# Bone Temperature Variation Using a 3D-Printed Surgical Guide with Internal Irrigation
## 4 Discussion  
The surgical guide with internal irrigation investigated in the present study has been proposed with the intent to provide an additional source of irrigation of the surgical field. The built-in irrigation tubes were designed to reach the body of the drill, underneath the guide sleeve. To the best of our knowledge, no other studies on this type of 3D-printed surgical guide has been published with the exception of a case report, where a similar guide was described [27]. Supposedly, a guide with such design should overcome one of the major issues of computer guided implant surgery, that is the risk of bone overheating. This study, however, did not support the initial hypothesis that supplementary irrigation may reduce the bone temperature increase, as we did not find any significant difference in the largest temperature variation among the different groups. Figure 8: Cone beam computerized tomography (CBCT) images of one of the surgical guides after the experimental phase. Condensed bone tissue partially occupies the final parts of the built-in irrigation tubes. Figure 7: Boxplots showing the average temperature variation (\(\Delta\)T) with respect to starting temperature at (**a**) Tc1, (**b**) Tc2, and (**c**) Tc3 in the four groups. It was observed that the median highest temperatures were found in the cortical bone (Tc1) with the use of the first drill, i.e., Cortical Perforator drill, in the majority of cases (Figure 4). This fact, which is in accordance with previous studies, may be explained by the impact of a relatively small drill on intact highly dense bone [17; 28]. Cortical bone is denser and more resistant than trabecular bone; it has a high coefficient of friction and a low thermal conductivity, which hinders heat dissipation [29]. The initial drill, in fact, encounters greater resistance and must remove considerable quantities of cortical bone at the drill entrance point. Subsequent larger drills, on the other hand, induce lower temperature variations as they have to remove smaller quantities of cortical bone tissue and the larger flute spaces can facilitate irrigation and bone chips removal [18; 30]. The lower temperature variations recorded at Tc2 and Tc3, which were located in the medullary bone, can be explained by the lower density and the lower cutting resistance while drilling this tissue compared to the cortical bone [31]. In a study by [PERSON] et al., a conventional external irrigation from the hand-piece, an internal tube in the surgical guide and a drill with an internal cooling were compared. The findings showed that the guide with the internal tube was a superior irrigation method than the conventional external irrigation, while it was less efficient than the internal drill cooling [32]. In the present study, however, the use of the supplementary internal irrigation (Group 1) did not significantly reduce the temperature variation compared with an external irrigation only (Group 2). No significant differences on temperature variation were found between Group 3 and Group 4 as well. A tendency, however, can be outlined for the main outcome, presenting the Group 3 with the highest temperature increase and Group 4 with the lowest. According to the assumption that the limit condition for bone regeneration is 47 \({}^{\circ}\)C for 1 min [8], the magnitude of temperature increase during the drilling procedure in this study seems to be rather safe in all groups. According to the temperature variation observed in this in vitro study, the threshold value was never exceeded. On the contrary the maximum temperature increase was lower than 5 \({}^{\circ}\)C in most cases. Such values are in accordance with previous in vitro and ex vivo studies. Investigating the effect of drilling speed and irrigation liquid temperature during guided and freehand implant surgery in bovine ribs, [PERSON] et al. recorded a maximum bone temperature increase of 2.10 \({}^{\circ}\)C [33].