## 4 Discussion
On the other hand, sonic and ultrasonic instruments, by leaving a cleaner osteotomy surface free of debris, avoid the closure of vascular spaces [28; 36]. In addition, clean surfaces can reduce inflammatory processes, and open vascular canals can improve tissue cell nutrition and facilitate bone healing [16; 17; 19; 25]. A further aspect to be considered in this study is the presence of indentations in restricted areas of some specimens in both piezoelectric devices, probably due to the operator's repositioning of the device tip during the cutting procedure (Figure 5). In addition to the indentations, scratches of different sizes and depths are observed only on the bone surfaces cut with the PM device, probably due to the roughness of the tip and the lower power of the PM (23 W) compared to the PP one (75 W) (Figure 6). In another experimental model (i.e., holes drilled in bovine cortical bone), [PERSON] et al. [38] described the presence of'slots' after 30 consecutive drillings with the piezosurgical device on the bone surface. Concerning the microcracks observed on bone-cutting surfaces, both piezoelectric devices have been shown to produce more microfractures compared to the RS (Figures 3b and 8). Interestingly, some authors described the presence of microcracks after the use of different cutting devices; they suggest that such microcracks might be related to compression of bone or excessive bone strain over certain threshold values [28; 38]. Moreover, bone microfractures result from bone drilling and may also occur in physiological loading conditions and under excessive loads [39; 40]. Some publications report that microcracks increase bone fragility, reducing the mechanical properties of bone tissue and leading to stress fractures [41; 42]. On the contrary, in a 2017 review, [PERSON] and [PERSON] [43] reported that repetitive strain applied to the bone can cause microfractures that, in turn, trigger the process of bone remodeling, thus removing and repairing the same microcracks. As it is well known, the bone remodeling that renovates the structure of skeletal segments for life is triggered by viable osteocytes [44; 45; 46] and is due to the coupled activity (both in spatial and temporal correlation) of osteoclasts and osteoblasts: initially, bone is eroded by osteoclasts and, on the same eroded surfaces, osteoblasts subsequently lay down new bone. With regard to osteocyte viability, it is very interesting to point out that in various papers [47; 48], the histologic analyses on the cutting surfaces with piezoelectric devices showed the presence of live osteocytes with a normal size and morphology. In contrast with the hypothesis that viable osteocytes are needed to trigger bone remodeling, [PERSON] and coworkers [40] demonstrated that microdamage, induced by in vivo fatigue loading (in a model using ulnae of Sprague-Dawley rats), correlates with osteocyte apoptosis near the bone damage and the subsequent cortical bone remodeling process, suggesting that cell apoptosis plays a substantial role in activating and/or targeting osteoclastic resorption. In another paper by [PERSON] and [PERSON] [39], the authors demonstrated in an equine model, in the neonatal period, that even habitual bone deformities (those below the microdamage threshold) cause microfractures in both cortical and trabecular bone; osteocytes close to microfractures become apoptotic, and subsequently, the early phase of the bone remodeling cycle takes place. In the present study, it is important to highlight that the numerous microcracks found in the samples cut with both piezoelectric devices should not be interpreted as a negative aspect. In fact, as described above, they can promote bone remodeling, thus leading to the formation of new bone, regardless of whether microfractures involve living/viable osteocytes or induce their apoptosis.