Biomechanics Of Clear Aligners: Discovering a New Orthodontic Technology

Table of Contents

1. Orthodontic Tooth Movement
2. Phases /Protocols of treatment
3. Biomechanics of clear aligners - Different types of tooth movements
4. Aligners vs. Braces Effectiveness In Treating Orthodontic Cases

Orthodontic Tooth Movement

The effectiveness of orthodontic tooth movement is directly correlated to the biology of tooth movement. Clear aligners have been developed and refined over the past few years to achieve desirable and optimal tooth movement. This article will shed light on the biomechanics of clear aligners and its effect on orthodontic tooth movement.

Push, not pull forces

A vital difference between how traditional braces move teeth and the biomechanics of clear aligners lies in the kind of force they exert on the tooth. Traditional brackets exert a pull force to shift teeth, whereas clear aligners exert a push force that gently guides teeth into the desired position. Aligners have the unique ability to simultaneously engage the various surfaces of the tooth since the tray encases the tooth's occlusal, buccal, and lingual surfaces. (7)

This allows aligners to apply compressive forces from all directions and allows attachments to have a good grip on the tooth and move it in the desired direction.

Shape Molding Effect

The shape molding effect involves molding the movement of target teeth to complement the shape of the aligner being used.

The aligner shape and dental geometry of the crowns have mismatches that are pre-established and mapped out with the help of advanced and sophisticated computer programs. These programs map out intimate areas of contact and relief points between the tray and the tooth surfaces. This generates a 3-dimensional force system that is distributed over the contact surfaces of the teeth. (1) (2).

Auxiliary Elements

Attachments can be used for retention of the aligner as well as to enhance or facilitate specific tooth movements (3).

Each attachment is custom designed for a specific movement on a specific tooth.It should be noted that in addition to the specified direction of force, the amount of force can be controlled by "preactivating" the aligner–attachment interface. (4)

Apart from attachments, Elastics and Temporary Anchorage Devices (TADs) can also be used to aid in achieving effective teeth movements.

Protocols of Orthodontic Treatment

1. Expansion

Expansion and proclination are generally the first step to gain space within the dental arch since this can be utilized to correct rotations and crowding. A study integrated 3D digital model measurements with CBCT measurements to assess the efficiency of arch expansion via aligners. It was reported that aligners could achieve arch expansion to various degrees through tipping and bodily movements up to a certain predictable amount (8).

2. Proclination

Another way to gain space in the arch is to procline retroclined teeth. The gradual segmented movement of teeth by clear aligners reduces the chances of round-tripping during treatment (9).

3. IPR

Interproximal reduction is a popular method to gain space within the dental arch. It involves the reduction of the proximal surfaces of the teeth and allows for the creation of minimal space between the teeth.

The gained space is used then to move teeth into more favorable positions. The IPR procedure is performed by passing abrasive strips or rotating discs in the interproximal region until the desired tooth reduction is achieved. (10)

4. Alignment

Correction of misaligned teeth comes in the corrective phase of orthodontic treatment and includes straightening of the teeth as well as alignment of the arches. Studies have reported that the clear aligner system effectively levels and aligns arches in mild, moderate and severe cases depending on multiple factors (11).

5. Retraction

Retraction is the most predictable way to close remaining spaces so it is generally one of the last stages of an orthodontic treatment setup. It involves space closure, reduction of the overjet, and elimination of residual spaces. This can be achieved by retracting the anterior teeth, or mesialization of posterior teeth, or a combination of both. Mesialization of posteriors is not favorable as it is less predictable.

The development of auxiliaries, along with the integration of advanced technologies such as CAD-CAM, has allowed aligners to achieve successful retraction of teeth. Retraction via aligners requires a clinician's expertise and correct patient selection (12).

Biomechanics of Clear Aligners - Different Types of Tooth Movements

1. Intrusion

A deep bite can be corrected by intrusion of anterior teeth or extrusion of posterior teeth, or both. Bite ramps are lingual prominences added to the palatal surface of the maxillary anterior region. This helps in disoccluding the posterior teeth, which subsequently encourages extrusion of posteriors or intrusion of anteriors (6).

