The Invisalign System

Question 1

How are impressions from Invisalign digitized?

Answer 1

CT (computed tomography) scanning is used, avoiding the need for impressions to be poured

Question 2

For Invisalign, is it better to scan directly in the patients mouth or to take an impression? Why?

Answer 2

The benefit of scanning the teeth directly in the patients mouth is the loss of slack between teh impression material and teeth.

It yields better accuracy

The manufactured thermoplastic has a better fit

Shortening of turnaround time

Information of the arches is sent without risk of distortions during shipping

Question 3

What are the downsides of scanning patient’s teeth rather than taking a PVS impression?

Answer 3

It could take longer (assuming there are no retakes of the PVS impressions.

Scanning is done in 20 minutes on average

Pts mouth gets dried out when scanning

(Another benefit of scanning is the lack of mess created.

Question 4

Why is it essential that the gingival margins of teeth are clearly obtained in the scan?

Answer 4

Trimming of the thermaformed plastic is done by robots and is guided by the level of the gingival margins

Question 5

What are the steps after the scan has been sent to Invisalign?

Answer 5

1) segmentation (virtual cutting of the teeth)
2) final setup (the desired end result of orthodontic treatment)
3) staging (how to incrementally get to the final result)
4) design and placement of attachments guided by the software that analyzes the forces transmitted to the virtual tooth roots
5) review (inspection of clincheck by the clinician
6) fabrication (production of SLA models and thermoforming the plastic to fabricate the Invisalign aligner)

Question 6

Practically, our current understanding of orthodontic biomechanics is limited to the first ___ of tooth movement. What affect the force systems which makes force analysis in orthodontics so difficult?

Answer 6

Millisecond

Movements of adjacent teeth

Nature of periodontal response to force

Precision of bracket placement

Wires

Attachments

Plastic

The way the patient handles the hardware

Question 7

What conditions in the gingiva and bone affect tooth movement?

Answer 7

Thick or thin character of collagenous structure of gingival tissue affects tooth movement

Bone too is compliant, except in cases such as idiopathic osteoscerotis, obstruction from overgrown, cystic formations, lining of the maxillary sinus

Question 8

True or false… teeth move faster when gingival tissues are inflamed and when the patient is pregnant.

Answer 8

True

Teeth move faster when the patient is pregnant due to the presence of the hormone relaxin in circulation. It relaxes collagenous tissues such as the PDL and sutures of adjoining bones

Question 9

Thermoplastic materials are viscoelastic; thus their properties are __-dependent.

Answer 9

Time

Forces generated by the aligner decrease with time. This process is accelerated in a moist and warm environment

Question 10

Does thicker aligner material yield better tooth movement?

Answer 10

Increasing the aligner material thickness does increase the stiffness. However, clinical trials provide no indication that a thicker aligner is better in finishing a case. Also, the increased thickness does not affect the quality of tooth movement during the active treatment phase.

Question 11

What are some factors that precipitate tipping when using aligners?

Answer 11

Roots are in maxillary sinus (the lining of the sinus doesn’t remodel readily)

Depth of over bite

Crown-root ratio

Gingival resistance

Sclerotic bone

Occlusal contacts

Crown morphology

Indifference of the orthodontist to carefully study the ClinCheck images

Question 12

True or false… complex attachement designs for teeth adjacent to extraction sites help reduce tipping, but, at present, sectional fixed appliances are necessary almost every time.

Answer 12

True

Question 13

What are some issues with closing premolar extraction cases with aligners?

Answer 13

After the extraction space is closed, marginal ridges are not level

first and second molar marginal ridges rarely align properly

Question 14

In premolar extraction cases, tipping of the ___ is most problematic and should not be allowed. To prevent tipping, the orthodontist may employ ___ attachments placed as ___ as possible.

Answer 14

Molars

Long and prominent molar

Mesially

Question 15

____ between the plastic and tooth surfaces is the reason for the improved control of tooth movement, better tracking, and better finishing.

Answer 15

Intimacy of forces

Question 16

For some movements, namely ___ and ___, attachments are required

Answer 16

Extrusion

Rotation

(Some movements need one attachment and some require two. There are some movements that require only a change in the shape of the aligner itself and no attachment)

Question 17

What are the drawbacks to aligner treatment?

Answer 17

1) by far, the weakest link is patient compliance
2) extrusion of teeth, especially the maxillary lateral incisor
3) inability to move the root apex, however, the addition of power ridges, optimized attachments, and Smart Track made torquing and translational movements possible)

Question 18

True or false… there are no reported studies of noticeable root resorption in patients treated with the Invisalign system

Answer 18

True.

This is probably due to less than 0.25mm of tooth movement per tray with mild forces. This distance does not obstruct the PDL blood flow, thus avoiding formation of necrotic regions

Question 19

True or false… the amount of IPR performed with Invisalign should be about the same as with fixed appliances

Answer 19

True

Exceptions aside, IPR should be limited to instances of Bolton’s discrepancy. Need to simulate physiological tooth attrition. Need to camouflage a skeletal deformity without surgery. Necessity to alter the tooth morphology

Question 20

True or false… IPR has shown to adversely affect dental and periodontal health

Answer 20

False

Initial concerns of root proximity, caries risk, and the like have not shown to be valid. Nonetheless a good depth of enamel must be present so that some of it could be removed.

Question 21

True or false… it is best to perform IPR before the intraoral scan.

Answer 21

False… it is best to perform IPR during the course of treatment, rather than before the scan/impression.

Question 22

In order to correct an anterior open-bite, the properly designed clincheck should exhibit a posterior openbite with about __-___mm of freeway space. Why?

Answer 22

This will not tangibly happen in the clinic, because the mandible will rotate forward and take up the room. Whereas clincheck images are static, the mouth is dynamic

The dynamic mindset of the orthodontist should envision that aligners might 1) block forces of the tongue on the lingual cingula of upper incisors, and 2) in the posterior segments, coverage of occlusal surfaces by the plastic trays aid in teh intrusion of posterior teeth

Question 23

what is the protocol that has been proven successful to extrude a tooth?

Answer 23

An optimized horizontal rectangular attachment with a gingival bevel (for anterior teeth)

If the clinician desires extrusion of posterior teeth, one must specifically ask for horizontal rectangular beveled attachments where the beveled surface faces the gingiva

Question 24

What are some special instructions that should be made to relate the Clincheck treatment plan for deep overbites with CAT?

Answer 24

Level curve of spee by extruding lower bicuspids for a total of 3mm and intruding the lower cuspid to cuspid a total of 4mm

Attachments on lower bicuspids: horizontal rectangular beveled gingival: 4mm wide, 1.5mm high, 1.25mm thick at the occlusal margin tapering to a thickness of 0.25mm at the gingival margin. Place as far occlusally without any interferences from the opposing arch

Question 25

What is the advantage of Invisalign over fixed appliances in the case of deepbites?

Answer 25

They can star correcting the overbite on both arches from the very beginning. Most typically with fixed appliances, upper anteriors would have to be flared out or intruded to create adequate clearance to place the lower brackets.

Question 26

What is the advantage of CAT over fixed appliances when treating cross bites?

Answer 26

The plastic covers the occlusal surfaces, so acting as a built-in bite raiser to disarticulate the occlusion during crossbite correction.