Subacromial Impingement Syndrome
The subacromial space lies directly inferior to the acromion, coracoid process, acromioclavicular joint, and coracoacromial ligament.
Lubricated by the subacromial bursa, the subacromial space in health is narrow, and the anatomical structures surrounding it are responsible for maintaining 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 narrows the space further.
Any pathological condition that further narrows this space (e.g., osteophyte, abnormal acromial anatomy, ligamentous calcification, or congenital defects of the acromion) increases the incidence of impingement. The most common causes of subacromial impingement are listed in the below table.
Causes of Subacromial Impingement Syndrome
|Rotator cuff tears|
|Abnormal acromial anatomy (e.g., type 2 acromion, type 3 acromion)|
|Congenital acromial defect (e.g., os acromiale)|
|Acquired acromial defects (e.g., displaced fracture)|
|Inflammatory arthritis of the acromioclavicular joint|
|Abnormalities of the superior aspect of the humeral head|
|Glenohumeral joint instability|
|Crystal arthropathies of the acromioclavicular joint|
|Frozen shoulder (adhesive capsulitis)|
|Tendinopathy of the coracoacromial ligament|
Similar to the congenital anatomical variant of the trefoil spinal canal being associated with a statistically significantly higher incidence of spinal stenosis, several common normal anatomical variants of the acromion often contribute to development of subacromial impingement syndrome. These include type 2 and type 3 acromions.
Although the “normal” type 1 acromion is relatively flat, the type 2 acromion curves downward and the type 3 acromion hooks downward in the shape of a scimitar. The downward curve of the type 2 and type 3 acromions markedly narrow the subacromial space.
In addition to these anatomical variations, a congenitally unfused acromial apophysis, the os acromiale, is often associated with subacromial impingement syndrome.
Patients with subacromial impingement syndrome present with diffuse shoulder pain, with an associated feeling of weakness combined with loss of range of motion.
Pain is often worse at night, and patients often complain 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 and gradually increasing shoulder instability and functional disability. In patients older than 50 years, progression of impingement often leads to rotator cuff tear.
What are the Symptoms of Subacromial Impingement Syndrome
A patient with subacromial impingement syndrome reports increasing shoulder pain with any activities that abduct or forward flex the shoulder, such as putting in a light bulb or reaching for dishes in a cabinet above shoulder height.
Patients with subacromial impingement syndrome have a positive Neer test, which is performed by having the patient assume a sitting position while the examiner applies firm forward pressure on the patient’s scapula and simultaneously raises the patient’s arm to an overhead position.
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 order magnetic resonance imaging (MRI) of the affected shoulder to clarify and strengthen the diagnosis.
How is Subacromial Impingement Syndrome diagnosed?
MRI and dynamic ultrasound imaging of the shoulder provides the best information regarding any pathological process of the shoulder. MRI is highly accurate and helps identify abnormalities that may put the patient at risk for continuing damage to the rotator cuff and humeral head.
MRI of the shoulder also helps rule out unsuspected pathological conditions that may harm the patient, such as primary and metastatic tumors of the shoulder joint and surrounding structures.
In patients who cannot undergo MRI, such as patients with pacemakers, computed tomography (CT) is a reasonable second choice. 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 tests consisting of complete blood cell count, erythrocyte sedimentation rate, and automated blood chemistry testing 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, 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. Primary and metastatic tumors of the shoulder and surrounding structures remain an ever-present possibility and should always be part of the differential diagnosis of patients presenting with shoulder pain.
Initial treatment of the pain and functional disability associated with subacromial impingement syndrome should include a combination of nonsteroidal antiinflammatory drugs (NSAIDs) or cyclooxygenase-2 (COX-2) inhibitors and gentle physical therapy. Local application of heat and cold also may be beneficial. For patients who do not respond to these treatment modalities, injection of the subacromial space with local anesthetic and steroid is a reasonable next step while obtaining MRI and other appropriate testing to clarify further the working clinical diagnosis. Ultrasound guidance may improve the accuracy of needle placement and decrease the incidence of needle related complications.
The use of physical therapy, including gentle range-of-motion exercises, should be introduced several days after the patient undergoes this injection technique for shoulder pain. Vigorous exercises should be avoided because they would exacerbate the patient’s symptoms.
For patients who do not respond to these treatment modalities or radiographically have shown anatomical subacromial impingement that is producing ongoing damage to the rotator cuff, open or arthroscopic acromioplasty is required.
Failure to diagnose subacromial impingement syndrome correctly puts the patient at risk for the missed diagnosis of other syndromes that may result in ongoing damage to the shoulder or lead to overlooked pathological processes in this anatomical region that may harm the patient, such as Pancoast tumor or primary or metastatic tumors of the shoulder.
MRI is indicated in all patients thought to have subacromial impingement syndrome, and aggressive treatment of surgically correctable causes of such impingement is generally indicated sooner rather than later to avoid ongoing irreversible shoulder damage.
The musculotendinous unit of the shoulder joint is susceptible to the development of tendinitis for several reasons. First, the joint is subjected to a wide range of often repetitive motions.
Second, the space in which the musculotendinous unit functions is restricted by the coracoacromial arch, making impingement a likely possibility with extreme movements of the joint. Third, the blood supply to the musculotendinous unit is poor, making healing of microtrauma more difficult. These factors can contribute to tendinitis of one or more of the tendons of the shoulder joint.
Calcium deposition around the tendon may occur if the inflammation continues, making subsequent treatment more difficult. Tendinitis of the musculotendinous unit of the shoulder frequently coexists with bursitis of the associated bursae of the shoulder joint, creating additional pain and functional disability.
Patients with untreated subacromial impingement syndrome continue to experience pain and functional disability and may continue to cause ongoing irreversible shoulder damage culminating in damage to the humeral head and rotator cuff tear.