The acromion is part of the scapula or shoulder blade that extends laterally over the shoulder joint. When one of the four ossification centers or growth plates of the acromion fails to fuse, an os acromiale forms. While most os acromiale do not cause any symptoms, a few can severely limit a patient’s shoulder function because of pain.
Who is at risk for developing an os acromiale?
Studies have shown that about 8% of the population have an os acromiale. Of the patients with an os acromiale, 30-60 % of them will have them on both sides. Os acromiale are more commonly found in blacks and males than whites and females.
Causes of os acromiale
The acromion develops from four ossification centers or growth plates. The growth plates generally fuse by age 18 but can fuse as late as age 25. When there is failure of the growth plates to fuse an os acromiale develops. Failure of fusion can occur at any of the ossification centers but more commonly occurs anteriorly or in the front of the acromion.
Symptoms for os acromiale
Most people who have an os acromiale do not have any problems. It is usually discovered when looking at the shoulder for another reason. Patients with symptomatic os acromiale can have pain with overhead activity. The pain will be located along the superior aspect of the shoulder. Symptoms can also occur at night. Along with pain, patients may have decreased shoulder motion and strength. Patients will be tender to palpation at the site of the os acromiale and there may also be frank movement of the bone at the site.
How does an os acromiale cause pain?
An os acromiale can cause pain in two ways. First there can be pain from the movement of the unfused bones. The movement of the bone from the pull of the shoulder muscles can also pinch the soft tissue within the shoulder. This impingement can irritate the muscles, tendons, and bursa (fluid filled sac between muscles and bones) resulting in pain with shoulder movement. Patients can also get impingement within the shoulder from other sources though and be asymptomatic from their os acromiale.
Diagnose for os acromiale
A patient’s history of symptoms and findings on physical exam can help the doctor make the diagnosis. X-rays will show the nonunion of the acromion. A MRI will also show the os acromiale along with other structures within the shoulder. The MRI may help differentiate the source of the pain.
Nonsurgical treatment options for os acromiale
Patients who do not have symptoms from their os acromiale do not require any treatment. For symptomatic os acromiale, the first goal of treatment is pain relief. Non-steroid anti-inflammatory drugs (NSAIDs) like Motrin®, ibuprofen, or Aleve® can help with pain control and decrease inflammation. Activities that cause symptoms should be avoided. A physical therapy program that focuses on establishing non-painful shoulder motion and strengthening of the shoulder muscles will also be helpful. Selective injection within the nonunion site or in the subacromial space (between the acromion and shoulder joint) may provide pain relief and will also help differentiate the source of the patient’s pain.
How long should I try nonsurgical treatments for os acromiale?
It is recommended to continue non-operative treatments for os acromiale for at least 6 months unless the patient has other conditions like a torn rotator cuff that may require earlier surgical intervention.
Surgical treatment options for os acromiale
There are three main options available for the treatment of os acromiale. Surgical treatment options are based on the size and location of the os acromiale, the patient’s symptoms, age, and functional demands.
Small fragments located in the anterior or front of the acromion can be surgically removed. Patient’s who fail more extensive procedures may also require excision of the fragment as a salvage procedure. Removal of larger fragments though, may impair the function of shoulder’s deltoid muscle. This in turn may limit the shoulder’s function.
Larger fragments may require fixation with screws and wires along with bone graft (usually taken from the patient’s hip) in order to get the two fragments to fuse. A second surgery may be required to remove the hardware if they irritate the patient. It generally takes 9-12 weeks for the bone to heal. There is always the risk that the bone does not heal though.
A third option is to debride the undersurface of the acromion and the bursa to help with pain relief. There are minimal risks with this procedure but results may be mixed with persistent pain.
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