Aligner manufacturing as an example of digital technology in orthodontics: A review

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INTRODUCTION

In the past decade, removable thermoplastic orthodontic aligners have become a common alternative to conventional fixed appliances [1] and other orthodontic devices used for occlusion correction by generating loads that induce specific tooth movements. Traditionally, clinical practice has relied on fixed appliances that include archwires and brackets. Recently, driven by the demand for minimally invasive alternatives, biomedical researchers have focused on developing appliances that combine efficiency in tooth alignment with comfort and esthetics [2]. Within contemporary orthodontics, clinicians increasingly prefer aligners to traditional bracket systems for occlusion correction. This preference is based on the unique advantages of aligners: they are removable, which simplifies oral hygiene; they are custom-designed for each patient’s dental arch; and their inconspicuous appearance significantly enhances psychological comfort. This practice has become more widespread and has provided solutions for many patients diagnosed with various temporomandibular joint dysfunctions, attributable to the joint’s complex structure and function. Notably, approximately one third of patients with orthodontic problems require specialized care, including the use of specific orthodontic correction appliances [3]. Thus, aligners represent a highly esthetic alternative to fixed orthodontic appliances, making them increasingly sought after, particularly among adults [4].

This review presents findings from Russian and international studies on removable orthodontic appliances for the correction, prevention, and treatment of malocclusion.

This review aimed to examine the approaches and methods for modeling removable orthodontic appliances—aligners—and to analyze their manufacturing technologies.

HISTORICAL OVERVIEW

Orthodontics is now approaching its fourth development stage since emerging as a dental specialty in the early 1900s. At that time, malocclusion was treated with metal bands cemented to the teeth to secure archwires used to apply corrective forces. By the 1970s, orthodontics saw the introduction of standardized brackets made of clear or translucent nonmetallic materials, representing substantial progress in the field. A decade earlier, in the 1960s, a major breakthrough had occurred with the introduction of the first bracket systems using stainless steel to secure archwires. These systems were notable for their advantages: high strength, durability, reduced friction between components, improved saliva drainage, decreased food accumulation, and relative ease of shaping and processing [5]. The first transparent brackets made of unfilled polycarbonate were soon replaced by brackets reinforced with ceramic, fiberglass, or polycarbonate and with metal inserts to minimize creep [6]. Later, ceramic brackets (monocrystalline sapphire and polycrystalline ceramics) were introduced, but their hardness caused enamel wear. Eventually, advanced materials were introduced to replace traditional steel archwires, ranging from Optiflex, composed of clear fibers, to various coated wires—including Teflon- and epoxy-coated types, titanium-reinforced plastic, and Bioforce [7]. In recent decades, spanning the late 20th and early 21st centuries, aligners were introduced into orthodontics. This innovation is considered a pivotal milestone in modern orthodontic science, often described as the third revolution in the specialty [8].

ALIGNERS: GENERAL CHARACTERISTICS, ADVANTAGES, AND LIMITATIONS

Aligners, manufactured as transparent removable trays, are designed to correct various discrepancies in the position of individual teeth, as well as the entire dental arch and occlusion once it has become permanent. These devices are engineered to exert controlled pressure on teeth, thereby aligning them and correcting malocclusion [9].

Clear aligners are customized removable appliances intended for comprehensive cosmetic orthodontic treatment of mild-to-moderate malocclusion [10]. They achieve gradual tooth movement through sequential sets of plastic aligners, which are worn and replaced at specific intervals throughout treatment [3]. In a study by Barone et al., a patient-specific system was developed to simulate orthodontic tooth movements using plastic aligners. The maxillary and mandibular arches were reconstructed through a combination of optical and radiographic methods. A finite element (FE) model was then created to analyze two different aligner configurations. The authors evaluated the impact of uneven aligner thickness and individualized initial misfit between the aligner and the patient’s teeth [1].

In 1998, Align Technology (USA) introduced the Invisalign system, consisting of transparent orthodontic trays. Unlike conventional brackets, these devices can be removed at specific times, such as during meals or oral hygiene procedures, making them particularly attractive for esthetic reasons. The trays are placed on the teeth to achieve correction without additional attachments, requiring daily wear with short breaks [9]. Geometric adaptation and material properties are among the key factors influencing device effectiveness, which in turn depend on material selection and manufacturing method [11]. As noted by Boyd et al., since its introduction in 1999, orthodontic treatment with clear aligners manufactured from translucent thermoplastic material using digital scanning and visualization technologies has become widely accepted [12].

