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Computer Aided Surgery
4:281?285 (1999)
Brief Technical Report
Real-Time Simulation of Tissue Deformation for the
Nasal Endoscopy Simulator (NES)
Uli Bockholt,
Dipl.-Math.,
Wolfgang Mu?ller, Dipl.-Inf., Gerrit Voss, Dipl.-Inf., Ulrich Ecke, M.D.,
and Ludger Klimek, M.D.
Interactive Graphics Systems Group (GRIS), Darmstadt University of Technology, Darmstadt (U.B.),
Department of Visualization and Virtual Reality, Fraunhofer Institute for Computer Graphics
(Fraunhofer-IGD), Darmstadt (W.M., G.V.), and Department of Otolaryngology, Head and Neck
Surgery, Mainz University Hospital, Mainz (U.E., L.K.), Germany
ABSTRACT Endonasal sinus surgery requires a great amount of training before it can be adequately performed. The complicated anatomy involved, the proximity of relevant structures, and the
variability of the anatomy due to inborn or iatrogenic variations make several complications possible.
Today, cadaver dissections are the ?gold standard? for surgical training. To overcome the drawbacks
of traditional training methods, the Fraunhofer Institute for Computer Graphics is currently developing a highly interactive medical simulation system for nasal endoscopy and endonasal sinus
surgery, in cooperation with the Mainz University Hospital.
For the simulation of a rhinoscopic procedure, not only are the realization of the 3D
interaction and the geometric representation of the anatomical structures necessary, but also a
real-time simulation of the deformation behavior constrained by the instrument collisions. The
challenge is to close the gap between a maximal degree of realism and the required real-time
conditions. Comp Aid Surg 4:281?285 (1999). �99 Wiley-Liss, Inc.
Key words: surgical simulation, skill training, virtual reality, soft tissue deformation, rhinoscopic
education
BACKGROUND
Endonasal surgery has become standard for the
treatment of diseases of the paranasal sinuses and a
variety of other pathologies that can be reached via
the nasal cavity. In general, the purpose of this
surgery is to relieve intractable sinus pain, to remove large expanding mucoceles or pyoceles, and
to prevent or control further central, orbital, or
external extensions of infection. Since its introduction, functional endoscopic sinus surgery (FESS)
has demonstrated success rates of 76% to 98%.3,6
Of the patients who failed to respond to both FESS
and initial medical therapy, only a small number
require revision endoscopic surgery (RESS).6 The
most frequent intraoperative findings, such as ad-
Received May 17, 1999; accepted August 23, 1999
Address correspondence to: Uli Bockholt, Interactive Graphics Systems Group (GRIS), Darmstadt University of Technology,
Rundeturmstrabe 6, D-64283 Darmstadt, Germany; Telephone: 49-(0)6151-155-283; Fax: 49-(0)6151-155-196. E-mail:
bockholt@igd.fhg.de.
This paper is based on a presentation at Medicine Meets Virtual Reality (MMVR) 7, held in San Francisco, California, in
January 1999.
�99 Wiley-Liss, Inc.
282
Bockholt et al.: Real-Time Simulation of Tissue Deformation
hesions, maxillary ostial stenosis, recurrent polyposis, and incomplete removal of diseased air cells,
often lead to a loss of normal anatomic landmarks.
Therefore, the surgeon must depend on his experience and knowledge of more general anatomic relationships.
The ?gold standard? today remains cadaver
dissection. Unfortunately, only a limited number of
cadavers are available for each trainee, and some of
these trainees may go on to perform surgery in
patients before having gained sufficient surgical
skills. Moreover, it is most likely that the trainee
will not see rare anatomic variations or perform
revision surgery in a cadaver: A normal variant
without disease will be found in most specimens,
the tissue is often changed by formalin preservation, and no bleeding occurs.
