Shoulder Impingement: Diagnostic Projections, Imaging, and Clinical Correlation

Which projection is useful in diagnosing shoulder impingement?

While no single radiographic projection is definitively diagnostic on its own, the Neer outlet view (also known as the supraspinatus outlet view) is widely considered the most useful projection for identifying anatomical factors contributing to shoulder impingement syndrome.

Understanding Shoulder Impingement Syndrome

Shoulder impingement syndrome occurs when the tendons of the rotator cuff (most commonly the supraspinatus tendon) and/or the subacromial bursa become compressed or "impinged" in the subacromial space. This space is located between the humeral head below and the acromion, coracoacromial ligament, and acromioclavicular (AC) joint above. Repetitive overhead activities, anatomical variations, and muscular imbalances can narrow this space, leading to inflammation, pain, and potential tendon damage.

The Role of Imaging in Diagnosis

Imaging plays a crucial role in the comprehensive evaluation of shoulder impingement. While the diagnosis is primarily clinical, based on patient history, physical examination, and specific provocative tests, imaging serves several vital purposes:

• Confirming Clinical Suspicion: Imaging can visualize the anatomical structures involved and identify potential sources of impingement.
• Identifying Contributing Factors: It helps to detect bony abnormalities (e.g., acromial morphology, osteophytes, AC joint spurs) or soft tissue changes (e.g., bursitis, tendinosis, tears).
• Ruling Out Other Conditions: Imaging can differentiate impingement from other shoulder pathologies such as rotator cuff tears, adhesive capsulitis, glenohumeral arthritis, or cervical radiculopathy.
• Guiding Treatment: Imaging findings can influence treatment decisions, from conservative management to surgical intervention.

Key Radiographic Projections for Impingement

While a standard shoulder series typically includes an Anteroposterior (AP) internal/external rotation, and a Scapular Y view, specific projections are more targeted for assessing impingement.

Neer Outlet View (Supraspinatus Outlet View)

This is the most critical radiographic projection for evaluating the subacromial space and acromial morphology.

• Purpose: To visualize the acromion, coracoacromial ligament, and the supraspinatus outlet in profile, allowing assessment of the shape of the acromion and the presence of any bony spurs that might narrow the subacromial space.
• Technique: The patient is positioned with the shoulder rotated 30-45 degrees anteriorly from the lateral position, with the X-ray beam angled caudally at 10-15 degrees. This projects the acromion in a true lateral profile.
• What it Reveals:
  • Acromial Morphology: The Neer outlet view is essential for classifying acromial types according to the Bigelow classification:
    • Type I (Flat): Least associated with impingement.
    • Type II (Curved): More common, can contribute to impingement.
    • Type III (Hooked): Strongly associated with impingement and rotator cuff tears due to significant subacromial narrowing.
  • Osteophytes/Spurs: Identifies bony spurs originating from the undersurface of the acromion or the AC joint, which can directly impinge on the rotator cuff.
  • Critical Shoulder Angle (CSA): While not exclusively from the outlet view, this angle (formed by a line from the inferior glenoid to the lateral acromion and a line along the glenoid fossa) can be assessed and correlated with impingement risk. A larger CSA (greater than 35 degrees) is associated with an increased risk of impingement and rotator cuff tears.

Other Useful Radiographic Projections

While the Neer outlet view is primary, other projections provide complementary information:

• Anteroposterior (AP) Shoulder View:
  • Purpose: Provides a general overview of the glenohumeral joint.
  • What it Reveals: Glenohumeral joint space narrowing, calcific tendinopathy (calcium deposits in tendons, often supraspinatus), AC joint arthritis, and any signs of superior migration of the humeral head (indicating a chronic rotator cuff tear).
• Axillary View:
  • Purpose: Visualizes the glenohumeral joint in the axial plane.
  • What it Reveals: Glenohumeral alignment, posterior impingement (e.g., internal impingement), coracoid impingement, and any bony lesions on the humeral head or glenoid.
• Scapular Y View:
  • Purpose: Primarily used to assess glenohumeral dislocations and general scapular orientation.
  • What it Reveals: Can give a general sense of the relationship between the humeral head and the acromion, though less specific for impingement than the Neer outlet view.

Beyond X-rays: Advanced Imaging Modalities

While X-rays are excellent for assessing bone, soft tissue structures are better visualized with advanced imaging:

• Magnetic Resonance Imaging (MRI):
  • Gold Standard: For visualizing soft tissues, including the rotator cuff tendons, labrum, joint capsule, and subacromial bursa.
  • What it Reveals: Tendinosis (degeneration), partial or full-thickness rotator cuff tears, bursitis (inflammation of the bursa), inflammation, and labral pathology. MRI is crucial for confirming the extent of soft tissue damage.
• Ultrasound:
  • Dynamic Assessment: Allows real-time, dynamic visualization of tendons and bursa during movement.
  • Cost-Effective: Often a more accessible and less expensive option than MRI.
  • What it Reveals: Can effectively identify rotator cuff tears, tendinosis, and bursitis. Its dynamic capability can demonstrate impingement as it occurs.

The Importance of Clinical Correlation

It is crucial to remember that imaging findings alone do not diagnose shoulder impingement. A comprehensive assessment always integrates:

• Detailed Patient History: Understanding the onset, nature, aggravating factors, and relieving factors of pain.
• Thorough Physical Examination: Including range of motion assessment, strength testing, and specific provocative tests (e.g., Neer's impingement sign, Hawkins-Kennedy test, Empty Can test) that reproduce the patient's symptoms.
• Imaging Results: To confirm clinical suspicions, identify anatomical contributors, and rule out other pathologies.

A significant number of asymptomatic individuals may have abnormal imaging findings, and conversely, some individuals with clear clinical impingement may have normal X-rays. Therefore, the clinical presentation must always guide the interpretation of imaging results.

Conclusion

The Neer outlet view stands out as the most valuable radiographic projection for assessing anatomical factors contributing to shoulder impingement syndrome, particularly for evaluating acromial morphology and subacromial space narrowing. However, a definitive diagnosis of shoulder impingement requires a holistic approach, combining clinical expertise from patient history and physical examination with appropriate imaging modalities like X-rays, MRI, or ultrasound, to provide a complete picture of the underlying pathology.