Summary

For almost 20 years, clear aligners have been used with growing popularity in addressing with success patients’ demands for esthetic orthodontic treatment. Many companies all over the world have developed methods of fabricating custom-made clear aligners designed to gradually and sequentially move teeth to their desired positions. Treatment efficacy with clear aligners has been reported to be good but further investigation of the various aspects of this kind of orthodontic treatment modality is needed for scientific evidence and further clinical improvement. This chapter presents an overview of the patients’ esthetic treatment demands, which led to the popularity of this modality of orthodontic therapy, as well as important aspects of the clear aligner method, the orthodontic tooth movement with clear aligners, and the material properties of clear aligners. Clear aligner material efficiency and effectiveness should continue to be studied, as a better understanding of the material properties and treatment outcomes could lead to better sequencing of tooth movement and more efficient treatment.

Introduction

Patients’ demands for esthetic orthodontic treatments have grown to include esthetic appliances, such as ceramic brackets, lingual orthodontics, and clear aligner therapy.^{1 , 2 , 3} If patients’ orthodontic treatment motivations are esthetically driven, they may prefer a more attractive appliance as well. More Invisalign patients reported seeking treatment to improve their appearance (85 vs. 67% for fixed appliance patients), whereas more fixed appliance patients reported seeking treatment because their dentist referred them (26 vs. 3% for Invisalign patients).⁴

Companies including Align Technology (Santa Clara, California, United States), Allesee Orthodontic Appliances (Sturtevant, Wisconsin, United States), and Smile Direct (Nashville, Tennessee, United States) have developed a method of fabricating custom-made clear aligners designed to gradually and sequentially move teeth to their desired positions.⁵ The short-term chemical and physical changes, as well as the structural conformation and leaching before and after use, have been previously studied on Invisalign (Align Technology).^{6 – 8} Invisalign changed in 2013 the material that was used in making their aligners to SmartTrack aligner material, which continues to be a polyurethane-based material but has been claimed to have increased elasticity and a more precise fit.⁹ To date, no studies have investigated the mechanical properties of the clear aligners manufactured by Allesee Orthodontic Appliances, including Simpli5 and Red, White and Blue, or that of Smile Direct. The latter is a relatively recent entry to the market place with no evidence in the scientific literature to verify its claims of efficacy and efficiency in treatment.

Treatment efficacy with clear aligners has been reported to be 41 to 59%, but further investigation in material behavior is needed for improvement.^{10 – 12} The force delivery properties of aligners are influenced by both the direction of displacement and the stiffness of the material used.^{13 ,​ 14} A more recent study has found that the orthodontic force produced by a thermoplastic material is strongly correlated with its hardness and elastic modulus. Therefore, any significant differences in the properties of clear aligners may have an impact on what aligner system the practitioner chooses to use.¹⁵ Material properties may even affect the treatment outcome, as it was found that patients wearing a harder aligner material for a 2-week activation time showed the best results in all measurements of occlusal and alignment improvement, although the difference was not statistically significant.¹⁶

It is also important to determine if the material’s properties change after use, as biofilm modification and oral environmental conditions may have effects on the hardness and viscoelasticity of the material.¹⁷ Previous studies have detected changes in the Invisalign material after use, including increased hardness, decreased mechanical properties, abraded cusp tips, integument adsorption, biofilm calcification, microcracks, delamination, and loss of transparency.^{6 – 8}

Esthetic Treatment Demands

Patients’ demands for esthetic orthodontic treatment have grown to include any type of esthetic appliances which are minimally visible.^{1 – 3} The appearance of orthodontic appliances plays a significant role in patients’ decisions to receive orthodontic treatment. A survey found that 33% of young adults would be unwilling to wear visible braces if needed.¹⁸ Another study noted that while traditional metal brackets were found to be esthetically acceptable to only 55% of adults, clear aligners were acceptable to over 90%.¹ Furthermore, they showed no difference in acceptability ratings when considering the appliances for their own treatment or for their children’s treatment, and they were willing to pay more for appliances they deemed more esthetic. Clear aligner preference extends to adolescents as well, as surveyed 15- to 17-year-olds rated clear aligners most acceptable and attractive over ceramic, self-ligating, traditional, and shaped brackets.¹⁹

As more adults are seeking orthodontic treatment, esthetic improvements of appliances may be a major factor in the increase of acceptability of orthodontic treatment in this group of patients. Perceived personal characteristics of adults may be influenced by their dental appearance and orthodontic appliance design: greater perceived intellectual ability was associated with the appearance of no appliance or aligner appliances compared to steel or ceramic appliances.³ This could likely influence the patient’s orthodontic appliance choice. If their treatment motivation is esthetically driven, they may prefer a more esthetic appliance as well.

