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Accuracy Evaluation of ComputedTomography–DerivedStereolithographic Surgical Guides inZygomatic Implant Placement inHuman CadaversBruno R. Chrcanovic, DDS*Davidson R. Oliveira, MSAntonio L. Custodio, PhD
Presurgical planning is essential to achieve esthetic and functional implants. For implant
planning and placement, the association of computer-aided design (CAD) and computer-
aided manufacturing (CAM) techniques furnishes some advantages regarding tridimensional
determination of the patient’s anatomy and fabrication of both anatomic models and
surgical guides. The goal of this clinical study was to determine the angular deviations
between planned and placed zygomatic implants using stereolithographic surgical guides in
human cadavers. A total of 16 zygomatic implants were placed, 4 in each cadaver, with the
use of stereolithographic (SLA) surgical guides generated by computed tomography (CT). A
new CT scan was made after implant insertion. The angle between the long axis of the
planned and actual implants was calculated. The mean angular deviation of the long axis
between the planned and placed implants was 8.06 6 6.40 (mean 6 SD) for the anterior-
posterior view, and 11.20 6 9.75 (mean 6 SD) for the caudal-cranial view. Use of the
zygomatic implant, in the context of this protocol, should probably be reevaluated because
some large deviations were noted. An implant insertion guiding system is needed because
this last step is carried out manually. It is recommended that the sinus slot technique should
be used together with the CT-based drilling guide to enhance final results. Further research
to enhance the precision of zygomatic implant placement should be undertaken.
Key Words: zygomatic implant, atrophic maxilla, image-based surgery,stereolithography, customized drill guides
INTRODUCTION
Presurgical planning is essential
to achieve excellent esthetic and
functional outcomes with dental
implants. Many conflicting vari-
ables such as the mandibular
canal, maxillary sinus, and adjacent teeth
Pontifıcia Universidade Catolica de Minas Gerais, MinasGerais, Brazil.* Corresponding author, e-mail: [email protected]: 10.1563/AAID-JOI-D-09-00074
RESEARCH
Journal of Oral Implantology 345
must be considered before implant surgery
is performed. Practitioners generally have
used conventional dental radiographs (pan-
oramic and periapical radiographs)1 and
conventionally fabricated surgical guides2,3
for implant placement. The panoramic radio-
graphs that are commonly used in implant
treatment planning are limited by their
characteristics of magnification and distor-
tion, as well as by lack of sharpness of the
image. Also, a panoramic radiograph is a 2-
dimensional image that provides little in-
formation about the buccal-lingual width of
the jawbones.4 The surgical guides conven-
tionally fabricated on diagnostic stone casts
do not provide information about the
varying thicknesses of the mucosa, the
topography of underlying bone, or the
anatomic structures, and they do not remain
stable during surgery because of reflected
soft tissue.2
Computed tomography (CT) is a helpful
tool for implant patients, especially in situa-
tions with anatomic limitations, insufficient
bone dimensions, and poor bone density.5,6
The use of CT imaging enhances the
correlation between implant planning and
actual implant placement compared with
conventional radiographic methods.7
Currently, few software systems use CT
scans to aid in planning surgery and to
produce surgical drilling guides. These
guides are manufactured in such a way that
they match the location, trajectory, and
depth of the planned implant with a high
degree of precision. As the dental practi-
tioner places the implants, the guides
stabilize the drilling by restricting the de-
grees of freedom of the drill trajectory and
depth. Earlier studies concluded that 3-
dimensional (3D) planning resulted in im-
plant positioning with improved biome-
chanics and esthetics.8–10 Use of such a
system usually prevents complications such
as mandibular nerve damage, sinus perfora-
tions, fenestrations, or dehiscences.8,11 Also,
computer-aided design (CAD) and compu-
ter-aided manufacturing (CAM) software may
improve the association between dental
implant planning and insertion, in terms of
3D determination of the patient’s jaw
anatomy and fabrication of both anatomic
models and surgical guides.8,12–14
Some studies have recently illustrated
promising results with stereolithographic
(SLA) surgical guides.12,15–18 The SLA consists
of a vat containing a liquid photopolymer-
ized resin.18 A laser mounted on top of the
vat moves in sequential cross-sectional
increments of 1 mm, corresponding to the
slice intervals specified during the CT for-
matting procedure. The laser polymerizes
the surface layer of the resin on contact.
