# Bone Temperature Variation Using a 3D-Printed Surgical Guide with Internal Irrigation
## 4 Discussion
In another ex vivo study in porcine ribs, bone temperature with open-flap and flapless guided implant surgery registered a maximum temperature increase of 4.81 \({}^{\circ}\)C [20]. It must be said that the actual maximum bone temperature at the site of drilling cannot be registered with thermocouples. The probe tips were located at 1.5 mm from the heat source, corresponding to the nominal implant surface. Bone has low thermal conductivity properties, and we could expect a low grade of heat distribution through the bone [34]. Therefore, temperatures at the immediate vicinity of an osteotomy may have been much higher, as demonstrated by [PERSON] et al. [31]. In this study, the persistence of an increased temperature was assessed by the average temperature increment from the starting temperature. Values of Group 1 were significantly higher than Group 2 in Tc2 and higher than groups 2, 3, and 4 in Tc3. This difference, even though of limited amount, may indicate that there was a heat concentration in the medullary bone and more specifically at the apex of the preparation for Group 1. It was speculated that drilling preparation caused the clogging of the internal irrigation tubes aperture facing the bone, as previously reported [11]. This theory was confirmed by CT observations carried out after the experimental phase. As matter of fact, the final parts of the built-in irrigation tubes were impacted by condensed bone tissue (Figure 8). This obstructive material could have negatively affected the temperature variation in two ways, since it would have impeded the irrigation flow from the internal tube and increased the friction with the drill. To reduce this phenomenon, it could be postulated that a more powerful irrigation should be performed. In the setting of the present study, the irrigation fluid flow originated from the hand-piece and it was distributed among the external and the internal built in tubes. It can be speculated that the fluid pressure at the end of the tubes was too weak. Another source of irrigation, distinct from that provided in the hand-piece may be recommended. Moreover, the drilling method may have contributed to the creation of the clog. Even though a recent study demonstrated that the intermittent drilling movement during a conventional osteotomy did not decrease the bone temperature [35], this method may be relevant during guided surgery, since it increases the irrigation flow and remove a major amount of bone debris from the drilling site [36]. With the present study, the novel irrigation method was tested in an ex vivo model using bovine ribs. This model was chosen since it resembles the morphological characteristics and thermal properties similar to human bone [34]. 3D-printing technology was used to fabricate surgical guides specifically designed for each experimental ex vivo substrate. Thanks to this approach, thermocouples positioning, which usually requires a standardized in vitro condition, was closer to clinical reality. The results on temperature variation can hardly be reproduced in an in vivo setting, with a different basal temperature and in the absence of the blood flow. Another limitation may rely on the fact that biological substrates were used, which differed one from another in terms of cortical thickness and medullary density. The lack of a constant drill load may represent a limitation, but the use of a testing machine could have affected a passive insertion of the drills through the sleeves. Therefore a manual drilling by a single operator was adopted. To obtain a more precise temperature recording, each osteotomy was performed with one continuous movement and then 30 s were waited. It must be said that his method is rarely reproduced in the clinical scenario, since the lag time from one drill to the subsequent is usually shorter. Further studies should test the guide under different conditions, such as an additional irrigation source and a different drilling method.