For a patient with right shoulder moderate acromioclavicular (AC) joint osteoarthritis, inferior distal clavicle spurring, mild glenohumeral osteoarthritis, limited active range of motion, pain with abduction and flexion, and tenderness over the lateral shoulder and upper scapula, which steroid injection—into the subacromial bursa, the glenohumeral joint, or the acromioclavicular (AC) joint—would be most beneficial?

Subacromial Bursa Injection is the Best Choice

For this patient with moderate AC joint osteoarthritis, inferior clavicle spurring causing mechanical impingement, limited active ROM with pain on abduction/flexion, and lateral shoulder tenderness, a subacromial bursa injection is most appropriate because the clinical presentation is dominated by impingement symptoms rather than isolated AC joint pathology.

Clinical Reasoning

Why Subacromial Over AC Joint Injection

The key discriminating features in this case point away from isolated AC pathology:

• Cross-body adduction test is only mildly positive - This is the primary provocative test for AC joint pathology 1. A truly symptomatic AC joint typically produces significant pain with this maneuver
• Pain with abduction and flexion - These movements primarily stress the subacromial space and rotator cuff, not the AC joint 1
• Limited active ROM - AC joint arthritis alone rarely causes significant ROM restriction; this suggests subacromial impingement or rotator cuff involvement
• Inferior clavicle spurring noted on imaging - The radiologist specifically identified this as a potential source of mechanical impingement, which occurs in the subacromial space 2

The AC Joint Paradox

Recent evidence reveals an important clinical reality: radiographic AC joint osteoarthritis often does not correlate with symptoms 3, 4. A 2024 study found no significant difference in outcomes after distal clavicle resection between patients who had good versus minimal response to preoperative AC injections, suggesting that distinguishing symptomatic from asymptomatic AC arthritis may be unnecessary 3. The presence of moderate AC arthritis on X-ray does not automatically make it the pain generator.

Why Not Glenohumeral Injection

The mild glenohumeral osteoarthritis is a secondary finding. Glenohumeral injections are indicated when:

• Primary glenohumeral pathology dominates
• Pain occurs throughout all ROM planes
• Night pain is prominent
• Conservative measures for GH OA have failed 5, 6

This patient's presentation—pain specifically with abduction/flexion and lateral tenderness—localizes to the subacromial region, not the glenohumeral joint.

The Subacromial Space as the Primary Pain Generator

The constellation of findings points to subacromial impingement as the dominant pathology:

1. Mechanical impingement from inferior clavicle spurring - explicitly noted on imaging 2
2. Pain with overhead activities (abduction/flexion) - classic for subacromial pathology 7
3. Lateral shoulder tenderness - corresponds to the subacromial bursa and rotator cuff insertion
4. Limited active ROM - suggests bursal inflammation or rotator cuff involvement

Fluoroscopic guidance can be useful for directing shoulder injections when anatomy is unclear 7, though ultrasound guidance is equally effective and can simultaneously assess for rotator cuff pathology 4.

Treatment Algorithm

Initial injection: Subacromial bursa with corticosteroid

• Addresses the mechanical impingement from clavicular spurring
• Treats likely concomitant bursitis
• Allows assessment of whether symptoms improve

If minimal response after 2-3 weeks:

• Consider diagnostic AC joint injection with local anesthetic 4, 1
• If AC injection provides >7 days of relief, the AC joint may be contributing
• However, even then, the impingement component still requires addressing 3

If good response to subacromial injection:

• Proceed with physical therapy focusing on scapular stabilization and rotator cuff strengthening
• Activity modification to avoid overhead positions
• Consider repeat injection if symptoms recur (though typically limit to 2-3 per year)

Common Pitfalls to Avoid

• Don't be misled by radiographic AC arthritis - it's extremely common in middle-aged and older adults and often asymptomatic 3, 4
• Don't inject the AC joint based solely on imaging - the clinical examination (especially cross-body test) should be strongly positive 1
• Don't ignore the mechanical impingement - the inferior clavicle spurring is the likely culprit and affects the subacromial space, not the AC joint itself 2

If conservative management including subacromial injection and physical therapy fails after 6 months, surgical options include arthroscopic distal clavicle excision to remove the impinging spur 2, 1, 8.