Since their introduction in 1998 by Align Technology, digitally created orthodontic aligners have seen remarkable advancements. Initially developed using thermoformed materials, these aligners were fabricated from staged orthodontic models designed with specialised software. This innovation provided patients with a discreet and removable alternative to traditional braces, marking a pivotal moment in orthodontic care.

By 2010, the growing accessibility of 3D printers and digital orthodontic software allowed dental practices to produce ‘in-house aligners’. This process involved 3D printing patient-specific resin models and using thermoforming materials such as polyethylene terephthalate glycol (PETG) and polyurethane to create orthodontic aligners. These developments gave dental practitioners greater control over production, potentially reducing costs and turnaround times. Consequently, ‘in-house’ aligners gained traction as an alternative to fixed appliances. However, their complex and time-intensive fabrication processes limited widespread adoption in busy orthodontic clinics. ^,

In 2021, another breakthrough in aligner therapy emerged when the Korean company Graphy introduced direct-printed aligners. Unlike thermoformed aligners, these were fabricated using Tera Harz TC 85, a printable photopolymer material that eliminated the need for physical models. Since then, they have introduced Tera Harz TA 28 and Tera Harz TR 07 which possesses different physical characteristics and is deemed an improvement over the Tera Harz 85. Recently, the LuxCreo company has also entered the market of direct-printed aligners. By streamlining production, direct printing reduced complexity while allowing for more consistent pressure application, potentially improving treatment outcomes. ^,

The introduction of shape memory polymers has revolutionised aligner materials. These advanced polymers offer two key advantages: reduced resin waste, making the process more environmentally sustainable, and the unique property of shape recovery. This means the material can undergo significant deformation and return to its original shape under specific conditions.

Although shape memory-like polymers lack the super-elastic properties of nickel-titanium alloy wires used in fixed orthodontics, they represent a substantial improvement over traditional thermoformed aligners. Their ability to deform over fairly large increments and return to their original shape while delivering light, continuous forces enhance the predictability of tooth movement. This consistency could broaden the scope of aligner-based treatments to address more complex cases.

The precision of 3D printing, coupled with the innovative force expression of these materials, positions shape memory polymers as a promising frontier in aligner technology. As advancements continue to evolve, they may redefine the possibilities of aligner-based orthodontic care.

Shape memory polymers (SMPs), utilised in the direct printing of aligners, are composed of a photopolymerisable matrix as their core. Methacrylate monomers are added to achieve the desired viscosity, while crosslinkers such as isocyanates are incorporated to preserve the shape memory function. These chemical combinations must be carefully formulated during the resin fabrication process to ensure a consistent and accurate polymerisation outcome.

The unique shape memory property of these resins enables them to undergo significant deformation and subsequently return to their original shape under specific conditions. In orthodontic aligner therapy, this transformation is primarily triggered by temperature differentials, in contrast to the physical deformation mechanisms of superelastic nickel-titanium (NiTi) wires.

Thermoformed aligners rely on linear relaxation of the stress induced by deformation and are limited in their range of activation. The range of activation typically used by conventional software is around 0.2 mm for linear movement and 2 degrees for angular and rotational activations. 3D-printed aligners have the potential to be deformed over longer spans and still deliver a light and continuous force ( Fig. 42.1 ).

Linear relaxation of different orthodontic materials.

Source: Reproduced from Bichu YM, Alwafi A, Liu X, Andrews J, Ludwig B, Bichu AY, et al. Advances in orthodontic clear aligner materials. Bioact Mater 2022;22:384–403. doi: 10.1016/j.bioactmat.2022.10.006. PMID: 36311049.

Due to their inherent rigidity and low elastic limit, thermoformed aligners cannot engage fully in dental undercuts and require block outs on the printed models to allow for intra-oral insertion. Due to their better plasticity, 3D-printed aligners have the capacity to engage undercuts and irregular geometry of the dentition allowing for larger areas and engagement and potentially reducing the number of attachments necessary to perform tooth movement ( Figs 42.2 and 42.3 ).

