Understand the Differences Among Clear Aligner Materials

Long gone are the days of wire-and-bracket braces being the only option for managing malocclusion, crowding, and other issues requiring orthodontic intervention. The development of clear aligner therapy offers patients a more esthetic alternative to traditional braces.

The basis for clear aligner therapy was first envisioned in 1945 by Harold Kesling, who said that “major tooth movements could be accomplished with a series of positioners by changing the teeth on the setup slightly as treatment progresses.”¹ Kesling advocated for the use of rubber-based tooth positioners based on wax models to sequentially reposition misaligned teeth. Although today’s aligners are made of thermoplastic materials instead of rubber, Kesling’s fundamental concepts laid the groundwork for the clear aligner therapy we use today.

Clear aligner therapy as we know it officially entered the market in 1998, when Align Technology launched its Invisalign removable polyurethane aligners. Although Invisalign remains a major player in the clear aligner game, many other companies have developed their own clear aligner therapies to much success.

And the need for clear aligner options is there. Clear aligners are now a mainstay of orthodontic care and have become increasingly popular. This is due to both impressive improvements in biomaterials and CAD/CAM technologies as well as an increase in adult patients looking for esthetic and comfortable alternatives to conventional fixed appliances.^2,3 The rapid spread of CAD/CAM technology has also accelerated the development of clear aligner technologies. The global clear aligner market, valued at $3.1 billion in 2021, is projected to increase to $11.6 billion in 2027.⁴

Although the numbers are impressive, the true success of clear aligner therapy can only be based on patient outcomes. The clinical performance of clear aligners is greatly affected by the materials used in aligner fabrication, and the mechanical, chemical, optical, thermal, and biological qualities of these materials are critical in the ultimate success of the clear aligner treatment.³ Findings from a 2022 study stated, “Advances in aligner material chemistry possess the potential to bring about radical transformations in the therapeutic applications of clear aligner therapy; in the absence of which, clear aligner therapy will be significantly limited [because of] its inherent biomechanical constraints and clear aligners would continue to underperform clinically, especially in comparison [with] conventional fixed orthodontic appliances.”³

Because the materials used in clear aligners have such a great effect on clinical performance, it’s important to understand the materials employed and the manufacturing processes used. “To improve the performance of aligners in orthodontic treatment, it is significant to investigate the properties of the materials and how they respond to various stresses and then develop the more performing ones,” researchers state.⁵

The Ideal Aligner

Clear aligners are subjected to numerous stresses in the mouth. They are subjected to temperature variations, moisture, chemical stresses of saliva and beverages, intermittent mechanical load stresses, and the thermal stress associated with the aligner creation process.⁵ To combat these stressors, an ideal aligner material should “exhibit high resilience, low hardness, sufficient elasticity, adequate resistance to varied stress and distortion, excellent transparency, low cytotoxicity, and high biocompatibility,” findings from one study state.⁵

Aligners also need to possess adequate stiffness to exert the force necessary to execute the planned tooth movement. However, if the material used for fabrication exhibits a high modulus of elasticity (stiffness levels that are too high), the aligner will be inflexible, resulting in the patient having difficulty placing and removing the aligner. Conversely, if an aligner is not stiff enough, if won’t be capable of generating adequate levels of force required to move the teeth.⁶

This fine line is one that has been carefully walked by clear aligner fabricators. Although most aligners are composed of thermoplastic materials (primarily polymers), different material compositions respond varyingly to the thermal and mechanical stresses. This has led to constant research and development as manufacturers explore the best materials and combinations for better outcomes.

Clear aligners have undergone numerous transformations in the search for optimized clinical efficiency and the ability to manage issues more esthetically, comfortably, and effectively. The first generation of aligners relied completely on the thermoformed plastic aligner material to move the teeth, without any additional features incorporated into the aligner system. Second-generation aligners incorporated attachments to provide more refined control of tooth movement. By 2013, enhancements were introduced to improve the predictability of deep bite correction.³ These developments have continued over the years, with new advancements in both systems and materials entering the market regularly.

