Os-acromiale is a loose / mobile acromion of the shoulder which may or may not cause pain. In the most cases the loose acromion may go undetected as it does not lead to any painful symptoms.
All bones have growth plates until a person stops growing (about 18 years of age). The growth plate is a part of the bone which consists of cartilage and its cells which cause the bone to lengthen during growth are situated in this non-solid part of the bone. The os-acromiale is nothing else but a persistent growth plate meaning that the growth plate did not fuse when growth ceased. It does not cause the bone to continue growing but remains as a strong fibrous union which can be disrupted with some force.
Os-acromiale occurs in a small percentage of the population and only a small part of those individuals have pain. It may become painful in time or sometimes due to excessive overhead sports or even an injury.