RANDOMIZED CONTROLLED TRIAL

Effect of clear aligner attachment design

on extrusion of maxillary lateral incisors:
A multicenter, single-blind randomized
clinical trial
Justin T. Groody,a Steven J. Lindauer,b Neal D. Kravitz,c Caroline K. Carrico,d Parthasarathy Madurantakam,e
Bhavna Shroff,b Manar Darkazanli,f and William Graham Gardnera,g
Richmond and South Riding, Va, and Charlotte, NC

Introduction: Extrusion of maxillary lateral incisors during aligner treatment is a difficult movement to achieve
accurately. Despite recommendations regarding attachment design, few studies and no prospective trials
compare predictability among attachments. This study aimed to compare the efficacy between optimized and
horizontal attachment designs for achieving maxillary lateral incisor extrusion during clear aligner treatment.
Methods: The study included maxillary lateral incisors in 3 orthodontic practices requiring at least 0.3 mm of
extrusion during the first series of 20-25 aligners in patients aged $16 years who were scheduled to begin clear
aligner treatment (Invisalign; Align Technology, San Jose, Calif). Teeth were randomly assigned to receive
optimized (O), rectangular horizontal nonbeveled (H), rectangular horizontal incisally-beveled (HIB), or
rectangular horizontal gingivally-beveled (HGB) attachments. After the first series, a blinded evaluator
measured extrusion using superimpositions with initial and predicted models. Linear models determined the
difference in the predicted extrusion percentage achieved on the basis of attachment design. Other
covariates were patient age, sex, number of trays, and self-reported compliance. Results: Forty patients
(74 teeth) were enrolled, and 38 patients (71 teeth) completed the study. Intraexaminer and interexaminer
reliability for extrusion measurements was high (intraclass correlation coefficient, 0.985 and 0.991, respectively).
The achieved extrusion was significantly less than predicted (mean, 73%; P \0.0001). The average achieved
extrusion was 62%, 79%, 78%, and 78% for O, H, HIB, and HGB attachments, respectively, with H significantly
more effective than O (P 5 0.0403). Horizontal attachments (H, HIB, and HGB combined) were significantly
more effective than O attachments (P 5 0.0060), with an average difference in achieved extrusion of 14% of
the predicted amount (95% confidence interval, 4-23; estimated 76% vs 62%). Horizontal attachments were
an estimated 22% more effective than O attachments for extruding maxillary lateral incisors. Conclusions: Hor-
izontal attachments are more effective than O attachments for predicted maxillary lateral incisor extrusion be-
tween 0.3 and 2.5 mm. The 3 horizontal attachment designs evaluated performed similarly for achieving
predicted extrusion. Trial Registration: This randomized clinical trial was registered and reported at
clinicaltrials.gov (NCT04968353). Protocol: The protocol was not published before trial commencement.
Funding: This study was funded in part by the Alexander Fellowship of the Virginia Commonwealth University
School of Dentistry, the Southern Association of Orthodontists, and the Virginia Orthodontic Education and
Research Foundation. No funding source influenced the study design, the collection, analysis or interpretation
of data, writing of the report, or the decision to submit the article for publication. (Am J Orthod Dentofacial Orthop
2023;164:618-27)

a
Formerly, Department of Orthodontics, School of Dentistry, Virginia Common- All authors have completed and submitted the ICMJE Form for Disclosure of
wealth University, Richmond, Va; currently, Private practice, Charlotte, NC. Potential Conflicts of Interest, and none were reported.
b
Department of Orthodontics, School of Dentistry, Virginia Commonwealth Uni- Address correspondence to: Steven J. Lindauer, Department of Orthodontics,
versity, Richmond, Va. School of Dentistry, Virginia Commonwealth University, 520 N 12th St,
c
Private practice, South Riding, Va. Richmond, VA 23298-0566; e-mail, sjlindau@vcu.edu.
d
Department of Public Health and Policy, School of Dentistry, Virginia Common- Submitted, January 2023; revised and accepted, July 2023.
wealth University, Richmond, Va. 0889-5406
e
Department of General Practice, School of Dentistry, Virginia Commonwealth Ó 2023 by the American Association of Orthodontists. This is an open access
University, Richmond, Va. article under the CC BY-NC-ND license (http://creativecommons.org/licenses/
f
School of Dentistry, Virginia Commonwealth University, Richmond, Va. by-nc-nd/4.0/).
g
Private practice, Richmond, Va. https://doi.org/10.1016/j.ajodo.2023.07.011

