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Interproximal reduction in the
refinement phase of Invisalign treatment:
A quantitative analysis
Tarek Abasseri, Tony Weir, and Maurice J. Meade
Adelaide, South Australia, Australia
Introduction: Interproximal reduction (IPR) is a common adjunct to contemporary orthodontic treatment. This
study aimed to carry out a quantitative analysis of IPR prescribed in the refinement phases of clear aligner ther-
apy with the Invisalign appliance (Align Technology, San Jose, Calif). Methods: The digital treatment plans
(DTPs) of a total of 330 patients treated by 11 orthodontists were evaluated. Relevant data regarding patient
age, gender, and prescription of IPR in the initial and refined DTPs were obtained from Align Technology’s digital
interface, ClinCheck. Computational analyses included descriptive statistics, Mann-Whitney U, and Kruskal-
Wallis tests. Results: Most (n 5182; 75.2%) of the 242 patients who satisfied inclusion criteria were females.
The median (interquartile range [IQR]) age was 29.2 (22.1-40.2) years. More than 60% of the contact sites pre-
scribed IPR related to the initial DTP (n 51312; 60.4%), with 39.6% (n 5859) recorded in the refinement DTPs.
A median (IQR) of 1.1 (0.6-2.1) mm of IPR was prescribed per patient in the initial DTP compared with a median
(IQR) of 0.6 (0.3-1.3) mm in the refinement DTPs. The most common site for prescribed IPR in all DTPs was the
mandibular anterior region. Almost half (n 5108; 44.6%) of the patients were prescribed IPR at the same contact
point site more than once during treatment. Conclusions: Almost 40% of the contact points that were prescribed
IPR were in the patients’refinement DTPs. Most IPR was prescribed for the anterior region of the mandible.
Almost half of the patients had IPR repeatedly prescribed at the same sites during treatment. (Am J Orthod
Dentofacial Orthop 2024;-:---)
Clear aligner therapy (CAT) has become an integral
part of contemporary orthodontics, and its use
has increased over the last 2 decades.
1,2
This is
partly credited to a greater proportion of adult patients
seeking orthodontic treatment and demanding more
comfortable and esthetic alternatives to fixed appli-
ances.
2,3
The popularity of CAT can also be attributed
to the rise of digital dentistry, in which the increased
use of extraoral and intraoral scanners has influenced
the ease and efficiency by which patient data can be ac-
quired and evaluated by the clinician.
4
The Invisalign appliance (Align Technology, San Jose,
Calif) appears to be one of the most commonly pre-
scribed CAT appliances globally.
5-8
Align Technology’s
proprietary software (ClinCheck Pro) is used by the
clinician to formulate a digital treatment plan (DTP),
which enables the manufacture of a series of aligners
intended to address specific treatment objectives.
9
ClinCheck Pro provides numerical data related to
various dental, intraarch, and interarch characteristics,
such as mesiodistal tooth widths. After treatment with
the initial series of aligners, $1 additional or refinement
phase may be needed. This involves the manufacture of
$1 additional series of aligners from $1 new DTP to
obtain the desired treatment outcomes. Several studies
have shown that $1refinement phases are routinely
required during Invisalign CAT.
9-11
Interproximal reduction (IPR) is an adjunctive tech-
nique commonly employed during CAT.
5,11,12
It involves
the permanent removal of proximal enamel from the
contact points between the teeth.
13
Purported reasons
for its use include the need to gain space for the relief
of crowding and tooth reshaping in addition to the man-
agement of open gingival embrasures and tooth size
From the Orthodontic Unit, Adelaide Dental School, Adelaide Health and Medical
Sciences Building, University of Adelaide, Adelaide, South Australia, Australia.
All authors have completed and submitted the ICMJE Form for Disclosure of Po-
tential Conflicts of Interest, and none were reported.
This work received no funding.
