Subacromial impingement occurs when the tendons of the muscles of the rotator cuff are compromised that they pass through the narrow confines between the acromion process and the coracoacromial ligament and the head of the humerus (Fig. 37.1). The narrow passage is known as the subacromial space. Most commonly the narrowing and resulting impingement is due to subacromial osteophytes, congenital or acquired variations in the shape of the acromion, from thickening and calcification of the subacromial ligament, and from subacromial bursitis. Dysfunction of the muscles of the rotator cuff can also contribute to narrowing of the subacromial space by allowing the head of the humerus to move superiorly.

Patients suffering from subacromial impingement syndrome often complain of generalized shoulder pain with an associated feeling of weakness combined with loss of range of motion. Sleep disturbance is common, with the patient often being unable to sleep on the affected shoulder. The onset of the symptoms associated with subacromial impingement syndrome is generally insidious without any identifiable inciting event, although the syndrome can begin acutely after shoulder trauma. Untreated, subacromial impingement syndrome can lead to progressive tendinopathy of the rotator cuff as well as gradually increasing shoulder instability and functional disability (Figs. 37.2 and 37.3). In patients above 50 years of age, progression of impingement often leads to rotator cuff tear. Any activity that requires the patient to abduct and/or forward flex the shoulder such as reaching overhead to put something away on a top shelf or paint a ceiling can exacerbate the patient’s pain symptomatology. Patients suffering from subacromial impingement will exhibit a positive Neer test. The Neer test is performed by having the patient assume a sitting position while the examiner applies firm forward pressure on the patient’s scapula and simultaneously raising the patient arm to an overhead position (Fig. 37.4). The Neer test is considered positive when the patient exhibits pain or apprehension when the arm moves about 60 degrees. While not completely diagnostic of subacromial impingement syndrome, a positive Neer test should prompt the examiner to obtain an ultrasound or MRI scan of the affected shoulder to more clearly identify the pathology responsible for the subacromial impingement.

The subacromial space lies directly below the acromion, the coracoid process, the acromioclavicular joint, and the coracoacromial ligament (see Fig. 37.1). The subacromial bursa provides lubrication for this narrow space but can become inflamed and in fact contribute to the impingement syndrome. 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. 37.5). Any pathologic condition that further narrows this space, for example, osteophyte, subacromial ligament thickening and calcification, and congenital or acquired abnormalities of the acromion, has the potential to increase the incidence of impingement (Fig. 37.6).

There are several common normal anatomic variants of the acromion that often contribute to the development of subacromial impingement syndrome. These include type 2 and type 3 acromions. While the “normal” type 1 acromion is relatively flat, the type 2 acromion curves downward and the type 3 acromion hooks downward in a shape reminiscent of a scimitar. The downward curve of the type 2 and type 3 acromions markedly narrows the subacromial space (Fig. 37.7). In addition to these anatomic variations, a congenitally unfused acromial apophysis termed os acromiale is also associated with subacromial impingement syndrome.

The benefits, risks, and alternative treatments are explained to the patient and informed consent is obtained. The patient is then placed in the sitting position with the shoulder relaxed and the forearm resting comfortably on the ipsilateral thigh. The acromioclavicular joint is then identified by palpation (Fig. 37.8). The skin overlying the acromion is prepped with antiseptic solution. A sterile syringe containing 2.0 mL of 0.25% preservative-free bupivacaine and 40 mg of methylprednisolone is attached to a 1½-inch, 22-gauge needle using strict aseptic technique. A linear high-frequency ultrasound transducer is placed in the coronal plane across the acromioclavicular joint (Fig. 37.9). After the acromioclavicular joint is
identified, slowly move the ultrasound transducer laterally to identify the hyperechoic margin of the acromion (Fig. 37.10). After the margin of the acromion is identified, the needle is placed through the skin at the middle of the anterior border of the coronally placed ultrasound transducer and is then advanced using an out-plane approach with the needle trajectory adjusted under real-time ultrasound guidance to enter the subacromial space just beneath the acromion (Fig. 37.11). When the tip of needle is thought to be within the subacromial space, a small amount of local anesthetic and steroid is injected under real-time ultrasound guidance to confirm needle placement. After needle tip placement is confirmed, the remainder of the contents of the syringe is slowly injected. There should be minimal resistance to injection. If synechiae, loculations, or calcifications are present, the needle may have to be repositioned to ensure that the entire subacromial space is treated.