Materials and Clinical Aspects of Orthodontic Treatment with Aligners

Abstract

<b>Background:</b> The use of orthodontic aligners to treat a variety of malocclusions has considerably increased in the last years. Yet evidence on forces and moments generated across difference aligners types, their efficacy and adverse effects relative to conventional fixed orthodontic appliances, accuracy obtained on specific tooth movements, the effect of aligner cleaners on their composition and mechanical properties and changes in the morphology, roughness and composition of attachment surfaces in contact with the aligners remain unclear.

<b>Aim:</b> The aim of the present thesis was to study: (i) the existing evidence on forces exerted by aligners, (ii) the efficacy of aligners compared to orthodontic treatment with fixed appliances, (iii) the accuracy of specific tooth movements, (iv) the effects of aligner cleaners and (v) the aligner/attachment surfaces during orthodontic treatment with aligners.

<b>Methods:</b> Two systematic reviews with meta-analyses were conducted to address the issues regarding the existing evidence on forces and moments generated by aligner type appliances and aligner efficacy and adverse effects relative to conventional treatment with fixed appliances. Seven and eight databases were searched without limitations for each topic respectively. Risk of bias assessment was based on the Cochrane Risk of Bias tool in all cases. A total of 13 in-vitro studies deemed eligible for inclusion and 2 were included in the quantitative synthesis for the former subject, whereas 11 studies (4 randomized/7 non-randomized) and 3 meta-analyses were conducted for the latter subject. Accuracy obtained from aligner treatment on specific tooth movements was measured by superimposing the predicted and achieved models over the initial ones in 20 Class I adult patients on posterior teeth. Moreover, the impact of three aligner cleaners was tested employing two alkaline peroxide solutions (Retainer Brite - RB; Retainer Cleaner - RC) and one peroxide-free (Steraligner - ST) on two different aligner companies, Clear Aligner (polyester) and Invisalign(polyester-urethane) for a two-week period. The acidity, changes in the chemical composition and changes in Martens Hardness (HM), elastic modulus (EIT), elastic index (nIT) and relaxation (RIT) were studied. Finally, attachments bonded with 2 different light-cured composite resins (sculptable and flowable) to 20 zirconia CAD/CAM frames and corresponding aligners, were examined before and after aligner removal and reseating in water, under (i) a stereomicroscope to identify morphological alterations, (ii) an optical profiler to measure the 3D-roughness parameters and (iii) by attenuated total reflection FTIR spectroscopy (ATR-FTIR) to determine changes in the molecular composition and degree of C=C conversion.

<b>Results:</b> When palatal tipping of the upper central incisor through PET-G aligners was considered, aligner thickness of 0.5, 0.625 or 0.75 mm was not associated with a significantly different moment to force (M/F) ratio. Aligner thickness does not appear to possess a significant role in forces and moments generated by clear aligners under specific settings, while the most commonly examined tooth movements are tipping and rotation. Moderate quality evidence indicated that treatment with orthodontic aligners is associated with worse occlusal outcome with the American Board of Orthodontics Objective Grading System and more patients with unacceptable results. No significant differences were seen for treatment duration. Horizontal movements of all incisors seemed to be accurate, with small (0.20-0.25 mm) or insignificant differences between predicted and achieved amounts. Vertical movements and particularly intrusions of maxillary central incisors were found to be less accurate, with a median difference of 1.5 mm (p<0.001). All achieved rotations were significantly smaller than those predicted, with the maxillary canines exhibiting the greatest difference of 3.05o (p<0.001). RB and RC aligner cleaners were weakly acidic (pH = 6.3), whereas ST was mildly acidic (pH = 4.8). The ATR-FTIR analysis demonstrated evidence of acidic hydrolysis of Clear Aligner in ST and Invisalign in RB. The IIT-derived properties of Invisalign were not affected by the cleaners. However, for Clear Aligner a significant change was found in HM (all cleaners), nIT (all cleaners) and RIT (RB, ST). Finally, characteristic abrasion-induced defects by removal and reseating of the aligners were detected without significant changes in the roughness parameters (control-tested), but with significant higher values in Sdr between materials within control or tested groups. The sculptable material appeared superior in terms of morphology and retention characteristics. Insignificant differences in the C=C conversion were found in the groups tested. However, in some specimens strong peaks or irreversibly absorbed water were detected indicating hydrolytic susceptibility of the superficial composite zone.