Ikumi used a MDCT scan for actual patients to calculate the CT values of the 1-mm surrounding area of the planned implant placement site using implant simulation software [25]. However, it is likely that the precise measurement cannot be performed in the case where the actual implant cavity was formed off the planned implant site because the density of the bone around the planned implant site was evaluated by preoperative simulation.

Therefore, we evaluated the bone quality 0.5 mm surrounding the implant, which was thought to have a strong effect on the primary stabilities, by CBCT scanning after forming the implant cavities.

Nkenke used an axial image to define the thickness of the cortical bone from the average thickness of the cortical bone around the implant measured by eye estimation and then evaluated its relationship with ITVs [26]. Whereas in this study, we measured the thickness of the cortical bone by setting the threshold for the voxel values around the implant for the purpose of securing reproducibility of the measurement of the thickness of the cortical bone. The threshold that we set was determined in the preliminary experiment so that it would highly correlate with the measurement values of the thickness of the cortical bone measured by eye estimation. Conversely, the limitation of this study is that the CBCT device is lacking some precision for estimation of bone density compared to quantitative computed tomography.

To date, there are several studies that investigated the relationship between the thickness of the cortical bone and the primary stability of the implant. Motoyoshi placed a total of 87 mini-implants, which were used as anchors for orthodontic treatment, in the buccal alveolar bone in the molar region of actual patients [27]. As a result, they reported that they found a positive significant correlation between the thickness of the cortical bone measured by using medical CT images and the torque value at the time of placement. Roze measured the thickness of the cortical bone of the jawbones in three human bodies using μCT and placed a total of 22 implants [28]. According to this report, there was a significant correlation between the thickness of the cortical bone and the ISQ values measured immediately after the implant placement. Furthermore, Isoda reported a significant correlation between the bone quality around the implants and the implant stabilities.