Dental implant surgery: planning and guidance
S. Lobregt 1, J. J. Schillings 1 and E. Vuurberg 1
This article describes a prototype application for The PISA project investigates and develops the
Dental implants interactive planning of dental implants, developed planning process and transfer to the patient as a
can be planned on the EasyVision clinical workstation, and whole, including many aspects of all the steps from
interactively on integrated with the existing software environment planning to the manufacture of a tool for the
the EasyVision of EasyVision. The application features a task- transfer of planning results to the patient.
workstation. oriented workflow which takes the clinical user
step by step through the process of (virtually) The tools developed within the PISA project
positioning dental implants, ending with the can lead to a significant improvement of the
design of a surgical aid (drill guide) for transfer of planning for implant placement, by optimizing
the planning results to the patient. The design of implant shapes and positions. Further
the shape of the drill guide is put completely in improvement can be expected with respect to
the hands of the clinical user, as shape design or the quality of the surgical intervention itself.
modification by the manufacturer is, in principle, Due to the use of surgical aids such as drill guides,
no longer necessary. The drill guide shape can the intervention will be better controlled,
be exported to a manufacturer in CAD/CAM as the guidance of surgical instruments will allow
compatible format. the surgeon to perform the operation exactly
according to the plan. It is expected that the
The work was carried out as part of the EC-funded time until replacement of the implant will be
PISA project, aimed at the development of tools increased considerably, and that fewer
for the design and manufacture of personalized replacements will be required. Additionally,
implants and surgical aids. the duration of the surgical intervention itself
will also be reduced.
The PISA project The work of Philips Medical Systems within the
PISA project focused on the clinical pre-operative
PISA (Personalized Implants and Surgical Aids) is planning for placement of dental implants, and on
The user is led a project aimed at developing tools and techniques the design of surgical aids in the form of drill
step by step to for improving the design and manufacture of guides to transfer the planning to the patient.
the design of personalized implants and surgical aids. Within the Interactive manipulation of computer-generated
a customized context of the PISA project, dental implants, hip implant models is used to simulate the placement
drill guide. or knee prostheses, pedicle screws, cranial plates of implants, and an automatic procedure has been
etc. are regarded as personalized’ when they are developed to generate a surgical aid. We have also
optimally adapted and positioned to a specific developed an interface between the medical
patient. A perfect fit and positioning of implants workstation and the CAD/CAM oriented
is of high importance. Sub-optimal implant environment, which produces the actual implants
positioning causes sub-optimal load transfer and surgical tools. The results of Philips Medical
between implant and bone, and is a major cause System’s work in the PISA project are described in
of implant failure and (too) early replacement. an internal report .
Although, to various extents, planning is possible
on currently available systems, there is in general Dental planning process
MIMIT EasyVision no means of transferring the planning to the
Advanced patient. However, this transfer is just as important The complete dental planning process consists
Development as the planning itself. Precise positioning of drilling of several steps. This section will give a global
Systems, Best, and cutting instruments is required if the desired overview of the whole planning process, while the
the Netherlands. result is to be obtained. various steps will be explained in following sections.
30 MEDICAMUNDI 45/4 November 2001
The diagram in Figure 1 assumes that patient data
of the relevant part of the anatomy is already
available, and the planning starts with the selection
of this data from the EasyVision database. The
patient data is derived from a CT scan, and is
visualized as cross-sections in user-definable
directions. Bone segmentations can be produced,
stored in the database and visualized as 3D’
renderings, together with the grayscale values.
After an adequate cross-sectional plane through the
mandibula or maxilla has been selected, a smooth
curve can be drawn to define a curved plane. This
is used to generate a panoramic view, as well as a
set of cross-sections perpendicular to the curved
plane, providing local information on the quality When the implants are optimally positioned with Figure 1.
Overview of the
of the bone. respect to the anatomy and to each other, an inter-
active function for generation of a drill guide can planning process.
If required, the lower alveolar nerve canal in the be started. A user-definable contact area on the The functionality
mandibula can be modeled and visualized, which is bone surface, together with the positioned implant
an interface to the
helpful for maintaining a safe distance between models, forms the input for an automatic procedure manufacturing
implants and this nerve canal. which generates a complete CAD/CAM-compatible environment,
for exchange of data
description of a suitable drill guide shape. This
Implants can be selected from a list of available representation can be exported to the technical compatible formats.
models. This can, for instance, be a list provided environment of a manufacturer, in principle with-
by one or more preferred suppliers. The selected out the need for further adaptations or optimiza-
implant model is placed in an initial position by a tions on the part of the manufacturer. If, for any
single mouse click, and can then be manipulated to reason, the manufacturer finds it necessary to make
optimize the position. The various images give modifications, or if a drill guide is designed at
direct feedback to the user by showing cross- another (clinical or technical) location, it is possible
sections of CT data and implants, as well as 3D to import, visualize and manipulate shape descrip-
renderings of anatomy with implants. tions of drill guides or other computer generated
objects in standard CAD-compatible format.
