Introduction
This study aimed to compare the accuracy of a novel 3-dimensional (3D)-printed attachment transfer technique with conventional composite-based methods, evaluating the effect of template material and thickness.
Methods
Three transfer methods were investigated: nonflowable composite resin, flowable composite resin, and 3D-printed attachments with a transfer carrier. In the conventional attachment transfer method, 4 different thermoplastic template sheet thicknesses were used: 0.3, 0.5, 0.75, and 0.8 mm. A standardized reference model with 6 digitally planned attachments was used for all groups. Using each method, attachments were transferred to 3D-printed dental models, which were then scanned and superimposed with the reference model. Root mean square and mean distance values were calculated using CloudCompare software (version 2.14; www.danielgm.net/cc/ ) for surface deviation analysis.
Results
Statistically significant differences were observed among the attachment transfer methods ( P <0.001, Kruskal-Wallis test). The 3D-printed attachment group exhibited the highest geometric accuracy across all evaluated sites. Pairwise comparisons showed that the 3D-printed group performed significantly better than both composite-based groups ( P <0.001, Dunn’s test). Composite viscosity showed no significant effect on transfer accuracy, except for attachments 16 and 23 in mean distance values ( P = 0.023-0.031) and attachment 16 in root mean square values ( P = 0.043). Template thickness significantly influenced transfer accuracy, with thicker aligners generally producing lower deviation values, particularly between Tristar (0.3 mm) and Track A (0.8 mm) ( P <0.05 to P <0.001).
Conclusions
The 3D-printed attachment transfer method demonstrated superior accuracy than conventional composite-based methods. Although the thinnest template exhibited greater deviations, accuracy varied among the different templates, suggesting that both material thickness and composition may influence transfer performance. Composite viscosity did not substantially affect transfer accuracy, as both flowable and nonflowable composite resins produced comparable results in most attachment sites.
Highlights
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The 3-dimensional printed attachment transfer method achieved the highest transfer accuracy.
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The thinnest templates yield the greatest deviations.
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Transfer accuracy is also influenced by the material’s mechanical behavior.
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Composite viscosity showed limited influence on attachment transfer accuracy.
The integration of advanced digital technologies such as computer-aided design (CAD) software, 3-dimensional (3D) printing, and digital scanning has significantly transformed the fabrication of clear aligners by enhancing both precision and efficiency. In addition, the growing demand for esthetically favorable orthodontic appliances has driven many clinicians to adopt CAD and computer-aided manufacturing technologies for in-office aligner production. The ability to establish a compact digital laboratory within the clinical setting, thereby eliminating the need for traditional plaster models and reducing reliance on bulky equipment, has further facilitated this transition. Consequently, in-house aligner manufacturing has become a practical and cost-effective solution for contemporary orthodontic practices.
The clear aligner technique can be broadly divided into 2 main approaches: the conventional thermoforming method and the direct fabrication of clear aligners from resin using 3D printing technologies. ,, In the thermoforming approach, composite attachments are used as auxiliaries to guide tooth movement and enhance aligner retention. A model containing the planned attachment geometries is produced, and a clear template aligner, known as an attachment transfer tray, is thermoformed over this model. Flowable or nonflowable composite resin is placed into the attachment reservoirs of this template, which acts as a mold to position the attachments on the teeth. The composite is light-cured through the template, allowing the attachments to bond to the enamel surface with their intended shape and orientation.
Clear aligners are effective in facilitating complex tooth movements, including torque, rotation, mesialization, and distalization of molars. , To achieve these movements efficiently, composite attachments with specific geometric designs are bonded to the tooth surface, playing a crucial role in force transmission. Although some attachments enhance aligner retention, others are strategically designed to facilitate controlled tooth movements such as rotation and translation. ,, The precise placement and design of these attachments are essential for the success of clear aligner therapy, as any deviation from their planned position may disrupt the intended biomechanics, leading to undesired tooth movements that cannot be fully corrected by subsequent aligners.
