Evaluation of Invisalign treatment effectiveness and efficiency compared with conventional fixed appliances using the Peer Assessment Rating index

Abstract

Introduction:
The purpose of this retrospective case-control study was to compare the treatment effectiveness and efficiency of the Invisalign system with conventional fixed appliances in treating orthodontic patients with mild to moderate malocclusion in a graduate orthodontic clinic.

Methods:
Using the peer assessment rating (PAR) index, we evaluated pretreatment and posttreatment records of 48 Invisalign patients and 48 fixed appliances patients. The 2 groups of patients were controlled for general characteristics and initial severity of malocclusion. We analyzed treatment outcome, duration, and improvement between the Invisalign and fixed appliances groups.

Results:
The average pretreatment PAR scores (United Kingdom weighting) were 20.81 for Invisalign and 22.79 for fixed appliances (P = 1.0000). Posttreatment weighted PAR scores between Invisalign and fixed appliances were not statistically different (P = 0.7420). On average, the Invisalign patients finished 5.7 months faster than did those with fixed appliances (P = 0.0040). The weighted PAR score reduction with treatment was not statistically different between the Invisalign and fixed appliances groups (P = 0.4573). All patients in both groups had more than a 30% reduction in the PAR scores. Logistic regression analysis indicated that the odds of achieving "great improvement" in the Invisalign group were 0.329 times the odds of achieving "great improvement" in the fixed appliances group after controlling for age (P = 0.0150).

Conclusions:
Our data showed that both Invisalign and fixed appliances were able to improve the malocclusion. Invisalign patients finished treatment faster than did those with fixed appliances. However, it appears that Invisalign may not be as effective as fixed appliances in achieving "great improvement" in a malocclusion. This study might help clinicians to determine appropriate patients for Invisalign treatment.

Full text

Evaluation of Invisalign treatment
effectiveness and efficiency compared
with conventional fixed appliances using
the Peer Assessment Rating index
Jiafeng Gu,
a
Jack Shengyu Tang,
b
Brennan Skulski,
c
Henry W. Fields, Jr,
a
F. Michael Beck,
d
Allen R. Firestone,
a
Do-Gyoon Kim,
a
and Toru Deguchi
a
Columbus and Mason, Ohio
Introduction: The purpose of this retrospective case-control study was to compare the treatment effectiveness
and efficiency of the Invisalign system with conventional fixed appliances in treating orthodontic patients with
mild to moderate malocclusion in a graduate orthodontic clinic. Methods: Using the peer assessment rating
(PAR) index, we evaluated pretreatment and posttreatment records of 48 Invisalign patients and 48 fixed appli-
ances patients. The 2 groups of patients were controlled for general characteristics and initial severity of maloc-
clusion. We analyzed treatment outcome, duration, and improvement between the Invisalign and fixed
appliances groups. Results: The average pretreatment PAR scores (United Kingdom weighting) were 20.81
for Invisalign and 22.79 for fixed appliances (P51.0000). Posttreatment weighted PAR scores between Invis-
align and fixed appliances were not statistically different (P50.7420). On average, the Invisalign patients
finished 5.7 months faster than did those with fixed appliances (P50.0040). The weighted PAR score reduction
with treatment was not statistically different between the Invisalign and fixed appliances groups (P50.4573). All
patients in both groups had more than a 30% reduction in the PAR scores. Logistic regression analysis indicated
that the odds of achieving “great improvement”in the Invisalign group were 0.329 times the odds of achieving
“great improvement”in the fixed appliances group after controlling for age (P50.0150). Conclusions: Our
data showed that both Invisalign and fixed appliances were able to improve the malocclusion. Invisalign patients
finished treatment faster than did those with fixed appliances. However, it appears that Invisalign may not be as
effective as fixed appliances in achieving “great improvement”in a malocclusion. This study might help clinicians
to determine appropriate patients for Invisalign treatment. (Am J Orthod Dentofacial Orthop 2017;151:259-66)
The Invisalign system, introduced by Align Technol-
ogy (Santa Clara, Calif) in 1999, involves moving
teeth in increments with a series of removable clear
polyurethane trays (aligners). Over the past few years,
Align Technology has seen significant growth, with
more than 3 million patients treated with Invisalign
worldwide.
1
Patients prefer Invisalign treatment over
conventional fixed appliances because of its superior
esthetics
2
and comfort.
3
However, in the era of evidence-based dentistry, the
scientific evidence on which to choose the treatment of
more than 3 million patients is limited. The most recent
systematic review of clear aligners only identified 11 rele-
vant scientific articles.
