Morphological and Three-Dimensional Analysis for the Clinical Reproduction of Orthodontic Attachments: A Preliminary Study

Abstract

In invisible orthodontics, the role of composite attachments in facilitating complex tooth movements is crucial. This study, which evaluates the efficacy of a novel clinical attachment procedure, holds significant implications for the field. The technique used two templates (one pre-drilled and the other pre-loaded with high-viscosity composites) and was compared with the standard procedure. Fifty attachments were planned for four dental arch prototypes. Dental impressions were taken using digital scans for virtual planning and after tested techniques. The stereolithographic files (STL) obtained were aligned with those of the virtual planning, and a colorimetric map was used to evaluate the composite resin’s maximum excess and defect deviation. The enamel–resin interfaces were observed by scanning electron microscopy (SEM). The Fisher test for the distribution of detachments and morphological defects and the Mann–Whitney test for the maximum values of excess and defect were used. No significant results were found between groups for morphological defects and detachments, and the maximum values of defect and excess were reported. SEM images for the experimental technique showed integrated adhesion. This innovative procedure, which has proven reliable and operationally straightforward, holds promise, instilling confidence in its practicality and potential to advance the field of orthodontics.

1. Introduction

The increasing popularity of removable clear aligners highlights the shifting priorities in orthodontic treatment toward aesthetics, convenience, and comfort [1]. Clear aligners are used more than ever and represent the result of new materials and technologies in the orthodontic field [2,3]. Several brands of orthodontic aligners are continually being improved to achieve even more complex tooth movements [4,5,6,7]. The choice of thermoplastic materials for aligners is crucial, as these materials need to be transparent, flexible, durable, biocompatible, and effective [3,8]. Different materials, including polyethylene terephthalate glycol-modified (PET-G) and thermoplastic polyurethane (TPU), exhibit varying properties that affect aligner performance [9,10]. Despite their advantages over traditional fixed appliances [11,12,13], clear aligners face challenges in achieving precise tooth movements [14]. Initially, aligners were limited to Class I crowding cases, avoiding enamel involvement with acid etching and maintaining its structural and optical properties [15,16,17]. However, when aligners expanded to more complex tooth movements in all three planes, using auxiliary elements (attachments) of composite resin bonded to the dental enamel became necessary [4,18,19,20]. Composite attachments, typically 2–5 mm in size and over 1 mm in thickness, are mainly bonded to the labial surface of multiple teeth [21]. These attachments have a high surface-area-to-volume ratio, increasing their interaction with the oral environment [22]. Unlike traditional orthodontic brackets that are bonded using a sandwich pattern to reduce exposure to the oral cavity, aligner attachments are more exposed, increasing their reactivity with oral fluids and materials [22].

Attachments play a critical role in the efficacy of clear aligner therapy. Various factors such as position, conformation, size, and number of attachments influence the fitting of the aligner in the oral cavity [23,24,25]. A precise bonding protocol is also essential, as inaccurate adhesion can cause incorrect tooth movements that can affect the efficacy of the therapy [26]. In orthodontic bonding, adhesives interlock with the enamel and the bracket base, creating a robust bond with minimal edge exposure [27]. However, for aligner attachments, the significant thickness and frequent interaction with aligners during fitting and mastication result in larger exposed surfaces and higher susceptibility to masticatory stresses [22]. Thus, while aligners offer aesthetic and functional benefits, the attachments’ exposure and stress factors present unique challenges compared to traditional brackets.

The composite resin used is critical for effective tooth movement and treatment outcomes due to its superior aesthetics and adhesion properties [28]. In addition to preserving its characteristics inside the oral cavity during the entire orthodontic treatment, the light-cured composite resin must faithfully reproduce the virtually planned attachments by filling the reservoirs of the template [29,30,31]. Regarding the viscosity of adhesive materials, an equivalent level of accuracy has been demonstrated in the shapes and volumes of attachments using composite resins with different viscosities [32].

The clinical reproduction of the attachments on the teeth represents a critical step that must guarantee their integrity [33]. Nevertheless, clinicians carry out this protocol according to their preferences, risking not completely filling the reservoirs of the templates, with consequent defects in the shapes or volumes in the attachments, or overfilling, causing an overflow of composite resin around the auxiliary elements [33]. Therefore, this research aims to create an innovative operative procedure to optimize the shapes of the attachments during their clinical positioning. This protocol uses two types of templates, each with different structural characteristics and functions. The evaluation also includes the hypothesis that this new procedure may limit the risk of creating excesses or defects in shape, causing the failure of specific dental movements or the adaptation of aligners.