Check STL files for any undercuts that may interfere with the proper seating of the aligner.

STL file obtained from an intra-oral scanner showing the deep interproximal spaces that will need to be blocked before thermoforming an aligner.

Block undercuts on STL files.

Undercuts have been reduced, and the model can now be printed to be used for thermoforming. The undercuts will not be used for retention in this case.

Shape memory aligners present significant differences when compared to thermoformed polyurethane, a widely used aligner material known for its favourable characteristics. Table 42.1 summarises the key distinctions in terms of pseudo-elasticity, stress–strain behaviour, durability and treatment efficiency. SMPs theoretically possess more advanced properties tailored for orthodontic tooth movement, offering potential advantages in clinical performance. ^,

SMPs may represent a significant advancement over conventional polyurethane in aligner therapy. Their pseudo-superelasticity, optimised stress–strain behaviour and superior durability contribute to more predictable and efficient treatment outcomes. While polyurethane remains a popular choice due to its established characteristics, the innovative properties of SMPs suggest their potential to redefine orthodontic care ( Fig. 42.4 ).

Difference in peak force between thermoformed and direct-printed aligners highlighting the light and continuous force delivered by direct-printed aligners.

Source: Reproduced from Hertan E, McCray J, Bankhead B, Kim KB. Force profile assessment of direct-printed aligners versus thermoformed aligners and the effects of non-engaged surface patterns. Prog Orthod. 2022 Nov23(1):49. doi: 10.1186/s40510-022-00443-2. PMID: 36443390; PMCID: PMC9705625 .

The introduction of ClinCheck software by Invisalign marked a groundbreaking moment in digital orthodontics. This innovative tool allowed orthodontists to visualise and plan the step-by-step movement of teeth using aligners, enabling a more predictable and efficient treatment process. Since then, numerous companies have entered the market, offering a diverse range of software solutions to meet the rising demand for orthodontic aligners. These programs are designed to simulate and facilitate precise tooth movement, empowering clinicians to create highly personalised treatment plans.

At present, most orthodontic software is specifically designed to be compatible with thermoformed aligners, reflecting their widespread use in modern orthodontic treatment. While thermoformed aligners are effective, they have inherent limitations in force delivery and activation during treatment. ^, For instance, the mechanical properties of thermoformed materials make them heavily reliant on experience-driven manual adjustments to achieve accurate and consistent force application.

The advent of 3D-printed aligners, produced directly through additive manufacturing, has created new opportunities to enhance the efficiency and customisation of aligner software. Unlike thermoformed aligners, 3D-printed aligners offer distinct advantages, including greater design flexibility, improved material properties and the reduced need for attachments. These advancements demand specialised software capable of fully harnessing the benefits of 3D printing technology.

Adapting software originally developed for thermoformed aligners often proves insufficient, as such programs fail to account for the unique capabilities of 3D-printed aligners. To optimise treatment planning, refine force calculations, and unlock the full potential of 3D-printed aligners, purpose-built software is essential. This next generation of software can revolutionise the orthodontic workflow, enabling clinicians to deliver more efficient, accurate and customised treatments.

‘The use of polymeric aligners introduces uncertainties regarding the location, distribution, and intensity of the loading system, which cannot be resolved by the current CAD-based practice’ ( Fig. 42.5 ).

Optimised attachments from Invisalign to create a force-driven system for better force applications.

Since its inception, software for thermoformed aligners has operated on a displacement-driven model. This method involves moving a tooth or group of teeth in small increments—typically 0.2 mm in linear displacement and around 2 degrees in angulation or inclination per aligner. It assumes that the aligner material, once deformed, will fully return to its intended shape and accurately express 100% of the virtual displacement calculated by the software, thereby guiding the teeth into their planned positions ^, ( Fig. 42.6 ).

