Average ISQ tended to increase during the healing period in all IT groups (Fig. 5). Average ISQ of the low IT group was 59.81 at 0 week, increasing significantly after ≥ 8 weeks (P < 0.01). The average ISQ values of the medium and high IT groups at 0 week were 73.25 and 68.85, respectively. The average ISQ increased in a time-dependent manner at each group.

Total average ISQ in this study was 73.3 ± 9.6; therefore, an expedient reference value was defined as the ISQ 73, then the percentage of specimens showing ISQ ≥ 73 was determined (Fig. 6). ISQ ≥ 73 was observed from 8 to 12 weeks in the low IT group and from 4 to 6 weeks in the medium and high IT groups. Significant differences in the incidence of ISQ ≥ 73 was recognized at the medium and high IT groups (corresponding t test, P < 0.05).

Relationship between IT and voxel value

In this study, conditions and environments for CT imaging in each facility were different. Bone quality was investigated at 18 treatment areas (8 in the maxilla, 10 in the mandible) on CBCT performed under standardized conditions, as shown below. CBCT was performed using a 3DX Multi-Image Micro CT FPD 8 system (J. MORITA MFG., Kyoto, Japan) (tube voltage, 80 kV; imaging area, 80 × 80 mm), and voxel values were measured with coDiagnostix™ 9.7 (dental wings, Montreal, Canada).

There was no difference of average voxel value between the maxilla and the mandible (Fig. 7). In comparison between average voxel values and IT groups, the low and medium IT groups showed no significant differences, but the high IT group showed voxel values ≥ 40% higher than the other groups (Fig. 8). A significant difference was observed between the low/medium IT groups and the high IT group (P < 0.05). A significant difference was observed between the low/medium IT groups and the high IT group (P < 0.05). Also, voxel values at each part of the implant (neck, middle apex) were compared with IT < 40 and ≥ 40. The results suggested that the neck and apex parts in the ≥ 40 IT group showed significantly higher voxel values than the middle and apex parts of the < 40 IT group (P < 0.05) (Fig. 9).