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British Journal of Oral and Maxillofacial Surgery xxx (2015) xxx–xxx
Unilateral sagittal split mandibular ramus osteotomy:
indications and geometry
Jacques Beukes a,∗ , Johan P. Reyneke b,c , Janalt Damstra d
a
Centre for Orthognathic Surgery and Implantology, Netcare Sunninghill, Cnr Witkoppen and Nanyuki rd, Sunninghill Park
Center for Orthognathic Surgery, Mediclinic Cape Town, 21 Hof street, Oranjezicht, Cape Town, 27A Higgo crescent, Higgovale, Cape Town
c Professor, Department of Maxillofacial and Oral Surgery, University of the Western Cape, Cape Town, South Africa
d Fourways Mall, M30 Mezzanine Level, Fourways, Johannesburg
b
Accepted 24 October 2015
Abstract
Small mandibular asymmetries may be corrected by unilateral sagittal split ramus osteotomy (USSO). This study had two objectives: first to
define the geometric changes in the mandibular condyle and the lower incisor teeth that result from the rotation of the major segment (n=26),
and secondly to examine in a clinical study the temporomandibular joints (TMJ) of 23 patients after correction of mandibular asymmetry by
USSO to find out if there were any long-term adverse effects. Small mandibular asymmetries (<5 mm) can be corrected by USSO. Secondary
anteroposterior changes as a result of setback or advancement on the operated side should be taken into account during the planning of
treatment. The small rotational changes of the condyle did not adversely affect the TMJ.
© 2015 The British Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.
Keywords: Unilateral Sagittal Split Mandibular Ramus Osteotomy
Introduction
Asymmetry of the face is a three-dimensional dentofacial
deformity and may involve the maxilla, the mandible, or
the chin, or a combination, and its correction may therefore
require repositioning of one, two, or all three bones. As in the
correction of other dentofacial deformities, the assessment
of the anteroposterior, vertical, and transverse positions of
the maxillary incisors and the cant of the maxillary occlusal
plane play extremely important parts in the planning of
treatment and are factors in deciding where the mandible
will finally be positioned. Once the maxilla and the mandible
∗
Corresponding author.
E-mail addresses: drjacquesbeukes@gmail.com (J. Beukes),
drjprey@global.co.za (J.P. Reyneke), janalt@drdamstra.co.za (J. Damstra).
are symmetrically positioned in the face, any remaining
asymmetry of the chin can be corrected by genioplasty.1
In some patients the facial asymmetry affects only the
mandible. In these cases only the lower jaw will need correction, and it would be the responsibility of the orthodontist
to align the upper dental midline and arch in all three spatial planes. When only the mandible will be operated on, the
upper (unoperated) dental arch will decide the final position
of the mandible and the symmetry of the face.
In most cases of mandibular asymmetry the correction will
require a bilateral sagittal split osteotomy (BSSO), which will
allow simultaneous correction of transverse, anteroposterior,
and vertical problems. The transverse position of the asymmetrical mandible will be dictated by surgica coordination of
the mandibular and the maxillary dental midlines, while the
anteroposterior and vertical positions will be dependent on
the ideal position of the maxillary incisor and dental arch.
http://dx.doi.org/10.1016/j.bjoms.2015.10.029
0266-4356/© 2015 The British Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Beukes J, et al. Unilateral sagittal split mandibular ramus osteotomy: indications and geometry. Br J
Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.bjoms.2015.10.029
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Fig. 1. Superior view of the mandible: A. Condylar rotation point. B. Correction of asymmetry. C. Anteroposterior change in a unilateral sagittal split
osteotomy. D. Unilateral advancement/setback.
There are, however, instances when mandibular asymmetry may be corrected by a unilateral sagittal split osteotomy
(USSO), and operating on only one side of the mandible will
require a rotation of the tooth-bearing segment (major segment) to coordinate the lower dental midline with the midline
of the upper arch. The rotation of this segment will take place
around the condyle on the unoperated side of the mandible
(Fig. 1), and there are certain geometrical limitations (and
possible functional implications) of the rotational change of
the condyle in the glenoid fossa.2
We investigated the indications for a USSO, evaluated the
geometry of the rotational movement of the mandible, and
evaluated retrospectively the long-term postoperative signs
and symptoms in the temporomandibular joints (TMJ) of 23
patients who had had USSO for the correction of mandibular
asymmetry.
