Optimizing printing protocols for the fabrication of 3D-printed attachments in clear aligner therapy: a comparative in vitro study

Abstract

Objective

This study aimed to evaluate the feasibility of a porcelain-reinforced 3D-printed resin as a potential next-generation attachment material for clear aligner therapy (CAT) and to investigate the effect of printing direction and post-curing time on its mechanical and biological performance.

Methods

Attachment specimens were fabricated using digital light processing (DLP) under two printing directions (parallel and perpendicular to the specimen’s long axis) and three post-curing durations (10, 20, and 30 min). Dimensional accuracy, surface morphology, water sorption, mechanical properties (elastic modulus, yield strength, hardness), and cytocompatibility were evaluated. Filtek™ Z350 XT served as the control material.

Results

Samples printed parallel to their long axis exhibited significantly better dimensional accuracy, elastic modulus, yield strength, and less water sorption compared with vertically printed samples (p < 0.05). Increasing post-curing duration improved hardness and cytocompatibility, with the 30-min group demonstrating the most favorable overall performance. Compared with Z350, the 3D-printed samples showed higher geometric fidelity, improved dimensional stability, less water sorption, and greater yield strength (p < 0.05), yet lower modulus and hardness (p < 0.05). Further clinical studies are needed to validate the long-term in vivo performance of this 3D-printed material.

Conclusion

The porcelain-reinforced 3D-printed resin demonstrates strong potential as an alternative material for clear-aligner attachments. Printing parallel to the specimen’s long axis, combined with a 30-min post-curing protocol, provides the most advantageous balance of mechanical performance, structural stability, and biocompatibility.

Introduction

Clear aligner therapy (CAT) is a digitally driven orthodontic modality that replaces conventional brackets and arch wires with a series of thermoformed transparent polymer appliances. These aligners are generated from three-dimensional (3D) digital dental models obtained through intraoral scanning and computer-aided design/manufacturing (CAD/CAM) workflows [1]. Each aligner induces incremental tooth movement through controlled elastic deformation and recovery of thermoplastic materials [2]. Compared with fixed appliances, CAT offers improved aesthetics, patient comfort, hygiene accessibility, and digitally simulated treatment planning [3, 4].

Despite its advantages, the biomechanical efficiency of CAT is highly sensitive to appliance–tooth conformity. Inadequate retention or uncontrolled force delivery may result in aligner “tracking loss”, where the appliance deviates from the planned tooth movement. Clinical evidence indicates that a substantial proportion of cases require mid-course correction or additional refinement stages, reflecting limitations in force transmission and geometric accuracy [5, 6].

To enhance three-dimensional tooth control, composite resin attachments are bonded to tooth surfaces [7] (Fig. 1A). These small geometrically designed auxiliaries create undercuts that improve aligner retention and modulate force vectors, thereby facilitating intrusion, torque expression, and rotational control. The performance of attachments depends critically on dimensional fidelity, mechanical integrity, and interfacial conformity with the aligner [8, 9]. However, conventional chairside fabrication relies on manual composite placement and polymerization, introducing variability in morphology, shrinkage distortion [10], and reproducibility. In this context, the emergence of 3D printing technology has introduced a novel digital pathway for attachment manufacturing.

Fig. 1.

Conceptual illustration of the design, clinical application, and advantages of 3D-printed attachments for clear aligner therapy. A Representative images of attachment design and clinical application. B Schematic overview of the major advantages of 3D-printed attachments

Compared with traditional composite attachments, 3D-printed attachments employ a distinct digital workflow on both the laboratory and clinical sides, enabling the direct fabrication of complex geometries with high precision [11]. This approach provides several unique advantages (Fig. 1B): (1) precise fabrication of complex attachment geometries, such as power arms and buccal buttons; (2) expanded potential for integrating advanced functional features, including antibacterial or shape-memory properties; (3) reduced technical sensitivity and operator-dependent variability, together with markedly improved chairside efficiency; and (4) superior dimensional accuracy, detail reproducibility, and structural consistency, resulting in enhanced clinical reliability.

Despite their theoretical advantages, research on 3D-printed attachments remains limited, and important knowledge gaps persist. Particularly, systematic evidence is lacking regarding how material composition, printing direction, post-curing protocols, and other processing parameters affect dimensional accuracy, mechanical performance, detail reproduction, and surface characteristics [12, 13].

