This study aimed to determine the factors influencing the lower lip profile
during Class III lower total arch distalization camouflage treatment.
The cohort included 45 patients diagnosed with skeletal class III
malocclusion. These patients underwent camouflage orthodontic treatment with
lower arch total distalization. Lateral cephalograms were taken at the
pretreatment and posttreatment stages. Cephalometric measurements were
analyzed to unravel the influencing factors of the lower lip profile. This
study defined new variables, i.e., CK and CKA, for detailed
soft tissue analysis.
Novel soft tissue measurements presented in this study showed statistically
significant changes after treatment. The CK angle and L1 to A Pog, L1 to NB,
and IMPA were statistically correlated. Simple regression analysis was
performed between variables. The formula was ΔCKA = 0.79(Δ L1
to NB) -1.6, and multiple regression resulted in ΔCKA = 0.5(Δ
L1 to NB) – 0.42 (‘LL to Eline’, before treatment).
The present study provided evidence that the lower lip evaluations, including
the novel CKA and CKHA, were significantly related to the mandibular incisor
position during total distalization of the mandibular dentition. The results
demonstrated that clinically significant lip retraction could be obtained.
Therefore, non-extraction with distalization must be suggested as the first
treatment option in mild skeletal class III malocclusion patients.
Keywords: Skeletal class III, Lower arch, Total distalization, Camouflage, Treatment, Lower lip response, CKA, CKHA.
The standard for orthodontic treatment is shifting from a hard tissue-based treatment
to a soft tissue-based treatment paradigm .
To establish a standard for soft tissue-based orthodontic treatment, several
researchers have proposed the ideal anterior and posterior positions of the soft
tissue of the face [2, 3]. In addition, Holdaway defined the harmony line (H-line) and
H-angle based on the position of the upper lip relative to the soft tissue Na-Pog line and set the standard for the upper
aesthetic value. The norm was 7-8 degrees .
A class III malocclusion is a skeletal disharmony in which the growth of the
mandible is relatively more dominant than that of the maxilla. In addition, the
incidence of class III malocclusion is much higher in Asians than in Westerners . Compensation treatment is known as one of
the representative treatment methods, along with orthognathic surgery for patients
with skeletal class III malocclusion . It
is known that a good soft tissue position can be obtained through compensation
treatment in patients with skeletal Class III malocclusion . However, unlike orthognathic surgery, compensation treatment
may cause an excessive labial tilting of the maxillary incisors and lingual tilting
of the mandibular incisors [7-9].
In many cases, mandibular extraction is necessary as part of the treatment plan to
obtain an appropriate occlusion for a class III malocclusion. Recently, as an
alternative to mandibular premolar extraction, class III malocclusion compensation
treatment has been actively performed with total distalization of the mandibular
arch using a TAD [10-13] (Fig. 1). Although such
rearward movement of the mandibular arch has the advantage of being a possible
treatment within the anatomical limitations, there has been no study that has
determined whether the change in the anterior and posterior soft tissue position
will be appropriately esthetic in the soft tissue-centered treatment planning
paradigm described above. Excessive retraction of the lower lip also has the side
effect of making the protrusion of the mandible more clearly visible. In evaluating
class III malocclusions before and after treatment, the protrusion of the lower lip
and the ratio of the upper lip should also be assessed.
The purpose of the present study was to evaluate the soft tissue index changes before
and after treatment in patients who underwent class III malocclusion compensation
treatment through total mandibular dentition posterior movement. Primarily, this
study introduced new variables, CK angle (CKA) and CKH angle (CKHA), for detailed
soft tissue analysis and aimed to analyze whether there is a correlation between the
amount of lower incisor lingual movement and retraction of the lower lip employing
known indicators and new CKA and CKHA. In addition, the usefulness of these two new
variables for diagnosis and comparison before and after treatment was also
2. MATERIALS AND METHODS
2.1. Study Sample
Among the patients who visited the Department of Orthodontics, Institute of Oral
Health Science, Ajou University School of Medicine, Suwon, South Korea, and
received orthodontic treatment, 45 patients (average age, 26.8 years; 19-40
years old) who met the inclusion criteria of this study and did not meet the
exclusion criteria were evaluated before treatment. Twenty-six females and
nineteen males were selected.
Posterior lateral head radiographs were obtained.
The inclusion criteria were as follows:
1) Patients who were diagnosed with a skeletal class III malocclusion (ANB <
2) Patients who had received class III malocclusion compensation treatment other
than jaw surgery.