618
Groody et al 619

M
any adults are pursuing orthodontic treatment to allow movement of teeth in the direction required
and seeking a more esthetic and comfortable and, thus, eliminate interferences to movement.
option than conventional fixed appliances.1 However, a recent retrospective study found no
Advancements in intraoral scanning, digital treatment significant differences in performance between
planning, and 3-dimensional (3D) technology, in gen- conventional and O attachments for achieving desired
eral, have contributed to the increased popularity of rotational or extrusive movements.14
clear aligner therapy.2 Invisalign, the first digitally- Because patients pursue aligner treatment instead of
based clear aligner system, was released .20 years ago conventional fixed appliances because of perceived
as an esthetic, no wire, no bracket, alternative to tradi- improvement in esthetics and comfort,1 many
tional fixed appliances. Software is used to formulate a practitioners may try to avoid placing large attachments
treatment plan, and computer-aided design and on maxillary anterior teeth. Visible attachments were
manufacturing stereolithographic technology to manu- shown to detract from the esthetic acceptability of clear
facture a series of clear aligners from a single digital aligners.15,16 Kravitz et al6 observed that maxillary lateral
impression.3 The clear aligners are designed to be incisors were the most common teeth requiring extru-
changed every 7 days, and Align Technology (San Jose, sion, and various experts recommend different, specific
Calif) software programs each successive aligner to attachment designs and protocols for achieving difficult
move and rotate teeth up to 0.25 mm or 2 , movements, including maxillary lateral incisor extrusion,
respectively.4 more accurately during treatment.17-20 A recent study
Since their introduction, ongoing improvements found that O attachments for extruding maxillary
have increased the complexity of conditions that can lateral incisors were the type most highly preferred by
be treated with clear aligners. Attachments were de- general dentists, whereas orthodontists preferred
signed to improve the predictability of the tooth move- gingivally-beveled horizontal (HGB) attachments.21 No
ments prescribed.4,5 Studies reported that the overall previous prospective studies have evaluated differences
predictability of specific tooth movements was 41% in in the effectiveness between O and horizontal
2009 and improved to 50% in 2020.6,7 Similarly, single attachments for extrusion during orthodontic aligner
activations of conventional fixed orthodontic appliances treatment.
rarely, if ever, result in perfect alignment. However,
although orthodontists can adjust conventional fixed Specific objectives and hypotheses
appliances to fine-tune treatment outcomes on a
visit-to-visit basis, it is not possible to make similar This randomized clinical trial aimed to determine
adjustments during aligner treatment. Aligners are whether there were differences in the efficacy of
preprogrammed and often require rescanning of the extrusion of maxillary lateral incisors between O
dentition to manufacture another series of aligners to attachments and 3 horizontal attachment designs:
finish patients properly. Studies have reported that a horizontal nonbeveled (H), horizontal incisally-beveled
majority of, or even all, aligner patients required at least (HIB), and HGB attachments. The null hypothesis was
1 series of refinement trays to achieve desired results.8-11 that there would be no significant differences in
Maxillary anterior teeth require the finest tooth posi- extrusion achieved among the attachment designs.
tioning precision because those are the most easily Secondarily, the effects of patient age, sex, number of
visible to patients and practitioners.12 To improve the trays, and self-reported compliance on extrusion
predictability of various tooth movements with aligners, accuracy were analyzed.
practitioners prescribe attachments to be bonded to
those teeth requiring more control during treatment.13
MATERIAL AND METHODS
The first attachments introduced were ellipsoid in
configuration, and they were largely replaced by more Trial design and any changes after trial
sharply defined and bulkier conventional attachments: commencement
horizontal, vertical, and beveled attachments, to The study design was a multicenter randomized clin-
improve control. Align Technology introduced ical trial with 4 parallel arms. The Institutional Review
optimized (O) attachments as a SmartForce feature to Board at Virginia Commonwealth University (VCU)
facilitate larger movements such as rotation .5 or granted permission to conduct this study (VCU Institu-
extrusion .0.5 mm on certain teeth.4 O attachments tional Review Board HM20021396). This randomized
are typically smaller than horizontal attachments but clinical trial was registered and reported at
are specifically designed to include a gap in the aligner clinicaltrials.gov (NCT04968353). There were no