The efficacy of tooth movement in clear aligner therapy is primarily determined by the material properties and thickness of the aligners, along with the geometric design and integration of auxiliary attachments. , Different attachment shapes have been reported to influence the magnitude and direction of forces exerted by thermoplastic clear aligners on the teeth. The accurate transfer of digitally planned attachments to the teeth is known to be affected by factors such as the type of composite resin and the thickness of the template plate used for shaping and delivering the attachments. Ensuring that attachments are accurately transferred in their intended form brings the treatment outcome closer to the planned digital orthodontic movement.
With recent advancements in 3D printing technology, permanent crown resins have been developed for use in restorative dentistry. Using these materials, which were originally designed for restorative applications, in the fabrication of orthodontic attachments and evaluating their transfer accuracy represents a novel and emerging area of research in orthodontics.
Conventional attachment transfer methods inherently present several limitations, regardless of variations in template thickness, material properties, or composite viscosity. The amount of composite required to fill the reservoir is difficult to standardize, often leading to inconsistent attachment formation. Excess composite around the attachment base is commonly observed and cannot be fully controlled, whereas the accuracy of attachment positioning cannot be verified before polymerization. These issues may lead to uncontrolled overflow and necessitate additional finishing procedures, which risk compromising enamel integrity. Moreover, residual composite may increase bacterial adhesion and promote the development of white spot lesions, whereas the release of bisphenol A from these materials raises concerns about biocompatibility and long-term health effects.
In contrast, 3D-printed attachments offer a standardized and dimensionally stable alternative. Their fabrication through a fully digital workflow allows for precise and consistent reproduction of predefined geometries, minimizing risks such as material overflow, deformation, and operator-dependent variability. Building on these advantages, this study aimed to compare the transfer accuracy of a newly introduced 3D-printed attachment transfer technique with that of conventional composite-based methods, while investigating the effect of template material type and thickness on the precision of attachment placement.
To systematically address these aims, the following alternative hypotheses were proposed: (1) there is a significant difference in attachment transfer accuracy among different techniques, and (2) the material type and thickness of the clear aligners used in these techniques significantly influence the geometric accuracy of attachment placement.
Material and methods
An a priori power analysis was conducted using G∗Power (version 3.1; Heinrich Heine University Düsseldorf, Germany) to determine the minimum sample size required for comparing 3 experimental groups. Assuming a medium to large effect size (Cohen’s f = 0.4), a significance level of 0.05, and a desired statistical power of 0.85, the minimum required total sample size was calculated to be 66. In this study, 40 specimens were included per group (total N = 120), which exceeds this requirement. In addition, a secondary power analysis was conducted for the 4 subgroups used in the tray thickness comparison. On the basis of the same assumptions (f = 0.4, α = 0.05, power = 0.85), the required total sample size was 76. As each thickness group included 20 specimens (total N = 80), the sample size was considered sufficient for this comparison as well.
A digital maxillary model was generated using Blenderfordental (B4D) software (Blenderfordental 2024, United Arab Emirates), incorporating anatomically accurate teeth from the Pablo anatomic collection. Attachments with predefined geometries and dimensions were positioned on teeth 16, 13, 11, 21, 23, and 26 using the Autoalign software (Diorco, Seoul, South Korea). To facilitate comparative evaluation, the model was exported in 2 formats: 1 with attachments and 1 without.
Both the models with attachments and those without were printed using the Ackuretta SOL 3D printer (Ackuretta Tech, Taipei, Taiwan) with Elegoo orthodontic resin (Elegoo, Shenzhen, China). The model printed with attachments was used as the reference model for superimposition. To ensure consistency in the printing process, both models were printed simultaneously with the build orientation set at a 60° angle to the platform. After printing, residual uncured resin was meticulously removed using the Ackuretta Cleani (Ackuretta Tech, Taipei, Taiwan), a 2-stage ultrasonic cleaning system, in which the models were immersed in a 99% isopropyl alcohol bath for 2 consecutive 3-minute cycles. To achieve complete polymerization, a standardized postcuring procedure was performed using the Curie curing unit (Ackuretta Tech, Taipei, Taiwan) for 5 minutes.
