The ecological impact of resin-printed models in clear aligner treatment

Clear aligner therapy is now a common treatment strategy in orthodontics. In addition to orthodontists prescribing this treatment, it is increasingly being carried out by general dental practitioners (GDPs).¹ The rise in uptake of this treatment is primarily due to increased societal pressure for straighter teeth.² Furthermore, patients have an expectation that this aesthetic improvement can be achieved without the need for unfashionable headgear or chunky metal brackets, discomfort, inconvenience and complicated oral hygiene requirements.³

A number of companies offer clear aligner treatment services including, Invisalign, Straumann, Dentsply Sirona and Ormco (Figure 1). Improvements in technology and small-scale resin printers have also allowed orthodontists to sequence and print their own aligner systems.

There is a limit to the tooth movement that can be achieved between one aligner and the next in the series, roughly 0.2–0.25 mm. Each case of aligner therapy typically uses 7–26 sets of aligners, with the number needed dependent on the presenting malocclusion: complex malocclusions often require more sets to achieve the desired result. Invisalign (Align Technology, Tempe, AZ, USA) reported a total of 2,358,645 case shipments in 2022, and 2,547,685 in 2021.⁴

Currently, the manufacturing process is the same for each system. A digital scan is taken and sent to the manufacturer; the sequenced models are 3D-printed, and aligners are thermoformed around these models.⁵ Each set of aligners requires an upper and a lower model to be printed. Owing to the 3D printing process and the resins used, models cannot be recycled and will ultimately be thrown away. The thermoset plastics used in aligners are irreversibly hardened, preventing re-melting or re-moulding.^5,6 In a case with 26 aligner sets, 52 models will be required. If this is multiplied by the number of cases treated by Invisalign in 2022, then the estimated number of discarded models will be approximately 122 million. This number is, however, likely to be higher because of the need for refinements and in-print failures. Furthermore, this figure only reflects the estimate for a single company, and there are now many companies that provide this service.

Invisalign has treated 14 million patients since its inception.⁷ Taking an average number of 26 aligners per case,⁸ this equates to up to 728 million potential models and a comparable number of aligners. Not only this, but each aligner pair has plastic packaging.

Following a 7–14-day wear period, patients are often encouraged to discard their aligners into general waste. Invisalign has stated that most of their production scrap and waste plastic generated at their manufacturing location in China is repurposed for reuse in floor tiles. However, they do not publish further data on how they dispose of the models of the manufacturing located in Mexico and Poland, and how much waste is produce each year.⁹

Undoubtedly, clear aligner therapy has revolutionized and been fully integrated into orthodontics. However, in a world focused more heavily on preservation and sustainability, it is essential that industry leaders act responsibly in future developments. This article aims to highlight a number of issues that contribute to material wastage in clear aligner therapy, and offers some practical solutions for individual clinicians, as well as solutions for future developments in this technology.

Recycling: the issue

3D-printed models can be printed through multiple techniques. The models produced by Invisalign and Dentsply Sirona are 3D-printed models. Most commonly in dentistry, they are produced by either stereolithography (SLA) or digital light processing (DLP), (Figure 2). These provide the accuracy needed for dental and orthodontic purposes. In both methods, there is the successive deposition of photo-sensitive resin material, cured solid with a laser in SLA printing, or projector, in DLP printing.¹⁰ Both types have individual advantages and disadvantages, but are similarly sufficient for model printing. Owing to the light curing process in both of these techniques, strong chemical bonds are formed that prevent the models from being recycled.

Aligners are deemed to be medical devices and, as a result, cannot be recycled owing to the biocontamination from patient wear. In addition, aligners are made from multi-layered polymers distinguished by their rigidity and elasticity to permit tooth movement, but another poorly recyclable material. Invisalign's aligners are made of a proprietary mixture of co-polyester and polyurethane (TPU), which requires unique recycling processes, rendering recycling in the typical method impossible. Furthermore, small plastics such as aligners and plastic aligner packaging are discarded by recycling plants as the energy to recycle them outweighs the material return.

By 2050, it is estimated that the world will generate 26 billion tons of plastic waste.^11,12 Of this, it is estimated that 20% has the potential to be recycled, but only 9% is recycled.¹³ The vast majority of plastics are incinerated or discarded into the environment, leading to plastic pollution in the soil, air, and water environment.¹⁴

Environmentally, plastics can threaten biodiversity by leaching harmful chemicals, entangling organisms and also leading to the spread of invasive species into new habitats by the process of ‘rafting’.¹⁵ Invasive species that ride plastic waste to new areas can reduce habitats of native species, carry various diseases, and put further strain on ecosystems – reducing biodiversity, ecosystem function and resilience.