American Journal of Orthodontics and Dentofacial Orthopedics November 2023  Vol 164  Issue 5
620 Groody et al

changes to the protocol after the trial commenced, but (nonbeveled and beveled attachments) and removed
some modifications to the statistical analysis were O attachments if an O attachment was originally desig-
made to combine study groups for reporting after pre- nated on a tooth randomly assigned for any other
liminary analysis. Results were reported both before horizontal attachments. The movement limit was set
and after groups were combined. to 0.25 mm maximum per aligner. Clinicians were
permitted to change interproximal reduction prescrip-
Participants, eligibility criteria, and settings tions, tooth angulation, and attachments on teeth
Patients were recruited from the Department of not included in this study and incorporate overcorrec-
Orthodontics at VCU and 2 private practice offices tions, as preferred.
(Richmond [W.G.G.] and South Riding [N.D.K.]) in Vir- Patients were instructed to wear each aligner for a
ginia. Both private practice orthodontists were Invisalign minimum of 22 h/d, 7 d/wk each, before moving to
VIP Diamond Plus Providers. Two orthodontists and VCU the next aligner, the standard aligner protocol used by
orthodontic residents treated all patients in the study. the participating providers. Proper demonstration of
Patients were recruited at the time of their consultation aligner insertion as instructed was observed. Participants
if they met the study criteria: patients aged $16 years to verbally confirmed compliance at each appointment,
be treated with either Comprehensive Invisalign or Invis- and compliance was self-recorded by participants in
align Teen, at least 1 maxillary lateral incisor requiring hours per day.
$0.3 mm extrusion, maxillary arch with #6 mm of Conventional horizontal attachments (H, HIB, HGB)
crowding or spacing, and all teeth present and fully were designed to be 4 mm wide mesiodistally; the
erupted (excluding third molars). Exclusion criteria incisogingival dimension and the angulation were
were: treatment plan requiring surgery or extraction of unaltered. Each attachment was placed in the incisal
any maxillary teeth, maxillary lateral incisors with pa- third and centered mesiodistally and incisogingivally. O
thology or large restorations (crowns, veneers, etc), attachments were used as received from Align. The
severely rotated maxillary anterior teeth ($15 ), and protocol for attachment bonding was standardized
presence of anterior crossbite. Patients with poor using the commonly accepted bonding procedures.
tracking requiring midcourse intervention or failure to
complete the aligners as prescribed were noted for re- Outcomes and any changes after trial
porting purposes but not included in the final analysis. commencement
Recruitment began in August 2021, and the final data Midcourse interventions to improve tracking, such as
collection was completed in June 2022. rescanning or introducing auxiliary appliances, were re-
corded and reported, but the teeth involved were not
Interventions analyzed as part of the corresponding group. Maxillary
All patients received the same treatment they would lateral incisors not tracking, noted by a minimum of 1
have received were they not in the study, except that mm of aligner material incisally when the aligner was
the designs of those attachments prescribed for fully seated, were recorded and reported but not
maxillary lateral incisor extrusion during the first analyzed within the group assigned.
20-25 aligners were assigned randomly. Each lateral Patients were evaluated only after the first series of
incisor requiring extrusion of $ 0.3 mm was assigned aligners (20-25 aligners). After that, treatment
to 1 of 4 different attachment groups: O, H, HIB, or proceeded as necessary, determined by patients and
HGB. Those 4 attachment designs were chosen on the individual practitioners, and was not recorded. Only
basis of a previous study which identified those maxillary arches were evaluated in this study.
attachments as the ones preferred most often by Pretreatment scans were taken with iTero scanners and
practitioners (including orthodontic specialists and sent to Align Technology to initiate the ClinCheck. The
general dentists) to extrude maxillary lateral incisors predicted posttreatment ClinCheck stereolithography
during treatment with clear aligners.21 All participants file was downloaded from Align Technology. In addition,
signed informed consent to participate in the study an actual posttreatment iTero scan was taken after the
and were offered a $50 gift card at completion to first series of aligners. Stereolithography files of the pre-
compensate them for their involvement. treatment, actual posttreatment, and predicted post-
Invisalign’s ClinCheck (Align Technology) software treatment were transferred to GeoMagic Control X by
was used to design each patient’s treatment sequence. 3D Systems (Rock Hill, SC). All models were standardized
Invisalign technicians were instructed to place O attach- to a global XYZ coordinate system, with the z-axis rep-
ments. The orthodontist prescribed the horizontal resenting the vertical axis.

