Clear Aligner Treatment

Terminology

• Aligner:

A transparent custom-made plastic orthodontic appliance that is molded to fit over the teeth to move them to the desired position and to correct their alignment; in a sequential manner. A clear aligner has minimal effect on the skeletal tissues.

• SmartForce Features:

Patented engineered solutions which are designed to create precise biomechanical forces on selected teeth. Features fabricated to allow clear aligners to facilitate simple and complex Orthodontic force systems to the teeth. The direction, point and magnitude of the forces are optimized to achieve the desired outcome. There are two types:

1. Built into the intrinsic shape of the aligners i.e. Power Ridge features and Pressure areas/points.
2. Require predefined Composite material to be bonded on the tooth surfaces i.e. Attachments.

• Attachments:

Characteristic shapes made up of composite that is temporarily placed onto the tooth surfaces to give aligners a means to enhance engagement, tooth movement and retention.

• ClinCheck:

A short 3D animation that allows you to see your predicted treatment result and how the teeth will move at each step before beginning the treatment process. It is a virtual set-up in a software that can be viewed by an Orthodontist.

• Staging:

The sequence and the speed at which teeth are moved with aligners is known as staging.

• X-Staging pattern:

Simultaneous movement of all of the teeth within each arch. The teeth are moved together from the initial stage through the final stage. Also known as simultaneous staging.

• V Staging pattern:

Distalization of the maxillary dentition with starting with the molars, followed by the premolars and ending with the retraction of the anterior teeth is known as V staging pattern.

•  A Staging pattern:

The anterior teeth move anteriorly followed by posterior teeth moving in the same direction is known A-staging pattern.

• M staging pattern:

It is defined for bicuspid extraction treatment. In this staging pattern, tooth movement starts by first closing the extraction spaces followed by the alignment of anterior teeth and finishing with molar movement.

• Midcourse correction:

A temporary pause in the treatment and a new scan or impressions are made. New aligners are fabricated and the treatment is then continued.

• Refinement:

The final perfecting stage of aligner therapy is known as refinement; it helps to achieve the ideal optimal position of teeth with minute adjustments. New impressions and scans are done and a series of new aligners are made near the end of the treatment.

Introduction

H.D Kesling proposed clear aligners as tooth moving appliances in 1945 by manually positioning teeth on plaster model in succession of minute movements till the required alignment was achieved. Nahoum introduced a series of vacuum-formed dental contour appliances with successive incremental changes to achieve major corrections. Finally, as the technology evolved, in 1999 Align technology launched the world’s first mass-produced custom made clear aligners, with automation and software innovation. The characteristic clear aligner system is based on the three components:

• Materials
• Attachments
• Software

 

Intra-oral Scanning

 The advent of intraoral scanners has eased the process of digitization and has eliminated the need for a third party to convert an impression into a virtual 3D model. There are four types of imaging technologies currently in use:

1. Triangulation:

This method measures the distances and angles from known points (laser source and sensor) with projected laser light. It requires a thin coating of opaque powder to be applied to the target tissue i.e. CEREC (Chairside Economical Restoration of Esthetic Ceramics or CEramic REConstruction) was the first intra-oral scanner produced that works on the concept of triangulation.  The angle of reflection and the distance from the laser source to the object’s surface as light reflects off the object is determined by The CEREC system.

2. Parallel confocal imaging:

A laser light projects through a pinhole to the target. The sensor is placed at the imaging plane where it is in focus (confocal) and a small opening present in the front of the sensor blocks any light from above or below. Only the focused light reflecting off the target tissue reaches the sensor for processing. This creates thousands of tomographic slices and stitches them together to form the three-dimensional picture.

3. Accordion fringe interferometry:

Two light sources are employed to project three patterns of light, called “fringe patterns, onto the teeth and tissue. The fringe pattern distorts and takes on a new pattern based on the shape of the object. Surface data points of the fringe curvature are recorded by a high-definition video camera which is offset from the projector by approximately 30°.

4. Three-dimensional in-motion video:

Three tiny high-definition video cameras are used at the lens to capture three precise views of the target. Llight energy is converted into electrical signals by a sensor behind the camera, which allows the distances between two data points to be calculated from two perspectives to create the three-dimensional data. The data points are captured in a video sequence which are then modeled in real time. Light powdering may be required to capture surface data points.

Three-Dimensional Printing

 The three-dimensional printers build the model of the teeth in layers. Printing time depends on the height of the model and the thickness. Objects may be printed from a variety of substances, depending on the intended use of the object and the type of the printer. The different types are:

• Stereolithography (SLA):

A liquid resin is held in a build tray. It is cured layer by layer using ultraviolet laser light that “draws” a cross-section or outline of each layer. It moves in a bottom-up sequence until the model is submerged in the resin bath by the thickness of the build layer for each pass of the laser.

• Fused deposit modelling (FDM):

It lays down layers of material heated just beyond its melting point, the material immediately solidifies as each layer is applied. The material is generally held on a replaceable spool.

