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 extrinsic compression login to view 4 more bullets intrinsic degeneration login to view 2 more bullets inflammatory process inflammation of the subacromial bursa due to abutement between the humerus and rotator cuff, and acromion and associated ligaments 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 meta-acromion (base) meso-acromion (mid) pre-acromion (tip) 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 insidious onset exacerbated by overhead activities and lifting objects away from body night pain login to view 1 more bullet Physical exam strength usually normal impingement tests (see complete physical exam of shoulder) positive Neer impingement sign login to view 1 more bullet positive Neer impingement test login to view 1 more bullet positive Hawkins test login to view 1 more bullet Jobe test login to view 1 more bullet Painful Arc Test login to view 1 more bullet Yocum Test login to view 2 more bullets Internal Impingement test login to view 1 more bullet Imaging Radiographs recommended views true AP of the shoulder login to view 2 more bullets 30° caudal tilt view login to view 1 more bullet supraspinatus outlet view login to view 1 more bullet findings common radiographic findings associated with impingement login to view 8 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 disorganized collagen fibers mucoid degeneration inflammatory cells inflammation of the subacromial bursa high levels of metalloproteases and other inflammatory cytokines Treatment Nonoperative physical therapy, oral anti-inflammatory medication, subacromial injections indications login to view 1 more bullet techniques login to view 4 more bullets Operative subacromial decompression / acromioplasty indications login to view 1 more bullet outcomes login to view 3 more bullets Technique Subacromial decompression and acromioplasty acromioplasty two-step procedure performed open or arthroscopically login to view 6 more bullets treatment of an os acromiale a two-stage procedure may be required with the presence of an os acromiale to avoid deltoid dysfunction caused by direct excision login to view 2 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