Shoulder Impingement Syndrome: Causes, Symptoms, and Treatment

What Is Shoulder Impingement?

Shoulder impingement syndrome — also called subacromial impingement — occurs when the tendons of the rotator cuff (especially the supraspinatus) and the bursa (fluid-filled cushioning sac) become pinched between the humeral head and the underside of the acromion bone as you raise your arm.

The acromion is the bony roof of the shoulder, and the space beneath it (the subacromial space) is only about 1–1.5 cm wide. Anything that reduces this already-narrow space — bone spurs, thickened ligaments, swollen bursa — can cause impingement.

Anatomy Behind Impingement

When you raise your arm, the humeral head naturally moves upward slightly. In a healthy shoulder, the rotator cuff muscles depress the ball and maintain space. In impingement:

The subacromial space narrows
The rotator cuff tendons are compressed or "pinched"
The subacromial bursa becomes inflamed (bursitis)
Repetitive compression causes tendon irritation and can lead to rotator cuff tears over time

Causes

Primary impingement: Structural narrowing of the subacromial space

Bone spurs on the undersurface of the acromion (common with aging)
Hooked acromion (type III acromion shape)
Calcific deposits in the tendon
AC joint arthritis encroaching from above

Secondary impingement: Functional narrowing due to muscle imbalances or instability

Weak rotator cuff muscles failing to depress the humeral head
Poor scapular control (scapular dyskinesis)
Shoulder instability
Tight posterior capsule pushing the ball forward and upward

Symptoms

Pain at the front and outer side of the shoulder and upper arm
Painful arc: Pain occurring at 70–120° of arm elevation (when the tendon passes through the narrowest subacromial space)
Weakness with overhead activities
Night pain, particularly when sleeping on the affected shoulder
Pain radiating down the outer arm (not typically below the elbow)
Clicking or popping with certain movements

Diagnosis

Clinical tests:

Neer's sign: Forward flexion of the arm with internal rotation reproduces pain
Hawkins-Kennedy test: Arm elevated 90°, then internally rotated — positive if painful
Empty can test: Arm at 90°, thumb pointing down, resistance applied — tests supraspinatus

Imaging:

X-ray: Evaluates acromion shape, bone spurs, AC joint, calcific deposits
MRI: Assesses tendon integrity (partial vs. full-thickness tears), bursal swelling

Treatment

Conservative Treatment (Very Effective in Most Cases)

Most impingement responds well to non-surgical treatment:

Physical therapy: Rotator cuff strengthening (especially external rotators), scapular stabilization, posterior capsule stretching
NSAIDs: Ibuprofen or naproxen reduce inflammation
Subacromial corticosteroid injection: Very effective for bursitis and acute flares; usually provides 2–6 months of significant relief
Activity modification: Avoid provocative overhead activities during treatment

Surgical Treatment

If conservative treatment fails after 3–6 months:

Subacromial decompression (acromioplasty): Arthroscopic removal of bone spurs and part of the acromion to increase subacromial space; often combined with bursectomy (removal of inflamed bursa)
Recovery: 4–8 weeks return to full activity (much faster recovery than rotator cuff repair)