📌 Subacromial Extrinsic Impingement

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Magnetic Resonance Imaging Hallmark Findings

1. Supraspinatus Tendon

Bursal-sided partial-thickness tear

• Fluid-signal defect or surface fraying
• Typical abrasion pattern from the coracoacromial arch
• Commonly associated with severe subacromial–subdeltoid bursitis
• Supports an extrinsic impingement mechanism

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2. Subacromial–Subdeltoid Bursa

Severe bursitis

• Bursal fluid distension or synovial thickening greater than 3 millimeters
• Chronic cases show wall thickening and synovial proliferation
• Loss of the subacromial fat plane
• Major contributor to shoulder pain and functional limitation

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3. Acromion

Acromial spur (anteroinferior enthesophyte)

• Inferiorly projecting bony spur narrowing the subacromial outlet
• Causes direct mechanical conflict with the supraspinatus tendon
• Magnetic resonance imaging shows a corticated spur with fat-plane effacement
• Often aggravated by inferior acromioclavicular joint osteophytes

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4. Coracoacromial Ligament

Ligamentous component of impingement

• Ligament thickening due to chronic repetitive stress
• Appears as a thickened low-signal band on all sequences
• Subligamentous edema-like signal supports active impingement
• Interpretation should consider the stage of rotator cuff disease

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One-line Radiologic Impression

Magnetic resonance imaging findings are consistent with subacromial extrinsic impingement, characterized by a bursal-sided supraspinatus tendon partial-thickness tear, severe subacromial–subdeltoid bursitis, acromial spur formation, and coracoacromial ligament thickening.

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