Chapter 8
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GENERAL DISCUSSION
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After answering the main questions posed at the end of the general introduction, some
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topics remain to be discussed with respect to the clinical applicability of TiP in impaction
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grafting for hip arthroplasty. Chapter seven showed that impacted TiP provided sufficient
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primary stability under loaded conditions in the challenging environment of a large
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acetabular reconstruction. Further, profound bone ingrowth was observed after fifteen
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weeks of implantation. Despite these promising results on the acetabular side, no realistic
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animal model was conducted to evaluate the applicability on the femoral side. Although
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chapter three describes low subsidence rates after 300,000 cycles of in vitro loading,
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femoral application poses surgical, biomechanical and biological challenges different to
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acetabular application. Firstly, the entrance to the femoral intra-medullary canal of the
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goat is relatively narrow. Secondly, despite uneventful in vitro application in the femur
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and uneventful in vitro as well as in vivo application in the acetabulum, TiP might turn
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out to be too hard for femoral application: the radially directed forces during impaction
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with a tapered impactor could result in a fissure or fracture of the usually osteopenic
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or osteoporotic femur. Further, the often sclerotic inner surface of the femoral shaft is
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biomechanically disadvantageous as it provides less interlocking with the TiP compared
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to the more cancellous structure of acetabular bone. Finally, there might be a biological
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challenge on the femoral side as bone graft incorporation is generally less than on the
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acetabular side. On the other side, these femoral conditions are also the same in the
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patients who have undergone femoral impaction grafting with bone grafts. Despite the
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less attractive circumstances, very satisfying long-term outcome has been reported after
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femoral bone impaction grafting.
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A very basic question concerns the standardization of the geometrical and mechanical
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properties of the implanted TiP. TiP are made from a bulk material with a wide range in
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size, porosity, pore sizes and micro stucture. From this bulk material, the TiP as applied in
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the studies described in this thesis were selected based on size, specific weight (porosity
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volume percentage) and macroscopic appearance. The production and selection method
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needs to be improved with respect to standardization of the presented fully synthetic
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potential grafting material of TiP. Mechanical properties (impactability and elasticity)
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