The primary advantages of aligners are their virtual invisibility when worn, making them highly esthetic, and their ease of removal, which simplifies oral hygiene. They are also safe for the oral mucosa and do not pose a risk of enamel damage, as no rigid metallic or ceramic elements are used; instead, soft, biocompatible attachments are applied. Rapid adaptation and absence of a foreign-body sensation in the oral cavity further distinguish aligners from other orthodontic correction methods [13]. The limitations of aligners include limited functional capacity, as they cannot correct severe malocclusion (diastemas), and their relatively high cost [3].

Despite the introduction of new materials and auxiliary components in aligner-based orthodontics, the majority of aligners are still manufactured through thermoforming [14].

3D PRINTING TECHNOLOGIES IN ALIGNER MANUFACTURING

Three-dimensional (3D) printing technologies are increasingly important for both clinical and research applications in dentistry. 3D printing has revolutionized surgical implantology, prosthodontics, restorative dentistry, orthodontics, implantology, and instrument manufacturing [15]. Unlike traditional manufacturing processes that rely on molding, machining, or other subtractive methods, this technology is unique in producing components by adding material layer by layer [16]. In orthodontic 3D printing, a variety of materials are used, including polycarbonate, titanium, steel, silver, photopolymers, wax, and polyamides such as glass-filled polyamide and nylon. Materials specific to stereolithography, such as epoxy resins, as well as other plastics like polylactic acid and acrylonitrile–butadiene–styrene (ABS) are also used [17].

This technology can be applied for the direct printing of clear aligners [18]. Despite improvements in 3D printing technology, decreasing costs, and its expanding use in other areas of dentistry, only a limited number of studies describe the direct 3D printing of clear aligners.

Several 3D printing processes can be used for the direct fabrication of clear aligners, including fused deposition modeling (FDM), selective laser sintering (SLS), selective laser melting (SLM), direct pellet extrusion (DPE), stereolithography (SLA), multijet photopolymerization (MJP), or continuous liquid interface production (CLIP) [19]. Currently, the most suitable option is 3D printing with photopolymerization of transparent resins.

Orthodontic aligners may be fabricated either through thermoforming on 3D-printed models or by direct 3D printing of aligners. Traditional thermoforming technology is inherently indirect [20]. Aligners are produced from sheets of transparent plastic that are thermally processed using pressure- or vacuum-forming equipment. Afterward, the aligners are trimmed and delivered to the patient [12]. With direct 3D printing, the aligner is digitally designed and fabricated directly from biocompatible transparent resins, eliminating the need for a dental model [14].

Studies of thermoforming in aligner fabrication have revealed specific features. During thermoforming, thickness varies depending on material properties. In a clinical trial conducted by Bucci et al., thermoplastic sheets with an initial thickness of 0.75 mm showed post-processing variation from 0.38 mm to 0.69 mm in different areas [21]. Comparable findings were reported by Hahn et al. [22]. Notably, a 10% reduction in aligner thickness may reduce elasticity by up to 30% [23].

To reduce polymer consumption, some researchers proposed hollow clear aligner models. However, when thickness decreased to <2 mm during thermoforming, deformation occurred, compromising the clinical applicability of the device [24].

The fabrication of aligners is a multistage process. Initially, the dentist takes an impression of the patient’s jaw to be treated. The impression is then subjected to 3D scanning [25]. If an intraoral scanner is used, the clinician immediately obtains a 3D model displayed on a computer. Upon completion of scanning, the dentist imports the data into specialized software, which gives the patient visualizations of pretreatment and post-treatment outcomes. The same software generates a series of 3D models of future aligners, which are subsequently printed and sent for tray production [26].

Unlike the traditional indirect method, direct 3D printing does not require fabrication of a physical dental model. Instead, the aligner is produced directly from digital 3D data stored electronically [27]. The materials used in this process differ substantially, with epoxy resins and photopolymers being the most common [28]. Multiple systems and 3D printing processes are used for this purpose, including stereolithography, fused deposition modeling, direct pellet extrusion, selective laser sintering, multijet photopolymerization, and continuous liquid interface production [27]. Direct laser processing and liquid crystal display (LCD) printing are rapid 3D printing processes that use a conventional light source to polymerize the entire resin, and these techniques have been widely adopted by 3D printer manufacturers [29].

