Orthodontic treatment with sequential aligners has seen a considerable surge in the last decades, and is currently used to treat malocclusions of varying severity. To enhance tooth movement and broaden the spectrum of malocclusions that can be treated with aligners, composite resin attachments are routinely bonded with the acid-etch technique on multiple teeth, a process known to impose irreversible alterations of the enamel structure, color, gloss, and roughness. Additionally, this clinical setting introduces a unique scenario of different materials applied in a manner that involves the development of friction and attrition between the attachment and the softer aligner material, all performing in the harsh conditions of the oral environment, which impact the aging of these materials. The latter may give rise to alterations of the aligners and the composite attachments and potential intraoral release of Bisphenol A, a known endocrine disrupting agent. Furthermore, at the final stages of contemporary aligner treatment, the removal of multiple, sometimes bulky, composite attachments with a volume and surface far greater than the remnant adhesive after debonding of brackets, through grinding that might be associated with pulmonary effects for the patient or staff. Because of the extensive enamel involvement in bonding, the release of factors from the attachment-aligner complex during service, the aging of these entities in the oral environment, and the laborious debonding/composite grinding process coupled with the hazardous nature of aerosol produced during the removal of these bulky specimens, appropriate risk management considerations should be applied and an effort to confine the application of multiple composite specimens bonded to enamel to the absolutely necessary should be pursued.
In the last decade, aligners have become an integral part of orthodontic armamentaria, and their use has been expanded to supposedly manage a wide range of malocclusions. At the same time, there is a populous list of companies offering aligner treatment either to professionals or directly to the public—sometimes even without the direct involvement of a dentist.
The establishment of aligners as a treatment modality for malocclusion was made possible with the introduction of bonded attachments, which enhanced the control of the crown’s spatial orientation, offering far more possibilities for treatment compared with the initial introduction of aligners when the tooth movement was limited to tipping and a small derotation of anterior teeth. Although the multiplicity of attachment shape and size opened new possibilities in the treatment of rotations, such as buccolingual inclination variations and vertical repositioning of teeth, their use is not free of concerns.
The purpose of this narrative review is to discuss the concerns arising from the increasing use of composite resins bonded onto the labial surface of teeth, focusing primarily on the biologic and environmental implications of their application.
Enamel involvement
One of the main advantages of the early phase of aligners was the absence of any involvement of enamel in the treatment of malocclusions. Although the application spectrum of aligner treatment was limited to Class I crowding cases, the fact that orthodontic treatment of these cases involved no acid etching–mediated bonding offered an advantage owing to the maintenance of the structural integrity of enamel, with favorable outcome on the potential for white spot lesions, decalcification, avoidance of the use of rotary instruments to grind the remnants of adhesives after debonding, and essentially a lack of long-term change of the enamel’s optical properties, because the absence of resin tags warrants that the surface would be intact.
Expanding the spectrum of indications for aligners to tooth movement in all 3 planes of space necessitated the use of grips bonded onto the enamel to generate buccolingual, mesiodistal, and incisocervical movement, which negated the advantage of an intact enamel surface. Such composite attachments used in conjunction with aligners have dimensions ranging from 2 mm to 5 mm and thickness that can exceed well 1 mm, are bonded most often on the labial surface of multiple teeth. Moreover, composite attachments used in aligner treatment possess a high surface-area-to-volume ratio, which affects their interaction with the environment.
In orthodontics, specifically in bonding orthodontic brackets, the adhesive application mode effectively alters the exposure pattern of the material to the oral cavity with a potential effect on reducing its reactivity with liquids and other materials. The sandwich pattern of application, where the adhesive is bonded to both enamel and bracket base, allows only the margins of the material to be exposed to the oral cavity. The acid-etched enamel interface on the one side and the morphologic irregularity of bracket base through the welded mesh wire or laser etching on the other side provide a mechanism for the interlocking of the polymeric material in both structures (tooth and bracket). The average thickness of the adhesive layer between the tooth and bracket has been estimated between 150 μm and 250 μm, depending on the morphologic condition of the bracket base, with smooth bases resulting in thinner adhesive layers owing to the homogeneous pressure and the lack of retentive sites for the entrapment of the adhesive, whereas rougher bases lead to thicker adhesive layers. Therefore, for a typical bracket with dimensions of 2.5 × 3.0 mm (height × width) bonded to enamel, the surface of the adhesive layer exposed in the oral cavity can be estimated to be somewhere in the 11-mm perimeter range or 11 mm × 200 × 10 −3 mm or 2.2 mm 2 adhesive surface area. This multiplied by 20 brackets—the average case—results in a sum of 44 mm 2 of material area exposed to the oral conditions. For wider brackets, which are introduced to provide better rotational and tipping control of teeth, these figures are expected to be higher.
