Os Acromiale Pain Syndrome

Os Acromiale Pain Syndrome

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 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 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.

One such congenital defect is caused by failure of the distal ossification center of the acromion to fuse. This failure to fuse is termed os acromiale and essentially results in a second acromial joint. This “second joint” can lead to impingement syndromes and exacerbate shoulder instability.

Patients suffering from os acromiale have 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. The clinical presentation is usually insidious, without a clear-cut history of trauma to the affected shoulder.

Affected patients tend to be younger than those with other causes of shoulder impingement syndromes.

Untreated, os acromiale 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 Os Acromiale Pain Syndrome

A patient with os acromiale 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 os acromiale have positive tests for shoulder impingement, such as Neer and Hawkins tests. Although not completely diagnostic of os acromiale, a positive Neer or Hawkins test should prompt the examiner to order magnetic resonance imaging (MRI) of the affected shoulder to clarify and strengthen the diagnosis.

How is Os Acromiale Pain Syndrome diagnosed?

MRI of the shoulder provides the best information regarding any pathological condition 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 the humeral head.

MRI of the shoulder also helps rule out unsuspected pathology 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.

Differential Diagnosis

Os acromiale is a clinical diagnosis supported by a combination of clinical history, physical examination, radiography, and MRI. Pain syndromes that may mimic os acromiale 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.

Adhesive capsulitis or frozen shoulder may confuse the diagnosis, as may idiopathic brachial plexopathy.

Acromial stress fractures and undiagnosed clavicular fractures also may mimic the clinical presentation of os acromiale, as may impingement syndromes caused by aberrant subacromial blood vessels. Primary and metastatic tumors of the shoulder and surrounding structures are an ever-present possibility and should remain as part of the differential diagnosis of patients with shoulder pain.


Initial treatment of the pain and functional disability associated with os acromiale 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 the working clinical diagnosis further. 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 symptoms.

For patients who do not respond to these treatment modalities or have radiographically shown anatomical subacromial impingement that is producing ongoing damage to the rotator cuff, open or arthroscopic acromioplasty is required.


Failure to diagnose os acromiale correctly puts the patient at risk for the missed diagnosis of other syndromes, which may result in ongoing damage to the shoulder or lead to an overlooked pathological condition 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 os acromiale, and aggressive treatment of surgically correctable causes of such impingement is generally indicated sooner rather than later to avoid ongoing irreversible shoulder damage.

Clinical Pearls

The acromion has three distinct ossification centers: (1) the basiacromion-metacromion, which is most proximal; (2) the metacromion-mesoacromion, which is in the middle; and (3) the mesoacromion-preacromion, which is most distal. Lack of fusion of the mesoacromion-preacromion is responsible for the development of os acromiale.

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 os acromiale 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.


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