## 7 What Kind of Information Can We Obtain from In Vivo and Ex Vivo Studies of Osteotomy?
Well documented experimental osteotomic studies were conducted in the late 1950s, when [PERSON] [119] reported the \"index of viability\", i.e., a parameter for evaluating the distance between the first normal osteocyte and the edge of the osteotomy; it indicated the drill's cutting effects upon the cellular elements of the bone. This index can be carried out on ex vivo and in vivo samples and this author well distinguished for the first time between ex vivo and in vivo studies on animals [119]. A zone of aseptic thermal necrosis was described by [PERSON] in 1958 [119], which was characterized by the degeneration of the osteocytes (pyknosis of the nuclei, complete disintegration of the cells, osteocyte lacunae devoid of cellular elements). Decreased bone damage is indicated by a low viability index value and larger bone damage by the higher values. In the mid-1960s, it was demonstrated that increased bur cutting speed with a cooling agent does not cause increased bone damage, i.e., decreased bone damage occurred if increased speed was applied [78]. Ultraspeed or increased speed was also supported by in vivo findings of [PERSON] [79], in terms of lesser inflammatory response and faster bone recovery. A rudimentary ultrasonic device was verified in the 1970s, found to be related to slower bone healing compared to rotational bur [80]. Ultrasonic and piezoelectric osteotomy was developed and rediscovered in the early 2000s; on the contrary, bone healing outcome proved to be faster with piezosurgical devices with respect to a traditional rotational bur [8]. In 1983, [PERSON] et al. investigated the friction and the heat generated from the metal-bone interface during drilling: it was eventually established that the lowest temperature threshold for thermal osteonecrosis is 47 \({}^{\circ}\)C for one minute. This study was particularly informative because it was conducted in an in vivo animal model (rabbit) and represents nowadays a pillar of the relevant literature [120]. Conflicting conclusions in terms of the effect of temperature in bone tissue can be seen by comparing in vivo and in vitro studies, as [PERSON] et al. [82] concluded in their study. The scholar revealed lower bone temperature in in vivo samples: bleeding flow could evacuate more thermal energy with respect to ex vivo settings [82]. Post-osteotomy callus formation and bone healing are influenced by the biological status and potency of the bone at the cut surfaces [121]. Excessive heat and thermal necrosis may irreversibly impair the bone healing process [122]. To limit the temperature increase, both bur and oscillating saws are usually cooled during the cutting process [123]. Thermal damage in the surrounding cortical bone depends not only on the maximum temperature value but also on the duration of temperature elevation and consequently on drilling time. High speed and increased force in drilling cause a small rise in temperature, due to the decrease in the drilling time [83]. Drilling with forces between 57 and 130 N is related to a minor increase in temperature, and similarly, the time interval in which the temperature remains above 50 \({}^{\circ}\)C is shorter [85]. Moreover, the results from [PERSON] contradict the work published by [PERSON] et al. [85] which had even shown that the cortical bone temperatures decreased between 1.5 and 9.0 N.  
Factors that influence heat generation during bur osteotomy were investigated by several studies and they include speed, drill force, irrigation, drill design and drill diameter. Another widely used technology as a standard tool for osteotomies is the oscillating saw [102]. From [PERSON]'s [86] paper to nowadays, most studies have compared the effects on bone of traditional rotatory osteotomes versus ultrasonic (piezosurgical) devices. The research interest in ultrasonic osteotomy is justified by the fact that piezosurgical osteotomy is extremely precise and provides arbitrary cut geometries, prompt handling, efficient bone cutting and minimal damage to adjacent soft tissue structures [102]. [PERSON] et al.