One of the hottest topics in dentistry is the perforation of the sinus floor that we find in posterior maxillary teeth.
The path of least resistance in the upper arch is usually the floor of the sinus. Rarely do we see a fistula on the palate. It can happen, and we can see the purulence on the buccal as well, but most of these draining lesions burrow a path through the floor of the sinus.
If there is good drainage through the osteum the patient generally has no symptoms. So most people can have issue with necrotic teeth or failing root canals and be completely unaware of it. This video below shows a comparison of the two maxillary sinuses.
It should be readily apparent that peri-apical x-rays do not reveal this type of information