# Using FEM to Assess the Effect of Orthodontic Forces on Affected Periodontium
## 4 Discussion
The authors carried out correlations between bone resorption level, type of applied forces (intrusion, extrusion, rotation, tipping, and translation), and decrease of force magnitude. They found that the highest apical and cervical stress was caused by translation and rotation, whereas intrusion determined the lowest cervical stress to be higher than apical stress. For in-intact periodontium, only intrusion and extrusion exhibited compressive stresses lower than maximum hydrostatic pressure (MHP) and maximum tolerable stress (MTS). In reduced periodontium, however, compressive stress (except for intrusion) exceeded these two parameters; thus, in the case of periodontal breakdown, lighter forces of 0.2 N are considered safer, and anything exceeding 0.6-1.2 N may compromise the desmodontium by producing stresses surpassing both maximum tolerable stress and hydrostatic pressure. The authors did not observe a correlation between the applied force, compressive stress increase, and periodontal breakdown, applicable to all five movements [36]. These results are confirmed by our studies as we could not find a correlation between these parameters. Lingualization in the lower frontal group is more important once the loss of attachment increases, both in terms of tensions in the system and in terms of the displacement _per se_, and, from a clinical point of view, in terms of unwanted side-effects. One of the possible deleterious effects is orthodontically induced inflammatory root resorption. When analyzing linear to nonlinear models, a study that considered pure intrusion, buccal tipping, and their combination, applied with either a light (0.25 N) or a heavy (2.25 N) force, found notable differences in stress distribution patterns and magnitude. None of the light-force models reached the critical compressive hydrostatic stress when a nonlinear periodontal ligament was considered, whereas all the heavy-force models reached it. Moreover, the authors observed that the areas of critical compressive hydrostatic stress corresponded to those with resorption craters in clinical studies [37]. Another potentially hazardous effect may occur, especially in the case of the LLL, because of the inherent root morphological characteristics and differences. Due to the anatomical features and location in the mandibular bone, the LLI is less protected when compared to the LCI. Our study emphasizes an increased risk of deleterious side-effects when applying a lingualization force to the LLI, with a potential emergence of the \"wedge effect\". This effect entails applying a higher amount of force on a significantly reduced surface, and it may be extremely damaging for the supporting periodontal tissues. When taking into account an affected periodontium, the periodontal prognosis may be extremely different between the two lower incisors; thus, clinicians should be aware of these important differences and apply a lighter force on the LLI. The point of force application is also important. For example, the results obtained by [PERSON] et al. showed that when the vertical force was applied on the lingual surface, a larger displacement was observed, and the type of the movement obtained was tipping. In addition, the authors stated that applying an intrusive lingual force to the lower incisor determines bodily movement and the same intrusive lialial force determines lialial tipping [38]. These observations also agree with another study that compared labial orthodontics to lingual orthodontics in terms of force magnitude. The differences in biomechanics of tipping were evaluated for a 50 g force applied on the labial and the lingual side, and the main stress patterns in the periodontal ligament for orthodontic tooth movement were recorded. The same force on the labial side caused approximately 0.0252 N/mm\({}^{2}\) maximum principal stress and on the lingual side it resulted in 0.0375 N/mm\({}^{2}\) maximum principal stress; thus, tipping for a lingually applied force requires about 1/3 less force when compared to a labial one [34].