Table 2.

Description and outcomes as observed in the studies selected for the systematic review.

Author and Year of Study	Study Design	Study Description/Characteristics	Study Outcome/Inference
Alexandropoulos et al., 2015 [10]	In vitro study	The current study’s objective was to describe the mechanical and chemical characteristics of modern thermoplastic orthodontic materials. Tests were conducted on Clear Aligner (Scheu-Dental), ACE and A+ (Dentsply), and Invisalign, four thermoplastic materials (Align Technology). Each material was used to create eight appliances, and a small sample of each was used to conduct an ATR-FTIR spectroscopy analysis.	Invisalign’s polyurethane base was discovered through ATR-FTIR analysis, while the other materials were based on polyester and polyethylene glycol terephthalate. In comparison to PETG-based products, Invisalign displayed higher hardness and modulus values, a slightly higher brittleness, and lower creep resistance.
Comba et al., 2017 [21]	In vitro study	The force system and displacement patterns produced by plastic aligners during bodily canine movement, both with and without composite attachments and Class II elastics, were described in this work using a finite element (FE) model.	The findings demonstrated that a vertical, rectangular attachment resulted in buccal displacement of the tooth and potential periodontal damage. Movements of intrusion and tipping were produced by configurations with and without an attachment.
D’Anto et al., 2022 [22]	Prospective study	This study compared the virtually intended and actual tooth movement at the end of stage 15, which is frequently the moment of initial refinement, in order to assess the predictability of CAT.	The first molars’ tip correction had the most under-performance (+2.3° ± 3.1°), the second molars’ torque correction had the largest over-performance (+2.3° ± 3.1°), and rotation corrections of all the teeth had the best accuracy.
Dasy et al., 2015 [23]	In vitro study	This study’s primary objective was to assess how well four different types of aligners stayed in place on a dental arch with different attachments. Three casts—one without any attachments to act as a control—were produced for this investigation.	Except when utilized with attachments, CA-soft, CA-medium, and CA-hard did not significantly boost retention. Furthermore, CA-hard and CA-medium required a great deal more energy to remove.
Ercoli et al., 2014 [24]	Comparative clinical trial	This research compared the Nuvola® and Fantasy® systems, looked at their material characteristics, and identified when to employ aligners. The Nuvola® aligner and the Fantasmino® system were used to treat two different patient groups.	Throughout the course of the therapy, the two types of aligners demonstrated variances. Although the Fantasmino® system has highly effective elastic qualities, its size did not promote compliance throughout the day.
Gomez et al., 2014 [25]	In vitro study	The purpose of this study was to describe the initial force system produced during physical movement of upper canines with plastic aligners that had and did not have composite attachments using a three-dimensional finite element (FE) model. A thermoformed plastic aligner, two light-cured composite attachments, and a CAD model of an upper right canine’s alveolar bone and periodontal ligament were created.	Without composite attachments, it was possible to see a compression area in the cervical third of the distal root surface and a tension area in the apical third of the mesial surface in terms of the stress distribution between tension and compression.
Hahn et al., 2010 [26]	In vitro study	The purpose of this study was to quantify the stresses and moments applied by three identically thick thermoplastic appliances to a maxillary central incisor during rotation. The three materials were used to create five identical appliances (Ideal Clear 1.0 mm, Erkodur 1.0 mm, and Biolon 1.0 mm).	The least moment was measured at a rotational speed of 20.17 mm (27.3 Nmm, 60.8), while the largest moment was measured at a deflection speed of 20.51 mm (271.8 Nmm, 62.5). The lowest intrusive force was measured at activation of 20.17 mm (0.0 N), while the highest intrusive force was assessed at rotation of 20.51 mm (25.8 N).
Ho et al., 2021 [27]	In vitro study	This study examined the behavior of a single tooth moving in an orthodontic aligner utilizing various aligner materials and attachment shapes. The first typodont models were created using a 3D printer and bicuspid extracted resin. Three different attachment types—a thick and thin ellipsoid and a bar—were created to fit the canine crown surface. Different aligners were made from three different types of aligner materials.	The BENQ group had a lesser shift in the long axis angle when the three aligners were compared, according to the changes in the canine’s long axis. The canine movement of the BENQ group did not exhibit noticeable movement, but the tipping canine movement of the PTG and TPU groups did.