Address correspondence to: Maurice J. Meade, Orthodontic Unit, Adelaide Dental
School, Adelaide Health and Medical Sciences Building, University of Adelaide, 4
North Terrace, Adelaide, South Australia, 5000, Australia; e-mail, maurice.
meade@adelaide.edu.au.
Submitted, November 2023; revised and accepted, February 2024.
0889-5406
Ó2024 by the American Association of Orthodontists. This is an open access
article under the CC BY-NC-ND license (http://creativecommons.org/licenses/
by-nc-nd/4.0/).
https://doi.org/10.1016/j.ajodo.2024.02.005
1
ORIGINAL ARTICLE
discrepancies.
14-19
IPR has also been used as an adjunct
to increase the stability of incisor alignment after
orthodontic treatment.
20
Current evidence suggests
that up to half of the enamel thickness may be removed
without adverse side effects. This equates to approxi-
mately 0.3 mm for each single maxillary incisor inter-
proximal contact point or surface, 0.2 mm for each
single mandibular incisor interproximal contact point
or surface, and 0.6 mm for a single contact surface of
a posterior tooth.
21
The ClinCheck Pro software can be used to identify
relevant sites and time and quantify the amount of
IPR required in the DTP.
11
The IPR recorded within
the software is for each contact area and not for the in-
dividual teeth associated with each contact area. A
2022 study reported that IPR was prescribed in 71%
of 500 patients in the initial DTP of Invisalign treat-
ment.
14
Furthermore, respondents in a recent survey
of orthodontists reported that they routinely prescribed
IPR in a mean of 55% of their annual caseload in the
initial accepted DTP and 31% of their annual caseload
in the additional refinement plans.
5
However, the evi-
dence also indicated that the quantity of IPR carried
out is less than that prescribed in the initial DTP for
treatment with the Invisalign appliance, with less
than half of the planned amount of IPR being carried
out.
14,16,17
This may be of clinical relevance, as short-
falls in this regard may have a deleterious impact
on treatment efficacy and patient and clinician
satisfaction.
To date, research regarding IPR in the refinement
phases of CAT is limited. One recent prospective study
evaluated the accuracy of IPR with the Invisalign appli-
ance, but direct evaluation of the refinement phase was
lacking.
18
This study aimed to quantitatively analyze the
IPR prescribed in the refinement phases of CAT with the
Invisalign appliance. A secondary aim was to compare
the characteristics of IPR prescribed in the refinement
phase with those in the initial phase.
MATERIAL AND METHODS
Institutional ethical approval was granted by the Uni-
versity of Adelaide Human Research Ethics Committee.
The study used data from the Australasian Aligner
Research Database (AARD). As of October 2023, the
AARD contained the relevant Invisalign treatment infor-
mation related to approximately 17,000 patients treated
by 17 orthodontists between 2013 and 2023. At the time
of data acquisition, the database included patients from
11 orthodontists.
Before commencing their Invisalign treatment, all
patients had provided consent for their records to be
used for research purposes. The orthodontists are
required to make the records of all patients treated
with the Invisalign appliance available to AARD.
The last 30 consecutive patients treated to comple-
tion by each of the orthodontists with the appliance
were selected. After selection, the patients were screened
with reference to the selection criteria. The inclusion
criteria entailed: (1) patients aged $18 years undergoing
nonextraction, dual arch CAT with the Invisalign appli-
ance; (2) patients who required $1refinement phase;
(3) the availability of ClinCheck Pro files after completing
the initial phase, and the completion of the refinement
phase; (4) IPR prescribed after the initial phase, to have
been completed by the end of the refinement phase;
(5) minimum 1-mm IPR in total per patient; and (6) min-
imum of 2 sites for IPR per patient.
Patients with missing teeth and IPR prescribed to the
second and third molars only were excluded from the
study.
