The maxillary sinuses are behind the cheeks on the inner side by the nose. They are the largest sinus cavity.
Posterior simply refers the back portion of the maxillary sinus. The front portion is the anterior. The maxillary sinus is one open space, so there is no tissue separating the anterior and posterior sections.
The sinuses control the temperature of air we breathe, heating it before it enters the lungs. They are also shock absorbers. If the face is injured by traumatic force, the sinuses act as crumple zones, much like a car's bumper and body. They minimize injury to the brain and other soft tissues.
The maxillary sinus can become inflamed by sinus infections. They can also have tumors or polyps grow inside them. Pressure behind the cheeks and thick green or yellow discharge are signs that something is wrong with the maxillary sinuses.
If only part of the maxillary sinus is affected, such as the posterior portion, a doctor may need to perform a CT scan to get a look at the entire sinus. Simply viewing the sinuses through an otoscope, a medical device with a light and magnifying lens, does not give the doctor a view of all of the sinuses.