2. Materials and Methods

2.1. Preparation of Dental Models

The present study’s sample size was determined to ensure 80% power and a significance level of 0.05. The eta-squared effect size was 0.42, resulting in 46 attachments.

Four dental arch models were made using extracted teeth. The dental elements were chosen based on the inclusion criteria (extraction for orthodontic or periodontal reasons and healthy dental crowns). Teeth with carious lesions, prosthetic crowns, amalgam, or composite restorations were excluded. Once extracted, all the teeth were preserved in a physiological solution and subsequently divided according to their morphological anatomy to create models of upper and/or lower dental arches with the extra-hard plaster base. All dental models were randomly divided into two groups (n = 2 experimental, and n = 2 control) to compare the different operative procedures for reproducing the attachments (

Table 1. Subdivision and description of the groups.

Groups	Dental Model	Number of Teeth	Number of Attachments to Place
Experimental	1	14	14
	2	12	12
Control	3	12	12
	4	12	12

). The prototypes were acquired using an intraoral scanner (iTero Element Flex, Align Technology, San Jose, CA, USA). Then, the impressions were sent to the aligner manufacturer (Lineo, Micerium Lab, Avegno, Genova, Italy) to fabricate the templates.

The experimental method included two types of templates. The first template was manufactured with holes corresponding to the shape and location of the attachments to be created. It was employed in enamel etching, namely in the technique known as “selective” etching. The second template included attachment reservoirs filled with a high-viscosity composite resin (ENAMEL with Hri Enamel, GDF GmbH, Germany) that was not polymerized. Filling of the reservoirs was carried out by the aligner manufacturer, after which this template was placed in suitable packaging away from ambient light and heat to be shipped for the in vitro experiment (Figure 1).

2.2. Operating Procedures in Attachment Reproduction Techniques

The attachment reproduction technique involves the same procedural steps (pre-treatment of the enamel, bonding, and realization of the attachments) in both groups. In detail, the following steps were performed:

• Pre-treatment of the enamel.
  • Experimental procedure: the pre-drilled template was positioned on the prototype’s teeth, and a 37% orthophosphoric acid gel (ENA Etch, Micerium S.p.A., Avegno, Genova) was applied for 30 s in correspondence with the dental enamel exposed by the template holes. After removing the template, the teeth were rinsed with plenty of water and dried with air.
  • Control: a 37% orthophosphoric acid gel (ENA Etch, Micerium S.p.A., Avegno, Genova) was applied for 30s on the buccal surface of the teeth. Subsequently, the teeth were rinsed with plenty of water and dried with air.
• Bonding.
  • Experimental procedure: a thin layer of bonding agent (ENA Bond, Micerium S.p.A., Avegno, Genova) was applied on all of the etched surfaces and light-cured for 20 s using a high-power lamp (Elipar DeepCure-L, 3M Oral Care, Tokyo, Japan).
  • Control: a thin layer of bonding agent (ENA Bond, Micerium S.p.A., Avegno, Genova) was applied on all of the etched surfaces and light-cured for 20 s using a high-power lamp (Elipar DeepCure-L, 3M Oral Care).
• Realization of the attachments.
  • Experimental procedure: the pre-loaded template was placed directly on the model’s teeth. Light pressure was applied around each attachment, and each template shape was light-cured according to the instructions for the composite material. No finishing and polishing were performed after the template was removed.
  • Control: before positioning the template, the reservoirs were filled with a low-viscosity flowable composite (ENAMEL plus HRi^®Flow HF, GDF GmbH, Essen, Germany). Then, the attachments were light-cured on the model’s teeth, following the composite instructions. No finishing and polishing operations were carried out after the template was removed.

2.3. Three-Dimensional Analysis

Digital scans of all prototypes were performed using a scanner (Itero Element Flex, Align Technology) to assess the accuracy of the procedures. These three-dimensional (3D) scans, in the form of STL files, were aligned with the virtual 3D attachment planning models provided by the aligner manufacturer using Cloud Compare 2.13 software. Cloud Compare is an open-source software that allows for the processing of 3D point clouds and triangular meshes to be obtained from 3D scans. The overlapping of the files was initially performed manually by selecting pairs of points, which made it possible to align the two models by selecting four pairs of equivalent points in both entities. The alignment was then refined through the Iterative Closest Point (ICP) algorithm [34]. The ICP automatically recorded the two 3D models. A colorimetric map was used to determine the deviation between the virtual and post-procedure planning files corresponding to the same prototype to obtain the excesses and defects of the composite.

