On top of the shoulder blade (scapula) sits a bony mass called the acromion. This composes the top of the shoulder and serves as the headquarters for the deltoid muscle – connecting to the collarbone (clavicle) to form the acromioclavicular joint or AC Joint. The AC joint is an important part of the shoulder as it connects the shoulder to the rest of the body.

This joint frequently incurs damage as a result of an impact to the outside of the shoulder – as often seen in contact sports as well as in biking and skiing accidents. When this happens it is called an AC joint separation.

As a result, patients may notice a visible bump on the top of the shoulder. While not extremely painful, this happens when the end of the collarbone becomes detached from the acromion and the arm correspondingly sags – making the bump more pronounced. The initial injury may cause pain in the AC joint area and may be accompanied by some bruising and swelling.

When a severe displacement occurs, patients experience a tremendous amount of weakness in the deltoid muscle – indicating more serious ligament damage.

An AC joint separation, or shoulder separation, is often confused with a shoulder dislocation. The primary differences lie in the bones affected. In a shoulder separation, the area where the clavicle and scapula connect is disrupted, but in a shoulder dislocation the humerus is displaced from the socket, which is formed by the scapula.

Those at Risk
Those involved in extreme biking, motocross racing and other action sports are frequently diagnosed with AC joint separation.

Diagnosis
The prominent bump at the top of the shoulder is a good indictor of AC joint separation. Patient history and detail of how the injury occurred followed by an assessment of pain and tenderness at the joint will also help in the diagnosis. An X-ray may be performed in order to confirm that there is no bone fracture.

Treatment
Most AC joint separation problems can be addressed conservatively and will include a period of rest, cold compression and anti-inflammatory medication in order to relieve pain and reduce swelling. Range of motion exercises begin shortly thereafter in order to rebuild shoulder strength and flexibility.

In the case of severe displacement and corresponding weakness of the deltoid muscle, an outpatient surgical repair may be performed in order to restore the ligaments, which maintain the AC region. Rest, cold compression and range of motion exercises follow the procedure.

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