The Materials

Although several different materials are used in clear aligner manufacturing, there are 4 primary categories to consider. These categories are thermoplastic polymers, polymer blends, 3D-printed materials, and bioactive materials.

Thermoplastic polymers

Based on their inherent molecular structure, thermoplastic polymers are classified as either amorphous or semicrystalline polymers. Amorphous polymers have irregularly arranged molecular structures with a low degree of molecular packing, whereas semicrystalline polymers comprise both irregularly arranged areas (or amorphous regions) and uniformly and tightly packed crystalline domains. Amorphous polymers are generally softer and transparent, with low shrinkage and good impact resistance. Semicrystalline polymers are harder and opaquer, with good chemical resistance and a sharp melting point. These properties are enabled by the crystalline domains, which confer hardness and rigidity to the material like a filler in a composite material.⁷

Across the board, the polymers most commonly used for clear aligners are polypropylene, polyurethane or copolyester, polycarbonate, polyvinyl chloride, ethylene vinyl acetate, and polyester.⁸ Within the polyester category, polyethylene terephthalate and its amorphous copolymer polyethylene terephthalate glycol are commonly used in clear aligner production because of their impressive optical and mechanical properties.⁹ Other extensively employed materials include thermoplastic polyurethane (which has favorable properties including strong mechanical and elastomeric qualities, abrasion and chemical resistance, and simplicity of manufacturing) and polycarbonate (known for its durability, transparency, and hardness).⁸

Polymer blends

When the powers of these materials combine, the mechanical properties of the polymers improve even more. Polymer blends of polyester, polyurethane, and polypropylene are commonly used in clear aligner manufacturing. This is due to study findings that show thermoplastic polymer blending results in improved mechanical and chemical properties, which can enhance clear aligners’ clinical performance.⁸ The mixture of these polymers is important; the blending ratio is essential in determining the features of the ultimate blend. For example, some blends have better mechanical properties, providing superior sustainable orthodontic forces, whereas others may have better tensile or impact strength.⁵

3D-printed materials

3D printing has changed the clear aligner game completely. 3D-printed aligners reduce the potential for errors from the analog impression or intraoral scan, subsequent 3D model, and ultimate thermoplastic process.¹⁰ In addition to improved accuracy and reduced errors, 3D printing can lower costs and shorten the time it takes to produce aligners.

Direct 3D-printing methods also avoid the adverse effects of the traditional thermoforming process, which can cause alterations of the material’s mechanical and esthetic characteristics during production. Thermoforming has been shown to decrease the transparency of thicker materials, increase water absorption and solubility, and enhance surface hardness of materials.¹¹

In short, 3D printing creates streamlined outcomes. As opposed to conventional fabrication, the direct 3D printing of clear aligners allows components to be created layer by layer rather than by traditional methods of moulding or machining.¹² This process uses different materials from traditional fabrication, such as acrylonitrile butadiene styrene plastic, stereolithography materials (epoxy resins), polylactic acid, polyamide (nylon), glass-filled polyamide, silver, steel, titanium, photopolymers, wax, and polycarbonates.¹² Several different 3D printing processes can be used for the direct printing of these materials, including selective laser sintering/melting, fused deposition modeling, direct pellet-based fused deposition, stereolithography, continuous liquid interface production technology, or multijet photo-cured polymer process.¹³

This results in better fit, higher efficacy of mechanical resistance, and improved geometric accuracy and precision.^10,11 Additionally, direct 3D printing results in softer aligner edges that don’t require trimming or smoothing, digitally defined undercut analysis, and customizable intraaligner thickness.¹²