Once the first slice is completed, a mechan-
ical table immediately below the surface
moves down 1 mm, carrying with it the
previously polymerized resin layer of the
model. The laser subsequently polymerizes
the next layer adjacent to the previously
polymerized layer.18 In this manner, a
complete SLA model of the maxilla and the
surgical guides is created.
Restoration of the atrophied edentulous
maxilla poses a great dilemma to the oral
and maxillofacial surgeon and the restorative
dentist. Patients with adequate maxillary
bone are ideal candidates for implants, but
they are the exception. Patients with mod-
erate to severe atrophy challenge the
surgeon to discover alternative ways to use
existing bone or resort to augmenting the
patient with autogenous or alloplastic bone
materials.
Various techniques have been described
for approaching the atrophic maxilla, includ-
ing the use of tilted implants in the parasinus
region19–22 and implants in the pterygoid
apophysis,23,24 grafting of the maxillary sinus
floor,25,26 and the use of short, wide im-
plants,27,28 different types of grafts,29–31 and
zygoma implants.32–40 The incidence of
implant loss in the severely resorbed pos-
Accuracy of 3D CT-Based Surgical Guides
346 Vol. XXXVI/No. Five/2010
terior maxilla is approximately 15% without a
sinus bone graft.41
The zygoma implant has been designed
by Branemark33,34,42 for those situations in
which there is insufficient bone in the upper
jaw, which would otherwise require onlay or
inlay (sinus) bone grafts. Zygomatic bone is
excellent for the anchorage of implants, as
has been validated in several anatomic
studies.43–46 These authors agree that the
quality of zygomatic bone is superior to that
of the posterior maxilla, and the importance
of the cortical portion of the zygomatic bone
for anchoring implants43 has been described.
Furthermore, zygomatic implants display
initial primary stability, because it has been
demonstrated that the zygomatic bone area
where the implant is inserted has wider and
thicker trabecular bone.45
Implant placement in the zygoma bone,
however, can be difficult because of the
variable anatomy and varying degrees of
atrophy possible in the maxillofacial re-
gion.47 The technique is not performed
without risk because the drill path is close
to important anatomic structures. A signifi-
cant error can be induced by only a slight
deviation of the drill path direction.48
The clinical effectiveness of the use of the
drill guide and the important advantage for
aesthetic outcome have been described.49
For zygoma implants, the accuracy of the
transfer of the preoperative plan to the
surgical field is even more crucial. Given an
appropriate visualization, 3D CT images
provide an unparalleled depiction of the
complex anatomic topography that has to
be respected when the trajectory of a
zygoma implant is decided.
Personalized drilling templates may be
fabricated by a computer-based transfer
from the available 3D CT planning data50;
this allows incorporation of all predeter-
mined biomechanical, esthetic, and ana-
tomic factors during the surgical proce-
dure.
The goal of this clinical study was to
determine the angular deviations between
planned and placed zygomatic implants in
human cadavers by using SLA-based drilling
guide technology.
MATERIALS AND METHODS
The study protocol was approved by the
Institutional Ethics Committee of the Federal
University of Minas Gerais. Four human
cadavers were considered for the study.
Standardized CT scanning procedures
were followed for each cadaver and were
performed by the same radiologist operating
a CT machine (Classic i-CAT, Imaging
Sciences International, Hatfield, Pa). CT data
for each cadaver were imported to the
planning software (Dental Slice software,
BioParts Prototipagem Biomedica, Brasılia,
Brazil), allowing the surgical team to simu-
late implant placement on the 3D model.