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Groody et al 621

The pretreatment models were used as the reference Interim analysis and stopping guidelines
data model. The actual posttreatment and predicted Participants were informed that they could
posttreatment were used as the measured data models. discontinue participation at any time and that it would
The reference data model was divided into regions: not affect their remaining treatment. Patients with
posterior teeth, lateral incisors, central incisors, and poor tracking requiring midcourse intervention or failure
gingiva. The posterior teeth region was used for the to complete the aligners prescribed were noted for
superimpositions. According to the method described reporting purposes but not included in the final analysis.
by Gr€unheid et al,22 a best-fit algorithm was used to
superimpose 2 models (pretreatment to predicted
posttreatment and pretreatment to actual post- Randomization
treatment) to compare individual tooth movements The statistician generated a stratified, block
planned and achieved in 3-dimensions with 50 iteration randomization list using the Excel random number
counts. The initial alignment algorithm was used before generator with blocks of 4 and stratified for the 3
the best-fit algorithm for superimposition. After practices based on anticipated recruitment from each
superimposition, the 3D compare tool was used to practice (40, 30, and 10 teeth). Randomization was at
measure the vertical distance of each maxillary lateral the tooth level rather than by the patient. Attachment
incisor between the pretreatment and predicted types were assigned when recruited patients consented
posttreatment models and the pretreatment and to participate in the study. The attachment type for
actual posttreatment models. Measurements were each tooth was revealed when the ClinCheck was
made at 3 distributed points in the middle third of received back from Align so clinicians could ensure
the incisal edge of the lateral incisor and averaged that the appropriate attachment was prescribed: O, H,
for the predicted extrusion and actual extrusion HIB, or HGB.
measurements.
The primary outcome was achieved extrusion relative Blinding
to the predicted extrusion (percent achieved) of the
A single evaluator completed the measurements and
maxillary lateral incisors among the 4 attachment
was blinded to the attachment used for each tooth.
groups (O, H, HIB, HGB). Secondary objectives included
Blinding of the treatment providers and patients was
the analysis of age, sex, number of trays, and reported
not possible. Digital pretreatment, prediction, and
compliance related to extrusion accuracy (percent
posttreatment models were deidentified for each tooth
achieved).
involved. Attachments were removed from the maxillary
After initial analysis, it was noted that the 3
lateral incisors for the final scan to ensure blinding
horizontal attachment designs (H, HIB, HGB) performed
during the superimposition and measurement process.
similarly to each other compared to the O attachments.
Because of the comparable clinical outcomes observed
Statistical analysis
and their visible design similarities, analysis was also per-
formed with the 3 conventional attachment designs Intraexaminer and interexaminer reliability were
combined, compared with O attachments. There were calculated using the intraclass correlation coefficient
no other changes to the initial trial protocol. (ICC) and the repeated measurements for the predicted
and actual extrusion. In addition, superimposition with
and without palatal rugae was compared for actual
Sample size calculation extrusion measures using ICC. Bivariate analyses were
Power analysis determined that 20 teeth/attachment conducted to determine if the 4 treatment groups were
would have 80% power to detect clinically meaningful equally balanced regarding patient sex, predicted
attachment differences. Based on data from prior extrusion, patient age, and self-reported compliance.
publications, the common standard deviation for These analyses were conducted using the Fisher exact
extrusion was assumed to be 1.25.23,24 A sample size test and analysis of variance methods on the basis of
of 20 per group could detect an effect size of 0.146 the variable type. The difference between actual and
and a variance of means of 0.114. These estimates reflect predicted extrusion was compared using a paired
small effect sizes and, therefore, could detect clinically t test. Linear models were used to determine the
meaningful differences in the extrusion ability among difference in the percent of the predicted extrusion
the 4 attachments.25 Sample size calculations were achieved on the basis of the attachment after adjusting
estimated with nQuery (version 8.5.2; Statistical for covariates. Covariates of interest included were
Solutions, Boston, Mass). patient age, sex, number of trays, and self-reported

