Keywords
malocclusion - mandibular setback - orthodontics - orthognathic surgery - skeletal
class III - skeletal relapse
Introduction
Skeletal class III is a skeletal discrepancy featured by mandibular prognathism with
or without maxillary retrognathism that is associated with a concave facial profile.
Southeast Asian populations show the highest prevalence rate (15.08%) of class III
malocclusion.[1]
Possible treatments to correct skeletal class III malocclusion are growth modification
in growing patients, dental camouflage by orthodontic treatment, and orthognathic
surgery combined with orthodontic treatment in nongrowing patients. Mandibular setback
surgery can be performed to move the mandible posteriorly and rotate and move it downward/upward anteriorly. The most popular surgical method to move the mandible posteriorly
is bilateral sagittal split osteotomy (BSSO). Obwegeser and Trauner were the first
to propose this surgery in 1957.[2] Subsequently, many modifications were proposed, such as those by Dalpont, Hunsuck,
and Epker.[2]
[3] In 2016, Tangarturonrasme and Sununliganon introduced the BSSO low Z plasty technique
(Prasan's modification). This method allows the mandible to be set posteriorly to
a larger extent and reduces possible complications.[4] With this technique, skeletal stability and satisfactory results can be achieved.
Chaiprakit et al then proposed the NM-Low Z plasty technique,[5] which removes bony interference on the internal surface of the proximal segment.
Thus, the surgeon can more easily conduct mandible surgery in three dimensions. Currently,
this technique is one of the surgical procedures used in our center.
Cephalometric study is one of the investigating tools for analysis of facial skeletal,
dental, and soft tissue profile of multistage treatment.[6]
[7] Skeletal relapse or skeletal instability is considered an undesired result of orthognathic
treatment. The cause for this relapse is thought to be multifactorial. One of the
contributions to skeletal relapse is the surgical technique. Kim et al[8] proved that BSSO and concomitant mandibular angle resection tended to influence
postsurgical skeletal instability. Various modifications of conventional BSSO can
lower the tension of pterygomasseteric slings and achieve more clinical aesthetic
results in mandibular prognathism patients with squared faces and prominent mandibular
angles. Kim et al[9] also reported a procedure to reduce postsurgical relapse after mandibular setback
using BSSO. They compared a technique including intended osteotomy of the distal segment's
posterior portion to the conventional one. This distal osteotomy technique has been
used since 1994. Without the distal osteotomy technique, the distal segment protrudes
into the gonial region as the mandible is set back. Consequently, lengthening the
pterygoid muscle causes relapse and reduces the posterior pharyngeal air space.
Although NM-Low Z plasty allows for less bony interference, which results in less
bony contact, there is no report regarding skeletal stability following mandibular
setback surgery. The aim of this research was to evaluate skeletal changes following
mandibular setback surgery performed with the NM-Low Z plasty technique in skeletal
class III patients.
Materials and Methods
This study followed the Declaration of Helsinki guidelines regarding medical protocol
and ethics, and the protocol was approved by the Ethics Committee of Thammasat University
(119/2563).
Thirty-eight skeletal class III patients (ANB angle lower than 0) who underwent the
NM-Low Z plasty technique for surgical mandibular setback procedure at Thammasat University
Hospital between January 2017 and March 2020 were included in the study: 29 patients
had two jaw surgeries, and 9 patients had one jaw surgery. An additional 14 patients
had genioplasty ([Table 1]). The same skilled surgeon conducted all of the orthognathic operations. The NM-Low
Z plasty technique was followed to the protocol described in previous case report.[5] Inclusion criteria were: a complete series of identifiable lateral cephalograms
and confirmation of growth completion by a cervical vertebral maturation status of
CS6.[10] Patients with craniofacial anomalies or facial bone fractures and incomplete diagnostic
data were excluded from the study.
