Os acromiale
The acromion is formed from multiple ossification centres with complete fusion between the ages of 22 and 25 years. If one of the ossification centres fails to fuse, an accessory ossicle is formed; the os acromiale. The os acromiale is mobile and is thought to contribute to impingement, but the reported incidence in normal population is between 1% and 15%. It is best assessed in the upper axial images Figure3)where a low signal space is detected between the high signal marrow of the distal acromion and the non-fused ossicle. On coronal oblique images it can be suspected by the presence of the so called “double AC joint”; the normal AC joint appears on the anterior image and the pseudo-articulation of the os acromiale appears on the posterior one.
Os acromiale. Axial T1-weighted image showing the os acromiale.
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