Hong et al., 2021 [28]	In vitro study	By comparing and contrasting the movement and rotation of teeth between a general attachment and an overhanging attachment, the authors of this study developed an attachment design that effectively induces tooth movement.	The outcomes demonstrated that the overhanging attachment aligner can successfully lessen crown tipping and prevent axial rotation for a central incisor’s targeted distal displacement.
Kohda et al., 2012 [13]	In vitro study	This study’s primary objectives were to quantify the forces applied by thermoplastic appliances constructed of three different materials and examine the impact of mechanical characteristics, material thickness, and activation level on these forces. We chose three thermoplastic materials with two different thicknesses.	Hardcast’s elastic modulus and hardness were much lower than those of Duran and Erkodur, whose characteristics were similar. Appliances made of thicker material (0.75 mm or 0.8 mm) consistently produced a lot more force than those made of thinner material (0.4 mm or 0.5 mm).
Kravitz et al., 2008 [29]	Prospective study	This study’s objective was to assess how attachments and interproximal reduction affected dogs receiving Invisalign rotational therapy. In this prospective clinical investigation, the virtual TREAT models of 31 people who had anterior Invisalign treatment were used to quantify 53 canines (33 maxillary and 20 mandibular).	With Invisalign, the average canine rotation accuracy was 35.8% (SD 26.3). According to statistical analyses, there was no appreciable distinction in accuracy between groups AO, IO, and N.
Kravitz et al., 2009 [30]	In vitro study	A total of 37 patients undergoing anterior Invisalign treatment were included in the study sample. On the virtual Treat models, measurements of 400 anterior teeth (198 maxillary and 203 mandibular) were taken.	With Invisalign, tooth mobility was 41% more accurate on average. Lingual constriction was the movement that was most correct (47.1%), and extrusion (29.6%)—specifically, extrusion of the maxillary (18.3%) and mandibular (24.5%) central incisors, followed by mesiodistal tilting of the mandibular canines (26.9%)—was the action that was least precise.
Kwon et al., 2008 [14]	In vitro study	This study’s goals were to assess the force and energy (resilience) delivery characteristics of thermoplastic overlay orthodontic materials and to identify how these characteristics changed following thermocycling or repeated load cycling. Materials of three sorts and three thicknesses were examined.	Amounts of 0.2 to 0.5 mm of deflection was required for the best force delivery. In the deflection range of the best force delivery, thin material exerted high energy. The force delivery properties after thermocycling were not different from those at the baseline but were different after repeated load cycling in the deflection ranges of optimal force delivery (0.2–0.5 mm).
Lombardo et al., 2017 [11]	In vitro study	This study’s major objective was to look into how 4 thermoplastic polymers used to make orthodontic aligners released stress after being deflected for 24 h straight. Two single-layer and two double-layer aligner materials were chosen.	Throughout the 24-h timeframe, all polymers examined produced a sizable amount of stress. The first 8 h had the greatest relief from stress, establishing a plateau that largely stayed steady.
Mohamad et al., 2022 [31]	In vitro study	This study detailed how the force in clear aligner attachments was topographically visualized. In an in vitro study employing resin models, the authors described a way for obtaining the topographical visualization of a clear aligner’s distributed force on the attachment using Prescale R pressure film and an image processing technology.	The average force for active aligners was between 6.2 and 6.3 N, while the average force for passive aligners was between 4.8 and 4.9 N. Therefore, during this study, a single transparent aligner attachment in a resin model received a net force of 1.3–1.4 N.
Valeri et al., 2022 [32]	In vitro study	The objective of this study was to evaluate the precision of the attachment bonding procedure used in aligner treatments. The authors used two transfer templates made of two different types of materials; the first, known as Leone-biocompatible thermoforming material hard/soft, had a lower Young’s modulus and was designated as soft, while the second, known as Leone-biocompatible thermoforming material, was designated as rigid.	From first with the lowest reproduction error to last with the worst performance, the data processing assigned the following performance ranking: C-Transbond, A-Transbond, C-Evoflow, and A-Evoflow are the first four. Contrary to popular belief, employing a rigid or flexible transfer template had less of an impact than using resin-based composites with various rheologies.