What will be seen on a lateral X-ray in a patient with painful elbow swelling after a fall?

Posterior Fat Pad Sign on Lateral X-ray in Elbow Trauma

In a patient who fell on their elbow with painful swelling, the lateral X-ray will most likely show an elevated posterior fat pad (option D), which is a reliable indicator of an occult elbow fracture. 1

Radiographic Findings in Elbow Trauma

Fat Pad Signs

• Posterior Fat Pad Sign:

  • Normally, the posterior fat pad is hidden within the olecranon fossa
  • When an elbow joint effusion occurs after trauma, the posterior fat pad becomes visible as it is displaced from the olecranon fossa
  • This sign has high predictive value for occult fractures, with studies showing up to 76% of children with this sign having an underlying fracture 2

• Anterior Fat Pad Sign:

  • While the anterior fat pad is normally visible (sail sign), its elevation or displacement also suggests joint effusion
  • However, the posterior fat pad sign is more specific for fracture

Clinical Significance

The American College of Radiology guidelines state that conventional radiographs are the first-line imaging modality for acute elbow pain following trauma 1. An elbow joint effusion identified on radiography with posterior and anterior fat pad elevation strongly implies an occult elbow fracture, particularly in the context of acute trauma.

Common Associated Injuries

When a posterior fat pad sign is present, the most common occult fractures include:

• Radial head/neck fractures (most common in adults, accounting for 50% of elbow fractures) 1
• Supracondylar fractures (especially in children) 2
• Coronoid process fractures
• Olecranon fractures

Diagnostic Approach

1. Initial Assessment: Lateral radiograph is crucial for visualizing the posterior fat pad
2. Additional Views: AP and oblique views should be obtained but may not show the fracture
3. Further Imaging: If radiographs show a posterior fat pad sign but no visible fracture, consider:
   • CT scan for suspected occult fracture 1
   • Follow-up radiographs in 2-3 weeks to look for healing response 2

Management Implications

The presence of a posterior fat pad sign should prompt treatment as if a nondisplaced fracture is present, even when no fracture is visible on initial radiographs 2. This typically involves immobilization and appropriate follow-up.

Common Pitfalls

• False negative results may occur if the elbow is not properly positioned at 90 degrees flexion 3
• Failure to recognize this sign may lead to missed fractures and improper management
• Point-of-care ultrasound can also identify the posterior fat pad sign when radiographic findings are subtle 3

The posterior fat pad sign is a valuable indirect sign of occult elbow fracture and should prompt appropriate management even when no fracture is directly visible on initial radiographs.