Anterior teeth intrusion:

Anterior teeth intrusion requires careful planning since it exerts a counteractive extrusive movement on the posterior teeth. This is counteracted by the provision of the following features:

• Bite ramps that disocclude posterior teeth while also applying intrusive forces on anterior teeth, especially the lowers.
• Providing attachments for anchorage on teeth adjacent to those being intruded, helps in preventing dislodging forces on the posterior part of the aligner. It is essential to provide attachments on canines, premolars, and first molars.  

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Posterior teeth intrusion:

Movements should be planned carefully and slowly to negate any counterproductive tooth movements.

• Attachments can be added to teeth adjacent to those being intruded to enhance anchorage.
• Auxiliaries such as TADS help in posterior intrusion depending on the amount of intrusion required.

2. Extrusion

Extrusion of teeth using aligners requires the use of attachments. This gives the aligner a surface to push against.

The type of attachments used depends on the position of the tooth within the arch.

• Anterior teeth: Extrusive attachments are used
• Posterior teeth: Horizontal rectangular bevelled attachments are used

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In some cases, a button bonded to the tooth together with an elastic can assist with the extrusion (17).

3. Uprighting or Tipping

Tipping is a relatively easy tooth movement with aligners, when compared to bodily movements. Vertical rectangular attachments and root control attachments can be used to control the amount of mesial-distal tipping of a tooth efficiently.

4. Torque

Torque is essentially a difficult orthodontic tooth movement and requires overcorrection of a tooth. During bodily tooth movement, a tooth can tip buccolingually. This can be prevented by using horizontal rectangular beveled attachments.

Horizontal rectangular beveled attachments ensure that every time a tooth tips or inclines buccally, a counteractive extrusive force is exerted to correct its inclination.

Torque Enhancers

Torque enhancers are planned ridges placed near the gingival margin of teeth undergoing torque movements. The ridges act in two ways.

• They toughen the gingival third of the aligner to make it more resilient.
• They provide additional force as close to the gingival margin as possible to prevent torque loss.  

The benefit of torque enhancers is that attachments need not be placed or removed. Additionally, they are more aesthetically pleasing to the patient. (1,5)

During anterior retraction, aligners apply a force on the incisal part, whereas torque enhancers apply force on the gingival part. These forces act concurrently in the same direction, ensuring a translational movement of the tooth.

4. Rotation

• Rotational attachments can also be used to perform canine and bicuspid rotations. These attachments are of different sizes and orientation  based on the direction and magnitude of force required to derotate a tooth (17). ​​​​​​
• To derotate anterior teeth, biplane attachments and root control attachments can be used.
• Highly challenging rotations are corrected with aligners by bonding buttons to the buccal and palatal surfaces and engaging the surfaces through elastics.

5. Translation

Translational movements may require the use of attachments to prevent unsightly crown tipping.

• Vertical rectangular beveled attachments are used to counteract unwanted crown tipping.
• Root control attachments can also be used during translatory anterior movements to achieve higher aesthetic satisfaction.

Aligners vs. Braces Effectiveness In Treating Orthodontic Cases

• Materials

Fixed orthodontic brackets are made of metal or ceramic and require the bonding of the bracket with the tooth.

Aligners are created using a thermoplastic material that can be vacuumed or pressure molded to fit the patient's teeth snugly. (13).

• Speed/Treatment duration

An important aspect of clear aligner therapy includes case selection and precise treatment planning under the supervision of a specialist. Studies report that with due case selection, treatment with clear aligners is more efficient and faster than with braces. (14).

• Predictability

Treatment with clear aligners has a stepwise approach. The foremost step is to virtually map a three-dimensional model of a patient's dentition and evaluate the predictability of the treatment outcome

• Pain

Aligners are increasingly being chosen by patients worldwide due to various reasons. One of the biggest reasons is the reduced discomfort or pain experienced by patients during the treatment. Studies have reported that orthodontic patients treated with clear aligners tend to experience lower pain levels during the first few days of treatment compared to fixed appliances (16).

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