Micro-computed tomography evaluation of aligner thickness and gap width after thermoforming process: A comparative study of three different materials

INTRODUCTION

Malocclusion represents one of the most prevalent oral health conditions, with its occurrence varying significantly across populations due to differences in geographic location, ethnicity, gender, and other demographic factors. Epidemiological studies have reported malocclusion prevalence rates ranging from 39% to 93%.^[1] In recent years, there has been a notable increase in the demand for orthodontic treatment among adults, driven by esthetic motivations, the pursuit of improved oral health, advancements in orthodontic techniques, and the need to address functional issues such as occlusal discrepancies.^[2]

In recent years, clear aligner (CA) therapy has emerged as a widely adopted alternative to conventional fixed orthodontic appliances, particularly among adult patients. This modality utilizes computer-aided design (CAD) and computer-aided manufacturing technologies to obtain digital scans of the dental arches, which are then used to fabricate a sequence of custom-fitted, transparent thermoplastic aligners. Treatment involves a progressive series of aligners, each designed to achieve incremental tooth movements. Patients are typically instructed to wear each aligner for a duration of 1–2 weeks, depending on the specific biomechanics and treatment protocol prescribed by the orthodontist.^[3]

Despite their growing popularity, CAs present several limitations compared to conventional fixed appliances. One of the primary drawbacks is the reduced ability to achieve certain types of tooth movements, such as controlled root movement, extrusion, and rotation. These limitations are largely attributed to the biomechanical constraints of aligner systems and the lack of in-treatment adjustability by the clinician, as the appliances cannot be modified once fabricated.^[4] Root movements, for instance, are particularly challenging due to the extensive bone remodeling required, while extrusive movements rely on tensile forces that are less effectively delivered by CAs.^[5] In addition, rotational movements – especially of cylindrical teeth – are difficult to achieve due to the absence of interproximal undercuts, which reduces aligner grip and causes slippage during attempted rotation.^[5]

Another critical factor influencing the predictability of clinical outcomes with CAs is their fit, defined as the gap between the inner surface of the aligner and the external tooth surface. The accuracy of fit is affected by multiple variables, including the parameters of the thermoforming process (such as pressure and temperature), the elastic modulus of the aligner material, the incorporation of divots or attachments, and the hygroscopic expansion that occurs on exposure to saliva.^[6]

Despite their clinical importance, key parameters of CAs – such as thickness and fit (i.e., gap width) – have been scarcely investigated even though these factors substantially impact their optical properties, biomechanical force delivery, retention, and the accuracy of prescribed orthodontic tooth movements.^[7] Ammann et al. have used Naturaligners made up of bio-based thermoformable material. In our study, we have used materials that have been widely employed in the orthodontic aligner industry (Polyethylene terephthalate glycol [PET G], polyurethane [PU], and multilayer polyurethane [MLPU]).^[8] Accordingly, the objective of our study is to quantitatively assess and compare the influence of the thermoforming process on aligner thickness and gap width using high-resolution micro-computed tomography (micro-CT), through a comparative evaluation of these three distinct aligner materials.

MATERIAL AND METHODS

Image analysis and measurement

Grids were superimposed on the micro-CT images using open-source scientific image analysis software at a resolution of 1024 × 1024 pixels. On each 2D grid, multiple reference points were identified. Measurements of aligner thickness and gap width were performed by tracing lines perpendicular to the tangent of the tooth surface at each reference point.

Image processing and 3D reconstruction were carried out using specialized software packages, including CTVOX and CTAn (NRecon). The micro-CT system operates by capturing multiple 2D X-ray projections of the rotating sample, which are computationally reconstructed into a high-resolution 3D volumetric dataset. Variations in X-ray attenuation, which depend on the density of the materials, enable visualization and quantification of internal structures without sample destruction. In [Figure 1], on each 2D grid, several reference points were identified: • 5 for the central incisor • 5 for the canine • 8 for the first molar.1 palatal gingival edge; 2 palatal surface centre; 3 incisal edge; 4 vestibular surface center; 5 vestibular gingival edge; 6 vestibular cusp; 7 palatal cusp; and 8 central groove.

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Figure 1: Identification of slice planes and two-dimensional reference points on the construction grid for the (a) incisor, (b) canine, (c) first molar. Eight points were identified: (1) Palatal gingival edge; (2) palatal surface center; (3) incisal edge; (4) vestibular surface center; (5) vestibular gingival edge; (6) vestibular cusp; (7) palatal cusp.