Therefore, this study aimed to investigate the printing parameters influencing the performance of 3D-printed attachments by systematically evaluating the effects of printing direction and post-curing duration on dimensional accuracy and mechanical properties, and by comparing these outcomes with those of clinically used composite attachment materials. The findings are expected to provide foundational evidence for assessing the feasibility of 3D-printed resins in attachment fabrication and to support future developments in personalized attachment design, integrated digital aligner systems, and more precise and controllable CAT strategies.

Materials and methods

Sample preparation

For the 3D-printed attachment group, the fabrication process consisted of four main steps (Fig. 2). (1) A digital model of the attachment was constructed using 3D modeling software (SketchUp, Trimble Inc., USA). The STL files were then imported into the BEGO CAMcreator software (BEGO, Germany) to configure the printing orientation and design the necessary support structures (Fig. 2A). (2) The designed models were fabricated using a BEGO Varseo X S DLP printer (Varseo XS, BEGO, Germany) with VarseoSmile Crownplus A2 ceramic-reinforced resin (BEGO, Germany) as the printing material (Fig. 2B). (3) After printing, the supported samples were removed from the build platform and immersed in 95% ethanol with agitation for 15 min to eliminate uncured resin monomers (Fig. 2C). (4) According to the experimental grouping, the samples were subsequently post-cured, after which the supports were removed and the surfaces were polished to obtain the final standardized specimens (Fig. 2D).

Fig. 2.

Workflow for the manufacturing of 3D-printed attachments. A modeling, in which the left subpanel shows specimens aligned parallel to the printing direction (Horizontal group), whereas the right subpanel shows specimens aligned perpendicular to the printing direction (Vertical group); B 3D printing; C elution; and D post-curing

For the Z350 group, the fabrication of attachments also involved four main steps (Fig. 3). (1) A cylindrical base (100 mm in diameter and 20 mm in height) was designed using CAD software (SketchUp 23.0.367, Trimble, USA), onto which rectangular and circular molds—identical in geometry to those of the 3D-printed groups—were constructed (Fig. 3A). (2) Master molds were fabricated via 3D printing (Fig. 3B). (3) Thermoformed attachment templates were produced using FusionAM sheets (LargeV, China) (Fig. 3C). (4) Dental composite resin (3 M Filtek™ Z350 XT, 3 M, USA) was manually filled into the templates, which were then firmly adapted onto flat smooth surface and light-cured for 40 s according to the manufacturer’s instructions. After curing, excess material was removed with abrasive paper, and surface finishing was performed to obtain standardized specimens (Fig. 3D).

Fig. 3.

Workflow for the manufacturing of traditional Z350 attachments. A computer-aided design of a master mold containing rectangular and circular specimen geometries; B 3D printing of the master mold; C thermoforming of plastic sheets over the master mold to generate standardized specimen molds; and D filling of Z350 resin into the corresponding mold cavities, followed by post-curing and demolding

Grouping

This study included one control group (Z350) and six experimental groups classified according to printing orientation and post-curing time: Hor10, Hor20, Hor30, Ver10, Ver20, and Ver30 (Table 1). All specimens were allocated according to testing requirements: rectangular specimens (25 × 2 × 2 mm) were used for accuracy evaluation, elastic modulus testing, yield strength testing, water absorption analysis, and dimensional stability assessment, with 15 specimens per group. Disk-shaped specimens (14 mm in diameter and 2 mm in thickness) were used for hardness testing, surface morphology observation, and cytotoxicity evaluation, with 5 specimens per group.

Table 1.

Classification of experimental groups

	Printing parallel to the long axis (Horizontal)	Printing perpendicular to the long axis (Vertical)
10 min post-curing	Hor10	Ver10
20 min post-curing	Hor20	Ver20
30 min post-curing	Hor30	Ver30

Characterization

Surface morphology and particle size distribution

The samples were mounted on aluminum SEM stubs using double-sided conductive copper tape and sputter-coated with a 5-nm gold layer using a carbon/gold coater (LEICA EM ACE200, Leica, Germany). The coated specimens were then examined using a field-emission scanning electron microscope (S-4800 FE-SEM, HITACHI, Japan) at an accelerating voltage of 5.0 kV and a working distance of 10 mm.

