As the demand for orthodontic treatment among adult patients continues to rise, many adults hesitate to wear traditional metal appliances due to their unaesthetic appearance. The unappealing look of metal appliances is also a concern for younger patients, which has prompted manufacturers to prioritise the development of aesthetic alternatives. Consequently, treatment options have evolved to include more visually appealing choices such as metal-free brackets, polycarbonate and porcelain brackets, and aesthetic wires. While the genuinely invisible lingual appliance offers excellent discretion in terms of visibility, the lengthy clinical hours required to manipulate these appliances have discouraged both orthodontists and patients from making them their first choice.

Clear aligners have emerged as a most helpful innovation in orthodontic armamentarium providing an aesthetically pleasing option for teeth alignment that offers removal flexibility at the user’s discretion. Recent advancements in precise aligner therapy have significantly transformed the available armamentarium of orthodontic appliances.

These appliances are nearly transparent, colourless and virtually invisible, allowing for discreet use by patients ( Fig. 41.1 ). Their removability enables individuals to avoid wearing them during social or professional engagements. Moreover, clear aligners facilitate improved oral hygiene, and most patients adapt to their use relatively quickly. The success of this treatment modality is closely dependent on patient compliance, which necessitates wearing the aligners for a minimum of 22 h per day and adhering to the prescribed schedule for changing the aligners.

The clear aligner is a removable appliance, which is nearly transparent, colourless and almost invisible.

A significant proportion of patients seeking clear aligner treatment includes individuals who previously underwent orthodontic treatment with fixed appliances and are now experiencing relapse or dissatisfaction with their treatment outcomes. Many of these individuals prefer not to undergo fixed appliance treatment again, particularly considering the aesthetic considerations pertinent to their professional lives. Furthermore, adolescents seeking enhancement of their appearance may favour clear aligners to avoid the aesthetic drawbacks of metal or partially clear fixed appliances.

Despite their advantages, the efficacy of clear aligners has been subject to extensive debate historically. However, increased insights from clinical experts and technological advancements have enhanced the understanding and effectiveness of clear aligner therapy in recent years. The growing body of published literature and clinical evidence substantiates the efficacy of clear aligners as a viable alternative to conventional full-fixed appliances for various types of malocclusion.

Numerous modifications and case reports in the literature highlight the successful treatment of diverse malocclusions through this therapeutic approach. Additionally, clear aligners can be effectively utilised with partially or fully fixed appliances when addressing complex malocclusions.

In 1924, Orrin Remensnyder developed the ‘Flex-o-Tite’, a gum-massaging device made of rubber for treating pyorrhoea. Later, Remensnyder noticed that it also caused minor tooth movement and subsequently filed a patent for this orthodontic appliance. ^,

In 1945, Harold D. Kesling introduced the concept of a ‘tooth positioner’ designed to prevent orthodontic relapse. This device, made of vulcanite, facilitates incremental tooth movement through its sequential use. The system for aligning teeth and addressing residual movement became known as the ‘Kesling Setup’. ^,

The Kesling’s setup procedure for making a tooth positioner begins with taking impressions when the orthodontic treatment is nearing completion. Each tooth, typically from the anterior to the first molars, is separated using a fine plaster cutting saw on the plaster models. The appliance is then denuded, and the teeth are arranged in an ideal occlusion, referred to as the Kesling ‘Finished Setup’. A rubber positioner is crafted in the lab during the final setup of the plaster models. When worn by the patient, this positioner guides the teeth into predefined positions according to the setup. It serves as a retainer to address minor relapses following orthodontic treatment ( Fig. 41.2 ).

Kesling’s setup.

(A) Initial study models; (B) stripping the tooth stumps with a steel bur, taking care to maintain the mesial–distal dimension of each tooth, without removing the dentogingival limit. (C) Mounting the teeth. (D) Setting the tooth stumps with heated red wax. (E) Mounting of teeth on the upper and lower left side as far as the first molars. (F) Finished setup model.

