Vertical skeletal changes after extraction and non-extraction treatment in matched class I patients identified by a discriminant analysis: cephalometric appraisal and Procrustes superimposition
Beit et al. Progress in Orthodontics
Vertical skeletal changes after extraction and non-extraction treatment in matched class I patients identified by a discriminant analysis: cephalometric appraisal and Procrustes superimposition
Philipp Beit 0 2
Dimitrios Konstantonis 0 1 2
Alexandros Papagiannis 1
Theodore Eliades 0 2
0 Clinic of Orthodontics and Pediatric Dentistry, Center of Dental Medicine, Faculty of Medicine, University of Zurich , Zürich , Switzerland
1 Department of Orthodontics, School of Dentistry, National and Kapodistrian University of Athens , Athens , Greece
2 Clinic of Orthodontics and Pediatric Dentistry, Center of Dental Medicine, Faculty of Medicine, University of Zurich , Zürich , Switzerland
Background: In the long-lasting debate of extraction versus non-extraction treatment, the impact of extractions on the skeletal vertical dimension remains rather unclear. The aim of this retrospective research study was to obtain a bias-free sample of morphologically similar borderline patients treated with or without extraction of the four first premolars and to retrospectively evaluate the vertical changes that occurred. Methods: A borderline sample of 83 patients, 41 treated with four first premolar extractions and 42 treated without, was obtained by means of discriminant analysis applied to a previously investigated parent sample of 542 class I patients. The pretreatment and posttreatment cephalometric radiographs were analyzed digitally, and seven measurements were assessed for vertical skeletal changes. Also, average tracings between the two treatment groups were evaluated using the Procrustes superimposition method. Results: The variables of SN to Go-Gn and Y-axis showed adjusted intergroup differences of − 0.91° and − 1.11° (P = 0.04). Comparing the mean intra-group differences of all the variables simultaneously, a significant difference was found between the two treatment groups (overall P value = 0.04). In the extraction group, only the gonial angle showed a significant decrease (P = 0.01) while the overall P value evaluating the intra-group differences between pre- and posttreatment was significant (overall P value < 0.01). In the non-extraction group, the variable of N-ANS/N-Me showed a significant decrease (P = 0.02) and the overall P value evaluating the intra-group differences between pre- and posttreatment was also significant (overall P value < 0.01). Differences in treatment duration were assessed using a log-normal model and showed that extraction treatment lasted significantly longer than non-extraction treatment (P < 0.01). Conclusions: The borderline group of patients identified by the discriminant analysis exhibited similar morphological characteristics at treatment's onset; therefore, the posttreatment changes could safely be attributed to the choice of extraction or non-extraction treatment and not to pre-existing differences. Treatment choice had an impact on the patients' vertical skeletal dimensions. Patients treated with four first premolar extractions showed a slight decrease in the vertical skeletal measurements, whereas non-extraction patient treatment showed a slight increase. The treatment time was also significantly higher in the extraction group.
Extractions; Vertical dimension; Class I; Discriminant analysis; Treatment time
Extractions are routinely implemented in orthodontics
mainly to address crowding and reduce dentoalveolar
protrusion. While the impact of extraction and
nonextraction treatment on the soft tissue response, smile
esthetics, stability, and other parameters has been
extensively investigated [
], the literature is rather
inconclusive with regard to the impact of the extractions on
the vertical dimension.
Control of the vertical dimension during orthodontic
treatment is challenging [
]. Still, the open bite
manifestations can range from a variety of skeletal, dental, and
functional features including increased mandibular plane
and/or gonial angle, altered anterior or posterior facial
heights, weak orofacial musculature, inadequate lip seal,
and anterior tongue position or thrust [
treatment techniques or extraction patterns have been
suggested to address the aforementioned discrepancies
]. Often, orthodontists tend to extract in patients
with increased anterior facial height. According to the
wedge hypothesis, the extraction of four premolars or
molars and the subsequent protraction of the posterior teeth
lead to a counterclockwise rotation of the mandible, thus
maintaining or increasing the overbite [
8, 10, 11
Although this theory is quite popular, it is not
evidencebased according to relative investigations .