Three representative SEM images were obtained from each sample at ×5.00 K magnification for particle size analysis, and the full field of view at this magnification was used as the measurement area. The images were analyzed using ImageJ software (Version 1.8.0_112, 64-bit; National Institutes of Health, Bethesda, MD, USA) after adjustment of brightness and contrast to improve particle edge definition. The maximum particle diameter was measured along the particle boundary with the line tool, and 1–2 measurements were recorded for each particle as appropriate. The resulting data were exported to Origin 2024 (Version 10.1; OriginLab Corporation, Northampton, MA, USA), where particle size distribution histograms were plotted and the mean, range, and standard deviation were calculated. Gaussian curve fitting was also performed to evaluate particle distribution uniformity.

Dimensional accuracy

The dimensions of the samples were measured using an electronic vernier caliper. For each specimen, the length, width, and height were recorded three times, and the average values were used for subsequent analysis. For further analysis, the data from Ver10, Ver20, and Ver30 were pooled as the vertical printing group, and the data from Hor10, Hor20, and Hor30 were pooled as the horizontal printing group. To evaluate manufacturing precision, dimensional accuracy was expressed as the percentage of the nominal CAD-designed dimension, calculated as the percentage of the nominal CAD dimension. A value of 100% indicated perfect fidelity to the design, whereas values > 100% and < 100% indicated oversizing and undersizing, respectively. The nominal design dimensions were 25 mm for length and 2 mm for both width and height.

Water sorption stability

Under ambient temperature and humidity, the initial mass of each specimen was recorded as W₀, with each weighing completed within 30 s. The samples were then immersed in distilled water maintained at 37 °C in six pre-cleaned beakers, ensuring that specimens did not overlap. After immersion for 12, 24, 48, 72, and 168 h (7 days), the samples were removed using clean tweezers, gently dried with sterile gauze, weighed, and recorded as W₁–W₅. At each time point, the length, width, and height of the specimens were measured three times using an electronic vernier caliper, and the mean values were calculated. Following measurement, the samples were immediately returned to the water bath. Water absorption was calculated as:

where W_n and W₀ are the specimen masses (mg) measured at each time point and at baseline, respectively, and V₀ is the initial specimen volume (mm³). W (Water sorption) is expressed in µg/mm³.

Mechanical properties

Three-point bending test

The mechanical properties of the specimens were evaluated using a universal testing machine (AG-IS 500 N, SHIMADZU Corporation, Japan), with the test configuration selected with reference to ISO 4049:2019 for polymer-based restorative materials. Each sample was positioned with a support span of 15 mm and tested at a crosshead speed of 0.5 mm/min. The elastic modulus was calculated using the equation:

where m is the slope of the initial linear region of the load–displacement curve (N/mm), L denotes the support span (mm), and b and h are the specimen width and thickness (mm), respectively.

Yield strength (R_e) was calculated separately using the formula:

where Fₑ represents the yield force (N) and S₀ denotes the initial cross-sectional area of the specimen (mm²).

Hardness

The Vickers hardness of the samples was measured using a microhardness tester (FM-700, FUTURE-TECH, Japan) under a 50 g load with a dwell time of 10 s. For each specimen, ten indentations were performed, and the mean value was recorded.

Biocompatibility assessment

Cytotoxicity was assessed using the CCK-8 assay on human oral keratinocytes (HOK cells). HOK cells were seeded into the lower chamber of a 24-well Transwell plate at a density of 0.5 × 10⁵ cells/well. Once the cells reached approximately 70% confluence, the 3D-printed samples and the Z350 control specimens were placed in the upper chamber. After 24 h of co-culture, 10 µL of CCK-8 reagent was added to each well, followed by incubation at 37 °C in the dark for an additional 4 h. The absorbance at 450 nm was then measured using a microplate reader. Cytotoxicity was quantified by calculating the relative cell viability (RCV) for each group:

Statistics

All statistical analyses were performed using IBM SPSS Statistics (Version 21, IBM, Armonk, NY, USA). For the 3D-printed groups, a two-way ANOVA was conducted to evaluate the main effects of printing direction and post-curing time, as well as their interaction. When significant effects were identified, Bonferroni post hoc tests were applied for multiple pairwise comparisons. Comparisons among all groups, including the Z350 control, were analyzed using one-way ANOVA followed by Bonferroni correction. Repeated-measures ANOVA was employed to analyze water absorption and dimensional stability over time. For data that did not conform to a normal distribution, the Kruskal–Wallis test or Mann–Whitney U test was used as appropriate. A p-value < 0.05 was considered statistically significant.