The exclusion criteria were as follows:
1) Patients with a history of orthodontic treatment,
2) Patients with impacted teeth,
3) Patients with a history of head and face trauma,
4) Patients with systemic disease that could affect tooth movements, such as
clavicle dysplasia and cleft palate,
5) Patients with previous orthognathic surgery and anterior segmental osteotomy
All patients were treated by the same orthodontist. The 0.022-slot MBT
prescription bracket was used, mostly followed by a 0.016 nickel titanium, 0.018
×0.025 Bioforce, and a 0.019 ×0.025 stainless steel wire
sequentially. Temporary Anchorage Devices (TADs) were used in the posterior
buccal shelf or retromolar area for posterior movement of the whole mandibular
dentition. This study was conducted with permission and approval from the
Institutional Review Board of Ajou University Hospital (IRB no.:
2.2. Cephalometric Measurement
The obtained radiographs showed the patient in a natural head posture, and all
radiographs were measured and analyzed by a researcher (BJ Kim) using V-ceph
22.214.171.124 (Osstem, Seoul, Korea) software. To verify the measurement error of the
operator, all lateral radiographs were repeatedly measured at 4-week intervals,
and the absence of errors was verified using Dahlberg’s formula .
Fourteen landmarks and seven reference lines were set for evaluation of the
anterior-posterior position of the face before and after compensation treatment
(Figs. 2 and 3). In this
study, one line and two angles obtained were presented for the evaluation of the
position of the lower lip at the initial visit and the position before and after
treatment. The linear measurements are illustrated in Figs. (4 and 5)
demonstrates the new lines and angles. In this study, the CK line was defined as
a line connecting the soft tissue pogonion and the lower lip anterior (LLA) as a
newly defined reference line. The angle between the CK line and the facial plane
is defined as the CK angle (CKA) A, and the difference between the H-angle and
the CK line is defined as the CKH angle (CKHA). A more protruded lower lip
relative to the upper lip results in high negative values for the CKHA, which is
similar to the ANB.
2.3. Sample Size Determination
Power analyses were performed using mean and standard deviation (SD) values of
the angulation change of the lower incisor to the occlusal plane (mean,
-2.8° and SD 3.6°) .
The R program (ver. 4.0.3, Vienna, Austria) suggested 16 as the minimum sample
size with power = 0.9.
2.4. Statistical Analysis
The SPSS (IBM SPSS Statics Version 25) and R (4.0.3 version) programs were used
for the statistical analysis of the measured values. The Shapiro–Wilk
test was conducted and presented with the normality of data distribution of each
variable. Paired t-tests with significance levels of 0.05 and
0.01 were performed to check whether the measured values changed before and
after the compensation treatment.
Pearson’s correlation coefficients with significance levels of 0.05 and
0.01 were used to analyze the variables expected to be related to the change in
measured values before and after compensation treatment. Afterward, simple
regression and backward stepwise multiple regression analyses of the variables
were also carried out.
SNA, SNB, and ANB were not significantly changed. Lower incisor retraction based on
L1 to NB was -2.5±2.84 mm, and TVL with the lower lip retraction was -1.5
±2.5 (Table 1). The
hard-to-soft tissue response ratio was 1.7:1 (60%).
The results of paired t-tests (Table 1) showed
statistically significant differences related to lower incisors before and after
treatment for the values of ODI, APDI, and AB to the mandibular plane, Wits, IMPA,
and L1 to A Pog, and L1 to NB. Among them, soft tissue factors related to changes in
the lower lip, CKA, CKH, true vertical line (TVL), and TVL soft tissue B also showed
statistically significant differences before and after treatment.
The results of a Pearson’s correlation analysis showed that before treatment,
the lower lip to E line (“LL to E”) was also correlated with the
retraction amount of CKA (P<0.01, r= −0.47). Initial lower lip protrusion
quantity was associated with that of lower lip retraction.
Among the variables that exhibited treatment changes, the factors associated with CKA
were investigated to evaluate which variables were correlated with lower lip
profile. The results for the CKA showed a statistically significant correlation with
L1 to A-Pog, L1 to NB, CKHA, Wits, IMPA, and APDI, and the CKH angle showed a
statistically significant correlation with L1 to A Pog, L1 to NB, CKA, and IMPA
As a result of the regression analysis of “L1 to NB” and CKA, which can
indicate the position of the mandibular incisors, the amount of change in CKA was
positively correlated with the amount of change in L1 to NB [ΔCKA =
0.79(Δ L1 to N B) −1.6], R2 = 0.81, p<0.001.
In a multiple regression analysis, the relationship between the amount of change in
CKA L1 to NB and “LL to E” (Initial value, Table 1) indicated that
ΔCKA = 0.5(ΔL1 to N B) - 0.42(LLtoE) −1.48, R2 =
Comparison of measurements at the initial and final stages.
Correlation analysis of L1 to A Pog, L1 to NB, CK, and CKH angles, and
potential influencing factors, r-value (P
L1 to A Pog
L1 to NB
L1 to A Pog
L1 to NB
Note:* P<.05, ** P<.01,
statistically significant; Pearson’s correlation coefficient.
Each value was obtained from the amount of change before and after the
Modern orthodontic treatment has been progressing as a paradigm of soft tissue-based
treatment . Holdaway published a baseline
called the harmony line (H-line) and presented a standard for a harmonious
appearance . Subsequently, in a paper
published in 1999, Arnett emphasized the importance of TVL based on a natural head
position and announced the ideal soft tissue position using a method called soft
tissue cephalometric diagnosis .
Camouflage treatment of skeletal class III malocclusion is the most suitable and
realistic treatment method for patients who want nonsurgical treatment [16, 17].
