Invisible aligners are medical devices, which allow repositioning of teeth through a treatment designed by the orthodontists. During this orthodontic treatment, patients use several aligners each for a couple of weeks. The aligner will apply a system of forces on the teeth to shift them to desired position. Since aligners exert forces thanks to their particular shape, it is important that during lifetime’s service they do not undergo significant deformations. This research aims to study the mechanical behavior of invisible aligners made by polyethylene terephthalate-glycol (PET-G), which is one of most used the plastic materials to produce such devices. In this study, cyclic compression tests in atmospheric environment (∼25°C) as well as in the presence of saliva (to simulate intraoral environment) were performed. The mechanical behavior of aligners with two different thicknesses (0.75 and 0.88 mm) was studied. In particular, each aligner was subjected to 22500 load cycles from 0 to 50 N. The chosen number of load cycles simulates the average load history to which an aligner is subjected during its lifetime. The tests were performed on a testing machine, using a hard resin dental cast properly fixed to the machine. Analysis of the results shows that the stiffness of the aligners increases during the cyclic test. In particular, a gradual reduction of the crosshead displacement was observed during the test, highlighting the occurrence of cyclic hardening phenomena. It was also found that the aligners show a residual strain recovery after removing the applied load. Moreover, in the analyzed range of load rate, the aligners show a low tendency to accumulate residual strains as loading cycles progress.

The analysis of forces, moments and pressure points has long been of great interest in orthodontics. Hence, we set out to define a method for measuring the pressure exerted by aligners on the teeth, and specifically to identify the precise points of pressure exertion. Intraoral scans were performed on a patient with optimal alignment and levelling before and after 2º vestibularisation of the upper central incisor. Pressure sensor film was placed in a dedicated housing between the aligner and teeth in order to record the pressure exerted after 15 s of aligner application. The images captured by the film were scanned, digitised, and subsequently analysed. Areas and amounts of pressure generated by the aligners were evaluated, and the net force of each was calculated, adjusted to take into consideration passive values. The method revealed the areas of contact by which the aligner transmits force on the teeth, and the pressures at which it does so. The pressure exerted by an aligner is not evenly distributed across the entire surface of the tooth during lingual tipping of an upper incisor. The areas of force concentration were not identical, as these are influenced by factors resulting from the manufacturing and casting processes.

The widespread adoption of clear aligners in orthodontic treatments in recent years has necessitated a more precise examination of the mechanical properties of the devices currently available in orthodontics. Recent studies indicate that aligners, when exposed to the forces exerted during swallowing, undergo fatigue-like phenomena, leading to chip formation and cracks. The cumulative damage results in a compromised fit between the tooth and aligner, which is crucial for the effective execution of orthodontic treatment. Additionally, the formation of chips poses a potential risk to patients, as there is a possibility of inadvertently ingesting microplastics that become detached from the aligner over time. This study attempts to assess the release of microplastics from the aligners subjected to cyclic compressive loading. Three different aligners (Essix Ace, Ghost Aligner and Invisalign) are tested to simulate swallowing conditions over the aligner usage period. The mechanical performance is studied in terms of the energy absorbed by the aligner, which shows that the Essix Ace has a stable energy absorption behaviour, while the energy absorbed by the Invisalign is significantly higher than their counterparts. Ghost Aligner did not perform well in the cyclic compression tests. The microplastics (MPs) released by the aligners are examined under an optical microscope. A dimensional analysis based on k-means image segmentation and edge detection algorithm is developed to analyse the MPs. The dimensional analysis of the MPs revealed that the ingestion of the MPs released by all the three aligners does not pose a health risk.

In this research work, the mechanical performance of a thermoformed clear dental aligner is studied. Its performance is evaluated under the cyclic compression test, which is designed to simulate the occlusal forces applied on the aligner during swallowing operations for its entire usage period. The mechanical results show that the aligner exhibit stable energy absorption and stiffness behaviour throughout its use period and thus can potentially be used for clinical applications. The microplastic released from the aligner due to the fatigue-like damage is analysed using optical microscopy. Most of the microplastics released have larger dimensions, which may be excreted from the gastrointestinal tracts and have less possibility to pass through the epithelium passively. Therefore, the use of aligner may not pose any cytotoxic health risks.

Background: The rife use of aesthetic appliance in orthodontic treatment requires the study of the properties of the materials they are made of.Objective: The aim of the present study was to evaluate the dimensional stability of clear aligners made of three different materials after the application ofin-vivodynamic stress andin-vitrostatic stress.Methods: Three different aligners made of different materials (PET-G; PET; SmartTrack^®), prepared on the dental arch of the same patient, were tested. For each material, three aligner samples were manufactured: one to be usedin-vivo, one to be testedin-vitro, and one to be used as a control.To evaluate the effects of the dynamic stress producedin-vivo, each aligner was worn by a single patient 22 hours per day, followed by a wash-out period of two weeks. To evaluate the effects of static stress, each aligner was exposed to thein-vitrocontinuous force of 50N. The tested and control aligners were scanned, then linear measurements were taken to evaluate their dimensional stability after different types of stresses.Results: PET seems to have the lowest percentage of deformation; PET-G and SmartTrack^®showed a reduced deformation going from the posterior to the anterior area. The contact with human saliva induces a greater deformation.Conclusion: Different materials show different behavior following application of static stresses and dynamic stresses in the oral cavity. PET showed the highest dimensional stability.

Osteosarcoma (OS) is one of the most aggressive primary bone cancers in young population. Therapeutic approaches to osteosarcoma include surgical resection and chemotherapy, leading to a current estimation of survival rate at 5 years of about 68%. The need for new therapies able to be more effective and less toxic becomes more and more urgent. In this general framework, the present study aims to evaluate the effect of US (UltraSound) on MG-63 cell line, a specific osteosarcoma cell line, in order to understand if this treatment can affect the viability of such a kind of cancerous cells. The MG-63 cell line (OS) was cultivated in a Dulbecco's Modified Eagle Medium with 10% of Fetal Bovine Serum. The SonoPore KTAC-4000 was used to generate and transmit US waves to the cells. The sonication process (OUT-Onco-Ultrasound-Tripsy) was performed at five different frequency levels namely: 400 kHz, 510 kHz, 582 kHz, 800 kHz and 1MHz. Further sonication tests were performed for the two frequency values for which the lowest live cell count (cell/ml) was obtained after treatment. 1 MHz frequency was found to be the one most impacting on the cell viability of MG-63 OS cell line.