Results

Characterization

Surface morphology and particle size distribution

Scanning electron microscopy showed differences in surface morphology among all groups. Compared with the other 3D-printed groups, the 30-min post-cured specimens tended to exhibit relatively larger and more clustered surface particles, although similar local clustered features could also be observed in some non-30-min groups, such as Ver10 (Fig. 4). In contrast, the Z350 group displayed a distinctly rough surface characterized by flake-like structures, which differed markedly from the morphological features observed in the 3D-printed groups (Fig. 4G).

Fig. 4.

SEM images of each group: A Hor10, B Hor20, C Hor30, D Ver10, E Ver20, F Ver30, and G Z350

Quantitative analysis based on particle size distribution further supported these findings. In both printing orientations, the particle size increased significantly after 30 min of post-curing (p < 0.05). The horizontally printed samples exhibited particle sizes of 0.69 ± 0.29 μm, 0.63 ± 0.35 μm, and 0.80 ± 0.35 μm after 10, 20, and 30 min of post-curing(Fig. 5A-C), respectively; the corresponding values for the vertically printed samples were 0.68 ± 0.19 μm, 0.63 ± 0.27 μm, and 0.74 ± 0.43 μm (Fig. 5D-F). The Z350 group presented a particle size of 0.89 ± 0.42 μm. Except for the two 3D-printed groups post-cured for 30 min, all remaining groups exhibited significantly smaller particle sizes compared with the Z350 group (p < 0.05) (Fig. 5G-H).

Fig. 5.

Particle size distribution and comparison among all groups. A–G Particle size distribution of each group: A Hor10, B Hor20, C Hor30, D Ver10, E Ver20, F Ver30, and G Z350. H Comparison of surface particle size among all groups. **** Indicates p < 0.0001; *** indicates p < 0.001; ** indicates p < 0.01; * indicates p < 0.05

Dimensional accuracy

Analysis of dimensional accuracy across the length, width, and height (Fig. 6) demonstrated that both horizontally and vertically printed 3D-printed specimens achieved substantially higher geometric fidelity than the Z350 group in all measured dimensions, indicating the superior precision afforded by additive manufacturing in the fabrication of orthodontic attachments. Within the 3D-printed groups, printing direction emerged as the predominant determinant of dimensional accuracy, whereas variations in post-curing duration exerted no statistically significant influence.

Fig. 6.

Dimensional accuracy of each group. A–C Impact of printing direction on the precision of 3D-printed samples. A Percentage of precision of length, B Percentage of precision of width, C Percentage of precision of height. D Impact of post-curing time on the precision of 3D-printed samples. **** Indicates p < 0.0001; ** indicates p < 0.01; ns indicates no statistically significant difference

A closer examination of individual dimensions revealed that printing direction had minimal impact on accuracy in the length and width directions, as no significant differences were observed between the vertical group (100.30 ± 0.24%, 101.00 ± 1.46%) and the horizontal group (99.93 ± 0.57%, 100.80 ± 1.05%) (p > 0.05) (Fig. 6A–B), In contrast, the Z350 group showed markedly greater deviation from the CAD dimensions, with values of 102.8 ± 4.10% in length and 123.46 ± 4.14% in width. In the height dimension, vertically printed samples exhibited significantly reduced accuracy (98.93 ± 2.15%) relative to their horizontally printed counterparts (100.00 ± 0.29%) (p < 0.05), whereas the Z350 group showed a value of 96.80 ± 4.01% (Fig. 6C).

Water sorption stability

As immersion time increased, the water absorption of all samples showed a gradual upward trend (Fig. 7A). After 7 days of immersion, the vertically printed samples exhibited a water absorption of 9.55 ± 1.85 µg/mm³, which was significantly higher than that of the horizontally printed samples (4.13 ± 0.54 µg/mm³) (p < 0.05), whereas both groups demonstrated significantly lower values compared with the Z350 group (14.74 ± 5.24 µg/mm³) (p < 0.05). Post-curing time had no significant effect on water absorption (p > 0.05). Figure 7B–D illustrate the dimensional changes of the samples after immersion in distilled water. After 7 days, all 3D-printed samples—regardless of printing direction or post-curing duration—exhibited significantly better dimensional stability in length, width, and height compared with the Z350 group (p < 0.05) (Table 2). Specifically, the Hor10, Ver20, and Hor20 groups demonstrated superior stability in the respective dimensions evaluated (p < 0.05).

