Injection Technique for Subacromial Impingement Syndrome

Indications and Clinical Considerations

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

The patient with subacromial impingement syndrome will report increasing shoulder pain with any activities that abduct and/or forward flex the shoulder, such as putting in a lightbulb or reaching for dishes in a cabinet above shoulder height. Patients with subacromial impingement syndrome will demonstrate a positive Neer test. The Neer test is performed by having the patient assume a sitting position, and the examiner applying firm forward pressure on the patient’s scapula and simultaneously raising the patient’s arm to an overhead position. The Neer test is considered positive when the patient exhibits pain or apprehension when the arm moves about 60 degrees. Although not completely diagnostic of subacromial impingement syndrome, a positive Neer test should prompt the examiner to obtain a magnetic resonance imaging (MRI) scan and/or ultrasound examination 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 and helps to identify abnormalities that may put the patient at risk for continuing damage to the rotator cuff and the humeral head ( Fig. 41.1 ). MRI scan 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 pacemakers, ultrasonography or computed tomography are reasonable next choices. Radionuclide bone scanning and plain radiography are indicated if fracture or bony abnormality, such as metastatic disease, is considered in the differential diagnosis.

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.

Subacromial impingement syndrome is a clinical diagnosis supported by a combination of clinical history, physical examination, radiography, ultrasonography, and MRI. Pain syndromes that may mimic subacromial impingement syndrome include subacromial bursitis, tendinopathy and tendinitis of the rotator cuff, calcification and thickening of coracoacromial ligament, and arthritis affecting any of the shoulder joints. 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 be 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 ( Fig. 41.2 ). 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 ( Box 41.1 , Fig. 41.3 ).

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 often contribute to the development of subacromial impingement syndrome. These include types 2 and 3 acromia ( Fig. 41.4 ). Whereas the normal type 1 acromion is relatively flat, type 2 acromion curves downward and type 3 acromion hooks downward in a shape reminiscent of a scimitar. The downward curves of types 2 and 3 acromia markedly narrow the subacromial space ( Fig. 41.5 ). In addition to these anatomic variations, a congenitally unfused acromial apophysis termed os acromiale is often associated with subacromial impingement syndrome (see Chapter 42 ).