In an era with attention directed toward enhancement technology, orthodontic intervention is closely linked to improving the quality of life (1). With a focus on providing a more acceptable dentofacial appearance and comfort “during the treatment”, not just considering “after the treatment” phase, esthetic-centered orthodontic appliances are being endorsed.

Clear aligners are tailored removable appliances intended for efficient comprehensive cosmetic orthodontic treatment of mild to moderate malocclusions (2). The appliance brings about incremental tooth movement by sequential sets of plastic aligner trays that are used and replaced at specific times throughout treatment. The transparent retainer worn by Zia Chisti after the completion of his conventional braces treatment was the brainchild of a global medical device company (3). In 1997, his team of Stanford University graduates launched a start-up in Silicon Valley called “Align Technology” that used Computer Aided Designing and Manufacturing (CAD/CAM). Over time, patient interest and the entry of a large number of companies have made it a thriving field in orthodontics (4).

As the primary biomechanical basis of the clear aligner force delivery relies on the shape-molding effect (5), the geometrical fit and material properties are among the major factors influencing the efficacy of the appliance. These in turn depend on the choice of the material and the method of manufacturing.

Orthodontic clear aligners can be manufactured by (i) thermoforming on the 3D printed models, or (ii) direct 3D printing of aligners. The conventional thermoforming technique is indirect in nature (6). After treatment planning, sequential dental models representing the patient's treatment progress will be printed, and thermally manipulated transparent plastic sheets will be molded against each dental model by a pressure- or vacuum-forming machine for aligner production. The retrieved aligners will be trimmed and finished for patient delivery and insertion (7) (Figure 1). On the other hand, the direct 3D printing technique designs and fabricates the aligner directly using biocompatible clear resins without requiring a dental model (8) (Figure 2).

The shortcomings related to the thermoforming technique of clear aligner manufacturing have been well documented. The process of thermoforming affects the thickness and the properties of the material. As noted from the observations of a clinical trial by Bucci et al., thermoplastic sheets of 0.75mm before processing were measured to vary between 0.38-0.69mm at different regions after processing (9). This has been further substantiated by Hahn et al., who indicated unintended local thickness variations with thermoformed aligners (10). A 10% reduction of aligner material thickness could reduce exerted forces by up to 30% theoretically (11). Thermoforming is an important way to alter material properties like transparency, surface hardness, water solubility, water absorption ability, flexural modulus, and elastic modulus of various aligner material brands and thicknesses (12–14).

Furthermore, literature evidence (11, 15) indicates that the dental model's height and thickness can affect the dimensional accuracy of the thermoformed appliance. Ihessen et al. (11) detected that aligners made from the high (i.e., 5 mm) base height of 3D-printed models were significantly thinner and more homogenous compared to those made from narrow ones (i.e., without a base). In an attempt to reduce the resin material quantity consumed, hollow models were investigated as an alternative. However, hollow models without adequate thickness (i.e., <2 mm) underwent deformation during thermoforming, thus compromising the clinical utility of the appliance (15).

Though improvements have been incorporated with newer materials and auxiliary designs in aligner orthodontics, the vast majority of manufacturing has remained to be the thermoforming technique so far (8). Leading global providers like Invisalign (Align Technology, San Jose, CA, USA), K-line (K Line Europe GmbH, Düsseldorf, Germany), and ClearCorrect (Straumann, Texas, USA) digitally plan tooth movement and print staged 3D models to make aligners using the thermoforming technique.

With the evolving digital technology and material science, the utilization of integrated systems manufacturing is widening the horizons of clear aligner manufacturing by directly printing the aligners rather than being limited to working model preparation. This article aims to concisely sum up the digital orthodontic workflow of the direct 3D-printed clear aligner manufacturing technique with literature evidence.

The sequential steps encompassing the process of direct 3D-printed clear aligner manufacturing are (i) data acquisition using scanning, (ii) virtual planning and computer-aided designing of clear aligners, (iii) 3D printing using the appropriate resin-printer combination, and (iv) post-processing (Figure 2). The description of each step is detailed as follows.

Studies involving the development and evaluation of physical, mechanical, and biological properties of direct 3D-printed aligners have been represented in Table 1.

With direct 3D-printing technology for clear aligner manufacturing, the errors related to the intermediate steps of making thermoformed aligners, such as model formation, molding, removal, and finishing, can be bypassed. Thus, it can be anticipated that aligner production will impart better precision between the initial set-up and the expected outcome. Establishing the offset for various printer and resin combinations will offer a greater advantage to control the desired and resultant thicknesses. Dimensionally controlled differential thickness can be programmed and processed according to the individual requirements of the case, resulting in better force application. The production can be with shorter supply chains, significantly shorter lead times, and lower costs. Since the thermoforming workflow generates a significant amount of non-biodegradable waste such as petroleum-based polymers used in model fabrication, the direct 3D-printing technique bestows a more environmentally friendly choice in the employment of digital technology.

The exploration of the components of system integration of direct 3D-printed aligners, such as digital scanners, virtual treatment planning modules, appliance designing software, and printing technologies, have yielded a wide range of clinically acceptable elements of an efficient standard. Currently, there is a need for more research and development in the field of material science. To come up with a cost-effective biocompatible material for intraoral contact of >20 h per day for 7–14 days (according to the protocol) that is capable of exerting optimal force levels on activation, withstand masticatory forces with adequate thickness, is esthetically nearly indistinguishable alongside teeth, and is also suitable for disinfection without any loss of material properties mandates collaborative work-up, development and testing.

The current level of research must progress with tested hypotheses to validate the evidence for the clinical application of direct 3D aligner-printing in-house technology. Further improvements and innovations will allow the delivery of easily accessible, cost-effective, digitally-driven orthodontic care in the modern era.