Source: Araújo TM, Fonseca LM, Caldas LD, Costa-Pinto RA. Preparation and evaluation of orthodontic set-up. Dental Press J Orthod. 2012;17(3):146–165.

Kesling’s diagnostic setup: When the Kesling’s setup is applied to pre-treatment models, it is commonly known as the ‘Kesling diagnostic setup’. This tool is invaluable in orthodontic treatment planning, as it helps visualise the final positions of teeth after orthodontic treatment in relation to the jawbone and offers insights into whether treatment should involve extraction or non-extraction. The Kesling’s diagnostic setup also illustrates the final occlusion in various scenarios involving tooth extraction, such as the extraction of lower incisors.

Nahoum, in 1964, introduced a clear thermoplastic appliance that could carry out tooth movements. Building upon this concept, Ponitz introduced the idea of an invisible retainer in 1971, which McNamara later refined in 1985. ^, Sheridan introduced the interproximal tooth reduction (IPR) technique in 1985 to address lower incisor crowding and aligning teeth using a clear labio-lingual plastic retainer known as the ‘Essix appliance’ in 1993. He utilised a 0.030-inch co-polyester thermoplastic sheet processed with the positive air pressure method, referred to as the Essix appliance. This system functions as both a retainer and a positioner. In 1997, Schwarz and Sheridan standardised, developed and patented an ‘in-office’ vacuum system to improve this technique. Sheridan’s method required the new dental study model at each stage of tooth movement to fabricate each appliance, making the process cumbersome and time-consuming for patients, orthodontists, and lab technicians.

The Essix appliance is indicated for correcting mild crowding or malocclusions. Tooth movement is achieved using spot thermo-forming techniques with specific pliers or a mounting procedure. In the latter method, the tooth surface requiring movement is modified by adding small composite layers sequentially. The appliance then pushes and trims these layers to create space for the desired tooth movement ( Fig. 41.3 ).

The Essix appliance, made from co-polyester thermoplastic sheet for minor tooth movements.

Fig. 41.4 shows a case of mild crowding treated using thermoforming aligners planned by Kesling’s setup.

A case of mild crowding treated with clear aligner system.

This case has been treated with Clear Smile system.

Source: Case courtesy Dr. Joseph Greenty Wollongong, NSW, Australia.

The Pre-finisher appliance marketed by T(ooth) P(ositioner) Orthodontics is based on the principles of diagnostic setup Essix appliance. It is a light, transparent and almost invisible removable device, snapped over teeth following orthodontic treatment to settle leftover rotations and spaces and to settle occlusion.

The contemporary methods of precise aligner fabrication were manual and involved labour-intensive processes like sequential wax setups. Therefore, these methods were deemed unsustainable for comprehensive orthodontic treatment and industrial scale aligner fabrication.

In the late 1990s, Zia Chishti and Kelsey Wirth conceived using computer-aided design and computer-aided manufacturing (CAD-CAM) technology for aligner manufacturing. Zia Chishti was an orthodontic patient who was inconsistent in wearing his clear retainers, which led to relapse. Resolving these issues ignited the idea of developing computer-aided technology for precise aligner fabrication as a means of orthodontic treatment to avoid braces. In 1997, they founded Align Technology with two orthodontists to provide clear, aesthetic solutions to align teeth.

In 1999, the Invisalign system was first established, which consisted of a series of clear thermoplastic aligners capable of carrying out 0.25–0.30 mm of tooth movement per aligner. Invisalign became widely popular among orthodontists due to its advanced treatment planning software (ClinCheck), patented SmartTrack material and treatment outcomes. Today, ‘Invisalign’ has become a synonym for aligners.

Aligners can be categorised into four basic categories: positioners and guides, including Kesling’s setup; activation aligners by thermoformed appliances and removable appliances; aligners with teeth manually set on dental models and aligners with teeth digitally set.

The initial versions of clear aligners were developed from polymer mixtures without using any auxiliaries to facilitate tooth movement, resulting in limited efficacy. Align Technology introduced composite attachments and single-layer polyurethane materials in the second generation of their aligners, Invisalign G2. These improvements enhanced the predictability of tooth movement, elasticity and adaptability. Clear aligners also began to be utilised in skeletal class III cases requiring single or bi-jaw surgery, expanding their clinical applications.