4
Of those, 6 were published more
than 5 years ago, and no evidence-based conclusions can
be drawn from those studies due to poor quality levels.
4
Randomized clinical trials have been conducted by a
research group to evaluate the effects of aligner material
stiffness and activation frequency on Invisalign treatment
completion and outcome.
5-7
The authors concluded that
patients with a 2-week activation protocol, no extractions,
and a low initial Peer Assessment Rating (PAR) score were
more likely to complete their initial series of aligners.
5
This
study supports Align Technology's 2-week activation time
a
Division of Orthodontics, College of Dentistry, The Ohio State University,
Columbus, Ohio.
b
Division of Dental Hygiene, College of Dentistry, The Ohio state University,
Mason, Ohio.
c
College of Dentistry, The Ohio State University, Columbus, Ohio.
d
Division of Oral Biology, College of Dentistry, The Ohio State University,
Columbus, Ohio.
All authors have completed and submitted the ICMJE Form for Disclosure of
Potential Conflicts of Interest, and none were reported.
Address correspondence to: Toru Deguchi, Ohio State University College of
Dentistry, 4088 Postle Hall, 305 W 12th Ave, Columbus, OH 43210; e-mail,
deguchi.4@osu.edu.
Submitted, February 2016; revised and accepted, June 2016.
0889-5406/$36.00
Ó2017 by the American Association of Orthodontists. All rights reserved.
http://dx.doi.org/10.1016/j.ajodo.2016.06.041
259
ORIGINAL ARTICLE

recommendation and also suggests that Invisalign is not
suitable for extraction patients and those with complex
treatment plans. Furthermore, the authors reported that
fixed appliances will be needed in premolar extraction
patients treated with aligners to correct dental tipping.
7
They also concluded that the aligners were most success-
ful in improving anterior alignment, transverse relation-
ships, and overbite; moderately successful in improving
midline and overjet; and least successful in improving
buccal occlusion.
6
Two retrospective cohort studies compared the
treatment results of Invisalign patients with those
with fixed appliances using the American Board of Or-
thodontics objective grading system.
8,9
The authors
reported that Invisalign patients lost 13 more
objective grading system points on average than did
fixed appliances patients and achieved a passing rate
27% lower than for fixed appliances.
8
The study indi-
cated that Invisalign and fixed appliances are similar
in correcting rotations, marginal ridge heights, space
closure, and root alignment, but fixed appliances are
superior in correcting occlusal contacts, posterior
torque, and anteroposterior discrepancies.
8
In their
follow-up study on postretention dental changes of
treated Invisalign patients, the authors reported that
patients treated with Invisalign relapsed more than did
those treated with fixed appliances, particularly in
maxillary anterior alignment.
9
Many of those studies were conducted several years
ago before Align Technology introduced changes to the
tray material, attachments, and treatment algorithms. Af-
ter 2008, improved technologies such as Precision Cuts,
Precision Bite Ramps, and Smart Force Attachments led
to innovations of Invisalign G3, G4, and G5 that resulted
in a possibly wider range and more precision for tooth
movement. It seems reasonable to reevaluate the effec-
tiveness and efficiency of this popular treatment system
after the introduction of what the company calls signifi-
cant advances in materials. Therefore, well-designed clin-
ical trials are needed to provide evidence for contemporary
Invisalign treatment.
An assessment of orthodontic treatment outcomes with
a quantitative index helps to establish goals, evaluate effec-
tiveness, and achieve a measureable finish for completed
patients. Several quantitative indexes have been developed
to evaluate the malocclusion severity and orthodontic
treatment need or treatment outcome.
10-14
One index,
the PAR, has been used widely for evaluating the
effects of treatment in a variety of circumstances.
15-17
The PAR is an occlusal index that not only measures how
much a patient deviates from ideal occlusion, but also
quantitatively evaluates orthodontic treatment outcomes
by comparing pretreatment and posttreatment casts.
13,14
The purpose of this retrospective case-control study
was to determine the effectiveness and efficiency of
the Invisalign system compared with conventional fixed
appliances in treating orthodontic patients with mild to
moderate malocclusion using the PAR index. The spe-
cific aims were to compare patients treated with Invisa-
lign and fixed appliances for (1) posttreatment PAR
scores, (2) posttreatment reduction in PAR scores, (3)
treatment duration, and (4) malocclusion improvement
between Invisalign and fixed appliances patients after
establishing 2 groups of Invisalign and fixed appliances
patients with comparable pretreatment characteristics.
See Supplemental Materials for a short video presenta-
tion about this study.
MATERIAL AND METHODS
The study protocol was reviewed and approved by the
Ohio State University Institutional Review Board.
