Shoulder impingement is a common cause of shoulder pain. Before jumping into the specifics of what “shoulder impingement” means, it is important to understand the anatomy of the shoulder.
Your shoulder joint is made up of the humerus (your upper arm bone) and your scapula (shoulder blade). The smooth, shallow depression on your scapula where your humerus sits is called the glenoid.
The acromion process is a piece of bone that extends off of the shoulder blade and creates a roof over the shoulder joint.
Shoulders are inherently unstable joints. The head of your humerus sitting on your glenoid is much like a golf ball sitting on a tee. This instability actually benefits us as humans because it translates to having a lot of freedom of movement, more than any other joint in our bodies. Without that extra movement we would not be able to swim, throw, reach behind our backs, etc.
Because the bones of the shoulder joint do not provide much structural stability, your shoulder is kept in its socket primarily by your rotator cuff muscles. The rotator cuff is the name given to the four muscles (supraspinatus, subscapularis, infraspinatus, teres minor) that surround the head of your humerus, and attach it to the shoulder blade. The long head of the biceps is sometimes grouped with the rotator cuff as it assists with stability in the front of the shoulder.
A lubricating sac called a bursa sits between the tendons of your rotator cuff muscles and the bone at the top of your shoulder (your acromion). The bursa allows your rotator cuff tendons to be able to glide smoothly as you move your arm.
Description and causes of shoulder impingement
Shoulder impingement is an umbrella term that describes irritation, pain, or injury of the rotator cuff muscles, long head of biceps, or subacromial bursa in the area between the humerus and the acromion (subacromial space).
Sources of pain can include:
The rotator cuff tendons and long head of biceps tendon can become inflamed or irritated. This is described at tendonitis.
The subacromial bursa can also become inflamed and will plump up with fluid.
As you raise your arm overhead or out to the side, the subacromial space narrows. This can cause pinching or “impingement” of the tendons or bursa that live in the subacromial space. The majority of the time, our bodies can tolerate this pinching and loading. However, there are some situations when it is more than those structures can tolerate:
People who do activities that involve repetitive reaching or lifting are vulnerable to injury in that area. This includes people who work in construction, painting, electricians, etc. It also includes athletes such as swimmers, baseball, tennis, and volleyball players.
This area can also be injured by trauma. If a person falls and lands on an outstretched arm, this can force the humerus upward, pinching the structures in the subacromial space up into the acromion process.
Slouched posture can also be problematic. Many of us sit at a desk looking at a computer all day. When we slouch forward, we further narrow our subacromial space placing the shoulder at a mechanical disadvantage and increasing the likelihood of pinching or impingement of the rotator cuff tendons or bursa.
Shoulder impingement most often causes pain and tenderness in the front of the shoulder. This pain may also extend down the front and side of the arm. It is particularly painful with lifting the arm forward and out to the side. It may also be painful with lowering arm from a lifted position. It may be difficult or painful to sleep on the shoulder. People also describe feeling that the painful arm is weak and that motion is limited.
Treatment often is focused on protected the irritated tissues. It is also important to strengthen upper back musculature, which helps improve upright posture. This opens up the subacromial space and decreases pinching in that area. As the irritated rotator cuff muscles become less painful, rehab usually includes strengthening those muscles to increase the amount of load they are able to tolerate moving forward.
If you believe you may be experiencing shoulder impingement, a PT can further evaluate you and personalize a rehabilitation program!