## 1 Introduction  
Despite the increasing use of clear aligner treatment (CAT) among patients and specialists, its predictability in the expression of orthodontic tooth movement (OTM) is still debated [1]. In their 2015 systematic review, [PERSON] et al. [2] specified a threshold for the predictability of CAT and limits in controlling OTM, and [PERSON] et al. [3] stated that the mean accuracy of the Invisalign(\({}^{\copyright\)}\) (Santa Clara, CA, USA) technique is about 50%, depending on the type of movement considered. Nonetheless, when the results of this study were evaluated to determine their clinical significance, 74% of randomly chosen patients had passed the American Board of Orthodontics cast evaluation system, demonstrating the possibility of obtaining clinically excellent results.  
Class II malocclusion involves the 25% of adults in the European populations, according to [PERSON] et al. [4]. The correction of the molar class II relationship with CAT could be obtained with extractions of the first upper premolars, with mandibular advancement or with distalization of the upper molars. Considering Class II non-extraction cases, the upper molars' distalization could be considered as one of the possible choices [5, 6]. According to Align Technology's \"best practice protocols\", distalization is effective when it is planned with sequentialization and attachments placed on teeth [7; 8; 9; 10]. Available evidence supports antero-posterior correction of 2.5 mm to 3 mm and suggests the use of attachments on all teeth from the canine to the second molar [6; 9].  
In upper distalization with CAT, one of the main problems is the loss of anterior anchorage, resulting in uncontrolled proclination movement of the anterior teeth, producing an overjet increase [11]. To prevent these side effects, Class II elastics are recommended. From the patients' point of view, the use of inter-arch elastics could be considered annoying because of possible smile esthetics interference during the orthodontic treatment, and one of the main risks in aligner orthodontics is the loss of patient compliance.  
Alternatively, it is possible to use temporary anchoring devices (TADs) positioned in the buccal interradicular space between the first and the second molars to use the Class I elastics configuration, to stretch an elastic from the precision-cut aligner to the TAD.  
TADs can also be used as an indirect anchorage, creating a pseudo-ankylosis by connecting the miniscrew to an adjacent tooth by means of a rectangular steel wire (miniscrew-supported pseudo-ankylosis) [12]. To analyze the biomechanical characteristics of these approaches and to provide the best indication for their clinical applications, the Finite Element Method (FEM) represents a valid tool.  
It is a numerical technique, approved and accepted in medicine, used to perform finite element analysis (FEA) of physical phenomena, and evaluate characteristics of biomaterials, devices and human tissues [13; 14; 15; 16; 17; 18]. Quantitative data are provided by simulating the physiological response of the dento-alveolar complex to increase the understanding of the reaction occurring after the application of orthodontic forces. Moreover, it is possible to separately analyze the response of individual tissues such as the periodontal ligament and the alveolar bone, and to make a comparison between different simulations [19].  
The aim of this study was to evaluate, through FEM analysis, the effects on the periodontal ligament, teeth and aligner, of the application of a distalization force on the upper first molar and second premolar using a skeletal interradicular miniscrew as a posterior anchorage preservation device. The study is required in order to better understand clear aligner biomechanics and to improve the daily clinical experience of orthodontists and patients.