Comparison of Optical Properties of Clear Aligners Before and After In-vivo Aging

1. Postgraduate Student, Department of Orthodontics and Dentofacial Orthopaedics, SRM Dental College, Chennai, Tamil Nadu, India. 2. Professor, Department of Orthodontics and Dentofacial Orthopaedics, SRM Dental College, Chennai, Tamil Nadu, India. 3. Head, Department of Orthodontics and Dentofacial Orthopaedics, SRM Dental College, Chennai, Tamil Nadu, India.

Abstract

Introduction: There is a rising demand for clear aligners among orthodontic patients and after the expiry of invisalign patent many indigenous aligners have been introduced in several part of the globe including India. The major advantage of the aligners is its invisibility which is attributed to its unique optical properties. The optical properties vary between different aligners and any changes in the optical properties during the clinical use is an important factor to be considered.

Aim: To evaluate and compare the optical properties of three different types of indigenous clear orthodontic aligners before and after in-vivo aging.

Materials and Methods: A prospective clinical study was conducted in the Department of Orthodontics and Dentofacial Orthopaedics, SRM Dental College, Chennai, Tamil Nadu, India, from August to December 2021. Total 36 volunteers participated in the study and were divided into three study groups. In group A subjects had Clearbite aligners (JJ Orthodontics Pvt. Ltd, Thrissur, Kerala), group B had Dentcare clear aligners (Dent Care Dental Lab Pvt. Ltd. Ernakulum, Kerala) and group C had Smile aligners (smile aligners Inc. Mumbai, Maharashtra). Polyvinyl siloxane impression of the maxillary arch was obtained and sent for the fabrication of two sets of clear aligners. One set of aligners were used to measure absorbance and transmittance before invivo aging and another set after an intraoral use of 14 days. The measurement of absorbance and transmittance were made at the wavelength range of 400-700 nm using a Shimadzu UV-3600i Plus UV-Vis-NIR spectrophotometer. Intergroup and intragroup comparisons were done using Independent sample t-test and One-way Analysis of Variance (ANOVA).

Results: In all the three groups of aligners evaluated, an increase in the absorbance values was noted at all the wavelengths after in-vivo aging but the increase was not statistically significant (p-value >0.05). Transmittance values of group A samples reduced after in-vivo aging but not significantly, whereas group B values reduced significantly between 400-440 nm (p-value <0.05) and group C values reduced significantly between 400-460 nm and 680-700 nm (p-value <0.05). Intergroup comparison of the mean absorbance and transmittance values of the group A, B and C samples before and after in-vivo aging showed no significant difference at all wavelengths (p-value >0.05).

Conclusion: The absorbance and transmittance values of all the three indigenous aligners did not change significantly after invivo aging at most of the evaluated wavelengths.

Absorbance, Orthodontic aligners, Polyethylene terephthalate glycol, Polyurethane, Spectrophotometry, Transmittance

Clear aligner therapy is an orthodontic treatment modality in which the patient wears a series of customised removable aligners that gradually moves the teeth to a desired position (1),(2). In the past few decades, there has been a substantial increase in the number of patients seeking clear aligner treatment because of its aesthetic superiority over labial orthodontics and improved comfort than lingual orthodontics (1),(2),(3),(4). Even though, labial and lingual orthodontic appliances provide better biomechanical advantage than clear aligners there has been an increasing trend in the practice of clear aligners across the globe as patients prefer the invisibility it provides (2),(5).

Clear aligners offer several advantages including reduced incidence of white spot lesions, caries, gingivitis or periodontal disease compared to patients undergoing fixed orthodontic and are less cumbersome to the orthodontist with substantial reduction in chair side time and total number of visits (5),(6).

