summary An Os Acromiale is an unfused secondary ossification center of the acromion that can be associated with shoulder impingement and rotator cuff tendonitis. Diagnosis is made with axillary lateral radiographs of the shoulder. Treatment is observation in the majority of cases with operative management reserved for patients who have persistent shoulder impingement or rotator cuff disease that have failed nonoperative management. Epidemiology Incidence common 8% bilateral in 60% Demographics more common in males more common in African American Anatomic location most common location is the junction of meso- and meta-acromion Etiology Associated conditions shoulder impingement rotator cuff disease meso-acromion is associated with rotator cuff tendonitis and full thickness tears (in 50%) Anatomy 3 ossification centers meta-acromion (base) origin of posterior portion of deltoid meso-acromion (mid) origin of middle deltoid pre-acromion (tip) origin of anterior deltoid fibers and coracoacromial ligament Blood supply acromiale branch of thoracoacromial artery Presentation History pain from impingement reduction in subacromial space from flexion of the anteiror fragment with deltoid contraction and arm elevation from motion at the nonunion site (painful synchondrosis) incidental finding on radiographs trauma can trigger onset of symptoms from previously asymptomatic os acromiale Imaging Radiographs recommended views best seen on an axillary lateral of the shoulder CT indications to better visualize the nonunion site to detect degenerative changes (cysts, sclerosis, hypertrophy) Treatment Nonoperative observation, NSAIDS, therapy, subacromial corticosteroid injections indications mild symptoms Operative two-stage fusion indications symptomatic os acromiale with impingement technique direct excision can lead to deltoid dysfunction a two-stage procedure may be required first stage - fuse the os acromiale ± bone graft second stage - perform acromioplasty preserve blood supply (acromiale branch of thoracoacromial artery) tension band wires, sutures, cannulated screws arthroscopic subacromial decompression and acromioplasty indications impingement with/without rotator cuff tear (where the os acromiale is only incidental and nontender) open or arthroscopic fragment excision indications symptomatic pre-acromion with small fragment salvage after failed ORIF results arthroscopic has less periosteal and deltoid detachment better excision results with pre-acromion Complications Deltoid weakness from fragment excision Persistent pain/weakness Prognosis Poorer outcomes after rotator cuff repairs in patients with meso-os acromiale