Shoulder impingement occurs when the top outer edge of your shoulder blade rub against or pinches your rotator cuff beneath it, causing pain and irritation.
As seen in the illustration, your rotator cuff is a group of four muscles that originates on the shoulder blade and attaches as a “cuff” of tendon onto your arm bone (humerus).
You use your rotator cuff to help raise your arm overhead and to rotate your arm towards and away from your body. You will notice in the illustration that the rotator cuff sits in a small space between two bones in the shoulder (acromion and the humerus). This arrangement makes the rotator cuff susceptible to being pinched or “impinged” between these bones, leading to what is called “impingement syndrome”.
When your rotator cuff is irritated or injured, it swells in much the same way your ankle does when it is sprained. However, because your rotator cuff is surrounded by bone, swelling causes other events to occur.
Swelling reduces the amount of space around the rotator cuff, leading to rubbing against the acromion. Like a viscious cycle, the rubbing of the rotator cuff tendons result in swelling and this further narrows the space below the acromion.
In some cases, bone spurs on the acromion bone can contribute to impingement by causing the space where the rotator cuff sits to be even more narrowed.
Symptoms of shoulder impingement syndrome include:
- Pain when your arms are extended above your head
- Pain when lifting your arm, lowering your arm from a raised position or when reaching
- Pain and tenderness in the front of your shoulder
- Pain that moves from the front of your shoulder to the side of your arm
- Pain when lying on the affected side
- Pain and achiness at night, which affects your ability to sleep
- Pain when reaching behind your back, like reaching into a back pocket or zipping up a zipper
- Shoulder and/or arm weakness and stiffness
Symptoms usually develops gradually over weeks to months.
Shoulder impingement is closely related to other common sources of pain in the shoulder called bursitis and rotator cuff tendonitis. These conditions can occur alone or in combination. Shoulder pain can also be a sign of a more serious injury to your rotator cuff, a small tear or hole called rotator cuff tear. If your rotator cuff has torn completely, you’ll likely have significant weakness and may not be able to raise your arm against gravity. In addition, you may have a rupture of your bicep muscle tendon as part of this continuing impingement process.
The causes of this impingement include:
- Your tendon is torn or swollen. This can be due to overuse from repetitive activity of the shoulder, injury or from age-related wear and tear.
- Your bursa is irritated and inflamed. Your bursa is the fluid-filled sac between your tendon and the acromion. Your bursa helps your muscles and tendons glide over your bones. Your bursa can become inflamed due to overuse of the shoulder or injury.
- Your acromion is not flat (you were born this way) or you have developed age-related bone spurs on your acromion.
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