The temperature variations occurred during the initial implant site preparation were examined. Drilling insert type, drilling depth, saline volume and saline temperature were the variables which were considered. The mean artificial bone sample baseline temperature was 22.6 ± 1.9 °C in both the ROI.

Overall, both the maximum recorded temperature (MT) and the highest temperature variation (ΔT) were observed in Group H (p < 0.001) with a mean value of 87.27 ± 38.42 °C and 64.59 ± 38.37 °C, respectively. Conversely, Group C recorded the minimum temperature increase or a decrease in temperature itself and Group A the smallest ΔT (p < 0.001), 19.33 ± 0.07 °C and − 1.09 ± 0.91 °C, respectively (Fig. 2).

In this figure, boxplots of the recorded temperature variations (ΔT), in °C, are showed, accordingly to the different study groups and drilling depth (4.5 and 9 mm). In particular: a shows ΔT at 4.5 mm in Groups A, B and C; b shows ΔT at 4.5 mm in Groups D, E and F; c shows ΔT at 4.5 mm in Groups G, H and I; d shows ΔT at 9 mm in Groups A, B and C; e shows ΔT at 9 mm in Groups D, E and F; f shows ΔT at 9 mm in Groups G, H and I

Table 1 shows the results derived from the analysis of temperature recorded during the 35-fold drillings. Generally, considering the same cooling saline temperature, it was not observed a statistically significant difference between the single and the double cooling system (p > 0.05). Conversely, temperature differed significantly when comparing the cooled saline with the not cooled one (p < 0.01). When considering Groups A, B, C, D, E and F, a temperature ≥ 40 °C was never recorded.

According to Spearman’s correlation, in Group A, strong correlations between ΔT at 9 mm and drill utilization times and MT at 9 mm and drill utilization times were reported (rho = 0.58, p < 0.001 and rho = 0.54, p < 0.01, respectively), while weak ones were observed between ΔT at 4.5 mm and drill utilization times and MT at 4.5 mm and drill utilization times (rho = 0.34, p < 0.05 and rho = 0.35, p < 0.05, respectively). In Group B, a strong correlation was only observed between ΔT at 9 mm and drill utilization times, resulting in a rho = 0.54, p < 0.001. In Group C, MT at 9 mm and drill utilization times were weakly correlated (rho = 0.34, p < 0.05). When considering Group D, a strong and a weak correlation were observed, respectively, for ΔT at 9 mm and drill uses (rho = 0.46, p < 0.01) and for MT at 9 mm and drill uses (rho = 0.37, p < 0.05). In Group E, ΔT at 9 mm and drill utilizations were strongly correlated, rho = 0.47, p < 0.01. In Group F, a weak correlation was detected between MT at 4.5 mm and drill usage (rho = 0.36, p < 0.05). When considering Groups G, H and I no correlations were observed among the studied variables (p > 0.05).

Drills’ wear was assessed using SEM at baseline (Fig. 3a) and after 10 (Fig. 3b) and 35 utilizations (Fig. 3c).

a Scanning electron microscope analysis of the used drills at baseline. No signs of wear were detected; b Scanning electron microscope analysis of the used drills after 10 uses. No signs of wear were detected on the cutting edges of rotary drills. Little presence of smear layer on the top of piezo-insert (*); c Scanning electron microscope analysis of the used drills after 35 uses. No signs of wear were detected on the cutting edges of rotary drills. Moreover, it was possible to notice the presence of water trees on the superficies of rotatory drills (§). Presence of smear layer on the top of piezo-insert (*)

Surface cutting edge degradation was evaluated in terms of influence of wear on heat genesis. As shown in Fig. 3, analyses were carried out using × 50, × 100 and × 500 magnification. Images were visually evaluated. No superficial differences were noticed among the drills at baseline and after 10 utilizations, apart from the little presence of a sort of smear layer on the top of piezo-inserts. The backscattered electron (BSE) detection showed the presence of elements with different atomic weight. Subsequentially, the energy dispersive X-ray analysis (EDX) was performed on the obtained images and apart from the chemical elements which compose steel (Iron, Manganese, Chromium, Silicon, Nickel, Aluminum, Sulfur, Nitrogen and Carbon), the presence of organic substance was revealed (Carbon, Phosphorus and Chlorine). The latter was related to the artificial bone block.

Analyzing drills’ surfaces after 35 utilizations, no signs of wear were detected on rotatory drills groups. When considering piezo-surgical insert groups, a bigger amount of smear layer was noticed, especially on the top of the insert. The BSE detection and the EDX analysis confirmed the presence of the organic substance that could be related to the artificial bone block.