Balancing incompatible endoprosthetic design goals: a combined ingrowth and bone remodeling simulation.
ABSTRACT In order to design a good cementless femoral implant many requirements need to be fulfilled. For instance, the range of micromotions at the bone-implant interface should not exceed a certain threshold and a good ratio between implant-bone stiffness that does not cause bone resorption, needs to be ensured. Stiff implants are known to evoke lower interface micromotions but at the same time they may cause extensive resorption of the surrounding bone. Composite stems with reduced stiffness give good remodeling results but implant flexibility is likely to evoke high micromotions proximally. Finding a good balance between these incompatible design goals is very challenging. The current study proposes a finite element methodology that employs subsequent ingrowth and remodeling simulations and can be of assistance when designing new implants. The results of our simulations for the Epoch stem were in a good agreement with the clinical data. The proposed implant design made of porous tantalum with an inner CoCrMo core performed slightly better with respect to the Epoch stem and considerably better with respect to a Ti alloy stem. Our combined ingrowth and remodeling simulation can be a useful tool when designing a new implant that well balances mentioned incompatible design goals.
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ABSTRACT: In this thesis we evaluated two different hip arthroplasty concepts trough in vitro studies and numerical analyses. The cortical strains in the femoral neck area were increased by 10 to 15 % after insertion of a resurfacing femoral component compared to values of the intact femur, shown in an in vitro study on human cadaver femurs. There is an increased risk of femoral neck fracture after hip resurfacing arthroplasty. An increase of 10 to 15 % in femoral neck strains is limited, and cannot alone explain these fractures. Together with patient specific and surgical factors, however, increased strain can contribute to increased risk of fracture. An in vitro study showed that increasing the neck length in combination with retroversion or reduced neck shaft angle on a standard cementless femoral stem does not compromise the stability of the stem. The strain pattern in the proximal femur increased significantly at several measuring sites when the version and length of neck were altered. However, the changes were probably too small to have clinical relevance. In a validation study we have shown that a subject specific finite element analysis is able to perform reasonable predictions of strains and stress shielding after insertion of a femoral stem in human cadaver femurs. The usage of finite element models can be a valuable supplement to in vitro tests of femoral strain pattern around hip arthroplasty. Finally, a patient case shows that bone resorption around an implant caused by stress shielding can in extreme cases lead to periprosthetic fracture.Acta orthopaedica. Supplementum 04/2012; 83(345):1-26.
- Recent Advances in Arthroplasty, 01/2012; , ISBN: 978-953-307-990-5
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ABSTRACT: Currently available implants for direct attachment of prosthesis to the skeletal system after transfemoral amputation (OPRA system, Integrum AB, Sweden and ISP Endo/Exo prosthesis, ESKA Implants AG, Germany) show many advantages over the conventional socket fixation. However, restraining biomechanical issues such as considerable bone loss around the stem and peri-prosthetic bone fractures are present. To overcome these limiting issues a new concept of the direct intramedullary fixation was developed. We hypothesize that the new design will reduce the peri-prosthetic bone failure risk and adverse bone remodeling by restoring the natural load transfer in the femur. Generic CT-based finite element models of an intact femur and amputated bones implanted with 3 analyzed implants were created and loaded with a normal walking and a forward fall load. The strain adaptive bone remodeling theory was used to predict long-term bone changes around the implants and the periprosthetic bone failure risk was evaluated by the von Mises stress criterion. The results show that the new design provides close to physiological distribution of stresses in the bone and lower bone failure risk for the normal walking as compared to the OPRA and the ISP implants. The bone remodeling simulations did not reveal any overall bone loss around the new design, as opposed to the OPRA and the ISP implants, which induce considerable bone loss in the distal end of the femur. This positive outcome shows that the presented concept has a potential to considerably improve safety of the rehabilitation with the direct fixation implants.Journal of biomechanics 06/2012; 45(11):1875-80. · 2.66 Impact Factor