Contradictory reports in the literature fail to reach a consensus
whether or not extractions have a definite effect on the
vertical dimension [
On a research note, often in the literature, the
matching process in retrospective surveys is rather inadequate
since the compared groups are not morphologically
8, 15, 21, 22
]. The improper matching of the
groups inevitably introduces susceptibility bias, which is
defined as the difference in prognostic expectations due
to pre-existing differences between and/or among
treatment groups. When contemplating between
differentmutually exclusive-treatment approaches or techniques,
a clinician takes into careful consideration the patient’s
morphological features. These features usually include
cephalometric and model measurements along with
other parameters like patient’s age and sex. In statistics,
such patient’s features that lead a clinician to a specific
treatment decision are called confounding variables.
Discriminant analysis is a statistical multivariate
technique that deals simultaneously with a large number of
confounding variables. In contemporary orthodontic
research, discriminant analysis has been used in order to
identify homogenous samples that cannot be
discriminated with regard to a specific treatment modality. The
homogeneity of the groups regarding the variables
included in the discriminant analysis can ensure that all
patients are borderline and equally susceptible to
alternative treatments. As shown in the literature, borderline
samples are ideal for various posttreatment comparisons
It was therefore the aim of the present retrospective
research study to evaluate a bias-free sample of borderline
patients treated with or without four first premolar
extractions and to assess the vertical skeletal changes occurred.
To overcome common methodological errors seen in
orthodontic literature and to eliminate susceptibility
bias, it was decided to obtain a borderline sample in
regard to extractions that derived from a large parent
sample of class I patients of a previous investigation
]. The parent sample consisted of 542 randomly
selected subjects, treated at the graduate Orthodontic
Clinic of the School of Dentistry of the National and
Kapodistrian University of Athens, Greece and in five
different private orthodontic offices in Athens, Greece.
The decision to select patients especially from a
university clinic where patients were treated by a numerous
residents and clinical instructors and from different
private clinical settings was made in order to eliminate the
possibility of selection and proficiency bias.
All patients were Caucasian male or female with a
class I dental and skeletal malocclusion, no transverse
discrepancies, and a full complement of teeth excluding
the third molars. They had no history of clefts or any
other dentofacial deformities, and they never had
received any previous orthodontic treatment or
orthognathic surgery. Still, the patients included did not
present with extensively decayed teeth that could
influence the clinician towards extraction, and according to
the charts, when extractions were decided, it was solely
for orthodontic purposes. Out of the 542 patients, of
which 331 were female and 211 male, 427 were treated
by non-extraction and 153 with extraction of the four
first premolars. The parent sample was collected in
2013, while the identification of the borderline sample
for the present investigation took place in 2017.
All patients received orthodontic treatment with
preadjusted edgewise appliances in both arches and had a
complete set of diagnostic records including initial and
final lateral cephalometric and panoramic radiographs
along with dental casts and detailed treatment charts.
Neither extra- nor intra-oral appliances or temporary
anchorage devices were used during treatment. However, in
regard to treatment mechanics in the extraction cases and
according to patients’ charts, after crowding was addressed
by the retraction of the anterior teeth, the implemented
biomechanics aimed at closure of the remaining spaces by
protraction of the posterior teeth. All cephalograms were
taken in the natural head position and were traced and
analyzed using ViewBox 184.108.40.206. The research protocol was
approved by the Ethics and Research Committee of the
National and Kapodistrian University of Athens, Greece
The parent sample was then subjected to a stepwise
discriminant analysis, which included all variables that
could possibly influence a clinician’s decision towards
extraction treatment. These variables were 26
cephalometric measurements, six dental cast measurements,
and the demographic variables of age and sex [
Hence, a reliable representation of all of the patient’s
dental, skeletal, and soft tissue traits that could possibly
swing the pendulum towards one of the two possible
treatment modalities was achieved.
