Biomechanical aging behaviour of clear aligners

Clear aligners have recently become increasingly popular for patients and dental practitioners.¹ In mild to moderate cases and compared with traditional fixed orthodontic appliances, clear aligners provide a more comfortable and aesthetic alternative for patients while benefitting clinicians with reduced chair time.² It has been reported that clear aligner patients usually experience relatively fewer scheduled appointments and emergency visits,³ fewer issues during treatment, and less anxiety if orthodontic emergencies were encountered during the Covid pandemic.⁴

Patients usually need to wear each clear aligner for 7 to 14 days to achieve the desired treatment outcome.⁵ By using a series of prefabricated thermoplastic aligners, teeth are incrementally moved to achieve the final treatment goal. However, there is great variation in the wear time of clear aligners dependent upon the dental practitioner and treatment goals.⁶

Orthodontic aligners are usually made from polyurethane or polyethylene terephthalate glycol as thermoplastic polymer materials.⁷ These materials have good biocompatibility, high flexibility and elasticity, and resistance to chemical and mechanical abrasion.^8,9 Aligner properties can be affected by moisture, temperature, and contact with salivary enzymes.⁹ It has been shown that the biomechanical properties of clear aligners can deteriorate after intraoral use due to an aging process that involves a loss in transparency and mechanical strength, as well as changes in hardness, abrasion resistance, distortion, and the introduction of cracks.^10–12 The biomechanical properties affect force expression and delivery during orthodontic treatment and hence affect treatment efficiency.¹³ Previous studies have investigated the biomechanical changes of clear aligners after intraoral exposure, none of which have reported the continuous biomechanical property changes during the entire aligner treatment period.^9,10,12,14

Therefore, the aim of this novel study was to characterise clear aligner aging behaviour over 21 days to provide an insight into the optimum wear time from a biomechanical perspective. It was intended to assess the biomechanical properties of flexural strength, translucency, roughness, hardness, and tensile strength.

Material and methods

Brand new clear aligners (Invisalign, Align Co., USA) “as-received” (0-day, n = 63) were used throughout the study. All of the aligners were immersed in distilled water and placed in an incubator at a constant temperature of 37°C. The clear aligner samples were immersed for 22 hr per day for 21 days to simulate an aging process. Each day, three clear aligner samples were removed and subjected to the following biomechanical tests (n = 3 in each group): flexural strength, translucency, internal and external surface roughness, tensile strength, and hardness.

Flexural strength

A customised digital tension gauge was used to measure the force needed for an aligner to expand buccally/palatally for 3 mm and 5 mm, respectively. The upper first molar gingival region was used as a reference for testing. The tests were repeated four times and a mean value was obtained.

Translucency

A translucency measurement was performed on the aligner region adjacent to the maxillary central incisor labial surface. The 3M Filtek Supreme XTE Universal restorative composite shade Enamel A3 was used to fill the internal of each chosen specimen in order to measure translucency.^15,16 The change was measured by a spectrophotometer (Easyshade V, VITA Zahnfabrik, BadSäckingen, Germany) according to the Commission Internationale de l’Eclairage (CIE) L*a*b* system, in which the L*, a* and b* correspond to luminosity, tonality and colour, respectively.^16,17 The specimens were blotted dry with absorbent paper and placed on a standard white or black background. The spectrophotometer was calibrated according to the manufacturer’s instructions prior to all measurements which were repeated three times on the facial surfaces adjacent to the central incisors. A light box was used to shield the spectrophotometer from external light sources while all measurements were taken at the same time each day for consistency and standardisation. The change in translucency parameter was determined by calculating the colour difference between the specimen over a white (Y: 74.49;L*: 85.61; a* – 5.1252; b* 9.7467) and black (Y: 7.63; L*: 31.30; a* – 2.2495; b* 4.2290) background.¹⁵ The translucency was calculated using the following formula:^15,16,18 $Translucency = \sqrt[2]{{(L_{w} - L_{b})}^{2} + {(a_{w} - a_{b})}^{2} + {(b_{w} - b_{b})}^{2}}$ \[\text{Translucency}=\sqrt[2]{{{\left( {{L}_{w}}-{{L}_{b}} \right)}^{2}}+{{\left( {{a}_{w}}-{{a}_{b}} \right)}^{2}}+{{\left( {{b}_{w}}-{{b}_{b}} \right)}^{2}}}\] in which w and b were the measurements taken against the white and the black background, respectively.¹⁵

