Numerically Simulated
Biomechanics
ROBERT EBERLEIN
THOMAS ZACHARIAS
SULZER INNOTEC
Sulzer Innotec – formerly corporate research and
development – today makes 70% of its sales to
customers outside the Corporation (see box on
p. 19). This is the reason why Sulzer Innotec is
still active in the field of medical technology,
although Sulzer had withdrawn from this business
with the spin-off of Sulzer Medica in 2001. One
example of the activities of Sulzer Innotec are
biomechanical calculations which facilitate the
development and licensing of orthopedic implants.
In the field of medical technology, Sulzer Innotec deals
with many different topics, especially in the fields of orthopedics,
the cardiovascular system, and the
photorealistic visualization of surgical techniques (e. g. with ultrasound and implantable signal generators). The customer projects
comprise the provision of both
support and advice in the mechan-
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SULZER TECHNICAL REVIEW 1/2004
ical design of implants and their
integration in specially developed
and tested calculation models of
biological tissue, such as bone,
muscle, and ligaments.
Numerical simulations play an
outstanding role in the solving of
these problems. Based on models
of the knee and the spine, this article gives an insight into the biomechanical calculation of orthopedic
4113
Simulation of the Knee Flexion
Behavior of the
materials
The most important aim of the implantation of a knee prosthesis is
to improve the patient’s quality of
life. This is only possible if the implant allows pain-free and nearly
physiological movement in the
joint. During the development
process, the implants are subjected to various tests. On the one
hand, stability tests are carried out
in order to assure that there is both
adequate freedom of movement
and no false positioning of the
joint resulting from extreme exter-
nal loading. On the other hand, by
means of in-vitro kinematic tests
(knee flexion, gait cycle) it is determined whether or not the artificial joint is suitable and capable of
restoring the natural range of
movement.
In order to accelerate the development of artificial knee joints and to
reduce the number of these costly
preclinical tests, Sulzer Innotec has
developed an idealized and experimentally validated model of the
human knee joint based on the finite element method (FEM). This
Simulation model
The Activities of Sulzer Innotec, the Center for Technology and Innovation
Overall behavior
1 A biomechanical analysis in
orthopedics leads from the
analysis of the behavior of the
materials used, through the
creation of a simulation model,
to the calculation of the behavior
of the implant in the body.
implants and their interaction with
the surrounding biological structures, and into the experimental
validation of these calculations
(Fig. 1). Both these projects were
carried out on behalf of Centerpulse Ltd*.
________________
* After its spin-off in 2001, Sulzer Medica
was renamed Centerpulse. On October 2,
2003, Centerpulse was acquired by the
American Zimmer Holdings, Inc.
Sulzer Innotec offers technological services, worldwide, for the solution of individual customer problems: in contractual research (innovative concepts and methods
for future products), in the development
of new business concepts, and in the fields
of production and quality assurance.
These services also include troubleshooting and the further development of existing products.
Contractual research is concentrated on:
Mechanics, fluid dynamics, materials,
and surfaces, as classical industrial
fields
Medical technology, as specifically applied in the above fields (biomechanics,
biofluidics, biocompatible materials,
implantable sensory devices, and ultrasound equipment)
business fields. Already in 2002, Sulzer
Innotec obtained more than two-thirds of
its total business from external companies
in the fields of energy, transport, chemical
industry, and medical technology.
Rapid, Flexible, and Interdisciplinary
Sulzer Innotec generates an additional
value in the rapid application of new technologies in the industrial environment
and in this way broadens the possibilities
of its customers, as well as of the universities and specialized professional schools.
Thanks to its around 140 highly qualified
specialists, projects are carried out competently, rapidly, flexibly, and in the strictest
confidence.
CONTACT
Successful Transformation
In all fields, Sulzer Innotec is able to call
on a great wealth of experience from more
than 50 years’ successful industrial research. Since 2001, Sulzer Innotec has been
on the way to becoming a self-supporting
business area that is making an important
contribution to the stability of the core
business and to the building-up of new
Sulzer Innotec
Hans-Walter Schläpfer
Postfach 414
8401 Winterthur
Switzerland
Phone +41 (0)52-262 21 51
Fax +41 (0)52-262 00 15
hans.schlaepfer@sulzer.com
www.innotec.ch
SULZER TECHNICAL REVIEW 1/2004
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Hip joint
Flexor muscle
of the knee
Femur
Extensor muscle
of the knee
Patella
Collateral ligaments
Tibia
Ankle joint
2 With this experimental model of a knee joint,
Sulzer Innotec validated its finite element model
for correctness.