While taking into consideration the anatomic
structures, the surgical team interactively
simulated the position of the implant on
each plane. Once the implant is planned, its
angulation can still be adjusted and its
dimensions adapted to obtain the optimal
position of the implant (Figure 1). The
implant is directed in a lateral and upward
direction with an angulation of 45 degrees
from a vertical axis. The end point has to
encroach into the zygomatic bone, which
has a thickness of about 10 mm. The zygoma
implant thus follows an intrasinusal trajec-
tory. After initial positioning of the implant,
several minor adjustments can be made until
the implant is surrounded by bone at its
entry and end points, to ensure that the
intermediate part does not perforate the
anterior maxillary wall. A rapid prototyping
machine based on the principle of stereo-
lithography was used to fabricate the SLA
models and guides. The aim was to create an
individualized drill guide that is suited to the
bone profile.
Chrcanovic et al
Journal of Oral Implantology 347
The SLA machine also read the diameter
and angulation of the simulated implants
and selectively polymerized resin around
them, forming a cylindrical guide corre-
sponding to each implant. Surgical grade
stainless steel tubes were attached to the
cylindrical guide. To prevent lateral angula-
tion of the drill during the drilling process,
drill guides (made by Peclab Ltda, Belo
Horizonte, Brazil) that perfectly adapted to
the stainless steel tubes were made (Fig-
ure 2). Drilling of the zygoma implants was
performed with the use of 4 drills. Conse-
quently, 4 sets of drill guides were provided.
The inner diameter of the drill guides is
0.3 mm greater than the diameter of the
corresponding drill. The angulation and
mesiodistal and buccolingual positioning of
each implant as planned with the use of 3D
computer simulation software were trans-
ferred to the SLA surgical guide.
The SLA bone-supported surgical guide
type was used. The surgical drill guide was
fitted onto the maxilla and was fixated with 2
or 3 osteosynthesis screws (10.0 3 1.5 mm).
The drilling procedures were performed with
the use of appropriate drills for each
corresponding implant according to the
manufacturer’s instructions. A total of 16
zygomatic implants were placed (SIN Sis-
tema de Implante, Sao Paulo, Brazil), all
4.0 mm of diameter ranging from 37.5 to
57.5 mm in length. Four implants were
placed in each cadaver, 2 in the canine
region, 2 in the first molar region, with the
use of SLA surgical guides generated from
CT. A new CT scan was made for each
cadaver after implant insertion.
FIGURE 1. The 3-dimensional computed tomography (CT) planning system. Axial, transversal, panoramic,and tridimensional CT slices are possible. Clinically relevant covisualization can be obtained.
FIGURE 2. Drill guides to every corresponding drillwere made to perfectly adapt into the cylindricalguide.
Accuracy of 3D CT-Based Surgical Guides
348 Vol. XXXVI/No. Five/2010
Adobe Photoshop Elements software
(version 2.0, Adobe Systems Incorporated,
San Jose, Calif) was used to match images of
planned and placed implants, and their
positions and axes were compared. Pre-
operative and postoperative CT scans in
anterior-posterior (Figure 3) and caudal-cra-
nial (Figure 4) views were aligned to allow
observation of the superposition of anatomic
markers. The angle between the long axes of
the planned and the actual implant (Fig-
ures 5 and 6) was calculated with the
VistaMetrix software (version 1.36.0, Skill-
Crest, Tucson, Ariz). Basic descriptive statistics
was employed to analyze the data obtained
using standard software (Excel, Microsoft
Corporation, Redmond, Wash).
RESULTS
In the right posterior implant of cadaver 4
(C4), the angular deviation between planned
and actual implant position in an anterior-
posterior view was 0.35 degrees, the smallest
deviation, but the left posterior implant of
cadaver 3 (C3) in a caudal-cranial view
showed the largest deviation (37.60 de-
grees). A more detailed presentation of the
angular deviation between planned and
placed implants in an anterior-posterior and
a caudal-cranial view in the 4 cadavers is
found in the Table.
