Half Moon Sign

1. High-Yield Overview & Pathognomonic Essence

• Morphological Hallmark: The half-moon sign on an anteroposterior (AP) radiograph of the shoulder represents the normal elliptical or crescentic radiopaque overlap shadow formed by the medial aspect of the humeral head projecting over the glenoid fossa (measuring approximately 5 to 6 mm in transverse width).
• Pathognomonic Significance: Complete absence or disappearance of this crescentic overlap shadow (the absent half-moon sign) is a pathognomonic plain radiographic indicator of posterior glenohumeral dislocation, resulting from posterior displacement of the humeral head out of the glenoid fossa.
• Critical Diagnostic Discriminator: Posterior shoulder dislocations are notoriously misdiagnosed or overlooked on initial emergency presentation in 60% to 80% of cases because the AP radiograph may deceptively appear reduced to an untrained eye. Complete loss of the half-moon overlap shadow mandates immediate orthogonal imaging (axillary lateral or Velpeau view) or computed tomography (CT).

2. Diagnostic Sign Summary Matrix

3. Clinical Case Imaging & Standardized Figures

4. Pathophysiology & Biomechanical Mechanism

The normal half-moon sign represents a projectional radiographic phenomenon arising from normal glenohumeral articular congruity. In standard AP projections of the shoulder girdle, the X-ray beam is angled tangentially to the thoracic cage rather than perpendicular to the scapular blade. Because the glenoid fossa is oriented approximately 30 to 45 degrees anteriorly relative to the coronal plane of the body, the convex spherical articular contour of the medial humeral head projects across the anterior and posterior cortical margins of the concave glenoid fossa. This overlap generates a dense, crescentic, half-moon shaped radiopaque shadow spanning 5 to 6 mm in transverse width and extending inferiorly to the lower pole of the glenoid.

Posterior glenohumeral dislocation occurs primarily through violent, uncoordinated muscular contractures (such as grand mal epileptic seizures, high-voltage electrical injuries, or electroconvulsive therapy) or direct high-energy trauma (such as motor vehicle collisions or sports collisions with an adducted, flexed, and internally rotated arm). During violent muscle activation, the internal rotators of the shoulder (subscapularis, latissimus dorsi, and pectoralis major) overpower the weaker external rotators (infraspinatus and teres minor). This imbalance drives the humeral head forcefully posterior over the posterior glenoid labrum.

Once displaced posteriorly, the humeral head rests behind the posterior glenoid rim and becomes mechanically locked in internal rotation. The medial articular surface of the humerus is completely removed from the glenoid cavity. In standard AP projection, the radiographic beam passes directly between the separated articular surfaces rather than penetrating overlapping bone contours. Consequently, the normal crescentic overlap shadow vanishes entirely (the absent half-moon sign). Simultaneously, the anterior glenoid rim becomes fully exposed, creating the vacant glenoid sign and widening the apparent glenohumeral space beyond 6 mm (the rim sign). If the anterior humeral head forcefully impacts the rigid posterior cortical rim of the glenoid, an impaction compression fracture develops (reverse Hill-Sachs lesion), producing the classic vertical trough line.

5. Modality-Specific Imaging Findings

Plain Radiography

Plain radiography represents the initial diagnostic modality, yet single AP radiographs account for the vast majority of missed posterior dislocations. The classic plain radiographic signs include:

• Absent Half-Moon Sign: Complete loss of the normal 5 to 6 mm crescentic overlap between the medial humeral head and the glenoid fossa on standard AP views.
• Lightbulb Sign (Ice Cream Cone Sign): Symmetrical, rounded appearance of the proximal humerus on AP projections caused by fixed internal rotation, which rotates the greater and lesser tuberosities into profile.
• Rim Sign: Pathologic widening of the radiographically visible glenohumeral joint space greater than 6 mm between the anterior glenoid rim and the medial articular margin of the displaced humeral head.
• Trough Line: A dense vertical sclerotic line running parallel to the medial articular surface of the humeral head, reflecting cortical compaction from a reverse Hill-Sachs lesion.
• Axillary Lateral View: The definitive plain film projection. Readily confirms posterior displacement of the humeral head relative to the glenoid fossa and reveals the presence and depth of any anterior impaction notch.
• Velpeau View: Essential alternative when intense pain or muscle spasm prevents abduction required for standard axillary positioning. With the patient leaning backward 30 degrees over the cassette, an angled superoinferior projection confirms glenohumeral relationship.

Computed Tomography (CT)

Computed tomography with multiplanar (axial, coronal, sagittal) reconstructions and 3D volume rendering serves as the diagnostic gold standard. Axial CT slices directly demonstrate posterior subluxation or complete dislocation of the humeral head relative to the glenoid fossa. Most importantly, axial CT precisely quantifies the percentage of humeral head articular involvement affected by the reverse Hill-Sachs defect (stratified as <20%, 20% to 40%, or >40%), detects posterior glenoid rim fractures (reverse Bankart lesions), and identifies associated tuberosity or surgical neck fractures.

