In the literature, in recent years, there was a growing interest on studying the efficiency of ultrasonic drilling in bone [6, 14, 15, 26-31]. However, each study had its own drilling parameters that made their outcomes specific. On the other hand, all of them investigated the application of ultrasonic drilling only in cortical bone. In this study, with considering optimum drilling parameters based on previous findings, we studied both the cortical and cancellous bone. In addition, we not only investigated the drilling force [6, 26, 28] and the temperature rise [14, 15, 27-31], but also assessed their effects: the necrosis and apoptosis of osteocytes through histopathologic evaluation [14, 15] in addition to the micro-cracks propagation and mechanical damages with SEM imaging. Finally, employing finite element modeling gave us an insight into the temperature distribution during each drilling method.
With regard to temperature, many previous studies on ultrasonic drilling in bone only focused on determination of temperature changes [27, 29-31] since its elevation is claimed as a marker for osteonecrosis [2, 32]. However, studies on the comparison of ultrasonic drilling and conventional drilling based on the temperature elevation are not consistent. Alam et al  studied ultrasonic drilling with frequency range of 5 to 30 KHz and their results showed lower temperatures for frequencies below 20 KHz in comparison to conventional drilling with the same drilling parameters. Ironically, here, with the frequency of 20 KHz, our results were thoroughly in a starling contrast to this outcome and the contrast was more significant in the lower feed-rates (Figure 2a). Furthermore, they claimed that vibration amplitude did not affect the temperature rise, which is in opposition to the results of our finite element analysis, again. There is also similar contrary view in some other studies [14, 26, 29]. Despite these contraries, there are also some supports for our results on comparison of temperature elevation in the literature [15, 30, 31]. As our finite element analyses also implied, the temperature distribution differed in the two drilling methods, which can explain the higher temperature elevation in the ultrasonic drilling. Bai et al  believe that in this method, the superposed motion intensifies the friction motion at the interface of the drill body and the wall of the bone hole, which leads to intensive heat generation there.
Concerning the mean force, our results on the competence of ultrasonic drilling in comparison to conventional drilling (Figure 2b) was completely in accordance to the previous studies [6, 28]. Shakouri et al  also indicated the same outcome for drilling speed of 1000 rpm. It is claimed that ultrasonic drilling causes lower drilling force and torque in comparison to conventional drilling by changing the mechanism of chip formation . However, in the study of Shakouri et al, the temperature rise for ultrasonic drilling with drilling speed of 2000 rpm was negligible and independent of feed-rate that our results disagreed over the both.
Comparing Figure 2a with Figure 2b, to explain the trends, we can state that increasing the feed-rate raises friction and therefore the mean force, which yields into generating more heat. However, as the feed-rate increases, the drilling time falls, resulting in lower transmission of generated heat to the bone and lesser temperature elevation. On the other hand, as the drilling speed goes up, the amount of frictional energy that is generated by drill bit increases; and since most of the drilling energy is converted to heat, increasing the rotational speed raises the temperature of the bone .
The attention-grabbing comparative point in the results, notably in the diagrams of Figure 2, is that in the one hand, we found lower temperature elevation for the conventional drilling, and on the other hand, the ultrasonic drilling had better results for drilling force. However, the temperature elevation and the force are not the main interests in application, and their influences on osteonecrosis and mechanical damages are what we should care for. As mentioned above, the former finding about the advantage of the conventional drilling could indicate less osteonecrosis yield in comparison to the ultrasonic drilling, especially since it was considerable for the feed-rate of 30 mm.min-1. Nevertheless, according to the later finding on the competitiveness of ultrasonic drilling in the required force, the believe that the osteonecrosis is because of drilling force and friction, and that the mechanical damages owing to applied force can cause osteocytes apoptosis could cast doubts on the superiority of the conventional drilling method. In addition, the cumulative effect of the force magnitude and the temperature rise was not clear. Therefore, our histopathologic observations, as explained in the result section, were interesting, especially since these observations were supported by the outcomes of SEM imaging. Consequently, we can conclude that the ultrasonic drilling was more comparatively advantageous based on the bone damages in both cortical and cancellous bone. The results showed death of osteocytes was not only due to temperature elevation, but also because of the drilling force, and both these parameters should be considered to assess the bone damages. This is in good agreement with some previous findings in the literature [15, 34].
However, more in-vivo studies on clinical success of the ultrasonic drilling are needed, both on animals and on human. Moreover, to have a better conclusion, consideration of further drilling parameters may be helpful.