Observations on the undersurface of the skeletalized human acromion in two populations

In late life, the humeral impingement disorder becomes common due to degenerative changes in the rotator cuff which allow the proximal humerus to press against the acromion in elevation of the arm, so producing facets on the tip region of the subacromial surface and on the lateral margin of the greater tubercle of the humerus. To discover what changes occur prior to those described, 184 acromions below the age of 55 years were examined. During the 18–25 year period, the subacromial surface showed patterns of formation of surface trabeculae of new bone, some associated with union of the epiphysis or with remodelling as a response to the presence of the proximal humerus at a distance of 10–15 mm; others indicated a subacromial attachment of the coraco-acromial ligament. As early as 30 years of age, flattening of the surface trabeculae of bone was noted in association with compression lipping of the outer margin of the greater tubercle of the humerus, the part that crosses closest to the subacromial surface during elevation of the arm. Although resembling changes found in the impingement disorder, in both populations by the age of 35 years the changes were too common to be so and were not associated with the erosive pitting of the upper surface of the tubercle regarded as the earliest bony criterion of degenerative change in the cuff. It is postulated that, in the normal shoulder joint, in raising the arm, pressure is exerted, perhaps transiently, in the subacromial bursa and adjacent soft tissues which induces the compressive changes described. Probably due to the physically arduous lifestyle of the islanders, these changes occurred earlier among them; so did (but later) cuff degeneration indicated by erosion of the humeral tubercle. Copyright © 1999 John Wiley & Sons, Ltd.