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What is subacromial impingement syndrome?

If you went to the doctor for shoulder pain–especially pain that had an unclear start—you were likely given a diagnosis of subacromial impingement syndrome (SIS). Somewhere between 44% and 65% of patients with shoulder pain are given such a diagnosis. Let’s talk about what it is.

The subacromial area is the space between the acromion process—a flat bit of bone that protrudes off of the shoulder blade—and the top of the humerus, the upper arm bone. The acromion sits like a roof over the humerus. Within the approximately 1-1.5-cm space there are bursae (fluid filled sacs) and rotator cuff tendons. Subacromial impingement syndrome is the name given to pain that originates from this area. Sufferers often feel pain in the front or side of their shoulder that often spreads down the upper arm, but rarely goes below the elbow. Pain can become quite sharp and intense depending on the severity. Usually movements, especially above shoulder height or behind the back, increase pain.

The pain itself can be due to inflammation of the bursae or rotator cuff tendon pathology. Although in many cases it is hard to say exactly what the painful tissue is. In that way, subacromial pain is a lot like most low back pain.

Also like back pain, the cause of SIS is not clear. We should take this opportunity now to discuss how subacromial impingement syndrome is a bit of a misnomer because it is not certain that impingement is actually the problem. When the syndrome was first proposed, Dr. Charles Neer theorized that repetitive pinching of the rotator cuff tendons and bursae in the subacromial space led to damage to these structures and eventual pain. We have since learned that may not be the case.

For one thing, impingement of the rotator cuff tendons and bursae between the humerus and acromion is completely normal. It happens in all shoulders, healthy or not, whenever one moves their arm in a certain way. Impingement is pain-free for most people so impingement by itself cannot be the problem.

For another, most of the pathological changes that we see in the rotator cuff tendons happen on the side facing the joint, not on the side facing the acromion. If pinching against the acromion was an injurious process, we would expect to see more tears on the acromial side of the tendon.

Lastly, removing the source of the mechanical impingement does not improve the outcomes. Studies have compared what happens when surgeons either remove the acromion process or don’t, and they were not able to show a difference. Patients felt the same amount of better whether they had an acromion or not, so impingement must not be that important in this shoulder condition. If it were, removing the acromion would make a big positive difference.

While impingement syndrome is the most common term, simply subacromial pain syndrome (SPS) is more accurate. This is an important distinction because it means that you can heal even without surgery. And there’s no need to fear moving your arm in the future, as you are very unlikely to damage your shoulder through the impingement mechanism.

How do we treat it? Even though impingement is probably not what created your shoulder pain, continuing to do movements that cause impingement may keep it irritated and painful. Therefore, a big part of the early rehabilitation will be to avoid movements and positions that cause impingement. The big ones are reaching above shoulder height, behind your back or head, or across your body. In the meantime, there are exercises you can do to stress your shoulder in a good way—which is very important—without worsening the condition.

Our job as physical therapists is to help identify the things in your life that may be contributing to your shoulder pain and modify them. Then we guide you through exercises that start small and progress you back to doing the activities that were previously painful for you. If you’re interested in how we can help click here.