3 FEM model of an artificial knee joint with collateral
ligaments and retained posterior cruciate ligament.
model consists of the femur, tibia,
patella, ankle, hip joint, and, by
means of springs, idealized collateral ligaments (Fig. 2). The knee
flexion is controlled by a vertical
shifting of the hip joint and stabilizing muscular forces. In the experimental study, the kinematics
of the individual components
were tracked by means of an optical tracking procedure.
Since it takes a matter of several
seconds and as the masses involved are comparatively small, a
typical knee flexion can be considered as quasi-static. During flexion, the knee joint undergoes
major shifts – mainly in relation to
the femur –, i.e., the FEM model
contains geometric non-linearities.
These are correctly taken into
account by the Abaqus/Standard
FEM program that is used, as are
also the major relative shifts in the
contact surfaces between femur
and tibia, and between femur and
patella.
According to the validation, the
FEM model can be used for various different studies. For example,
with this mode, it is possible to
study the ligament tensions that
are to be set during an operation,
or the effect of the retention of a
posterior cruciate ligament on the
knee kinematics (Fig. 3). Moreover,
new implant designs can be incorporated into the model and tested
without any problem. This considerably reduces the time required
and the costs of the development.
Lateral collateral
ligament
Evaluation of a Mobilityretaining Spinal Implant
Posterior cruciate
ligament
Medial collateral
ligament
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SULZER TECHNICAL REVIEW 1/2004
In order to be able to carry out an
evaluation of spinal implants, it is
necessary to have a basic understanding of the natural biomechanics of the human spine. This
is especially true in the case of the
Dynesys™ spinal implant (see
STR 1/1999, p. 10) considered
here, as it maintains the mobility
of the spine, i. e., it stabilizes the
vertebrae without stiffening them
(Fig. 4).
The effect of a Dynesys implant on
a degenerated mobile segment (set
up by two adjacent vertebrae) of
the lumbar spine was compared
with the intact kinematics. The
conceptional procedure selected
for this is divided into three steps.
In step 1, all the relevant material
parameters and material models,
for example for the tissue of the intervertebral discs, were procured.
These data formed the basis for
subsequent biomechanical modeling of the mobile spinal segment.
In step 2, this mobile segment was
subjected to various FEM analyses.
Specifically investigated was the
4 The Dynesys spinal implant
consists of screws, tubular spacers,
and an in-lying tie-rod.
also allows – besides the simple
evaluation of the implant itself –
improvements to the implant design.
Simplification of Approvals
The validated calculation models
provide valuable support in the
initial development or further development of high-grade implants
and prostheses. Besides their use
in development and design, the
numerical calculations described
here also have great potential for
simplifying and accelerating CE
and FDA approvals and for making this process less costly. They
offer the possibility of testing and
demonstrating the efficacy and
safety of new and further developments in a largely “natural” environment and thus of contributing
to increased cost-effectiveness in
the health sector.
5 Simulation of two vertebrae together with the Dynesys
spinal implant.
6 Calculations show that the Dynesys
spinal implant limits the unnaturally
high mobility of a spinal segment in
flexion to physiological values.
Intact segment
Degenerated segment
Segment stabilized with Dynesys
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180
Mobility of the segment (%)
biomechanical response of the intact mobile segment, but also the
effect of an almost completely
degenerated intervertebral disc,
which has only minimal residual
load-bearing capacity. The deformation behavior of a mobile spinal
segment is not trivial and can
therefore only be simulated exclusively by means of efficient numerical methods such as FEM.
Therefore in the present case, as in
the previously described knee
analysis, the Abaqus/Standard
FEM program was used.
Finally, in step 3, the effect of the
Dynesys implant on the lumbar
spine was analyzed. This was
done by incorporating the CAD
data for the Dynesys spinal implant into the already available
calculation model of the mobile
segment (Fig. 5). Then, as in step 2,
FEM analyses of the mobile segment instrumented with Dynesys
were carried out.
With the biomechanical FEM
analysis, numerous different loadings simulating a real (physiological) loading of the mobile segment
in the human body were investigated. One example is flexion
(bending forward of the upper
part of the body). The resulting rotation of the upper spinal segment
compared with the lower is represented for an intact segment, a degenerated segment, and a segment
stabilized with a Dynesys implant
(Fig. 6). The diagram shows how
here the Dynesys implant supports the restoration of the natural
kinematics of the particular spinal
segment. Naturally, the various
components of the implant must
first be adjusted to the physiological mobility of two adjacent vertebrae. The biomechanical FEM
analysis makes this possible and
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CONTACT
120
Sulzer Innotec
Thomas Zacharias
Postfach 414
8401 Winterthur
Switzerland
Phone +41 (0)52-262 42 11
Fax +41 (0)52-262 00 85
thomas.zacharias@sulzer.com
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