A flat part of a pig's ilium was used as the implant placement site to secure as vertical implant placement and an equal depth of insertion to the bone surface as possible (Figure 1).

GXCB-500® (GENDEX, Des Plaines, IL, USA) was used as the CBCT device to obtain almost identical voxel values to the CT values that could be obtained from MDCT. Scanning conditions were as follows: the tube current, 5 mA; the tube voltage, 120 kV; the field of view (FOV), 85-mm diameter to secure a complete size for scanning; and the voxel size, 0.1 mm³. In addition, the scanning was performed by immersing the samples in a polypropylene container with water in it to create as similar scanning environment as possible to the one that had soft tissues around the bone model. Of note, the scanning was performed immediately just after the formation of the implant cavity to accurately measure the voxel values around the inserted implants.

External Hex Implants for laboratory use (SETiO®, GC Company, Tokyo, Japan) having a general threadlike shape with a mechanically polished surface were used. They were two kinds of diameters (3.8 mm, 5.0 mm) and two kinds of lengths (7.0 mm, 12.0 mm) consisting a total of four groups, and 25 pieces of implants were used in each group (Figure 2).

When forming an implant cavity, it is necessary to move the drill back and forth along the insertion direction. This process is usually done by an operator with freehand, but it is likely to cause unevenness in the diameter of the implant cavity. Therefore, an implant cavity-forming device was used to prevent this in our experiment (Figure 3). This device is able to adjust the up-and-down movement speed and the rotation speed of the drill. In this study, the implant cavity was constructed at a moving speed of 5 mm/s that was close to the actual up-and-down movement speed obtained from the preliminary experiment and at 800 rpm according to the manufacturer's protocol. It was also confirmed in the preliminary experiment that using this device would significantly reduce the unevenness of the diameter of the implant cavity.