Clear Aligner Therapy

Companies including Align Technology and Allesee Orthodontic Appliances have developed a method of fabricating custom-made clear aligners designed to gradually and sequentially move teeth to their desired positions (Fig. 1.1).⁵ The short-term chemical and physical changes, as well as the structural conformation and leaching before and after use have been previously studied on Invisalign (Align Technology).^{6 ,​ 7} However, Invisalign has recently changed the material that was used in making the aligners to SmartTrack aligner material, which continues to be a polyurethane-based material.⁹ Align Technology states that the SmartTrack material delivers a lower initial insertion force for improved patient comfort, while maintaining more constant force over the 2-week wear. Additionally, it is claimed to have higher elasticity and a more precise fit. This is beneficial in that it improves tracking and control of tooth movements.²⁰ No studies to date have investigated the mechanical properties of the clear aligners manufactured by Allesee Orthodontic Appliances, including Simpli5 and Red, White and Blue. Both systems use the company’s highly esthetic proprietary material and are designed to treat minor to intermediate anterior misalignment, only differing in the number of aligners the patient has to wear to correct the misalignment.²¹

The aligner manufacturing process differs for the two companies. Align Technology uses stereolithography technology to create plastic resin models from photoactivated polymer.⁵ The patient’s polyvinyl siloxane (PVS) impressions are scanned and convertedb into three-dimensional electronic models, where the teeth are electronically separated and moved by a technician. Alternatively, the models can be fabricated directly from the patient’s intraoral scan.²² Each stage of treatment is converted into a physical model with a stereolithography apparatus, and an automated aligner system heats, forms, and laser-marks sheet plastic over each model.⁵ Ridges from the model formed by stereolithography can be seen in the finished aligner material, and the tray is scalloped along the gingival margin (Fig. 1.2). Conversely, Allesee Orthodontic Appliances fabricates their aligners from stone models where the individual teeth are manually sectioned by lab technicians and repositioned with wax.^{23 ,​ 24} The finished product is highly transparent with a straight-line finish instead of scalloping the gingival margins (Fig. 1.2). Each system produces clear aligners from the models, each corresponding to a 2- to 3-week interval of treatment. Progressive alignment of 0.25 to 0.5 mm is designed into each aligner.²⁵ Aligner systems including ClearSmile and Raintree Essix allow more displacement in each aligner (0.5–1 mm) compared to the Invisalign system (0.25–0.33 mm).^{25 ,​ 26}

Benefits of clear aligner therapy include esthetics, comfort, oral hygiene improvement, and reduced chair time.²⁷ Adult Invisalign patients have reported less pain and fewer negative impacts on their lives than those with fixed appliances.^{4 ,​ 28} Those with fixed appliances took more pain medication during the first week of orthodontic treatment than the Invisalign patients.⁴ Adolescents also have positive attitude to aligners. The vast majority did not limit foods, avoid communication, or feel self-conscious while wearing the aligners.²⁹ After 3 months, 70% had seldom or never experienced discomfort, and 80% had seldom or never used pain relievers. As treatment progressed, the patients reported even less discomfort.²⁹ In addition to improved comfort, clear aligners also show favorable consequences for periodontal health compared to fixed appliance treatment.³⁰ After 24 months, teenagers using Invisalign Teen aligners had the plaque index decreased by 15.1% in the maxilla and 16.6% in the mandible.²⁹

Orthodontic appliances must be selected on the basis of more than appearance, as the appliances must have desirable functional properties and treatment outcomes. A systematic review in 2005 determined that there was not sufficient evidence to adequately evaluate Invisalign treatment effects, and that high-quality clinical evidence was needed.³¹ Since then, there have been numerous studies that have looked at the efficacy and treatment outcomes of Invisalign treatment. Treatment efficacy with clear aligners has recently been reported to range from 41 to 59%.^{10 – 12} While the reported treatment efficacy numbers are low, case reports have shown successfully completed moderate to difficult orthodontic malocclusions, including open bite, extraction, and surgical cases.^{32 – 37} Furthermore, resolving moderately severe anterior crowding can be successfully accomplished with Invisalign.³⁸