Table 1
Demographic variables of the sample
|
Demographic variables (n = 38)
|
|
|
Age (y), mean ± SD (range)
|
24.8 ± 4.3 (16–37)
|
|
Women, % (n)
|
65 (25)
|
|
With two jaws surgery, % (n)
|
76 (29)
|
|
With one jaw surgery, % (n)
|
24 (9)
|
|
With genioplasty, % (n)
|
36.8 (14)
|
Abbreviation: SD, standard deviation.
A set of three standardized lateral cephalograms (T0: prior to surgery, T1: immediately
following surgery, T2: 6 to 12 months after surgery) were obtained from patients.
All cephalograms were digitized, traced, and measured into variables by one observer
using Dolphin Imaging software, the reliability of which for both hard and soft tissue
points equaled those traced by manual landmark plotting.[11]
[12]
[13]
[14] Moreover, problems with time consumption and various errors due to improper tracing,
measurement, and calculation were diminished using computerized cephalometric software.
The mean skeletal changes were defined in the variables at different stages: immediate
postsurgical changes (T1-T0) and postsurgical stability (T2-T1). Reference lines were
constructed for linear measurements. The vertical reference line was described as
a line perpendicular to the line 7 degrees below the sella-nasion (SN) line. A horizontal
reference line was drawn between the cephalometric marks porion (Po) and orbitale
(Or), as shown in [Table 2].[15] Skeletal relapse was defined as forward movement in the horizontal direction at
the deepest midline point on the bony curvature of the anterior mandible between the
infradental and pogonion (point B) ([Fig. 1]).
Fig. 1 Reference points and reference lines used in cephalometric analysis.
Table 2
Definition of the variables utilized in the study
|
Measurement
|
Definition
|
|
B-SN7perp (mm)
|
The perpendicular distance from B point to the line below 7° to the SN line
|
|
Pog-SN7perp (mm)
|
The perpendicular distance from pogonion to the line below 7° to the SN line
|
|
SNB (degrees)
|
The angle between the SN line and NB line
|
|
ANB (degrees)
|
The difference between angles SNA and SNB
|
|
Me-SN7 (mm)
|
The distance from Me to the line below 7° to the SN line
|
|
B-SN7 (mm)
|
The distance from B point to the line below 7° to the SN line
|
|
SN-GoGn (degrees)
|
The angle between the SN line and mandibular plane
|
|
IMPA (degrees)
|
The angle between line of lower incisor axis and mandibular plane
|
Abbreviations: IMPA, incisor mandibular plane angle; SN, sella-nasion.
To analyze the reliability test, 6 films from the 38 patients were randomly selected.
At 2-week intervals, the cephalograms were retraced and measured, and the Dahlberg
formula was used to compare linear measurements between two time periods.
(FIG), where D is the difference between two measurements and N denotes the number of double determinations.[16]
The distribution of skeletal changes at point B was examined. The statistics showed
that the measurement was normally distributed according to the Shapiro–Wilk test and
Q-Q normality plot ([Figs. 2] and [3], [Table 3]).
Table 3
Tests of normality of horizontal and vertical measurements
|
Normality tests
|
|
Kolmogorov–Smirnov
|
Shapiro–Wilk
|
|
Statistic
|
df
|
Significance
|
Statistic
|
df
|
Significance
|
|
BSN7perpT0
|
0.100
|
36
|
0.200
|
0.971
|
36
|
0.460
|
|
BSN7perpT1
|
0.084
|
36
|
0.200
|
0.982
|
36
|
0.817
|
|
BSN7perpT2
|
0.107
|
36
|
0.200
|
0.974
|
36
|
0.558
|
|
BSN7T0
|
0.082
|
36
|
0.200
|
0.976
|
36
|
0.622
|
|
BSN7T1
|
0.090
|
36
|
0.200
|
0.967
|
36
|
0.343
|
|
BSN7T2
|
0.090
|
36
|
0.200
|
0.963
|
36
|
0.275
|
Abbreviations: df, degree of freedom; T0: prior to surgery, T1: immediately following
surgery, T2: 6 to 12 months after surgery; 95% confidence interval.