The sample for this retrospective case-control study
was selected from approximately 1500 conventional or-
thodontic patients and 250 Invisalign patients in the ar-
chives of the Division of Orthodontics at Ohio State
University College of Dentistry. All patients were started
and completed by orthodontic faculty and residents be-
tween 2009 and 2014. The patients were chosen without
regard to their history or final treatment results. Selec-
tion was based on the following criteria: (1) available
pretreatment and posttreatment records including digi-
tal models (OrthoCad) and photos; (2) age, 16 years or
older when treatment started; (3) no auxiliary appliances
other than elastics used during treatment; (4) no extrac-
tion patients; (5) no orthognathic surgery or syndromic
patients; and (6) full permanent dentition except third
molars. The only patients not included were those who
were debonded early or terminated (Invisalign patients,
because of compliance, hygiene, or transfer). After the
initial review, 62 fixed appliances patients and 61 Invis-
align patients met the criteria. To match the pretreat-
ment malocclusions between the 2 groups and
eliminate any early termination patients, 48 subjects
from each group were selected. With a nondirectional
alpha risk of 0.05 and an estimated standard deviation
of 4.316, our power to detect a difference of 63 units
of weighted post-PAR score was 0.92.
Patients in the fixed appliances group were treated
with fixed orthodontic appliances with various prescrip-
tions, but all appliances were straight-wire edgewise ap-
pliances.
The PAR index (United Kingdom weighted PAR,
which includes the mandibular anterior
18
) was used in
this study to assess 8 components: maxillary anterior
segment alignment, mandibular anterior segment
260 Gu et al
February 2017 Vol 151 Issue 2 American Journal of Orthodontics and Dentofacial Orthopedics

alignment, anteroposterior discrepancy, transverse
discrepancy, vertical discrepancy, overjet, overbite, and
midline.
Digital models were used to determine the PAR
scores. Previous research has demonstrated that PAR in-
dex scores derived from digital models are valid and reli-
able measures of malocclusion.
19
One investigator
(J.S.T.) was trained and calibrated for the PAR index
and performed all the PAR measurements. This investi-
gator was blinded to the group assignment (Invisalign
or fixed appliances) to which the models belonged. In-
traexaminer reliability was assessed with intraclass corre-
lation coefficients determined by duplicate scoring of 12
randomly selected subjects from each category (total of
24) 2 months after initial data collection.
In their original article to evaluate the PAR index,
Richmond et al
13
determined that “at least a 30 percent
reduction in PAR score was required for a case to be
considered as improved and a change of 22 PAR points
brought about great improvement.”However, not every
patient starts with s PAR score above 22. To include all
subjects, we redefined “great improvement”as either
(1)weighted PAR score reduction of 22 points or more
or (2), if the initial PAR score was less than 22, a
weighted PAR score after treatment equal to 0.
Statistical analysis
All statistical analyses were performed with soft-
ware (version 9.3 SAS, Cary, NC). Subject characteris-
tics were compared for the 2 treatment groups using
chi-square or Fisher exact tests for categorical variables
and randomization tests for continuous variables. Mul-
tiple comparisons were adjusted using the step-down
Bonferroni method of Holm. The Fisher exact test
was also used to compare the distribution of percent
reduction of weighted PAR scores between the 2
groups.
Logistic regression was used to evaluate differences
in great improvement outcome due to treatment group
after controlling for age. The level of statistical signifi-
cance for all analyses was set at a50.05.
RESULTS
The calibrated examiner demonstrated good intra-
examiner reliability. (The intraclass correlation coeffi-
cient scores ranged from 0.66 for posttreatment
overbite to $0.98 for the remaining variables.)
The basic sample description and characteristics are
presented in Table I. The Invisalign group had a mean pre-
treatment age of 26.0 69.7 years (average 6standard de-
viation), and the fixed appliances group had a
pretreatment age of 22.1 67.9 years. This difference
was statistically significant (P50.0374). The differences
between the 2 groups for the following variables were
not statistically different: sex, race/ethnicity, percentage
of Class I molar patients, or percentage of retreatment pa-
tients (previous orthodontic treatment) (P.0.05; Table I).
The mean weighted pretreatment PAR scores be-
tween the Invisalign and fixed appliances groups were
not statistically different (P51.0000; Table II). There
were no statistically significant differences between the
2 groups for the 8 individual components of pretreat-
ment PAR scores (Table II).
After treatment, weighted PAR scores for both groups
were less than 5 and did not differ significantly
(P50.7420; Table III). None of the 8 individual compo-
nents of posttreatment PAR scores differed significantly
between the groups (Table III).
Neither of the weighted PAR reduction scores after
treatment or the reduction scores of the 8 individual
Table I. Comparison of baseline characteristics between Invisalign and fixed appliances groups
Variable
Treatment groups
All Invisalign Fixed appliances
Pvalue(n 596) (n 548) (n 548)
Age (y) 24.0 (SD, 9.0) 26.0 (SD, 9.7) 22.1 (SD, 7.9) 0.0374*
Sex 0.6695
Female 62 (65%) 32 (67%) 30 (63%)
Male 34 (35%) 16 (33%) 18 (37%)
Race/ethnicity 0.8813
African American 16 (17%) 7 (15%) 9 (19%)
Asian 4 (4%) 2 (4%) 2 (4%)
White 73 (76%) 38 (79%) 35 (73%)
Hispanic 3 (3%) 1 (2%) 2 (4%)
Class I molars 65 (68%) 35 (73%) 30 (63%) 0.2751
Retreatment 7 (7%) 4 (8%) 3 (6%) 1.0000
*Significant difference between groups (P\0.05).
Gu et al 261
American Journal of Orthodontics and Dentofacial Orthopedics February 2017 Vol 151 Issue 2

PAR components was statistically different between
the 2 groups (Table IV). The treatment time for the
Invisalign group (13.35 months) was significantly
shorter than that for the fixed appliances group
(19.08 months) (P50.0040; Table IV). To compare
the treatment efficiency of the groups, we evaluated
the weighted PAR reduction per month. The Invisalign
group was not significantly different compared with
the fixed appliances group in reducing the weighted
PAR score per month of treatment (P50.2318;
Table IV).
In our study, all patients in both groups were
improved; ie, there was at least a 30% reduction in the
PAR score. However, logistic regression analysis indicated
that, using our definition of “great improvement”as either
a reduction in PAR score of 22 points or a final PAR score
of 0, the odds of achieving “great improvement”in the In-
visalign group were 0.329 times the odds of achieving
“great improvement”in the fixed appliances group after
controlling for age (95% confidence interval, 0.133-
0.815; P50.015; Table V). We further analyzed the
distribution of the percent reduction in weighted PAR
scores between the 2 groups. Our data indicated that fixed
appliances are significantly more effective to reduce
weighted PAR scores than Invisalign (P50.0322; Fig).
DISCUSSION
In our study, the patients in the Invisalign group were
significantly older than those in the fixed appliances
group, 26.0 and 22.1 years, respectively. This might indi-
cate that older people tend to prefer the cosmetic benefit
of the Invisalign system and also that older people might
feel more confident that they could wear their trays as
directed. Walton et al
2
reported that older subjects tended
to rate clear orthodontic appliances higher than did
younger subjects. However, there is also potential selec-
tion bias that attending faculty in our program possibly
placed higher consideration in the Invisalign treatment
option to older patients. In our study, both the Invisalign
and the fixed appliances patients were older than 16 years,
and had generally finished growth, so that it was unlikely
that growth played a significant role in the treatment
Table II. Pretreatment PAR scores between Invisalign and fixed appliances groups
Variable
Invisalign (n 548) Fixed appliances (n 548)
PvalueMean SD Minimum Maximum Mean SD Minimum Maximum
UANT 5.25 2.07 1 10 6.23 2.09 1 11 0.2313
LANT 4.65 2.41 0 10 5.13 2.45 0 9 1.0000
AP 2.48 1.34 0 4 2.44 1.29 0 4 1.0000
TRANS 0.17 0.66 0 3 0.33 0.81 0 3 1.0000
VERT 0.02 0.14 0 1 0 0 0 0 1.0000
OJ 5.75 5.25 0 24 4.88 6.27 0 24 1.0000
OB 1.92 1.65 0 8 2.54 2.36 0 10 1.0000
MID 0.58 1.65 0 8 1.25 2.05 0 8 1.0000
PREWPAR 20.81 6.79 9 43 22.79 7.72 9 38 1.0000
UANT, Maxillary anterior segment alignment; LANT, mandibular anterior segment alignment; AP, anteroposterior discrepancy; TRANS, transverse
discrepancy; VERT, vertical discrepancy; OJ, overjet; OB, overbite.; MID, midline; PREWPAR, pretreatment weighted PAR.
Table III. Posttreatment PAR scores between Invisalign and fixed appliances groups
Variable
Invisalign (n 548) Fixed appliances (n 548)
PvalueMean SD Minimum Maximum Mean SD Minimum Maximum
PUANT 0.44 0.94 0 5 0.42 0.79 0 3 1.0000
PLANT 0.04 0.2 0 1 0.08 0.35 0 2 1.0000
PAP 2.06 1.29 0 4 1.69 1.27 0 4 1.0000
PTRANS 0.06 0.43 0 3 0.06 0.43 0 3 1.0000
PVERT 0 0 0 0 0.02 0.14 0 1 1.0000
POJ 0.75 2.94 0 12 0 0 0 0 1.0000
POB 0.56 0.9 0 2 0.17 0.56 0 2 0.3060
PMID 0.17 0.81 0 4 0.25 0.98 0 4 1.0000
POSTWPAR 4.08 4.35 0 19 2.69 2.23 0 9 0.7420
PUANT, Posttreatment maxillary anterior segment alignment; PLANT, posttreatment mandibular anterior segment alignment; PAP, posttreat-
ment anteroposterior discrepancy; PTRANS, posttreatment transverse discrepancy; PVERT, posttreatment vertical discrepancy; POJ, posttreat-
ment overjet; POB, posttreatment overbite; PMID, posttreatment midline; POSTWPAR, posttreatment weighted PAR.
262 Gu et al
February 2017 Vol 151 Issue 2 American Journal of Orthodontics and Dentofacial Orthopedics

outcome. Djeu et al
8
also found that Invisalign patients
were relatively older than fixed appliances patients.
Otherwise, our treatment groups were balanced for all
other demographic components (sex, race/ethnicity, Class
I molar relationship, and retreatment percentage). Previ-
ous research indicated that more female patients choose
Invisalign treatment; however, in our study there was
no statistical difference in sex between Invisalign and
fixed appliances patients (P50.6695).
20
Our 2 treatment groups were comparable in maloc-
clusion severity with regard to pretreatment weighted
PAR scores and all 8 individual components (Table II).
The average weighted pretreatment PAR scores of our In-
visalign and fixed appliances patients were similar to
those in the study by Miller et al,
21
although only some
of their patients were from a university clinic. Our fixed
appliances patient weighted pretreatment PAR score
was also comparable with previously reported dental
school orthodontic patients.
22
Thus, our sample seemed
consistent with other reported groups of orthodontically
treated patients. The Invisalign and fixed appliances pa-
tients in this study were treated by orthodontic residents
under supervision of different orthodontic faculty. In
contrast, Djeu et al
8
analyzed only the first 50 Invisalign
patients treated by 1 orthodontist. Therefore, our study
provides greater generalizability.
The average weighted posttreatment PAR score of the
Invisalign group was higher than that of the fixed appli-
ances group by lessthan 2 points; this was not statistically
different.None of the individual componentsof posttreat-
ment PAR scores demonstrated a significant difference
between the Invisalign group and the fixed appliances
group. However, regarding deep overbite correction with
either posterior extrusion or anterior intrusion, previous
reports only recommend Invisalign to treat simple maloc-
clusions with small overbite discrepancies
4
and indicate
that significant correction of a deep overbite with Invisa-
lign appears unlikely.
6,23
Recently, Align Technology
introduced Invisalign G5 with optimized attachments
and precision bite ramps to improve deep overbite
correction. However, this innovation was introduced
after the time period covered in this sample, so its effect
on the results of deepbite treatment is unknown. The
PAR index does not differentiate between deep overbite
and open bite. Boyd
24
and Boyd et al
25
reported that
Table IV. PAR score reduction after treatment and improvement between Invisalign and fixed appliances groups
Variable
Invisalign (n 548) Fixed appliances (n 548)
PvalueMean SD Minimum Maximum Mean SD Minimum Maximum
DWPAR 16.73 6.78 5 38 20.1 8.06 7 38 0.4573
MOS 13.35 8.63 4 48 19.08 5.92 6 31 0.0040*
DWPAR/MOS 1.55 0.8 0.25 3.5 1.16 0.69 0.43 4.33 0.2318
DUANT 4.81 2.09 1 10 5.81 2.21 1 11 0.4573
DLANT 4.6 2.44 0 10 5.04 2.42 0 9 1.0000
DAP 0.42 1.18 2 3 0.75 1.34 2 4 1.0000
DTRANS 0.1 0.52 0 3 0.27 0.71 0 3 1.0000
DVERT 0.02 0.14 0 1 0.02 0.14 1 0 1.0000
DOJ 5 4.85 0 24 4.88 6.27 0 24 1.0000
DOB 1.35 1.76 2 8 2.38 2.53 2 10 0.4573
DMID 0.42 1.7 4 8 1 2.1 4 8 1.0000
Improved 48/48 (100%) 48/48 (100%) 1.0000
Great improvement 11/48 (22.9%) 22/48 (45.8%) 0.0150*
DWPAR, Weighted PAR score reduction; MOS, months in treatment; DWPAR/MOS, average weighted PAR score reduction per month; DUANT,
maxillary anterior segment alignment score reduction; DLANT, mandibular anterior segment alignment score reduction; DAP, anteroposterior
discrepancy score reduction; DTRANS, transverse discrepancy score reduction; DVERT, vertical discrepancy score reduction; DOJ, overjet score
reduction; DOB, overbite score reduction; DMID, midline score reduction; Improved, subjects with at least a 30% reduction in PAR score; Great
improvement, either weighted PAR score reduction of 22 points or more, or weighted PAR score after treatment equal to 0.
*Significant difference between groups (P\0.05).
Table V. Logistic regression analysis for “great improve-
ment”(either PAR reduction of 22 points or to 0 if initial
PAR \22) with patient age as a random variable
Effect Num DF Den DF Chi-square
Pr
.chi-square
Type III tests of fixed effects
Group 1 93 5.92 0.015
Comparison Estimate DF 95% confidence limits
Odds ratio estimates
Group Invisalign
vs fixed
0.329 93 0.133 0.815
Num DF, numerator degrees of freedom; Den DF, denominator de-
grees of freedom; Pr, probability.
Gu et al 263
American Journal of Orthodontics and Dentofacial Orthopedics February 2017 Vol 151 Issue 2

Invisalign may have advantagesin correcting mild anterior
open bite because of the intrusive effect on the posterior
teeth from the double thickness of the aligner trays.
Another point to note is posttreatment overjet,
although there were no statistical differences between
the 2 groups. Fixed appliances corrected overjet in all pa-
tients, but Invisalign still left some patients with signifi-
cant overjet (maximum score, 12.00; Table III). This
result agrees with previous observations on overjet and
anteroposterior correction by Invisalign.
6,8
To overcome
Invisalign's deficiency in overjet and anteroposterior
correction, auxiliary appliances might be used.
We further evaluated efficiency between Invisalign
and fixed appliances by considering treatment duration.
In our study, there was a statistically significant differ-
ence between the treatment duration of the 2 groups:
Invisalign treatment was on average 5.7 months faster
than that of the fixed appliances group (Table IV). This
result was comparable with previous reports between In-
visalign and fixed appliances on nonextraction treat-
ment with mild to moderate malocclusion.
8,20
One
study indicated that there is no detailing or finishing
phase for Invisalign patients that might have resulted
in less treatment time. Although there was no
statistically significant difference in the post-PAR scores
between the Invisalign (4.1) and fixed appliance (2.7)
groups, this difference may be more obvious when
analyzing more severe cases, which may result worse
PAR scores in Invisalign patients who did not require fin-
ishing and detailing. The other suggested reasons for
shorter treatment durations for Invisalign patients may
be related to the difference in the method of tooth
movement. In patients with Invisalign, the occlusal force
would be much greater than the orthodontic force and
result in faster tooth movement than that of the tooth
positioner. However, the average reduction in weighted
PAR score was not significantly different between the
2 groups; neither was treatment efficiency.
Richmond et al
13
reported that at least a 30% reduc-
tion in PAR score is required for a patient to be considered
as “improved.”All subjects in our Invisalign and fixed ap-
pliances groups improved by at least 30% (Table IV and
Fig). The mean PAR score percentage reduction in our In-
visalign and fixed appliances groups were 80.3% and
87.0%, respectively. However, a change of 22 PAR points
is needed to be considered a “great improvement.”
13
Since not every patient has more than 22 PAR points at
pretreatment, to include more subjects, we redefined
“great improvement”to include those with a pretreat-
ment PAR score less than 22 points if their posttreatment
PAR score was 0. Using this definition of “great improve-
ment,”significantly more subjects were classified as
“greatly improved”in the fixed appliances group
(P50.0150; Table IV). Even if we considered only sub-
jects with the original definition of “great improve-
ment”—a change of 22 PAR points—there were 9 of 48
subjects in the Invisalign group and 19 of 48 subjects in
the fixed appliances group who met this definition. How-
ever, this difference did not reach statistical significance
(P50.0628). The distribution of percent reduction in
Fig. Distribution of percent reduction in weighted PAR scores (WPAR) between the Invisalign and
fixed appliances groups.
264 Gu et al
February 2017 Vol 151 Issue 2 American Journal of Orthodontics and Dentofacial Orthopedics

weighted PAR scores between the 2 groups further indi-
cated that fixed appliances are significantly more effective
at reducing weighted PAR scores than Invisalign
(P50.0322; Fig), and it appears that Invisalign may be
about half as effective as fixed appliances.
