Download Influence of bone-cut position in intraoral vertical ramus osteotomy

Journal of Dental Sciences (2014) 9, 272e276
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ORIGINAL ARTICLE
Influence of bone-cut position in intraoral
vertical ramus osteotomy on skeletal
stability after mandibular setback
Chun-Ming Chen a, Steven Lai a, Kun-Rong Hsu b,
Shiu-Shiung Lin c*
a
Department of Oral and Maxillofacial Surgery, Kaohsiung Medical University Hospital,
Kaohsiung Medical University, Kaohsiung, Taiwan
b
Department of Family Dentistry, Kaohsiung Medical University Hospital, Kaohsiung Medical
University, Kaohsiung, Taiwan
c
Orthodontic Department, Kaohsiung Chang Gung Memorial Hospital and Chang Gung University,
College of Medicine, Kaohsiung, Taiwan
Received 23 June 2012; Final revision received 9 August 2012
Available online 27 July 2013
KEYWORDS
bone-cut position;
intraoral vertical
ramus osteotomy;
long-term stability;
mandibular
prognathism;
sagittal split ramus
osteotomy
Abstract Background/purpose: Postoperative skeletal stability is associated with osteotomy
design of orthognathic surgery. The purpose of this study was to investigate osteotomy siterelated factors of intraoral vertical ramus osteotomy (IVRO) related to skeletal relapse in a
2-year postoperative follow-up.
Materials and methods: Twenty-seven patients with mandibular prognathism underwent surgical mandibular setback with IVRO. Cephalometric radiographs of the patients were collected
after completing preoperative orthodontic treatment (T1), at the stage immediately after surgery (T2), and in the 2-year postoperative follow-up (T3). Pir was located at the posterior most
and inferior most ramus point. Io was the inferior most osteotomy point of the mandible.
Relapse was defined as forward movement of menton (Me) in the 2-year follow-up. Hierarchical modeling analyses were used to assess changes in the variables, including the amount of
postoperative relapse (MeT32), the quantity of surgical setback (MeT21), the available setback
horizontal distance (PireIo), and the available setback ratio (MeT21/PireIo).
Results: The mean setback of Me was 12.6 mm, and the mean relapse was 0.9 mm (7.1% Z 0.9/
12.6). In the 1-by-1 and 1-by-2 models, there were no significant differences between the
relapse and other variables. However, we found a significant difference in the 1-by-3 model.
The MeT21 and MeT21/PireIo were significant factors in postoperative relapse.
* Corresponding author. Orthodontic Department, Kaohsiung Chang Gung Memorial Hospital and Chang Gung University, College of
Medicine, Number 123, Da-Pi Road, Niaosong District, Kaohsiung City 83301, Taiwan.
E-mail address: [email protected] (S.-S. Lin).
1991-7902/$36 Copyright ª 2013, Association for Dental Sciences of the Republic of China. Published by Elsevier Taiwan LLC. All rights reserved.
http://dx.doi.org/10.1016/j.jds.2013.06.002
Bone-cut position in IVRO
273
Conclusion: We found that multiple factors contributed to postoperative relapse of
IVRO. Our study also confirmed the 2-year stability of IVRO in treating mandibular
prognathism.
Copyright ª 2013, Association for Dental Sciences of the Republic of China. Published by Elsevier Taiwan LLC. All rights reserved.
Introduction
In recent years, many operations were designed to address
mandibular prognathism.1,2 Early evolution of orthognathic
surgical procedures, such as subcondylar osteotomy, ramus
osteotomy, and mandibular body osteotomy or step
osteotomy, were routinely used to address mandibular
prognathism. Subcondylar osteotomy and horizontal
osteotomy of the ramus resulted in significant postoperative relapse due to the deficiency in bone-adjoining
sphere. Disadvantages of mandibular body osteotomy are
potential damage to the inferior alveolar nerve and
forfeiture of bilateral bony segments in molar areas. In
addition, divergences in the cross-distance between the
bilateral second molars and second premolars are inordinate. Therefore, mandibular body osteotomy is now rarely
used to handle mandibular prognathism.
The blood supply to the mandible is one of the main
problems during surgery. There has been concern regarding
the safety of complex mandibular osteotomies because the
inferior alveolar artery plays a predominant role. The work
of Bell and Levy showed that blood flow through the
mandibular periosteum tended to maintain a sufficient
blood supply to the teeth in a mobile segment.3 This even
held true in cases where the labial periosteum was
degloved. This phenomenon is well evidenced by the
rapidly increasing applications of orthognathic surgery.