Table I compares the surface area of adhesives or composites exposed to the oral environment in the routine case of orthodontic bonding and the corresponding values of aligner treatment with the use of attachments. The assumption for the aligner scenario was the use of 12 attachments per case with varying shape depending on their position (4 trapezoid, 4 rectangular, and 4 elliptical attachments). However, the actual clinical use of composite for aligner attachments differs vastly from the provided crude theoretical comparison of exposed surfaces because of the following two reasons. First, the adhesive during bracket bonding is applied in a sandwich pattern, which decreases exposed surfaces and its potential reactivity with the oral environment. Second, attachments in aligner treatment possess considerable thickness to assist in tooth positioning, and as such, they are exposed to a daily snagging of the aligner during fitting or mastication. Thus, apart from the larger surface exposed relative to the edges of the adhesive, aligner attachments are also subjected to masticatory stresses during eating and stresses arising from the fitting of the aligner on the teeth multiple times daily, which brings us to the next issue.
| Procedure | Assumptions | Estimated exposure area |
|---|---|---|
| Bracket bonding |
|
Margin exposure: (periphery 11.0 mm) × (thickness 200 × 10 −3 mm) (2.2 mm 2 per tooth) × (20 teeth) Total area = 44 mm 2 |
| Attachments with aligners |
|
Beveled (Exposed surface 11 mm 2 ) × (4 x attachments) = 44 mm 2 + Rectangular (Exposed surface 11 mm 2 ) × (4 × attachments) = 44 mm 2 + Ellipsoid (Exposed surface 3.1 mm 2 ) × (4 × attachments) = 12 mm 2 Total area = 100 mm 2 |
In vivo-induced alterations of aligners and attachments
Newly delivered aligners that have not been in clinical use have similarly rough surfaces on both sides because of the reproducible industrial manufacturing process of stereolithography, milling, and polishing that does not include much human interference. Intraoral use of aligners during treatment has been reported to reduce the surface roughness of the aligner coming in contact with the composite attachments. This can be seen as soon as the first week of service and leads to a decreased coefficient of friction and subsequently reduced micromechanical retention of aligners with the tooth and its attachments. In contrast, the lower roughness of as-retrieved aligners after 1-or 2-week exposure could be explained by the intraoral wear of both the aligner attachment and lingual area, with composite and enamel, respectively, indicating a polishing effect from the contact of the aligner with the much harder enamel or composite resin attachment surfaces.
Attachments used for aligner treatment are usually made from composite resins with a hardness of 400-700 N/mm 2 Marten’s hardness (HM) ( Table II ), , whereas human enamel has a hardness of about 2866 N/mm 2 . This translates to a 6-fold increase of hardness for composite resin and a 23-fold increase for human enamel compared with Invisalign aligners. It can be therefore anticipated that aligners will be subject to severe wear by their contact with the attachments or the labial and/or lingual enamel, leading to smoother surfaces of as-retrieved aligners. In addition, as there is a big difference in HM of composites and enamel, lingual surfaces might present more intense wear, but the surface morphology and roughness may, in this case, play a more decisive role. In particular, the composite resin attachment area is microscopically characterized by a striation pattern perpendicular to the tooth axis as a positive remnant of the thermoplastic transfer template. In contrast, the tooth enamel surface is lacking this abrasive texture and appears to be smoother than the nonpolished composite resin. Relevant research from Barreda et al identified attachment surface alteration depending on the hardness and filler loading of the composite used. Specifically, even if in the majority of patients the shape of the alignments was only slightly changed, noticeable changes were observed in the attachments’ texture for most patients, which might include composite cracks or fractures. In addition, significantly greater attachment wear was seen with a micro-filled composite with 76% filler content compared with a nano-filled composite with 72.5% filler content.