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RESULTS

The data were entered using MS Excel version 2021 and analyzed using IBM Statistical Package for the Social Sciences statistics for Windows (Version 25). Continuous variables were represented as mean and standard deviation, and the inferential statistics were computed using one-way ANOVA statistics and post hoc Tukey’s honestly significant difference (HSD).

[Table 1] presents the comparison of mean width (w) measurements for incisor, canine, and molar teeth across three different materials: PET-G, PU, and MLPU, each with a sample size of 9 (n = 9), totaling 27 observations per tooth type.

In summary, significant differences in width were observed for incisor and canine regions showing higher values with PU. However, no such difference was noted in the molar region and [Graph 1] represents the same.

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Graph 1: Graphical representation of aligner gap width.

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[Table 2] provides a comparative analysis of thickness (t) measurements at the incisor, canine, and molar regions across three material groups: PET-G, PU, and MLPU, with each group comprising 9 samples (n = 9), making a total of 27 observations for each tooth type and [Graph 2] represents the same.

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Graph 2: Graphical representation of aligner thickness.

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For canines, PU exhibiting the greatest mean thickness compared to PETG and MLPU. In contrast, molars showed no significant differences in aligner thickness across the three materials (F = 0.735, p = 0.490), with mean values being comparable.

The Tukey HSD post hoc test was conducted to identify pairwise differences among the three materials – PET-G, PU, and MLPU – for width measurements at the incisor, canine, and molar regions. The results are interpreted based on mean differences, standard errors, p-values, and confidence intervals as shown in [Table 3] and [Graph 3] represents the same.

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Graph 3: Comparison between the groups for mean aligner gap width measurements (mm) with their standard deviations of three aligner materials measured at incisor, canine, and molar regions using post hoc Tukey’s honestly significant difference.

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In summary, PU consistently showed significantly higher width values at the incisor and canine regions, particularly when compared to MLPU and PET-G. However, no significant differences were found among materials at the molar region, indicating uniformity in performance across materials in that region.

The Tukey HSD post hoc analysis was performed to examine pairwise differences in thickness (t) across the three materials – PET-G, PU, and MLPU – for the incisor, canine, and molar regions. The test reports the mean differences, standard errors, p-values, and confidence intervals to assess the significance of observed differences.

[Table 4 and Graph 4] show that a significant difference in thickness was found only in the canine region between PU and PET-G, with PU showing superior thickness. For the incisor and molar regions, no significant differences were detected among the three materials.

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Graph 4: Comparison between the groups using post hoc Tukey’s honestly significant difference mean aligner thickness measurements (mm) with their standard deviations of three aligner materials measured at incisor, canine, molar regions.

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DISCUSSION

CAs are indeed becoming increasingly popular, experiencing a surge in usage due to their comfort, esthetics, and convenience. Early aligners were limited in their ability to perform complex tooth movements, but newer generations have incorporated features such as attachments and optimized force delivery to expand their capabilities.^[9] In CA treatment, both the thickness and gap width of the aligners play crucial roles in their effectiveness. Aligner thickness affects the force applied to teeth, influencing the extent and efficiency of tooth movement whereas the gap width, it dictates how well the aligner can transfer force to the teeth.^[10]

This study quantifies the thickness and gap width of CAs made of PU, PETG and MPU at different sites of teeth. To evaluate the aligner thickness and the gap width between the aligner and cast, tomographic microphotographs of three virtual slices corresponding to three tooth types investigated (Central incisor, canine, and first molar), yielding a total of two slices, were obtained for each sample and analyzed. Grids were constructed on the microphotographs thereby obtained at a relative magnification of 3200 for each tooth, that is, 3 grids per sample. On each 2D grid, several reference points were identified.

Over the past few decades, various imaging techniques – such as micro-CT, scanning electron microscopy (SEM), light microscopy, and confocal laser scanning microscopy – have been developed to evaluate CA properties. Optical coherence tomography offers high-resolution, non-invasive 3D imaging but is limited by poor contrast at certain material interfaces. SEM provides detailed surface morphology but is costly and may introduce artifacts. Micro-CT was employed in this study to evaluate aligner thickness and gap width due to its non-destructive nature, high spatial resolution, and ability to generate detailed 3D images. Unlike traditional sectioning methods, micro-CT preserves the structural integrity of the aligner material during both measurement and analysis. This imaging modality enables precise visualization of internal and external geometries, allowing for accurate quantification of aligner thickness and the interfacial gap between the aligner and tooth surface – even in anatomically complex regions. Accurate measurement of aligner thickness is clinically relevant, as it directly influences the magnitude and consistency of force transmission to the dentition. Similarly, assessment of the gap width is essential for evaluating the fit of the aligner, which is critical for effective force delivery and predictable tooth movement. Ammann et al. previously have researched using hard X-ray tomography for assessment of the gaps between aligners and teeth, whereas, in our study, micro-CT was employed since it offers higher spatial resolution, better contrast, lower radiation dose, and greater accessibility compared to hard X-ray tomography, allowing nondestructive, high-resolution three-dimensional analysis of small specimens using laboratory-based systems.^[8]

