The path of least resistance in the maxilla is the floor of the sinus

This is probably the single most important thing to learn as a practicing clinician, who does not utilize a CT scanner. Posterior maxillary peri-apicals are generally useless as they do not convey the proper information due to overlap. It is impossible to see a perforation of the sinus floor with a PA. Moreover, patients are asymptomatic as it is draining so there is no chance of pressure build up in the maxillary antrum