The age, gender, and number of refinement phases of
patients satisfying inclusion and exclusion criteria were
recorded. The resultant patient files were opened with
the ClinCheck Pro facility, and millimeter values for pre-
scribed IPR between tooth contacts were recorded.
The prescribed IPR sites were recorded and grouped
according to the following DTP categories: initial DTP
(I), first refinement DTP (R1), second refinement DTP
(R2), third refinement DTP (R3), fourth refinement
DTP (R4), combined refinement DTPs (R1-4), and total
(I 1R1-4).
The categories were classified into the following sub-
groups: maxillary (anterior and posterior), mandibular
(anterior and posterior), anterior (incisors and canines),
posterior (premolars and first molars), maxillary anterior,
maxillary posterior, mandibular anterior, and mandib-
ular posterior.
The contact area between the first premolar and
canine was classified as a posterior contact. Conse-
quently, the anterior subgroup contained 5 contact
points, and the posterior subgroup contained 6 contact
points. All data were entered into a Microsoft Excel
spreadsheet (Microsoft, Redmond, Wash).
Statistical analyses
Statistical analysis was conducted via SPSS software
(version 29; IBM, Armonk, NY). The significance was
set to P\0.05. The Shapiro-Wilk test was performed
to determine the normality of the distribution of the
data. The test indicated that the results did not follow
a normal distribution. Frequencies were presented in
medians and percentages. The Mann-Whitney U test
and the Kruskal-Wallis H test were carried out to
2Abasseri, Weir, and Meade
-2024 Vol -Issue -American Journal of Orthodontics and Dentofacial Orthopedics
determine whether the differences between the medians
of groups and subgroups were significant.
RESULTS
A total of 242 (73.3%) patients were evaluated after
the exclusion of 88 patients who did not satisfy the se-
lection criteria. The sample comprised 182 (75.2%) fe-
males and 60 (24.8%) males. The median (interquartile
range [IQR]) age for the overall sample was 29.2 (22.1-
40.2) years, with no significant differences in the median
ages of males and females (P.0.05).
More than 60% of the contact sites prescribed IPR
related to the initial DTP (n 51312; 60.4%), with
39.6% (n 5859) recorded in the refinement DTPs.
Table I outlines the location of IPR sites according to
the DTP in which the IPR was prescribed. IPR was pre-
scribed for a total of 2171 contact point sites. The
maximum number of refinement DTPs requiring IPR
was 4. Overall, 15 patients required 4 refinement DTPs,
and 22 patients required 3 refinement DTPs.
Table II shows that IPR was prescribed at a median
(IQR) of 4 (2-7) sites in each patient in the initial DTP
compared with a median (IQR) of 3 (1-5) sites per patient
in the refinement DTPs.
Figure 1 illustrates the number of prescribed IPR
sites, according to location, in the initial DTP. The site
related to the contact points between the central incisor
and lateral incisor (n 5111 and 27.2% in the maxilla;
n5278 and 30.8% in the mandible) was the individual
site at which IPR was prescribed most frequently.
Figure 2 shows that the contact point between the cen-
tral and lateral incisor was the individual site at which
IPR was prescribed most commonly in the refinement
of DTPs in the maxilla (n 597; 30.3%). The contact
point between the central incisors was the individual
site in which IPR was prescribed most commonly in the
mandible (n 597; 30.3%) in the refinement DTPs.
The most common site for IPR across all phases of
treatment was the mandibular anterior zone, with a pre-
scription rate of 50.1% (n 5657) from the initial DTP
and 49.36% (n 5424) from the refinement DTPs.
Most of the IPR prescribed in the refinement DTPs was
related to the R1 (n 5600; 27.6%) and the R2 DTPs
(n 5223; 10.3%). Figure 3 shows that minimal amounts
were performed in the R3 (n 532; 1.5%) and R4 DTPs
(n 54; 0.2%).
Table III shows that there was a median (IQR) of 1.1
(0.6-2.1) mm of IPR prescribed per patient in the initial
DTP compared with a median (IQR) of 0.6 (0.3-1.3) mm
in the refinement DTPs.
