## 4 Discussion
The authors pointed out two potential causes of the inefficacy (or even a potential disadvantage) of the internal cooling system: (i) the internal coolant duct was partially clogged by bone chips (as revealed by CBCT), and (ii) the irrigation fluid delivered by the physiodispenser was distributed between the external and the built-in coolant pipes--therefore, the pressure gradient along the internal coolant duct could have been too low [23]. We can further speculate regarding the poor performance of internal cooling in the experiments of [23]. Although it was carefully conducted, with surgical templates specifically designed to fit each bone piece, it is not clear whether the precise fit did not impede the evacuation of the internally routed irrigation fluid. Using both internal and external irrigation, it was not possible to separately observe the flow of the fluid that exited the internal coolant duct. [PERSON] et al. tested a worst-case scenario of in vitro osteotomies performed in bovine metatarsal sections, along the cortical layer [24]. At a depth of 6 mm, the mean temperature rise caused by a drill of 1.9 mm in diameter was 26.4 \({}^{\circ}\)C for the conventional surgical template and 5.8 \({}^{\circ}\)C (significantly lower, \(p\) < 0.05) for an innovative surgical template with two internal coolant pipes that targeted the drill's point of entry. Moreover, the authors created an actual dental implant drill guide of a similar concept and demonstrated its clinical benefits [24]. In an experimental model inspired by reference [23], [PERSON] et al. clarified the role of coolant evacuation in the context of internally conducted irrigation [25]. Guided drilling performed with surgical guides that had both an entry channel and an exit channel built into the guide resulted in a mean temperature rise of less than 2 \({}^{\circ}\)C at 6 mm depth and 1 mm from the osteotomy margin. In the absence of the exit channel, the mean temperature elevation was higher by about 0.5 \({}^{\circ}\)C, and this difference was statistically significant [25]. Compared to our previous study of the thermodynamic performance of surgical guides with internal cooling [26], the present work looked at a different, sequential drilling protocol. Moreover, instead of porcine femur pieces, here we used standardized bone models that replicate the bone density of the human mandible (D1 for the cortical layer and D2 for the cancellous regions [29]). Unlike in [26], the mean temperature rise observed in the case of the surgical guide with internal cooling was significantly lower than in the case of the classical guide for each of the three drills (Figure 6). A potential cause of this discrepancy might be the lack of biological variability in the present investigation. Nevertheless, these two studies have the common feature that the guide with internal irrigation tubing ensures a narrower distribution of the temperature elevations than the classical guide, which indicates a more consistent cooling performance (compare Figure 5 of reference [26] with Figure 5 of this article). The standard deviations of the temperature elevations were relatively high compared to the means, especially in the context of drillings performed with classical guides (Figure 6). The large coefficients of variability indicate that, even in the absence of biological factors, many sources of error are involved in these measurements. We sought to control several experimental conditions (specimen temperatures, drilling speed, axial force, coolant flow rate, osteotomy locations, thermocouple positioning), but vibrations caused by drilling might have affected the thermal contact between the thermocouple and the bottom of its housing. Also, the precise path of the irrigating fluid was beyond our control. The present study suggests that an internal coolant pipe brings about a significant decrease in bone heating during guided osteotomy even in the absence of a dedicated irrigating fluid evacuation channel, provided that the surgical template's bottom does not rest on the target surface.