Type 4 acromion: a new classification.

The three types of acromial processes previously described in the literature include type 1, in which the undersurface is straight, and types 2 and 3, in which the undersurface is concave. In this report, a fourth type of acromion is described in which the undersurface is convex near its distal end. The frequency of a type 4 acromion in the normal population is discussed based on a retrospective review of the MR examinations of 30 patients that originally were read as normal.