Over the years, many amendments have been applied to
ameliorate postoperative stability, such as sagittal split
ramus osteotomy (SSRO) and intraoral vertical ramus
osteotomy (IVRO). The most crucial advantage of IVRO
compared with SSRO is its much lower relative incidence of
trauma to the inferior alveolar nerve.4,5 Hence, we prefer
using IVRO to correct mandibular prognathism. Our
department formulated a modified IVRO procedure.6
Therefore, the current research was to analyze the relationship between postoperative relapse and the osteotomy
length achieved by the modified IVRO, as appraised by
consecutive cephalograms in the 2-year follow-up.
Materials and methods
Twenty-seven patients with mandibular prognathism (22
females and 5 males) were treated with the modified IVRO
procedure to correct their mandibular prognathism. Their
mean age was 20.4 years (range: 17e27 years). All operations were carried out at the Department of Oral and
Maxillofacial Surgery, Kaohsiung Medical University Hospital, from January 1991 to December 1998. The selection
criteria for patients in this study satisfied the following
standards: (1) all patients had skeletal Class III
developmental malformations of mandibular prognathism
with natural dentition; (2) patients with craniofacial
anomalies were excluded from the analysis; (3) neither
injuries nor acknowledged syndromes were etiologic factors; (4) none of the patients was in active development
stage at the time of operation; (5) all patients accepted
preoperative and postoperative orthodontic treatment; (6)
all patients were surgically treated with modified IVRO
technique by a single surgeon; and (7) an acrylic interocclusal splint and maxillomandibular fixation were used
for 6 weeks postoperatively.
Cephalograms were collected and appraised at the
following three intervals: preoperatively after completion
of presurgical orthodontic movement (T1), immediately
postoperatively (T2), and at 2 years postoperatively (T3).
The following items were examined: sella (S), nasion (N),
the posterior most and inferior most ramus point (Pir), the
inferior most osteotomy point (Io), and menton (Me).
Because of the magnification differences between the left
and right sides of the mandible, intermediate outlines of
bilateral projected images of mandibular contour were
traced and identified. In our IVRO method, the lower
portion of proximal segment was excised. The Io landmark
on T2 cephalometric tracing was located and transferred
onto T1 cephalogram by superimposing it between T1 and
T2 cephalometric tracings. The Pir landmark was identified
on T1 cephalogram as the intersection between the lower
half portion of ramus contour and the longest projected
line perpendicular from the vertical reference line
described below. For analysis, an xey coordinate axis was
fabricated. The frame of reference was established with its
source at nasion, and x axis was aligned at an angle of 7
upward to the source line (NeS) as the horizontal axis
(Fig. 1). The vertical reference line (i.e., y axis) was aligned
perpendicular to this line through sella. Cephalometric
tracings of the preoperative stage (T1), changes immediately after surgery (T21), and at the 2-year postoperative
stage (T32) were superimposed to assess differences.
Changes in positions of the landmarks were compared with
reference lines.
Relapse was specified as an advancing movement of Me
during the 2-year follow-up period. All alterations of each
measurement were examined by a paired t test. The PireIo
(the available setback horizontal distance at T1) was
measured between Pir and Io along the y axis. The available
setback ratio was defined as MeT21/PireIo. Hierarchical
modeling analyses were used to survey differences in variables (MeT32, MeT21, PireIo, and MeT21/PireIo) and
investigate factors responsible for postoperative stability.
Hierarchical modeling was composed of seven models at
three levels (1-by-1, 1-by-2, and 1-by-3). Differences at a
level of P 0.05 were considered significant.
274
C.-M. Chen et al
Table 1
(n Z 27).
Summary of variable changes in patients
Variable (mm)
MeT21 (horizontal)
MeT32 (horizontal)
MeT21 (vertical)
MeT32 (vertical)
PireIo
MeT21/PireIo
Figure 1 Landmarks, references lines, and linear measurements applied in this study. The x axis was constructed by
drawing a line through nasion 7 upward from the SeN line. The
y axis was constructed by drawing a line through sella (S)
perpendicular to the x axis. Me (menton) Z the inferior most
point on the mandibular symphysis; Io Z the inferior most
osteotomy point along the mandibular border; Pir Z the posterior most and inferior most point of the ramus.
