Alterations in the surface roughness and porosity parameters of directly printed and Invisalign aligners after 1 week of intraoral usage: An in vivo prospective investigation

Introduction

Direct printing of clear aligners could be the next paradigm shift in modern orthodontics and can potentially overcome the limitation of the indirect production method. This study investigated the effects of 1 week of intraoral usage on the surface roughness parameters of directly printed aligners (DPAs) and commercially produced Invisalign aligners compared with their unused control counterparts using confocal laser scanning microscopy.

Methods

The study consisted of 4 groups with 34 samples per group. Unused control aligners were allocated to the control groups (DP-Ctr and INV-Ctr). Sixty-eight patients undergoing clear aligner therapy were allocated to group DP-Clin (patients in therapy using DPAs made from TC-85 DAC resin) and group INV-Clin (patients provided with Invisalign aligners). After 1 week of intraoral usage, the aligners were retrieved from the patients in groups DP-Clin and INV-Clin. Samples were made from the buccal surface of the maxillary right central incisor of each aligner and underwent surface roughness and porosity measuring using confocal laser scanning microscopy. The arithmetic mean deviation of the profile, root mean square deviation, maximum peak height, maximum valley depth, maximum height difference among the highest peak and deepest valley, void volume, and void count were measured. Descriptive analysis and median (quantile) regression models were used for data analysis of this experiment.

Results

One week of intraoral usage significantly increased the surface roughness and porosity of DPAs. In contrast, a significant reduction in the surface roughness and porosity parameters of Invisalign aligners was recorded after intraoral service.

Conclusions

This study suggests an increase in the surface roughness and surface porosity of DPAs following 1 week of intraoral usage, which might lead to an increase in bacterial adhesion and biofilm formation in these aligners.

Highlights

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Directly printed aligners (DPAs) could present the next paradigm shift in modern orthodontics.
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DPAs might have potential advantages regarding sustainability and waste reduction in the production process.
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The production of DPAs is a technique-sensitive, complicated, and time-consuming process.
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Intraoral service for 1 week increased the surface roughness and surface porosity of DPAs.
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These effects might increase biofilm formation and bacterial adhesion.

Direct 3-dimensional (3D) printing of orthodontic aligners is emerging as the next paradigm shift in clear aligner therapy. This technique is being explored as a solution to address the drawbacks of the traditional thermoforming process, which can be time-consuming and labor-intensive and result in geometric inaccuracies, thermal deformations, and inconsistent fitting and layer thickness. ^,

Initial investigations into the feasibility of directly printing orthodontic aligners used readily available materials such as dental Long Term resin (Formlabs, Somerville, Mass). However, this resin, originally developed to produce rigid dental appliances, such as dental splints, lacks the flexibility and transparency necessary for optimal aligner fabrication. Therefore, it was not until the advent of Tera Harz TC-85 Direct Aligner Clear (DAC) resin (Graphy, Seoul, Korea), a photopolymerizable polyester-urethane polymer with biocompatible CE Class IIa status and approval from both the European Commission and Korean Food and Drug Administration, that direct printing of orthodontic clear aligners became a viable option in 2019. Despite the potential advantages offered by this new material, the available body of evidence on its properties is currently limited because of its recent introduction to the market.

Nevertheless, Tera Harz TC-85 DAC has shown promising results in recent studies, and several claims have been made regarding its potential advantages such as simplifying the manufacturing process, sustainability and waste reduction, enhancing fitting precision and more accuracy as well as allowing for customization of aligner thickness. ^,

The surface roughness of aligners is a crucial factor affecting plaque accumulation, biofilm adhesion, loss of transparency, and aligner discoloration. An in vivo study on aligner surface roughness has shown significantly higher surface roughness parameters in directly printed aligners (DPA) compared with Invisalign aligners (Align Technology, San Jose, Calif) after 1 week of intraoral service. However, it is worth noting that this study, along with many other currently available studies on this material, employed the earlier protocol of aligner polymerization in the presence of oxygen, which inhibits the complete polymerization of the resin. Incomplete polymerization could adversely affect the mechanical properties of DPA and increase their surface roughness, which might act as an initiator in the leaching of the aligner resin in the oral cavity. Curing the resin under a nitrogen atmosphere creates an oxygen-free environment and prevents oxidation inhibition. This could increase the degree of polymerization and thereby improve the mechanical properties and surface roughness parameters of DPA. Therefore, it is necessary to conduct further in vivo studies that incorporate curing under nitrogen to understand its effects fully.

This study investigated the effects of 1 week of intraoral usage on the surface roughness and porosity parameters of DPA and Invisalign aligners compared with their unused control counterparts using confocal laser scanning microscopy (CLSM).

Material and methods

This in vivo, prospective experimental study was approved by the ethics committee of Johann-Wolfgang Goethe University in Frankfurt, Germany (no. 2022-856) and registered in the German Clinical Studies Register (DRKS00030417). Signed informed consent was obtained from all study participants. A single specialist orthodontist with certification for in-house production of aligners using Tera Harz TC-85 DAC resin (Graphy, Seoul, Korea) treated all patients in a private orthodontic practice. Only patients who met the inclusion criteria were included in the study, requiring participants aged 18-48 years to have complete healthy permanent dentition, healthy periodontium, excellent oral hygiene, and good compliance, with little to moderate anterior crowding. Patients with a history of craniomandibular disorders, parafunctions such as bruxism or clenching, systemic diseases, smoking, medication use, pregnancy or breastfeeding, poor oral hygiene, or insufficient compliance were excluded from the study.