The central block in Figure 1 represents the
functionality for visualization and manipulation of All information such as segmentations, positions
all involved objects, providing a complete overview and shapes of implants, nerve canal models and Data is
of the situation. The viewing geometries of the drill guides, can be stored in the EasyVision available in
different viewports are coupled in order to facilitate database together with the patient data. This allows CAD/CAM-
user-friendly navigation. Objects such as implant the user to interrupt a planning process and compatible
models, nerve canal models or paths can be selected continue later, or to exchange patient and planning formats.
and manipulated. data with other EasyVision users.
Selection of relevant
using EasyVision’s 3D
MEDICAMUNDI 45/4 November 2001 31
Segmentation plane. Distances between these cross sections can
be varied interactively, and the whole set of cross
Various semi-automatic tools are available for sections can be repositioned along the curve by a
Semi-automatic obtaining a desired segmentation result, including simple mouse click. The curve can easily be
segmentation thresholding, region growing and watershed modified, by adding, removing or repositioning
tools are separation. If required, the result of these semi- control points. The panoramic view and cross
provided. automatic tools can be manually edited and refined sections change with it. For positioning of implants
in a user-friendly and interactive way. Editing can in the mandibula, it is important that the nerve
be performed in 2D as well as in 3D with direct canal should be clearly shown in the panoramic
feedback from the screen. Figure 2 shows an view.
example of segmentation editing in 3D, where one
mouse click is used to indicate the part of the Nerve canal modeling and implant positioning
segmented bone which is to be kept.
For implants in the mandibula, it is of great
Curved path definition importance to keep a safe distance from the nerve
canal (lower alveolar nerve). Modeling and
It is important that implants are positioned at visualization of the canal is provided for better
A panoramic locations where the bone is strong. A panoramic assessment of the spatial relationship between
cross section cross-section along a curved plane through the implants and the nerve canal.
provides an mandibula or maxilla, combined with cross-
overview for sections locally perpendicular to this curved plane, Accurate automatic segmentation of the nerve
assessing bone provide an adequate overview for assessment of canal would be difficult, because of the open
quality. bone quality. structure of the surrounding bone, and it is
doubtful whether a clinician would be prepared to
The first step is to define a suitable cross-sectional spend a lot of time trying to manually optimize
base plane through the dataset, such that the such an approximate segmentation. We therefore
relevant part of the mandible or maxilla is clearly chose to provide a fast and simple option for
visible (Figure 3). This base plane can have any modeling the canal with a 3D polybezier path
orientation with respect to the original slices. Next, (Figure 4). Such a path can be created with a small
a smooth polybezier path, which passes number of mouse clicks in the panoramic view as
approximately through the center of the bone, is well as in the cross-sectional views, and modified if
created by a few mouse clicks. This results in the required. The resulting path models the centerline
generation of a panoramic view on a curved plane of the nerve canal and is displayed in all available
through the created path and perpendicular to the views, so that the distance between implants and
selected base plane, as well as a number of cross nerve canal can be evaluated visually during
sections perpendicular to this base plane and also placement or repositioning of implant models.
locally perpendicular to the curved panoramic Possible future extensions of functionality could go
Panoramic view plus
cross sections along
curve through the
32 MEDICAMUNDI 45/4 November 2001
The display shows all
cross sections of the
are shown together
3D renderings and
in the direction of automatic evaluation and Drill guide generation and interface with the
restriction of the distance between implant models manufacturer
and nerve canal.
The drill guide is generated automatically, based
The optimal position for implants is determined on the planned positions and sizes of the implants The drill guide
interactively by placing and repositioning and an area of contact between bone and drill is generated
geometric models of implants with respect to the guide. The contact area indicates where the drill automatically.
segmented bone and, if applicable, the nerve canal. guide is allowed to touch the bone. A proposed
The required implant can be selected from a list of contact area is generated automatically, after which
available implant sizes and lengths, which can be it can be modified interactively in 3D by the user,
customized to represent, for example, the range of if required (Figure 5).
implants which is available from a preferred
supplier, or the set of implants which is commonly The building of a suitable drill guide from
used in a particular institution. implants and contact area is a complicated process
involving manipulation of the surface of the
The implant models can be virtually placed jawbone as well as geometrical primitives
within the patient image data by a single mouse representing implants and supporting cylinders.
click in the base plane through mandibula or We developed an automatic solution for this,
maxilla. The implant is then positioned at the which uses representations of bone, contact area,
indicated location and perpendicular to the base implants, and drill guide in the form of a
plane. Cross sections or 3D renderings of the triangulated surface.