Patients were predicted to belong to the extraction or
the non-extraction group according to their discriminant
score. The discriminant score for each patient was the
sum of the multiplication of the discriminating variables
with their standardized canonical discriminant function
coefficients. Subsequently, the discriminant score can be
considered a weighted linear combination (sum) of the
According to the discriminant analysis, each patient
was assigned a discriminant score that ranged from −
3.48 to + 3.07. Patients that received a negative score
were predicted to be treated with four premolar
extractions, whereas patients receiving a positive score
were more likely to be treated by non-extraction
(Fig. 1). The further away a patient’s score was drawn
from 0 (the cutoff point), the more definite the treatment
decision was, thus classifying the patient to either the
“clear-cut” extraction or non-extraction group.
Conversely, patients with discriminant scores around 0
exhibited a significant degree of morphological similarity and
therefore could not be clearly classified to either one of
the two groups.
To assess the impact of the two different treatment
approaches, seven commonly used variables that
measure the skeletal vertical dimension were employed. Of
the seven variables, six describe angles and are shown in
degrees and one variable is a ratio (N-ANS/N-Me). The
orientation of the mandibular plane to the anterior
cranial base was described by the following angles: FMA
(Frankfurt horizontal (FH) plane to mandibular plane
(MP) derived by the line connecting the landmarks
gonion and menton); SN to Go-Gn angle that is formed
at the intersection of the lines passing from the
landmarks Sella to nasion and gonion to gnathion; and
Y-axis which is the angle formed at the intersection of
the line Sella to Gnathion to Frankfurt horizontal plane.
Also, the gonial angle formed by the points
mentongonion-articulare was assessed. The anterior facial height
was further evaluated by the variable of
N-ANS/ANSMe derived by the ratio of the projections of both
measurements to the perpendicular on FH plane.
Additionally, the cant of the palatal plane (PP) in relation to
the cranial base and the mandibular plane was evaluated
utilizing the measurements of palatal plane to SN and to
MP respectively. All cephalometric landmarks, planes,
and lines used can be seen in Fig. 2.
Regarding the sample size calculation, we assumed
1.7° mean difference in FMA measurements between
the two groups, with the standard deviation assumed
to 2.75 in both groups. Setting the significance level
at 5%, to achieve 80% power, 42 individuals were
required in each group.
Descriptive statistics for the pretreatment variables for
both groups, as well as descriptive and inferential statistics
to assess the intra- and intergroup differences in vertical
dimension changes, were performed. Since these variables
were used in the previous discriminant analysis to identify
the borderline cases, baseline differences between the two
groups were quite unlikely. The mean differences from
pretreatment to posttreatment measurements that
occurred in each group were compared through regression
models adjusting for baseline age and sex. Furthermore,
paired t tests were calculated to assess the differences
between the pretreatment and posttreatment
measurements for the two groups separately. The joint significance
of differences was measured using F tests. The significance
level was predetermined at 5%.
All cephalometric tracings were performed by the
principal investigator (XX). Additionally, all tracings were
averaged and then superimposed using generalized
Procrustes superimposition [
]. Procrustes superimposition
was performed on the inter- and intragroup averaged hard
tissue profile. Fifteen skeletal points were used as
reference for the superimposition (A point, B point, sella,
sheno-ethmoidale, nasion, orbitale, porion, basion,
articulare, gonion, antegonial notch, menton, pogonion, anterior
nasal spine, posterior nasal spine). More specifically, in
our study, Procrustes superimposition takes two shapes,
resizes them, and aligns them to minimize the sum of the
squared distance between corresponding cephalometric
points. This is a mathematically defined procedure,
characterized by validity and repeatability of the results [
Additionally, evaluations were performed for both
random and systematic errors of the method. To
evaluate intra-examiner repeatability, with a table of random
numbers, 20 subjects were selected—10 from each
treatment group—and were retraced 3 weeks later by the
same investigator. Also, to assess inter-examiner
agreement, 20 subjects—10 non-extraction and 10
extraction—were randomly selected, and the principal
investigator was evaluated against the second
investigator (XX). The intra-class correlation coefficient (ICC)
based on the variance components from a one-way
analysis of variance was used. All statistical analyses
were performed in STATA (version 13.0; Stata Corp,
College Station, TX).
