Injection Technique for Os Acromiale Pain Syndrome

Indications and Clinical Considerations

Patients with os acromiale pain syndrome will have diffuse shoulder pain with an associated feeling of weakness combined with loss of range of motion. The pain is often worse at night; patients often report that they are unable to sleep on the affected shoulder. The clinical presentation is usually insidious without a clear-cut history of trauma to the affected shoulder. Untreated, os acromiale pain syndrome can lead to progressive tendinopathy of the rotator cuff as well as gradually increasing shoulder instability and functional disability ( Fig. 42.1 ). In patients older than 50 years of age, progression of impingement often leads to rotator cuff tear.

The patient with os acromiale pain syndrome will report increasing shoulder pain with any activities that abduct and/or forward flex the shoulder, such as putting in a light bulb or reaching for dishes in a cabinet above shoulder height. Patients with os acromiale pain syndrome will demonstrate a positive Neer or Hawkins test result. Although not completely diagnostic of subacromial impingement syndrome, a positive Neer or Hawkins test result should prompt the examiner to obtain a magnetic resonance imaging (MRI) scan of the affected shoulder to further clarify and strengthen the diagnosis.

MRI of the shoulder provides the clinician with the best information regarding any disease of the shoulder. It is highly accurate in identifying os acromiale and helps to identify abnormalities that may put the patient at risk for continuing damage to the rotator cuff and the humeral head ( Fig. 42.2 ). MRI of the shoulder will also help the clinician rule out unsuspected disease that may harm the patient, such as primary and metastatic tumors of the shoulder joint and surrounding structures. In patients who cannot undergo MRI scanning, such as those with a pacemaker, ultrasonography and computed tomography are reasonable next choices. Radionuclide bone scanning, single-photon emission computed tomography/computed tomography (SPECT/CT), and plain radiography are indicated if fracture or bony abnormality, such as metastatic disease, are being considered in the differential diagnosis ( Fig. 42.3 ).

Screening laboratory testing consisting of complete blood cell count, erythrocyte sedimentation rate, and automated blood chemistry should be performed if the diagnosis of subacromial impingement syndrome is in question. Arthrocentesis of the glenohumeral joint may be indicated if septic arthritis or crystal arthropathy is suspected.

Os acromiale pain syndrome is a clinical diagnosis supported by a combination of clinical history, physical examination, radiography, ultrasonography, computed tomography, and MRI. Pain syndromes that may mimic os acromiale pain syndrome include subacromial impingement syndrome, subacromial bursitis, tendinopathy and tendinitis of the rotator cuff, calcification and thickening of the coracoacromial ligament, and arthritis affecting any of the shoulder joints ( Fig. 42.4 ). Adhesive capsulitis or frozen shoulder may confuse the diagnosis, as may idiopathic brachial plexopathy (Parsonage–Turner syndrome). The presence of primary and metastatic tumors of the shoulder and surrounding structures remains a possibility and should always remain as part of the differential diagnosis of patients with shoulder pain.

Clinically Relevant Anatomy

The subacromial space lies directly inferior to the acromion, the coracoid process, the acromioclavicular joint, and the coracoacromial ligament. Lubricated by the subacromial bursa, the healthy subacromial space is narrow, and the anatomic structures surrounding it are responsible for maintaining both static and dynamic shoulder stability. The space between the acromion and the superior aspect of the humeral head is called the impingement interval, and abduction of the arm will further narrow the space (see Fig. 42.2 ). Any pathologic condition that further narrows this space (e.g., osteophyte, abnormal acromial anatomy, ligamentous calcification, congenital defects of the acromion) will increase the incidence of impingement. Os acromiale is a congenital defect that is a result of the failure of the distal ossification center of the acromion. The failure of the distal acromion to fuse essentially results in a second acromial joint, which can foster shoulder instability and impingement.

Much as the congenital anatomic variant of the trefoil spinal canal is associated with a statistically significant higher incidence of spinal stenosis, there are several common normal anatomic variants of the acromion that contribute to developing subacromial impingement syndrome. These include types 2, 3, and 4 acromia ( Fig. 42.5 ). Whereas the normal type 1 acromion is relatively flat, type 2 acromion curves downward, type 3 acromion hooks downward in a shape reminiscent of a scimitar, and type 4 acromion has a convex shape. The downward curves of types 2 and 3 acromia markedly narrow the subacromial space (see Fig. 42.5 ). In addition to these anatomic variations, a congenitally unfused acromial apophysis termed os acromiale is often associated with subacromial impingement syndrome (see Figs. 42.1 and 42.2 ).