The concept of using transparent tooth positioner was pioneered by Kesling HD, followed by clear retainers by Pontiz RJ, vacuum formed dental contour appliance by Nahoum HI and the Essix retainers by Sheridan J (5),(7),(8),(9). In 1997 Align Technology introduced Invisalign and since then it dominated the world market of clear aligners for two decades and was holding more than 40 patents (10),(11). The patents expired in October 2017. This marked a sudden influx of aligner companies across the globe including India (11).

The absorbance and the transmittance value of the clear aligners is mainly determined by the chemical composition of the materials used in the manufacturing of the aligner (12),(13),(14). From an aesthetic point of view, the colour stability and transparency of orthodontic clear aligners are expected to be stable throughout the treatment (15). The initial aligners systems used single layer of rigid polyurethane sheets for fabrication of clear aligners. Later Polyvinyl Chloride (PVC), Polyethylene Terephthalate Glycol (PET-G) and elastomer reinforced materials with superior aesthetic and mechanical properties were introduced (16),(17),(18),(19),(20),(21),(22),(23). Each manufacturer use one of these materials for the fabrication of clear aligners but the specific composition is mostly kept as a trade secret, hence the optical and mechanical properties of a clear aligner cannot be concluded based on the generic material used (12),(13),(14),(15),(16),(17),(18),(19),(20),(21),(22),(23).

The mechanical properties of various clear aligners have been widely investigated with in-vitro and clinical studies (18),(20),(24),(25),(26). Though, studies evaluating the optical properties of clear aligners are there in the literature, the studies were done only under laboratory conditions where exact oral environment including masticatory stress, varying oral temperature and pH could not be simulated and there are no published studies on the indigenous aligners manufactured in India (7),(13),(14),(16),(25).

Considering this lacunae in the existing literature the current study was designed to evaluate and compare the absorbance, transmittance and staining of three indigenous clear aligners; Clearbite aligners (JJ Orthodontics Pvt. Ltd, Thrissur, Kerala), Dentcare clear aligners (DentCare Dental Lab Pvt. Ltd. Ernakulum, Kerala), and Smile aligners, (smile aligners Inc. Mumbai, Maharashtra) after in-vivo aging. The result of this study will help us to determine the aesthetic stability of the three indigenous aligners and also it will help us to understand if these values of clear aligner is a prerequisite for the clear aligner selection.

Material and Methods

A prospective clinical study was conducted in the Department of Orthodontics and Dentofacial Orthopaedics, SRM Dental College, Chennai, Tamil Nadu, India, from August to December 2021. This study was approved by Institutional Review Board and Institutional Ethical Committee (SRMDC/IRB/2019/MDS/No.107). This clinical study is registered in clinical trial registry of India with a registration number CTRI/2021/08/035866.

Sample size calculation: The sample size was calculated using G Power software. The estimated sample size with power of 85% and α error of 0.05% was 36 with 12 in each group. The data for sample size determination was obtained from the study published by Lombardo L et al., in 2015 (7). Convenience sampling technique was used, 36 volunteers who fulfilled the inclusion criteria and gave informed consent to participate in the study were selected.

Inclusion criteria: Subjects falling in the age group of 18-28 years with Decayed, Missing and Filled Teeth (DMFT) score not more than 2, plaque index not more than 2, without previous history of orthodontic treatment or bruxism were included in the study. The 36 volunteers were divided into three groups of 12 each in order of their enrollment.

• Group A (n=12): Subjects received aligners from Clearbite (JJ Orthodontics Pvt.Ltd, Thrissur, Kerala).

• Group B (n=12): Subjects received aligners from Dentcare (Dent Care Dental Lab Pvt.Ltd. Ernakulam, Kerala).

• Group C (n=12): Subjects received aligners from Smile aligners (smile aligners Inc. Mumbai, Maharashtra).

Study Procedure

The maxillary impression of the 36 subjects belonging to three study groups were obtained with polyvinyl siloxane material and sent to the respective laboratories for the fabrication of clear aligners. Two sets of aligners with a thickness of 0.8 mm were fabricated from each impression for the clinical study (Table/Fig 1).