Surface roughness

The labial surface of the maxillary central incisor region of the aligners with a dimension of 3 mm × 3 mm was chosen for both external and internal surface roughness tests. This sample region was selected because of its relatively flat geometry which can yield more accurate measurements. A profilometer (Polytec TopMap Micro. View, Baden-Württemberg, Germany) was used. Three measurements per sample were carried out on both the external and internal surfaces. The mean surface roughness values (Ra) were determined and expressed in micrometres.

Tensile strength

The clear aligner specimens were prepared according to the ISO standard ISO527-2 (Plastics - Determination of tensile properties. https://www.iso.org/standard/56046.html) for the tensile strength test. The test was measured using a universal testing machine (Model 3369, Instron) with a 500N load cell. The specimens were stretched at the speed of 1 mm/min until the elastic limit was reached. Data for each specimen were acquired for the maximum load and the corresponding relative tensile strength was calculated: $σ_{\max} = P_{\max} / A_{0} (N / {mm}^{2})$ \[{{\text{ }\!\!\sigma\!\!\text{ }}_{\max }}={{\text{P}}_{\max }}/{{\text{A}}_{0}}\left(\text{N}/\text{m}{{\text{m}}^{2}} \right)\] in which σ_max is the tensile strength, P_max is the maximum load at failure, and A₀ is the original cross-sectional area of the specimen. The tensile strength was calculated from the average value of four specimens being tested and expressed as N/mm².

Hardness

The labial surface of the maxillary central incisor aligner region with a dimension of 5 mm × 5 mm was chosen. Vickers hardness (VHN) was measured using a universal testing machine (Model 3369, Instron) with a Vickers diamond indenter of 50 N and a loading time of 10 s. Four continuous indentations were made on the labial surface of the specimen, and the VHN was the mean value obtained from the four measurements. The dimensions of the indentation were measured using a light microscope (Nikon SMZ800N, Japan), the Vickers numbers (VHN) were then calculated using the following formula:^9,19 $VHN = \frac{1.8544 F}{d^{2}} (kgf / mm 2)$ \[\text{VHN}=\frac{1.8544\text{F}}{~{{\text{d}}^{2}}}(\text{kgf}/\text{mm}2)\] in which F is the applied load and d is the mean diameter of the indentation.

Statistical analysis

Statistical analysis was performed using SPSS (Version 23.0, IBM Corporation, Chicago, USA). Comparisons were performed using one way ANOVA and the paired t-test. A p value less than 0.05 was considered as statistically significant.

Results

Flexural strength

The flexural strength at 3 mm and 5 mm distance were both influenced by the aging process (Figure 1). The aligner material demonstrated the highest flexural strength at day 0: 3 mm (0.26 ± 0.01N) and 5 mm (0.41 ± 0.01N). After 7 days of simulated intra-oral aging, the flexural strength significantly decreased to 0.22 ± 0.01N for 3 mm and 0.35 ± 0.01N for 5 mm (p < 0.05 for both).

Translucency

Means and standard deviations at the 95% confidence interval for translucency are listed in Table I. The results showed no statistically significant changes in translucency during the 21-day artificial aging period (p > 0.05 for all).

Table I.

Translucency and surface roughness of clear aligners did not change significantly during the 21-day study (P > 0.05 for all, n = 3 in each group)

