The frontal right knee x-ray shows a fractured patella and an irregularity in the region of tibial spine.

The HBL left knee view shows air in the knee joint; this can be mistaken for a lipohaemarthrosis (fat-fluid level).

The patient underwent urgent surgery to washout the knee and fix the patellar fracture.

On a plain x-ray:

• Air appears black.
• Fat appears dark grey.
• Soft tissues appear light grey.
• Bone & metallic objects appear white.