Structural Analysis of Replacement Knee Design - Ansys

CASE STUDY
Structural Analysis of Replacement Knee Design
DEPUY, A JOHNSON & JOHNSON COMPANY
EXECUTIVE SUMMARY
Challenge:
To analyze two sizes of a
replacement knee design at
different angles of articulation
Solution:
Use ANSYS to analyze the design
Benefits:
Allowed DePuy to attain a good
indication of the performance of
the product before testing
Introduction
DePuy is the oldest manufacturer of orthopaedic
implants in the United States. The company was
founded in 1895 when Revra DePuy, a salesman,
revolutionized the fracture management industry
by introducing wire splints to replace the
makeshift wooden splints then in use for
stabilizing fractures. DePuy is one of the world’s
leading orthopaedic companies, with a reputation
for innovation in new product development.
DePuy has patented different replacement knee
systems, first of which was developed more than
20 years ago. One of the types incorporates a
state-of-the-art mobile-bearing which offers a
wide range of options to allow the surgeon to
match the implant to the patients’ anatomy. The
graphic to the right illustrates a typical replacement
knee.
Challenge
The scope of this work was to analyze two sizes of
a replacement knee design at different angles of
articulation using ANSYS. Initially the finite
element results were compared with the known
experimental measurements obtained on one of
the two sizes at three angles of articulation. Once
the correlation had been achieved, the same
methodology was used to analyze the other
design at various angles.
Solution
The replacement knee design comprises two
components: the femoral component and the
bearing. The graphic to the right shows the solid
geometry of the design in ANSYS after
importation of the CAD model in Parasolid
format.
Both the femoral component and the bearing
were meshed with 3-D higher order
tetrahedral elements. The meshing of the two
parts was made fully parameterized. The mesh on
the underside of the femoral component was
made sufficiently fine to ensure minimal loss of
accuracy in the geometry of the curved contact
surfaces.
A coarser mesh was used in the interior and on
the upper side of the femoral component, since its
material was significantly stiffer than that of the
bearing, and consequently, very little structural
deformation was expected. Another option was to
mesh the contact surfaces of the femoral
component with rigid target and the load applied
to a pilot node.
A similar approach was used for the bearing, as
the size of the elements was more critical in the
“In addition, it permitted us to gain detailed information on stress and
deflection which can be difficult to detect in physical tests.”
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CASE STUDY
“This validation has allowed us to extend the application of this
methodology to the evaluation of a range of new implant designs,
providing feedback accurately and in a short timeframe.”
contact region than other non-contacting
surfaces. However, a mesh density even finer
than that on the contact surfaces of the femoral
component was desirable in the bearing to
ensure a good resolution of the contact area and
stresses. An indiscriminate refinement of the
mesh on all the upper surfaces of the bearing
proved to be computationally too expensive and
a new meshing procedure was developed and
tested by IDAC, a finite element analysis and
computer-aided engineering consulting firm
and the leading UK provider of ANSYS and
DesignSpace software.
A preliminary contact analysis was first run
with the original mesh density prescribed to the
bearing, then the elements that were in contact
with the femoral component were further
refined for the subsequent solution. An example
of this mesh is depicted in the figure on the
right.
The graphic above illustrates the stress
distribution in contact area between the bearing
and the femoral component. These stress
distribution plots can be created in the ANSYS
program for any point in time during the
nonlinear solution.
It was found that excessive geometric
penetration at setup produced stress
singularities and, therefore, the contact pair
should be checked prior to the solution.
Localized peak contact stresses also could be
produced by the discretization of the otherwise
smooth contact surfaces. The mesh refinement
level for the elements in the vicinity of contact
after the preliminary contact analysis may be
increased, but at the expense of a longer solution
time.
Apart from the contact stresses, the total contact
area was also an important result item. The total
contact area was obtained from summing the
areas of all contact elements showing partial or
full contact. This generally leads to an
overestimation of the actual contact area
although it was considered insignificant given
the high mesh density in the contact area.
All of the analysis work described here was
performed on Intel based personal computers
running the ANSYS Revision 7.0 program.
DePuy are users of ANSYS and the parametric
models created here by IDAC have been supplied
to DePuy for their engineers to perform
further analyses and modifications in-house.
Benefits
James Brooks, a senior mechanical design
engineer at DePuy, was impressed with the
results of the study. “Following on from this
study, and working with IDAC, a number of our
own engineers have been able to do further
comparisons of a new design against an existing
product in various loading conditions. This has
rapidly allowed us to get a good indication of
the performance of the product before testing.”
Fiona Haig, a mechanical designer at DePuy,
also adds, “IDAC’s macro allowed us to quickly
and consistently replicate physical testing
which would normally have taken weeks to
undertake in our labs. In addition, it permitted
us to gain detailed information on stress and
deflection which can be difficult to detect in
physical tests. The macro has proved an invaluable
tool in the comparison and validation of
new implant designs as well as proving a
highly effective learning aid for our core team
of FEA users.”
She continues. “The results achieved using
IDAC’s analysis method closely correlated to
the results of those physical tests previously
undertaken in our labs. This validation has
allowed us to extend the application of this
methodology to the evaluation of a range of
new implant designs, providing feedback
accurately and in a short timeframe.”
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