# Finite Element Analysis of Mandibular Anterior Teeth with Healthy, but Reduced Periodontium
## 4 Discussion  
The findings of this study suggest that the degree of displacement is correlated with the PDL's thickness. These results are in agreement with other studies [15]. Furthermore, stress increases in the PDL depending on the PDL's degree of thickness. In our study, for the maximum stress in the PDL, for the lateral incisor, it was observed that increasing the force by 25% determined an insignificant increase in the stress. Therefore, it was not justified to use a force of 1 N instead of a force of 0.8 N. These parameters depend on the particular anatomy that was captured by CBCT-based modeling and by object-oriented FEA (real case). These results are similar to those obtained in orthodontic practice. From a medical point of view, it is recommended to use lower values because higher values could trigger root resorption, particularly in cases with abnormal root shape and increased root length [16; 17]. Taking into consideration that the PDL has an absorption function and facilitates the displacement and alignment of teeth during orthodontic treatment [18] in the areas with decreased thickness, the displacement is accelerated or significantly increased. The correlation between the PDL's thickness and displacement is displayed throughout our study. Analyzing the three graphic representations for the lateral incisor (Figure 9a-c), it can be noticed that the displacement is higher in the area where the maximum bone stress is highest, and in area 5-6, the displacement is increased. This outcome is due to the minimal ligament thickness. Thus, these aspects should be well known before treatment planning in order to apply the necessary therapy according to the particularities of each patient. In order to analyze the effects of forces on the atypical anatomy of the root, FEA is a suitable, non-invasive method with results that can be subsequently applied in practice, in orthodontic treatment. Regarding the type of displacement obtained, it can be observed, in the case of the three teeth (central incisor, lateral incisor and canine), that a bodily displacement with tipping was obtained. This aspect is highlighted by the shape of the curve. A horizontal line of the curve means that the movement is a pure bodily movement. Nevertheless, in our study, the line is descendent, which means that the movement is not a pure bodily movement, and it is a bodily displacement with tipping (Figures 9c, 10c and 11c). Comparing the stress values in the ligament and in the bone, it was observed that the highest stress developed in the lateral incisor due to the minimal ligament thickness in area 4. Our results are comparable with the findings of [PERSON] et al. [19], who found the highest stress in the central incisor. The results of our study would have been similar to those from previous studies, provided that the anatomy of the mandibular teeth would not be atypical compared to the maxillary teeth. This highlights the need for studies on the anatomical features of the teeth and periodontium, FEA being one of the most recommended methods. The highest maximum stress was found in area 1, in proximity to the cervical margin. This is in agreement with other studies [19]. The orthodontic-periodontal interrelationship is well known and systematically studied [20; 21; 22], particularly since the reduced periodontium is considered at risk regarding orthodontic treatment [23]; thus, the necessity of studies in this field is justified. This type of periodontium has low resistance to mechanical stress [24]. It is well known from practice and the literature that the most common root resorption occurs at the level of lateral and central incisors [17; 25]. The explanation of this phenomenon is given by the radiological anatomy of these teeth (curved shape), the position on the arch (it is positioned with predilection towards lingual), the thinner cortex at this level and, especially in the case of the mandible, in the hypomochlion area, the PDL is thinner.