Welcome to our regular monthly feature of sharing informative tutorials on the concept of Aligners
What are Aligners? — Clear aligners are a series of custom-made tooth ‘aligners’ which gradually straighten & align teeth. Each aligner in the series is formed with small incremental movements which cause the aligner material to deflect and stretch over the teeth when they are inserted. As the plastic in the aligner attempts to spring back to its original shape, force is delivered to the teeth thereby producing tooth movement.
Why do I need to learn about Aligners? — Many dentists have a “misconception that aligners automatically treat teeth” and there is no need to understand the concept — but that is a major mistake for any dentist — Aligner treatment is a tool but has to be fully managed and controlled by a trained dentist or orthodontist who understands its proper utilization of force application, engagement and its biomechanical restrictions It is important to understand that aligner treatment is a technique, not a product.
Therefore, a clear understanding of the similarities and differences between fixed appliances and clear aligners is essential for the clinician when making a decision whether to treat a case with fixed appliances or clear aligners.
Aligner Treatment is a Technique — Not a Product
As in any tooth movement, a force is applied on the tooth to move. but the fundamental difference between the way a bracket and wire system moves teeth and the way clear aligners move teeth is that
Fixed appliances PULL teeth — When an archwire is engaged onto a lingually erupted tooth, the elasticity in the archwire causes the archwire to return to its original arch form. As the archwire returns to its original shape, it pulls on the lingually erupted tooth to move it into the arch
Application of Force in Aligners is by PUSHING the tooth
Clear aligners PUSH teeth — When an aligner is inserted over teeth, minor differences between the positions of the teeth intraorally and the positions of the teeth in the aligner. The aligner deforms over the teeth, and the elasticity in the aligner material pushes the teeth into position
Pull teeth to move
Push teeth to move
Fixed appliances engage teeth via an archwire ligated into the bracket slot. The thicker and more rigid the archwire, the better the engagement, we start off with softer round wires and slowly progress upwards to thicker wires which provide more control to induce the required tip, torque, and in-and-outs bends to finish a case
Brackets engage a portion of the tooth – While the Plastic in the aligners wrap around the whole tooth
Clear aligners engage teeth by having the plastic aligner material wrapped around teeth. The more plastic aligner material wrapped around a tooth, the better the engagement
The more thicker the archwire in each bracket slot – the better engegement
The more aligner plastic covering the tooth – the better the engagement
In fixed braces the most common anchorage model is that of reciprocal anchorage. One segment of teeth will act as an anchorage unit for another segment of teeth. But most commonly the segment with larger root surface area will resist movement while the other segment may loose anchorage
In clear aligner treatment, the anchorage segments can be individually pre-determined for each tooth. At each stage of treatment anchor units can be deliberately controlled to loose tooth position or control each tooth in each aligner tray delivered to your patient
Anchorage segments can be pre-determined and on each aligner
In fixed appliances, extrusion of a single tooth may be accomplished easily. However, because all the teeth in the arch are connected by an archwire, there are reciprocal movements of the adjacent teeth. Eventually as the treatment progresses into more rigid archwires, the occlusal plane will level out..
Extrusion of a teeth in fixed braces are controlled by rigidity of each archwire
Extrusion of a single tooth is a moderately difficult tooth movement for clear aligners, depending on the amount of extrusion required. At times, some auxiliary treatment such as buttons and elastics may have to be placed to assist with single-tooth extrusion.
Single Tooth extrusion possible – as arch wire help reinforce the dentition
Difficult to extrude a single tooth, need to place attachments on other teeth to help
In fixed edgewise appliances, dental arches are leveled through relative intrusion with reverse curves in the archwire. As the anterior teeth intrude, there is some concurrent extrusion of the posterior teeth.
In clear aligner treatment, entire segments of teeth may be intruded successfully, or selective intrusion of individual teeth to correct an occlusal cant or level out gingival margins. clear aligners offer extremely good vertical control.
Selective tooth intrusion
In fixed edgewise appliances, torque is built into the bracket slot. The amount of torque expressed is related to the size of the archwire and the amount of torque built into the bracket slot. There are varying torque prescriptions for different bracket systems. Some clinicians will use different torque prescriptions for individual patients depending on the initial malocclusion. Additional torque may be added by making torqueing bends in the archwire.
Pressure points applied on the aligners can help control Root Torque
Clear aligners offer the power ridge features which is physical deformations made to plastic trays using which can provide some degree of force to help ensure lingual root torque.
Labial / Lingual Root Torque via Bracket engagement
Lingual Root Torque via Power ridges / deformation
In fixed edgewise appliances, tip is built into the bracket slot. If further adjustment to root inclinations is required, then root-tip bends may also be made in the archwire
In clear aligner treatment, optimized root control attachments offer control of root inclinations varying degree of attachments
will offer control of root inclination
Carefully placed attachments can allow for adjusting root position in Aligners
Control through Bracket Positioning
Control through Optimized Attachments
To help summarize the above details one can take a glance at this sheet