Clinical

Practice

Clinical Limitations of Invisalign Contact Author

Xiem Phan, BSc, DDS; Paul H. Ling, DDS, MDS Ortho, MOrth, FDS, RCS

Dr. Ling
Email: paulling@rcsed.ac.uk

ABSTRACT
Adult patients seeking orthodontic treatment are increasingly motivated by esthetic considerations. The majority of these patients reject wearing labial fixed appliances andare looking instead to more esthetic treatment options, including lingual orthodontics
and Invisalign appliances. Since Align Technology introduced the Invisalign appliance in1999 in an extensive public campaign, the appliance has gained tremendous attention
from adult patients and dental professionals. The transparency of the Invisalign appliance enhances its esthetic appeal for those adult patients who are averse to wearing
conventional labial fixed orthodontic appliances. Although guidelines about the typesof malocclusions that this technique can treat exist, few clinical studies have assessed theeffectiveness of the appliance. A few recent studies have outlined some of the limitations
associated with this technique that clinicians should recognize early before choosing
treatment options.

For citation purposes, the electronic version is the definitive version of this article: www.cda-adc.ca/jcda/vol-73/issue-3/263.html

In 1945, Kesling1 introduced the tooth positioning appliance as a method of refining
the final stage of orthodontic finishingafter debanding. A positioner was a onepiece pliable rubber appliance fabricated on
the idealized wax set-ups for patients whosebasic treatment was complete. The practicaladvantage of the positioner lay in its ability
to position the teeth artistically and to retain
the alignment of the teeth achieved throughbasic treatment with conventional fixed appliances. Various minor tooth movements could
be incorporated into the positioner. Kesling
predicted that certain major tooth movements
could also be accomplished with a series of
positioners fabricated from sequential tooth
movements on the set-up as the treatment progressed. In 1971, Ponitz2 introduced a similar
appliance called the “invisible retainer” madeon a master model that prepositioned teeth
with base-plate wax. He claimed that this appliance could produce limited tooth move-

ment. Sheridan and others3 later developed atechnique involving interproximal tooth reduction and progressive alignment using clear
Essix appliances. This technique was basedon Kesling’s proposal, but almost every tooth
movement required a new model set-up andtherefore a new set of impressions at almost
every visit, making the technique excessively
time-consuming.
In 1997 with the introduction of theInvisalign appliance, available to orthodontists in 1999, Align Technology made Kesling’s
proposal much more practical. Instead of necessitating a new set-up for each new aligner,
creation of an Invisalign appliance involves
computer-aided-design and computer-aidedmanufacturing (CAD-CAM) technology,
combined with laboratory techniques, to fabricate a series of positioners (aligners) thatcan move teeth in small increments of about
0.25 to 0.3 mm.

JCDA • www.cda-adc.ca/jcda����� ��• April 2007, Vol. 73, No. 3 •263

––– Ling –––What is the InvisalignAppliance?
The Invisalign appliance involves
a series of aligners made from a transparent, thin (typically less than 1 mm)
plastic material formed with CADCAM laboratory techniques. Thesealigners are similar to the splints
that cover the clinical crowns andthe marginal gingiva (Fig. 1). Eachaligner is designed to move the teeth
Figure 2: Finishing and detailing
Figure 1: On closer inspection, some milda maximum of about 0.25 to 0.3 mm
with the Invisalign appliance varies
esthetic limitations become apparent. The
in precision. Closure of extraction
Invisalign appliance is not completely invisover a 2-week period, and is worn inspaces is often prone to excessive tipible. An inherent translucent space is visible
a specific sequence. The Invisalign apping, as seen in the lower canine andalong the incisal edges.
pliance is currently recommended for
premolar. Vertical interarch settling is
adults and for adolescents with fully
also difficult to complete.
erupted permanent teeth who meetan acceptable standard of compliance.
Excellent compliance is mandatory
since the appliance has to be worn aminimum of 20 to 22 hours a day and each aligner should
Current Technique
Fixed orthodontic appliances have been the backbone be worn 400 hours to be effective.
of orthodontic biomechanical technique. However, thereluctance to wear buccal braces because of their poor
esthetic has been a driving force for the development ofalternative treatment options for the adult population.
Some current treatment options include Essix retainers,
Trutain retainers, lingual orthodontics and Invisalignappliances.
Because of their removable nature, Essix retainers
and Trutain retainers are indicated for mild nonskeletal
malocclusions. Essix appliances have conventionally beenused as anterior retainers from cuspid to cuspid. They
are fabricated from vacuformed plastic sheets, and havea physical memory and flexibility that allows them tosnap onto the anterior teeth, extending into gingival
undercuts. With minor modification, Essix appliances
can achieve small tooth movements, and serve as temporary bridges and bite planes.
The Invisalign appliance alone is also generally indicated for mild nonskeletal malocclusions. It was successfully used by Boyd4 in conjunction with segmental fixed
appliances, or with full fixed appliances used immediately before and after surgery for certain skeletal Class III
malocclusions.
Fixed lingual orthodontic appliances, on the other
hand, can be used for complex malocclusions. Lingual
orthodontics uses the same concept as conventional fixedbraces, but with bracket placements on the lingual rather
than the buccal surfaces of teeth. This approach improves
the esthetic look of the appliance, but has been slow togain popularity in North America because of insufficienttraining and knowledge of the technique.
264

