Os acromiale is seen with some frequency. There is failure for part of the acromion to fuse with basi-acromion.
There are 3 spaces where the primary ossification centers may fail to fuse:
(1) between the os pre-acromiale and the acromion
(2) between the os meso-acromiale and the acromion
(3) between the os meta-acromiale and the acromion
Risk factors:
(1) race (more frequent in Blacks)
(2) familial
(3) physical exercise involving the shoulder, especially overhead (tennis player, swimmer, etc)
(4) blunt trauma
Clinical findings:
(1) shoulder pain localized to the anterolateral acromion, which may occur at night
(2) decreased shoulder motion
(3) decreased shoulder strength
(4) tender to palpation
(5) mobile piece of bone on palpation
The loose bone is best seen in an axillary view.
Differential diagnosis:
(1) fracture
