In a patient with a traumatic shoulder injury whose pain worsens when the arm is unsupported, is the condition more likely an acromioclavicular (AC) joint separation or a glenohumeral dislocation?

Clinical Differentiation: AC Separation vs. Glenohumeral Dislocation

In a traumatic shoulder injury where pain worsens when the arm is unsupported, acromioclavicular (AC) joint separation is more likely than glenohumeral dislocation.

Key Distinguishing Clinical Features

AC Separation Presentation

• Pain increases dramatically when the arm hangs unsupported because gravity pulls the weight of the arm downward, stressing the disrupted AC and coracoclavicular ligaments 1, 2
• Patients typically support the affected arm with their contralateral hand to relieve the gravitational stress on the injured ligaments 1
• The mechanism is usually a direct impact on the lateral shoulder with the arm in adduction 1, 2
• Physical examination reveals point tenderness directly over the AC joint and visible/palpable deformity (step-off) at the superior shoulder in higher-grade injuries 3

Glenohumeral Dislocation Presentation

• Patients hold the affected arm rigidly in their contralateral hand and resist any movement attempts, not specifically because of unsupported weight but because of severe pain with any motion 3
• The shoulder appears squared-off with loss of normal deltoid contour 3
• A palpable humeral head may be felt in the axilla (anterior dislocation) or a dimple is visible inferior to the acromion laterally 3
• Pain is severe with any attempted motion, not specifically related to gravitational stress 3

Diagnostic Algorithm

Step 1: Mechanism Assessment

• Direct blow to the top or lateral aspect of the shoulder with arm adducted → AC separation 1, 2
• Fall on outstretched hand, contact sports collision, or high-impact trauma → glenohumeral dislocation 3

Step 2: Pain Pattern Analysis

• Pain specifically worse when arm hangs freely and relieved by supporting the arm → AC separation 1
• Pain severe with any shoulder motion regardless of support → glenohumeral dislocation 3

Step 3: Physical Examination

• Focal tenderness over AC joint with visible step-off deformity → AC separation 3
• Squared-off shoulder contour with palpable humeral head displacement → glenohumeral dislocation 3
• Preserved shoulder contour with localized AC prominence → AC separation 1

Step 4: Neurovascular Check

• Palpate radial and ulnar pulses immediately—absence requires urgent vascular imaging after any shoulder dislocation or high-energy trauma 4

Imaging Protocol

Initial Radiography (Both Conditions)

• Obtain upright radiographs (three views minimum: AP internal rotation, AP external rotation, and axillary or scapular-Y view) as supine positioning underrepresents shoulder malalignment 5, 4
• The axillary or scapular-Y view is essential because standard AP views alone miss up to 50% of glenohumeral and AC dislocations 4

AC Separation-Specific Imaging

• Add Zanca view (AP with 10-15° cephalic tilt) centered on the AC joint 6
• Consider bilateral stress views with weights to assess coracoclavicular ligament integrity in equivocal cases 1

Critical Pitfalls to Avoid

• Do not assume a single diagnosis—18.2% of high-grade AC separations have concomitant intra-articular injuries including SLAP tears (14.3%) and rotator cuff tears 7
• Do not miss glenohumeral dislocation by focusing only on obvious AC deformity—always obtain the axillary view 4
• Do not perform reduction maneuvers without confirming the diagnosis radiographically first 3
• In patients over 35-40 years with trauma, consider concomitant rotator cuff tears which may be masked by the acute injury pain 4

Age-Specific Considerations

• Patients under 35-40 years: AC separations are more common from sports injuries; also maintain high suspicion for instability and labral pathology 4, 1
• Patients over 35-40 years: Consider associated rotator cuff tears with either injury pattern; these patients may have pre-existing degenerative changes 4