1. Radiological Fat Pads: Elbow
Fat pads raise due to increased joint fluid pressure.
In trauma
– haemarthrosis
– hidden radial head or humeral supracondylar fracture
No trauma
- Inflammatory effusion
Anterior fat pad
Usually visible
Raised away from bone in abnormal film (sail sign)
Posterior fat pad
Always abnormal
Normal
Abnormal
wikipedia

2. Radiological Fat Pads: Calcaneal (Kagner’s) Fat Pad)
Causes: trauma, oedema and haemorrhage.
Suspect: Fracture, soft tissue injury, inflammatory condition, infection, neoplasm
Clearly delineated: normal
Distortion or obliteration of borders: pathological

3. Radiological Fat Pads: Knee
Normal horizontal beam
Lipid layer
Radiology materclass
Blood layer
Fracture with lipohaemarthrosis:
Blood and fat leak from bone marrow into joint space.
Alternatively: obliteration of the hypodense fat pads indicates effusion.
Tibial Plateau fracture

4. Radiological Fat Pads: Distal Forearm (Pronator Quadratus Sign)
Fat pad overlying pronator quadratus. Seen ~90% of the time.
Bowing, obliteration or displacement can occur in pathology.
radiopaedia
Wide ranges of sensitivities and specificities have been recorded for this sign. It is one of the less reliable fat pad signs.