American Journal of Orthodontics and Dentofacial Orthopedics November 2023  Vol 164  Issue 5
622 Groody et al

compliance (average hours of aligner wear per day). using O attachments, 19 (27%) with H, 15 (21%) with
Clustering was used in the analysis to account for HIB, and 14 (20%) with HGB. The predicted (prescribed)
patients with 2 teeth involved in the study, usually extrusion ranged from 0.31 mm to 2.46 mm, with an
randomly assigned to different attachment groups. All average of 0.84 mm, and did not differ
pairwise comparisons were adjusted using Tukey’s significantly among the 4 groups (P 5 0.2550). The
adjustment to account for multiple comparisons. SAS distributions of patient sex, age, number of trays, and
Enterprise Guide was used for all analyses (version 8.2; self-reported compliance did not differ significantly
SAS Institute, Cary, NC). The significance level was set among the 4 attachment groups. The baseline
at 0.05. characteristics of the 4 attachment groups are presented
in Table I.
Error of the method
After 2 weeks, the measurements for 14 models Primary and secondary outcome analyses
(25 teeth) were repeated, and intraexaminer reliability Intraexaminer and interexaminer reliability for extru-
was evaluated. This involved performing the sion measurements was calculated and found to be very
pretreatment to prescribed posttreatment and high (ICC, 0.985 and 0.991, respectively). The average
pretreatment to posttreatment superimposition intraexaminer measurement difference between evalua-
procedures again and remeasuring the outcomes. tions was 0.006 mm (95% confidence interval [CI],
Interexaminer reliability was established by having a 0.017 to 0.004). The average interexaminer measure-
second blinded examiner superimpose and measure the ment difference between evaluations was 0.002 mm
14 models (25 teeth). (95% CI, 0.014 to 0.019). Extrusion measurements
A subsample of 10 teeth was measured for extrusion by were, on average, 0.03 6 0.12 mm larger without the
superimposing with and without the palatal rugae as sta- palatal rugae for reference. Results from the equivalence
ble landmarks to quantify the potential measurement bias testing determined the 2 measures (superimposition on
introduced by superimposing models on the posterior the best fit of posterior teeth and superimposition on
teeth. 26,27 This was done because the rugae were removed palatal rugae) were equivalent within the preset bounds
during ClinCheck processing and were absent in the pre- of 0.1 mm (90% equivalence bounds, 0.09 to 0.03).
dicted posttreatment models. The differences in the extru- The amount of actual extrusion achieved was signif-
sion with and without the rugae were compared with 2 icantly less than the predicted extrusion by an average of
one-sided t test method. Equivalence bounds were preset 0.21 mm (95% CI, 0.17-0.25; P \0.0001). The average
at 0.1 mm and evaluated at the 0.05 level to generate 90% achieved extrusion was 73% of the predicted extrusion
equivalence bounds. (95% CI, 68-78). A graphical depiction of the association
between actual and predicted extrusion by attachment
RESULTS type is shown in Figure 2.
Participant flow The amount of extrusion achieved compared with the
predicted was significantly associated with the attach-
Sixty-eight patients were assessed for eligibility for ment type (P 5 0.0240). Although the overall test was
this study. Three patients declined to participate. Forty significant, the only pairwise comparison (adjusted for
patients (74 maxillary lateral incisors) met the inclusion multiple comparisons) that yielded a statistically signif-
criteria and were enrolled in the clinical trial. A total of icant comparative difference was the H vs O attachment
71 maxillary lateral incisors from 38 patients were with an average difference of 17% of the predicted
included in the final analysis. One patient (2 maxillary extrusion (95% CI, 1-33; P 5 0.0403). Teeth with H at-
lateral incisors: H, HGB) was excluded because of poor tachments extruded an average of 79% 6 21% of the
tracking and inadequate compliance and switched to predicted amount, compared with O attachments at
braces. One patient (1 maxillary lateral incisor: HGB) 62% 6 22%.
had poor compliance and tracking, requiring midcourse Because the 3 horizontal attachment designs were
correction. The last data collection was in June 2022. similar in shape and clinically demonstrated an esti-
The Consolidated Standards of Reporting Trials mated average extrusion of 79%, 78%, and 78% (for
Guidelines flow diagram is presented in Figure 1. H, HIB, and HGB, respectively), compared with the O
attachment at 62%, the 3 conventional attachments
Baseline data were combined into 1 group, and the model was refit.
Of the 71 maxillary incisors included in the study and This full model used clustering to account for instances
analyzed, 23 (32%) were randomly assigned and treated when 2 teeth from the same patient were involved in the

