Os Acromiale

The roof of the shoulder (the acromion) has different growth centers made of cartilage. During development, these growth centers grow, transform into bone, and fuse together to form one bone. Occasionally, one or more of these growth centers does not completely fuse with others. The end of the acromion which does not fuse, separated by a layer of cartilage and fibrous tissue from the remainder of the bony acromion and scapula, is a called an os acromiale. In most cases, an os acromiale does not move and does not cause symptoms. However, this os acromiale, the unfused portion of the bony acromion, may be unstable and move, especially after traumatic injury, which may pinch the rotator cuff tendon or bursa, cause symptoms of rotator cuff inflammation. Alternatively, this entity may not cause any pain or problems.