Shoulder impingement is common in sports that involve repetitive overhead motions or throwing, such as swimming, surfing, baseball, softball, water polo, and volleyball. During normal shoulder motion, the rotator cuff and subacromial bursa travel smoothly beneath the acromion in the subacromial space (the space between the acromion and humeral head). Additionally, the subacromial bursa, a small fluid-filled sac, helps the rotator cuff travel smoothly beneath the acromion and AC joint. In shoulder impingement, however, the rotator cuff and bursa get pinched or impinged underneath the acromion during overhead activities, resulting in pain.
Several factors can contribute to shoulder impingement. Structural or anatomic abnormalities might result in a narrower subacromial space. For example, some people are born with a curved or hook-shaped acromion that narrows the subacromial space. With aging, development of AC joint arthritis and bony spurs underneath the acromion can also narrow the subacromial space. The less room there is for the rotator cuff and bursa to travel, the more likely it is that these structures get pinched during shoulder motion.
A second factor is inflammation. Overuse or repetitive irritation of the rotator cuff underneath the acromion can lead to inflammation and swelling of the rotator cuff tendons and overlying bursa (tendinitis and bursitis). Not only are the inflamed tendons and bursa painful, but pain is aggravated when these inflamed and swollen structures get pinched or impinged underneath the acromion during overhead motions.
A third factor is shoulder instability, especially in young athletes. If the structures of the shoulder are ineffective in stabilizing the humeral head within the socket (glenoid fossa) during overhead motions, the humeral head might migrate upward out of the socket, causing impingement. Underlying shoulder instability is likely a primary cause of impingement symptoms in young athletes.