Bioactive materials

A considerable concern with any orthodontic treatment is oral hygiene. Proponents of clear aligner therapy present clear aligners as a more hygienic option than traditional orthodonture because the appliances are removable. However, because the patient’s teeth and gingiva are covered by the aligner for 20 to 22 hours per day (except for removal for eating, etc), there could be an increased risk of bacterial growth, causing tooth or periodontal damage.³ This microbial accumulation of oral pathogens such as Streptococcus mutans and Porphyromonas gingivalis, a common problem of orthodontic treatment, has spurred a series of studies examining materials and potential applications of nanoantibacterial materials and bioactive properties that could be integrated within them.¹⁴

One such research study examined the application of gold as an antibacterial property. In the study, researchers coated 4,6-diamino-2-pyrimidinethiol–modified gold nanoparticles over clear aligners. In a suspension of P gingivalis, this coating provided antibacterial effects that slowed biofilm formation and showed favorable biocompatibility.¹⁵

A second research group turned to essential oils, infusing a cellulose-based clear aligner material with cinnamaldehyde. This material demonstrated antimicrobial properties against Staphylococcus epidermidis as well as S mutans. Results from the study found that cinnamaldehyde reduced biofilm formation in a laboratory setting. As a bonus to its antimicrobial properties, cinnamaldehyde also increased hydrophobicity of some materials, which could further decrease early adhesion and delay the formation of biofilm.¹⁶

Although the perfect balance of antimicrobial properties and biocompatibility has yet to be achieved in current nanomaterials, studies are ongoing.

The Right Combination?

Although many different permutations of materials are on the market, there’s no right answer when it comes to defining the perfect aligner. Each material has its own beneficial properties, and advancements continue to refine and improve clear aligner materials. Although thermoplastic polymers and polymer blends continue to dominate the commercial clear aligner market, different companies are taking different approaches:

Invisalign (Align Technologies).Invisalign currently uses its SmartTrack™ material LD30. This material is composed of a multilayer aromatic thermoplastic polyurethane from methylene diphenyl diisocyanate and 1,6-hexanediol plus additives. Compared with prior material EX30, LD30 has a more amorphous structure and greater elastic recovery.

Clarity (3M ESPE). Clarity aligners incorporate 2 different materials, depending on treatment goals. 3M’s Clarity™ Aligners Flex’s flexible 5-layer material is indicated for a wire-sequencing approach, whereas Clarity Aligners Force, a rigid material composed of a proprietary 5-layer copolymer blend, is indicated for segmental mechanical approaches. Designed so that the outer layers resist staining and scratching, the inner layers provide flexibility and resilience.

ClearCorrect (Straumann). ClearCorrect aligners are composed of its ClearQuartz material, a Zendura polyurethane trilayer proprietary blend. ClearQuartz features an elastomeric layer between 2 resilient, low-porosity shells. The outer layer has low porosity, allowing the aligner to be tough and stain resistant and help grip teeth firmly. With its enhanced elasticity, the inner layer is engineered to provide consistent and continuous force.

SureSmile (Denstply Sirona). SureSmile aligners are made of Essix PLUS or Essix C+, composed of polypropylene/ethylene copolymer (> 95%) and stabilizers (< 5%). The Essix C+ material is a more flexible plastic that provides additional strength to help withstand mastication pressure when making aligners for bruxers.

Although these formulas serve as current examples on the market, as materials continue to develop, aligner material formulas will certainly change so that companies can continue to bring the best properties to clear aligners. Understanding these materials and the properties they feature can help clinicians make informed decisions regarding the best fit for their patients and guarantee the most favorable outcomes.

References

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10. Jindal P, Juneja M, Siena FL, Bajaj D, Breedon P. Mechanical and geometric properties of thermoformed and 3D printed clear dental aligners. Am J Orthod Dentofacial Orthop. 2019;156(5):694-701. doi:10.1016/j.ajodo.2019.05.012
11. Ryu JH, Kwon JS, Jiang HB, Cha JY, Kim KM. Effects of thermoforming on the physical and mechanical properties of thermoplastic materials for transparent orthodontic aligners. Korean J Orthod. 2018;48(5):316-325. doi:10.4041/kjod.2018.48.5.316
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