The pronator quadratus sign, also known as MacEwan sign, can be an indirect sign of distal forearm trauma. It relies on displacement of the fat pad that lies superficial to the pronator quadratus muscle as seen on a lateral wrist radiograph.
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Pathology
Displacement, anterior bowing, or obliteration of the fat plane in the setting of trauma may indicate a distal radius or ulna fracture. Various studies have described a high specificity but wide-ranging sensitivity for fracture ranging from 26-98%; thus a negative pronator quadratus sign does not exclude a fracture 1,2,4.
Radiographic features
Plain radiograph
On lateral wrist radiographs, the pronator fat pad normally appears as a thin radiolucent triangle, with its base attached to the palmar surface of the distal radius 1,2. It is observed ~90% of the time 1. One study has suggested <8 mm (female) and 9 mm (male) on lateral x-rays as a cut-off for normal 4.
History and etymology
It was first described by Douglas W MacEwan (fl. 2021) 6, a Canadian radiologist, in 1964 3,5.
Differential diagnosis
In the absence of trauma, there are other causes for a positive pronator quadratus sign:
muscle strain or haematoma
inflammatory conditions
infectious conditions, e.g. osteomyelitis, cellulitis
septic arthritis of the wrist