To overcome the limitations of these traditional training methods, VR training simulators are
being developed for several endoscopic procedures.4,5
Simulation of surgery in a virtual environment for educational purposes has several advantages over cadaver dissection: VR simulators allow
for unlimited numbers of procedures with a single
system, every anatomic variation can be simulated,
and conditions like massive polyps or a postoperative situation with missing landmarks can be included in the virtual environment.
METHODS
The Nasal Endoscopy Simulator (NES) prototype
consists of a graphics workstation (SGI 02), a
tracking system, surgical instruments, and a plastic
model of the head (Fig. 1). The visual feedback and
the control of the training session is realized by a
graphical user interface, which includes the possibility to record and replay a training session (Fig.
2). During navigation in the virtual endonasal sinus
system with the endoscope, the trainee is able to
test various endoscope optics. Collisions between
instruments and anatomical structures are detected,
and the trainee is able to deform soft tissues with
the instruments. In the development of the Nasal
Endoscopy Simulator (NES) several goals have to
be achieved:
?
Patient-specific CT slices have to be reconstructed to form a 3D representation of the
anatomical structures. For performance reasons, triangulated surface models are used for
the geometric representation (Fig. 3). The
trainee can choose several cases with different
Fig. 1. VR interface of NES.
pathologies. To give the geometric models a
realistic appearance, rhinoscopic live images
are mapped onto the surfaces.
? The 3D interaction has to be realized, i.e., the
different interaction capacities of the typical
scissors, gators (biopsy forceps), and absorbers. We use a electromagnetic tracking system
to register position and orientation of the surgical instruments. The opening angle of the
gator is measured by a small potentiometer
(Fig. 4).
? Models have to be developed that describe the
relevant physiological behavior of the anatomical structures. For example, the simulation of bleeding and of deformation behavior
is important for the endoscopic procedure. In
the first step, the simulation of the deformation behavior resulting from the forces exerted
by the instruments on the endonasal tissues is
realized (Fig. 5).
Simulation of Tissue-Specific Deformation
Behavior
In a rhinoscopic operation, the surgeon is deforming the endonasal tissues with the surgical instruments, e.g., by pulling with scissors or pushing
with a probe. To simulate these procedures we have
implemented and tested different approaches. The
tissue-specific characteristics should be taken into
account, the realism of the simulation should be as
Bockholt et al.: Real-Time Simulation of Tissue Deformation
283
Fig. 2. Graphical user interface of NES.
high as possible, and the real-time condition should
not be lost. In the initial approach, the deformation
behavior is simulated by some smooth interpolation
functions describing the deformed virtual situs. In
other approaches, a mass-spring model is used for
the simulation.
Fig. 3. The virtual situs.
284
Bockholt et al.: Real-Time Simulation of Tissue Deformation
Fig. 4. Simulation of surgical instruments.
Simulation Using the Smooth Function
Method
In this approach the deformation behavior is described by ?bump weighting functions?,1 whereby
a smaller inner region, surrounding the collision
points of the instrument and the anatomical structure, is moved according to the instruments? constraint. The outer region of the surface at a distance
from the collision points remains undeformed. A
smooth interpolation function (?bump weighting
function?) is then used to interpolate between these
two regions. According to this interpolation function, the nodes in the area between the inner and
outer regions are moved, so that the smoothness of
the surface is obtained. Tissue-specific deformation
characteristics can be considered in this approach
by means of the inner and outer radii of the deformed and undeformed regions, by the shape of
these regions, and by the interpolation function.
The advantage of this method is its ability to provide a real-time simulation. Combined with the
collision detection, the simulation of deformation
with the bump-weighting function hardly influences the performance of the medical training system.
Simulation Using the Mass-Spring System
For the simulation of the deformation of anatomical
structures, Finite Elements Methods (FEM) are applied. The simulation is very accurate, but the solving of the equations requires much computational
power. Consequently, efforts have been made to
overcome these drawbacks by using simplified
FEM models which can be solved under real-time
conditions.2 Mass-spring systems represent such
simplified FEM models connecting two mass
points by a spring. In this way, a physiological
model of the virtual situs is generated via mass
points and springs describing the elastodynamic
behavior of the anatomical structures. The elasticity
of the spring is described by spring constants, and
the masses of the points are controlled by the mass
Fig. 5. Simulation of 3-D interaction (left: pulling, right: pushing).
