Why are knee x-rays performed for the purpose of trauma obtained as a cross-table shoot through?
Firstly the patient is usually not keen to stand, but more importantly it gives you the opportunity to visualise a fat-fluid interface of a lipohaemarthrosis, suggesting an intra-articular fracture.
What causes this appearance?
Avulsion fracture at the attachement of the arcuate complex.
What internal structure is most commonly injured?
Cruciate ligaments : posterior cruciate most commonly.
Best seen on the anterior projection is a small curvilinear calcific opacity immediately superior to the fibular styloid. The adjacent tibia is intact.