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 [21] 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, osteoporosis, mental disorder, alcohol- or drug dependence, or smoking habit; failure to follow treatment directions; presence of severe periodontal disease, shedding disorder, trismus, malocclusion, or bruxism in the oral cavity; or failure of implant treatment.

The implant body used in this study was the Genesio® Plus implant with Aanchor surface (GC, Tokyo Japan). The implant body had been processed to create a rough surface by sandblasting and acid etching (Fig. 1). The implant body for the treatment of each participant was selected from two diameters (3.8 or 4.4 mm) and three lengths (8.0, 10.0, or 12.0 mm). The treatment area and the implant size used in this study are shown in Table 1.