The mean angular deviation of the long
axis between planned and placed implants
was 8.06 6 6.40 (mean 6 SD) for the
anterior-posterior view, and 11.20 6 9.75
(mean 6 SD) for the caudal-cranial view.
Minimal and maximal values for the anterior-
posterior view were 0.35 degrees and 21.20
degrees, and 0.76 degrees and 37.60 degrees
for the caudal-cranial view.
DISCUSSION
The strength of the anchorage in the
zygoma compensates for the bad quality of
the bone, mostly type IV in the posterior
maxilla. From a biomechanical point of view,
it has been demonstrated that if the zygoma
fixtures are connected to the anterior
FIGURES 3–6. FIGURE 3. Preoperative and postoperative computed tomography (CT) scans in an anterior-posterior view were aligned while the superposition of anatomic markers was observed. FIGURE 4.Preoperative and postoperative CT scan superposition in a caudal-cranial view. FIGURE 5. Superpositionof the planned and placed implants in a caudal-cranial view (cadaver 4). FIGURE 6. Superposition of theplanned and placed implants in an anterior-posterior view (cadaver 1).
Chrcanovic et al
Journal of Oral Implantology 349
implants, masticatory forces applied to the
fixed prosthesis are transferred to the
zygoma.44
The CT scanning template is the principal
key to the system because it permits the
transfer of the predetermined prosthetic
setup to the actual implant planning. The
scanning template is an exact replica of the
desired prosthetic outcome; this allowed
both surgeon and restorative dentist to base
implant planning on the desired prosthetic
outcome. The treatment plan is thus driven
by the prosthetic end result.16,51
Other studies have assessed the magni-
tude of error in transferring the planned
position of implants from CT scans to a
surgical guide. In the in vitro study by
Besimo et al,52 the deviation between the
positions of the apex of proposed implants
on cross-sectional CT images and on the
corresponding study cast was measured at
77 sites. Transfer errors for the maxilla and
mandible were 0.6 6 0.4 mm and 0.3 6
0.4 mm. However, investigators concluded
that the transfer errors noted in their study
were not clinically relevant because other
factors involved in transferring positional
and angular measurements from CT images
to the actual surgical area may result in
greater errors. Another in vitro study by
Sarment et al12 included 50 implants placed
into 5 epoxy resin edentulous mandible
models. Each epoxy resin mandible received
5 implants on each side. On the right side, 5
implants were inserted using a conventional
surgical guide, whereas on the left side, 5
implants were inserted using an SLA surgical
guide. When compared with conventional
guides, significant improvements were evi-
dent in all measurements taken with SLA
surgical guides. Investigators stated that the
clinical significance of this result may be
relevant when multiple parallel distant im-
plants are placed, and where the degree of
accuracy is critical for obtaining a single
prosthetic path of insertion. Their studies,
although very relevant, were made with
normal dental implants only in the jaws,
not in the zygoma.
Van Assche et al53 placed 12 implants in 4
formalin-fixed cadaver jaws. Upon compar-
ison with the planned implants, investigators
noted average angular deviation of 2 6 0.8
degrees and mean linear deviation of 1.1 6
0.7 mm at the neck and 2 6 0.7 mm at the
apex in the placed implants. Another human
cadaver study by Van Steenberghe et al44
included 6 zygoma implants with surgical
drilling guides based on CT data. Researchers
matched preoperative CT scans with post-
operative CT scans to evaluate the deviation
between planned and placed zygoma im-
plants. Investigators reported that the angu-
lar deviations in the axis for 4 planned and
placed implants were less than 3 degrees,
whereas 1 implant showed 3.1 degrees and
the last one showed 6.9 degrees angular
deviation in the axis.
