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.

The acromion is a projection from the scapula (shoulder blade) and is located on the top and front (antero-superior) of the shoulder. It can be felt as a bony tip on the top of the shoulder. The acromion is like a “roof” over the rotator cuff. The acromion serves as the attachment of the deltoid muscle. The deltoid muscle extends from the acromion to the upper humerus and is a main elevator of the arm.

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.

On x-rays the mobile unfused bone can usually clearly be seen.

The pain is due to either motion of the bony fragment or due to the fact that this part of the bone then “impinges” on the underlying rotator cuff tendon.

It is therefore one of the causes of “impingement” of the rotator cuff. The doctor examining the patient would decide that this was impingement and then would have to make sure if the pain was from the bone itself or from the mobile segment impinging on the underlying tendon. This can usually be done by injecting local anaesthetic into the subacromial space (between the tendon and acromion (see injections) or under ultrasound control the local anaesthesia can be injected into the defect itself which would then lead to immediate pain relief proving the origin of the pain.