Study subjects

Eleven participants, nine men and two women between 28 and 42 years of age (mean age 31.3 ± 4.6), were recruited as the study subjects. All participants provided informed consent verbally. Study information was disseminated to participants both verbally and in written form. The inclusion criteria were as follows:

• Generally healthy subjects

• Dentate subjects without ill-fitting restorations (≥ 24 teeth including both first upper molars)

• No dental treatment planned during the study

• Non-smoker or no use of tobacco for at least 6 months prior to the study enrolment

• No use of systemic antibiotics in the 6 months prior to the study enrolment or daily use of mouthwashes

Study design

An impression of the upper jaw from each participant was taken to create a hard resin splint (Palapress® vario, Heraeus Kulzer, Wehrheim, Germany), and six implants were mounted on both buccal aspects of each splint and bonded with resin caps that could hold the top and apex of the implant (Fig. 1). The implants used in this study were commercially available rough surface implants (GC Aadva® implant, GC, Tokyo, Japan, 3.3-mm diameter, 8-mm length, Sa value 2.0–2.3) and machined surface implants that were identical in design other than the surface roughness (not commercially available, Sa value 0.3–0.5) (Fig. 2). When the implants were mounted on the splint, the cover screws were tightened to prevent plaque accumulation inside the implant bodies. The participants were instructed to wear the splints for 24 h/day during the 4-day experimental period except for mealtimes. During eating and drinking, the splint was taken off and kept in a provided splint box to avoid drying. The participants were allowed to brush their teeth twice a day but were not allowed to use any kind of mouthwash during the entire experimental period.