Three hundred fourteen cone-beam computed tomograms (CBCTs) from 168 (53.5%) females and 146 (46.5%) males from the database of the Department of Cranio-Maxillofacial and Oral Surgery, Center of Dental Medicine, University of Zurich, Switzerland, from the years 2008 to 2013 were analysed. Patients were divided into 4 age groups: group 1 = 0–20 years, group 2 = 21–40 years, group 3 = 41–60 years and group 4 = 61 years and older.

Inclusion criterion was the presentation of the entire mandible. All images were taken with the 3D eXam CBCT (KaVo Dental GmbH, Biberach, Germany) and evaluated with the KaVo eXamVision Software (Version 1.9.3.13, KaVo Dental GmbH). The X-ray parameters were 120 kV at 3–8 mA (pulsed); image resolution was between 0.25- and 0.4-mm voxel side length. The data was processed in DICOM format on a PC (IntelR CoreTM i5-2500 CPU @ 3.3 GHz, 4 GB RAM, Windows 7 operating system) and evaluated on a diagnostic monitor.

Clinical indication for the CBCTs included the assessment of wisdom teeth, pre-implant assessment, diagnosis of intraosseous pathologies and exclusion of fractures. Exclusion criteria for this study were an impaired continuity of the IAN or an ambiguous course of the nerve and an irregular bone volume caused by pathologies in the area of the planned measurements.

The CBCTs were evaluated under standardized conditions by an investigator, being familiar with the software. First, the complete course of the IAN was evaluated in every CBCT, then bone thickness lateral to the nerve canal (spongious bone + cortical plate) was measured along the full length of the IAN. Measurements were performed at 2-mm intervals on both sides of the mandible in a sagittal direction along the whole course of the nerve, starting 2-mm distal to the posterior edge of the mental foramen on each side and ending at the mandibular foramen. At each measuring point, the lowest vestibular bone thickness (bt) was recorded as the shortest possible horizontal distance between the lateral wall of the mandibular canal and the buccal mandibular compact bone (see Fig. 1).