Houston Methodist. Leading Medicine

Shoulder FAQ


I've just been diagnosed with something called subacromial impingement syndrome also known as SIS for short. What causes this to develop? I've been active all my life but not overly so. I'm not a grade A athlete or anything like that. I have 57-years-old so I know things are going to start falling apart. I just wasn't prepared for it to happen quite so soon.


One of the most common causes of shoulder pain is this problem you mentioned: subacromial impingement syndrome or SIS. The term impingement tells us something is getting pinched. Subacromial impingement syndrome occurs when the rotator cuff tendons rub against the roof of the shoulder, the acromion. Although SIS is one term, it actually represents a wide range of underlying pathologies. There could be a bursitis, rotator cuff tendinopathy, fracture, calcific tendinitis, or other change in the local anatomy contributing to the problem. There are many factors that when present combine together to result in subacromial impingement syndrome. Aging with its many degenerative processes isn't always very kind to the shoulder. Bone spurs form, the rotator cuff and other soft tissues fray and wear thin, and trauma all add to the development of mechanical shoulder pain. Loss of blood supply to the area is another reason why these problems occur. Subacromial impingement syndrome and rotator cuff degeneration go hand-in-hand together. Much debate and controversy exist over the connection between these two conditions. Which comes first? Does the impingement cause tearing of the rotator cuff? Or does the rotator cuff degenerate and weaken over time resulting in impingement? Orthopedic surgeons have looked carefully for an exact source of external compression. They have tried removing different parts of the bone around the shoulder in an effort to stop acute bursitis and the impingement process. Studies have been done using cadavers (human bodies preserved after death) to try and solve the question of cause and effect. The effects of age and shape of the acromion have been examined as possible contributing factors. There's been an effort to find outside (referred to as extrinsic) factors for the rotator cuff degeneration and subacromial impingement. Low blood supply to the supraspinatus tendon of the rotator cuff has been blamed for tendon degeneration. But some experts suggest just the opposite -- the impaired blood flow to the tendon may develop because the tendon has been damaged first. It is likely that the cause of subacromial impingement syndrome (SIS) is really multifactorial. Each patient will have his or her own unique combination of reasons why they developed an impingement syndrome. Alicia K. Harrison, MD, and Evan L. Flatow, MD. Subacromial Impingement Syndrome. In Journal of the American Academy of Orthopaedic Surgery. November 2011. Vol. 19. No. 11. Pp. 701-708.

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