2.4. Scanning Electron Microscope Analysis

After 3D analysis, all teeth with bonded attachments in each group were subjected to scanning electron microscopy (SEM). The samples were vertically sectioned at the attachment’s center using a water-cooled diamond disk to reveal the interface between the composite resin and enamel. The specimens obtained were fixed with 2% glutaraldehyde (Santa Cruz Biotechnology, Inc., Dallas, TX, USA) in a 0.1 M phosphate buffer (Sigma-Aldrich, Germany) [35]. Once fixed, they were dehydrated with ethanol and amyl acetate. The samples were dried using the critical point method (Leica Microsystems GmbH, Wetzlar, Germany) after coating the fractured surface using a Plasma Sciences CrC-100 Turbo-Pumped sputtering system. Then, the samples were observed using a Phenom G2 Pro scanning electron microscope (Phenom-World B.V., Eindhoven, The Netherlands) [36,37,38]. After examining all SEM scans, three specimens from each group were selected for a detailed description. This approach was used because the observed features were consistent across all samples within each reference group.

2.5. Statistical Analysis

Qualitative variables were reported as percentages (%) and numerical variables as means and standard deviations (SD). The Kolmogorov–Smirnov test observed a non-normal distribution, and a non-parametric approach was applied. Fisher’s exact test compared the distribution of detachments and their morphological defects after attachment reproduction in the two groups. In addition, the Mann–Whitney U test was also used to compare the maximum excess and defect values of the composite resin around the attachments. A value of p < 0.05 was considered statistically significant. Statistical analyses were performed using SPSS 25.0 (IBM SPSS Statistics, New York, NY, USA) software for the Windows system.

3. Results

Descriptive statistics showed that the experimental group had three attachments with morphological defects (11.5%), corresponding to premolars (n = 1) and molars (n = 2). In comparison, the control group showed altered morphologies in six attachments (25%) on incisors (n = 4), canines (n = 1), and molars (n = 1). The Fisher test showed no significant differences (p = 0.281) (Figure 2a). Regarding the detachment of the auxiliary elements from tooth surfaces after their placement, the experimental group showed a detached attachment (3.8%), corresponding to a molar tooth. In the control group, two detached attachments corresponding to incisors (n = 1) and canines (n = 1) were observed (8.3%). The Fisher test showed no significant differences (p = 0.602) (Figure 2b).

From the analysis of deviations, a colorimetric map was obtained showing the maximum deviations from the reference model. The composite defects (missing areas) around the attachment are represented in blue, while the excesses of composite resin are highlighted in red (Figure 3).

Table 2. Maximum values of composite excess and defect around attachments for the teeth of the dental models.

Tooth	MODEL 1		MODEL 2		MODEL 3		MODEL 4
	Max Excess [mm]	Max Defect [mm]	Max Excess [mm]	Max Defect [mm]	Max Excess [mm]	Max Defect [mm]	Max Excess [mm]	Max Defect [mm]
Tooth 17	0.349	−0.557	/ a	/ a	/ a	/ a	/ a	/ a
Tooth 16	0.214	−0.541	0.418	−0.378	0.448	−0.603	0.291	−0.647
Tooth 15	0.535	−0.742	0.428	−0.579	0.499	−0.571	0.331	−0.548
Tooth 14	0.556	−0.742	0.428	−0.579	0.499	−0.641	0.485	−0.591
Tooth 13	0.556	−0.547	0.425	−0.545	/ b	/ b	0.405	−0.514
Tooth 12	0.555	−0.546	0.360	−0.404	1.380	−0.564	0.405	−0.514
Tooth 11	0.416	−0.449	0.363	−0.491	0.674	−0.574	0.204	−0.502
Tooth 21	0.429	−0.455	0.284	−0.565	0.674	−0.605	0.277	−0.652
Tooth 22	0.508	−0.782	0.340	−0.627	0.694	−0.568	/ b	/ b
Tooth 23	0.508	−0.782	0.746	−0.932	0.694	−0.719	0.527	−0.747
Tooth 24	0.421	−0.942	0.767	−0.950	0.601	−0.719	0.399	−0.512
Tooth 25	0.476	−0.984	1.254	−1.029	0.601	−0.599	0.346	−0.548
Tooth 26	0.476	−1.102	/ b	/ b	1.380	−0.820	0.313	−0.647
Tooth 27	0.556	−1.102	/ a	/ a	/ a	/ a	/ a	/ a

