Three acromiai ossification centers, a pre-acromion, a meso-acromion and a meta-acromion appear in the 15 to 18-year-old person and fuse with the base of the acromion at about 22 to 25 years. When there is failure of union of any one of these centers to its neighbor, the resulting separate bone is called os acromiale.
We could only find one report in Japan about the incidence of os acromiale although there are several reports from other countries. Kitamura in 1986 reported the incidence of os acromiale in Japan as 2.9%. We investigated the incidrnce and appearance of os acromiale by reviewing axial roentgenograms of 341 shoulder girdles.
We found 6 cases of os acromiale with an incidence of 1.8%. According to Folliasson's classification, 5 cases took the form of separation between the meso-acromion and meta-acromion that is type A. One case, type c, had two separations between the pre-acromion and meso-acromion and between the meso-acromion and meta-acromion.