Eventually, the fate of discarded plastics is small (<5 mm) fragmented pieces of plastics known as microplastics.¹⁶ Through various processes, these microplastics are ingested and inhaled by organisms. Through bioaccumulation and biomagnification, larger concentrations of microplastics are found in organisms higher up the food chain, ultimately ending in apex organisms, such as humans.¹⁷

Although the direct effect on humans has yet to be thoroughly researched, recent research has demonstrated the propensity for this to cause organ toxicity in mammals, including endocrinedisrupting effects and decreased locomotor activity.^18,19

Landfill space is limited and often plastics are burned to ‘save space’. This disposal method, known as incineration, is often viewed as energy recovery owing to the ability to generate electricity through this process.²⁰ However, many of these plastics contain additive hazardous materials which, when burned, along with CO₂, produce carcinogenic substances that can affect human health. These toxic vapours also contribute to greenhouse gas accumulation and global warming.²¹

Plastic pollution alters habitats, reducing ecosystems' ability to adapt to climate change, which directly affects organisms within the ecosystem and has socio-economic implications for humans.²²

If dental models, aligners and packaging were manufactured to reduce the resources required for their manufacture and the impact of their waste, there would be significant biological, environmental and social benefits worldwide.

Possible solutions for current issues

Development of a environmentally sustainable material

One of the main issues with aligners is the inability to recycle used materials. The key manufacturing companies need to develop materials that can be recycled to reduce the burden on the environment. One such material proposed for models is nylon, which is more easily recycled into pellets. These pellets can be used in a number of ways, including in the manufacture of non-cosmetic parts of cars and in other machinery.

3D printer type

As mentioned previously, in SLA and DLP printing, resin layers are joined by chemical bonds that cannot be recycled. Fused deposition modelling (FDM), printers are a novel alternative (Figure 3). These printers work by extruding thermoplastic filament through a heated nozzle, melting the material, which is applied in layers. This plastic is deposited one layer at a time until the object is complete.

FDM printers are not currently used in dentistry mainly because of a lack of precision and inaccuracy. The print method of extrusion can lead to voids between layers and results in more defects than other traditional 3D printing methods. With each successive layer, there is a risk of introducing inaccuracies, leading to lower build quality. However, the filament used can be more easily recycled, and prints can be turned into pellets to be made back into filament. If the shortcomings of FDM printers are addressed, the application of this technology in dentistry could be a solution to the excessive plastic waste by allowing for cyclic recycling.

Model design

When created, models are solid to reduce deformation and improve accuracy when the aligners are thermoformed around them. This uses large amounts of resin. Waste could be reduced if the models were produced and printed hollow. Although this is not a long-term solution, it would reduce plastic usage in the interim while a better solution is developed.

Direct 3D printing aligners

In Korea, Graphy Inc (Seoul, Republic of Korea) has developed a material and the technology to 3D print aligners directly without the need for models (Figure 4). This process has many advantages, and if adopted widely, it would drastically reduce material wastage as millions of models would not need to be printed. It would also reduce the environmental impact of transporting/posting aligners across large distances, with the ability to produce aligners in house.

This technology is still in its infancy, but initial studies are promising. This technology will become more widespread as companies, such as FormLabs and SprintRay, develop their direct aligner resin.

Shape memory thermoformed aligners

Smart materials, such as shape-memory aligners, overcome the rate-limiting staging restriction of conventional aligner materials and have demonstrated the possibility of achieving the same clinical outcomes with fewer aligners.²³

One company (ClearX, Düsseldorf, Germany) already offers a thermoresponsive, shape memory, polyurethanebased thermoplastic material. ClearX states that each aligner is used for 3 weeks and has, accordingly, three shapes. These aligners recover their original thermoformed shape after a reforming step of thermal activation, achieved by immersing the aligner in hot water at 67oC for 10 minutes within a booster device. This method reduces the number of clear aligners needed in a standard aligner treatment by almost 50%. Moreover, this company is committed to raising awareness among its customers about the need to reduce plastic in the aligner industry. The company is climate neutral and cooperates closely with Ocean Blue Project, an ocean clean-up organization.¹³

Direct aligner recycling

In the UK, Spotlight Oral Care was a programme partnered with Terracycle with the ambition to recycle any clear aligner and its plastic packaging. This programme is no longer available however, owing to financial reasons and a poor uptake among practices.