• Digital light processing (DLP):

It is based upon a chip technology developed by Texas Instruments where the process is similar to SLA modelling. But an entire layer is cured at once which results in faster build times and a smoother surface finish.

• PolyJet photopolymerization (PPP):

The printers are similar to inkjet printers, but they work in three dimensions rather than two.  The material is sprayed out of nozzles and cured immediately with ultraviolet light. In some models, the build layers can be quite thin which results in minimal surface stratification.

 

Materials

 The aligner materials currently used range from rigid polyurethanes (Zendura) and proprietary copolyesters (Essix) to multilayer polyurethanes and proprietary copolyesters (Smart Track). Acomparison of different materials showed that multilayer foils are better than monolayer foils through the thermoforming process. Ideal properties for the material used in clear aligner treatment are:

• Better adaptability to the dental arch.
• High formability to conform precisely to teeth and to any attachments that may exist.
• Consistency in the application of Orthodontic forces
• Transparent for esthetics.
• Stain resistant.
• Display good elasticity with low insertion force.
• Prolonged range of activation without permanent deformation.
• Prolonged period of force decay.
• .

 

Attachments

 Aligners hold teeth by wrapping around them. Attachment enhance the engagement by providing the aligners a handle to hold on to the convex surface of the tooth or a pressure point to facilitate certain types of movements.

Attachments can be used for the retention of the aligner and to enhance or facilitate specific tooth movements. It aims to provide a ledge for the aligner to grip that is perpendicular to the direction of displacement. The size should be sufficient enough provide surface area to offset the force delivered. The attachment should be placed far enough away from the gingival margin so that the aligner will not spread or stretch and slip off the attachment.

Attachments are classified into two categories:

• Conventional Attachments:

Handles/surfaces that allow the aligner to engage and hold a tooth better.

• Optimized Attachments:

Pressure points for aligners to push against the tooth to move in a specific direction. The aligners are manufactured at a more acute angle than the active surface of an attachment. Therefore, on insertion active pressure is exerted on the active surface of an optimized attachment to move tooth in a specific direction.

Type	Tooth	Movement	Feature
Conventional Attachments
Ellipsoid Attachment	-	Retention and anchorage.
Rectangular Attachment-Horizontal	-	Root control especially labial root torque and to increase retention.
Rectangular Attachment-Vertical	-	Root Control; root tipping movements.
Bevelled Rectangular Attachment-Horizontal	-	·       Gingival bevel for extrusion. ·       Occlusal bevel for intrusion
Bevelled Rectangular Attachment-Vertical	-	Bevelled on the mesial or distal for rotation.
Optimized Attachments
Optimized Rotation attachment	Upper and lower canines and premolars	Rotation.
Optimized Extrusion Attachment	Upper and lower incisors, canines, premolars and molars	Extrusion.
Optimized Anterior Extrusion Attachment	Upper central and lateral incisors	Incisor extrusion.
Optimized Root Control Attachment	Upper incisors, upper and lower canines and premolars	Mesio-Distal root tip.
Optimized Multiplane Attachment for upper lateral incisor	Upper lateral incisor	Extrusion, crown tipping and rotation Or Rotation with intrusion or extrusion.
Optimized deep bite Attachment	Upper and lower premolars. For retention can be placed on canine or premolar.	Anchorage during anterior intrusion.
Optimized Multiplane Attachment	Upper and lower molars	Rotation with intrusion or extrusion.
Optimized Anchorage Attachment	Upper and lower molars and second premolars	Anterior retraction.

 

Biomechanics of Aligner Treatment

 The plastic encapsulates and surrounds the tooth and it must provide both retention and activation to move the teeth.  The natural undercuts of the teeth provide the retention and the active component to move teeth by the elastic deformation of the aligner to facilitate the tooth movement.

The elastic deformation of the aligner cannot be as great as to overcome the retention forces and certain directions allow the aligner a greater inherent ability to undergo elastic deformation.

In the facio-lingual movement, the entire body of the aligner is elastically distorted. It then returns to its original shape, carrying along the tooth with it. The entire movement is then subdivided in such a way that the aligners remain within this range of elastic deformation. To achieve the desired results and movements a sequence of aligners is made.

SmartForce Features

• Power ridges:

Power ridges are engineered corrugations placed at specific locations to enhance the undercut near the labial gingival margin of teeth for torqueing upper and lower incisor roots. They function in two ways:

1. Stiffen the gingival third of the aligner to make it more resilient.
2. Provide additional force close to the gingival margin to increase the effective moment arm of the aligner.

• Pressure areas:

Present on the lingual surface of maxillary and mandibular centrals and laterals and the lower cuspids to direct intrusive forces more closely down the long axis of the tooth when any of these teeth are intruding more than 0.5 mm.

• Precision bite ramps:

Prominences on the lingual surfaces on the upper aligners in the anterior region to assist in the correction of deep overbites.

• Precision cuts:

Cuts in the aligner to facilitate wearing of the elastics.

Reasons for midcourse correction

1. Lack of patient compliance.
2. Midtreatment restorative work that made the current set of aligners unusable as a result of changes in tooth morphologic structure.
3. Lack of one or more teeth tracking with aligners and predicted tooth movement making the fit of the aligners unacceptable.
4. A change in the treatment plan.

Ideal properties of aligners as an Orthodontic appliance

• It should not interfere with function.
• It should cause no harm or irritation to the oral tissues.
• No interfere with the maintenance of good oral hygiene.
• It should be as light and unnoticeable as possible for superior aesthetics.
• It should be sufficiently strong to withstand masticatory forces.
• Exhibit good retention.
• It should be capable to exert an appropriately controlled force in the correct direction and deliver this force for sufficiently long time between adjustment visits
• Allow control of anchorage to minimize tooth movements other than those intended.

Indications of Clear Aligner Treatment

• Mild crowding (1–5 mm).
• Treatment that can be accomplished with lateral or antero-posterior expansion.
• Cases that require minor interproximal tooth reduction.
• In severe crowding that require removal of a lower incisor.
• Spacing (1–5 mm).
• Class II division 2 type malocclusions associated with deepbite.
• Narrow arches that require expansion.
• Relapse cases after fixed orthodontic treatment.
• Minor rotations.
• Absolute intrusion of one or two teeth.
• Molars that require distal tipping.

Contra-indications of Clear Aligner Treatment

• Crowding greater than 5 mm.
• Skeletal anterior-posterior differences greater than 2 mm (discrepancies are measured in comparison to Class I canine relation).
• Discrepancies between Centric-relation and centric-occlusion.
• Severe rotation (greater than 20 degrees).
• Anterior and posterior open bites.
• Tooth extrusion.
• Correction of tipped tooth more than 45 degrees.
• Tooth with short clinical crowns.
• Multiple missingng teeth.

Advantages of Clear Aligner Treatment

• Less impact on the quality of life with superior esthetics.
• More tolerable in terms of pain compared to traditional Orthodontic braces.
• Better oral hygiene compared to conventional braces and less gingival or periodontal problems.
• Simultaneous movements in several planes of space resulting efficient treatment.
• Strength of clear aligners lie in controlling the tooth movement in vertical dimension.
• Maximum anchorage appliance.
• Segregation of individual tooth movements is possible.
• Less emergencies with clear aligner treatment
• Clear aligners can be used as fluoride application trays.
• Less chair side time.
• More convenience than fixed appliances.

Disadvantages of Clear Aligner Treatment

• Multiple movements are considered unpredictable.
• Poor at slipping anchorage and mesialisation of posterior teeth.
• Dependent upon the patients’ wear and compliance.
• The aligners are not very effective at correcting more severe orthodontic maoocclusions than traditional braces.
• The extrusion of anterior teeth, rotations of rounded teeth and anterior buccolingual inclination improvement are unpredictable with aligners.
• As Lower premolars have round morphology it might be challenging for aligners to grab and rotate.
• During space closure the aligners have a limited ability to maintain teeth upright.
• Limited root movement control.
• There is a chance of misplacing the appliance.
• The physician has no capacity to change the appliance during treatment since the aligners are manufactured by the company from treatment start to treatment conclusion.
• Clear aligner therapy is high priced compared to conventional treatment.
• In rare instances, an allergic reaction may occur.

Instructions

• Wear aligners for at least 22 hours a day.
• After every two weeks the patient should move to the next aligner.

 

References

Graber LW, Vig KW, Huang GJ, Fleming P. Orthodontics: current principles and techniques. Elsevier Health Sciences; 2022 Aug 26.

Dalaie K, Ghaffari S. Importance of attachments in treatment with clear aligners: a narrative review. Journal of Dental School, Shahid Beheshti University of Medical Sciences. 2020 Jan 1;38(1):41-7.

Graber LW, Vanarsdall RL, Vig KW, Huang GJ. Orthodontics: current principles and techniques: Elsevier Health Sciences; 2016.

Tamer İ, Öztaş E, Marşan G. Orthodontic treatment with clear aligners and the scientific reality behind their marketing: a literature review. Turkish journal of orthodontics. 2019 Dec;32(4):241.

Rossini G, Parrini S, Castroflorio T, Deregibus A, Debernardi CL. Efficacy of clear aligners in controlling orthodontic tooth movement: A systematic review. The Angle Orthodontist. 2014;85(5):881-9.

Weir T. Clear aligners in orthodontic treatment. Australian dental journal. 2017 Mar;62:58-62.

El-Bialy T, Galante D, Daher S. Orthodontic biomechanics: treatment of complex cases using clear aligner. Bentham Science Publishers; 2016 Jun 1.

Aljabaa A. Clear aligner therapy––Narrative review. Journal of International Oral Health. 2020;12(7):1-4.

Shetty S, Shaikh N, Clear aligner therepy – A review. J Dent Spec 2021;9(2):46-52

 

Written By:

Dr. Maham Munir

BDS, FCPS Part-II Training in Orthodontics

CMH Lahore Medical College