Fig. 7.

Water absorption and dimensional stability of each group. A Measurement of water absorption at multiple time intervals. B Length changes at multiple time intervals. C Height changes at multiple time intervals. D Width changes at multiple time intervals

Table 2.

Water absorption and dimensional stability in length, width, and height of the specimens after 168 h of immersion

Group	Water absorption (µg/mm³)	Dimensional stability in length (%)	Dimensional stability in width (%)	Dimensional stability in height (%)
Ver10	9.44 ± 2.83	100.38 ± 0.05	100.82 ± 0.46	102.16 ± 1.38
Ver20	8.72 ± 0.98	100.26 ± 0.05	101.03 ± 3.06	101.31 ± 1.27
Ver30	10.50 ± 1.03	100.23 ± 0.03	100.69 ± 1.25	101.50 ± 1.06
Hor10	3.84 ± 0.53	100.33 ± 0.27	101.17 ± 0.71	101.08 ± 1.23
Hor20	3.98 ± 0.47	100.27 ± 0.10	101.18 ± 1.05	100.52 ± 2.59
Hor30	4.58 ± 0.42	100.21 ± 0.07	101.88 ± 0.82	101.89 ± 0.55
Z350	14.74 ± 5.24	100.94 ± 0.12	108.12 ± 4.33	108.10 ± 6.53

Mechanical properties

Elastic modulus

Statistical analysis indicated that both printing direction and post-curing time had significant effects on the elastic modulus of the materials (Fig. 8). Among these factors, printing direction exerted a more pronounced influence. At the same post-curing durations (10, 20, and 30 min), the elastic modulus of the horizontally printed samples (6.23 ± 0.37 GPa, 6.51 ± 0.27 GPa, and 6.36 ± 0.23 GPa) was consistently and significantly higher than that of the vertically printed samples (4.41 ± 0.36 GPa, 5.45 ± 0.54 GPa, and 5.20 ± 0.76 GPa) (p < 0.05) (Fig. 8A).

Fig. 8.

Elastic modulus of each group. A Impact of printing direction on the elastic modulus of the 3D-printed samples. B Impact of post-curing time on the elastic modulus of the 3D-printed samples. C Comparison of the elastic modulus between the 3D-printed samples and the Z350 group. **** Indicates p < 0.0001; ** indicates p < 0.01; * indicates p < 0.05; ns indicates no statistically significant difference

With respect to post-curing time, the elastic modulus of samples subjected to 20 min of post-curing—regardless of printing orientation—was significantly higher than that of those cured for 10 min (p < 0.05); however, no statistically significant differences were observed between the 20- and 30-min groups (p > 0.05) (Fig. 8B). Among all experimental groups, the Z350 resin exhibited the highest elastic modulus (14.32 ± 1.64 GPa), which was significantly greater than that of all 3D-printed groups (p < 0.05) (Fig. 8C).

Yield strength

Statistical analysis revealed that the yield strength of the horizontally printed group after 10 min of post-curing (106.80 ± 25.56 MPa) was significantly higher than that of the vertically printed group (88.32 ± 24.69 MPa) (p < 0.05) (Fig. 9A). For the remaining 3D-printed groups, no statistically significant differences were observed between different printing directions or among different post-curing times (p > 0.05) (Fig. 9A–B). When compared with the Z350 group (87.13 ± 31.79 MPa), the Hor20 (120.40 ± 39.85 MPa), Hor30 (123.00 ± 35.66 MPa), and Ver20 (112.50 ± 24.88 MPa) groups exhibited significantly higher yield strength (p < 0.05) (Fig. 9C–D).

Fig. 9.