A total of 27 samples were analyzed, comprising three different aligner materials: PET-G, PU, and MLPU. For each material, nine samples were fabricated and evaluated. In aligner brands, PET-G, PU, and MLPU are commonly used materials due to their unique properties:

PETG is a commonly preferred material for CAs due to its higher translucency, fatigue resistance, and dimensional stability. PU is a ductile elastomer with high elasticity and formability, which provides good wearability and shock absorption. Multilayer PU materials have been introduced to overcome the drawbacks of single-layer materials. Materials with a combination of hard outer shells and a soft inner shell show improved mechanical strength of maximum load in the tensile test and water absorption rate.^[10]

In CA treatment, both the thickness and gap width of the aligners play crucial roles in their effectiveness. Aligner thickness affects the force applied to teeth, influencing the extent and efficiency of tooth movement. The gap width is another critical factor, as it dictates how well the aligner can transfer force to the teeth.^[11]

PU showed the highest mean gap width in both tooth types. These results indicate that PU may exhibit less precise adaptation or greater gap formation in these regions compared to PETG and MLPU. In contrast, no significant differences in gap width were observed among the three materials for molars (F = 0.259, p = 0.774), with mean values being similar across groups. This suggests that material selection does not significantly impact the marginal fit as measured by gap width. A significant difference in thickness was found only in the canine region between PU and PET-G, with PU showing increased thickness. For the incisor and molar regions, no significant differences were detected among the three materials as shown in Graphs 2 and 3.

Our results are in agreement with Lombardo et al., who reported that aligners generally exhibit thinner sections and narrower gaps at anterior teeth and thicker regions with wider gaps in the posterior segments.^[6] Interestingly, their analysis of PU aligners suggested a more homogeneous distribution of thickness compared to PET-G, which displayed greater variability. This partly contrasts with our data, as our PU samples showed the largest gap widths, indicating that thermoforming characteristics and brand-specific polymer formulations may play a role.^[12]

Palone et al. also confirmed that thermoforming reduces aligner thickness, particularly at cusp tips and incisal edges, which are critical load-bearing regions. This supports our finding that the canine region showed the most marked thickness differences among materials.^[12] Similarly, Mantovani et al. highlighted thickness inhomogeneity within Invisalign aligners, reinforcing the clinical importance of accurate measurement methods such as micro-CT. [13]

When comparing single-layer and multilayer systems, Park et al., who showed that multilayer thermoplastic polyurethane aligners demonstrated superior fit and smaller gap widths than PET-G in micro-CT analysis. In line with their results, our MLPU samples exhibited lower variability and consistent adaptation, underscoring the mechanical advantage of layered designs in resisting deformation during thermoforming.^[14]

In addition, Elshazly et al. demonstrated that both material composition and thickness strongly affect the distribution of orthodontic forces, reinforcing the clinical significance of even small dimensional differences. Our data support this notion, as PU’s greater thickness in the canine region could potentially influence localized force levels, while its wider gap width may reduce overall force transfer efficiency.^[15]

Most studies, including the present one, are conducted in vitro and cannot fully replicate intraoral conditions such as saliva, temperature fluctuations, and masticatory forces. Few studies have longitudinally tracked dimensional changes using micro-CT during real patient wear, although Bucci et al. showed measurable thinning after intraoral use. Moreover, while thickness and gap width are valuable physical parameters, their direct correlation with clinical outcomes such as movement predictability, retention, and patient comfort remains underexplored.^[16]

Overall, our study adds to the body of evidence by simultaneously quantifying both aligner thickness and gap width across different tooth regions and materials, using high-resolution micro-CT. The results reinforce that PU may provide less precise adaptation compared to PET-G and MLPU, with the latter offering the most stable and clinically favorable properties.

CONCLUSION

PU showed the highest mean gap width and aligner thickness in both tooth types. These results indicate that PU may exhibit less precise adaptation or greater gap formation in these regions compared to PETG and MLPU. PETG and MLPU show a favorable property.