One set of aligners from each group were sent to the laboratory for measurement of absorbance and transmittance before in-vivo aging. The next set of aligners were delivered to the study subjects and were instructed to wear the aligners for 24 hours for 14 days except while brushing and eating and to clean the aligner with soft texture tooth brush under running water once in the morning and once at night. The aligners were retrieved at the end of 14 days and transported to the lab for measurement of absorbance and transmittance after in-vivo aging.

The absorbance and transmittance were measured using a Shimadzu UV-3600i Plus UV-Vis-NIR spectrophotometer (Table/Fig 2). The aligners were sectioned from canine to canine to remove the lingual portion by using a rotating saw before spectrophotometer analysis to expose the labial wall. The aligners were mounted on the holder and placed inside the spectrophotometer for the measurement (Table/Fig 3). The absorbance and transmittance was measured in the wavelength of 400-700 nm, within the visible spectrum of light at intervals of 20 nm.

Statistical Analysis

Descriptive and Inferential statistics were analysed using Statistical Package for Social Sciences (SPSS) (IBM Corp. Released 2011. IBM SPSS Statistics for Windows Version 20.0. Armonk, NY:IBM Corp). Paired t-test was done for intra group readings and Oneway Analysis of Variance (ANOVA) was used to compare the values among the groups.

Results

In all the three groups of aligners evaluated an increase in the absorbance value was noted at all the wavelengths after in-vivo aging in patient’s mouth for 14 days but the increase was not statistically significant (p-value >0.05) [Table/Fig-4-6]. The lowest absorbance value of 0.8873±0.006 was recorded in the group C samples at the wavelength of 420 nm and the highest of 1.000±0.000 was recorded in group B samples at 440 nm before aging. But one-way ANOVA comparing the mean absorbance of the samples belonging to the three experimental groups at all wavelengths measured before In-vivo aging showed no significant difference between the values obtained (Table/Fig 7).

The lowest absorbance value of 1.0270±.235 was recorded in the group B samples at the wavelength of 660 nm and the highest of 1.85±0.543 was recorded in group C samples at 440 nm after aging. But an one-way ANOVA comparing the mean absorbance of the group A, B and C samples after in-vivo aging showed no significant difference between the values obtained at all wavelengths (Table/Fig 8).

Transmittance values of group A samples reduced after 14 days of intraoral use but the difference was not statistically significant (Table/Fig 9). The transmittance values of group B reduced significantly between 400- 440 nm above which the reduction was not significant (Table/Fig 10). In group C, the reduction was significant only at wavelengths between 400-460 nm and 680-700 nm (Table/Fig 11). The lowest transmittance value of 8.7627±0.032 was recorded in the group A samples at the wavelength of 620 nm and the highest of 11.9787±1.64587 was recorded in group C samples at 420 nm before aging.

The lowest transmittance value of 5.2017±1.56050 was recorded in the group B samples at the wavelength of 420 nm and the highest of 9.5747±5.23260 was recorded in group A samples at 420 nm after aging. The mean transmittance values of the samples from the group A, B and C before aging and after aging were compared using oneway ANOVA test. The comparison did not reveal a significant difference between the mean values at all wavelengths (Table/Fig 12),(Table/Fig 13).

Discussion

The current study demonstrated no significant difference in the optical properties of the three aligners evaluated before and after in-vivo aging for 14 days as measured by absorbance and transmittance values. They exhibited similar optical properties after in-vivo aging though there was a trend of increased absorbance and reduced transmittance noted in all the three aligners.

The properties of the aligners are hugely dependent on the chemical composition, thickness of the material used and the manufacturing process (7),(12),(15). Amorphous polymers like polyurethane, PET-G, polyvinylchloride and polysulfone exhibit high translucency and preferred as aligner materials over crystalline polymers which are highly opaque and unaesthetic (12),(15),(24),(26),(27). All the aligners are not created equal, and those currently on the market differ in terms of their material, thickness and manufacturing process. Alexandropoulos A et al., evaluated the chemical and mechanical properties of three contemporary thermoplastic orthodontic materials (polyurethane, polyester and polyethylene glycol terephthalate) and observed significant differences in their chemical structure and mechanical properties and therefore anticipated differences in their clinical behaviour (28).