Table IV shows that a median of 0.3 mm of IPR was
prescribed per contact point in the initial DTP. The
Kruskal-Wallis test indicated that there was significantly
more IPR prescribed per contact point in the initial DTP
compared with each of the refinement DTPs (P\0.001).
Almost half (n 5108; 44.6%) of the patients were
prescribed IPR at the same contact point site more
than once.
Figure 4 shows that IPR was prescribed in 3 DTPs be-
tween the central incisors in the maxilla 9 times, whereas
Table I. Location of IPR sites according to DTP (n 5242)
Group Total
DTP
Initial R1 R2 R3 R4 R1-4
Mx 1Md 2171 (100) 1312 (60.4) 600 (27.6) 223 (10.3) 32 (1.5) 4 (0.2) 859 (39.6)
Mx 728 (100) 408 (56.1) 214 (29.4) 96 (13.2) 10 (1.4) 0 (0.0) 320 (44.0)
Md 1443 (100) 904 (62.7) 386 (26.8) 127 (8.8) 22 (1.5) 4 (0.3) 539 (37.4)
Mx 1Md anterior 1608 (100) 941 (58.5) 471 (29.3) 170 (10.6) 22 (1.4) 4 (0.3) 667 (41.5)
Mx 1Md posterior 563 (100) 371 (65.9) 129 (22.9) 53 (9.4) 10 (1.8) 0 (0.0) 192 (34.1)
Mx anterior 527 (100) 284 (5.0) 170 (32.3) 68 (12.9) 5 (1.0) 0 (0.0) 243 (46.1)
Mx posterior 201 (100) 124 (61.7) 44 (21.9) 28 (13.9) 5 (2.5) 0 (0.0) 77 (38.3)
Md anterior 1081 (100) 657 (60.8) 301 (27.9) 102 (9.4) 17 (1.6) 4 (0.3) 424 (39.2)
Md posterior 362 (100) 247 (68.2) 85 (23.5) 25 (6.9) 5 (1.4) 0 (0.0) 115 (31.8)
Note. Values are presented as n (%).
Md, mandibular; Mx, maxillary.
Table II. Number of sites prescribed per patient in the
initial and refinement DTPs
DTP Median (IQR)
Initial
Maxillary 3 (1-5)
Mandibular 5 (3-7)
Maxillary 1mandibular 4 (2-7)
Refinement
Maxillary 2 (1-4)
Mandibular 4 (2-7)
Maxillary 1mandibular 3 (1-5)
Note. Values are presented as median (IQR).
Abasseri, Weir, and Meade 3
American Journal of Orthodontics and Dentofacial Orthopedics -2024 Vol -Issue -
Figure 5 shows that 3 patients had IPR prescribed be-
tween the central incisors at the beginning of 4 accepted
DTPs.
DISCUSSION
This study is the first to investigate IPR prescription in
the refinement of DTPs of CAT with the Invisalign appli-
ance. The findings indicated that almost 74% of the
sample were prescribed IPR in the initial and refinement
DTPs and that almost 40% of the sites prescribed IPR
occurred in the refinement DTPs. In addition, the find-
ings demonstrated that IPR was most frequently
prescribed in the anterior region of the mandible in the
initial and refinement DTPs and that almost half of the
patients were prescribed IPR at the same contact site
more than once.
The lack of research regarding IPR in the refinement
phases of CAT limits direct comparison with other
studies. Therefore, a large number of patients was cho-
sen, as there was little relevant information to compute a
number to power the investigation. However, analysis of
our findings in relation to studies evaluating IPR in the
initial phase of CAT is likely to provide valuable insight.
The 300 patients initially evaluated in this study
compared with 30-500 patients evaluated in similar
Fig 1. Number of prescribed IPR sites, according to location, in the initial DTP.