The traditional method for class III malocclusion is to extract the mandibular
premolars and then retract the mandibular incisors, which has a risk of root
exposure or root resorption due to excessive retroclination [7-9]. A systematic study
on soft tissue changes after extraction reported that lower lip retraction amounted
to 2 mm to 4.5 mm based on “LL to Eline” in adult patients . A class III research on increasing class
III patients with lower second premolars extraction indicated that -2.8mm lower
incisors retraction resulted in 0.9mm lower lip retraction . After developing orthodontic TADs, posterior movement of
the mandibular anterior teeth has been widely used as an alternative treatment to
mandibular extraction for patients with class III malocclusion [10-13].
Obtaining an esthetic look with compensation treatment as good as expected with
orthognathic surgery may be difficult, but patient satisfaction will be higher if
soft tissue changes can be maximized .
However, it is difficult to predict the position before and after the treatment
because the soft tissues of the face, including the lips, are viscoelastic .
This study aimed to determine the factors affecting the ratio of the lower labial
protrusion to lower lip change with compensation treatment of class III
malocclusions and quantify the expected retraction amount of the lower lip. The CKA,
CKH angle, TVL, lower lip, TVL, soft tissue B, B’ to Pog’, and LLA to
Pog’ showed statistically significant differences when the measured values
before and after the treatment of patients who received the class III malocclusion
compensation treatment were compared (Table 1). The rearward
movement of the mandibular anterior incisor, which is currently mainly used for the
compensation treatment of class III malocclusion, was strongly correlated with the
amount of lower lip retraction, CKA (Table 2).
An additional regression analysis was performed on the CKA, which was found to be
statistically correlated with L1 to NB, indicating the position of the mandibular
incisors. An interpretation of the results suggested that the lower lip position,
which changes according to the movement of the mandibular incisors, can be
quantitatively predicted through the CKA. In other words, when the mandibular
anterior teeth move 1 mm posteriorly, the lower lip can retrovert about 79% compared
to the facial plane (N’-Pog’). This can be expected even with
posterior movement of the mandibular incisors through non-extraction, suggesting
that the mandibular full-arch distalization treatment will be highly effective in
soft tissue changes.
In multiple regression analysis, ΔCKA = 0.5(ΔL1 to N B) −
0.42(LLtoE) −1.48. was formulated. The result explained that the protrusive
lip retracted further, while the lower incisors moved backward, and the retraction
ratio was 42%. When the initial lower lip protrusion became constant, a lower
incisor, 1 mm retraction, resulted in 0.5° retraction of the lower lip (CKA).
As the initial lower incisor position affected the initial lower lip position, the
effect of lower incisor movement decreased when the lower lip became constant.
A limitation of this study is that it was conducted only on patients who had
undergone treatment for posterior movement of the anterior teeth of the mandible,
that is, non-extraction treatment. Although 3.7 mm retraction on average of the
lower incisors presented significant lower lip retraction (2.1mm), which was
comparable to the reported amount with extraction [
15, 16], a comparison study with
extraction or mandibular ASO will provide further insight into the differences and
limitations between extraction and non-extraction treatments.
The results of this study showed ODI, APDI, and AB to be the mandibular plane. IMPA,
L1 to A Pog, L1 to NB, and TVL, CKA, and CKHA were significantly correlated with
lower incisor retraction in patients who underwent class III malocclusion
compensation treatment. Among them, changes in the CK and CKH angles before and
after treatment were correlated with L1 to A Pog, L1 to NB, CKA, and IMPA. The
regression analysis results confirmed that the CKA could be predicted by the amount
of mandibular anterior movement and initial lower lip position. Moreover, the amount
of lower lip retraction was significantly correlated with that of lower incisor
retraction during the treatment. The amount of the initial lower lip protrusion also
influenced the lower lip profile change. Therefore, the CKA and CKHA newly presented
in this study can be guidelines for soft tissue-centered diagnosis and treatment
LIST OF ABBREVIATIONS
= Lower Lip Anterior
= Standard Deviation
= True Vertical Line
ETHICS APPROVAL AND CONSENT TO PARTICIPATE
This study was approved by the institutional review board (IRB) of the Ajou
University Hospital (IRB No: AJIRB-MED-MDB-20-265).
HUMAN AND ANIMAL RIGHTS
No animals were used in this research. All human research procedures were conducted
following the ethical standards of the committee responsible for human
experimentation (Institutional and National) and followed the Helsinki Declaration
of 1975, as revised in 2013.
CONSENT FOR PUBLICATION
All the participants read and signed informed consent.
STANDARDS OF REPORTING
STROBE guidelines have been followed.
AVAILABILITY OF DATA AND MATERIALS
The data supporting the findings of the article is available in the
https://github.com/pfChae/01042023.git, reference number 01042023.
CONFLICT OF INTERESTS
The authors declare that they have no conflict of interest.
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Open-Access License: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: https://creativecommons.org/licenses/by/4.0/legalcode. This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Orthodontics, Institute of Oral Health Science, Ajou University School of
Medicine, Suwon, South Korea; E-mail: