Research design and study participants

This prospective study was conducted jointly by Tokyo Dental College (Tokyo, Japan) and Fukuoka Dental College (Fukuoka, Japan) from January to December 2015. All study protocols were conducted in accordance with the Declaration of Helsinki and were approved by the ethics committees of Tokyo Dental College (approval #416) and Fukuoka Dental College (approval #213).

Participants comprised patients at Tokyo Dental College Hospital or Fukuoka Dental College Hospital who were ≥ 20 years old, desired implant treatment in the posterior region, and consented to the details of the study protocols. The participants of this study were selected without randomization.

In this study, implant treatment was performed on 33 tooth extracted sites. The reasons for tooth extraction were periodontal diseases (14), caries (12), root fractures (7), and teeth had already been extracted (8). Tooth extraction was carried out with normal technique without socket preservation method. All participants were followed up for more than 4 months after tooth extraction and X-ray examined with multi-slice CT (MSCT) or cone beam CT (CBCT). Consequently, it was confirmed that sufficient bone mass exists to insert the implant body without bone augmentation in all treatment site. CT imaging equipment was different for each facility.

Exclusion criteria were untreated systemic disease, diabetes, cardiovascular disease,

Evaluation of treatment

IT, ISQ, and voxel value were measured in this study. IT was measured immediately after the implant insertion using a torque wrench (GC, Tokyo, Japan).

ISQ was measured throughout the experimental period. To measure ISQ, a Smartpeg Type 21 (Osstell AB, Gothenburg, Sweden) was connected at 5 N cm to the implant body, measured using an Osstell ISQ™ (Osstell AB) three times from the buccal side. Average values were used for the evaluations. ISQ was measured immediately (0 week), and 4, 6, and 12 weeks after surgery in all cases, and also at 2 and 8 weeks after surgery where possible.

In the following cases, the implant body was excluded from the evaluation.

• If motion and/or rotation was observed in the implant body.

• If the bone surrounding the implant body showed absorption.

• If inflammation was observed in tissue surrounding the implant.

• If a mandatory ISQ measurement was not performed.

In this study, we performed X-ray image diagnosis using a multi-slice CT (MSCT) or a cone beam CT (CBCT) to confirm the healing of the bone form and volume after the tooth extraction. CT imaging equipment was different for each facility (two models of CBCT and one model of MSCT). It was difficult to make the same evaluation on voxel values obtained from different equipment. The X-ray examination performed in different two models of CBCT at 18 treatment sites and 8 treatment sites. Seven treatment sites were X-ray examined in MSCT. Therefore, bone quality was investigated at 18 treatment areas (8 in the maxilla, 10 in the mandible) on the CBCT performed under standardized conditions, as shown below.

The 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). The voxel values were calculated based on CT images for bone quality diagnosis. Voxel values were measured three times at 12 locations covering the mesial, distal, buccal, and lingual sides of each of the neck, middle, and apex parts of the implant treatment area, then average voxel values for each part and for the whole treatment area were calculated (Fig. 2).

Statistical analysis

All numerical data obtained in this research were statistically analyzed with one-way analysis of variance and multiple comparison tests by Bonferroni et al. Fisher’s exact test and the corresponding t test were used for statistical analysis of insertion torque values and the relationship between ISQ and insertion torque, respectively.