Classic Radiological Signs: Trauma

The posterior fat pad sign is highly specific for an intra-articular fracture. The posterior fat pad sits in the olecranon fossa and is normally NOT visible on lateral elbow X-ray. When visible, it indicates an effusion has elevated it out of the fossa - in the context of trauma, this means there is an intra-articular fracture even if not visible on X-ray. Management: Treat as an occult fracture with above-elbow backslab immobilisation, analgesia, and follow-up X-ray in 7-10 days when periosteal reaction may reveal the fracture. Consider the supracondylar region carefully as this is the most common site of occult fracture in children.

This is the FBI (fat-blood interface) sign, also called liphaemarthrosis. The horizontal layer shows fat floating on blood in the suprapatellar bursa. This is pathognomonic of an intra-articular fracture - the fat comes from bone marrow and can only enter the joint if there is a fracture communicating with the joint space. In the knee, the most common causes are tibial plateau fracture, distal femoral fracture, or patellar fracture. I would carefully review the X-ray for these fractures, and if not visible, CT would be indicated to identify the fracture. This finding confirms significant injury even if the fracture line is subtle.

This is the 'fleck sign' and it indicates a Lisfranc injury until proven otherwise. The fleck represents an avulsion fracture at the base of the second metatarsal where the Lisfranc ligament attaches, connecting the medial cuneiform to the second metatarsal base. This injury is commonly missed and has serious consequences if not treated. I would look for other signs: widening greater than 2mm between the 1st and 2nd metatarsal bases, malalignment of the metatarsal bases with their respective cuneiforms, and associated fractures. If X-rays are equivocal but clinical suspicion is high, weight-bearing X-rays (if tolerated) or CT would be indicated. Missed Lisfranc injuries lead to midfoot arthritis and disability.