The error of the method in this study was assessed by
replicating standardized tracings of all cephalograms
several months after the first tracing. This was to assess the
intraexaminer error. The error study determined the extent
to which a single rater obtained the same result using the
same instrument to measure the objects. Therefore, the
intraexaminer error was studied in terms of landmark
identification on cephalograms during tracing. Whenever
the disparity between two values of any point or angle was
>0.5 mm or >1 , respectively, the point or angle was noted
and measured again. Other registrations were compared
with the third one. The outlier was not included in the data.
In addition, the mean value was determined by the two
approximate values. Therefore, our outcomes and variables
were considered to be reliable.
Results
The changes in Me at three stages (T1, T2, and T3) are shown
in Table 1. The mean changes in Me were 12.6 mm backward
and 0.8 mm downward. In the 2-year follow-up, the relapse
rate of Me was 0.9 mm on average, and this amount represented 7.1% of the total range of the mean setback. In the
Mean
12.6
0.9
0.8
0.6
14.7
0.9
Standard deviation
3.30
2.99
1.73
1.64
3.81
0.29
vertical direction, Me was 0.6 mm higher in 2 years postoperatively. The horizontal distance of PireIo was 14.7 mm.
The available setback ratio (MeT21/PireIo) was 0.9.
Results of hierarchical modeling analyses are demonstrated in Table 2. The P values of individual 1-by-1 models
were 0.4315 (MeT21), 0.6984 (PireIo), and 0.77 (MeT21/
PireIo). The P values of 1-by-2 models were still not significant (Table 2). In the 1-by-3 model, however, P values of
MeT21 and MeT21/PireIo revealed significant differences.
Because of the high correlation in three independent variables, we subsequently checked the collinearity with Jump
7 software (SAS Institute, Cary, NC, USA). Collinearity is a
statistical phenomenon in which an independent variable is
a linear combination of other independent variables.
However, we found that the largest condition index (42.88)
was >30. We then used the centered scores to avoid
collinearity, which would tend to involve the interaction.
Finally, the largest condition index (7.06) was <10 and the
outcomes were the same in the 1-by-3 model. This
confirmed that MeT21 and MeT21/PireIo were significant
factors responsible for postoperative stability.
Discussion
A research into the impact of surgery on the vascular supply
has greatly benefited osteotomy designs of the mandibular
Table 2
MeT32 in the hierarchical model test.
Variable (model level)
95% Confidence level
P
Model 1: MeT21
Model 2: PireIo
Model 3: MeT21/PireIo
Model 4
MeT21
PireIo
Model 5
PireIo
MeT21/PireIo
Model 6
MeT21
MeT21/PireIo
Model 7
MeT21
PireIo
MeT21/PireIo
( 0.23, 0.51)
( 0.26, 0.38)
( 4.92, 3.68)
0.4315
0.6984
0.77
( 0.26, 0.53)
( 0.32, 0.37)
0.4929
0.8856
( 0.34, 0.44)
( 5.47, 4.98)
0.7896
0.9234
( 0.19, 0.71)
( 7.50, 2.90)
0.2518
0.3706
(0.13, 2.16)
( 1.70, 0.03)
( 27.39, 0.93)
0.0286 *
0.0578
0.0371 *
*P < 0.05.
Bone-cut position in IVRO
ramus. Surgeons traditionally believed that the inferior
alveolar artery played a key role in nourishing the mandible.
However, Bell and Schendel showed that blood supply from
the surrounding soft tissues was sufficient.7 The blood supply tended to be maintained when the inferior alveolar artery was obstructed. In 1957, Trauner and Obwegeser8
initially described the procedure of SSRO, and it was successively adjusted by Bell and Schendel,7 Dal Pont,9
and Epker.10 Furthermore, Moose identified the intraoral
median approach to a subcondylar osteotomy in 1964,11 and
in 1968, Winstanley described the intraoral lateral approach
to a subcondylar osteotomy.12 Later on, Hall and
McKenna further elaborated the method of IVRO.13 It is
widely admitted that a certain level of skeletal alteration
invariably occurs after treating mandibular prognathism.
Numerous studies reported an acceptable postsurgical stability of both SSRO and IVRO techniques.4,14e16 Nevertheless, the postoperative skeletal stability is a matter of
debate even among the most seasoned surgeons.