Note: B-SN7perp indicates horizontal measurement; B-SN7 indicates vertical measurement.
Fig. 2 Normal Q-Q plot of horizontal measurement.
Fig. 3 Normal Q-Q plot of vertical measurement.
To compare mean skeletal changes in variables at different stages of the NM-Low Z
plasty technique, the paired t-test was performed. Two-sided p-values < 0.05 were considered significant. IBM SPSS Software Version 22 (International
Business Machines Corporation, Armonk, New York, United States) was used to conduct
this statistical study.
Results
Anteroposterior Skeletal Relationship
At point B, the average mandibular setback was 9.78 mm, and the average skeletal relapse
was 2.61 mm in this group of patients, indicating a 26.69% skeletal relapse rate (2.61/9.78).
The statistical analysis showed that the difference between the before surgery and
immediate postsurgical changes was significant (p < 0.05). Obviously, the mandible was significantly setback repositioned (B-SN7 perp,
Pog-SN7 perp, SNB, p < 0.05), and significantly relapsed forward during the postsurgical period (B-SN7
perp, Pog-SN7 perp, SNB, p < 0.05). This indicates that the mandible has relapsed significantly forward during
the postsurgical follow-up period.
Vertical Skeletal Relationship
There was a trend of decrease in vertical dimension postsurgically. B point showed
significant upward relapse (B-SN7, p < 0.05) after surgery and follow-up period. Since sagittal and vertical skeletal relationship,
the mandible demonstrated a forward-upward rotation or autorotation.
Dental Relationship
The incisor mandibular plane angle (IMPA) decreased because of autorotational counterclockwise
movement of the mandible immediately after removal of the stabilization splint (IMPA,
p < 0.05). The mandible seemed to relapse in the forward direction, and class III mechanics
were used to prevent this situation. Consequently, the lower incisors were lingually
inclined. During the postsurgical period, the lower incisors retroclined to maintain
proper overjet, while the mandible had a tendency to relapse in the anterior position.
Discussion
In patients with skeletal III relationship, the mandible was moved backward and inferiorly
repositioned during mandibular setback surgery ([Table 4]). After surgery, the mandible then moved in forward and superior directions.
Table 4
Comparison of skeletal and dental variables according to patients who underwent NM-Low
Z plasty at various stages
|
Variables
|
Measurement
|
T0
|
T1
|
T2
|
p
|
|
Mean
|
SD
|
Mean
|
SD
|
Mean
|
SD
|
T1-T0
|
T2-T1
|
T2-T0
|
|
Horizontal skeletal variable
|
B-SN7perp (mm)
|
81.46
|
13.52
|
71.68
|
12.45
|
74.28
|
12.7
|
0.000[a]
|
0.000[a]
|
0.000[a]
|
|
Pog-SN7 perp (mm)
|
83.29
|
14.96
|
75.33
|
14.02
|
77.82
|
13.93
|
0.000[a]
|
0.000[a]
|
0.000[a]
|
|
SNB (degrees)
|
86.88
|
4.39
|
83.5
|
4.15
|
84.8
|
4.37
|
0.000[a]
|
0.000[a]
|
0.000[a]
|
|
ANB (degrees)
|
–3.65
|
2.65
|
2.09
|
1.68
|
1.17
|
2.04
|
0.000[a]
|
0.000[a]
|
0.000[a]
|
|
Me-SN7 (mm)
|
–128.04
|
15.75
|
–124.74
|
13.1
|
–122.78
|
14.4
|
0.017[a]
|
0.077
|
0.001[a]
|
|
Vertical skeletal variable
|
B-SN7 (mm)
|
105.32
|
12.99
|
102.1
|
10.95
|
99.96
|
11.57
|
0.009[a]
|
0.041[a]
|
0.000[a]
|
|
SN-GoGn (degrees)
|
31.82
|
5.94
|
32.58
|
6.5
|
32.08
|
7.12
|
0.255
|
0.228
|
0.726
|
|
Dental variable
|
IMPA (degrees)
|
85.88
|
7.6
|
79.32
|
6.56
|
79.46
|
7.75
|
0.000[a]
|
0.839
|
0.000[a]
|
Abbreviations: SD, standard deviation; T0, prior to surgery; T1, immediately following
surgery; T2, 6 to 12 months following surgery.
