There are several X-ray findings to consider: the radius-capitate-lunate alignment, which if absent may indicate a scapholunate injury. Knowing the normal shapes of the scaphoid and lunate is important to help identify X-ray abnormalities that may suggest a scapholunate injury. The scaphoid on the anterior-posterior (AP) view is normally the shape of a boat. The lunate on lateral view appears like a half-moon while on the AP view it appears roughly square-shaped. A change in any of these shapes may signify a scapholunate injury. The signet ring sign of the scaphoid is a rounded appearance of the cortex of the scaphoid tubercle on the AP view of the wrist, suggesting a subluxation. The pizza sign or piece of sign is the triangular appearance of the lunate on the AP view, suggesting a lunate dislocation. A widening of the scapholunate space of 3 to 5 mm on the AP view is suggestive of a scapholunate dissociation. This has been termed the Terry Thomas Sign and the David Letterman Sign: The gap between these celebrities’ dental incisors appears wide like a widened scapholunate space on the AP view.¹⁰ A clenched fist view may be necessary to diagnose a significant scapholunate ligament sprain.¹¹ To pick up a subtle scapholunate dissociation and distinguish it from baseline physiologic widening of the scapholunate space, consider bilateral X-rays of the wrist, as some people have baseline physiologic widening.

SLAC Injury

One of the reasons that it is important for us to pick up scapholunate injuries in the ED is that some patients may progress to scapholunate advanced collapse (SLAC), a consequence of untreated scapholunate dissociation or complete rupture of the ligament. In SLAC the capitate collapses toward the radius, resulting in chronic arthritis and pain.

Occult Scaphoid Fracture

The occult scaphoid fracture is the most common occult carpal-bone fracture.¹² Like lunate ligamentous injuries, understanding age-related prevalence is important for scaphoid fractures, which occur more commonly in young adults. Anatomical snuffbox tenderness, scaphoid tubercle tenderness on the palmar aspect of the wrist, and pain on axial compression of the thumb should always be carried out on physical exam in young adults after a FOOSH. Sensitivity for each of these tests has been reported to be 100 percent, with a specificity of 74 percent when all are positive.¹³ Axial compression of the thumb has been shown to have the weakest diagnostic performance of the three tests, likely because many older individuals have arthritis in the first carpometacarpal joint, leading to a false-positive test.¹⁴ Two nuances in performing these physical exam tests are key: snuffbox tenderness should be performed with the patient’s wrist in ulnar deviation to bring the proximal scaphoid into the snuffbox, and palmar scaphoid tenderness should be elicited at the base of the thenar eminence with the wrist in neutral or radial deviation to bring out the scaphoid. Two additional tests for scaphoid fracture include pain on resisted supination of the wrist, which has been shown to have a 100 percent sensitivity, and the clamp sign, where the patient uses a pincer grasp around their scaphoid with their thumb in the snuffbox and index finger of the palmar scaphoid tubercle when asked where their point of maximal pain is.¹⁵ This test has a high positive likelihood ratio for scaphoid fracture.¹⁶