Typical software used to move teeth incrementally for aligner therapy

(DentOne, Korea).

While this model is effective for moderate or simple malocclusions, it oversimplifies the complex biomechanical processes underlying tooth movement. Several research papers report that the average rate of correction using thermoformed aligners is around 50%. ^, Current software does not adequately account for factors such as anchorage control, bone remodelling, occlusal forces or the variability in biological responses among patients. These limitations reduce its ability to reliably manage complex malocclusions or deliver the precise force systems required for optimal outcomes.

Additionally, the physical properties of thermoformed aligners impose further constraints. As vacuum-formed plastic sheets, they have limited capacity to vary material thickness or mechanical properties, which restricts their application in advanced orthodontic cases. These inherent material and software limitations underscore the need for more advanced solutions to address the demands of complex tooth movements and improve the predictability of outcomes ^, ( Fig. 42.7 ).

A ClinCheck used to correct a malocclusion with the Invisalign appliance.

Note the significant number of attachments necessary to produce the desired force system.

The introduction of 3D-printed aligners, made from SMPs, represents an opportunity to overcome some of the limitations of thermoformed aligners. SMPs allow for better adaptation to dental anatomy, improved engagement with undercuts, and the delivery of light, continuous forces during the deactivation phase of treatment ( Fig. 42.8 ).

The importance of engaging undercuts to increase the amount of contact between the aligner and the surface of the tooth.

However, to fully realise these advantages, more refined materials and specialised software must be developed to optimise the biomechanical potential of these materials. ^,

Such software would shift the focus from simple displacement-based calculations to a biomechanical force-driven model. This new approach would not only define the target position of the teeth but also calculate the optimal forces and moments required to achieve that movement. By considering anchorage requirements, staging protocols and the fundamental biomechanical principles of orthodontics, the software could deliver more balanced treatments with fewer side effects and increased predictability. ^,

To fully harness the unique properties of 3D-printed aligner materials, optimised software must incorporate specific features tailored to their capabilities. One essential component is the creation of a module capable of accurately calculating the forces and moments required for each tooth movement. This module would stage movements while considering the light, continuous forces and moments delivered by shape memory polymers, optimising bone remodelling and respecting biological limits to ensure safe and effective treatment.

A second module should integrate Newton’s Third Law of Motion, which states that for every action, there is an equal and opposite reaction. This module would calculate the distribution, magnitude and duration of forces applied to the dentition. By leveraging this principle, it would optimise anchorage, minimise unwanted side effects such as round-tripping and improve overall force efficiency during treatment.

In addition, the ability to vary aligner thickness, shape, contact surfaces and material properties opens new horizons in treatment planning and software design. Such features enable a dynamic treatment approach based on sound biomechanical principles. By applying these strategies to specific stages of treatment, clinicians could potentially reduce treatment time, expand the scope of cases to include more complex malocclusions and deliver improved clinical outcomes.

Force decoupling and targeted force application

Another significant advantage of these advancements is force decoupling , or the isolation of forces in terms of direction and magnitude. This allows for more precise combinations of forces and moments, ensuring better control during treatment. The shape memory effect of 3D-printed aligners enables targeted pressure application on specific areas of the tooth crowns, guided by software calculations designed to achieve the desired displacement.

Force decoupling is achieved by creating pressure zones within the aligner. These zones can be formed by either adding or removing aligner material in specific areas, allowing for highly controllable force application. Several tooth movements, such as rotations and large displacements like molar distalisation, can benefit from these decoupling patterns. By placing differential stiffness in the aligner, shape memory aligners can reduce undesirable forces, optimise the load on the tooth being displaced and minimise stress on the anchorage units. This results in more controlled and predictable tooth movement, ultimately enhancing treatment outcomes ^, ( Fig. 42.9 ).

Strategic applications of forces to produce a force-driven system to produce planned tooth movement.

Courtesy: Dr. Juan Francisco Gonzalez.