## 3 Results
This aspect, despite not being clearly addressed in FEA studies, was visible when interpreting and correlating the results with clinical data and anatomical and physiological constants [7,10,11,13,24,25]. Thus, [PERSON] et al. [22], in an intact periodontium FEA study employing multiple failure criteria (VM, S1, S3, and HP), reported in the apical third of PDL and root multiple hydrostatic pressure stresses exceeding 16 KPa of physiological hydrostatic pressure and signaling ischemic, resorptive, and further periodontal loss risks for a tipping force of 0.35-0.5 N (which normally do not appear in clinical practice) [17]. Other FEA studies [33,34,35,36,37] employing hydrostatic pressure failure criteria in intact PDL reported optimal orthodontic forces of 0.28-3.31 N (but with significant differences from one study to another for the same tooth and movements, contradicting both clinical knowledge and [PERSON]'s reports [17]), and quantitative hydrostatic pressures exceeding the physiological one by far (i.e., MPa vs. KPa). None of these studies addressed these inconsistencies that contradicted clinical knowledge,with the only obvious explanations being non-adequate failure criteria employed, since there are reported differences for criteria analyzing the same material [12]. There are failure criteria specially designed for homogenous (Von Mises) and non-homogenous (Tresca) materials, which supply 15-30% quantitative differences when applied to the same material [7; 10; 11; 13]. The reduced sample size of most FEA studies is another issue that must be addressed, evaluating if and how the results' accuracy is influenced. Most FEA studies (despite various failure criteria being employed) used a sample size of one (one patient, one model, few simulations) [9; 12; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37], mostly examining an idealized model of the tooth and bone. Nevertheless, we were able to draw valid conclusions from their simulations (i.e., accuracy of the results was not influenced by the reduced number of analyzed models but rather by the misuse of failure criteria, extremely high forces, or/and other boundary condition issues). FEA being based on a mathematical algorithm, the number of models is less important than the input data and boundary conditions, since the results will remain identical independently of the number of models/sample size if the information uploaded to the software suffers from bias. In our study, the sample size was nine (nine patients, eighty-one models, 405 simulations), all supplying comparable results, thus enabling us to draw valid conclusions. Moreover, the use of numerous 3D models in the FEA implies difficulties related to time consumption and computing power (the manual segmentation technique far superior in providing anatomical accuracy when compared with the automated software reconstruction method). Far from being the most correct method in the study of tissues, FEA has numerous limitations related to the fact that it cannot accurately reproduce clinical situations (e.g., complex interactions and movements between the components of anatomical tissues, identical internal architecture of human tissues, complex associations of orthodontic movements). Additionally, more variables should be taken into careful consideration such as force levels [38] and mechanical characteristics of orthodontic wires [39]. Thus, correlating FEA results with in vivo and in vitro studies for a better image of the biomechanical behavior of tissues is of importance. Nevertheless, FEA is the only possible method that provides a detailed individualized analysis for each anatomical component of the tissues (i.e., not allowed by other methods that supply a general view).