The Evolution of Clear Aligner Materials – What Orthodontists Need to Know

As orthodontists, we consider the ideal properties for clear aligner materials. With the growing popularity of clear aligner therapy, it’s crucial to understand the key characteristics that make for an effective aligner material and how recent innovations have impacted treatment outcomes and patient safety.

The Ideal Aligner Material

The ideal aligner material must strike a delicate balance between rigidity and elasticity. It should be rigid enough to apply consistent force to the teeth but elastic enough to maintain a snug fit and not crack or break. Other important properties include:

1. Durability – Able to withstand wear from daily use
2. Clarity – Maintains transparency for aesthetic appeal
3. Biocompatibility – Safe for intraoral use
4. Force retention – Maintains force delivery over time
5. Formability – Can be thermoformed into precise shapes

Common Aligner Materials

Polyethylene terephthalate glycol (PETg)

PETg materials like ZenduraTM are widely used for aligners and retainers. ZenduraTM has been repackaged as ClearQuartzTM by Clear Correct, TruGenTM by SparkTM, and MasterControl STM by Angel Aligner. They offer good clarity and durability. However, studies have shown PETg materials may experience more rapid force decay than newer options.

Polyurethane

Polyurethane materials can offer improved elasticity and force retention compared to PETg. The elasticity allows them to maintain a better fit and more consistent force delivery over time. However, some polyurethane materials may be less rigid than PETg.

SmartTrack Material

Invisalign’s proprietary SmartTrack material is a multi-layer polyurethane and PETg material specifically engineered for aligner therapy. According to Align Technology, SmartTrack is designed to provide gentler, more constant force and improved control of tooth movements compared to its previous single-layer EX30 material.

Material Properties and Performance

A study by Wheeler et al. compared SmartTrack to Invisalign’s previous EX30 material. They found that SmartTrack achieved significantly more tooth movement (73.06% vs 42.75% of planned movement) over 14 days. The SmartTrack material appeared to provide more consistent force delivery over time.

Alexandropoulos et al. compared the mechanical properties of several aligner materials. They found that Invisalign’s SmartTrack material showed higher hardness and elastic modulus values than PETg-based products. This study suggests SmartTrack may offer improved force delivery and control.

Skaik et al. examined the effects of time and aligner removal frequency on force delivery for different PETg materials. They found that forces decreased over time and with increased removal frequency for all materials tested. However, a modified PETg material with higher elasticity and abrasion resistance showed more stable force delivery than conventional PETg.

Thickness and Homogeneity

A recent study by Mantovani et al. used micro-computed tomography to evaluate the thickness homogeneity of Invisalign aligners. They found that the actual thickness of aligners after thermoforming ranged from 0.566 mm to 0.644 mm, less than the manufacturer’s stated 0.75 mm. Interestingly, they observed that the gingival-lingual region of molar aligners was significantly thinner than the occlusal region. This thickness variation is an inherent challenge in the thermoforming process and could explain challenges in achieving predictable tooth movements in the molar region with clear aligners.

Intraoral Aging Effects

Lira et al. investigated the effects of intraoral aging on Invisalign’s SmartTrack material. After 14 days of use, they found:

– No significant chemical changes to the material
– Gradual increase in water absorption and volume
– Significant color changes (darkening and yellowing)
– Decreased elastic modulus and hardness
– Slight increase in surface roughness and biofilm formation

These findings highlight how the oral environment can impact aligner material properties over time, which may affect treatment efficacy and aesthetics.

Safety and Biocompatibility

An important consideration for any orthodontic material is its safety and biocompatibility. Eliades et al. conducted a study on the cytotoxicity and estrogenicity of Invisalign appliances. Their findings showed no evidence of cytotoxic effects on human gingival fibroblasts and no estrogenic activity in their in-vitro assay. This study is reassuring for clinicians and patients, suggesting that Invisalign aligners do not release harmful substances or disrupt endocrine function.

Patient Comfort and Experience

Material advancements may provide benefits beyond just occlusal outcomes. A study by Brascher et al. found that patients reported less pain intensity, pain duration, and pressure on insertion with SmartTrack compared to the previous aligner material. This improved comfort could lead to better patient compliance and satisfaction.

Conclusion

The need to optimize force delivery, improve the predictability of tooth movements, and enhance patient comfort has driven the evolution of clear aligner materials. Skilled clinicians can achieve excellent results with various aligner materials and appliances, and current evidence shows promising mechanical properties and force delivery characteristics with materials like SmartTrack. We hope to see more comparison studies between aligner brands and materials as emerging brands and competitors launch their products.

As orthodontists, it’s crucial to understand the properties of the aligner material you’re using and how it may impact treatment. When selecting an aligner system, consider factors like planned tooth movements, patient comfort, wear time, and the potential effects of intraoral aging.

While advancements in materials science have improved aligner performance, it’s important to note that material properties are just one factor in successful clear aligner therapy. Proper diagnosis, treatment planning, and clinical expertise remain paramount in achieving optimal outcomes. As materials science continues to advance, we can look forward to even better options in the future, potentially leading to more efficient and effective clear aligner treatments for our patients.

References:

1. Wheeler T, Patel N, McGorray S. Effect of aligner material on orthodontic tooth movement. J Aligner Orthod. 2017;1(1):21-27.

2. Alexandropoulos A, Al Jabbari YS, Zinelis S, Eliades T. Chemical and mechanical characteristics of contemporary thermoplastic orthodontic materials. Aust Orthod J. 2015;31(2):165-170.

3. Skaik A, Wei XL, Abusamak I, Iddi I. Effects of time and clear aligner removal frequency on the force delivered by different polyethylene terephthalate glycol-modified materials determined with thin-film pressure sensors. Am J Orthod Dentofacial Orthop. 2019;155(1):98-107.

4. Lira LF, Vargas EO, da Silva EM, et al. Effect of oral exposure on chemical, physical, mechanical, and morphologic properties of clear orthodontic aligners. Am J Orthod Dentofacial Orthop. 2023;164(2):e51-e63.

5. Brascher AK, Zuran D, Feldmann RE, Benrath J. Patient survey on Invisalign® treatment comparing the SmartTrack® material to the previously used aligner material. J Orofac Orthop. 2016;77(6):432-438.

6. Mantovani E, Parrini S, Coda E, et al. Micro computed tomography evaluation of Invisalign aligner thickness homogeneity. Angle Orthod. 2021;91(3):343-348.

7. Eliades T, Pratsinis H, Athanasiou AE, Eliades G, Kletsas D. Cytotoxicity and estrogenicity of Invisalign appliances. Am J Orthod Dentofacial Orthop. 2009;136(1):100-103.