Yield strength of each group. A Impact of printing direction on the yield strength of the 3D-printed samples. B Impact of post-curing time on the yield strength of the 3D-printed samples. C–D Comparison of the yield strength between the 3D-printed samples and the Z350 group. * Indicates p < 0.05; ns indicates no statistically significant difference

Hardness

In terms of hardness, printing direction and post-curing time exhibited distinct patterns of influence. At any given post-curing duration (10, 20, or 30 min), no significant differences were observed between the horizontally and vertically printed groups (p > 0.05) (Fig. 10A). However, the hardness of the samples increased progressively with longer post-curing time. The horizontally printed specimens exhibited hardness values of 20.95 ± 1.10 HV, 23.58 ± 0.78 HV, and 28.69 ± 1.20 HV after 10, 20, and 30 min of post-curing, respectively (p < 0.05). Similarly, the vertically printed specimens showed hardness values of 21.71 ± 1.95 HV, 24.28 ± 0.91 HV, and 28.29 ± 1.13 HV at the corresponding time points (p < 0.05). In addition, the Z350 group demonstrated a hardness of 49.74 ± 6.17 HV, which was significantly higher than that of all 3D-printed groups (p < 0.05) (Fig. 10B–C).

Fig. 10.

Hardness of each group. A Impact of printing direction on the hardness of the 3D-printed samples. B–C Impact of post-curing time on the hardness of the 3D-printed samples, and comparison with Z350 group. * Indicates p < 0.05; ns indicates no statistically significant difference

Biocompatibility assessment

The biocompatibility results are shown in Fig. 11. The cell viability of both the horizontally printed and vertically printed groups subjected to 10-min post-curing was significantly lower than that of the other groups (p < 0.05). In contrast, no significant differences in cell viability were observed among the 3D-printed groups after 20-min, 30-min post-curing and the Z350 group, indicating that adequately post-cured 3D-printed materials exhibit comparable cytocompatibility to clinical resin materials.

Fig. 11.

Cell viability of all 3D-printed groups compared with the Z350 control group. * Indicates p < 0.05

Discussion

Effects of different printing directions

Existing studies have clearly demonstrated that printing direction is one of the most critical determinants of geometric fidelity, internal structural uniformity, and macroscopic mechanical performance in photopolymer-based additive manufacturing [14–16]. Given that vat-photopolymerization inherently relies on a layer-by-layer stacking process, orientation directly influences the pattern of error accumulation: when interlayer interfaces are unfavorably aligned, they become more prone to interfacial gaps and weakened bonding, resulting in orientation-dependent geometric deviations and mechanical deterioration [17, 18]. In terms of dimensional accuracy, no meaningful differences were observed between the two printing directions, except for the height dimension, where horizontal printing achieved significantly higher accuracy. The reason might be the material loss during support removal. In vertical printing groups, all supports lay beneath the specimens, therefore their removal caused basal material loss that disproportionately affected the 2-mm height accuracy. In contrast, although the same loss existed in the horizontal printing groups, it had negligible impact on the 25-mm length accuracy, resulting in no significant orientation-related differences. On the other hand, the 3D-printed attachments demonstrated significantly higher geometric fidelity compared with those fabricated in the conventional way. The manual, template-based approach resulted in substantial and clinically unacceptable oversizing (e.g., > 20% in width), highlighting a key advantage of the digital 3D-printing workflow in reproducibly manufacturing attachments to their exact design specifications.

The water sorption behavior and mechanical properties of the printed specimens also exhibited pronounced orientation-dependent characteristics. Horizontally printed samples demonstrated lower water sorption and higher elastic modulus than those printed vertically, a trend that aligns well with previous findings on 3D-printed resins [19, 20]. From a mechanistic perspective, although the vertical group involved fewer layers, each layer possessed a substantially larger cured cross-sectional area, resulting in reduced light penetration and nonuniform edge curing [21], which in turn facilitated interfacial debonding, shear slip, and early microcrack initiation, allowing infiltrating water molecules to diffuse more readily along weak interfacial regions and further exacerbate structural degradation [22]. In contrast, horizontal printing promotes favorable alignment of polymer chain segments and interlayer fusion interfaces along the principal loading direction, yielding a more homogeneous network architecture and more efficient load-transfer pathways [23].

Notably, the influence of printing direction on yield strength was evident only in the 10-min post-curing group and gradually diminished in the 20–30-min groups. This pattern suggests that sufficient post-curing enhances interlayer cross-linking density, stabilizes interfacial regions, and alleviates anisotropy—consistent with previous reports showing that extended post-curing increases network conversion and structural integrity in photopolymer systems [24].

Collectively, these findings indicate that printing direction governs not only geometric fidelity but also exerts profound influence on mechanical performance by modulating interlayer cohesion, polymer-network homogeneity, and microscale defect distribution. Based on the comprehensive results of this study, horizontal printing should be regarded as the preferred fabrication strategy for clear-aligner attachments, offering superior dimensional accuracy, more consistent mechanical properties, and enhanced long-term performance under clinical loading conditions.

