4.1 IT, RT, and ISQ Values

The primary stability of inserted implants in the two groups was evaluated by immediate IT, RT, and ISQ after their successful placement in the PFBs, and the details are listed in Table 2. Accodring to the Shapiro–Wilk test analysis results, the data are normally distributed. As shown in Table 2, the IT values in the OD group significantly increased. The IT values in CD and OD groups are 7.77 ± 0.21 and 11.73 ± 0.45, respectively, with a significant difference (p < 0.001). Likewise, the OD group has a higher RT value than the CD group (9.28 ± 0.45 and 6.65 ± 0.19, respectively), demonstrating a significant difference (p < 0.001). However, ISQ values in the four directions (buccal, lingual, mesial, and distal) show no significant differences between the CD and OD groups. The mean ISQ values for the CD and OD groups were 46.71 versus 47.08 (buccal), 45.67 versus 46.96 (lingual), 49.08 versus 49.71 (mesial), and 48.17 versus 49.92 (distal). The OD group consistently showed a slightly higher average ISQ value compared to the CD group.

4.2 Entrance Characteristics of Prepared Implant Sites

The entrance characteristics of implant sites were assessed using a material-type upright laser confocal microscope. In Figure 3, the entrance characteristics were varied when using different drills to prepare the implant sites under the same condition ($$N$$ = 300 r/min, $${F}_{z}$$ = 0.08 mm/z, depth = 10 mm, without irrigation). In the CD group, burrs, tears, and potholes were observed at the prepared sites. However, only a few burrs appeared in the OD group. During clockwise drilling, the cutting action of the tool generated burrs on the prepared surface. When the implant was inserted, burrs were further displaced due to torque, leading to tearing or the formation of depressions. In the counterclockwise mode, the tool compresses the material, causing localized fractures and adhesion of material to the prepared site walls. This compression increased the material's resistance to deformation during implant insertion, resulting in fewer occurrences of tearing or potholes, and only some burrs.

4.3 Spin Speed and Thrust Force

During drilling, the thrust force (in the axial direction) reflects different material machining stages. Figure 4a shows the thrust force of BLT drills under dry drilling during the following routine clinical procedures. With this method, the osteotomy is generally performed using bits of three different diameters (2.2, 2.8, and 3.5 mm). A 2.2 mm pilot drill bit was used to form a pre-hole. The 2.8 and 3.5 mm drill bits were used to form the final implant hole to complete the osteotomy. The A-I dashed line indicates the critical state when the pilot drill bit is ready to enter the PFB. The A-II dashed line indicates the state at which the drill bit reaches the bottom of the PFB. Moreover, A-Ⅲ represented the state of the drill bit as it exited the PFB. The thrust force produced by the BLT drill bits was relatively small, in the range of 0–6 N. The thrust force was 1.53 (pilot drill bit), 2.22 (2.8 mm drill bit), and 2.77 N (3.5 mm drill bit), respectively. However, using the same drilling parameters, the thrust force of the Densah drill bits is higher than that of the BTL drill bits (Figure 4b). The thrust force of the 1.6 mm pilot drill bit was 0.97–1.42 N (49.20–85.49%) higher than that of 2.2 mm pilot drill bit under the clinical routine method. The working mode of the pilot drill bit was clockwise cutting. The 2.2 mm BLT drill bit was built with more spiral grooves than the 1.6 mm Densah drill bit. As shown in Figure 4b, the thrust force has an evident tendency to increase. The thrust force of OD VT-1828 and OD VT-2838 was 0.32–0.67 N (21.10–61.58%) and 0.18–4.44 N (7.64–134.86%) higher than that of the 2.8 and 3.5 mm BLT drill bits, respectively. For drilling parameters: $$N$$ = 300 r/min; $${F}_{z}\,$$= 0.04 mm/z; implant depth: 16 mm. The thrust force produced by the BLT drill bits was 0–6 N. The thrust force was 1.53 (pilot drill bit), 2.28 (2.8 mm drill bit), and 2.65 N (3.5 mm drill bit). All three drill bits were rotated clockwise. Subtractive manufacturing has also been used. Force analysis of the PFB was conducted according to the theory of the three deformation zones. Because of the structural characteristics of the material, the cutting forces in the three deformation zones were relatively small. Therefore, the thrust force from the deformation zones was small.

However, the thrust force of Densah drill bits was 2.24 (pilot drill bit), 2.34 (OD VT-1828), and 2.99 N (OD VT-2838) (Figure 4c). In the osseodensification method, the tool is changed from clockwise to counterclockwise, and the material is squeezed by the tool instead of cutting. The thrust force increased by 46.41, 2.63, and 12.83%, respectively. For drilling parameters: $$N$$ = 1500 r/min, $${F}_{z}$$ = 0.04 mm/z; Implant depth = 16 mm. Thrust force was 0.44 (2.2 mm drill bit), 1.86 (2.8 mm drill bit), and 2.46 N (3.5 mm drill bit), respectively (Figure 4d). With an increase in the spin speed, the extrusion between the tool and material was reduced. Therefore, the thrust forces were reduced by 80.36, 20.51, and 17.73%, respectively.

