From the case:
Fractured neck of radius
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Fractured neck of radius with sail sign due to joint effusion. The posterior fat pad is also visible.
Case Discussion
Radial head/neck injuries are common, often resulting from a fall onto an outstretched hand. Sometimes if undisplaced they can be difficult to see. The presence of a visible posterior fat pad and an elevated anterior fat pad are indicative of joint effusion. Even if a fracture is not visible, if a joint effusion is present this should raise suspicion of a fracture. Follow-up imaging may be beneficial in these cases.