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What is Shoulder Impingement Syndrome?

The dictionary defines impingement as "a sharp collision," but in the shoulder this is usually a gradual process that can cause a lot of shoulder pain, especially when using the hands above the level of the head. Shoulder impingement syndrome occurs when the supraspinatus tendon rubs against a part of the scapula called the acromion. This process is similar to the way that a rope frays and unravels as it travels through a rough pulley. The mechanical impingement of the tendon on the undersurface of the acromion causes a chemical inflammation in the tendon that is felt as shoulder pain by the patient. The inflammation of the tendon results in tendinitis (AKA tendonitis) of the rotator cuff.

The lubricating sack between the rotator cuff and the acromion, known as the bursa, is a special structure that helps to reduce the amount of friction as the tendon slides back and forth. Bursal sacks are found in several other places in the body where a tendon runs over a bone. The bursa in the shoulder is called the "subacromial bursa". Shoulder impingement can cause the inflammation and irritation of the bursa in the shoulder or "bursitis".

As with tendinitis, many people (especially athletes, construction workers, and laborers) who perform a lot of overhead activities suffer from shoulder pain caused by shoulder impingement syndrome. The tendons of the rotator cuff rub between the head of the humerus and the shoulder blade when you use your hands above the level of your head or when your arm is held out to the side of the body.

Symptoms of shoulder impingement often involve one or all of the following:

  • The tendons of the rotator cuff either are being pinched or are rubbing on a bone spur
  • There is inflammation of a lubricating sac (bursa) located just over the rotator cuff, a condition called bursitis
  • There is inflammation of the rotator cuff tendons, called tendonitis or tendinitis
Shoulder impingement, tendinitis or tendonitis, and bursitis usually happen at the same time.  Bone spurs can create bursitis.

Bone spurs on the undersurface of the acromion can create inflammation and bursitis.

Each of these three conditions, shoulder impingement, tendinitis or tendonitis, and bursitis usually happen at the same time. As a result, the terms are often used interchangeably. The reasons why these conditions may not get better by themselves are related to the frequency with which the cuff is irritated and to the changes that occur in our bodies as we grow older. Repetitive activity that irritates the rotator cuff may increase the amount of inflammation, and the aging process makes it more difficult for the inflammation to heal. Doctors believe that the blood supply to the tendon may decrease as we get older, causing the tendon to get weaker and possibly making it more difficult for the body to heal small tears or inflammation.

While most patients usually try to ignore the first signs of pain, they often visit a doctor after the pain has started to prevent them from sleeping well. Shoulder pain that makes it difficult to fall asleep is a common symptom of impingement syndrome and rotator cuff tendinitis. Most patients with this type of shoulder pain do not have a history of a bad shoulder injury, a dislocation, or an episode of the shoulder giving way. At first, most people notice only minor pain and a slight loss of strength. Subtle changes in range of motion, and especially in the ability to lift the arm overhead, may be ignored for a while. However, as symptoms progress, the shoulder pain and limited motion become more difficult to ignore.

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