Material selection depends on the specific category. One major category includes products composed of combinations of vinyl acetate elements and polymer compounds, making them highly popular. The most common form is flexible sheets made of vinyl acetate and ethylene polymers, with thicknesses ranging from 2 mm to 4 mm, available in both transparent and colored variants [26]. The Australian company Myofunctional Research Co. applies a unique combination of nylon and silicone to manufacture trainers, which serve as an alternative to aligners and represent one of the distinctive approaches to appliance fabrication. Polyurethane deserves special mention, ranking third among commonly used materials. As a complex compound derived from polyols and isocyanates, polyurethane is lightweight and resistant to acids and solvents. These features allow manufacturers to design specific friction parameters at the production stage, yielding products with distinctly low or, conversely, high performance characteristics [30].

With advancements in digital technologies and materials science, integrated manufacturing systems are increasingly applied to expand the possibilities of direct aligner fabrication beyond the preparation of working models.

The workflow for 3D-printed aligner fabrication includes the following sequential stages: data acquisition through scanning, virtual treatment planning and automated design of clear aligners, 3D printing with the appropriate resin–printer combination, and postprocessing.

To obtain patient data such as the condition of teeth, gingiva, and occlusion, digital scanners are used, operating on either direct or indirect principles. The direct method uses intraoral scanners to acquire data for 3D model creation, whereas the indirect method involves scanning impressions or gypsum casts with a desktop or a computed tomography–based scanner [31]. The use of direct intraoral scanners, such as class I medical electrical devices compliant with ANSI/IEC 60601-1 standards, eliminates the need for alginate or elastomeric impressions and provides greater patient comfort compared with the traditional approach [32].

Digital scanners can record, store, and transmit information, as well as perform occlusal assessment, basic measurements, and model analysis. The resulting data are exported into treatment planning software.

Software designed for automated aligner design enables segmentation of individual teeth and their gradual repositioning into the desired configuration. The output can be transferred to slicing software to initiate 3D printing of clear aligner trays.

3D printing is a method of rapidly creating physical prototypes through automated manufacturing. Rapid Prototyping technologies are divided into additive and subtractive methods. Additive manufacturing builds an object layer by layer, whereas subtractive manufacturing removes material to produce the same object [33]. 3D printers used for additive manufacturing operate along 3 axes, including 2 planar dimensions (X: right–left; Y: anterior–posterior) and the Z dimension (superior–inferior). The printing technology, thickness of each printed layer (Z-resolution), print orientation (0°, 25°, 45°, or 90°) relative to the build surface, and the total height of the object determine the time required for printing.

Among processing technologies, vat polymerization currently receives the most attention for aligner fabrication. Vat polymerization is a 3D printing method in which liquid polymer resin is poured into a vat and cured by ultraviolet (UV) light. Key printing technologies based on this method include stereolithography, digital light processing (DLP), and liquid crystal display (LCD) printing [34]. The principle of stereolithography involves using a concentrated UV laser beam to cure a small defined area at a time [33]. Using these methods and subsequent processing, various mechanical properties of aligner layers have been obtained depending on the specific printing technology applied [18].

Various thermoplastic materials or their combinations are used in aligner fabrication owing to their superior properties [35]. These include polyvinyl chloride (PVC), polyurethane (PU), polyethylene terephthalate (PET), and polyethylene terephthalate glycol (PETG) [36]. The current development of clear dental aligners is characterized by a highly technological approach. Initially, to establish an accurate treatment plan, either direct intraoral 3D scanning or scanning of a gypsum model transferred into digital format is performed. Further planning of dental arch correction is carried out using specialized software [37]. Each aligner in the treatment series requires the creation of a unique 3D model. This is achieved through technologies such as 3D printing, stereolithography, or inkjet techniques. These models then serve as the basis for forming aligners from transparent thermoplastic material. The process includes thermoforming or vacuum forming, followed by trimming to the required dimensions. The entire workflow is time-consuming, labor-intensive, and costly [16]. Additionally, the environmental impact of resins used in thermoplastic 3D printing has not been fully evaluated. Important factors to consider include energy consumption, waste generation, and environmental pollution [17]. One potential solution is the use of recycled materials in 3D printers to improve the ecological sustainability of 3D printing.

