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https://the.emergencyphysio.com/wp-content/uploads/knee-lip-lateral.png
What do you see?
There is no clear fracture line
Much like ice floats on water, fat also floats on water/blood because it is less dense.
An intra-articular fracture may allow for blood and fat to exit the bone marrow and settle in the joint space.
This is called a lipohemarthrosis.
Best seen with a cross-table horizontal lateral view x-ray.
Go back to the image and examine the supra patellar pouch.
Most commonly seen in the knee in presence of a tibial plateau fracture.
Seen in approximately 1/3rd of tibial plateau fractures
If you see this without a clear fracture, consider CT of knee which can help detect the hidden fracture.
Remember the DDx of knee hemarthrosis with negative plain films:
Meniscal tear, ligament tear (usually ACL), patellar dislocation and osteochondral fracture.
Except for meniscal tearing (>6h) these other pathologies cause rapid onset swelling (<2h).