## 5 Which Animal Model Is Suitable for Bone Investigation of Osteotomic Effects?
At the same time, small animals such as rodents are often suitable in terms of feasibility, but they might not match the needed osseous characteristics, especially because of their small size. Considering all the possible elements, the ideal animal models are non-human primates (NHPs), followed by dogs, although the ethical issues raised recently have made their involvement in animal experimentation increasingly difficult [71]. Nevertheless, it has to be considered that in vivo experimentation does not always imply the sacrifice of the animal--for example, in the case of an evaluation of microcirculatory parameters with intravital microscopy [72]. Bone macroscopic and microscopic characteristics deeply influence the choice of animal model: rodent bones can hardly be a suitable model for developing a novel dental implant, due to their small size. On the contrary, notwithstanding their dimensions, murine models can provide insights into the processes that underlie the biocompatibility of biomaterials [73; 74; 75] and bone healing, also based on multiple specific strains that have been selected over time [60; 76]. Larger animals have many more clinical, biomechanical and dimensional similarities to humans. This is reflected also by the different bone turnover rates, which are close to the human ones in dogs or pigs and are much quicker in rodents and rabbits. Therefore, they can be crucial to study the steps and the clinical aspects of bone healing. A question to be raised is whether a specific animal bone site (i.e., a specific region of a particular skeletal segment) is more suitable than another one in the study to obtain inferences about a specific human bone site. The most used sites in animal models are the femur, tibia, calvaria, mandible and vertebra, and each one has particular features that vary depending on the animal. A great variety of properties has to be considered in different subsites within the same bone, in term of bone density and cortico/cancellous thickness; for example, anterior and posterior mandible specimens show peculiar characteristics, as well as femoral diaphysis, metaphysis, epiphysis, et al. To our knowledge, there is no clear evidence in the scientific literature of the better adequacy of a specific anatomic site in an animal model for speculation about a human bone site. The investigator has to take into consideration all the above-mentioned aspects. For instance, we chose as an animal model the in vivo rabbit calvaria in our previous paper [8], considering:* The osteotomy instruments that we wanted to test (two different piezosurgical devices and one conventional rotary osteotomy device), which required high dense bone. We consequently excluded mice, dog alveolar bone and rabbit mandible or femur. * The space that we needed in order to perform several osteotomic lines, which helped us in lowering the number of animals to sacrifice and led us to exclude mouse or rat as potential animal models due to their small size (Reduction in \"the three Rs\" [70]);
* The European Communities Council Directive, which led us to exclude large animal models (sheep, pork, dog) on an ethical basis [8].