The acromion is the bone you can feel on top of the shoulder. It develops as four different parts know as ossification centres.
These centres are supposed to fuse when you stop growing. In some people this does not happen and a cartilage bridge between two bony parts remains.
Often there are no symptoms and the condition is only discovered by chance when an X-ray is done. In some cases, however, this does cause problems.
The os acromiale can cause impingement and a rotator cuff tear. One can also get arthritic changes at the unfused site.
Trauma can cause a previously non-problematic os acromiale to become symptomatic.
Treatment starts with pain killers and physiotherapy. If this does not help, the os acromiale can be surgically fused.