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Dentomaxillofacial Radiology
DMFR 174—14/11/2006—11:19——235188
Dentomaxillofacial Radiology (2007) 36, 1–6
q 2007 The British Institute of Radiology
http://dmfr.birjournals.org
RESEARCH
Transtomography for implant placement guidance in
non-invasive surgical procedures
F Bousquet*,1, P Bousquet2 and L Vazquez3
1
3
Private Practice, Montpellier, France; 2Department of Perio Implantology, University of Montpellier, Montpellier, France;
Department of Stomatology, Oral Surgery and Dentomaxillofacial Radiology, University of Geneva, Geneva, Switzerland
Objectives: To illustrate the use of transtomography for the placement of implants using a
radiopaque radiographic guide and to evaluate the accuracy of transtomography.
Methods: The study included 11 implants inserted with minimally invasive procedures. Pre-, intraand post-operative examinations were performed with a ProMax panoramic unit implemented with
transtomographic technique (Planmeca Oy, Helsinki, Finland). At each implant site, cross-sectional and
longitudinal intraoperative transtomograms were taken through a radiopaque reference guide to control
and adjust the drilling axis. The effective axis on post-operative transtomograms was compared with the
planned axis correction estimated on intraoperative images. Radiopaque guides, used as the gold
standard, were measured on intraoperative cross-sectional slices to evaluate image distortion.
Results: Intraoperative transtomograms, with the reference guide inserted in the bone, gave clear
images of the cortical plates and accurate information of drilling length and axis which allowed the
surgeon to adjust pilot drilling axis in 6/11 (54.5%) sites, including sites with narrow bone ridges. The
implant axis on the post-operative tomogram compared with the planned axis correction showed an angle
difference ranging from 0.88 to 3.48. The image distortion on cross-sectional slices ranged from 0.03 mm
to 0.52 mm, resulting in a distortion ratio ranging from 0% to 6% when expressed in percentages.
Conclusions: Transtomographic examination performed with a radiographic reference guide
during implant surgery can provide the necessary and accurate information for implant placement.
Transtomography distortion appears to be less important than in other conventional tomographic
systems and comparable with CT scan distortion.
Dentomaxillofacial Radiology (2007) 36, 1–6. doi: 10.1259/dmfr/91082519
Keywords: dental implants, tomography, distortion, mandible
Introduction
Various imaging techniques, including conventional radiography and CT, are used for the pre-surgical planning of
implant placement. Conventional tomography is the
method of choice to obtain cross-sectional information of
small regions of interest for implant planning.1 Tomograms
before dental implant placement will help determine the
height, width, inclination and undercut of the alveolar
bone, as well as the location of anatomical structures such
as the mandibular canal, the submandibular gland fossa,
the maxillary sinus and the nasal fossa.2,3 Cross-sectional
images, using films or photostimulable phosphor (PSP)
plates obtained with panoramic radiographic units, showed
similar image performance and are acceptable for dental
Q1
*Correspondence to: Frédéric Bousquet; E-mail: [email protected]
Received 30 November 2005; revised 3 May 2006; accepted 18 June 2006
implant placement planning.4 In a study on phantom heads,
Welander et al5 tested transtomography, which employs
the narrow beam of an advanced direct digital panoramic
machine. They showed that this new tomographic scanning
technique, which cannot be used in other panoramic units
with a sensor technique, gave images with properties
comparable with those of conventional tomography.
In conventional implant placement surgery, a large
full-thickness periosteal flap is reflected to visualize the
bone contour before drilling, whereas flapless and
minimally invasive procedures maintain a better blood
supply to the site by leaving the periosteum intact on the
buccal and lingual aspects of the ridge.6,7 However, these
“blind” procedures make it more difficult to evaluate the
shape of the alveolar bone; as the risk of cortical plate
perforation is increased, flapless surgery is limited to
straightforward cases with a favourable bone crest.8 – 10
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The aim of this article is to illustrate how transtomography, when used with a radiopaque radiographic guide,
can assist the surgeon during implant placement. The
protocol developed in the present study allowed to evaluate
the accuracy of transtomography.
Methods
Between January and October 2005, nine partially
edentulous patients (six females and three male patients),
age ranging from 31 to 76 years (mean age 53 years), were
treated using flapless surgery or a minimally invasive
procedure. The implants were inserted in 11 sites (in one
incisor site in the maxilla, in seven molar and three
premolar sites in the mandible).
Pre-operative planning included clinical and radiographic examination. All patients were examined using a
ProMax panoramic unit implemented with transtomographic technique (Planmeca Oy, Helsinki, Finland). This
digital panoramic unit can make image layer thickness
ranging from 1 mm to 36 mm and combines a longitudinal
and cross-sectional transtomograms in one composite
image. An image layer thickness of 3 mm was chosen for
this study. The exposure values were 66 kV, 1 mA and 8 s.