Patients, material, and methods
Unilateral sagittal split osteotomy of the mandible
It is not often possible to correct mandibular asymmetry by
a USSO without a maxillary operation. Surgical coordination of the dental arches in these cases demands tedious and
long orthodontic preparation in an attempt to align the dental
arches in such a fashion that the teeth will fit into occlusion
after the USSO.
Operating on only one side of the mandible will allow for
rotation of the midline to either the left or the right by either
advancing or setting back the mandible. The unoperated side
will be rotated around the condyle and will have secondary
anteroposterior implications, which have to be considered
when planning treatment.
When a USSO is done on the left side of the mandible
the midline can either be rotated to the right by advancing
the major segment, or to the left by setting the segment back
(Fig. 1). Rotation to the right will result in slight advancement
of the mandible, while rotation to the left will set the mandible
back. The opposite will apply when the USSO is done on the
right side (Fig. 1). These secondary anteroposterior changes
will have aesthetic implications that should be considered
when planning treatment. The final anteroposterior position
of the upper incisor and maxilla will be dictated by the anteroposterior position of the lower incisor after correction of the
midline and slight anterior or posterior alteration. This is not
a consideration after a BSSO, in which case the surgeon will
not be “limited” by the unoperated side of the mandible.
The degree of anteroposterior change at the mandibular
midline will be influenced by the relation between the length
of the mandible, the amount of rotation required to achieve
symmetry, and the shape of the mandible. We studied the relation between these factors by comparing the dimensions and
shapes of 26 scanned mandibles. The anteroposterior changes
in relation to the amount of transverse correction (amount
of rotation) were correlated, as was the effect that the shape
of the mandible would have on the dimensional changes.
Geometry
To calculate the condylar changes after USSO, 26 randomly selected scanned mandibles were analysed using
SimPlant®Ortho Pro 2.1 software (Materialise Dental, Leuven, Belgium). The computed tomographic (CT) images
were originally acquired from patients to produce surgical guides for dental implants. Permission to use the data
for research purposes was obtained before scanning. The
mandibles were scanned at 0.5 mm voxel resolution, and surface models were generated by a commercial segmentation
company (SimPlant®, Johannesburg, South Africa) using a
thresholding-based method. Requirements for inclusion were
full acquisition of the condylar heads during the scanning and
segmentation process, and the presence of at least four anterior teeth and two molars. Twenty-six scans met the inclusion
criteria and were included in the study. No distinctions were
made between men and women, and the investigators were
unaware of the identity of the patients.
The CT data and surface models were downloaded in a
DICOM multifile format and imported into SimPlant®Ortho
Pro 2.1 software (Materialise Dental, Leuven, Belgium). The
landmarks used in the study are described in Table I (Fig. 2).
To reduce any error in measurement, digital markers3 with
an Ø of 1.2 mm were imported as stereolithographic (STL)
files and placed in the landmark configuration described
in Table 1 (Fig. 2). The landmarks were digitised using
the middle of the digital markers as references. Coronal,
sagittal, and axial reference planes were constructed with
the “cephalometry” tool of the software. The positions of the
surface models were standardised by using the “set natural
head position” function of the software. An occlusal plane
was constructed by connecting the tip of the lower incisor
with the tips of the mesiobuccal cusps of the lower first molar
Please cite this article in press as: Beukes J, et al. Unilateral sagittal split mandibular ramus osteotomy: indications and geometry. Br J
Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.bjoms.2015.10.029
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Fig. 2. A three-quarter view of the mandible. The digital landmarks (black)
were placed on the images: 1. Co Lat. (R), 2. Co (R), 3. Co Med. (R), 4. Co
(L), 5. Split point (SP) 1, 6. Split point (SP) 2, and 7. Incisor point (IP) (see
Table 1).
teeth. This plane was used to reposition the surface model at
an angle of 9o with the axial reference plane. The standard
angle of the occlusal plane to the Frankfort horizontal pane
is 9o and was used as reference and was used as reference.4
Table 1
Landmarks and measurements used in this study.