The third generation of Invisalign G3 brought the SmartForce system, which included optimised attachments, pressure area indentations and power ridges. These features allowed for more precise application of orthodontic forces, such as moments of couple and moment of force, making aligners more effective in handling complex cases. The introduction of multi-layered aromatic polyurethane and co-polyester materials in Invisalign G4 marked another leap forward, providing consistent low-grade force application and achieving better outcomes in cases like open bite and deep bite. Subsequent innovations in the fifth and sixth generations of Invisalign G5 and G6 introduced features such as precision bite control and bite ramps. These advancements enhanced the ability to treat cases requiring intrusion, root parallelism and canine retraction, making aligners a reliable option for managing bimaxillary protrusion cases that involved extractions and root control during retraction.

With time, aligners were also designed for adolescent patients to improve outcomes in deep bite cases by intruding lower anterior teeth and levelling the curve of Spee through aligner activation. Further innovations, such as aligners for patients in mixed dentition, the Invisalign mandibular appliance and the Invisalign palatal expander, extended the scope of aligner therapy. Additionally, the integration of cone beam computed tomography (CBCT) in treatment planning has enhanced the precision and efficiency of aligner-based treatments. Aligners continue to evolve with advancements in materials, software and treatment protocols. Each new generation introduces refinements that improve versatility and effectiveness, solidifying aligners as practical and efficient tools in modern orthodontic care ( Fig. 41.5 ).

The evolution of the Invisalign system.

Source: Courtesy Align Technology. https://www.dentaltribune.com/up/dt/2021/10/OTMEA_0521_FINAL.pdf .

According to a systematic search for brands of aligners, more than 75 companies provide the option of clear aligners worldwide. Table 41.1 tabulates the evolution of clear aligner therapy and treatment mechanics over the years.

Over the years, aligner materials have evolved significantly. Initially, rubber-based tooth positioners were used, followed by vacuum-formed appliances. Modern clear aligners have advanced from single-layered plastics to second-generation polyurethane materials and now to multi-layered materials. These multi-layered aligners combine a soft layer, which provides elastic deformation for smooth seating, with a hard layer, which ensures strength and durability. Aligner materials can be broadly categorised into thermoplastic materials formed through vacuum moulding and materials produced directly via 3D printing. Thermoplastic polymers are further classified into two types: amorphous polymers, which are transparent, soft and impact-resistant, and semi-crystalline polymers, which are hard, translucent and chemically resistant.

Polyesters such as polyethylene terephthalate (PET) and polyethylene terephthalate glycol (PET-G) are commonly used due to their excellent mechanical strength and optical clarity. Thermoplastic polyurethane (TPU), primarily composed of di- and tri-isocyanates and polyols, offers high mechanical strength, elasticity, chemical resistance, abrasion resistance and ease of processing. These characteristics make TPU another widely used material in aligner fabrication. Invisalign has upgraded its materials from a single-layer polyurethane (Exceed-30) to multi-layer aromatic thermoplastic polyurethane/co-polyester SmartTrack. This enhances the elasticity of the aligner, applies consistent forces and improves clinical efficiency. Polymer blends such as polyester, polyurethane and polypropylene are often used to enhance mechanical properties, and provides an optimal balance of strength, flexibility and durability.

Orthodontic treatment in the present era has changed and can be broadly divided into the following categories:

Overview of steps in clear aligner treatment ( Fig. 41.6 ):

• 1.

  Case records and case selection

• 2.

  High-quality impressions or intra-oral scans

• 3.

  3D virtual setup and treatment progress stages

• 4.

  Approval of treatment steps on the web

• 5.

  Manufacturing of aligners’ and delivery to treating doctor

• 6.

  Issue of aligners and review

• 7.

  Finishing and retention

• 8.

  Follow-up

Workflow of fabrication of clear aligners by thermoforming process.

