These three cross-sectional images, in which the thickness of the Schneiderian membrane was previously measured, were also used for the measurement of the angle of the maxillary sinus. A segment DG (point D is the deepest point of the floor of the maxillary sinus) is created, vertical to the horizontal plane with stable length equal to 9.9 mm. The mean of 9.9 mm was chosen, because of a limitation of the NNT® software’s limitation. A linear segment EGF (point E is the point where the segment EGF intersects with the buccal wall of the maxillary sinus, and point F is the point where the segment EGF intersects with the lingual wall of the maxillary sinus) is created which vertical to segment DG and parallel to the horizontal plane. The points D, E, and F designate a triangle named DEF. The angle which is measured is EDF (Fig. 3).

Using the built-in ruler of the NNT Viewer®, we measured the angle EDF for each cross-sectional image created by the aforementioned method. This angle indicates the width of the maxillary sinus in each location. As this angle increases, so does the width of the maxillary sinus.

According to the value of this angle, the sinus width was classified in one of the three following groups (small-, moderate-, and large-sized sinus):

Angle ≤ 65^o: small

65^o < angle < 80^o: moderate

Angle ≥ 80^o: large

The CBCT images were initially evaluated by an oral surgeon and then they were re-evaluated and measured by two experienced dentists who were involved in the patients’ treatment and follow-up. In case of any disagreement, both parties re-evaluated and remeasured the related CBCT imaging.

All analyses were performed using Stata version 13.1 (Stata Corp., TX, USA).