Effects of different post-curing time

Post-curing is a critical step in optimizing the performance of photocurable resin systems, as it affects not only the degree of polymer network conversion but also the evolution of surface microstructure [25]. In the present study, extending the post-curing time from 10 to 30 min increased the mean surface particle size of the 3D-printed resin from the submicron range of approximately 0.63–0.80 μm, while the Z350 group showed a value of approximately 0.89 μm. Previous reviews on dental resin composites have shown that commonly used glass fillers are often within the micrometer range (e.g., approximately 0.6–10 μm), whereas smaller and more uniformly distributed nano-/submicron-scale fillers are generally associated with improved hardness, wear resistance, surface smoothness, and mechanical stability [26]. However, no universal “optimal” particle size has been established for all resin systems, because the final performance depends on filler size distribution, filler loading, interfacial coupling, and matrix composition [27].

Notably, although no significant difference in elastic modulus was detected between the 20- and 30-min groups, the 30-min group still exhibited larger mean particle size, greater surface densification, and higher hardness. This suggests that the polymer network may have approached a bulk conversion plateau by 20 min, whereas additional post-curing continued to improve surface maturation and local mechanical stability. For clear-aligner attachments, a higher and more stable surface hardness is generally desirable because it helps resist wear, local plastic deformation, and loss of attachment morphology during repeated insertion-removal cycles [28]. In this regard, materials with hardness approaching clinically used attachment resins, such as Z350 (3 M Filtek™ Z350 XT Universal Restorative), Z350L (3 M Filtek™ Z350 XT flowable resin), may still be regarded as reliable for clinical application. Whether further increases in hardness will necessarily translate into superior orthodontic efficacy, however, remains to be verified in future clinical studies. In future work, we will continue to modify the resin formulation, for example by optimizing filler loading and composition, in an effort to further improve hardness and overall material performance.

Water-sorption analysis revealed no significant differences among the various post-curing durations, consistent with the findings of Altarazi, who noted that post-curing time primarily affects mechanical behavior rather than diffusion-controlled sorption processes [29]. In contrast, Z350 exhibited markedly higher water uptake, likely due to its high-surface-energy nanofiller architecture and more hydrophilic composite interface—features commonly reported in nano-hybrid resin composites [30].

Regarding biocompatibility, specimens cured for only 10 min showed substantially reduced cell viability, whereas ≥ 20-min post-cured samples demonstrated cell activity comparable with that of Z350. This observation is consistent with the well-established notion that insufficient post-curing leads to incomplete monomer conversion and increased release of cytotoxic residual monomers [31]. Our findings similarly confirm that adequate post-curing (20–30 min) substantially mitigates this risk and ensures cytocompatibility.

Considering mechanical performance, surface densification, and biological safety, a post-curing duration of 30 min appears optimal for this porcelain-reinforced photocurable resin. It provides enhanced cross-link quality, increased hardness, and favorable long-term performance without compromising biocompatibility, thereby demonstrating strong potential for clinical application in clear-aligner attachments.

Consistent with these theoretical and empirical observations, the present study found that horizontally printed specimens significantly outperformed vertically printed ones across four key domains—dimensional accuracy, resistance to water sorption, elastic modulus, and yield strength (Table 3).

Table 3.

Comprehensive analysis

	Effect of printing direction	Effect of time of post-curing
Characterization
Surface Morphology	No significance	evident particle agglomeration and increased particle size after 30 min of post-curing
Accuracy	Horizontal groups showed significantly higher height accuracy than vertical groups	No significance
Water Resistance	Horizontal groups showed significantly lower water absorption than vertical groups	No significance
Mechanical Properties
Elastic Modulus	Horizontal groups showed significantly higher elastic modulus than vertical groups	20–30 min posturing groups showed higher elastic modulus than the 10-min group
Yield Strength	Horizontal groups showed significantly higher yield strength than vertical groups after 10 min of post-curing	No significance
Hardness	No significance	Hardness increased with longer post-curing time
Biocompatibility assessment
Biocompatibility	No significance	20– 30 min post-curing groups showed higher biocompatibility than the 10-min group
Conclusion	Horizontal groups and 30-min post-curing groups showed superior overall performance.