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Groody et al 623

Fig 1. Consolidated Standards of Reporting Trials flow diagram.

Table I. Characteristics of attachment groups at baseline

Characteristic O (n 5 23) H (n 5 19) HIB (n 5 15) HGB (n 5 14) P value
Predicted extrusion, mm 0.7 6 0.3 0.9 6 0.4 0.9 6 0.5 0.9 6 0.6 0.2550
Age, y 32.1 6 14.3 30.4 6 15.2 31.4 6 15.0 31.2 6 15.6
No. of trays 21.7 6 2.3 22.4 6 3.8 23.7 6 4.2 22.6 6 3.9
Compliance, h/d 17.9 6 4.2 17.3 6 3.9 18.1 6 3.7 18.7 6 1.8
Sex (female, male) 14, 9 13, 6 13, 2 11, 3

Note. Values are presented as mean 6 standard deviation.

study and was adjusted for the number of trays, patient Harms
age, patient sex, self-reported compliance, and a vari- There were no adverse effects reported during the
able to indicate horizontal vs optimized attachment. study period. Two patients did not complete the first
The attachment type (horizontal vs O) was significantly series of trays as prescribed, and the 3 teeth enrolled in
associated with achieved extrusion (P 5 0.0060). On the study from these patients were not analyzed.
average, horizontal attachments achieved an estimated However, those patients resumed comprehensive
76% of predicted extrusion, compared with optimized orthodontic treatment, one with conventional fixed
attachments, which achieved 62% of predicted extru- appliances and the other with aligners.
sion. This 14% difference (95% CI, 4-23) equated to
an estimated 22% improvement using a horizontal DISCUSSION
attachment design compared with an O attachment for
Main findings in the context of the existing
maxillary lateral incisor extrusion. None of the other fac-
literature, interpretation
tors were significantly associated with achieved extru-
sion. Results of the full refit model are presented in This was the first randomized clinical trial comparing
Table II. the efficacy of various attachment designs on achieving

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624 Groody et al

Fig 2. Achieved vs predicted extrusion for the 4 attachment designs. Note that points below the diag-
onal line indicate achieved extrusion less than prescribed.