Previously, the mean accuracy of tooth movement
with Invisalign was reported to be about 41%,
23
and
those authors also reported that 70% to 80% of clini-
cians need either midcourse correction, refinement, or
conversion to fixed appliances to finish treatment.
Recently, Simon et al
26
reported that overall mean ef-
ficacy of tooth movement was 59%. They also demon-
strated that incisor torque, premolar derotation, and
molar distalization can be performed using Invisalign.
In our study, the refinement rate for Invisalign was
37.5%, which is much lower than reported previ-
ously.
23
This significant difference may reflect the
improvements of the aligner material and attachment
features, or also indicate increased clinician experience
with the system.
Refinement features with the Invisalign system may
partially account for the disparity in treatment effec-
tiveness between Invisalign and fixed appliances.
Treatment with fixed appliances is continuous. In
contrast, treatment with Invisalign may be interrupted
by requesting midcourse correction or refinement. Pa-
tients who choose Invisalign for esthetic reasons may
well also prefer shorter treatment times, and thus be
more likely to prefer to avoid refinement or any fixed
appliance treatment to complete difficult tooth move-
ment. The higher PAR scores and shorter treatment
times may indicate greater patient autonomy with In-
visalign treatment. The PAR index was developed based
on the judgment of dental professionals and may not
be fully coincident with patient values.
There are several shortcomings with this study. Most
significantly, it is a retrospective study. This introduces
selection bias. Only subjects with complete records
were included, so early termination patients because of
poor cooperation or hygiene were excluded. Addition-
ally, the residents who delivered care had greater experi-
ence with fixed appliances. Furthermore, whereas
developments in materials and protocols with fixed
appliance systems have been modest during and after
the time period encompassed in this investigation, there
have been major changes in materials and protocols to
the Invisalign system since 2009, and the results of
this study do not reflect them.
Like fixed appliances, Invisalign can achieve
outstanding outcomes with appropriate patients. How-
ever, the clinician's orthodontic knowledge and clinical
experience, as well as the patient's compliance and moti-
vation, all play significant roles in the process.
Practitioners with limited experience with Invisalign
should be conservative in patient selection with this sys-
tem. Further research with randomized clinical trials is
warranted to compare the treatment outcomes between
Invisalign and fixed appliances.
CONCLUSIONS
In this study, we examined the treatment effectiveness
and treatment efficiency between Invisalign and fixed ap-
pliances with the weighted PAR index. We made the
following conclusions.
1. Final occlusal scores did not differ between the 2
systems.
2. Fixed appliances improved malocclusion more
effectively than did Invisalign.
3. Treatment with Invisalign was finished on average
30% (5.7 months) faster than treatment with fixed
appliances.
4. However, the likelihood of achieving “great improve-
ment”in a malocclusion appears to be better with
fixed appliances.
SUPPLEMENTARY DATA
Supplementary data related to this article can be
found online at http://dx.doi.org/10.1016/j.ajodo.2016.
06.041.
REFERENCES
1. Align Technology. Invisaline for adults and teens. Available at:
www.invisalign.com/braces-for-adults-and-teens. Accessed on
August 25, 2015.
2. Walton DK, Fields HW, Johnston WM, Rosenstiel SF, Firestone AR,
Christensen JC. Orthodontic appliance preferences of children and
adolescents. Am J Orthod Dentofacial Orthop 2010;138:
698.e1-12: discussion, 698-9.
3. Fujiyama K, Honjo T, Suzuki M, Matsuoka S, Deguchi T. Analysis of
pain level in cases treated with Invisalign aligner: comparison with
fixed edgewise appliance therapy. Prog Orthod 2014;15:64.
4. Rossini G, Parrini S, Castroflorio T, Deregibus A, Debernardi CL. Ef-
ficacy of clear aligners in controlling orthodontic tooth movement:
a systematic review. Angle Orthod 2015;85:881-9.
5. Bollen AM, Huang G, King G, Hujoel P, Ma T. Activation time and
material stiffness of sequential removable orthodontic appliances.
Part 1: ability to complete treatment. Am J Orthod Dentofacial Or-
thop 2003;124:496-501.
6. Clements KM, Bollen AM, Huang G, King G, Hujoel P, Ma T. Acti-
vation time and material stiffness of sequential removable ortho-
dontic appliances. Part 2: dental improvements. Am J Orthod
Dentofacial Orthop 2003;124:502-8.
7. Baldwin DK, King G, Ramsay DS, Huang G, Bollen AM. Activation
time and material stiffness of sequential removable orthodontic
appliances. Part 3: premolar extraction patients. Am J Orthod Den-
tofacial Orthop 2008;133:837-45.