The evaluation and diagnosis of an orthodontic case require a comprehensive set of essential pre-treatment records. This includes study models, clinical photographs, orthopantomograms (OPGs), cephalometric radiographs, and any other relevant X-rays necessary for diagnosing malocclusion and planning treatment for the specific case.

A tentative plan needs to be formulated and discussed with the patient. Clear aligner treatment (CAT) requires high-quality impressions or intra-oral scans. High-precision dental impressions must be made of polyvinyl siloxane (PVS) material. The occlusion bite is also recorded on a wax sheet. The impressions should extend beyond the last tooth, cover a fair extent of soft tissues and be without voids or defects. The physical impressions are converted into digital format through 3D scanning and must be sent to the selected vendor.

Highly sophisticated hardware and software functions allow the vendor to scan these impressions, create virtual images of each of the teeth and create study models for visualisation, such as 3D records and a virtual setup. The higher functions of the software allow data segmentation and create a virtual setup similar to the Kesling’s set in many permutations and combinations to arrive at a final occlusion.

Alternatively, records are obtained through intra-oral scanners; the virtual records are shared to the vendor through the cloud or the web. The accuracy of virtual setup and aligners depends on the quality of the dental impressions or the quality of scans obtained through an intra-oral scanning process.

The experts use sophisticated software functions to calculate arch length tooth size discrepancy, including possible alignment of the teeth, various extraction plans and recommendations sent by the treating orthodontists and the patient’s needs. Once the treatment plan and outcome are prepared, the treating orthodontist is requested to access the plan and consider for approval or modifications, if any ( Fig. 41.7 ). After approval, the treatment steps are created, symbolising the stages of progression of orthodontic tooth movement and corresponding to the number of aligners.

ClinCheck: The interactive treatment planning system by Invisalign.

Source: Available from: http://www.clinicalandete.com/fotos/clincheck_invisalign_clinica_dental_landete_mostoles.jpg .

These aligners are manufactured, numbered, packed and shipped to the treating doctor.

The aligners are issued to the patient with specific instructions and plans, and a follow-up protocol is observed. The treatment response is evaluated from time to time, and in case this does not follow the expected outcome, there is a possibility of an alternate redo at least once. On achieving the desired occlusion after the active phase, a retention protocol is initiated.

With the newer improvements in the attachments and integration with the miniscrew implant anchorage savers system, there are almost no contraindications on the type of malocclusion ‘per se’ in clear aligner therapy. However, certain aspects are more challenging to handle, and it is important to anticipate the dental movements and possible problems that may need to be encountered.

Anatomy of the crown: The area of contact between the aligner and the crown will influence the success of the aligner and, thus, the predictability of the movements. For example, it is not uncommon to find partially erupted clinical crowns in adolescent patients. Thus, the placement of attachments is essential to improve the retention of the aligner. On the other hand, in adult patients who may present with gingival recession problems, it is recommended to cut the aligners more occlusal and limit the number of attachments to facilitate insertion and removal of the aligners.

In some cases, it is recommended to utilise elastics and buttons and to include a precision cut in the prescription of the particular case. Regarding anteroposterior correction, the most predictable results are obtained in 2–4 mm class II and with sufficient clinical crowns. In cases of more than 4 mm class II, a combination of treatment with distaliser appliances is recommended. Subjects with spaced dentition respond quickly and are treated easily. However, some more complex situations, like class II division 1, would require attachments.

In using the aligners, the learning curve can be long, and it is advisable to start with cases exhibiting spacing and minor crowding.

• 1.

  Clear aligners are removable appliances, making them easy for patients to wear and maintain, thus enhancing compliance.

• 2.

  They are generally comfortable and well-accepted by patients due to their minimal impact on daily activities and discreet appearance.

• 3.

  The risk of enamel demineralisation and white spot lesions is significantly reduced compared to fixed appliances, as aligners allow for better oral hygiene maintenance during treatment.

• 4.

  Aligners minimise the risk of trauma to soft tissues, as they do not have sharp edges or brackets that could irritate the cheeks or lips.

• 5.

  They are particularly advantageous for patients with a history of bruxism, as they provide a protective barrier for the teeth during treatment.

