## 4 Discussion  
Regenerative endodontics has several advantages over apexification and root canal treatment, such as revitalisation and further root development of the immature necrotic teeth [2]. However, the newly formed tissue lacks the presence of odontblast-like cells [7] and, as a result, the root maturation is based on reparative tissues [27], such as cementum-like tissue or osteedentine. Our objective was to determine the biomechanical implications over supporting tissues of the maturation of the root with cementum instead of dentine, when the tooth is subjected to orthodontic loads. The results of this FEA study showed that the biomechanical performance of the periodontal ligament and tooth structure of the dentine- and cementum-reinforced teeth models was similar in all scenarios, and that the periodontal ligament acted by reducing the mean stress values in all models. The periodontal ligament acts as a mechanical connection between the tooth and the alveolar bone; therefore, the delivery of orthodontic loads to the tooth causes stress in the periodontal ligament, and it is related to resorption of the periodontal tissues [28]. On the pressure side, the external mechanical force induces collagenous extracellular matrix resorption of the periodontal ligament and bone [29] due to elastic activity. The recruited osteoclasts resorb and remodel the alveolar bone during orthodontic movement [29]. A disadvantageous stress distribution with peak stress values concentrated in some areas of the periodontal ligament could lead to a pathological process where resorption is not  
Figure 5: Stress distribution in the tooth (**A**) and in the periodontal ligament (**B,C**) of Models 1, 2 and 3 in the rotation scenario. Similar stress patterns are observed between all tissues of Models 2 and 3. Maximum tensile stress of the teeth (**A**) and ligaments (**B,C**) are coloured in red. controlled and affects the tooth's hard tissues. Root resorption is prone to occur in areas of stress concentration at the compression site during the process of orthodontic tooth movement [30]. Consequently, the question arises as to whether teeth previously treated with regenerative endodontics and that have completed their root development with cementum can endure orthodontic treatment or if they are at risk of root resorption [19]. In our simulation, the mean stress values of the periodontal ligament were similar among the dentine and cementum-reinforced teeth. Moreover, the stress distribution patterns in the periodontal ligament were similar among the dentine- and cementum-reinforced teeth as well. These results suggest that the performance of previously revitalised teeth subjected to orthodontic loads might be comparable to that of physiologically developed teeth, although this needs to be confirmed in in vivo situations. Thus, controlled clinical trials and/or experimental studies are needed. The results of this study also showed that the periodontal ligament acted by dissipating the mechanical loads incurred during simulation of orthodontic loads. Moreover, stress distribution patterns demonstrated that peak stress values were reduced from the teeth to the ligament. Cementum has poorer mechanical properties than dentine [13]. The amount of cementum surrounding the root of physiologically developed teeth (i.e., with dentine) has implications in the mechanical performance of teeth, since mechanical stresses increase at the root apex with increasing thickness of the apical cementum [31]. A previous computational study showed a disadvantageous stress distribution along the root in teeth that have completed their root development with cementum compared to physiologically developed teeth [14]. However, it must be noted that these differences in peak stress values are remarkable in high magnitude forces, such as those applied during biting or traumatic dental injuries and are mild with low magnitude forces, such as those applied in orthodontics [14].