1.1 Goals of Procedure

Subacromial impingement is one of the most common causes of anterior shoulder pain and occurs when the undersurface of the anterior acromion compresses the subacromial bursa, supraspinatus insertion, and biceps tendon during elevation of the arm. This was long considered the primary cause of rotator cuff tearing, ¹ but is now thought to be a consequence of rotator cuff dysfunction resulting in loss of effective humeral head depression. Decompression of the acromion’s undersurface increases the size of the subacromial space, allowing for unfettered movement of the rotator cuff and bursa as well as diminished pain. An arthroscopic approach to acromioplasty has many advantages over open approaches and is now the most popular method of decompression.

1.2 Advantages

Historically, subacromial decompression and acromioplasty were done through an open approach. An oblique incision was made from the anterolateral corner of the acromion to just lateral to the coracoid, to expose the coracoacromial (CA) ligament and the undersurface of the anterior acromion. ² This approach required detachment and reattachment of the deltoid, and longer postoperative rehabilitation, and could result in chronic deltoid dehiscence. Arthroscopic treatment is a minimally invasive technique that, when compared to open acromioplasty, allows for earlier return to work, reduced complications, and superior long-term functional results. ^{3 , 4} Arthroscopy also allows for visualization of the glenohumeral joint as well as the subacromial space, providing a more comprehensive assessment of the shoulder. Rotator cuff and biceps pathology can be concurrently treated through the same portals used for decompression

1.3 Indications

Subacromial decompression is indicated for patients with symptoms of shoulder impingement that are refractory to nonsurgical treatment. This procedure is often done in conjunction with rotator cuff repair and biceps tenodesis. The diagnosis of shoulder impingement is made based on history and physical examination. Patients typically report pain on the anterior aspect of the shoulder often radiating to the biceps and/or the deltoid insertion. Symptoms are typically exacerbated by elevation of the arm above shoulder level and with internal rotation. Several tests for impingement have been described and found to be sensitive, but not specific. ⁵ In addition to impingement tests, a comprehensive physical examination should include rotator cuff strength testing, as well as palpation and provocative testing of the biceps.

Radiographs should include a true anteroposterior view of the shoulder (Grashey view), axillary view, and a scapular outlet view. The scapular outlet view allows for assessment of acromial morphology and pathology. Bigliani et al classified the acromion morphologically: type I being flat, type II curved, and type III hooked. ⁶ Type III morphology has the strongest association with full-thickness rotator cuff tears.

The critical shoulder angle as described by Moor et al is another exciting radiographic assessment of the shoulder. ⁷ This angle is subtended by a line drawn along the vertical inclination of the glenoid and a tangent to the most lateral extent of the acromion. Larger angles are associated with rotator cuff pathology, whereas smaller angles may correlate with the development of osteoarthritis. Further investigation of this measurement is needed before it can alter the method of routine acromioplasty, however. MRI is useful for defining the severity of other shoulder pathology including the extent of rotator cuff tearing.

The initial treatment of shoulder impingement is nonoperative. Patients can be offered a subacromial cortisone injection and physical therapy, both of which have been shown to be successful in the management of shoulder impingement. When nonsurgical management has failed, a subacromial decompression can be considered. In the absence of full-thickness rotator cuff tearing, physical therapy lasting 6 to 12 weeks and one or two injections are reasonable trials of nonoperative management.

1.4 Contraindications

The main contraindication to a complete subacromial decompression is an unrepairable anterosuperior rotator cuff tear. In the absence of the anterosuperior rotator cuff, the CA arch prevents superior escape of the humeral head. Loss of the arch, typically from resection of the CA ligament, results in complete loss of elevation because the humerus has no stable foundation to act as a fulcrum during elevation (superior escape). In the setting of such a tear, a limited decompression can be done with preservation of the CA ligament and superior escape avoided.

Os acromiale is typically an asymptomatic, incidental finding, but has been implicated in shoulder impingement. This is thought to be due to separate bony components of the acromion moving in opposition and causing compression of the anterior shoulder tendons during elevation. ⁸ If the os acromiale is stable (i.e., the components move as one unit), subacromial decompression can be utilized, but care should be taken to avoid excessive bony resection leading to a painful mobile os. An unstable os acromiale are those that are tender to palpation and have significant mobility between components. Prior to decompression of an unstable os acromiale, fusion with internal fixation is recommended to avoid persistent pain at the fibrous connection.