ANATOMY OF THE NORMAL ACROMION INVESTIGATED USING MRI

The shape of the acromion is strongly associated with impingement synd rome and with rotator cuff tears. It is notoriously difficult to image the acromion with conventional radiography. We have developed MRI tec hniques to depict the acromion in its longitudinal axis. Furthermore, we have measured the subacromial space in both external and internal r otation. In previous studies, three types of acromial shape have been described with the type III or hooked acromion being present in 66% of cases with rotator cuff tears. We studied 31 normal shoulders in 29 p eople using MRI. Within this population aged 24-36 years, mean age 31 years, no type III acromions were found. Twenty-one were type I (67.7% ) and 10 were type II (32.3%). In addition, we found no difference in subacromial height in external or internal rotation. Low rates of intr a- and interobserver error were found. These results imply that the ho oked acromion is not present in the normal population and is, therefor e, likely to be an acquired abnormality.