Recycling aligners is a controversial strategy and can be viewed as greenwashing. Since there is a global surplus of plastic waste, pellets from recycled aligners and their plastic packaging are probably worthless for other industries, and therefore, it may be pointless to recycle aligners and their plastic packaging.

First, the collected aligners are sent to the material recovery facilities and sorted based on material characteristics and composition. At this stage, the materials are cleaned and sent to third-party processing plants to process the material into usable forms. The plastics are melted down and reformed into pellets, flakes, and powder to manufacture products, such as outdoor furniture, storage containers, athletic tracks and shipping pallets.¹⁹ This repurposing is commonly phrased ‘downcycling’ as the material cannot be used cyclically in new aligners.

Environmental considerations for treatment planning with clear aligners

Split aligner orders

Since the mean accuracy of Invisalign for all tooth movements is 50%, the longer the planned treatment, the greater the chance of the tooth failing to track during the treatment of complex cases.^24,25 If this occurs, additional aligners must be produced, and the original sets become redundant. To prevent this, one strategy is to order half the aligners at a time. The remaining aligners can be ordered when the clinician is satisfied with the treatment progression, teeth tracking and patient compliance.

Case selection

Understanding the limits of clear aligner systems is imperative to ensure that there is no wastage of material should treatment outcomes not be met halfway through treatment. Examples of complex movements include excessive distalization (greater than 3 mm), significant extrusion,²⁶ complex rotations, overcorrections and molar uprighting. This is not an extensive list, but it highlights the need for a level of orthodontic knowledge to ensure treatment outcomes can be met efficiently.

Increased treatment efficiency from case selection, use of auxiliaries and effective planning will indirectly reduce material wastage as refinements may not be required.

Another often overlooked factor is patient compliance. Patients attending review appointments without their aligners could be a sign that they are wearing them for less than the prescribed time. This is likely to lead to refinements and, therefore, aligner wastage. Continual patient motivation is important, and one method to positively reinforce patients who are compliant is reducing the wear period per aligner, for example from 10 days to 7 days. All patients should be advised to keep their previous aligners in case they are required to wear them for longer for tracking issues. Patients who choose aligners should be consistent with their decision, follow the instructions for use and be cooperative to avoid unnecessary refinements.

Communication and education

Aligner manufacturers should prioritize communication of the environmental impact associated with their products to prevent misuse by aligner providers and users. Currently, it seems that Invisalign does not address this communication strategy in its education programme. By providing aligner providers and patients with the necessary tools to minimize the excessive use of aligners and avoid wastage, the manufacturing cost and overall environmental impact of the product can be significantly reduced. This approach would lead to a decrease in the number of refinements required and foster increased collaboration between providers and patients.²⁷

However, the responsibility for making sustainable treatment choices also lies with orthodontists and patients. Orthodontists should pro-actively familiarize themselves with the environmental impact of aligner treatment in comparison to fixed orthodontics. They should strive to work efficiently with aligners and educate their patients about this aspect, ensuring they are informed about the associated disadvantages.²⁷

One study revealed that one out of every six patients switched from Invisalign to conventional braces during the course of treatment.²⁸ This indicates that appropriate case selection by the practitioner prior to the consenting process could have potentially prevented excessive plastic waste and improved the overall treatment duration for the patient.

Conclusion

There has been rapid development and advancement in dental technology in the past decade, improving patient outcomes, but with a heavy environmental toll.

The growing demand for clear aligner therapy has resulted in millions of models and aligners being discarded yearly. There is an urgent need for a sustainable and environmentally conscious solution.

Considering the difficulty of recycling plastic waste worldwide, the surplus of plastics from other industries and the poor quality/performance of products derived from recycled plastic, the aligner industry and its customers should focus on reducing the amount of plastic used in both aligners and their packaging.

There are solutions each clinician can consciously make to reduce the environmental burden of each clear aligner treatment. However, the onus should lie with the companies that produce aligners. Companies should be held responsible and accountable for their production processes, and pressure should be applied for more sustainable practices.

The aligner industry should involve dentists and patients by raising awareness to reduce the environmental impact of this kind of orthodontic system.

Finally, further research must be undertaken to quantify the impact of clear aligner therapy on the environment so that policies, regulations and laws can be implemented, and production methods can be altered. By adopting eco-friendly practices and materials, the clear aligner industry can pave the way for a more sustainable future, preserving both our planet and our smiles for generations to come.