Although the torque and ISQ values of 06S were greater compared with those of 06D, only the former were statistically significant. These results suggest that although the difference between 06D and 06S in primary stability was small, the risk of slackening was significantly greater for 06D compared with that of 06S. Further, the discrepancy between the statistical significance of the torque and ISQ values may be partly explained by the lower sensitivity of ISQ compared with that of torque. The ISQ values of 06D and 06S compared with that of 12S were 5% and 8% higher, respectively, which were much lower than those of increased torque values. Sakoh et al., who evaluated the primary stability of a conical implant and a hybrid cylindrical screw-type implant according to torque and ISQ values, found that only torque values but not ISQ were significantly different. Resonance frequency analysis (RFA) using the Osstell System and Periotest is commonly used. However, neither system optimally measures stability nor defines a successful implant upon implant placement; and measuring IT prevails over the ISQ and Periotest.

The IT values reported here ranging from 13 to 22 N cm were much lower than the clinically recommended torque value of 35 N cm. The lower torque values can be explained by our use of artificial bone, mimicking type-4 bone, and a cylindrical non-self-tapping design limiting the interface between thread and bone. Moreover, the IT and RT values of 12S were consistent with those of the standard implant (Straumann) measured in our previous study.

Here, the RT value of each implant was lower compared with their respective IT values, consistent with other reports. The IT and RT values of 06S were highest, followed by 06D and 12S. In contrast, the differences between RT and IT values were highest for 12S, followed by 06D, and in 06S. We reported that the RT decreased more than IT. Thus, 06S had the lowest rate of decline (IT-RT) in the present study. Therefore, the single thread with a smaller pitch and helical angle is suitable for immediate loading. These inverse results may reflect the relation between lead angle or pitch length and the risk of slackening.

Thread design is a critical factor associated with primary implant stability. FEA loading studies show that relative vertical displacement is affected by thread pitch, torsion angle, and compactness. Under normal load, displacement was positively correlated with thread pitch and helix angle, and negatively with compactness. In low bone density jawbones, implant pitch, helix angle, and compactness have been reported to affect stability. Few studies have clarified the relationship between multiple threads and primary stability. A main advantage of an implant with multiple threads is quicker installation. However, this may be misunderstood by clinicians, because the double-threaded implant recommended by a manufacturer is used for all four types of immediate treatment loading. For example, our comparison of 12S to 06D led us to a different conclusion (i.e., better stabilities of 06D vs 12S and 06S vs 06D). When single- and double-threaded implants with the same pitch were compared, double-threaded implants were less stable because of greater damage to bone tissue damage, which is attributed to the high lead angle reported here as well as by others. Clinicians are advised to recognize the risk associated with using a multithreaded implant with a high lead angle, which may compromise primary stability because of greater bone tissue damage despite faster insertion. The question of placement speed warrants further consideration. Indeed, another advantage of double-threaded implants is placement speed. The implantation speed of 06D was twice that of 06S, and implantation was completed twice as fast. Nevertheless, while plastic bottles and emergency valves have double-threaded screws for faster opening and closing, the effect of placement speed for dental implants on initial stability parameters, such as torque and ISQ values, has not been investigated and remains unknown. It was also interesting that the debris generated with 06D were larger than those generated with 06S, despite higher placement speed. Implant design is the same as that of industrial screws with multiple threads that are mass-produced. This is required because the lead angle is large and the installation speed is fast. Multithreading allows for a lower number of rotations, so even if the installation speed is reduced, the implant is embedded faster compared with a single thread. The issue of placement speed on initial stability warrants further investigation.

Interestingly, not all surfaces of the implant body and implant cavity were in tight contact with each other, and gaps and bone fragments were observed in some portions. Although gaps are unlikely to be beneficial for initial implant stability, bone fragments are thought to be beneficial for bone union. However, it is unclear how the size and number of debris affect bone union. The present study found that different thread designs are associated with different sizes and numbers of debris, but how this affects bone union remains to be addressed.

Of note, artificial bone models simulate physical properties of the real bone, such as density, compressive strength, tensile strength, and elastic modulus, but are distinct in that they have an entirely homogeneous structure. Although bone models composed of a combination of cortical and cancellous bone are also available, this study used a single bone model with a homogeneous density to eliminate any effect of cortical bone and evaluate only the effect of design features on torque and ISQ values. Experimental implant placement using artificial bone models is generally conducted to simulate in vivo implant placement in the jaw bone, and must therefore be designed to provide clear insights into the effects of relevant factors. Regarding observation of the interface between the implant body and the artificial bone using divided blocks, the use of divided blocks is indispensable. To minimize artifacts due to division, the number of divisions should be minimized, and accordingly, two-block division is considered best. Although it is impossible to deny that debris is likely to occur even with two-block division, this is a common error that can occur in all test pieces, and the comparison of implant bodies is therefore considered possible.

Implantation using a bone model is different from in vivo implantation in that implantation is performed under a dry environment, the environmental temperature is not the oral but room temperature, and no physiological reactions occur, such as osteolysis and osteogenesis. Therefore, torque and ISQ values obtained with a bone model cannot be directly extrapolated into in vivo conditions, but can be relatively compared with the corresponding values obtained from other simulation studies under the same conditions. The present study was also conducted for such purposes.

Despite their advantage of faster insertion, double-threaded implants with a higher lead angle may have diminished primary stability, because they cause increase damage to bone tissue despite faster insertion.