Di Giacomo et al15 evaluated the match
between the positions and axes of planned
TABLE
Angular deviations between planned and actual zygomatic implant positions in 4human cadavers*
Cadaver C1 C2 C3 C4
View A-P C-C A-P C-C A-P C-C A-P C-C
RegionFirst right superior molar 12.30 4.20 1.46 14.80 6.66 14.20 0.35 8.59Right canine 2.70 14.70 4.22 18.30 11.20 0.76 5.05 6.30First left superior molar 3.62 3.14 1.54 24.70 21.20 37.60 9.88 6.39Left canine 8.79 5.74 4.99 14.10 18.50 1.22 16.50 4.41
*A-P indicates anterior-posterior view; C-C, caudal-cranial view.
Accuracy of 3D CT-Based Surgical Guides
350 Vol. XXXVI/No. Five/2010
and inserted implants when an SLA surgical
guide was used. They inserted 21 implants in
4 patients using 6 SLA surgical guides and CT
data, measuring the deviation between
planned and inserted implants. Investigators
noted an average angular deviation of 7.25
6 2.67 degrees between planned and
inserted implant axes. This average angular
deviation was higher in the study of Vrielinck
et al48—10.46 degrees (range: 0–21.0 de-
grees)—also an in vivo study. Other in vivo
studies tried to determine deviations in the
position and inclination of planned and
placed implants using SLA surgical guides
and to compare 3 different types (tooth-
supported, bone-supported, and mucosa-
supported) of SLA surgical guides.18 Under
the guidelines of this study, CT-derived SLA
surgical guides supported by tooth, bone, or
mucosa provided a precise tool for both
flapless and conventional flap implant inser-
tion. Naitoh et al54 found angular deviations
between planning and placement ranging
from 0.5 to 14.5 degrees, with an average of
5.0 degrees, using teeth-supported conven-
tional guides. The CT data were used to
transfer only the position and/or inclination
of the implants to a laboratory-made tem-
plate placed on working plaster models.
The main goal of the study was to
evaluate the possibilities of skeletally sup-
ported drill guides for zygomatic implant
placement in patients with severely atrophic
maxillas, while still providing a predictable,
permanent, and successful treatment result.
The mean angular deviation of the long axis
between planned and placed implants was
8.06 6 6.40 (mean 6 SD) for the anterior-
posterior view, and 11.20 6 9.75 (mean 6
SD) for the caudal-cranial view. This aim was
not completely met by this treatment con-
cept, in terms of angular deviations between
planned and placed implants. Despite the
fact that deviations between planned and
placed implants could be quite substantial,
in some cases this may not affect the ability
of the restorative dentist to design and
fabricate a prosthetic suprastructure onto
these deviated implants. Deviations with
good clinical results also occurred, as can
be observed with the left side on cadaver 1
(Figures 7 and 8).
It is not easy to make direct comparisons
between in vitro studies and the present
human subject, as in vitro studies provide
improved control of all contributing pa-
rameters. However, it was observed as large
angular deviations, probably because of
FIGURES 7 AND 8. FIGURE 7. Good clinical results. Left side of cadaver 1. FIGURE 8. Good clinical results. Leftside of cadaver 1.
Chrcanovic et al
Journal of Oral Implantology 351
poor fit of the surgical guide, between other
causes. The precision of the whole procedure
depends largely on the ability to position
accurately the drill guide on top of the bone,
and to maintain that stable position during
the whole procedure. The difference of
osteosynthesis screws in fixing the surgical
guide onto the maxilla bone can also have
an important role. In this study, 1 mm length
screws were used—a half length when
compared with the study of Vrielinck et
al.48 The number of screws was also lesser: 2
or 3 against 4 or 5 in the study of Vrielinck et
al.48 Asymmetric distribution of the screws or
uneven tightening of the screws could bring
the drilling template out of balance. Further-
more, a certain error is induced as the
diameter of the steel tubes is slightly larger
than the drill diameter. Finally, the largest
error is probably due to the fact that the final
step in the procedure is carried out manu-
ally. Implant placement cannot be done
through the surgical drill guide because of
present mechanical limitations. The drill
guide, therefore, has to be removed before
the implant is actually inserted, leaving the
possibility of additional deviation.