Note: A paired t-test was conducted.
a
p < 0.05 by paired t-test.
NM-Low Z plasty surgical osteotomy has several benefits over other modifications operated
at mandible. By removing the medial side of the proximal segment, bony interference
was reduced. This allows for more mandibular movement (set back or advancement) and
minimizes the risk of interfering with essential structures such as the facial nerve
and styloid process. Additionally, it minimizes the risk of compromising the pharyngeal
airway space as compared with other techniques.[5]
In this research, there was significant difference in vertical (B-SN7, p < 0.05) and horizontal skeletal variables (B-SN7 perp, p < 0.05) during the postsurgical period. These findings would explain why upward and
forward movement of the mandible occurred during the postsurgical period. Our findings
conformed to many previous studies,[17]
[18]
[19] which reported a greater horizontal relapse and more forward chin projection caused
by bite closure after postsurgical orthodontic treatment. Additionally, Hsu et al
reported that the most sagittal relapse in the postsurgical period contributed to
the forward-upward rotation of the mandible.[20]
There was a decrease in the vertical skeletal relationship in the postsurgical period.
According to Ko et al, it was hypothesized that the vertical dimension was decreased
because of autorotation of the mandible with the occlusal settling process of the
posterior teeth in postsurgical orthodontic treatment, especially in cases with severe
occlusal interferences.[17] Furthermore, Imerb et al stated that a high mandibular plane angle had less stability
than a normal mandibular plane angle.[21]
However, there were benefits in performing dental decompensation before surgery using
the conventional approach.[22]
[23] From our finding that the lower incisors proclined after decompensation and retroclined
after the postsurgical period, round-tripping tooth movement was observed, which was
considered an adverse effect. This was in agreement with Capelozza et al, who reported
dental decompensation to obtain more skeletal correction in patients with skeletal
class III.[23]
With rotational relapse (forward and upward movement of the mandible) immediately
after surgery, there might be a benefit to patients with skeletal class III and hyperdivergent
patterns (open-bite tendency) to decrease facial height. In contrast, hypo-/normodivergent
patients must be treated with caution. Vertical control of overbite and anterior facial
height are recommended in contemporary orthodontics.[24]
In the finishing stage of comprehensive orthodontic treatment, normal overbite is
one of the requirements before the removal of full fixed appliances.[25] To correct excessive overbite, two factors must be addressed: (1) the vertical relationship
between upper lip and upper incisors and (2) anterior facial height. If there is an
appropriate display of the upper frontal teeth while smiling, the position of the
upper incisors needs to be maintained, and the overbite needs to be corrected by intruding
the lower incisors instead of the upper incisors. If the display of the upper frontal
teeth is excessive, intrusion of the upper incisors is suggested. In patient with
a short facial height, extrusion of the lower posterior teeth is acceptable. If the
patient has a long facial height, intruding the incisors is indicated. As with other
issues, planning for impending events is critical while dealing with skeletal and
dental relapses.
Various types of osteotomy modification, fixation, and postoperative protocol could
explain the different relapse rate. Our study, NM-Low Z plasty, could confirmed that
skeletal stability was comparable to the conventional Hunsuck-Epker procedure in the
finite element analysis.[26] The results in this study were limited to NM-Low Z plasty surgical technique. Further
study needs to be conducted to compare the amount and direction of skeletal changes
with conventional surgical technique as well as biomechanical studies.
Conclusion
Mandible was significantly relapsed in forward and upward direction following mandibular
setback surgery using NM-Low Z plasty technique. Lower incisors were retroclined after
surgery to compensate autorotational counterclockwise of mandible. Patient selection,
overcorrection, and orthodontic mechanics to deal with relapse must all be considered.
