External Impingement of the Shoulder

The topic describes structural (outlet) impingement in detail, but examiners expect the primary-versus-secondary distinction — and it explains why exercise works and bone surgery often does not.

• Primary (structural / outlet) impingement. A fixed, anatomical narrowing of the subacromial outlet — a Bigliani type-3 hooked acromion, a subacromial or AC-joint osteophyte, a thickened coracoacromial ligament, or an os acromiale. The mechanical block is the primary problem (the classic Neer model).
• Secondary (functional / non-outlet) impingement. The outlet is anatomically normal, but it is dynamically narrowed because the humeral head rides up: from rotator-cuff weakness or fatigue (loss of the head-depressor force couple), scapular dyskinesis (loss of acromial upward rotation and posterior tilt), or glenohumeral instability (the head translates under load, especially in young throwers). Here impingement is a symptom of another problem.
• Why the distinction matters. Secondary impingement is treated by restoring dynamic control (rotator-cuff and scapular rehabilitation, addressing instability) — not by removing bone. This is a key reason the high-level trials (CSAW, FIMPACT) found decompression no better than placebo: most "impingement" has a large functional component that bone resection does not address, whereas structured exercise does.
• Practical rule. Reserve any thought of decompression for genuine primary/structural impingement (a defined mechanical lesion); a normal-outlet shoulder with weakness, dyskinesis or instability is secondary and is a rehabilitation (or instability) problem. Scapular dyskinesis and instability are developed in their own topics.

The surgical section says to "preserve the coracoacromial ligament to maintain arch integrity" — worth developing, because releasing it has a specific, feared consequence.

• The arch is a static superior restraint. The coracoacromial arch (acromion + coracoacromial ligament + coracoid) is the passive roof of the subacromial space. With an intact rotator cuff it is only a minor buttress, but in a cuff-deficient shoulder it becomes the last static restraint against anterosuperior escape of the humeral head.
• The danger of over-decompression. Aggressive acromioplasty with coracoacromial-ligament release in a shoulder that has (or later develops) a large or massive cuff tear can precipitate anterosuperior escape — the head subluxes up and forward through the deficient roof — a difficult, often effectively irreparable complication.
• The corollary for practice. Decompression should therefore be conservative and cuff-status-aware: release the coracoacromial ligament sparingly (or preserve it) when the cuff is deficient, remove only enough acromial undersurface to relieve a genuine mechanical spur, and protect the deltoid origin. It is a further argument (alongside CSAW/FIMPACT) against routine, aggressive bony decompression.

Global epidemiology. Shoulder pain is among the three most common musculoskeletal presentations worldwide; subacromial pain syndrome (the contemporary umbrella term for external impingement with an intact cuff) accounts for the majority of these consultations. Prevalence rises with age and overhead occupational or sporting load and is broadly similar across high- and limited-resource settings.