Indications for the Invisalign Appliance
Joffe5 suggested that the Invisalign appliance is mostsuccessful for treating mildly malaligned malocclusions(1 to 5 mm of crowding or spacing), deep overbite problems (e.g., Class II division 2 malocclusions) when the
overbite can be reduced by intrusion or advancement ofincisors, nonskeletally constricted arches that can be expanded with limited tipping of the teeth, and mild relapse
after fixed-appliance therapy.
Conditions that can be difficult to treat with an
Invisalign appliance or are contra-indicated altogether
include:• crowding and spacing over 5 mm
• skeletal anterior-posterior discrepancies of morethan 2 mm (as measured by discrepancies in cuspid
relationships)• centric-relation and centric-occlusion discrepancies• severely rotated teeth (more than 20 degrees)• open bites (anterior and posterior) that need to beclosed• extrusion of teeth• severely tipped teeth (more than 45 degrees)• teeth with short clinical crowns• arches with multiple missing teeth.
Use of the Invisalign appliance is relatively new fororthodontists and is still being developed. Currently, few
clinical studies and case reports have assessed the effectiveness of this technique. Although Align Technology hassuggested guidelines for its appropriate use, clinicians
have encountered numerous limitations when using theappliance.

JCDA • www.cda-adc.ca/jcda • April 2007, Vol. 73, No. 3 •––– Invisalign –––Clinician Involvement
Although diagnostic preparation for treatment with
the Invisalign appliance is similar to that for treatment
with conventional fixed orthodontic appliances, clinicians play a more limited role during treatment with theInvisalign appliance. Preparation includes initial assessment, diagnosis, treatment planning and completion of
pretreatment records (e.g., panoramic and lateral cephalometric radiographs, bite registration, photos and polyvinyl siloxane impressions), all of which must be sent to
Align Technology in California where simulated virtual
treatment is formulated by proprietary 3-dimensional
CAD-CAM technology. Clinicians then download thevirtual treatment set-up from the Internet to evaluate theproposed final positioning of the teeth. Clinicians can request modifications at this time, but once the aligners are
made, they cannot alter the appliance during the treatment. As a consequence, clinicians must prospectively
formulate a precise treatment plan. If the results are unsatisfactory, clinicians may use auxiliary appliances (e.g.,
fixed braces) or contact Align Technology for adjustment
and fabrication of new aligners.
Compliance
Since the Invisalign appliance is removable, patient
motivation is critical to achieving the desired result. For
the appliance to be effective, patients must wear it at least
22 hours a day. They may remove it only when eating;when drinking hot beverages that may cause warpingor staining, or beverages that contain sugar; and whenbrushing and flossing. The transparency of this appliance
may increase the likelihood of its being misplaced when
it is removed. In their 1998 study comparing Essix and
Hawley retainers, Lindaurer and Shoff 6 found that one
sixth of their patients lost their appliances; the majorityof these losses were ascribable to the appliances being
clear and removable. Aligners from the Invisalign appliance have very similar properties to those of Essix
appliances.
Extraction Cases
Patients having premolar extractions may not be suitable candidates for treatment with the Invisalign appliance because the appliance cannot keep the teeth uprightduring space closure (Fig. 2). Bonded restorative attachments on the buccal surfaces can assist in limited movements, but clinical results have suggested only partial
effectiveness. 5 Bollen and others7 reported excessive tipping around premolar extraction sites. They found that
only 29% of those with 2 or more premolars extracted
were able to complete space closure with the initialaligners; none completed the overall treatment. Miller
and others, 8 in their case study of lower-incisor extrac-