Ideal mechanical properties and chemical stability is a basic requisite of clear aligners. The transparency of the aligner is the major key to their success and popularity (1),(2),(7),(12),(13),(14),(16),(17),(18). Studies evaluating the optical properties of clear aligners are there in the literature but there are no published studies on the indigenous aligners manufactured in India (Table/Fig 14) (7),(13),(22),(25),(29).

Most aligner companies recommend a 14 days consecutive wear of appliance for a minimum of 22 hours per day. The transparency of rthodontic clear aligners should be stable during this period or else the aligners may become less aesthetically appealing during this time period which may be of a clinical concern [12,27]. Clear aligners are exposed to various masticatory stress, salivary enzymes, staining food and beverages, mouthwashes in the oral environment during their two weeks of continuous wear [11,13,22]. These variables are very difficult to simulate in an experimental setup and in-vitro protocols exaggerate the time of exposure of the aligners to the staining agents. Hence, in this study the optical properties of the aligners were evaluated after in-vivo aging for 14 days in patients mouth.

The absorbance and transmittance before and after in-vivo aging and in-vitro staining was measured using a Shimadzu 3600 plus UV-VIS-NIR spectrophotometer using the method recommended by Lombardo L et al., to expose the labial wall (7). Absorbance is defined as a measure of the capacity of a substance to absorb light of a specified wavelength. Transmittance is the fraction of incident light, at an established wavelength that passes through the material. Greater the transmittance, the more transparent the material and greater the absorbance value, less transparent the material (7).

The result of the current study is in contradiction with the previous study conducted by Lambardo L et al., Liu CL et al., and Bernard G et al., who absorbed significant differences in their optical properties and colour stability of different clear aligner brands (7),(25),(29). This may be due to the reason that these studies were conducted under in-vitro conditions where the aligners underwent prolonged exposure to the food stains which were greater than the average time to which the aligners are exposed to stains during intraoral use. Further, the aligners used in these studies differed in their chemical composition, thickness and the method of processing which may affect the optical properties to a great extent.

The clearbite aligners were made from Polyurethane and Polyethylene Terephthalate Glycol (PET-G) where Dentcare aligners and Smile aligners were fabricated from PET-G. Though, the composition of the aligner materials used to fabricate the three aligners evaluated in the current study differed slightly. The thickness of the material (0.8 mm) and processing methods (thermoforming) were essentially same (30),(31),(32).

Gracco A et al., investigated the optical properties of clear aligners before and after intraoral use and noted that the intraoral conditions influence the optical properties and chemical stability of the aligners. (22). A similar study conducted by Zafeiriadis AA et al., assessed the in-vivo colour alterations of two different clear retainers and observed that used retainers exhibited greater colour change and the colour change increased with time for both materials (13).

The fact that the current study did not demonstrate any such difference may be attributed to the huge improvement in the quality of materials available for aligner fabrication and better methods of aligner processing and finishing available in the current era.

Limitation(s)

This study did not include the dietary variables that affect the staining characteristics of the aligner material.

DOI and Others

DOI: 10.7860/JCDR/2022/59186.17033

Date of Submission: Jul 21, 2022
Date of Peer Review: Aug 22, 2022
Date of Acceptance: Sep 06, 2022
Date of Publishing: Oct 01, 2022

AUTHOR DECLARATION:
• Financial or Other Competing Interests: None
• Was Ethics Committee Approval obtained for this study? Yes
• Was informed consent obtained from the subjects involved in the study? Yes
• For any images presented appropriate consent has been obtained from the subjects. No

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