The study included 3 main groups (G1-G3), each comprising 40 maxillary arch models, for a total of 120 samples. G1 and G2 involved conventional attachment transfer methods using nonflowable and flowable composite resins, respectively. In both groups, models were further divided into 4 subgroups (n = 10 each) according to the brand and thickness of the thermoformed template aligner materials: Tristar (0.30 mm, polyethylene terephthalate glycol [PET-G]), Taglus (0.50 mm, PET-G), CA Pro (0.75 mm, copolyester and thermoplastic elastomer), and Track A (0.80 mm, PET-G). The only variable differentiating G1 and G2 was the type of composite resin used to fill the attachment reservoirs: G1 used a nonflowable composite (G-aenial Posterior; GC Corp, Tokyo, Japan), whereas G2 used a flowable composite (GC Aligner Connect; GC Corp, Tokyo, Japan). This subgrouping structure enabled evaluation of the potential effects of both template thickness and material composition on transfer accuracy. In contrast, G3 represented a digital protocol using 3D-printed attachments with carriers, applied to 40 models. The detailed group and subgroup structure is summarized in Table I .
Table I
Group and subgroup structure of the study based on composite type and template aligner material characteristics (N = 120)
| Template thermoplastic sheet (transfer aligner) material (thickness; brand) | Subgroup | n |
|---|---|---|
| Nonflowable composite resin (G-aenial Posterior, GC Corp, Tokyo, Japan) (G1) | ||
| Taglus (0.50 mm; Vedia Solutions, Mumbai, India) | Taglus (PET-G) | 10 |
| Tristar (0.30 mm; Visivest Corp, Kuala Lumpur, Malaysia) | Tristar (PET-G) | 10 |
| CA Pro (0.75 mm; Scheu-Dental, Iserlohn, Germany) | CA Pro (Copolyester + TPE) | 10 |
| Track A (0.80 mm; Forestadent, Pforzheim, Germany) | Track A (PET-G) | 10 |
| Flowable composite resin (GC Aligner Connect, GC Corp, Tokyo, Japan) (G2) | ||
| Taglus (0.50 mm; Vedia Solutions, Mumbai, India) | Taglus (PET-G) | 10 |
| Tristar (0.30 mm; Visivest Corp, Kuala Lumpur, Malaysia) | Tristar (PET-G) | 10 |
| CA Pro (0.75 mm; Scheu-Dental, Iserlohn, Germany) | CA Pro (Copolyester + TPE) | 10 |
| Track A (0.80 mm; Forestadent, Pforzheim, Germany) | Track A (PET-G) | 10 |
| 3D-printed attachment (G3) | – | 40 |
TPE , Thermoplastic elastomer.
In G1 and G2, a standardized clinical protocol was followed for composite placement and adaptation. A plastic filling instrument and microbrush were used to shape and remove excess material for both composite types. In G1, the nonflowable composite was incrementally placed into the attachment reservoirs, whereas in G2, the flowable composite was injected in 2 stages. In patients of overflow, the excess material was gently adjusted using the same instruments. After the composite application, all transfer templates were thermoformed using a pressure-forming machine (Ministar, Scheu-Dental, Iserlohn, Germany) at 3.5 bar pressure, seated onto the models, and subsequently light-cured for 5 seconds using a light-emitting diode curing unit (Valo; Ultradent Products Inc, South Jordan, Utah), without any additional manipulation during the curing phase.