Implant position and length were estimated by measuring
the crestal width and the distance from the top of the ridge
to the critical anatomic structures (cortical plates, undercuts, concavities or mandibular canal) on the panoramic
image (1.2 magnification factor) and on a composite image
(1.4 magnification factor; Figure 1).
All measurements were taken with the computed
Planmeca measurement program which allows the
calibration of the image on the screen. The ProMax
panoramic with transtomographic digital unit offers
three picture grabbing modes. The three resolution
modes are: normal resolution (one picture pixel equals
four physical pixels), enhanced resolution (one picture
pixel equals three physical pixels) and high resolution
(one picture pixel equals two physical pixels). The size
of the physical pixel is 33 mm. After picture calibration
Figure 1 Pre-operative longitudinal and cross-sectional transtomograms. The horizontal lines are drawn with the Planmeca program to
improve the assessment of critical anatomic structures. The distance from
the top of alveolar crest to the lingual undercut is 10 mm. The distance
from the top of the alveolar crest to the superior border of the mandibular
canal is 13 mm. The occlusal registration key, used to position the patient,
is also visible
Dentomaxillofacial Radiology
for panoramic or transtomographic images (1.2 or 1.4
magnification factor, respectively), measurements were
carried out in normal resolution mode (i.e. 132 mm
representing the size of four physical pixels), leading to
values precised to two decimals. Angle measurements
were carried out with the Dimaxis computerized
compass program joined to the ProMax panoramic
unit, which gave degree values precised to one decimal.
Consequently, to ensure accuracy of the measurement
system, results were not rounded off.
An individualized silicon occlusal registration key was
used for patient positioning during pre-, intra- and postoperative transtomography. Throughout surgery, to avoid
injury to the inferior alveolar nerve and other critical
anatomic zones, the drilling length through the mucosa and
the bone before transtomographic control was maintained
2 mm shorter than the estimated distance to these zones.
The first drilling, performed with the pilot drill, respected
the axis corresponding to the planned implant axis.
A custom-made radiopaque radiographic reference
guide made of titanium alloy (Ti6Al4V) was inserted in
the bone after the first drilling. The reference guide
consists of a 2 mm diameter intrabony part, either 5.50 mm
or 9.60 mm long. The intrabony part of the 9.60 mm long
guides has notches which make it easier to take
measurements on the images (Figure 2). The extrabony
part of the radiographic guide is 1.5 mm high, 4 mm in
diameter and protrudes out of the mucosa. At each implant
site, cross-sectional and longitudinal intraoperative transtomograms were taken with the radiopaque reference guide
securely sutured to the adjacent teeth or to the surrounding
mucosa. The position of the reference guide on the
transtomographic images allowed three-dimensional control of the planned surgical drilling trajectory (Figure 3), so
the drill’s axis could be adjusted as necessary.
After full drilling and enlargement, implants were
inserted. Ten implants were Branemark Tiunite Mk3
(Nobelbiocare France, Paris, France) and one was a
Replace Select Tapered (Nobelbiocare France). 5/11
(45.4%) implants were 13 mm long, 3/11 (27.3%) implants
were 11.5 mm long and 3/11 (27.3%) implants were
10 mm long.
Figure 2 Three radiographic reference guides. The intrabony part, with
or without notches, measures 9.60 mm for the long guides and 5.50 mm
for the short one. The extrabony part may be sutured to the mucosa
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Frédéric Bousquet et al
Figure 3 Intraoperative transtomogram showing the axis of the
radiopaque reference guide needing an estimated angle correction of
108. The bone height available for full drilling was controlled on the crosssectional view and estimated to 13 mm
For the purposes of this study, post-operative transtomography (Figure 4) was used to evaluate the inserted
implant; the effective axis of the implant on the postoperative images was compared with the planned axis
rectification estimated on intraoperative images.
The radiopaque guide, used as the gold standard in this
study, was measured on the intraoperative cross-sectional
slices to evaluate the image distortion. The measurements
were carried out thrice on the screen using the Planmeca
program after calibration of the image; the recorded value
was the measure having the biggest difference with the
gold standard. Measurements of the intrabony part of the
reference guide on the slices were compared with the real
intrabony length of the guide.
Results
Comparison of the pre-, intra- and post-operative transtomograms showed no artefact produced by either the
titanium guide or the dental implant. Composite images
(cross-sectional and longitudinal slices), with optional
alignment of the different zones on the two images,
improved the visualization of the bone contours and
anatomic limits at each site (Figure 1). In 7/11 (63.6%)
sites, the crest width was less than 7 mm (ranging from
5.2 mm to 6.5 mm) and a lingual undercut or a buccal
concavity was obvious on pre-operative tomograms in 8/11
(72.7%) sites.