Translucency Parameter					Roughness – internal surface (μm)				Roughness – external surface (μm)
		Lower	Upper			Lower	Upper			Lower	Upper
Day	Mean±SD	95% CI	95% CI	P-value	Mean±SD	95% CI	95% CI	P-value	Mean±SD	95% CI	95% CI	P-value
0	3.69 ± 0.66	2.05	5.34	P > 0.05	5.53 ± 0.26	4.89	6.17	P > 0.05	1.34 ± 0.04	1.23	1.44	P > 0.05
1	4.97 ± 3.32	-3.27	13.20	P > 0.05	5.30 ± 0.23	4.74	5.87	P > 0.05	1.53 ± 0.10	1.28	1.77	P > 0.05
2	4.31 ± 4.25	-6.24	14.86	P > 0.05	5.64 ± 0.8	3.65	7.64	P > 0.05	1.72 ± 0.06	1.58	1.87	P > 0.05
3	3.10 ± 1.84	-1.47	7.67	P > 0.05	5.65 ± 0.67	3.97	7.32	P > 0.05	1.44 ± 0.50	0.19	2.69	P > 0.05
4	4.93 ± 2.91	-2.30	12.17	P > 0.05	5.39 ± 0.41	4.37	6.4	P > 0.05	1.60 ± 0.54	0.27	2.93	P > 0.05
5	4.85 ± 0.91	2.58	7.12	P > 0.05	5.33 ± 0.58	3.9	6.77	P > 0.05	1.45 ± 0.30	0.7	2.2	P > 0.05
6	4.50 ± 1.59	0.54	8.46	P > 0.05	5.45 ± 0.49	4.24	6.65	P > 0.05	1.45 ± 0.25	0.84	2.07	P > 0.05
7	3.87 ± 0.75	2.01	5.73	P > 0.05	5.58 ± 0.21	5.05	6.1	P > 0.05	1.29 ± 0.19	0.82	1.75	P > 0.05
8	4.72 ± 0.35	3.86	5.59	P > 0.05	5.47 ± 0.49	4.25	6.68	P > 0.05	1.38 ± 0.50	0.14	2.62	P > 0.05
9	3.20 ± 1.59	-0.76	7.15	P > 0.05	5.27 ± 1.07	2.61	7.94	P > 0.05	1.48 ± 0.45	0.37	2.59	P > 0.05
10	2.35 ± 0.84	0.27	4.44	P > 0.05	5.23 ± 0.65	3.61	6.86	P > 0.05	1.65 ± 0.06	1.51	1.79	P > 0.05
11	3.89 ± 1.61	-0.11	7.89	P > 0.05	5.64 ± 0.03	5.55	5.73	P > 0.05	1.72 ± 0.05	1.59	1.85	P > 0.05
12	2.35 ± 0.19	1.86	2.83	P > 0.05	5.39 ± 0.19	4.92	5.87	P > 0.05	1.52 ± 0.13	1.2	1.84	P > 0.05
13	4.76 ± 1.43	1.20	8.32	P > 0.05	5.54 ± 0.08	5.33	5.74	P > 0.05	1.34 ± 0.51	0.07	2.61	P > 0.05
14	2.51 ± 1.06	-0.12	5.14	P > 0.05	5.45 ± 0.52	4.15	6.74	P > 0.05	1.55 ± 0.45	0.42	2.67	P > 0.05
15	2.53 ± 0.33	1.72	3.34	P > 0.05	5.80 ± 0.03	5.73	5.87	P > 0.05	1.27 ± 0.05	1.14	1.41	P > 0.05
16	3.62 ± 1.96	-1.24	8.49	P > 0.05	5.57 ± 0.52	4.27	6.86	P > 0.05	1.75 ± 0.25	1.14	2.36	P > 0.05
17	2.36 ± 0.39	1.40	3.32	P > 0.05	5.53 ± 0.19	5.07	6.00	P > 0.05	1.72 ± 0.10	1.46	1.98	P > 0.05
18	2.65 ± 1.30	-0.57	5.88	P > 0.05	5.78 ± 0.27	5.1	6.46	P > 0.05	1.63 ± 0.24	1.04	2.23	P > 0.05
19	5.97 ± 1.35	2.63	9.32	P > 0.05	5.41 ± 0.77	3.5	7.31	P > 0.05	1.81 ± 0.09	1.58	2.05	P > 0.05
20	3.75 ± 1.88	-0.91	8.41	P > 0.05	5.28 ± 0.78	3.34	7.21	P > 0.05	1.68 ± 0.18	1.23	2.14	P > 0.05
21	4.49 ± 0.75	2.64	6.34	P > 0.05	5.31 ± 0.78	3.36	7.25	P > 0.05	1.70 ± 0.25	1.07	2.32	P > 0.05

Surface roughness

The surface roughness of both the inner and outer surface is shown in Figure 2. The internal and external roughness were not significantly affected when compared with the baseline value (p_Internal = 0.99; p_External = 0.57).