In G3, individually designed 3D-printed transfer carriers were created in B4D and customized for each tooth. Each carrier incorporated two 0.2 mm diameter connectors to secure attachments during the design phase, with intentionally thinned edges allowing breakage on removal after bonding ( Fig 1 ). In addition, 0.05 mm spacing was incorporated at the attachment base to optimize composite adaptation and minimize excess material extrusion during bonding. The attachments were 3D-printed using Senertek permanent crown resin (Senertek, İzmir, Turkey) on an Ackuretta SOL (Ackuretta Technologies, Taipei, Taiwan) printer with 0° and 50 μm layer height to ensure detailed geometry. After printing, spray alcohol was applied to remove uncured resin, and the attachments underwent postcuring for 5 minutes. The complete attachment-carrier units were then positioned on teeth and bonded using flowable composite resin (GC Aligner Connect), followed by the same curing protocol as the other groups.
Workflow of 3D-printed attachment transfer.
After the attachment transfer procedures, all models containing both composite and 3D-printed attachments were digitized using the 3Shape TRIOS 3 intraoral scanner (3Shape, Copenhagen, Denmark). Initial model alignment was performed in the alignment module of B4D using the landmark method. Anatomic landmarks were manually placed on both the moving and destination models, followed by transformation and refinement using the Iterative Closest Point algorithm to improve the initial superimposition ( Fig 2 , A ).
Steps for visualizing deviations between attachments in B4D and CloudCompare: A, Landmark-based alignment between the reference model and the transferred attachments; B, Using the circle cutting tool in B4D; C, Paint-based alignment between the reference model and the transferred attachments; D, Final alignment in B4D; E, Fine registration alignment in CloudCompare and generation of a color-coded deviation map with a ±200 μm color scale range ( Blue , inward deviations; red , outward deviations)
The aligned models were joined, and the attachment regions were segmented using the cutting tools in the model designer module. All unrelated parts of the model were removed to isolate only the regions of interest for focused analysis ( Fig 2 , B ). Subsequently, localized alignment refinement was performed using the paint method ( Fig 2 , C ). Attachment regions on both models were manually painted, and an additional Iterative Closest Point registration was applied with parameters set to 50 iterations, 20% outlier removal, and no downsampling ( Fig 2 , D ).
The aligned attachment meshes were imported into CloudCompare software (version 2.14 alpha; www.danielgm.net/cc/ ) for quantitative deviation analysis because of its point cloud-based architecture and native support for surface deviation metrics. Final alignment was achieved through fine registration, as shown in Figure 2 , E Quantitative evaluation was performed using root mean square (RMS) and mean distance (MD) values, which reflect the average magnitude and consistency of deviations among surfaces. Color-coded deviation maps were generated using a ±200 μm scale, in which red indicates outward deviation and blue indicates inward deviation ( Figs 3-7 ).
Deviation maps for 6 attachments in the 3D-printed attachment group. The color scale ranges from −200 μm ( blue , inward deviation) to 200 μm ( red , outward deviation), with green indicating minimal deviation.
Deviation maps of 6 orthodontic attachments created using flowable and nonflowable composite resins with the CA Pro transfer aligner. The color scale ranges from −200 μm ( blue , inward deviation) to 200 μm ( red , outward deviation), with green representing minimal or no deviation.
Deviation maps of 6 orthodontic attachments created using flowable and nonflowable composite resins with the Taglus transfer aligner. The color scale ranges from −200 μm ( blue , inward deviation) to 200 μm ( red , outward deviation), with green representing minimal or no deviation.
Deviation maps of 6 orthodontic attachments created using flowable and nonflowable composite resins with the Tristar transfer aligner. The color scale ranges from −200 μm ( blue , inward deviation) to 200 μm ( red , outward deviation), with green representing minimal or no deviation.
Deviation maps of 6 orthodontic attachments created using flowable and nonflowable composite resins with the Track A transfer aligner. The color scale ranges from −200 μm ( blue , inward deviation) to 200 μm ( red , outward deviation), with green representing minimal or no deviation.
