It has not been determined whether acromial morphology is an innate an
atomic characteristic or whether it represents a degenerative process
with type I acromions changing to type III acromions over time. The pu
rpose of this study was to determine the incidence of the three types
of acromial morphology with respect to age. The acromial morphology of
272 patients (176 patients with no signs of mechanical impingement or
rotator cuff disease and 96 patients with mechanical impingement) was
determined by supraspinatus outlet radiographs. In the entire group o
f 272 patients there was a significant increase in the incidence of ty
pe III acromions and a decrease in the incidence of type I acromions i
n patients older than 50 years of age (p = 0.046). In 176 patients wit
h no symptoms there was a very significant increase in the number of t
ype III acromions in those patients older than 50 years of age (p = 0.
004) with a corresponding decrease in the incidence of type I acromion
s. The incidence of the three acromial types also varied between patie
nts with and without symptoms of mechanical impingement. The incidence
of the three acromial types varies with respect to the age of the pat
ient and whether he or she has symptoms of mechanical impingement. Thi
s raises the possibility that type I acromions may progress to type II
acromions and then further change into type III acromions over time.