All intraoperative transtomograms gave clear images of
the cortical plates and information of drilling length and
Figure 4 Post-operative transtomogram showing the inserted implant
with an effective angle correction of 10.88
3
axis which allowed the surgeon to adjust pilot drilling axis
in 6/11 (54.5%) sites, including sites with narrow bone
ridges. The effective axis of the implant on the postoperative transtomogram, compared with the estimated
axis rectification planned when necessary on the intraoperative transtomography, showed an angle difference
ranging from 0.88 to 3.48. Post-operative transtomograms
confirmed that all implants had been placed as planned and
that the implants did not intrude on critical anatomic
structures.
Measurements of the guide on cross-sectional slices
compared with the gold standard showed a distortion
ranging from 0.03 mm to 0.52 mm (maximum overestimation 0.03 mm, maximum underestimation 0.52 mm),
resulting in a distortion ratio ranging from 0% to 6%
when expressed in percentages.
Discussion
Welander et al5 described how advanced panoramic
machines that combine a translational movement with a
pendular movement of the beam and detector make direct
digital transtomographic images and can be utilized for the
same purposes as conventional tomography. Transtomography produces almost instant results and allows quick
measurements on the screen with a computed program.
This made transtomography more appropriate in our study
than conventional tomography using films or PSP plates. In
addition, the panoramic machine’s compact size allowed it
to be installed in the operating room. The surgeon rapidly
and conveniently performed intraoperative investigations
in the shortest time (with no discomfort to the patient) as
the panoramic machine was within a few paces of the
operating table. Positioning of the patient was facilitated
by means of the individualized silicon key.
The present image distortion (maximum overestimation
0.03 mm, maximum underestimation 0.52 mm) can be
compared with other previous studies on distortion using
different tomographic systems. In a conventional spiral
tomography study on human cadavers, Serhal et al11 used a
digital calliper for the measurements and reported a
maximum overestimation of measured distances of
1.05 mm and a maximum underestimation of 1.36 mm.
The 0% – 6% distortion reported with CT scan12 and the
0% – 5.2% distortion reported with limited cone-beam CT12
are similar to our results with transtomography (distortion
ranging from 0% to 6%).
As the distortion with transtomography, using a ProMax
panoramic unit implemented with transtomographic technique, and the CT scan distortion are similar, transtomography may be considered as an imaging tool providing
accurate information for implant planning.
In this study, the radiographs were produced with
66 kV, 1 mA, during 8 s and the emitted dose was 51 mGy
cm22. Absorbed dose for a composite image (one
longitudinal slice and one transversal slice) was 6 mSv –
8 mSv and 3 mSv – 4 mSv for a single slice (information
given by the manufacturer, Planmeca Oy). These absorbed
doses are comparable with results reported by Lecomber
Dentomaxillofacial Radiology
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et al.13 The absorbed dose was 2 mSv for one linear crosssectional tomogram slice and 314 mSv for a CT scan
examination. Other previous works showed absorbed doses
ranging from 4 mSv to 6.1 mSv for a conventional spiral
cross-sectional tomography14 and 3 mSv for one slice of
conventional tomography.15
Intraoperative transtomography using a radiographic
guide may be considered as a form of navigation surgery;
three-dimensional information allows the surgeon to
rectify the drill’s axis when needed. We believe it
is more flexible than reported navigation surgeries for
dental implants needing a CT scan, a computer software
and a template for implant guidance.16 – 19 Navigation
surgical procedures requiring a CT scan appear to be
better suited for large edentulous zones or full arch
rehabilitations, whereas a transtomographic navigation
protocol, resulting in a reduced absorbed dose, may be
considered a safe procedure for implant placement in
small edentulous zones.
This transtomographic navigation protocol may also
improve flapless surgical procedures; currently reported,
limitations with these blind procedures are a narrow
alveolar ridge and the presence of anatomical undercuts.8 – 10 When an undercut of more than 158 is detected
radiologically, or when the width of the ridge is less
than 7 mm, flapless procedures are no longer possible; a
traditional flap reflection is then recommended to allow
better visibility.8 – 10 With a radiographic guide and
transtomography, the risks of critical anatomic zones
injury and of cortical plate perforation were minimized,
allowing implant placement in all cases, including sites
presenting a narrow ridge, a buccal concavity or a
lingual undercut.
In conclusion, transtomographic examination performed with a radiographic reference guide during implant
surgery can provide the necessary and accurate information for implant placement. This protocol may be
especially valuable during blind surgical procedures as
cortical plates or critical anatomic structures may be
approached when needed without increasing the risk of
damaging them. Distortion of the images appears to be less
important than with other conventional tomographic
systems and comparable with CT scan distortion.
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