Finally, the borderline sample was comprised of 83
patients that were chosen around the optimal cutoff
point (0) of the discriminant scores and within one
standard deviation as shown in Fig. 1. Of the patients,
42 that were treated by non-extraction and 41 were
treated with four first premolar extractions. Of the
nonextraction patients, 24 were female and 18 were male;
meanwhile, out of the extraction patients, 23 were
female and 18 were male. The mean age for the
extraction group was 13.71 years (SD 3.28) and for the
nonextraction group 14.62 years (SD 3.84).
Descriptive statistics of the pretreatment variables for
both extraction and non-extraction groups are listed in
Table 1. As expected, the P values (t test for independent
samples) showed no statistically significant differences
between the two groups at treatment’s onset. This
outcome was further confirmed by the P value for
pretreatment differences in all outcomes (P = 0.59).
The intra-group differences between pre- and
posttreatment measurements were also examined through
paired t tests for the two groups separately. In the
extraction group, all variables apart from the palatal plane
to SN (mean difference, 0.38; 95% confidence interval
(CI), − 033, 1.09; P = 0.29) showed a decrease, but
statistical significance was found only for the gonial angle
(mean difference, − 1.06; 95%CI, − 1.80, − 0.32; P = 0.01).
Still, the overall P value evaluating the differences
between pre- and posttreatment in the extraction
patients was significant (overall P value < 0.01).
In the non-extraction group the variables of FMA, SN
to Go-Gn, Y-axis and palatal plane to SN showed an
increase whereas the variables of N-ANS/N-Me, gonial
angle, and palatal plane to MP showed a decrease.
However, of all these variables, only N-ANS/N-Me
showed a statistically significant change (mean
difference, − 0.52; 95%CI, − 0.97, − 0.08; P = 0.02). Yet, when
all measurement differences were simultaneously
evaluated in the non-extraction patients, the overall P value
showed a significant change between pre- and
posttreatment values (overall P value < 0.01). The results of the
intra-group differences can be seen in Table 2.
Differences in treatment duration were assessed using
a log-normal model, which showed that extraction
treatment lasted significantly longer than non-extraction
treatment (P < 0.01) (Table 1).
When we compared the two groups, the differences of
the mean change values for five out seven cephalometric
variables were not statistically significant (Table 2).
Contrariwise, the variables of SN to Go-Gn and the Y-axis
showed adjusted differences of − 0.91 (95%CI, − 1.77, −
0.06; P = 0.04) and − 1.11 (95%CI, − 2.19, 0.03; P = 0.04)
between the two groups. Considering the mean
intragroup differences of all variables simultaneously, the F test
suggested a statistically significant difference between the
two treatment groups (overall P value = 0.04). The
comparisons of the intra-group differences can also be seen in
Table 2. Also, the intra-group trajectories of the extraction
and non-extraction patients are depicted in Fig. 3. Average
tracings and superimpositions between the two treatment
groups can be seen in Figs. 4 and 5.