The original Branemark protocol creates a
sinus window technique for placement of
these zygoma dental implants. Stella and
Warner’s55 published ‘‘sinus slot technique’’
significantly simplified the original Brane-
mark protocol. The ‘‘sinus slot’’ is a guide
window made directly through the buttress
wall of the maxilla, whereby the zygoma
implant is guided through the maxilla to the
apex insertion at the junction of the lateral
orbital rim and the zygomatic arch. This
lateral sinus slot allows greater potential for
bone-to-implant interface because of this
lateral position, and eliminated the sinus
window and sinus lining elevation for place-
ment of the implant. This lateral window
allows direct vision to the base of the
zygoma bone and helps control the implant
position by direct vision.56 The fact that we
have done these surgeries in a cadaver
without direct vision through the maxillary
sinus may also have influenced the results.
Moreover, some deviation may occur
when the actual implant entry point is
considered compared with initial treatment
planning. This may be due to the brittle and
soft consistency of bone in the maxillas with
severe bone atrophy.48
On the left side of cadaver number 3, 1
implant emerged in the infratemporal fossa
(Figure 9) and the other one inside the orbit
FIGURES 9 AND 10. FIGURE 9. Poor clinical results. One implant emerged in the infratemporal fossa ofcadaver 3. FIGURE 10. Poor clinical results. One implant emerged inside the orbital cavity of cadaver 3.
Accuracy of 3D CT-Based Surgical Guides
352 Vol. XXXVI/No. Five/2010
(Figure 10). This great deviation of course
occurred because of the long length of the
zygomatic implants (37.5–57.5 mm)—3 to 4
times that of oral implants—which means
that even minute angular deviations lead to
important discrepancies at the extremity.44
According to Vrielinck et al,48 it should be
noted that the common practice today is to
position zygoma implants without any form
of physical control of the drilling trajectory.
However, care has to be taken to ensure a
proper mesiocranial direction for the im-
plant. If the implant is planned too much
laterally, it would emerge in the infratem-
poral fossa. If, on the contrary, it is planned
too much mesially it would end up in the
nasopharynx or the sphenoid sinus. If the
inclination of the implant is too much in the
cranial direction, it would enter the fossa
pterygopalatina. For an implant directed too
much horizontally, no bony structures will be
encountered.48 Based on the dimensional
variability of the zygoma bone, such errors
might, even with accurate 3D CT-based
planning and transfer, create potential dan-
gers.
It must be emphasized that in the
preliminary feasibility study, implant plan-
ning may be done solely on the basis of
available bone volume (ie, implant planning
may not take into account information
conveyed through a preoperative prosthetic
set-up, because the quantity of present bone
may be minimal because of the atrophy).
Consider that the ‘‘wrong’’ implants position
should not affect the ability to design and
fabricate a prosthetic suprastructure onto
these deviated implants, despite the angular
deviation.
CONCLUSIONS
The results of this study demonstrate that
the use of the zygomatic implant, in the
context of this protocol, should probably be
reevaluated because some large deviations
were noted. An implant insertion guiding
system is needed because this last step is
carried out manually. It is recommended that
utilization of the sinus slot technique
together with the CT-based drilling guide
would enhance the final results. The tridi-
mensional CT is a helpful tool for patient
candidates to zygomatic implants, because
the drill path is close to important anatomic
structures. The reported results may be
surprising and should stimulate further
research to enhance the precision of zygo-
matic implant placement, even given that
better results were obtained by former
studies.
ABBREVIATIONS
CAD: computer-aided design
CAM: computer-aided manufacturing
CT: computed tomography
SLA: stereolithographic
3D: three-dimensional
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