Glenohumeral Lipohaemarthrosis

This AP xray of the left shoulder shows a transverse fracture of the surgical neck of the humerus with impaction of fragments. There is inferior displacement of the humeral head on the glenoid. This appearance (pseudosubluxation) is commonly seen in association with fracture and suggests joint effusion. There is a fat-fluid level (click image for arrows) consistent with a lipohaemarthrosis. This indicates the presence of intraarticular injury.
Lipohaemarthrosis seen on plain radiographs is unusual in the glenohumeral joint, although in one series it was seen in 25% of cases. It was first reported in 1962 (compare with 1929 for knee lipohaemarthrosis). It is only seen on erect views. Lipohaemarthroses have also been described in elbow and hip joints.
Note that a fluid-fluid level may be seen in a simple haemarthrosis on MRI. The upper fluid must show magnetic characteristics of fat (eg drop signal on fat-sat sequences) before you can call it a lipohaemarthrosis.
References:
1. Sorenson SM, et al. The FBI Sign: CT, MRI, and Radiographic Appearance of Lipohaemarthrosis. www.gentili.net
2. Arger PH, Oberkircher PE and Miller WT. Lipohemarthrosis [of the shoulder]. AJR 1974 121(1):97-100
Credit: Dr Laughlin Dawes
http://www.radpod.org