movements; it was specifically noted that larger

Table II. Full repeated measures model predicting
attachments with sharper edges seemed to be more
achieved extrusion, adjusting for attachment, patient
effective for controlling tooth rotation.30 In addition,
age, sex, number of trays, and self-reported compli-
an in vitro study found that rectangular beveled
ance
attachments significantly improved aligner retention
Variables Estimate 95% CI P value compared with ellipsoid or no attachments.31 This was
Intercept 0.50 0.02-0.97 0.0410 consistent with the findings of this study, in which the
Attachment larger, conventional rectangular attachments were
Horizontal 0.14 0.04-0.23 0.0060
significantly more effective in accomplishing maxillary
Optimized Reference
Sex lateral incisor extrusion as prescribed.
Female 0.08 0.04 to 0.21 0.1832 It was previously reported that extrusion was the
Male Reference most difficult type of tooth movement to control with
No. of trays 0.01 0.01 to 0.02 0.4158 aligners.32 Kravitz et al6 observed that incisor extrusion
Age (10 y increase) 0.04 0.08 to 0.00 0.0538
was the least accurate movement, with the maxillary
Compliance 0.00 0.01 to 0.02 0.6350
lateral incisors extruding only 28.4% of the amount
Note. H includes H, HIB, and HGB combined.
prescribed. A follow-up study 11 years later showed
improvement to 53.7%.7 In contrast, a retrospective
maxillary lateral incisor extrusion during aligner study evaluating the accuracy of clear aligners reported
treatment. Attachments were introduced to increase that maxillary incisor intrusion was the least accurately
aligner retention and improve control of various tooth predicted movement, whereas incisor extrusion did not
movements.1,4,5,28 A systematic review recommended differ significantly from what was intended.33 Krieger
that additional-novel attachments might improve the et al34 found that all changes in the anterior region after
effectiveness of Invisalign for various movements, aligner wear were within acceptable bounds except for
including maxillary incisor extrusion.29 However, a improvement of overbite. Al-Nadawi et al,35 in a
more recent systematic review specifically examining randomized clinical trial, found no differences in
the effects of attachments on clear aligner therapy effectiveness for any incisor movements, including
concluded that there was a lack of evidence to support incisor extrusion, when comparing a 7-day to 14-day
the role of attachments in improving control of wear protocol for aligners. In this trial, we observed an
extrusion or vertical movements in general, though estimated effectiveness of 76% with conventional
they were deemed effective for achieving other rectangular horizontal attachments (beveled or not)

