Summary Subacromial impingement is the most common cause of shoulder pain which occurs as a result of compression of the rotator cuff muscles by superior structures (AC joint, acromion, CA ligament) leading to inflammation and development of bursitis. Diagnosis can be made on physical examination with a positive Neer and Hawkins tests, and can be supplemented with MRI studies. Treatment is a trial of nonoperative measures including NSAIDs, physical therapy and corticosteroid injections. Arthroscopic subacromial decompression with possible acromioplasty is indicated in patients who fail conservative measures. Epidemiology Incidence subacromial impingement is the most common cause of shoulder pain accounts for 44-65% of shoulder disorders Etiology Pathophysiology subacromial impingement is thought to be a combination of login to view 8 more bullets inflammatory process login to view 1 more bullet Subacromial impingement is the first stage of rotator cuff disease which is a continuum of disease from impingement and bursitis partial to full-thickness tear massive rotator cuff tears rotator cuff tear arthropathy Associated conditions hook-shaped acromion os acromiale posterior capsular contracture scapular dyskinesia tuberosity fracture malunion instability Anatomy Acromion 3 ossification centers unite to form the acromion login to view 3 more bullets failure of the ossification centers to fuse results in an os acromiale Classification Bigliani classification studies have shown classification system has poor inter observer reliability Bigliani classification of acromion morphology (based on a supraspinatus outlet view) Type I Flat Type II Curved Type III Hooked Presentation Symptoms pain login to view 4 more bullets Physical exam strength login to view 1 more bullet impingement tests (see complete physical exam of shoulder) login to view 15 more bullets Imaging Radiographs recommended views login to view 7 more bullets findings login to view 9 more bullets MRI useful in evaluating the degree of rotator cuff pathology subacromial and subdeltoid bursisits often seen CT arthography can also accurately image the rotator cuff tendons and muscle bellies Ultrasound can also accurately image the rotator cuff tendons and muscle bellies Studies Histology tendinopathy histology shows login to view 3 more bullets inflammation of the subacromial bursa login to view 1 more bullet Treatment Nonoperative physical therapy, oral anti-inflammatory medication, subacromial injections login to view 7 more bullets Operative subacromial decompression / acromioplasty login to view 6 more bullets Technique Subacromial decompression and acromioplasty acromioplasty login to view 7 more bullets treatment of an os acromiale login to view 3 more bullets Complications Deltoid dysfunction resulting from a failed deltoid repair following an open acromioplasty or an excessive acromionectomy during an arthroscopic procedure secondary to direct excision of an os acromiale Anterosuperior escape avoid acromioplasty and CA ligament release to preserve the coracoacromial arch in patients with massive, irreparable rotator cuff tears