Treatment outcomes of Invisalign have been compared to fixed appliances using the objective grading system of the American Board of Orthodontics. Compared to traditional braces, Invisalign lost an average of 13 more points and had a 27% lower passing rate.³⁹ While the strengths of Invisalign included its ability to close spaces and correct anterior rotations and marginal ridge heights, it was deficient in correcting large anteroposterior discrepancies and occlusal contacts.³⁹ Evaluation of dental casts of patients treated with Invisalign and their comparison to patients treated with fixed appliances immediately after end of treatment and 3 years posttreatment concluded that patients treated with Invisalign relapsed more than those treated with fixed appliances, particularly in the maxillary anterior region. Even though the Invisalign group relapsed more, the mean alignment was superior to the fixed appliance group before and after the retention phase.⁴⁰

Orthodontic Tooth Movement with Clear Aligners

The type of desired tooth movement influences the efficacy of treatment with clear aligners. When looking at dental improvements, aligners were most successful in improving anterior alignment, transverse relationships, and overbite.¹⁶ Aligners were least successful at improving buccal occlusion and only moderately successful at improving midline and overjet.¹⁶ One study reports lingual constriction to be the most accurate movement (47.1%) and extrusion to be the least accurate (29.6%).¹⁰ Additionally, it was determined that canine rotation accuracy was significantly lower than for other teeth and lingual crown tip was significantly more accurate than labial crown tip. This study was done without auxiliaries in order to provide a baseline value of what can be achieved with aligners alone. A relatively recent study investigated the efficacy of Invisalign aligners in tooth movements deemed difficult with aligners and analyzed the influence of auxiliaries, including attachments and Power Ridges.¹⁴ It was found that premolar derotation showed the lowest accuracy (40%), while molar distalization was the most effective movement (87%). No statistically significant difference was found with the use of attachments in the efficiency of premolar derotation or molar distalization. Furthermore, no substantial difference was observed if incisor torque (42% mean accuracy) was supported with a horizontal ellipsoid attachment or a Power Ridge. A very recent systematic review looked at 3 randomized clinical trials and 8 prospective and 11 retrospective studies.⁴¹ It concluded that Invisalign aligners is a viable alternative to conventional orthodontic therapy in the correction of mild to moderate malocclusions in nongrowing patients that do not require extraction.⁴¹ This technique is good at levelling, tipping, and derotating some teeth, but has limited efficacy in arch expansion through bodily movement, extraction space closure, correction of occlusal contacts, and larger anteroposterior and vertical problems.

Other factors can influence orthodontic tooth movement with clear aligners, such as age, gender, root length, and bone levels. A quadratic (U-shaped) relationship between age and tooth movement was found for women, indicating an increase in tooth movement in younger and older women.¹¹ However, a more linear relationship was found for men, with decreased movement at older ages. This study also found a significant negative correlation between tooth movement and the measurement of the apex to the center of rotation, but bone quality was not correlated with tooth movement.¹¹ This may account for individual differences in treatment efficacy during clear aligner therapy.

As tooth movement with aligners has been distance-based as opposed to force-based with fixed appliances, recent studies have attempted to quantify the force delivery properties of aligners. Initially, it was determined that median force values for intrusion during rotation of an upper central incisor at the low activation range of ±0.17 mm were between 0.0 and –0.8 N, with the highest intrusive force being –5.8 N for a rotation of –0.51 mm.^{13 ,​ 14} A few years ago, it has been reported that initial mean moments were 7.3 N mm for maxillary incisor torque, 1.0 N mm for distalization, and 1.2 N mm (without attachments) to 8.8 N mm (with attachments) for premolar rotation.²⁵ While the recent findings suggest that bodily tooth movements and torque can be performed with aligners since they deliver the necessary force systems, the ideal values (0.35–0.6 N for rotation and tipping, and 0.1–0.2 N for intrusion) were exceeded.⁴²

The force delivery properties of aligners are influenced by both the direction of displacement and the stiffness properties of the material used.^{13 ,​ 14} At lower activation ranges, different chemical and physical material properties might be responsible for the different force levels. The local deformations of the material and friction at the contact areas may be of relevance.^{13 ,​ 14}

