The study sample included 76 patients, of which 39 were females and 37 were males. In total, 120 sinuses (44 both left- and right-sided, 21 right-sided, and 11 left-sided) were evaluated as suitable for the present study and were measured. The total sample was classified in four age groups (below 45 years, 45–54 years, 54–64 years, and over 65 years of age). The mean age value of the sample is 58 years (ranging from 19 to 65 years of age).

All patients were Greek adults, and all CBCT examinations were performed to evaluate the posterior maxilla for future implant surgery. All patients were either patients of our clinic or had been referred to our clinic by their private dentist for implant or other pre-implant surgery. The minimum acceptable alveolar process was 5 mm. Patients with previous implant therapy or/and maxillary sinus augmentation in their history as well as patients with active periodontal disease, cysts, polyps, sinusitis, allergic rhinitis, or other pathological entities in their maxillary sinuses were excluded from our study. Other exclusion criteria were the presence of systematic diseases impacting the metabolism and quality of the bones in patients’ history, such as thyroid disease, hyperparathyroidism, diabetes, chronic renal disease, osteoporosis, and no development or acquired craniofacial or neuromuscular deformities. None of the patients previously or currently received medication including vitamin D, human growing hormone (HGH), or bisphosphonates. All the collected data were anonymous.

The CBCT images were obtained with NewTom VGI Tomograph (NewTom, Verona, Italy) with a voxel size of 0.3 mm. Operating parameters were set at 5.28 mA, 110 kV, exposure time 3.60 s, and field of view (FOV) of 15 × 12 or 12 × 8 cm. Panoramic reformatted images with thickness of 1 mm were chosen for the present study.

All measurements were performed in millimeters using the ruler contained in the NNT Viewer®.

The distance between the medial and the lateral walls of the maxillary sinus in the panoramic reformatted images was divided in four equal segments. Thus, three points were created; AR, BR, and CR for the right sinus and AL, BL, and CL for the left sinus (Fig. 1). For each of the aforementioned points, a cross section of 1 mm thickness was performed in the middle of the alveolar bone. In the cross-sectional images, the thickness of the Schneiderian membrane was measured in the deepest point of the sinus floor (point D). Thus, three different measurements were performed for each sinus (Fig. 2).