Tensile strength

The value of tensile strength fluctuated between 18 and 22 N/mm² during the 21-day artificial aging period, without a statistical significance noted between the different time points (p > 0.05 for all, Table II)

Table II.

Tensile strength and maximum load of the clear aligners did not change significantly during the 21-day study (P > 0.05 for all, n=3 in each group)

Tensile strength (N/mm²)					Maximum load (N/mm²)
		Lower	Upper			Lower	Upper
Day	Mean±SD	95% CI	95% CI	P-value	Mean±SD	95% CI	95% CI	P-value
0	20.85 ± 1.08	18.18	23.53	> 0.05	104.26 ± 5.38	18.18	23.53	> 0.05
1	20.55 ± 2.75	16.17	24.92	> 0.05	102.74 ± 13.75	16.17	24.92	> 0.05
2	21.38 ± 3.38	16.00	26.76	> 0.05	106.89 ± 16.91	16.00	26.76	> 0.05
3	19.87 ± 3.10	14.93	24.80	> 0.05	99.34 ± 15.50	14.93	24.80	> 0.05
4	20.28 ± 2.44	16.41	24.16	> 0.05	101.42 ± 12.18	16.41	24.16	> 0.05
5	20.98 ± 3.20	15.88	26.08	> 0.05	104.91 ± 16.02	15.88	26.08	> 0.05
6	20.01 ± 2.61	15.85	24.16	> 0.05	100.05 ± 13.06	15.85	24.16	> 0.05
7	19.02 ± 0.48	18.26	19.78	> 0.05	95.11 ± 2.39	18.26	19.78	> 0.05
8	21.70 ± 3.90	15.49	27.91	> 0.05	108.50 ± 19.51	15.49	27.91	> 0.05
9	20.11 ± 3.01	15.32	24.91	> 0.05	100.57 ± 15.06	15.32	24.91	> 0.05
10	18.98 ± 4.34	12.07	25.88	> 0.05	94.89 ± 21.69	12.07	25.88	> 0.05
11	22.02 ± 2.46	18.10	25.93	> 0.05	110.08 ± 12.30	18.10	25.93	> 0.05
12	18.22 ± 3.48	12.67	23.76	> 0.05	91.08 ± 17.42	12.67	23.76	> 0.05
13	18.95 ± 2.84	14.43	23.46	> 0.05	94.74 ± 14.18	14.43	23.46	> 0.05
14	18.82 ± 1.56	16.33	21.31	> 0.05	94.08 ± 7.82	16.33	21.31	> 0.05
15	21.11 ± 2.61	16.95	25.26	> 0.05	105.53 ± 13.05	16.95	25.26	> 0.05
16	20.43 ± 4.67	13.01	27.85	> 0.05	102.15 ± 23.33	13.01	27.85	> 0.05
17	19.45 ± 3.11	14.51	24.40	> 0.05	97.27 ± 15.53	14.51	24.40	> 0.05
18	18.93 ± 0.32	18.41	19.44	> 0.05	94.65 ± 1.62	18.41	19.44	> 0.05
19	21.08 ± 1.75	18.29	23.86	> 0.05	105.38 ± 8.75	18.29	23.86	> 0.05
20	18.40 ± 2.80	13.94	22.86	> 0.05	91.99 ± 14.01	13.94	22.86	> 0.05
21	17.63 ± 3.58	11.94	23.32	> 0.05	88.17 ± 17.88	11.94	23.32	> 0.05

Hardness

Figure 3 shows the means of the VHN (kgf per square mm) during the 21-day artificial aging period. The surface hardness significantly reduced after 10 days from 6.60 ± 0.43 kgf/mm² to 4.37 ± 0.48 kgf/mm² (p < 0.05) when compared with the baseline value.