implant are visualized in all other viewports as
well. The segmented bone in the 3D visualization The whole procedure of drill guide generation is
can be rendered as transparent if desired. After very user friendly and fast, and results in a
placement of an implant model, the position and description of a complete drill guide, which can be
orientation of the model can be modified exported in STL format to any manufacturer.
interactively in any of the viewports until the Knowledge of the application by the manufacturer
position and orientation are considered to is not required, as production from this description
be optimal. is straightforward and will result in a drill guide
MEDICAMUNDI 45/4 November 2001 33
from left to right:
posed contact area,
and contact area as
modified by the user.
from left to right:
for export to the
which will be exactly as planned by the clinical possibility to export bone segmentations to a third
The design of expert who performed the planning. This puts the party for design of a drill guide.
the drill guide is design of the drill guide shape completely in the
in the hands of hands of the clinical user, as shape design or First evaluation
the clinical user. modification by the manufacturer is, in principle,
no longer necessary. In our opinion, only the We have performed a first evaluation of the
clinical expert can be responsible for the final shape described method by planning the placement of
of the drill guide. implants in data acquired from an edentulous
patient, and evaluating the fit of the resulting
Export to and import from a manufacturer is drill guide on a model of the mandibula for
possible in the form of triangulated surface which it was designed. Three implants were
descriptions in STL format. This format is a positioned in the mandibula and, based on this,
commonly used standard in CAD/CAM a drill guide was designed and manufactured
environments. The EasyVision visualization by Materialise, which is a partner in the
software was extended with functionality to render PISA project.
such triangulated surfaces in combination with
voxel-based segmentation results. In addition to the planning data, the CT data
were also sent to Materialise in order to produce a
Conversion of voxel-based segmentations to model of the mandibula itself (Figure 6). The fit
triangulated surface descriptions was also included between drill guide and the mandibula model
in the tools we developed. This provides the appeared to be perfect.
34 MEDICAMUNDI 45/4 November 2001
The next step in evaluation of the described We have developed a prototype application for Figure 6.
prototype will be an experiment to perform the planning dental implant placement which can serve
a drill guide,
planning of implants on an animal cadaver as a basis for surgical planning in a more general designed according
mandibula. This evaluation will cover the sense. The software has been fully integrated in to planning results,
is fitted on a model
complete process from CT scan to drill guide the EasyVision clinical workstation environment.
of the mandibula
production, including the automatic drill guide Many of the tools which were developed as part of for which it was
generation part. Data acquisition will be done this work can be applied in a broader context designed.
while the bone is embedded in soft tissue. The within future EasyVision products. Examples of
mandibular bone will be cleaned and prepared these are the functionality to visualize and
after data acquisition to evaluate the fit of the manipulate triangulated surfaces in combination
drill guide on the bone. with voxel-based data, and the functionality for
data exchange with CAD/CAM based
Conclusion and further work environments. Examples of applications, which
may use the developed tools are the positioning of
In this article we have described the design of a pedicle screws, orthopedic implant planning,
drill guide which rests on the bone surface. The manufacturing of anatomical models for various
same software can, however, be used to produce a purposes, and planning for navigation systems or
drill guide which rests on available teeth, or even robotic systems.
on the soft tissues. These options will remove the
necessity to expose the bone surface during Acknowledgements
operation. Using soft tissue to support the drill
guide may, on the other hand, have a negative Some of the concepts for simulation of dental
effect on accuracy. This will have to be determined implant positioning originated from earlier re- The software
by further evaluation. search by the ESAT department of the University tool can applied
of Leuven , which was one of the partners in in a broader
An additional option, which becomes available this project. The technology for manufacturing of clinical context.
with the tool for exporting segmentation the drill guides was provided by Materialise B.V.
results as CAD/CAM compatible triangulated , also from Leuven. The authors would also like
surfaces, is the export of segmentations of to thank Professor F.W. Zonneveld and Mrs
anatomical structures to a manufacturer for the A.G.M Huitema of Philips Medical Systems’ CT
more general purpose of producing 3D anatomical department, who provided valuable support with
models, for instance for assessment of complex respect to the clinical science and application
situations, rehearsal of surgical procedures, aspects. The work as a whole is part of the EC-
education etc. funded PISA project.
 Lobregt S, Vuurberg E, Schillings JJ. Dental Implant An Image-Guided Planning System for Endosseous
Planning in EasyVision, Technical End Report PISA Oral Implants. IEEE Transactions on Medical
Project, Philips Publication XPR 072-010024.00, Imaging 1998; 17: 842–852.
 Materialise Rapid Prototyping & Manufacturing,
 Verstreken K, Van Cleynenbreugel J, Martens K, Leuven. SurgiCase: Drilling Templates for Dental
Marchal G, Van Steenberghe D, Suetens P. Implantology.
MEDICAMUNDI 45/4 November 2001 35