Abbreviation
Landmarks:
Co Lat (R)
Co (R)
Co Med (R)
Co (L)
Split point (SP) 1
Split point (SP) 2
Incisor point (IP)
Measurements:
Anteroposterior
Transverse
Direct split
Definition
The most lateral point of the right head of the
condyle
The most superior point of the right head of the
condyle
The most medial point of the right head of the
condyle
The most superior midpoint of the left head of
the condyle
A midpoint on the buccal surface of the
mandibular body indicating the posterior border
of the virtual sagittal split
A midpoint on the buccal surface of the
mandibular body indicating the anterior border
of the virtul sagittal split
The most inferior contact point between the left
and right mandibular incisors
The distance between the coronal reference
plane and IP, measured parallel to the sagittal
and axial reference planes
The distance between the sagittal reference
plane and IP, measured parallel to the coronal
and axial reference planes
The shortest distance between SP1 and SP2
3
Fig. 3. A superior view of the mandible. The rotations and measurements
used for the 3-dimensional surgical simulation: (i) the rotation point at the
centre of a line connecting Co Lat. (R)and Co Med. (R), (ii) clockwise coronal rotation at the condyle (positive), (iii) counterclockwise coronalrotation
at the condyle (negative), (iv) anteroposterior change, (v) transverse change
at the lower incisors, and (vi) the unilateral surgical advancement measured
at the vertical osteotomy lines.
A line was constructed to connect the left and right landmarks
of condylion (Co) and used to align the mandibular surface
models parallel to the coronal and axial reference planes.
A virtual unilateral sagittal split was created on the left
ramus of the surface model using the “surgery” tool of the
software. The midpoint on a line that connected the mesial
and lateral points of the right condylar head was used as
a fixed rotation point to rotate the condylar head virtually
(Fig. 3). For statistical reasons clockwise rotations at the
condyle were considered “positive” and counter clockwise
rotations as “negative”. Two virtual surgical groups were
created. Positive or clockwise rotation of the condylar head
indicated unilateral advancement while negative or anticlockwise rotation indicated unilateral setback (Fig. 3). Dental
midline corrections of 1, 2, 3, 4, and 5 mm by USSO were
simulated. The change of the angle of the condyle (rotation)
and secondary anteroposterior changes at the lower incisors
were recorded. Virtual surgery and measurement were done
by the same operator (JD).
Statistical analysis of the geometric study
All statistical analyses were made with the aid of SPSS for
Windows (version 16, SPSS Inc, Chicago, IL). The measurement error was calculated and the sample measured a second
time after 2 weeks by the same operator (JD). Differences
between the measurements were calculated and the measurement error established using Dahlberg’s formula.5 The
measurement error was small (0.21 mm), which confirmed
the accuracy of the method.
The mean (SD) and 95% CI for the transverse, anteroposterior, and direct split differences were calculated for the
Please cite this article in press as: Beukes J, et al. Unilateral sagittal split mandibular ramus osteotomy: indications and geometry. Br J
Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.bjoms.2015.10.029
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unilateral advancement and setback simulations. Possible differences between the unilateral sagittal split advancement and
setback groups were also investigated.
setbacks. However, the anteroposterior changes are slightly
greater than for unilateral advancements.
Clinical
Clinical study
Twenty-three patients (11 females and 12 males, mean
(range) age 29 (15-46) and 25 (17-54) years, respectively,
who required USSO for correction of facial asymmetry
(<5 mm at the lower incisor) were included in this retrospective study. Unilateral mandibular setbacks and advancements
were included. The vector of advancement and setback for
the correction of the mandibular midline were recorded at the
time of the operation. All the measurements were made and
operations done by a single operator (JR). Bicortical screws
were used for internal fixation and patients were instructed to
use intermaxillary (1/4 , 3.5 oz) elastics for 3-4 weeks postoperatively. All patients were followed up for a period of at
least six months and the TMJ examined for pain, clicking,
mobility, or discomfort.
Eight patients had unilateral setback procedures and in 15
the mandible was advanced on one side. Twelve USSO were
on the left, of which nine were unilateral advancements and
three unilateral setbacks. Eleven USSO were on the right,
of which 6 were unilateral advancements and 5 unilateral
setbacks. In one case the mandibular operation was done as
a single procedure, and in 22 it was in conjunction with Le
Fort I maxillary osteotomies. In four cases a genioplasty was
required to correct the symmetry of the chin.
Results
Geometry
The positive and negative rotations in unilateral advancements and setbacks are shown in Table 2. As the mandibular
midline moves around the condyle (the point of rotation)
the advancement of the incisors decreases the further the
rotation. The opposite is true for unilateral mandibular
Table 2
Asymmetry: positive and negative rotations in unilateral advancements and
setbacks. Data are mean (SD).
