The principle maxillary sinus ostium (PMO) and accessory sinus ostium (i.e., accessory ostium) are the anatomical structures of the middle meatus that are referred to as the osteomeatal complex. This complex contributes to the final drainage pathway of the anterior ethmoidal, maxillary and the frontal sinuses. The principle maxillary sinus ostium is present throughout the population; however, the accessory ostium is not. The accessory ostium only occurs in 30 to 40 percent of the population.
Typically, the maxillary sinus is located close to the alveolar ridge of the first molar and second premolars; therefore, it is not uncommon to find the roots of the first and second molars in the maxillary antrum. However, when these teeth are lost, the sinus expands into the remaining alveolar bone. As time passes, the floor of the sinus becomes the crest of the remaining alveolar ridge; thus, the level of the bone and the floor of the nasal cavity may be nearly identical. In a case such as this, before dental implants can be placed, bone grafts may be necessary.
See: Ostium (maxillary sinus)