Black eyebrow sign is the description given on plain facial radiographs to intra-orbital air.
Air rises into the most superior aspect of the orbit, almost always in the context of a facial fracture, in a linear fashion, giving the appearance of an eyebrow. The fracture is usually an orbital blowout fracture, where air leaks from either the maxillary or ethmoidal sinus after a fracture.
The appearances are more easily observed on CT – the primary, more sensitive imaging modality in facial injuries, albeit sometimes only after plain radiographs have been undertaken.
The black eyebrow sign indicates orbital emphysema, typically from a orbital fracture, with air having entered from the adjacent sinuses.