Variable
Change in
condylar angle (o )
Positive rotations (unilateral advancement):
0.75 (0.07)
1 mm
2 mm
1.50 (0.13)
2.24 (0.19)
3 mm
2.98 (0.25)
4 mm
5 mm
3.71 (0.31)
Negative rotations (unilateral setback):
1 mm
0.76 (0.07)
1.52 (0.13)
2 mm
2.29 (0.2)
3 mm
4 mm
3.07 (0.27)
3.85 (0.33)
5 mm
Anteroposterior
change (mm)
0.66 (0.03)
1.31 (0.07)
1.91 (0.12)
2.52 (0.14)
3.09 (0.17)
0.68 (0.04)
1.38 (0.13)
2.08 (0.1)
2.80 (0.13)
3.58 (0.34)
Nine patients had unilateral advancements on the left with a
mean (range) anterior vector of 2.8 (1.5-4.0) mm. Six patients
had unilateral advancements on the right with a mean (range)
anterior vector of 2.9 (2.0-5.0) mm. Eight patients had unilateral setback osteotomies, three on the left with a mean
(range) setback of 3.7 (2.5-5) mm, and five on the right with a
mean (range) setback of 3.9 (1.5-6) mm. None of the patients
presented with any long-term symptoms in the TMJ.
Discussion
Control of the proximal segment and accurate positioning
of the condyle in the glenoid fossa remain not only critical
but challenging during a sagittal split ramus osteotomy.6 The
inevitable rotation of the condyle of the major segment during correction of mandibular asymmetry by a USSO should
therefore be a consideration. The rotation of the major segment showed inevitable anteroposterior changes that should
be considered when planning treatment.
Rotation of the condyle
The geometric study indicated that relatively small rotations
of the condyle can be expected on the major segment after
correction of mandibular dental asymmetry of < 5 mm by
USSO. The natural adaptability of the mandibular condyle
to small changes in its relation to the glenoid fossa is certainly advantageous, and the possible reason for the lack of
postoperative symptoms in the TMJ in the patients studied.
The position of the condyle on the minor segment should,
however, not change and be carefully controlled during the
operation. Advancement of the major segment will result in
a defect anteriorly between the segments, while a setback of
the segment will cause a bony defect posteriorly. Any force
applied between the segments of bone during placement of
rigid fixation may result in peripheral condylar sag. The use
of a bone clamp and lag screws should be avoided, and serious
intersegmental bony defects managed by either placement of
small bone grafts in the defects, contouring of the bone, or
fracturing the retromolar segment before placement of rigid
fixation.7
Anteroposterior changes
The inevitable anteroposterior changes of the lower central
incisors as a result of the rotation of the major segment
must be considered when planning treatment. Correction
of 5 mm mandibular asymmetry in the dental midline by
advancement of a major segment (rotation) will result in
3.17 mm advancement of the lower incisors, while a setback
of the major segment will cause the lower central incisors
Please cite this article in press as: Beukes J, et al. Unilateral sagittal split mandibular ramus osteotomy: indications and geometry. Br J
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5
to move posteriorly by 3.56 mm. This anteroposterior dental
change must be dealt with by either maxillary advancement
or setback. Maxillary setback is, however, not aesthetically
pleasing and seldom indicated. The aesthetic requirements
of the case will govern the decision.
Ethics statement/confirmation of patients’ permission
Indication
References
Skeletal mandibular asymmetry is usually corrected by
BSSO. However, USSO should be considered for patients
who require maxillary advancement for the correction of a
Class III malocclusion when there is relatively little mandibular asymmetry with a dental midline discrepancy of <5 mm.
Operating on only one side of the mandible reduces possible mandibular complications by half, and also reduces the
duration of treatment.
We conclude that correction of relatively small mandibular
asymmetries by USSO does not seem to cause adverse complications in the TMJ, but secondary anteroposterior changes
in the lower central incisors should be considered when planning treatment.
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Conflict of Interest
Only routine records of orthognathic surgery were used. All
data were anonymous.
We have no conflict of interest.
Please cite this article in press as: Beukes J, et al. Unilateral sagittal split mandibular ramus osteotomy: indications and geometry. Br J
Oral Maxillofac Surg (2015), http://dx.doi.org/10.1016/j.bjoms.2015.10.029