Fat–Fluid Leveling in Acute Knee Trauma: A Critical Red Flag for Intra-Articular Fracture

Definition

Lipohemarthrosis refers to the presence of fat and blood within the joint space, resulting in a fat–fluid level due to density differences.
It is a strong indirect radiographic sign of an intra-articular fracture, even when no fracture line is visible.

Imaging Findings

• Standing lateral knee radiographs
  • Best projection for demonstrating fat–fluid leveling
  • Sensitivity reported up to ~95%
• Standing AP radiographs
• Fat–fluid level may also be visible
• Fat–fluid leveling is usually not detected
• Required in non-ambulatory patients
• Leveling is seen only on the lateral view
• Supine AP radiographs
• Supine cross-table lateral

⚙️ Pathophysiology

Following an intra-articular fracture:

• Fat from bone marrow rises to the non-dependent (lucent) portion
• Blood settles in the dependent (opaque) portion
• Upright positioning enhances gravitational separation, improving detectability

Key Teaching Points

• A visible fat–fluid level should prompt strong suspicion of an intra-articular fracture
• Absence of a fracture line does not exclude fracture
• Standing radiographs, when feasible, reduce the risk of missed occult fractures
• In non-ambulatory patients, true cross-table lateral imaging is mandatory

Take-home Message

• Fat–fluid leveling due to lipohemarthrosis is a red flag for intra-articular knee fractures.
  Whenever possible, obtain standing knee radiographs to avoid missed diagnoses.

·        ·  European Radiology
The utility of standing knee radiographs for detection of lipohemarthrosis
https://link.springer.com/article/10.1007/s00330-023-10163-8

·  Clinical Radiology
Fat–fluid level on standing lateral knee radiographs as a reliable indicator of occult fractures
https://www.clinicalradiologyonline.net/article/S0009-9260(25)00391-5/fulltext