Fig 2. Number of prescribed IPR sites, according to location, in the refinement DTPs.
4Abasseri, Weir, and Meade
-2024 Vol -Issue -American Journal of Orthodontics and Dentofacial Orthopedics
investigations.
11,14,16-19
The median age of the patients
was 29.2 years, which was similar to the 28.5-31.4 years
recorded in corresponding studies.
9,11,18
In addition, this
study investigated patients treated by 11 orthodontists,
which compared with 1-10 in the other evalua-
tions.
14,16,18,19
More IPR was prescribed in the initial DTP than in the
refinement DTPs. This was similar to the findings of a
2022 survey of orthodontists in which respondents re-
ported that they prescribed IPR more frequently in the
initial DTP compared with in the refinement DTPs.
5
In
addition, virtually all of the prescribed IPR occurred by
the third refinement DTP—the timepoint, according to
a 2022 study, in CAT with the Invisalign appliance, at
which no further improvement in treatment outcomes
can be expected.
10
IPR was prescribed at a median of 3 sites in the
maxilla and mandible per patient in the refinement
DTPs. This compared with a median of 4 sites in the
initial DTP of the present study and 6.92 in the initial
DTP reported in a study by Weir et al.
14
The median
amount of prescribed IPR in the refinement DTPs of
each patient was 0.6 mm, which was less than the me-
dian of 1.1 mm in the initial DTP of this study. It also
compared with a mean of 0.28-2.16 mm of IPR pre-
scribed in the initial DTPs in similar investiga-
tions.
14,17,19
Prescribed IPR in the refinement DTPs was compara-
tively more common in the mandible than in the maxilla.
This was similar to the findings related to the initial DTP
of this study and other similar research.
14,16
Moreover, a
greater amount of IPR was prescribed in the mandibular
anterior zone, and this was in accordance with the find-
ings from other investigations.
14,15,17
It also aligned
with the findings from a survey of orthodontists in the
Republic of Ireland, in which respondents reported that
the lower labial segment was the area in which IPR
was most commonly performed.
12
This might be due
to the greater prevalence of tooth size discrepancies
observed in the lower anterior region compared with in
the corresponding teeth in the maxilla.
14,22
It might
also have reflected the need to address the open gingival
embrasures that may emerge as previously imbricated
Fig 3. Frequency of IPR prescription according to DTP.
Table III. Prescribed IPR (mm) per patient in the initial
and refinement DTPs
DTP Median (IQR)
Initial
Maxillary 0.8 (0.4-1.5)
Mandibular 1.5 (0.9-2.3)
Maxillary 1mandibular 1.1 (0.6-2.1)
Refinement
Maxillary 0.4 (0.3-1.0)
Mandibular 1.0 (0.4-1.6)
Maxillary 1mandibular 0.6 (0.3-1.3)
Note. Values are presented as median (IQR).
Table IV. IPR (mm) prescribed per contact point ac-
cording to DTP
DTP Median (IQR) Maximum Minimum
Initial 0.3 (0.2-0.4) 0.1 1.1
R1 0.2 (0.2-0.3) 0.5 0.1
R2 0.2 (0.2-0.3) 0.5 0.1
R3 0.2 (0.2-0.4) 0.2 0.4
R4 0.2 (0.2-0.2) 0.2 0.2
Abasseri, Weir, and Meade 5
American Journal of Orthodontics and Dentofacial Orthopedics -2024 Vol -Issue -
teeth gradually align during treatment. In addition, it
might reflect an approach of waiting for greater align-
ment of the teeth before commencing IPR.
23
Several studies have indicated that IPR routinely
underperformed compared with that prescribed in the
initial DTP.
