Implants

The type of implant used for the experiments and the characteristics of its design are shown in Table 1 and Fig. 1, respectively. Figure 1 shows that the implant is compressed longitudinally to one third. The outer surface of the implant is indicated with a solid line, and the inner surface of the implant is indicated with a dotted line.

Preparation of an implant socket in artificial bone

For artificial bone, rigid polyurethane foam (Solid Rigid Polyurethane Foam 20 pcf; Sawbones; Pacific Research Laboratories Inc., USA) measuring 18 × 4 × 13 cm and approaching the maxilla molar part bone density (0.32 g/cc) and physical properties (compressive strength, 8.4 MPa; tensile strength, 5.6 MPa; shear strength, 4.3 MPa; coefficient of elasticity, 284 GPa) [24] was employed. An implant socket was formed by making an interval of more than 2 cm, while avoiding axis wobbling as much as possible, with a drill press (ASD-360; Ashina, Hiroshima, Japan) in the artificial bone.

Measurement of torque-time curves

For measurement of torque-time curves, a torque measurement system capable of high-speed sampling at 1 sample/ms (PC torque analyzer TRQ-5DRU; Vectrix, Tokyo, Japan) was used. The rotational speed at the time of insertion was 15 rpm, the load was 500 g, and the maximum torque value indicated in the torque-time curve (and following implant torque curve) when inserting the implant (Osseoset 200; Nobel Biocare Japan, Tokyo, Japan) was assumed as the insertion torque value. In addition, the removal torque value (RT) was obtained from the removal torque curve when the implant was reversed immediately after insertion.

Measurement of the rate of torque rise

The average torque rise rate (N · cm/s) in each region was obtained from the point that the origin and torque rose immediately after implantation, with both ends of the region indicating lines, both ends of the region indicating a quadratic function, and the torque values and implant time of both ends indicating a logarithmic function becoming gradual on the implant torque curve, and mean values and SD were calculated.

Statistical analysis

It was confirmed that the measurement results for the insertion and removal torque values of each implant were normally distributed, and their significant differences were examined by Student’s t test and the Tukey–Kramer method (JMP software; SAS Institute Japan, Tokyo, Japan). The significance level was set at p = 0.05.