Material Properties of Clear Aligners

A better understanding of the material properties could lead to better sequencing of tooth movement and more efficient treatment.⁴³ It has been found that there is great variety in mechanisms among the initial force systems during clear aligner therapy, as an aligner with high initial force may be followed by an aligner with a low force, resulting in tooth movement that is not constant.¹² Additionally, as the order of sequential aligners increases, aligner strains relating to force delivery increase.⁴⁴ A relevant study has found that the orthodontic force produced by a thermoplastic material is strongly correlated with its hardness and elastic modulus; therefore, any significant differences in the properties of clear aligners may have an impact on what aligner system the practitioner chooses to use.¹⁵ Material properties may even affect the treatment outcome, since patients wearing a harder aligner material for a 2-week activation time showed the best results in all measurements of occlusal and alignment improvement, although the difference was not statistically significant.¹⁶

It is also important to determine if the material’s properties change after use, as biofilm modification and oral environmental conditions may have effects on the hardness and viscoelasticity of the material.¹⁷ During the time the aligners are worn, they are exposed to salivary enzymes, byproducts of oral flora, liquids, and trauma caused by swallowing, speech, and bruxism.⁶ In vitro testing conditions are unable to simulate the intraoral conditions the aligners are exposed to, including plaque accumulation. Therefore, retrieval analysis obtains critical information since it investigates the material in its intended environment.¹⁷

Previous studies detected changes in the Invisalign material after use. Differences were found in the surface morphology of aligners after use, including abraded cusp tips, integument adsorption, biofilm calcification, microcracks, delamination, and loss of transparency.^{7 ,​ 9} Delamination of the material can lead to loss of mechanical strength of the aligner.⁶ The loss of transparency may be caused by trauma from chewing and bruxism.⁶ Additionally, buccal segments showed an increase in hardness and a decrease in mechanical properties, which may be caused by masticatory-induced cold work.^{7 ,​ 8}

Although polyurethane is biocompatible, it is not an inert material, as it is sensitive to heat, humidity, and salivary enzymes.⁶ No traceable byproducts were detected after Invisalign aligners were stored in artificial saliva or an ethanol aging solution.^{6 ,​ 7} Furthermore, no evidence of cytotoxicity or estrogenicity was found at various concentrations of aligner eluents.⁴⁵ This could be related to the material’s structure, as it is composed of polyurethane with added methylene diphenyl diisocyanate and 1,6 hexanediol.⁷ The diphenyl structure provides stability and sufficient reactivity to form a polymer free of byproducts.⁷ Also, unlike the aromatic rings in bis-GMA, polyurethane has short rigid portions joined by short flexible hinges and long flexible portions.⁴⁵ However, the in vitro testing conditions may have underestimated the material’s chemical stability.

Past studies have shown that clinical recommendations can be determined from their material study’s results. One study found that as the order of sequential aligners increases, aligner strains relating to force delivery increase. It was concluded that final aligners should be thicker or worn for a longer period of time.⁴⁴ However, another study determined that thin material (0.508 mm) can deliver higher energy than thick materials (0.762–1.1016 mm), and it, therefore, recommended that thin material be selected to move teeth efficiently.²⁶

Other investigations have found changes in the aligner material after use. In the deflection ranges of optimal force delivery (0.2–0.5 mm), the force delivery properties of aligners were different after repeated load cycling, but were not different after thermocycling.²⁶ Additionally, both thermocycling and load cycling influenced Vickers hardness values. This may be significant because if aligners become hard during use, they may cause discomfort to the patient and induce changes in their force delivery properties. Another study has found that intraoral aging adversely affected the mechanical properties of Invisalign. Contrary to the previous study, used aligners had significantly lower hardness values, with higher elastic index and creep indentation values.⁸ The decreased hardness indicates a less wear-resistant material, where the increased elastic index implies the material become more brittle.⁸

While changes in material properties after clinical use have been reported, it has also been found that neither material fatigue nor difference in stiffness plays a significant role in the rate or amount of tooth movement. No significant difference was found in the amount of orthodontic tooth movement between those who wore the same aligner for 2 weeks and those who changed to a new duplicate aligner after 1 week.⁴³ Another study did not find a substantial difference in the treatment completion rate when comparing hard and soft appliances, with completion rates being 32 and 27%, respectively.⁴⁶ However, neither material was the same as the material used by Align Technology. The hard material used was twice as stiff as the commercial material, while the soft material was one-tenth as stiff.

These differing and sometimes contradicting findings suggest that orthodontic tooth movement with clear aligners needs further study. A better understanding of the clear aligner material properties remains pivotal to this ongoing investigation and could potentially lead to better sequencing of tooth movement and more efficient treatment. However, comparisons between studies may be a challenge, as the material used in future studies may not be the same material used in past studies, since the companies continue to evolve the material and its characteristics.