Outlook

In this study, the commercial nano-hybrid composite resin Z350 was used as a conventional benchmark to assess the potential of a porcelain-reinforced 3D-printed resin as an alternative material for clear-aligner attachments. Compared with the traditional template-based filled technique, 3D printing offers substantial advantages in design flexibility—enabling the direct fabrication of complex geometries such as customized buccal buttons, power arms and cantilevers—and markedly streamlines the clinical workflow. The conventional filling method typically requires multiple steps, including chairside resin filling, bonding and manual finishing; this process commonly consumes 3–5 min per tooth and is sensitive to operator skill and template adaptation. In contrast, 3D-printed attachments can be produced with precise pre-defined morphology, eliminating resin filling and finishing steps, and shortening chairside time.

Beyond workflow efficiency, the material-level findings of this study demonstrate several clear advantages of the 3D-printed resin over Z350. First, 3D-printed attachments exhibited superior dimensional accuracy across all three spatial axes, corroborating the observations of Bellocchio, who reported significantly improved geometric reproducibility in printed orthodontic components [11]. Second, the 3D-printed material showed lower water sorption and more favorable dimensional stability under humid conditions, suggesting a more orderly and volume-stable cross-linked network that better withstands the thermohumid environment of the oral cavity. Moreover, the 3D-printed resin demonstrated higher yield strength than Z350, indicating enhanced resistance to plastic deformation. This may be attributable to a combination of its ceramic-reinforced composition and the anisotropic properties imparted by the layer-by-layer fabrication, which, when optimized (i.e., horizontal printing), can result in favorable mechanical performance [15]. Although Z350 exhibited higher elastic modulus and hardness, these properties largely stem from its higher inorganic filler content and dense nanocomposite architecture [32], features that may concurrently predispose the material to increased water uptake, stress concentration and microcrack initiation at the filler–matrix interface.

Taken together, this study provides a systematic evaluation of how printing direction and post-curing time influence the performance of 3D-printed attachment materials; however, several limitations remain. First, the experiments were conducted under controlled in vitro conditions and did not fully replicate the complex clinical environment of long-term aligner wear, including repetitive insertion–removal cycles, frictional wear, and thermal–hygroscopic cycling. Second, long-term aging behavior, fatigue resistance, and microbiological stability were not assessed. Future studies incorporating dynamic loading models, accelerated aging protocols, and in vivo or in situ evaluations will be necessary to further validate the long-term durability and clinical applicability of this material system.

Conclusions

This study systematically evaluated the influence of printing direction and post-curing time on the performance of a porcelain-reinforced 3D-printed resin for clear-aligner attachments, the main findings are as follows:

1. Printing parallel to the specimen’s long axis demonstrated superior dimensional accuracy, mechanical behavior, and wet-state stability compared with printing perpendicular to the specimen’s long axis.

2. Extended post-curing time improved hardness and cytocompatibility, with 30 min yielding the most favorable outcomes.

3. Relative to the conventional Z350 resin, the 3D-printed material exhibited clear advantages in geometric fidelity, yield strength, water sorption and dimensional stability, supporting its feasibility as an alternative attachment material.

Abbreviations

CAT

Clear aligner therapy

3D

Three-dimensional

CAD/CAM

Computer-aided design/manufacturing

DLP

Digital light processing

SEM

Scanning electron microscopy

HV

Vickers hardness

CCK-8

Cell Counting Kit-8

HOK

Human oral keratinocytes

OD

Optical density

RCV

Relative cell viability

ANOVA

Analysis of variance

Authors’ contributions

X.W.: Conceptualization, Methodology, formal analysis, Writing - Original Draft. H.G.: Data Curation, Validation, Writing - Original Draft. P.M.: Methodology. Y.B.: Resources, Supervision, Funding Acquisition. D.F.: Conceptualization, Writing - Review & Editing, Supervision, Funding Acquisition. All authors reviewed the manuscript.

Funding

This work was supported by This work was supported by the Beijing Hospital Authority Clinical Medicine Development Special Funding Support (Grant No. ZLRK202330) and Beijing Natural Science Foundation (Grant No. L222115).

Data availability

All data supporting this study are included in the article.

Declarations

Ethics approval and consent to participate

The study was conducted in accordance with the Declaration of Helsinki, and approved by the Ethics Committee of Beijing Stomatological Hospital, Capital Medical University (NO. CMUSH-IRB-KJ-PJ-2022-43).

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.