tion, found similar excessive tipping around extraction
sites using panoramic radiographs.
Anterior Open Bites
Treatment of anterior open bites with the Invisalignappliance has had limited success. A few authors havereported difficulty achieving ideal occlusion during
treatment of cases of anterior open bite. After retreatment of anterior crowding and open-bite relapse with theInvisalign appliance, Womack and others9 found that theposition of the maxillary central incisors was superior tothat of the canines and posterior teeth. Although they
noted anterior extrusion, it was not enough to achieve
ideal overbite. In their 2003 randomized clinical trial,
Clements and others10 reported similar limitations; theyfound no significant improvement in anterior open bite
after treatment.
Overbite
Although Joffe5 suggested that deep overbite problems
can be corrected with the Invisalign appliance, others
have provided evidence to the contrary. Kamatovic,11 ina retrospective study, concluded that the Invisalign appliance did not correct overbite relationships. The peer
assessment rating (PAR) index was below 40%.
Occlusion
Many authors have suggested that removable appliances have limited potential to correct buccal malocclusions. The lack of interarch mechanics may explain thislimitation. In 2003, Clements and others10 demonstrated
that correcting buccal occlusions with appliances similar
to the Invisalign appliance was least successful; for somepatients, their buccal occlusions were worse after treatment. Djeu and others12 found that fixed appliances weresuperior to the Invisalign appliance for treating buccolingual crown inclinations, occlusal contacts, occlusal
relationships, and overjet. Only 20.9% of their patientstreated with the Invisalign appliance met the predetermined passing standard, compared with the 47% of those
who had fixed appliances.
In addition, Kamatovic11 found that the Invisalign appliance in general did not reduce the PAR index and concluded that the appliance did not correct buccal segment(antero-posterior and transverse) relationships. Vlaskalic
and Boyd13 also concluded that conventional fixed appliances could achieve better occlusal outcomes than theInvisalign appliance.
Posterior Dental Intrusion
Because of the thickness of the Invisalign appliance, intrusion of posterior teeth is often observed.
Compensating for such intrusion must be accomplishedin the retention period when the teeth are allowed to

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––– Ling –––erupt freely into occlusion. Womack and others9 claimed
that intrusion could occur from 0.25 mm up to 0.5 mm.
This degree of intrusion was also confirmed by Boyd andcoworkers in their 200014 and 200213 studies.
Tooth Movement
Because it is a removable appliance, the Invisalignappliance has very limited control over precise tooth
movements. Root paralleling during space closure after
extraction, tooth uprighting, significant tooth rotations
and tooth extrusion have been inconsistently successful.
Bollen and others 6 indicated that the Invisalign appliance yielded the most predictable results with tipping
movements.
Intermaxillary Appliances
The Invisalign appliance, because it is removable,
wraps around the teeth, which can inhibit the use ofinterarch mechanics (e.g., Class II and Class III elastics).
Some clinicians have suggested using elastics on buttons
bonded to the buccal surfaces as adjuncts to tooth movement, but retention of the appliance when wearing theseelastics may be compromised. 5
Treatment Time
The clinician’s treatment time can be lengthened because of the additional time required for documentation
during Invisalign case preparation. The treatment plan
must include the sequential movements for every toothfrom the beginning to the end of treatment.
If changes are needed after treatment starts, significant additional time and documentation are required
to modify the treatment plan. In addition, the lag time
between formulating a treatment plan and inserting theappliance can be up to 2 months. This lag time can cause
further delays if the dental changes are significant because of the additional time needed for planning anddocumenting the treatment again, in addition to theextra waiting period required to make new aligners. In
their 2002 case study, Womack and others9 describedsevere limitations that prevented their completion of apatient’s mandibular alignment because of the delay between planning the virtual treatment and the delivery of
the appliance.
Conclusion
The Invisalign appliance may be a treatment option
for simple malocclusions, as Joffe5 suggests, but it has
some limitations. Achieving similar results to those of
more conventional fixed appliances may be difficult. The
use of the Invisalign appliance in combination with fixed
appliances has been explored to reduce the time needed
to wear fixed appliances, but may result in considerably higher professional fees overall. Conversely, the266

Invisalign appliance can provide an excellent esthetic
during treatment, ease of use, comfort of wear, and superior oral hygiene. 5 Additional research and refinementof the design should allow further development of thisworthwhile treatment. a
THE AUTHORS
Dr. Phan is a general dental practitioner in Waterloo,
Ontario.

Dr. Ling is adjunct clinical professor, University of Western
Ontario, London, Ontario, and postgraduate examiner, Royal
College of Surgeons of Edinburgh, Edinburgh, U.K.
Correspondence to: Dr. Paul Ling, 89 Surrey St. E, Guelph, ON N1H 3P7.
The authors have no declared financial interests in any company manufacturing the types of products mentioned in this article.
This article has been peer reviewed.

References
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