One drawback of the thermoplastic process is that the heating used to form the material around the teeth causes significant changes in its properties. Ryu et al. [38] examined changes in 4 types of thermoforming materials after processing. The study showed that thermoforming reduces transparency in thicker materials, increases water absorption and solubility, and can alter the surface hardness of certain plastics. Thermoforming reduces the thickness of aligners compared with the original dimensions of the thermoplastic foil [20]. The mechanical properties of thermoplastic materials used in clear aligner fabrication play a decisive role in achieving desired clinical outcomes in complex orthodontic movements, along with factors such as aligner thickness uniformity and geometry [40]. Clear aligners fabricated by thermoforming may vary in thickness from 0.5 mm to 1.5 mm, affecting their physical properties and clinical effectiveness in tooth movement through surface pressure. Thickness uniformity is critical to the magnitude of applied forces: variations affect accuracy and adaptation to teeth. The mechanical properties of the polymer used, the frequency of daily removal, and the degree of activation most strongly affect the forces generated [40].

Thermoplastic aligners respond to the intraoral environment during use. Body temperature, oral humidity, and salivary enzymes significantly affect aligners, altering their initial dimensions and mechanical properties [41]. After storage in artificial saliva, both elastic modulus and yield strength decreased, thereby reducing the overall mechanical properties of the polymers tested [39].

One study [42] evaluated the in vitro cytotoxicity of primary human gingival fibroblasts after 14 days of exposure to 4 thermoplastic materials: Duran (Scheu-Dental GmbH, Germany); Biolon (Dreve Dentamid GmbH, Germany); Zendura (Bay Materials LLC, USA); and SmartTrack (Align Technology, USA). Thermoformed materials exhibited higher cytotoxicity because of the release of monomers with increasing temperature during thermoplastic process. An alternative approach involves clear aligners fabricated by 3D printing with specialized resins [43].

Cai et al. noted that clear aligner therapy has its own biomechanics, which differ from those of conventional orthodontics. The orthodontic forces generated by clear aligners primarily result from elastic deformation of the appliance [44].

Gomez et al. confirmed that composite attachments help create a force system that mimics natural tooth movement. When an aligner segment shifts distally without attachments, the tooth experiences a clockwise moment and distal tipping. Composite attachments counteract tooth inclination by generating a counter-moment, thereby facilitating physiologic canine movement. The magnitude of this counteracting moment depends on the displacement applied to the aligner segment and is generated by a complex system of forces acting on the active surfaces of the attachments. Supporting this concept, zones of compression were identified on the active attachment surfaces, specifically the mesial aspect of the gingival attachment and the distal aspect of the incisal attachment [45].

3D printing enables the layer-by-layer fabrication of components, as opposed to conventional manufacturing methods based on machining, molding, and other subtractive processes [46]. Modern materials used for orthodontic 3D printing include acrylonitrile–butadiene–styrene (ABS), stereolithography-specific materials (epoxy resins), polylactic acid (PLA), polyamide (nylon), glass-filled polyamide, silver, steel, titanium, photopolymers, wax, and polycarbonate [47]. The use of directly 3D-printed clear aligners eliminates cumulative errors that may arise during analog impression-taking and subsequent thermoforming [48]. Direct printing also provides other advantages, including shorter supply chains, reduced turnaround time, and lower costs. Moreover, it is a more environmentally sustainable process, generating substantially less waste than subtractive and thermoforming techniques [49].

Several patents describe the direct 3D printing of aligners using specialized printable materials [50]. Jindal et al. reported that the transparent resin Dental LT (Formlabs, USA) demonstrated comparable performance to the thermoplastic materials Duran and Durasoft (Scheu-Dental GmbH, Germany) when subjected to nonlinear compressive forces equivalent to human occlusal cycles [51]. This suggests that aligners fabricated through direct 3D printing possess sufficient mechanical strength to withstand external loads without compromising clinical performance.

Dental LT resin is an approved class IIa biocompatible material with high resistance to fracture, making it particularly suitable for gnathological splints, dental retainers, and other rigid orthodontic devices fabricated directly for functional use [27].