Discussion

The biomechanical properties of clear aligners are crucial for the clinical efficacy of orthodontic tooth movement. Biomechanical aging can affect force delivery and the wear protocol of aligners. The present study evaluated the changes in biomechanical properties including the flexural strength, surface roughness, translucency, tensile strength, and hardness over 21 days of an in vitro aging process. Significant reductions were observed in flexural strength (both 3 mm and 5 mm) and hardness, while translucency, surface roughness, and tensile strength did not show significant changes over the 21-day period. The changes in biomechanical properties can be directly related to the force delivered by the aligners during treatment, which will affect treatment outcome.²⁰ Although many studies have investigated the biomechanical properties of aligners after an intraoral aging process, the exact process and the continual change has not been explored.

The biomechanical aging effect on Invisalign aligners has been investigated previously.¹¹ Polyurethane, as the main polymeric component used in Invisalign aligners, is not inert and can be affected by routine intraoral use. However, the literature has shown that the molecular composition is stable and remains unchanged,^10,14 but several factors can potentially induce an aging process including contact with heat, moisture, masticatory force and salivary enzymes.¹¹ Intraoral use can also result in microcracks, abrasion, and calcified deposits.^10–12 It has been further shown that traceable chemicals, particularly constituent monomers, are not released by the aging process,^9–12 but one study identified the trace elements of aluminium (Al) and silicon (Si).¹⁴

In the present study, the aging process produced a significant reduction in the hardness of the aligners after 10 days. A similar trend of incrementally reduced hardness due to intraoral aging has also been observed.¹⁰ However, alternative studies have found that material hardness increased after the aging process.^9,11 This was attributed to the material cold working during mastication¹¹ while it was further suggested that the change was a result of increased hyperplasticity.⁹

The flexural strength at both 3 mm and 5 mm demonstrated a gradual reduction over time, particularly after 7 days of aging. This trend indicated that the force delivered by aligners likely decayed with time. Previous studies agreed that the mechanical properties of clear aligners can affect the delivery of orthodontic forces.²¹ During clinical application, the aligners should be able to deliver continuous and controlled forces to generate the planned orthodontic movements.⁹ If the material properties deteriorate over time, the expression of force will similarly be affected and the patient may not realise the full benefit of a 14-day wear protocol.

The optimum aligner wear protocol is still debated. The evidence to date is limited but a recent study compared orthodontic tooth movement efficacy following different protocols and concluded that there were no clinically significant linear discrepancies between a 7-day and 14-day wear protocol but the 14-day regime was better at delivering intrusive and rotational tooth movements.⁶ It has been reported that the majority of orthodontic tooth movement occurs during the first week,²² favouring a 7-day wear protocol; however, a contrary study suggested that a 14-day wear protocol had a higher level of success compared to a 7-day protocol.²³ Based on the outlined biomechanical changes, it is reasonable to recommend that clear aligners should be changed before a significant reduction in flexural strength after 7 days and hardness after 10 days. A better understanding of the aligner properties can allow improved management of the biomechanics and treatment plan leading to better treatment efficiency and treatment outcome.

There are limitations associated with the present study. Due to the nature of the in vitro design, there was no account of acidity, salivary enzymes, nor masticatory forces, and that the findings may vary compared to those encountered in the true intraoral environment. Only one material (i.e. Invisalign clear aligners) was used, which may limit the generalisation of the findings to other clear aligner products. Future studies could consider an in vivo design, including more influencing factors, and investigate which have the most aging impact on the biomechanical properties of aligner material.

Conclusion

The arch flexural strength and hardness of the aligner material showed significant reduction after 7 days and 10 days, respectively. The biomechanical properties of surface roughness, translucency, and tensile strength showed no significant change over the 21 days of the artificial aging process.

eISSN:: 2207-7480
Lingua:: Inglese

Frequenza di pubblicazione:: Volume Open
Argomenti della rivista:: Medicine, Basic Medical Science, other

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Biomechanical aging behaviour of clear aligners

Pubblicato online: 20 mar 2024

Pagine: 60 - 66

Ricevuto: 01 dic 2023

Accettato: 01 feb 2024

DOI: https://doi.org/10.2478/aoj-2024-0006

© 2024 Li Mei et al., published by Sciendo

This work is licensed under the Creative Commons Attribution 4.0 International License.

Figure 1.

Figure 2.

Figure 3.