The results of the evaluations for random and
systematic errors showed excellent agreement: ICC 0.99
(95%CI 0.99–1.00) for intra-examiner agreement and
ICC 0.98 (95%CI 0.98–1.00) for inter-examiner
In this investigation, the use of the discriminant analysis
ensured the morphological homogeneity of the
extraction and non-extraction samples, thus eliminating the
P value evaluating the significance of all differences in the extraction group < 0.01; corresponding P value for the non-extraction group < 0.01
aDifferences between post- and pretreatment measurements
bAdjusted for age and sex differences between the two groups
cTests whether all differences between treatment groups equal zero by using an F test
Gonial angle (Ar-Go-Me)
susceptibility bias commonly seen in orthodontic
retrospective surveys [
8, 15, 19, 28
]. The borderline spectrum
was comprised of patients with similar skeletal and
dental features along with the vertical skeletal
measurements. These statistically “unclassified” borderline
patients with regard to extractions received extraction or
non-extraction treatment that was decided by the
orthodontist that they happened to visit. The effect of the
orthodontic treatment on the vertical dimensions in the
borderline spectrum of patients was then assessed
through a cephalometric analysis which provided an
objective assessment of the vertical skeletal changes that
occurred. Due to the aforementioned methodology, the
treatment outcomes can be safely attributed to the
choice of treatment modality and to not to pre-existing
dental or skeletal differences among patients.
Furthermore, Procrustes superimposition was
conducted so that shape differences could be described.
According to this mathematical model, in order to
compare two configurations, we firstly adjust for size, and
then align them, so that any effect of translation and
rotation are removed. This method exhibits the
advantage over the classical cephalometric approaches
that all points are considered equally significant.
Conventional superimposition planes, such as the Frankfurt
horizontal (FH) plane, or the anterior cranial base plane
(SN), although widely encountered in orthodontic
literature, would imply that two points out of the whole
would be of greater importance over the others.
Additionally, by adjusting for size, pure shape differences
could be observed, not affected by the scale factor.
As shown by the discriminant analysis applied to the
parent sample of 542 class I patients, measurements that
assess the vertical dimension like the Y-axis, FMA,
ANS-Me did not show any discriminating power
between the extraction and non-extraction group of
patients. That occurred despite the fact that these
measurements differed significantly between the two
treatment groups [
]. Subsequently, with regard to the
treatment decision, the vertical variation between
patients was not taken into consideration by the
clinicians upon treatment planning, despite the fact that the
literature reports a possible vertical effect when a case is
treated with extractions [
]. Still, the variable of lower
crowding was the most important variable in deciding
extractions as shown by the magnitude of its
standardized canonical function coefficient (0.728) followed by
the lower lip to E-plane (− 0.407), upper crowding
(0.347), and overjet (− 0.219) [
Our results showed that in regard to the vertical
skeletal measurements, the patients treated with extractions
showed a slight decrease, which bordered at the
traditional 5% level of statistical significance (P = 0.04)
compared to their non-extraction counterparts. Despite
the statistical significance though and because of the
small-scale intergroup differences, it is open to
discussion whether or not the results are clinically significant.
However, of the seven variables, only two (SN to
Go-Gn and Y-axis) differed significantly between the two
groups at the end of treatment, with them being
decreased in the extraction group and increased in the
non-extraction group of patients. Aras [
open-bite cases and reported a significant decrease of
the SN to Go-Gn angle after the extraction of the four
second premolars and also after the extraction of the
four molars but, in disagreement with our findings,
reported a non-significant decrease after extraction of
the four first premolars. Also, in contrast to our findings,
Meral et al. [
] did not observe a significant intergroup
posttreatment difference for the SN to Go-Me angle in
their study. Furthermore, Kumari et al. [
] did not find
any significant mean change difference for the Y-axis
between the extraction and non-extraction cases.
Moreover, the intergroup posttreatment differences for the
Y-axis in Luppanapornlarp et al. [
] investigation can
rather be attributed to the morphologically dissimilar
pretreatment groups rather than to the treatment
modality itself. In regard to the FMA angle, our results are
in agreement with previous investigations that reported
non-significant changes between the two treatment
]. Kocadereli et al.  also reported a
slightly higher but not significant increase of the FMA
and SN to Go-Gn angles in the non-extraction group of
patients. Regarding the slight closure of the palatal plane
to mandibular plane angle, our findings are in agreement
with the findings of Kirschneck et al. [
Intergroup mean tracings were superimposed using
Procrustes superimposition. The Procrustes method was
chosen in order for all landmarks to be treated equally,
and therefore, no points were arbitrarily considered to be
of greater significance when compared to the others.
