"Blow out" fracture of orbit involve -

These generally result from blunt trauma to the orbit by a large object such as a cricket ball, tennis ball, human fist. The force of the blow causes a backward displacement of the eyeball and an increase in the intraorbital pressure; with a resultant fracture at the weakest point of the orbital wall. Usually this point is the orbital floor , but this may be the medial wall also. Ref: Parson's 22 nd edition , page no. 499