Four groups were included in this study: (1) DP-Clin consisted of DPAs retrieved after 1 week of intraoral service (DP-Clin), (2) Invisalign aligners after 1 week of intraoral use (INV-Clin), (3) control for DP-Clin (DP-Ctr), and (4) control for INV-Clin (INV-Ctr). Both control groups (DP-Ctr and INV-Ctr) consisted of unused aligners.

The study by Papadopoulou et al was used for sample size calculation for this study. The statistical analysis resulted in an effect size of one. However, because this study measured a larger area and anticipated greater variability, an effect size of 0.5 was employed. Consequently, 34 aligners per group were required to detect this effect size at a statistical significance level of 5% with a power of 80%. Because only one aligner was obtained from each patient, 68 patients were recruited for this investigation through consecutive sampling.

The clear aligner treatment used in this study focused solely on aligning the anterior teeth and did not involve using any auxiliaries or attachments. Patients were given detailed instructions to wear the aligners for at least 20 h/d and properly clean the aligners using a soft toothbrush at least once daily. To evaluate patient compliance, a 3-point Likert scale was employed.

The DPAs were printed by a certified orthodontist using the Sprintray Pro 55 3D printer (Sprintray, Los Angeles, Calif) and Tera Harz TC85A aligner resin (Graphy, Seoul, Korea). The aligners were oriented vertically with minimal supports and printed in successive layers of 100-μm thickness. The excess resin was removed by a centrifugation process for 6 minutes at 600 rpm. The aligners were then cured for 20 minutes at level 2 under nitrogen using the Graphy Tera Harz Cure THC 2 UV curing system (Graphy, Seoul, Korea). Two aligners were made per each patient undergoing treatment with DPAs. One aligner was used for the intraoral service (DP-Clin), and the other was allocated to the control group (DP-Ctr).

The retrieved aligners underwent a gentle, nonchemical, and nonabrasive cleaning procedure, including rinsing under water and plaque removal using a soft dental brush.

The buccal surface of the maxillary right central incisors from the aligners was selected for sample preparation as this portion has a relatively flat geometry that enables more accurate assessments. The internal surface of the aligners was evaluated using the CLSM with Keyence VK-X100 microscope (Keyence, Osaka, Japan). The 1000.0 × 1419.9 μm acquisition area was measured using a 10× nominal magnification lens. The expert module of the program was used for image acquisition, and the VK-analyzer software (Keyence, Osaka, Japan) was used to measure the surface roughness and porosity parameters. The automatic tilt correction was applied with the secondary curved surface function to compensate for the surface curvature. The Gaussian regression filter and short- and long-wavelength cutoff filters were applied according to the ISO 25178 standard to suppress the image noise and separate the surface texture from the form.

After surface roughness and surface porosity parameters were measured for each sample: height measurements (arithmetic mean deviation of the profile [Sa], root mean square deviation of the profile [Sq], maximum peak height [Sp], maximum valley depth [Sv], and maximum height difference between highest peak and deepest valley [Sz]) and functional volumetric parameters (void volume [Vvv] and void count [Vvc]) for evaluation of surface porosity.

SPSS software (version 29; IBM, Armonk, NY) was used to perform the statistical analysis of this study. Shapiro-Wilk test was used to assess the normality of the data. Median (quantile) regression analysis was used to evaluate the effects of material type and intraoral usage on the surface roughness parameters. Predictors were set at Invisalign (yes/no) and intraoral usage (yes/no). Alpha was set at 0.05.

Results

In our study, patient allocation followed a consecutive sampling approach until 34 eligible patients were included in each group. However, 7 patients in the DP-Clin group were ultimately excluded because they were not wearing the aligners for the full 7 days, with 2 experiencing aligner breakage and 5 reporting discomfort or speech distortion. There were no dropouts observed in the INV-Clin group.

The DP-Clin group included 26 females and 8 males with a median age of 29.7 ± 8.0 years. Group INV-Clin consisted of 23 females and 11 males with a mean age of 30.7 ± 8.7 years.

The acquired microscopic images and 3D demonstration of the surface roughness of the samples are depicted in Figures 1 and 2 . Observation of the samples under the microscope revealed the more porous and rougher appearance of samples in group DP-Clin compared with other groups. DP-Ctr and INV-Clin groups exhibited less porous surfaces, whereas group INV-Ctr exhibited a corrugated surface texture.

The descriptive statistics are presented in Table I and Figure 3 . The quantile regression model and its interpretation are shown in Table II . Significant interaction effects were observed regarding all surface roughness and porosity parameters of both predictors (aligner material and intraoral use). Our results show that the group DP-Ctr had the lowest surface roughness and porosity ( P <0.001). The highest surface roughness and porosity parameters were recorded in the INV-Ctr group ( P <0.001). Compared with the INV-Ctr group, the median values of all surface roughness and porosity parameters in group INV-Clin were significantly lower ( P <0.001), which indicates a reduction in surface roughness and porosity after intraoral use of the Invisalign aligners. In contrast, intraoral usage significantly increased all surface roughness and porosity parameters of the DPAs ( P <0.001). Although group DP-Clin showed higher surface roughness and porosity than INV-Clin, its parameters remained lower than INV-Ctr group ( P <0.001).

Table I

Descriptive statistics of surface roughness and porosity parameters in groups DP-Ctr, DP-Clin, INV-Ctr, and INV-Clin

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