Furthermore, all tracings superimposed were scaled to a
standard size. As a result, pure shape changes could be
detected, not affected by the influence of size [
regard to the vertical dimension, the average tracings at
the end of treatment showed a slight difference between
the extraction and the non-extraction patients (Fig. 5).
Still, the main changes were localized at the perioral area
and were manifested as retraction of the anterior
dentoalveolar units followed by subsequent retraction of the lips.
In contrast to our results, Garlington et al. [
in the cephalometric superimpositions a forward rotation
of the mandible in 17 out of the 23 cases that were though
treated with extractions of the four second premolars.
In the extraction group, almost all of the vertical
measurements showed a non-significant decrease except
for the gonial angle that was reduced significantly.
Nevertheless, when all measurements were taken into
consideration, the overall intra-group difference between
pre- and posttreatment showed statistical significance. In
regard to the FMA [
8, 15, 19, 30
] and the palatal plane
to MP angle [
14, 17, 29
], our results are in agreement
with those of other authors who also suggested a
decrease after extraction treatment. Also, Kim et al. 
reported a non-significant decrease of the palatal plane
to MP angle after four first premolar extraction
treatment in contrast to four second premolar extraction
treatment that resulted in a significant increase of the
aforementioned plane. Concerning the Y-axis, Kumari et
] showed a significant intra-group increase for the
extraction patients in contrast with our study where a
slight non-significant decrease was found.
In regard to the gonial angle, a similar observation of a
decrease but at a higher value (− 2.5, SD 4.5) was made
by Kirschneck et al. [
] in an extraction group of
patients. Other authors could not confirm any similar
], while most did not assess changes in the gonial
angle. Gonial angle though is an important parameter of
the craniofacial complex giving an indication about the
vertical parameters and symmetry of the facial skeleton.
Any change in the gonial angle might be attributed to
two distinctive causes. Either there is a true morphologic
change in the angle between the ramus and the base of
the mandible or there is a change in the location of the
derived cephalometric landmark of articulare due to
rotation of the mandible. Because of the curvature in the
mandibular condyle, any minute change in the
identification of articulare can have an impact on the gonial
angle. Even though not statistically significant, most
changes for vertical parameters were negative in the
extraction group, thus showing a vertical decrease,
which could lead to the alteration of the gonial angle.
In the non-extraction group, the angular
measurements that assess the vertical skeletal changes in regard
to the orientation of the mandible to the anterior cranial
base showed a non-significant increase, thus being in
accordance with the findings of other authors [
14, 15, 17
The statistical significance of the overall intra-group
differences though shows that the treatment had a definite
impact on the vertical skeletal dimension of the
nonextraction patients by increasing it. In contrast to our
study and other authors, Meral et al. [
] found a
significant decrease in the palatal plane to MP angle (mean
diff. − 1.5, P < 0.05) and in the SN to Go-Me angle (mean
diff. − 1.5; P < 0.01) after a non-extraction therapy. Our
results showed a decrease of − 0.52 for the anterior facial
height ratio (N-ANS/N-Me), therefore differing from the
reports of Kumari et al. [
] who observed a significant
increase (mean diff. 1.1, P = 0.005) and of Sivakumar et
] who also assessed a slight but not significant
increase of 0.08 (P = 0.81) after non-extraction therapy.
