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Updated: Jun 3 2021

Subacromial Impingement

Images tear arthropathy.jpg acromiale supraspinatus outlet view.jpg os acromiale.jpg acromiale.jpg
  • 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
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