^a missing tooth on the prototype. ^b tooth with attachment loss.

shows the results of overflow and composite defects around attachments for each tooth in both procedures. The Mann–Whitney U test found no statistically significant difference for the maximum values in excess (p = 0.831) and defect (p = 0.609).

The deviation results were exported and processed to obtain the mean and standard deviation values for each pair of scanned models (

Table 3. Mean values and standard deviations (SDs) of the dental prototypes’ deviations (mm).

Prototypes	Mean ± SD (mm)
Model 1	−9.91 × 10−5 ± 4.89 × 10−4
Model 2	−7.27 × 10−5 ± 5.34 × 10−4
Model 3	−1.12 × 10−4 ± 5.29 × 10−4
Model 4	3.32 × 10−5 ± 7.01 × 10−4

). Figure 4 reports the distribution of standard deviations for all dental arch models.

SEM images of the enamel–resin interface in the experimental and control groups are shown in Figure 5 and Figure 6. The experimental technique determined that there was compact adhesion between bonding materials and the enamel structure, and no gaps or air bubbles were observed (Figure 5). In contrast, the control group showed non-integrated contact between the enamel and the resin with gaps and air bubbles within the composite resin (Figure 6).

4. Discussion

It is now well established that the attachments play a fundamental role in performing specific dental movements in invisible orthodontics. One of the basic requirements is to clinically report the exact position and orientation of the attachments [25,39,40], which has been planned virtually. However, scientific research has only described this concept in descriptive studies [21]. Currently, the clinical procedure used for reproducing attachments involves using a transfer template with planned shapes, which will be filled with composite resin. After treating the vestibular surface of the teeth with adhesive techniques, the template is placed in the dental arch for attachment–tooth adhesion. Finally, each positioned auxiliary element is polymerized [36].

The bonding process requires careful attention to minimize errors. Key factors include the choice of resin-based composite [30] and the correct use of the transfer template [26]. Unfortunately, there is currently no standard clinical protocol that includes the ideal type of composite for attachments. D’Antò et al. [32] indicated that flowable, conservative, and orthodontic composites are all suitable, although orthodontic composites may have higher overflow. Barreda et al. [30] found that bulk-fill resins maintained their shape but showed surface alterations over six months, with acceptable clinical performance. Mantovani et al. [31] suggested that bulk-fill resins are better suited to aligners than flowable resins. Recently, Gazzani et al. [28] showed that the higher viscosity of conventional composites is more suitable for clinical procedures as they had the best wear performance. In addition, the recent study by Valeri et al. [41] reported that the clinician should focus more on the choice of composite resin than on the type of transfer template (rigid or soft). In fact, errors with flowable composites can be due to air bubbles during bonding, whereas viscous resins prevent material loss [41]. The absence of a standardized protocol is an important issue to address because it reflects the inherent variability among practitioners in clinical practice, adapting available attachment placement procedures according to individual experience and available resources.

To try to reduce operational errors during the attachment reproduction procedure, this research evaluates in vitro an innovative clinical experimental technique that is simple, effective, and predictable. Our study recognizes the variability among clinicians and aims to contribute to the development of a more standardized approach by rigorously testing and documenting the results of this technique under controlled conditions. The first template is exposed to etching only the tooth surface involved by the holes with the same attachment shape. Selective enamel etching could also result in more precise adhesion of the adhesive resin and composite. Subsequently, the pre-loaded template is immediately placed in the dental model, and polymerization is performed for each attachment. Comparisons between the proposed technique and the commonly used procedure reported interesting results. First, the experimental group had a lower incidence of morphological defects in attachments than the control group (11.5% vs. 25%), but the difference was not statistically significant. This suggests that both methods may have similar efficacy in preventing morphological defects. Detachment of the attachments was minimal in both groups, with only one observed in the experimental group (3.8%) and two in the control group (8.3%). Furthermore, the detachment could be attributed to the fact that there were several attachments to be placed equal to that of the teeth present for both groups; therefore, the templates were very retentive. Interestingly, the posterior teeth in the experimental group were more involved in detachment and morphological defects, unlike the control group. In parallel to this study, these techniques were evaluated using a new quality index (CorAl) that uses the differential entropy of point clouds to measure the accuracy of attachment placement [42]. The research found no significant discrepancies between the experimental and control groups regarding morphological defects or detachments, indicating comparable accuracy. The experimental technique showed good alignment in attachment placement, suggesting reliable reproduction of intended attachments. Therefore, clinicians could benefit from the ease of use of this innovative technique, which simplifies the process of bracket placement without sacrificing accuracy.