| Material/manufacturer | Martens hardness (N/mm 2 ) |
|---|---|
| Transbond LR (3M, Espe) | 785 |
| Transbond XT (3M, Espe) | 568 |
| Accolate (Danville materials) | 276 |
| IPS Empress direct dentin (Ivoclar Vivadent) | 487 |
| ZNano (Danville materials) | 375 |
| Brace paste (American Orthodontics) ∗ | 456 |
| Enlight LV (Ormco Corp) ∗ | 427 |
| G-aenial (GC Corp) ∗ | 337 |
| Flow Tain (Reliance Orthodontics) | 268 |
| Wave (SDL) ∗ | 260 |
∗ Does not contain 2,2-bis[4-(2-hydroxy-3-methacryloy-loxypropyl)-phenyl]propane (Bis-GMA).
Intraoral aging likewise has a significant effect on the mechanical properties of orthodontic aligners, which can be seen even after 1 week. Invisalign aligners received after periods of 1 or 2 weeks of intraoral use show reduced HM and indentation modulus compared with new aligners, whereas the measured values fall into previously reported ranges. The decrease in hardness indicates a material with reduced wear resistance that is more vulnerable to attrition under occlusal forces. Used Invisalign aligners showed significantly increased relaxation index compared with as-received aligners, which to our knowledge, has not been studied extensively in vivo because of the requirement of bulky specimens for relaxation testing. This finding is associated with material softening or residual stress relaxation and is specifically important for aligners that, like other orthodontic appliances, are preactivated (ie, prestrained) and then inserted into the mouth to release orthodontic forces. Under constant deformation, the exerted force is lower, whereas under constant strain, the material is relaxed.
Several explanations exist for this deterioration of the mechanical properties of aligners after intraoral use, with the first lying with the material itself. Fourier transform infrared spectroscopies have revealed that Invisalign aligners are made of a polyurethane-based material, , , and thus under clinical conditions, might suffer from a polyurethane softening mechanism. This finding is based on the fact that thermoplastic polyurethane consists of a 2-phase microstructure with hard and soft segments, where the latter tend to be oriented perpendicular to the applied stresses and then break into smaller pieces to receive further deformation. Other factors proposed to be explanatory of the intraoral deterioration of the aligners’ mechanical properties include the possibility of residual stresses from by the manufacturing process and the leaching of matrix plasticizers.
According to the results of another study employing mechanical testing of various aligners ( Table III ), Invisalign aligners show significantly higher values compared with the other materials used in the laboratory (like A+, Clear Aligner, or Essix ACE Plastic) in terms of hardness, modulus, and elastic index but lower creep resistance. This finding can be ascribed to the different chemical structure between the materials. Significant differences are also seen among different polyethylene terephthalate glycol copolymer (PETG) materials used for in-lab aligners, which might be attributed to 2 factors: (1) different molecular weights of the various PETG polymers and (2) the thermoforming effect on the mechanical properties. Thermoforming may influence the molecular orientation, mean molecular weight, and residual stresses due to rapid cooling of the thermoplastic materials on the stone models. The HM of commonly used aligner materials varies considerably and lies within the range of 80 to 160 N/mm 2 . , Previous studies have reported that PETG materials have higher wear resistance compared with polypropylene materials, but there is no similar comparison between PETG and polyurethane-based materials (like the one Invisalign uses). Likewise, great variability exists in the indentation moduli of aligners, which range between 1500 MPa and 2700 MPa. Generally, a higher modulus of elasticity is preferred, because it increases the force delivery capacity of appliances under constant strain. Otherwise, aligners from materials with a higher modulus of elasticity can be constructed with smaller thickness to provide similar forces. Invisalign aligners have a significantly higher elastic index than other materials (2467 MPa vs 2112-2374 MPa), which indicates a slightly more brittle material. At the same time, the higher indentation creep of Invisalign aligners implies that under constant occlusal forces exerted by the occlusion, they are more likely to deform and therefore attenuate the applied orthodontic forces. However, in summary, these data indicate that Invisalign aligners show a preferred combination of higher hardness and higher modulus, but at the same time, higher creep resistance than other aligner materials.