The analysis in Figure 4e indicates that the thrust force initially increases and then decreases with rising spin speed (300–1500 r/min), for both BLT and Densah drill bits. At 300 r/min, the thrust force of Densah drill bits was approximately 47.22% higher than that of BLT drill bits, while at 800 r/min, it was 13.00% higher. At 1500 r/min, the thrust force for BLT drill bits was marginally higher (0.08%) than for Densah drill bits. The thrust force increased with feed per tooth for both BLT and Densah drill bits. At a 10 mm implant depth, the thrust force for BLT drill bits increased by 47.21% and 12.12%, while for Densah drill bits, the increase was approximately 22.73% and 23.72%, respectively.

4.4 Temperature

The temperature data collected in various intervals, i.e., 0–70 s, were used to analyze the heat product affected by different machining parameters. Figure 5A,B show the temperature changes under certain drilling parameters (300 rpm, 0.04 mm/z and 1500 rpm, 0.04 mm/z); Without irrigation, the average maximum temperature ($${{\rm{T}}}_{\max }$$) of BLT drilling reached 12.66°C at 300 rpm and 0.04 mm/z; at 300 rpm and 0.08 mm/z, the $${{\rm{T}}}_{\max }$$ slightly increased to 12.71°C. Conversely, at 1500 rpm and 0.04 mm/z, $${{\rm{T}}}_{\max }$$ increased to 10.91°C, demonstrating a 13.82% reduction attributed to decreased friction at higher spin speeds. During drilling with irrigation, as shown in Figure 6C, the $${{\rm{T}}}_{\max }$$ reached 10.67°C under 300 rpm and 0.04 mm/z, while under 300 rpm, 0.08 mm/z and 1500 rpm, 0.04 mm/z, the $${{\rm{T}}}_{\max }$$ reached 11.57°C and 10.39°C, respectively, reflecting the cooling effect of irrigation. The same trend was observed with increasing feed per tooth and spin speeds. Notably, irrigation reduced $${{\rm{T}}}_{\max }$$ by 15.72% (12.66°C–10.67°C) at 300 rpm and 0.04 mm/z, and by 8.97% (12.71°C–11.57°C) at 300 rpm and 0.08 mm/z.

For Densah drills, Figure 5G–L illustrates the same trends. Under drilling without irrigation, $${{\rm{T}}}_{\max }$$ was 12.74°C at 300 rpm and 0.04 mm/z, slightly increasing to 12.81°C at 300 rpm and 0.08 mm/z. However, at 1500 rpm and 0.04 mm/z, $${{\rm{T}}}_{\max }$$decreased to 12.62°C. This trend suggests that while higher feed rates slightly increase the heat due to thicker chips, higher spin speeds reduce friction, resulting in lower $${{\rm{T}}}_{\max }$$. With irrigation, the $${{\rm{T}}}_{\max }$$ reached 12.68, 12.07, and 12.05°C, respectively. The $${{\rm{T}}}_{\max }$$ for drilling with saline showed 5.26 (from 12.74°C to 12.68°C), 5.78 (from 12.81°C to 12.07°C), and 4.52% (from 12.62°C to 12.05°C) decreased. The use of saline effectively reduced the heat; however, less temperature decrease in the OD group was identified.

Figure 6 highlights temperature differences ($$\unicode{x02206}{{\rm{T}}}_{\max }$$) at various depths. For BLT drills, at a depth of 5 mm under drilling without irrigation, $$\unicode{x02206}{{\rm{T}}}_{\max }$$ decreased by 3.52% with increasing spin speed, attributed to reduced tool-material friction. A similar trend was observed at 10 mm depth. Under irrigation, at 5 mm depth, $$\unicode{x02206}{{\rm{T}}}_{\max }$$ decreased by 13.59%, 15.22%, and 16.49%, with the maximum reduction of 21.12% observed at 10 mm depth (from 3.5°C to 0.90°C). However, increasing feed per tooth slightly increased $$\unicode{x02206}{{\rm{T}}}_{\max }$$, attributed to the greater material thickness removed per revolution. For Densah drills, $$\unicode{x02206}{{\rm{T}}}_{\max }$$ followed a different trend under drilling without irrigation. Initially, it increased with spin speed due to extrusion friction, rising by 0.70% (from 3.94°C to 4.03°C at 5 mm depth). Beyond a threshold speed, $$\unicode{x02206}{{\rm{T}}}_{\max }$$decreased by 3.07% (from 4.02°C to 3.62°C), as reduced contact time lessened the extrusion effect. Under irrigation, $$\unicode{x02206}{{\rm{T}}}_{\max }$$ decreased by 14.51% (from 4.03°C to 2.14°C) at 5 mm depth. While the trend of temperature reduction with increasing spin speed was similar to BLT drills, the decrease for Densah drills was less significant.

As summarized in Table 3, both BLT and Densah drills benefited from saline irrigation in reducing thermal damage during dental implant site preparation. However, BLT drills showed greater temperature reduction efficiency compared with Densah drills. This highlights the importance of optimizing drilling parameters and irrigation methods to minimize thermal damage and improve clinical outcomes.

4.5 Optimized Drilling Parameters

Comparative analysis revealed distinct mechanical differences between OD and CD. The OD protocol generated elevated thrust forces and cutting resistance during cortical bone preparation, accompanied by increased thermal generation. However, this technique demonstrated superior performance in implant stability metrics, showing significantly enhanced insertion and removal torque compared to CD.

A three-dimensional function involving speed, feed per tooth, and heat production was established in this study to explore the optimal parameters, aiming to identify the optimal parameters that minimize heat production during drilling. Based on the results, this study suggested a set of parameters could be used in clinical OD performance: the spin speed and feed per tooth were set as 1500 rpm and 0.04 mm/z, respectively (Figure 7).