3D printing also provides precise control over the thickness of dental aligners, and therefore the magnitude of applied orthodontic forces. Edelmann et al. observed increased overall aligner thickness compared with the corresponding design file, particularly when transparent Dental LT resins were used [52]. Dimensional inaccuracy in printed aligners may lead to undesirable tooth movement. McCarty et al. demonstrated that dimensional accuracy of 3D-printed aligners can be affected at the early stages of printing, for example during curing of biocompatible resins [53]. Jindal et al. further showed that postprocessing conditions strongly influence the mechanical properties of printed aligners: curing time and temperature are critical for maintaining resin resistance to compressive loads [54]. Proper postprocessing is therefore essential to ensure adequate strength and biocompatibility of the final 3D-printed product. Additional polymer cross-linking ultimately improves the mechanical performance of printed materials and reduces residual stress.

Unlike subtractive processes, additive manufacturing creates components layer by layer from liquid photopolymer resin to produce solid polymers. An intrinsic property of 3D layer-by-layer deposition is mechanical anisotropy; the mechanical properties of printed parts are influenced by environmental changes, postcuring time, and alterations within the resin vat. Shanmugasundaram et al. analyzed mechanical anisotropy in stereolithography-based additive manufacturing. They demonstrated that printed parts classified as isotropic had significant advantages, including higher accuracy and superior clinical performance [55].

Materials used for 3D printing are highly toxic before the printing process, but their toxicity gradually decreases after polymerization. Postpolymerization and postprocessing are essential to reduce toxicity levels, as recommended by 3D printing material manufacturers. Ahamed et al. conducted an in vitro cell viability study and found that Invisalign material was more biocompatible compared with materials used for direct aligner printing [56]. Although many transparent resins are marketed for 3D-printed dental appliances, none are specifically designed for direct 3D printing of clear aligners. The resin E-Ortholign has been described as biocompatible, dimensionally stable, flexible, and durable for direct 3D printing [19]. Currently, no photopolymer resin approved for commercial sale is suitable for direct printing of clear aligners, although interest in orthodontics is growing, especially with the development of certified biocompatible resins [28].

The polymeric material used for 3D printing must possess sufficient physical, mechanical, optical, and biological properties for aligner fabrication. One such material is Tera Harz TC-85, developed by Graphy Inc. (Republic of Korea). It is an aliphatic vinyl ester-polyurethane polymer functionalized with methacrylate with shape-memory properties [57]. Additionally, vinyl acetate–based materials such as Duran (Scheu-Dental GmbH, Germany) and Bioplast (Scheu-Dental GmbH, Germany) remain among the most popular choices due to their optimal balance of cost-effectiveness and functionality. Final preparation of printed aligner materials includes procedures such as cleaning of uncured residual resin, removal of supports, and postcuring.

CONCLUSION

Aligner therapy represents a modern approach to treating malocclusion, offering not only effective correction but also greater comfort for the patient. This review examined the materials used in aligner fabrication, as well as modeling methods and technologies such as 3D printing. Although aligners have certain limitations related to restricted indications and high cost, they also hold significant future potential because of their simplicity and practicality.

ADDITIONAL INFORMATION

Funding sources: No funding.

Disclosure of interests: The authors have no relationships, activities, or interests (personal, professional, or financial) with third parties (for-profit, not-for-profit, or private entities) whose interests may be affected by the content of this article. The authors also report no other relevant relationships, activities, or interests within the past three years.

Author contributions: S.A. Demyanenko: supervision, writing—original draft, writing—review & editing; A.L. Morozov, Ya.Yu. Penkova: resources, writing—original draft. All authors approved the version of the manuscript to be published and agreed to be accountable for all aspects of the work, ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

About the authors

Svetlana A. Demyanenko

Email: dc.kvalitet@gmail.com
ORCID iD: 0000-0002-2743-498X
SPIN-code: 9692-7083

MD, Dr. Sci. (Medicine), Professor

Yana Yu. Penkova

Author for correspondence.
Email: yanapenkova2003@mail.ru
ORCID iD: 0009-0007-7973-4689
Russian Federation, Simferopol

Andrey L. Morozov

Email: moyar@list.ru
ORCID iD: 0009-0007-7871-9081
SPIN-code: 2737-5787
Russian Federation, Simferopol

References

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