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DePuy is the oldest manufacturer of orthopaedic implants in the United States. The company was founded in 1895 when Revra DePuy, a salesman, revolutionised the fracture management industry by introducing wire splints to replace the makeshift wooden splints then in use for stabilising fractures. DePuy is one of the world's leading orthopaedic companies, with a reputation for innovation in new product development. DePuy have 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.
Description of Work
The scope of this work was to analyse two sizes of a replacement knee design at different angles of articulation using the general purpose finite element software 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 analysis the other design at various angles. The photograph to the left shows the replacement knee design. The femoral component (top part) is made of cobalt chrome alloy and the bearing (bottom part) is made of polyethylene.
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 3D higher order tetrahedral elements. The meshing of the two parts was made fully parameterised. 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 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 CADFEM UK CAE Ltd..
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 below 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. Localised peak contact stresses could also be produced by the discretisation 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 program. DePuy are users of ANSYS and the parametric models created here by CADFEM UK CAE Ltd. have been supplied to DePuy for their engineers to perform further analyses and modifications in-house.
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 CADFEM UK CAE Ltd., 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. “CADFEM UK CAE Ltd.'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 CADFEM UK CAE Ltd.'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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