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 of the rotator cuff between the humeral head and anterior acromion coracoacromial ligaments acromioclavicular joint intrinsic degeneration supraspinatus attrition of the supraspinatus leads to inability to balance the humeral head on the glenoid causing superior migration and narrowing of the subacromial space 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 poor indicator of successful nonoperative management Physical exam strength usually normal impingement tests (see complete physical exam of shoulder) positive Neer impingement sign positive if passive forward flexion >90° causes pain positive Neer impingement test if a subacromial injection relieves pain associated with passive forward flexion >90° positive Hawkins test positive if internal rotation and passive forward flexion to 90° causes pain Jobe test pain with resisted pronation and forward flexion to 90° indicates supraspinatus pathology Painful Arc Test pain with arm abducted in scapular plane from 60° to 120° Yocum Test positive if pain reproduced with elbow elevation while ipsilateral hand placed on contralateral shoulder sensitive but nonspecific Internal Impingement test positive if pain is elicited with abduction and external rotation of the shoulder Imaging Radiographs recommended views true AP of the shoulder useful in evaluating the acromiohumeral interval normal distance is 7-14 mm 30° caudal tilt view useful in identifying subacromial spurring supraspinatus outlet view useful in defining acromial morphology findings common radiographic findings associated with impingement proximal migration of the humerus as seen in rotator cuff tear arthropathy traction osteophytes calcification of the coracoacromial ligament cystic changes within the greater tuberosity Type III-hooked acromion associated with impingment os acromiale best seen on axillary lateral 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 first line and mainstay of treatment of subacromial impingement alone without rotator cuff tear techniques aggressive rotator cuff strengthening and periscapular stabilizing exercises an integrated rehabilitation program is indicated in the presence of scapular dyskinesia which aims to regain full shoulder range of motion and coordinate the scapula with trunk and hip motions platelet-rich plasma injections most recent meta-analysis showing insufficient evidence to support use Operative subacromial decompression / acromioplasty indications subacromial impingement syndrome that has failed a minimum of 4-6 months of nonoperative treatment outcomes poor subjective outcomes have been observed after acromioplasty in patients with workers' compensation claims anxiety and depression Technique Subacromial decompression and acromioplasty acromioplasty two-step procedure performed open or arthroscopically an anterior acromionectomy is performed first the anterior deltoid origin determines the extent of the acromionectomy when performed arthroscopically and must remain intact an anteroinferior acromioplasty to smooth the undersurface of the acromion follows as the second step of the procedure a bone rasp is used if performed open a shaver or burr is used if performed arthroscopically the deltoid is meticulously repaired to bone in open procedures 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 the os acromiale is first fused with bone graft and allowed to heal an acromioplasty is then performed as a separate second procedure 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