[Finite element simulation of the dilatation of coronary vascular implants]. / Finite Elemente Simulation der Dilatation Koronarer Gefässimplantate.

Intravascular stents are small tube-like structures expanded into stenotic arteries to restore blood flow. The stent is mounted on a balloon catheter and delivered to the site of blockage. First the balloon is inflated, so the stent expands and is pressed against the stenotic wall of the coronary artery. After balloon deflation the stent remains in the coronary artery and the balloon is removed. In this paper, the finite element method is used to simulate the expansion and recoil after ballOon deflation of the stent as well as the cyclic loading of the stent caused by the changing blood pressure.

[New compounds for improving wear behavior of a tumor knee endoprosthesis]. / Neue Werkstoffpaarungen zur Verbesserung des Verschleissverhaltens einer Tumor-Knieendoprothese.

Wear of the central bushing made of ultra-high molecular weight polyethylene (PE-UHMW) of the hinged knee endoprosthesis of a tumour-resection system is the leading reason for revision. The aim of the study was to optimize the wear characteristics of the endoprosthesis on the basis of the tribological properties of new materials and an additional finite element (FE) calculation taking account of the given design. In screening tests the reference combination of PE-UHMW bushing and CoCr axis--used in the clinical setting--was first tested. The PE-UHMW bushing was then replaced by one made of each of the materials reinforced high-density polyethylene (PE-HD) and carbon fibre-reinforced epoxy resin (CFRP). In addition, a new material combination with an alumina ceramic bushing and a CFRP axis was investigated. In comparison with the reference combination PE-UHMW/metal, the combination of ceramic bushing and CFRP axis showed less wear. However, with the particular design of the prosthesis studied here, high mechanical loading applied experimentally resulted in mechanical failure. FE calculations confirmed these experimental results. Improvement of the wear characteristics of this specific implant caused therefore be achieved only by optimizing the bearing design.

Mechanical simulation of composite hip stems.

Calculation of new orthopedic implants prior to manufacturing of prototypes can be economic in the case of complex production processes. The use of composite materials for highly loaded hip stems is one application of the Finite-Element Method. Due to the anisotropic behaviour of composite structures, special routines had to be programmed for element alignment and failure analysis. Stability of carbon fibre-reinforced epoxy hip stems could be confirmed by experimental results. The risk of neck fracture was found to be one of the critical features in the design process.