Impingement occurs when the supraspinatus tendon or the region where the supraspinatus tendon is attached to the humerus, contacts the under surface of the acromion and the coraco-acromial ligament.
In patients with impingement, pain occurs as the arm is lifted up away from the side of the body. It typically occurs at approximately 90 degrees abduction and commonly occurs as a painful catch.
Impingement can occur for a variety of reasons. Injury to the rotator cuff such as a tear or strain injury can cause the supraspinatus tendon to swell in which case it can be more prone to catching on the under surface of the acromion.
A spur may be another cause for impingement and can develop on the under surface of the acromion as part of the normal aging process. This spur can then dig into the superior surface of your rotator cuff.
Arthritis of the AC joint may also cause or contribute to impingement due to spurs forming on the under surface of the acromioclavicular joint.
Impingement can also occur with bursitis