14-18
This might have been a factor in this
study. The amount of IPR performed by the clinicians
might have been less than that prescribed by them in
the initial DTP and in previous refinement DTPs. One
of the purported reasons for the underperformance of
IPR is the choice of instrument to carry out the
procedure. Lagan
aetal
19
contended that it was the
use of mechanical oscillation that enabled the amount
of IPR performed to be matched by the actual amount
carried out in their study. However, a 2013 in vitro
study
24
indicated that mechanical oscillation was not
superior. Recent surveys,
5,11,12
in any case, have sug-
gested that mechanical oscillation is rarely used by or-
thodontists.
Further evidence that supports the hypothesis that
underperformance of IPR prescribed in previous DTPs
results in the requirement for further prescribed IPR is
Fig 4. Distribution of frequency of repeated IPR prescriptions among patients in the maxilla.
Fig 5. Distribution of frequency of repeated IPR prescriptions among patients in the mandible. FDI,
F
ed
eration Dentaire Internationale.
6Abasseri, Weir, and Meade
-2024 Vol -Issue -American Journal of Orthodontics and Dentofacial Orthopedics
apparent from the findings of this investigation. The
procedure was prescribed more than once at the same
site in almost half of the surveyed patients. If the cumu-
lative amount of prescribed IPR was accurately per-
formed in many of the patients in this study, it might
have surpassed the suggested relatively small maximum
amounts of IPR before the procedure results in irrevers-
ible iatrogenic harm.
23,25
All retrospective investigations are at a high risk of
selection bias. To minimize the risk in this study, the
last 30 patients who had completed treatment by all or-
thodontists contributing to AARD were evaluated. In
addition, it was not known what instrument was used
by the treating clinician and the reason why IPR was pre-
scribed. Furthermore, the prescribed amount of IPR was
presented per contact point within the ClinCheck Pro
interface. An assumption might be made that the IPR
was equally divided between the 2 teeth adjoining the
contact point. However, this cannot be concluded with
certainty, as different methods for IPR can result in un-
equal pressures between the teeth, resulting in an un-
equal distribution of IPR. This might have contributed
to the need for additional refinements, as the imbalance
of IPR might predispose to the development of open
gingival embrasures.
Future randomized and prospective research should
include a detailed determination of whether the repeated
prescribed IPR in this study is replicated more widely. In-
vestigations should also aim to ascertain what factors in-
fluence orthodontists’decision-making processes
regarding the use and timing of IPR.
The findings of this investigation are of clinical rele-
vance. Although the quantities of prescribed enamel
removal are often small, the treatment is not without
risk. Careful consideration of the timing, quantity,
and location of IPR is essential to optimize treatment
efficacy and to ensure patient and clinician satisfaction
with treatment outcomes. The findings provide
valuable information to clinicians and patients
regarding the use of IPR in CAT with the Invisalign
appliance. In addition, this study contributed baseline
information for further relevant investigations by
researchers.
CONCLUSIONS
The present study was the first to quantitatively
assess the IPR prescribed in the refinement phases of
CAT with the Invisalign appliance. Almost 40% of the
contact points that were prescribed IPR were in the pa-
tients’refinement DTPs. Most of the IPR was prescribed
for the anterior region of the mandible. Almost half of
the patients had IPR repeatedly prescribed at the same
sites during treatment, with some having IPR prescribed
at the site up to 4 times during treatment.
ACKNOWLEDGMENTS
The authors thank the orthodontists who contributed
their patient records to the Australasian Aligner Research
Database.
AUTHOR CREDIT STATEMENT
Tarek Abasseri contributed to formal analysis, inves-
tigation, methodology, software, validation, visualiza-
tion, original draft preparation, and manuscript review
and editing; Tony Weir contributed to conceptualiza-
tion, data acquisition, data curation, formal analysis,
methodology, software, validation, visualization, orig-
inal draft preparation, manuscript review and editing,
and supervision; and Maurice J. Meade contributed to
conceptualization, formal analysis, methodology, proj-
ect administration, resources, software, validation, visu-
alization, original draft preparation, manuscript review
and editing, and supervision.
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