The issue of major concern in orthognathic surgery is the
long-term skeletal stability. In previous studies, it was
found that the mean amount of surgical setback was
approximately 4.87e8.4 mm for SSRO,14,15,17e19 and
approximately 5.3e8.4 mm for IVRO.14,15,20 According to
our research,21 the average amount of Me setback was
12.7 mm, which is greater than those in the previous reports. Compared with the amount of setback, the range of
relapse potency was from 7.1% (0.6/8.4 mm) to 51.4%
(2.87/5.58 mm) for SRRO, and 11.7% (0.7/6.3 mm) to 24.5%
(2.06/8.4 mm) for IVRO. Our current report demonstrated a
relapse potency of 11.8% (1.5/12/7). In addition, our patients who underwent surgical mandibular setback greater
than 10 mm showed no obvious relapse. This indicated that
the modified IVRO surveyed in this study presented good
skeletal stability even when treating patients with severe
prognathism of the mandible.
The postoperative stability of both IVRO and SSRO has
been extensively evaluated. However, the causes of relapse
remain unclear. The most frequently proposed cause of
relapse is the amount of mandibular setback. Conclusions
about the correlation between the amount of mandibular
setback and the tendency to relapse are still controversial.
Phillips et al,14 Kobayashi et al,18 and Schatz and Tsimas19
found that the amount of surgical setback was correlated
with relapse. By contrast, no relationship between the
amount of surgical movement and the degree of relapse
was found by Sorokolit and Nanda22 and Mobarak et al.23
Our findings indicated that the changes in Me (T32) between the immediate postsurgical stage and the 2-year
postoperative stage revealed no significant difference.
Anatomical differences of individual patients and the
preference of surgeon to carry out the procedure are two
factors that affected the length of the osteotomy. Tornes
investigated the osteotomy length and the postoperative
stability of 80 patients treated with vertical subcondylar
ramus (VSCR) osteotomy.24 He concluded that the osteotomy length appeared to be a minor factor in postoperative
stability. However, the osteotomy length of IVRO is greater
than that of VSCR osteotomy and can go beyond the gonial
point on the lower border of the mandible.
Despite previous findings, in 1-by-1 hierarchical
modeling analyses, PireIo and MeT21/PireIo did not
275
significantly differ in terms of the relationship between the
osteotomy length of IVRO and postoperative skeletal
relapse. In the 1-by-2 model, there was still no significant
difference regarding the postoperative skeletal relapse.
However, it might have been significant if there had been
more combined variables that contributed to the relationship between the osteotomy length and postoperative stability. Finally, there was a significant difference in the
combined variables (MeT21, PireIo, and MeT21/PireIo),
which correlated with the postoperative stability. The
MeT21 and MeT21/PireIo were significant factors responsible for postoperative stability. Therefore, we found that
the combination of osteotomy design and its available
setback capacity could be primary factors affecting its
postoperative stability.
Research methods and surgical approaches can greatly
vary among surgeons even in the same hospital. This is a
major problem for surgeons when they try to select the best
techniques to minimize postoperative skeletal changes.
Interosseous semirigid or rigid fixation is commonly used
with SSRO technique. However, fixation between distal and
proximal segments was found to be difficult, and hence it
cannot be mandatory for IVRO technique. Instead of
interosseous fixation, an acrylic interocclusal splint and
maxillomandibular fixation were used in this study to
immobilize the jawbones for 6 weeks after surgery. The
bilateral temporomandibular joint gradually and physiologically adapts. None of our patients reported any complications of temporomandibular disorders after surgery.
The postoperative relationship of the jawbones of our patients was stable, and the relapse rate was less than 10%.
Furthermore, reports regarding postoperative permanent
complications of our patients were rare. It is important for
surgeons to evaluate postoperative complications to refine
the surgical technique. A thorough investigation of temporary and permanent postoperative complications will be
conducted in the future.
In conclusion, our study demonstrated 2-year stable
surgical outcomes by applying a modified IVRO to correct
mandibular prognathism. The postoperative stability of
IVRO was found to be affected by multiple variables rather
than merely a single factor. The results showed that three
combined factors of MeT21, PireIo, and the ratio (MeT21/
PireIo) presented a statistically significant influence on
postoperative stability.
Conflicts of interest
The authors have no conflicts of interest relevant to this
article.
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