Lateral view of the elbow shows elevated anterior and posterior fat pads giving leading to ‘sail sign’. There is an undisplaced transverse fracture of the radial neck.

In its normal position, the anterior fat pad is seen closely approximated to the anterior distal humerus lying within the radial and coronoid fossae. When joint effusion distends the joint capsule, the anterior fat pad is lifted up.

The posterior fat pad normally rests within the deeper olecranon fossa and as such is not normally visible on a lateral elbow x-ray. If the posterior fat pad visible or lifted up, it indicates an effusion in the joint.

Reference: Imaging of Elbow Fractures and Dislocations in Adults, www. emedicine.medscape.com