• 1.

  The high cost of CAT is largely attributed to significant laboratory charges involved in aligner fabrication.

• 2.

  Traditional fabrication methods involve multiple dental models for every treatment stage, leading to increased material and resource wastage. However, the adoption of direct 3D-printed aligners is expected to address this issue by reducing waste.

• 3.

  Successful treatment outcomes heavily rely on patient compliance, as aligners must be worn daily for the prescribed duration.

• 4.

  Extended treatment duration may result from low predictability in certain cases, necessitating refinements or midcourse corrections to achieve desired outcomes.

• 5.

  Aligners show limited efficacy in cases requiring severe rotation corrections, often yielding suboptimal results in such scenarios.

• 6.

  The aligners may stain over time due to poor maintenance and lose their aesthetic value.

Comprehending the biomechanics of aligner tooth movement is essential for precise treatment planning, appropriate patient selection and attaining predictable, realistic treatment outcomes.

Orthodontic treatment with aligners involves gradually moving the teeth with a push or twin force system through custom-made successive aligners. Each aligner in the sequence is designed to incrementally reposition the teeth in small amounts, facilitating the overall movement of the teeth throughout treatment. The fundamental difference between conventional braces and aligners is that aligners work on the push principle while orthodontic appliances work on the pull principle. So, the push force generated by plastic determines the difficulty of each tooth’s movement.

Tooth movement with aligners can be carried out primarily by two mechanisms:

• 1.

  Shape moulding effect: It refers to the capability of aligners to gradually modify the shape and alignment of targeted teeth according to the shape of aligners. This is achieved by positioning the targeted teeth as desired and creating aligners in that same configuration. This process induces deflection and activation of the aligners in the patient’s mouth, leading to incremental tooth movement. The shape-moulding effect drives aligner-based tooth adjustments, facilitating effective force transmission across a larger tooth surface area.

• 2.

  Attachments: Attachments enhance the predictability of tooth movement by exerting forces at specific areas of the tooth. It produces forces by passively ‘getting in the way’ of aligner plastic, causing elastic deformation arising from a mismatch between tooth position and aligner material, establishing a force vector that subsequently affects the tooth. The stress generated by attachments is lower compared to the shape moulding effect. So, attachments do not carry out tooth movement by themselves but facilitate it.

Attachments are designed in various morphologies, sizes and orientations to assist different types of movements or to conform to the morphology of dental crowns. The initial shapes of attachments were ellipsoid and rectangular, with vertical and horizontal orientations. The optimal attachment design is used for specific tooth movements, such as rotation corrections, root movements and multiplanar movements ( Fig. 41.8 ).

Conventional and optimised attachments for different types of tooth movements.

Parts of the attachment include an active surface, a passive surface and the base of the attachment. An active or functional surface generates the force vector by contacting the plastic of the aligner. The force vector is perpendicular to the active surface. The passive surface is the remaining portion of the buccal surface of the attachment, which provides stability to the attachment and facilitates the wearing of the aligners. The base of the attachment is the portion that adheres to the clinical crown of the tooth. ^,

The bevel of the attachment is the oblique cut of the attachment, which makes it a smooth inclined plane and assists in seating the aligners. It was designed to overcome the fitting issues with conventional rectangular attachments. The tooth movement is opposite to the active surface of the bevel. Occlusal bevel results in intrusion of the tooth, and gingival bevel results in extrusion of the tooth.

The optimal attachment design can be determined by:

• 1.

  Orientation of active surface: It determines the force direction.

• 2.

  Location: As the distance between C _Res and attachment (line of action) increases, tipping movement increases, and rotational movements reduce.

• 3.

  Size: Large attachment offers increased retention and better force delivery due to increased surface area.

So, attachments provide aligner retention, avoid slippage of aligners and deliver predetermined force vectors.

The attachments are made of composite resin housed in the template aligner. The polymerisation of composite attachments during bonding is influenced by the mode and duration of polymerisation.

• •

  First order control

• •

  Second order control

• •

  Third order control

• •

  Vertical control