Magnetic Resonance Imaging (MRI) & Ultrasound

Magnetic resonance imaging is indicated following reduction to evaluate associated capsulolabral and rotator cuff pathology. MRI demonstrates posterior labral tears (reverse Bankart lesion), posterior capsule stripping or attenuation, posterior capsular sleeve avulsion of the humerus (reverse HAGL lesion), and subscapularis tendon avulsion or interstitial tearing. Fluid-sensitive sequences (T2 fat-suppressed and STIR) demonstrate extensive bone marrow edema within the anteromedial humeral head corresponding to the reverse Hill-Sachs impaction. Diagnostic musculoskeletal ultrasound can confirm dislocation and empty glenoid fossa dynamically in bedside triage, but remains operator dependent and secondary to CT.

6. Diagnostic Utility, Sensitivity & Specificity

The clinical and radiographic diagnostic accuracy of solitary plain shoulder films has been evaluated extensively across trauma series. Posterior glenohumeral dislocations represent only 2% to 5% of all shoulder dislocations, contributing to low clinical suspicion and a reported 60% to 80% initial failure of diagnosis at initial emergency room evaluation.

When evaluated individually on standard AP radiographs, the absent half-moon sign demonstrates a diagnostic sensitivity of approximately 75% to 85% and a specificity exceeding 90%. However, when the triad of plain radiographic signs (absent half-moon sign, lightbulb sign, and rim sign >6 mm) is recognized concurrently, the diagnostic sensitivity rises to greater than 90%. Obtaining true orthogonal projections (Grashey true AP plus axillary lateral or Velpeau view) approaches 100% diagnostic accuracy on plain films, while cross-sectional CT achieves 100% sensitivity and specificity.

7. Differential Diagnosis & Radiographic Mimics

8. Clinical Significance & Management Implications

Early identification of the absent half-moon sign dictates immediate management and protects against severe long-term complications:

• Prevention of Chronic Locked Dislocation: Missed posterior dislocations rapidly transition into chronic locked dislocations within 2 to 3 weeks. Chronic dislocation leads to permanent articular cartilage destruction, severe capsulolabral contractures, and extensive humeral head osteonecrosis.
• Acute Closed Reduction Technique: In acute dislocations diagnosed within several days, closed reduction under conscious sedation or general anesthesia is indicated. Gentle longitudinal axial traction is applied in forward flexion with lateral translation of the humeral head. External rotation must never be forced while the arm is adducted, as the impacted reverse Hill-Sachs notch will leverage against the sharp posterior glenoid margin, causing an iatrogenic humeral neck fracture.
• Operative Pathway for Small Defects (<20% Articular Surface): Structurally stable following closed reduction; treated nonoperatively with immobilization in neutral rotation or slight external rotation (10 to 15 degrees) for 3 to 4 weeks.
• Operative Pathway for Moderate Defects (20% to 40% Articular Surface): Structurally unstable with high risk of redislocation. Surgical reconstruction is indicated, utilizing subscapularis tendon transfer into the defect (McLaughlin procedure), transfer of the lesser tuberosity with subscapularis insertion (modified Neer procedure), or anatomic osteochondral allograft reconstruction.
• Operative Pathway for Large Defects (>40% Articular Surface or Chronic Dislocation): Irreparable joint incongruity with severe risk of osteonecrosis. Primary indication for hemiarthroplasty, total shoulder arthroplasty, or reverse total shoulder arthroplasty based on glenoid bone stock and rotator cuff integrity.

9. Master Class Pro-Tip

Master Class Pro-Tip: The most catastrophic diagnostic pitfall in upper extremity trauma is mistaking a locked posterior shoulder dislocation for adhesive capsulitis or a severe contusion because the solitary AP radiograph appears superficially aligned. Whenever assessing an AP shoulder radiograph in a patient unable to externally rotate past neutral following a seizure, electric shock, or high-energy trauma, rigorously interrogate the medial glenohumeral overlap. If the classic 5 to 6 mm crescentic half-moon overlap shadow is absent, the joint is dislocated until proven otherwise. Never discharge the patient without obtaining a true axillary lateral or Velpeau view. During closed reduction under anesthesia, remember the cardinal rule: never forcefully externally rotate the adducted arm, as the impacted reverse Hill-Sachs notch will leverage against the rigid posterior glenoid rim and snap the humeral neck. Apply axial traction in forward flexion, gently translate the head laterally and anteriorly, and only then gently rotate into neutral.

Master Class Radiological Pearl: Avoiding the Missed Posterior Dislocation Trap

10. Evidence-Based Synthesis

11. References

1. Nobel W. Posterior traumatic dislocation of the shoulder. J Bone Joint Surg Am. 1962;44(3):523-538. DOI [Level IV: Case Series]
2. Schild H, Müller HA, Klose K. The halfmoon sign. Australas Radiol. 1982;26(3):273-277. DOI [Level IV: Imaging Investigation]
3. Cisternino SJ, Rogers LF, Stufflebam BC. The trough line: a radiographic sign of posterior shoulder dislocation. AJR Am J Roentgenol. 1978;130(5):951-954. DOI [Level IV: Case Series]
4. Kowalsky MS, Levine WN. Traumatic posterior glenohumeral dislocation: classification, pathoanatomy, diagnosis, and treatment. Orthop Clin North Am. 2008;39(4):519-533. DOI [Level IV: Narrative Review]
5. Rouleau DM, Hebert-Davies J, Robinson CM. Acute Traumatic Posterior Shoulder Dislocation. J Am Acad Orthop Surg. 2014;22(3):145-152. DOI [Level IV: Diagnostic Review]