Subacromial Impingement

There's a narrow corridor at the top of your shoulder — sandwiched between the ball of the upper arm bone below and the bony shelf at the top of your shoulder blade (the acromion) above. The rotator cuff tendons and a small fluid-filled cushion (the bursa) all have to slide through that corridor every time you raise your arm. Impingement is when that corridor narrows and the tendons get pinched against the bone. The narrowing can come from a hook-shaped acromion you were born with, a bone spur that built up over time, weak rotator cuff muscles letting the ball drift upward, or a thickened bursa from chronic irritation.

It's one of the most common causes of shoulder pain in adults over 40 and in overhead athletes (swimmers, painters, baseball pitchers). It sits on a spectrum: at one end, simple inflammation of the bursa; at the other, full rotator cuff tears.

• Pain on the outside of the shoulder and upper arm, especially when reaching overhead or out to the side
• A painful arc: lifting the arm hurts most between about chest height and ear height, then eases above that
• Pain when lying on the affected shoulder at night
• Weakness or difficulty with overhead tasks — reaching into a cabinet, brushing your hair, or throwing
• A catching or pinching sensation at the top of the shoulder during certain arm movements
• Gradual onset without a single injury — the pain builds over weeks to months of repetitive overhead use

Pain with overhead activities and a painful arc — the range from chest-height to ear-height when raising the arm out to the side hurts the most. Your provider does specific maneuvers (the Neer and Hawkins-Kennedy tests) that drive the tendons up against the bony roof to reproduce the pain. X-rays may show a hooked or spurred acromion or visible narrowing of the corridor. MRI shows whether the rotator cuff is just inflamed or has actually torn, and how thickened the bursa is.

1. Activity modification

   Cut back on overhead lifting and the activities that load the shoulder upward while the cuff and shoulder-blade muscles catch up.

Surgery comes into play when pain persists despite a real course of PT and injection, and imaging shows a clear structural reason (a hooked acromion, a bone spur, an associated cuff tear). The procedure shaves down the bony roof to widen the corridor (an acromioplasty), with cuff repair added if the cuff is torn.

External patient-education references and related OSI pages for additional background:

• Britannica — Shoulder (anatomy)
• AAOS OrthoInfo — patient education library
• OSI glossary — orthopedic terminology in plain language