8. Djeu G, Shelton C, Maganzini A. Outcome assessment of Invisalign
and traditional orthodontic treatment compared with the
Gu et al 265
American Journal of Orthodontics and Dentofacial Orthopedics February 2017 Vol 151 Issue 2

American Board of Orthodontics objective grading system. Am J
Orthod Dentofacial Orthop 2005;128:292-8.
9. Kuncio D, Maganzini A, Shelton C, Freeman K. Invisalign and
traditional orthodontic treatment postretention outcomes
compared using the American Board of Orthodontics objective
grading system. Angle Orthod 2007;77:864-9.
10. Cangialosi TJ, Riolo ML, Owens SE Jr, Dykhouse VJ,
Moffitt AH, Grubb JE, et al. The ABO discrepancy index: a
measure of case complexity. Am J Orthod Dentofacial Orthop
2004;125:270-8.
11. Casko JS, Vaden JL, Kokich VG, Damone J, James RD,
Cangialosi TJ, et al. Objective grading system for dental casts
and panoramic radiographs. American Board of Orthodontics.
Am J Orthod Dentofacial Orthop 1998;114:589-99.
12. Little RM. The irregularity index: a quantitative score of mandib-
ular anterior alignment. Am J Orthod 1975;68:554-63.
13. Richmond S, Shaw WC, Roberts CT, Andrews M. The PAR index
(peer assessment rating): methods to determine outcome of ortho-
dontic treatment in terms of improvement and standards. Eur J Or-
thod 1992;14:180-7.
14. Richmond S, Shaw WC, O'Brien KD, Buchanan IB, Jones R,
Stephens CD, et al. The development of the PAR index (peer assess-
ment rating): reliability and validity. Eur J Orthod 1992;14:
125-39.
15. Firestone AR, Beck FM, Beglin FM, Vig KW. Evaluation of the
peer assessment rating (PAR) index as an index of orthodontic
treatment need. Am J Orthod Dentofacial Orthop 2002;122:
463-9.
16. Deguchi T, Honjo T, Fukunaga T, Miyawaki S, Roberts WE, Ta-
kano-Yamamoto T. Clinical assessment of orthodontic outcomes
with the peer assessment rating, discrepancy index, objective
grading system, and comprehensive clinical assessment. Am J Or-
thod Dentofacial Orthop 2005;127:434-43.
17. Dyken RA, Sadowsky PL, Hurst D. Orthodontic outcomes assess-
ment using the peer assessment rating index. Angle Orthod
2001;71:164-9.
18. DeGuzman L, Bahiraei D, Vig KW, Vig PS, Weyant RJ, O'Brien K.
The validation of the Peer Assessment Rating index for malocclu-
sion severity and treatment difficulty. Am J Orthod Dentofacial Or-
thop 1995;107:172-6.
19. Mayers M, Firestone AR, Rashid R, Vig KW. Comparison of peer assess-
ment rating (PAR) index scores of plaster and computer-based digital
models. Am J Orthod Dentofacial Orthop 2005;128:431-4.
20. Buschang PH, Shaw SG, Ross M, Crosby D, Campbell PM. Compar-
ative time efficiency of aligner therapy and conventional edgewise
braces. Angle Orthod 2014;84:391-6.
21. Miller KB, McGorray SP, Womack R, Quintero JC, Perelmuter M,
Gibson J, et al. A comparison of treatment impacts between Invis-
align aligner and fixed appliance therapy during the first week of
treatment. Am J Orthod Dentofacial Orthop 2007;131:302.e1-9.
22. Firestone AR, Hasler RU, Ingervall B. Treatment results in dental
school orthodontic patients in 1983 and 1993. Angle Orthod
1999;69:19-26.
23. Kravitz ND, Kusnoto B, BeGole E, Obrez A, Agran B. How well does
Invisalign work? A prospective clinical study evaluating the effi-
cacy of tooth movement with Invisalign. Am J Orthod Dentofacial
Orthop 2009;135:27-35.
24. Boyd RL. Esthetic orthodontic treatment using the Invisalign
appliance for moderate to complex malocclusions. J Dent Educ
2008;72:948-67.
25. Boyd RL,Oh H, Fallah M, VlaskalicV. An update on present and future
considerations of aligners. J Calif Dent Assoc 2006;34:793-805.
26. Simon M, Keilig L, Schwarze J, Jung BA, Bourauel C. Treatment
outcome and efficacy of an aligner technique—regarding incisor
torque, premolar derotation and molar distalization. BMC Oral
Health 2014;14:68.
266 Gu et al
February 2017 Vol 151 Issue 2 American Journal of Orthodontics and Dentofacial Orthopedics