The amount of initial crowding should also be taken
into consideration when assessing the posttreatment
vertical skeletal changes since these changes are closely
related to the extent of tooth movement. In studies
assessing the impact of extractions on the vertical
dimension, the initial crowding ranges from slight to
8, 24, 28, 29
], while in most studies, it is not
9, 12, 15, 17, 18, 30, 32
]. Clinically, an
orthodontist initially addresses the anterior crowding by moving
the teeth into the extraction sites and then it is decided
whether or not to close the remaining spaces from the
posterior, the anterior, or reciprocally. In cases with
severe tooth-arch discrepancies, almost the entire
extraction space is used to address the crowding leaving very
little margin of dental maneuvers. As Konstantonis et al.
 showed, crowding is a major factor in the
decisionmaking process when an orthodontist contemplates
between extraction and non-extraction treatment. In this
investigation, the borderline cases presented with
similar amounts of crowding: − 2.51 and − 2.93 mm
(P value = 0.448) of maxillary crowding and − 4.95
and − 5.37 mm (P value = 0.164) of mandibular
crowding for the non-extraction and the extraction
cases respectively (data not shown). The
aforementioned amounts of crowding in the extraction cases
allows the implementation of the desired
biomechanics with regard to treatment goals in which vertical
control plays a major role. Actually, more side effects
are expected in borderline cases with a mild to
moderate crowding rather than in clear-cut extraction
cases with severe crowding.
Treatment duration varied significantly between the two
groups in the present study. Extraction treatment lasted
2.79 years, whereas non-extraction treatment lasted
1.8 years. The four premolar extraction treatments lasted
an average of 1 year or 55% longer than non-extraction
treatments. Kim et al. [
] reported a mean treatment time
of 2.3 years for extraction therapy with four first
premolars; however, there was no control group treated without
extractions. Longer treatment time for extraction
therapies is also in concordance with the conclusions of Maveras
et al. [
], but nevertheless, other factors like the
implemented biomechanics, the operator’s experience, and
patient’s compliance might have an additional impact on
the treatment’s duration [
The main limitations of the present investigation are
due to its retrospective nature. To overcome this, a large
parent sample obtained from a previous study was used
and a discriminant analysis was performed to reduce
selection and susceptibility bias. A randomized control
study would be ideal for such posttreatment comparisons,
but randomization with regard to extractions is neither
easy nor easily ethically justifiable. Furthermore, the study
evaluated only class I extraction and non-extraction
patients with a mean FMA of 28.37° (SD 3.96) and 26.85°
(SD 5.86) respectively. Hence, no conclusion can be made
about high-angle patients, although in these patients
vertical control would be especially desirable. As in all
cephalometric studies, landmark identification error is also a
concern. Therefore, intra- and inter-examiner reliability
was evaluated for both random and systematic errors. Still,
since the majority of the patients were adolescents,
different patterns of growth and variation between males and
females should also be considered [
Since stability of the results achieved is a major goal
for every orthodontist, long-term comparisons between
the extraction and non-extraction treatment groups
should also be considered. Extractions have an impact
on the vertical skeletal dimension as shown in the
present research study, but the possibility of relapse to
the original dimension remains an issue to be clarified.
Discriminant analysis was successful in identifying a
group of morphologically similar patients, which were a
borderline in regard to extractions. The choice of
extraction or non-extraction treatment had an impact on the
patients’ vertical skeletal dimensions. The overall
difference between the two groups was significant with the
extraction patients exhibiting a decrease in the vertical
dimension when compared to the non-extraction
patients. Patients treated with extractions of the four first
premolars showed a slight decrease in the skeletal
vertical measurements, whereas patients who received
nonextraction treatment showed a slight increase. However,
due to the small-scale intergroup differences, the results
might be of little clinical significance. Still, treatment
time was significantly higher in the extraction than in
the non-extraction group. Further studies are needed to
investigate the stability of the results achieved.
PB participated in its design and coordination, and acquired data and wrote
the manuscript. DK was responsible for acquisition of data and proofread the
article. AP performed the statistical evaluation and participated in the design
of the study. TE conceived the objectives of the study, participated in data
acquisition and helped in the interpretation of the results. All authors read
and approved the final manuscript.
Ethics approval and consent to participate
An ethics and research committee approval from the Committee of Ethics of
the Dental School of the National and Kapodistrian University of Athens in
Athens, Greece, was obtained (ref. 311/21.09.2016) on October 24, 2016.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
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