From SEM observations of the enamel–resin interfaces, the experimental technique showed a compact bonding interface without gaps compared to the standard procedure (Figure 5 and Figure 6). These results are consistent with the comparative study by Chen et al. [36], where the bonding interfaces of high-viscosity composites were compact and gap-free. In contrast, the flowable composite had small cracks. This finding suggests that the experimental technique may offer advantages regarding attachment stability and longevity, potentially reducing the risk of detachment during orthodontic treatment. In addition, the colorimetric map obtained from the deviation analysis illustrated the distribution of deviations from the reference model. The lack of significant differences in deviation parameters suggests that both techniques achieve similar levels of accuracy in composite resin placement around attachments (Figure 4).

Several limitations should be considered to improve the effectiveness of this procedure and its applicability in orthodontic practice. First, this study utilized a limited sample size, which may impact the generalizability of the findings. Future research with larger sample sizes could provide more robust evidence. Moreover, the effectiveness of the experimental procedure was evaluated in the short term, and longitudinal studies are necessary to assess long-term performance, durability of the reproduced attachments, maintenance requirements, and potential complications over extended periods of orthodontic treatment. This study used prototype dental arches with extracted teeth in an in vitro environment. The clinical applicability of the results may be influenced by factors such as patient anatomy, intraoral conditions, and operator skill, which have not been fully replicated in the experimental setting. Large-scale clinical studies are needed to validate the efficacy of the experimental procedure in real-world orthodontic practice by focusing on attachment stability and longevity, patient comfort, treatment outcomes, and overall satisfaction among patients.

To minimize morphological defects, the manufacturers must develop advanced transfer templates with enhanced precision and accuracy, focusing on innovative designs and manufacturing techniques to reduce variability in the reproduction of attachments. In addition, choosing the most suitable composite resin is also fundamental. Therefore, exploring composites’ mechanical properties, bond strength, and wear resistance is necessary to identify formulations that ensure bond stability, shape integrity, and optimal aesthetic results.

Adequate training and experimental technique proficiency are essential to maximize its effectiveness and minimize procedural errors. So, training programs and standardized protocols should be developed, such as educational workshops, hands-on training sessions, and proficiency assessments, to ensure clinicians are equipped with the necessary skills and knowledge to implement the procedure effectively. It is also appropriate to promote interdisciplinary collaboration between orthodontists, materials scientists, engineers, and dental technicians to refine the experimental technique, addressing the challenges related to attachment adhesion, morphology, and clinical reproducibility.

Finally, the lack of standardized clinical protocols presents a challenge, but this study’s controlled methodology, acknowledgment of operator variability, and consistent in vitro results offer a strong foundation for the technique’s validation. This study aims to contribute to the establishment of standardized protocols in the future, with the recognition that further research and clinical trials are necessary to fully realize this goal.

5. Conclusions

The reproduction of orthodontic attachments using two templates—one pre-drilled and the other pre-loaded with composite resin—has proven to be simple, predictable, and accurate. The pre-drilled template allows for selective etching of the dental enamel, which enhances the bonding phases of the composite already loaded inside the shapes in the second template. SEM images showed that this method results in integrated bonding between the dental tissue and the attachment’s composite.

This experimental technique improves adhesion and prevents morphological defects. Using the two templates contributes to precise composite resin placement, especially in anatomically challenging areas like the curvature of the dental arch. This precision can improve clinical outcomes and increase patient satisfaction with orthodontic treatments. Additionally, the technique offers a standardized approach to attachment reproduction, reducing operator variability, and ensuring consistent clinical outcomes. This is particularly beneficial in multi-operator settings or for less experienced clinicians.