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Conclusions
Recommendations
Introduction Orthodontic treatments have helped to correct discrepancies in dento-maxillary with the aim to modify the teeth positions and maxilla-facial growth, by means of mechano-biological intervention of related structures. In orthodontic treatment, teeth are moved to a favorable position, a procedure that can be done in months or years, depending on final location. Orthodontic tooth movement (OTM) is due to biological events that take place in the alveolar bone when mechanical forces are applied to the teeth. The alteration of structural environment in the bone structure produces changes on it during the treatment [16,31]. Alveolar bone fraction and tissue mineral density can vary by age, during OTM and metabolic processes among others [6,9,37,40]. The fact that bone tissue is an ever-adapting structure, responding to a wide range of external and internal stimuli, contributes to the complexity involved in studying its behavior. Researchers face a daunting task when seeking better understanding of how different factors affect its metabolism, especially because continuous modeling, re-modeling and p97 processes of bone structures inherently imposes the need for repeated 
intra-subject assessment of bone mineral density (BMD). To study the effects of age [13,25], sex [5,39], diet [11,28,29], physical loading [7,20], systemic health [2,12,22], environmental factors [3,15,18], on BMD, reliable measurements must be performed on live human subjects. Techniques for BMD assessment include Radiogrammetry (RG), Compton Scattering Technique; 
Radiographic Photodensitometry (RP); Dual-energy Photon Absorptiometry (DPA) among others [8]. In general, the majority of current techniques are considered invasive, since quantification of bone density involves the use of ionizing radiation (X rays), validating the need for development of a more conservative method to evaluate variations in BMD that permit repeated intra-subject valuation. Electromechanical phenomenon provided by piezoelectric transducers (PT) have evidenced a great potential in different engineering applications. In the structural field, PT helps to evaluate, identify, classify and estimate different structural conditions as it has been proven in different engineering fields as for example; Non-Destructive Evaluation (NDE), Structural Health Monitoring (SHM) and Control among others [23,32,34,38,42]. In these fields, different methodologies and techniques have been developed to integrate and to use PT in the structures due to the electromechanical coupling that these present naturally. Specially, we can mention a technique that has gained widespread attention in last two decades as a result of its high local sensitivity, easy implementation and the nonparametric analysis that are constructed with noncomplex theories; this is defined as the electromechanical impedance technique (EMI). EMI technique is applied in structural identification and monitoring conditions in real-time which has demonstrated a great capacity and sensitivity of capturing structural variations with high frequency vibrations [14,21,41]. Actually, different applications are being explored in the bio-medical field, such as the use of biomedical sensors for monitoring condition of bones, through experimental studies on human and rabbit bones as it was experimented in the study of [4]; where the changes in the EMI signatures correlated fairly well variations in the condition of the bones. Additionally [26,27], proposed a technique for monitoring dental implant stability applying the EMI technique. The method involved bonding a piezoelectric transducer to the implant in which the electrical admittance was measured to evaluate its stability, which in turn was correlated with the mechanical parameters of the bone. This shows that exists an opportunity to apply the EMI technique in the monitoring of different biological structures.
Materials and methods