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Groody et al 625

and 62% with O attachments for maxillary lateral incisor palatal tissue to serve as a reference. Results showed
extrusion, using the commonly accepted, 7-day wear minimal differences averaging 0.03 mm between the
protocol, thus equating to a 22% improvement in effec- 2 methods, within the preset equivalence bounds of
tiveness for rectangular compared with O attachments. 0.1 mm.
The studies described demonstrated general improve-
ment in outcomes over time as aligner treatment strate- Limitations
gies and materials evolved. In addition to attachment
This study was designed to evaluate the efficacy of
design, differences in effectiveness among studies could
small extrusive movements prescribed for maxillary
also be affected by the amount of movement prescribed,
lateral incisors during aligner treatment, thus limiting
length of treatment or number of trays evaluated, partic-
the direct application of the findings to patients
ipant cooperation in wearing aligners, and other study
requiring similar magnitudes of extrusion. Predicted
design factors.
extrusion averaged 0.84 mm (range, 0.31 to 2.46 mm),
Adults seeking orthodontic treatment consider
with the extrusion achieved being 73% of what was
appliance esthetics, discomfort, and cost, among other
predicted. Importantly, the study did not evaluate the
factors, in selecting a treatment modality.1 Lingual fixed
ability of aligner treatment to achieve actual treatment
appliances have the best esthetic appearance among
goals for individual patients. Practitioners may have
orthodontic options, but aligners offer superior comfort
intentionally prescribed overcorrections for extrusion,
and adaptability.36 Interestingly, the number of teeth
anticipating that the appliance would not deliver
with lingual attachments was the most influential factor
100% of the amount predicted.
in patients reporting negative experiences during aligner
Only vertical movements of the maxillary lateral
treatment.37 However, compared with traditional fixed
incisor incisal edge were evaluated in this study. Buccal
buccal appliances, altered speech production persists
or palatal displacements or inclination changes may
longer for patients wearing aligners.38 Although aligners
have been prescribed intentionally or occurred
have been shown to be more esthetically acceptable than
inadvertently, but they were not assessed because the
conventional orthodontic appliances, the appearance of
objective was to evaluate maxillary lateral incisor
visible attachments attracted attention and reduced the
extrusion. Participating clinicians were not instructed
esthetic acceptability of aligners.15,16,37 Using larger,
to restrict other types of movement that they judged
conventional rectangular attachments may improve
to be needed for individual patients. In addition, it is
the predictability of certain movements, such as the
possible that other factors, such as initial spacing or
extrusion of incisors, but may also be more obvious to
crowding in the arch, could have affected extrusive
observers. One study found that evaluators preferred
movements during alignment, but these occlusal char-
the appearance of ceramic brackets to aligners when
acteristics appeared to be evenly distributed among the
accompanied by multiple attachments,15 whereas most
attachment groups studied.
participants agreed they would accept reduced appliance
Another possible limitation of this study was the
esthetics if a better outcome could be achieved. Another
reliance on participating patients to wear their aligners
study found no significant differences during treatment
as prescribed. Though compliance was checked at each
for quality of life reported in adolescents between fixed
visit and reported verbally by patients, there was no
appliances and aligners, though females using Invisalign
objective measurement of compliance other than the
said they felt more attractive.39
failure of the aligners to track properly, as noted at
This study evaluated lateral incisor extrusion by
follow-up visits. Reported compliance averaged 17-19
superimposing digital models (actual and predicted)
h/d, despite requesting 22 h/d of wear. In addition, it
using the posterior region as a stable reference, as
is known that actual compliance is generally lower
done in a previous study.22 It was anticipated that little
than reported compliance, but self-reporting is still
or no posterior tooth movement would occur in the short
generally considered a useful assessment.41,42 Two
timeframe analyzed (20-25 sets of aligners). Align
patients (3 teeth) were excluded because of inadequate
Technology software (ClinCheck) removes the palatal
compliance and poor tracking.
tissue during processing, so the validity of this
assumption was checked by using the rugae to
superimpose the actual pretreatment and posttreatment Generalizability
models and compare the results with superimpositions The results of this study can be applied to compliant
done using the posterior region.26,27,40 This would be orthodontic patients undergoing Invisalign treatment
analogous to superimposing the initial actual and for mild malocclusions with a prescription for maxillary
starting ClinCheck models and digitally transferring the lateral incisor extrusion of between 0.3 and 2.5 mm.

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626 Groody et al

CONCLUSIONS eye-tracking technology. Am J Orthod Dentofacial Orthop 2020;

158:400-9.
1. Horizontal attachments (H, HIB, or HGB) were 16. F€orsch M, Krull L, Hechtner M, Rahimi R, Wriedt S, Wehrbein H, et al.
Perception of esthetic orthodontic appliances: an eye tracking and
significantly more effective at achieving prescribed
cross-sectional study. Angle Orthod 2020;90:109-17.
maxillary incisor extrusion (76% of the predicted 17. Costa R, Calheiros FC, Ballester RY, Gonçalves F. Effect of three
amount) compared with O attachments (62% of different attachment designs in the extrusive forces generated by
the predicted amount) in patients with mild thermoplastic aligners in the maxillary central incisor. Dental Press
malocclusion undergoing Invisalign treatment. J Orthod 2020;25:46-53.
18. Glaser BJ. Dr. Glaser’s 10 commandments of attachment design:
2. The 3 horizontal attachment types performed
ten simple rules to keep your patient’s invisalign treatment on
similarly to achieve prescribed extrusion. track. Orthod Prod 2016;30-5.
19. Bowman SJ, Celenza F, Sparaga J, Papadopoulos MA, Ojima K,
Lin JC. Creative adjuncts for clear aligners, part 2: intrusion, rota-
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