Flattened Acromion on Shoulder MRI: Clinical Significance
A flattened acromion (Type I) on shoulder MRI is generally a favorable finding associated with significantly lower risk of rotator cuff tears compared to curved or hooked acromion morphologies, and typically does not require specific intervention unless other pathology is present.
Acromial Morphology Classification
The acromion is classified into three distinct types based on its shape, which has direct implications for rotator cuff pathology 1:
- Type I (Flat): Acromial angle 0-12 degrees - associated with the lowest risk of rotator cuff tears 1
- Type II (Curved): Acromial angle 13-27 degrees - intermediate risk 1
- Type III (Hooked): Acromial angle >27 degrees - 89% association with rotator cuff tears (p < 0.001) 1
Clinical Significance of Flattened Acromion
The flattened acromion represents the most benign acromial morphology and is protective against rotator cuff disease 1. Key clinical implications include:
- Lower mechanical impingement risk: The flat configuration provides more subacromial space, reducing mechanical compression of the rotator cuff during shoulder elevation 1, 2
- Reduced likelihood of rotator cuff pathology: Type I acromions demonstrate significantly lower rates of rotator cuff tears compared to hooked morphology 1
- Better prognosis: When rotator cuff pathology does occur with a flat acromion, it is more likely related to intrinsic tendon degeneration rather than extrinsic mechanical impingement 2
Diagnostic Approach When Flattened Acromion is Identified
When a flattened acromion is noted on MRI, focus should shift to evaluating other potential causes of shoulder pain 3, 4:
- Assess for rotator cuff pathology: Despite favorable acromial morphology, evaluate for full-thickness tears, partial-thickness tears, or tendinosis using MRI without contrast as first-line imaging 5, 6
- Evaluate for labral pathology: Consider MR arthrography if clinical examination suggests labral tears, as this has 86-100% sensitivity for labral injury 5
- Examine for os acromiale: Look for unfused acromial epiphysis, which can cause chronic shoulder pain independent of acromial shape 7, 8, 9
- Consider alternative diagnoses: Functional impingement, early tendinopathy, subtle labral pathology, or referred cervical spine pain may be present despite normal acromial morphology 4
Important Caveats
Plain radiographs are equally effective as MRI for determining acromial type 1. The outlet view radiograph provides adequate visualization of acromial morphology with good interobserver reproducibility (coefficient of variation 0.1) 1. MRI offers no additional benefit over plain radiographs specifically for acromial classification 1.
Do not dismiss shoulder pain simply because a flat acromion is present 4. While the flat morphology is protective, patients can still develop:
- Intrinsic rotator cuff degeneration unrelated to impingement 2
- Labral tears from trauma or instability 5
- Subacromial bursitis 3
- Os acromiale with symptomatic synchondrosis 7, 8, 9
Treatment Implications
When shoulder pain occurs with a flat acromion, the treatment approach differs from impingement-related pathology 4:
- Structured rehabilitation focusing on rotator cuff strengthening and scapular stabilization is first-line treatment for 3-6 months 4, 6
- Acromioplasty is rarely indicated since mechanical impingement from acromial morphology is not the primary pathology 1
- Subacromial corticosteroid injection may be both diagnostic and therapeutic if conservative management fails after 4-6 weeks 4
- Surgical referral should focus on addressing intrinsic tendon pathology or labral tears rather than acromial decompression 6