In the present study, 64 CBCT images (128 maxillary sinuses), taken between 1 January 2013 and 31 December 2013 at the Department of Cranio-Maxillofacial and Oral Surgery at the University of Zurich, were included. The inclusion criterion of each CBCT scan was the presence of two complete maxillary sinuses; the osseus borders of both sinuses had to be entirely visible.

The scans were performed using a KaVo 3D eXam CBCT (Biberach, Germany). The settings were 5.0 mA and 120 kV, with a voxel size of 0.125, 0.25, 0.3, or 0.4 mm (exposure time 26.9 s for 0.3/0.4 voxel size or 26.9 s for 0.125/0.25 voxel size). The field of view (FOV) ranged between a height of 10–13.3 cm (patient-adjusted) with a constant diameter of 16 cm.

For the measurement of the maxillary sinus volume, the CBCT images were imported as DICOM files into SMOP, an implant planning software (Swissmeda AG, Baar, Switzerland). This software allows the calculation of volumes (mm³) and surfaces (mm²) of a 3D object (e.g., maxillary sinus) by interpolating closed curves.

Using this tool, the volume of each maxillary sinus was calculated by drawing parallel-oriented closed curves in the same coronal plane. Volume measurements were performed in a standardized manner. First, the most posterior and anterior part of each maxillary sinus was defined by placing a curve each in the coronal plane. Next, the space between the two curves was divided into equally sized slices of 2 mm by placing further curves. As a result, a single sinus consisted of 15–25 curves (depending upon the size of the maxillary cavity), having an intercurve distance of 2 mm (Fig. 1).

Some CBCT scans showed sinus cavities that were radiographically partially or fully obliterated indicating a swelling of the mucosa or a sinus pathology. In order to calculate the exact volume of this obliteration, two measurements were performed: first, the sinus volume within the osseus borders was measured by placing the curves on these osseus boundaries. Second, in cases of obliteration, the curves were placed on the mucous borders within the osseus maxillary sinus, measuring the remaining pneumatized cavity. Next, subtracting the two volumes, the obliterated sinus volume was calculated (Figs. 2 and 3).