Shape of the acromion: congenital or acquired--a macroscopic, radiographic, and microscopic study of acromion.

Debate continues as to whether the differing shapes of the acromion are congenital or acquired. This has been investigated by neonatal cadaver study, adult cadaver study, radiographic study, magnetic resonance imaging study, or various other means. No one, to our knowledge, has investigated this by histologic study. A macroscopic and histologic study of 22 cadaveric shoulder joints was carried out to establish what, if any, developmental changes occur in the differing patterns of acromion. The cadaveric shoulders were dissected and examined macroscopically. All of the acromion processes were transected and photographed, and the histology of the anterior and inferior surfaces was studied. In all of the curved and hooked types of acromion, a common pattern of degeneration of collagen, fibrocartilage, and bone was observed, consistent with a traction phenomenon. None of the flat acromions exhibited these changes. Extensive histologic changes were noted on the anterior surface of acromion as compared to the inferior surface in curved or hooked acromion. We conclude that the different shapes of acromion are, therefore, acquired as a response to traction forces applied via the coracoacromial ligament and are not congenital in origin.