenched-fist and PA views with the wrist in ulnar deviation, the scapholunate gap is increased. The scaphoid rotates to a more transverse position when the ligaments between the lunate and scaphoid are interrupted, increasing the scapholunate angle to greater than 60°.
[2] This rotation causes the scaphoid to be viewed end-on, producing the classic signet-ring sign (see the image below).
Scapholunate dislocation. The scapholunate space is usually greater than 4 mm, a scenario also known as the Terry-Thomas sign. Rotation of the scaphoid causes the scaphoid to be viewed end-on, producing the classic signet-ring sign.
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Lunate dislocation: On the usual PA image, the lunate has a trapezoidal shape that changes with flexion and extension. In this type of dislocation, the lunate is displaced volarly and rotated with the capitate. The rest of the carpal bones are in a normal anatomic position in relation to the radius. On the lateral radiograph, the lunate has the classic "spilled-teacup" sign from the disruption of the 4 C s. On the PA image, the lunate has a triangular or pie shape as shown below.
Magnetic resonance imaging (MRI) can be considered for patients with wrist pain or instability but who have normal radiographic findings. MRI may be less important in patients with a ligament injury; in these patients, arthrography may be considered.
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Lunate dislocation. Posteroanterior projection of the wrist showing the pie shape of the lunate.
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Perilunate dislocation. On the posteroanterior radiograph, crowding is evident between the proximal and distal carpal bones.
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Perilunate dislocation. The lunate is in a normal anatomic position with respect to the radius. The rest of the carpal bones are displaced dorsally.
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Scapholunate dislocation. The scapholunate space is usually greater than 4 mm, a scenario also known as the Terry-Thomas sign. Rotation of the scaphoid causes the scaphoid to be viewed end-on, producing the classic signet-ring sign.
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Progressive perilunar instability pattern as reported by Mayfield et al. Stage I involves scaphoid instability; stage II, scaphoid and capitate instability; stage III, scaphoid, capitate, and triquetrum instability; and stage IV, lunate dislocation.
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On a normal lateral radiograph, the 4 Cs should be easily visualized. The 4 Cs are the convexity of the distal radius, the convexity and the concavity of the lunate, and the convexity of the capitate. A longitudinal axis aligns the radius, the lunate, the capitate, and the third metacarpal bone. The scapholunate angle is normally 30-60 degrees.
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