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What is the diagnosis, and what is the most likely mechanism?
There is an avulsion fracture of the PCL at its insertion on the posterior tibial eminence, most likely secondary to an impact of the tibia on the car's dash, translating the tibia posterior relative to the femur. This mechanism makes sense because this fracture involves the non accelerating leg, which is typically flexed in position while driving, creating a good bio-mechanical scenario for shearing the tibia posterior relative to the femur in a frontal impact situation.
There is a lucent line traversing the posterior tibial eminence with cortical disruption and mild offset of the fragment. A small suprapatellar effusion is noted.