The borders of the maxillary sinus are formed of thin cortical bone which appear as thin radiopaque lines on periapical radiographs. The size of the maxillary sinus varies considerably although the right and left sinuses are typically symmetrical. The floor of the sinus is seen on periapical radiographs near the apices of the molars and premolars, and may extend down as far as the crest of the alveolar ridge, particularly in edentulous areas. Radiopaque lines traversing the sinus either horizontally or vertically are septae, bony projections from the floor and wall of the antrum. Septae give the sinus the appearance of being divided into compartments, although this is not the case. The radiolucent compartments formed by the septae sometimes mimic periapical pathoses.
Occasionally small bone nodules on the floor of the maxillary sinus may imitate root tips. One way to differentiate the two is to look for trabecular pattern; a nodule will show trabecular pattern while a root tip will not.
The zygoma appears as a U-shaped radiopaque line with the round portion superimposing the area of the first and second molars. Depending on the angle in which the x-ray beam passes through the zygoma, it will vary in size, width, and definition.
The nasolabial fold may appear as an oblique line traversing the premolar region. The line of contrast is well-defined and the area of increased radiopacity is caused by the superimposition of the cheek tissue. This feature increases with age and can be used to identify the side of the maxilla if the area is edentulous.
The medial and lateral pterygoid plates lying immediately posterior to the maxillary tuberosities have a variable appearance(another view of the pterygoid plates), often not being visible at all. Typical appearance is a single radiopaque shadow with no trabecular pattern. The hamulus may be seen extending inferiorly from the medial pterygoid plate and does show trabecular pattern.