Bockholt et al.: Real-Time Simulation of Tissue Deformation
values. In the first step, the mass points are attached
to the surface nodes, and the springs to the surface
edges. In addition, some springs are positioned
through the deformable volume, connected to mass
points on the opposite side of the surface. The
system is initialized in such a way that the energy
minimum of the mass-spring model represents the
undeformed shape of the physiological model.
When surgical instruments are manipulating the
surface of the anatomical structure, the colliding
faces are deformed in response to the external stimuli with a physically-based behavior. In this case
the equilibrium of the mass-spring model is disturbed, and the motion equation has to be solved for
each mass point. The solving of the differential
equation is realized by the Adams Bashforth
method. This high-order approach requires the
computationally expensive functional iteration only
once. In the second step, the Delaunay triangulation
is used to establish a full volume mass-spring
model represented as consisting of many more
mass points. The uses of such a complex model are
manifold, but it needs much more computational
power. Moreover, the elastodynamic characteristics
of the tissue types can be described by a variety of
parameters using a mass-spring model, e.g., spring
constants, mass values, and the relevant damping
constant of the springs.
DISCUSSION
The nasal endoscopy training simulator (NES) represents an advanced training system incorporating
Virtual Reality and multimedia for training and for
quality control in endonasal sinus surgery. The
trainees are able to practice various surgical techniques without having to advance their learning
curve on humans. The simulation of deformation is
a step towards interactive realism in computerassisted training. Current work focuses on the integration of a haptic device, in order to feel the give
and resistance of the anatomical structures. In that
context, an adequate and more sophisticated description of the tissue-specific elastodynamic characteristics is necessary.
285
In addition, the simulation of virtual cutting is
under development. These medical simulators are
on the way to founding an educational base which
will perhaps be as important to surgery as flight
simulators are to aviation.
ACKNOWLEDGMENTS
We thank Prof. Dr. h.c. Dr.-Ing. Jose? L. Encarnac?a?o
and Prof. Dr. med. Wolf Mann for providing the
environment in which this work was possible. We
also thank all our colleagues and students at our
laboratory, especially Kristina Wittig and Harald
Hechler; without their work we would not have
been able to achieve the results presented herein.
Part of this work was funded by the German Research Society (Deutsche Forschungsgemeinschaft)
DFG.
REFERENCES
1.
Bryson S. Paradigms for the shaping of surfaces in a
virtual environment. In: SIGGRAPH ?92, 19th International Conference on Computer Graphics ?Interaction Techniques?, Course Notes 9, 1992.
2. Deussen O, Kobbelt L, Tu?cke P. Using simulated
annealing to obtain good nodal approximations of
deformable bodies. Proceedings of Sixth Eurographics
Workshop on Simulation and Animation, September
1995.
3. King JM, Caladarelli DD, Pigato JB. A review of
revision functional sinus surgery. Laryngoscope 1994;
104:404 ? 408
4. Ku?hnapfel U, Krumm H-G, Kuhn C, Hu?bner M, Neisius B. Endosurgery simulations with KISMET. Virtual Reality World?95, Stuttgart, 1995.
5. Mu?ller W, Bockholt U. The Virtual Reality Arthroscopy Training Simulator. In: Westwood JD, Hoffman
HM, Stredney D, Weghorst SJ (eds): Proceedings of
Medicine Meets Virtual Reality 6, San Diego, CA,
January 1998. Amsterdam: IOS Press; 1998. p 13?19.
6. Schaitkin B, May M, Shapiro A, Fucci M, Mester SJ.
Endoscopic sinus surgery: 4-year follow-up on the
first 100 patients. Laryngoscope 1993;103:1117?
1120.
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