During the measurement process, when the insertion load was gradually reduced in 0.5 N increments, a threshold was reached beyond which either implant insertion could no longer be accomplished or the implant exhibited slippage at the entrance of the bone despite maintaining the predetermined rotational speed (Fig. 4). The minimum insertion loads were 2.0 N for S12, 3.5 N for S06, and 4.0 N for D06, with significant differences among them (p < 0.05).

An example of a torque‒time curve of the blue line on an implant under free-spinning conditions for placement

For both the ITV and RTV, across all the loads (minimum insertion load, 5.0 N, 10.0 N, and 15.0 N), S06 presented the highest values, followed by D06 and then S12, with significant differences observed among the implant types. In contrast, no significant differences were observed among the various insertion loads within each implant type or between the ITV and RTV (p < 0.05) (Figs. 5 and 6).

ITV at different insertion loads

RTV at different insertion loads

The recorded torque–time curves can be categorized as shown in Fig. 7. For all the implants, as the insertion load decreased, the torque–time curves shifted to the right (Figs. 8a, b, and c). For S12, the slope of the linear portion of the torque–time curve was consistently approximately 0.36–0.39 N cm/sec regardless of the insertion load. However, the time taken for the torque to begin rising linearly varied. At the minimum insertion load of 2.0 N, a delay of 5 s was observed relative to the other loads (Table 1). The insertion time, which is the time required to reach the maximum torque value, was the longest at an insertion load of 15.0 N, followed by 10.0 N, 5.0 N, and then the minimum insertion load of 2.0 N (Fig. 8a).

Pattern analysis of the insertion torque–time curve for minimizing the insertion load. The figure illustrates the torque–time curves of three types of implants as an example under minimum insertion load conditions. The figure shows the initial insertion torque point (red arrow), the insertion-starting point (black arrows), and the insertion endpoint (white arrows). From the initial insertion torque point to the insertion-starting point, the curve represents the free-spinning zone, during which the implant does not engage with the prepared site, resulting in no increase in the insertion torque, indicating a free-spinning state. From the insertion starting point to the insertion endpoint, the curve represents the constant insertion zone, where the implant is steadily inserted into the prepared site, and the insertion torque consistently increases. The insertion process is completed at the insertion endpoint, and the time from the initial insertion torque point to the insertion endpoint is defined as the insertion time

The torque‒time curves for different insertion loads at SI2, D06 and S06. a: S12, (b): D06, (c): S06

For D06, similar to S12, the torque–time curves shifted to the right with decreasing insertion load (Fig. 8b). The slope of the linear portion was 0.63–0.68 N cm/sec. Differences in the time required for the torque to rise linearly were observed. A delay of 14 s was noted at the minimum insertion load of 4.0 N, and a delay of 6 s was noted at an insertion load of 5.0 N (Table 2). The trend in the insertion time for D06 was similar to that observed for S12, with longer times at 15.0 N, followed by 10.0 N, 5.0 N, and then 4.0 N.

For S06, the slope of the linear portion was similar to that of S12 (approximately 0.37–0.39 N cm/sec). Additionally, a shift of 20 s was observed at an insertion load of 3.5 N, and 9 s was observed at 5.0 N (Table 3). The insertion time for S06 followed the same trend, being the longest at 15.0 N, followed by 10.0 N, 5.0 N, and finally 3.5 N (Fig. 8c).

When the insertion load was varied from the minimum insertion load to 15.0 N, the overall shape of the torque–time curves differed depending on the implant design. However, the slopes of the linear portions remained unchanged. A rightward shift of the torque–time curves was observed at lower insertion loads, with maximum time shifts of 5 s for S12, 14 s for D06, and 20 s for S06. Furthermore, in all three implant types, increasing the insertion load was associated with a higher initial insertion torque value, which is defined as the torque value observed immediately after insertion.

At the contact interface between the coronal portion of the implant and the artificial bone, cavities created by the passage of the leading thread were observed (Fig. 9). These cavities indicate the position at which the thread tip engaged the artificial bone, and with increasing insertion load, the cavity was observed to shift further in the apical direction. At the minimum insertion load, the depth of thread engagement corresponded to approximately half the lead of each implant (Fig. 9). At an insertion load of 15.0 N, the void created by the passage of the first thread was located deeper than the lead length of the implant. At the interface between the implant body and the artificial bone, voids, which are located primarily in the thread troughs, resulting from compression-induced fracturing and the subsequent displacement of artificial bone fragments, were observed. The number of voids measured at the minimum insertion load, 5.0 N, 10.0 N, and 15.0 N, was as follows: for S12, 6, 7, 7, and 7; for D06, 14, 15, 15, and 18; and for S06, 8, 9, 14, and 14 (Fig. 10). The total void area increased proportionally with the insertion load (Fig. 11), and the void area for D06 increased to a greater extent than those for S12 and S06. Furthermore, the rate of increase in void area when the insertion load was increased from 5.0 N to 10.0 N was greatest at 48.2% for S12, 21.0% for D06, and 33.3% for S06.

Artificial bone‒implant interface at the upper region of the implants under various insertion loads (magnification, × 50). This figure illustrates the bone‒implant interface at the upper region of the implants (S12, D06, S06) under different insertion load conditions: minimum insertion load, 5 N, 10 N, and 15 N. The minimum insertion load for each implant is as follows: S12: 2.0 N, D06: 4.0 N, S06: 3.5 N. The arrows indicate the voids at the first thread that passed during implant insertion. The values (e.g., 596.25 μm, 421.88 μm) represent the depth of the contact area at the bone–implant interface under each load condition

Void area at the bone‒implant interface for each implant and insertion load (magnification, × 20). This figure shows the void area at the bone‒implant interface for three implant types (S12, D06, and S06) under various insertion load conditions: minimum insertion load, 5 N, 10 N, and 15 N. The minimum insertion load for each implant is as follows: S12: 2.0 N; D06: 4.0 N; and S06: 3.5 N. The values displayed in the red area (e.g., 152 μm2, 520 μm2) represent the measured void area for each thread at the bone–implant interface under the respective load conditions

Total void area at the bone‒implant interface under various insertion loads for different implant types