Statistical analysis
Descriptive statistics were presented as median, standard deviation, and minimum-maximum values. Normality of the data distribution was assessed using the Shapiro-Wilk test, and homogeneity of variances was evaluated with Levene’s test. Because the data were not normally distributed, the Kruskal-Wallis test was used to determine differences among groups, and Dunn’s post-hoc test was applied for pairwise comparisons. The level of statistical significance was set at P <0.05. Statistical analyses were performed using SPSS software (version 28.0; IBM, Armonk, NY), and the figures were created with BioRender ( https://BioRender.com ).
Attachment transfer procedures were performed by a single operator (S.B.) to eliminate interoperator variability. To assess intraoperator reliability, 12 randomly selected models were resuperimposed 1 month later. The consistency of these repeated measurements was evaluated using the intraclass correlation coefficient (ICC), with the minimum ICC value calculated as 0.829, indicating good reliability. For interoperator reliability, the minimum ICC value was 0.742, also indicating good reliability.
Results
The trueness of attachment transfer was evaluated across 3 groups: Nonflowable composite resin (G1), flowable composite resin (G2), and 3D-printed attachment with transfer carrier (G3), using MD and RMS values ( Table II ). The Kruskal-Wallis test revealed statistically significant differences among groups for all attachments in RMS measurements ( P <0.001), whereas MD measurements showed significance for 5 of 6 attachments (16, 13, 11, 23, and 26), with attachment 21 were not statistically significant ( P = 0.869). The 3D-printed group consistently demonstrated superior accuracy, with significantly lower RMS values and MD values closest to zero. The flowable group exhibited the highest deviations compared with other groups, particularly for attachments 13 and 23 in terms of RMS values ( Table II ) ( Fig 8 ).
Table II
Comparison of attachment trueness among different attachment transfer methods
| Variable | G1 | G2 | G3 | |||||
|---|---|---|---|---|---|---|---|---|
| Median | Min-Max | Median | Min-Max | Median | Min-Max | Test statistic (H) | P value | |
| 16 (MD) | 0.032 | 0.001-0.081 | 0.020 | −0.016 to 0.073 | −0.005 | −0.020 to 0.069 | 46.273 | <0.001 |
| 13 (MD) | 0.005 | −0.008 to 0.056 | 0.010 | −0.023 to 0.073 | −0.001 | −0.027 to 0.090 | 15.916 | <0.001 |
| 11 (MD) | 0.042 | 0.006-0.092 | 0.032 | −0.029 to 0.071 | 0.002 | −0.030 to 0.065 | 23.531 | <0.001 |
| 21 (MD) | 0.012 | −0.008 to 0.092 | 0.022 | −0.031 to 0.073 | 0.014 | −0.011 to 0.076 | 0.281 | 0.869 |
| 23 (MD) | 0.032 | 0.007-0.059 | 0.014 | −0.021 to 0.054 | −0.005 | −0.023 to 0.045 | 36.345 | <0.001 |
| 26 (MD) | 0.024 | 0.000-0.098 | 0.026 | −0.011 to 0.071 | 0.002 | −0.011 to 0.023 | 50.449 | <0.001 |
| 16 (RMS) | 0.114 | 0.088-0.143 | 0.096 | 0.016-0.151 | 0.066 | 0.047-0.155 | 57.047 | <0.001 |
| 13 (RMS) | 0.086 | 0.071-0.154 | 0.109 | 0.011-0.153 | 0.055 | 0.026-0.109 | 48.043 | <0.001 |
| 11 (RMS) | 0.116 | 0.086-0.148 | 0.114 | 0.021-0.142 | 0.087 | 0.050-0.134 | 25.129 | <0.001 |
| 21 (RMS) | 0.104 | 0.052-0.160 | 0.119 | 0.024-0.153 | 0.066 | 0.035-0.137 | 21.386 | <0.001 |
| 23 (RMS) | 0.110 | 0.093-0.138 | 0.115 | 0.026-0.137 | 0.066 | 0.031-0.108 | 44.679 | <0.001 |
| 26 (RMS) | 0.103 | 0.062-0.155 | 0.110 | 0.075-0.139 | 0.074 | 0.045-0.100 | 62.155 | <0.001 |
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