Der acetabuläre Pressfit bei äquatorialer Beschichtung der zementfreien Hüftpfanne – eine finite-Elemente-Analyse Pressfit of equatorially roughened cementless acetabular components – a finite elements analysis

Shear stress and von Mises stress distributions in the periphery of an embedded acetabular cup implant during impingement

As literature implies, daily activities of total hip arthroplasty (THA) patients may include movements prone to implant-implant impingement. Thus, high shear stresses may be induced at the acetabular implant-bone interface, increasing the risk of implant loosening. The aim of the current study is to determine whether or not impingement events may pose an actual risk to acetabular periprosthetic bone. An existing experimental workflow was augmented to cover complete three-dimensional strain gage measurement. von Mises and shear stresses were calculated from 1620 measured strain values, collected around a hemispherical cup implant at 2.5 mm interface distance during worst-case impingement loading. A shear stress criterion for acetabular periprosthetic bone was derived from the literature. At the impingement site, magnitudes of von Mises stress amount to 0.57 MPa and tilting shear stress amount to -0.3 MPa at 2.5 mm interface distance. Conclusion can be drawn that worst-case impingement events are unlikely to pose a risk of bone material failure in the periphery around fully integrated cementless acetabular hip implants in otherwise healthy THA patients. As numerical predictions in the literature suggested, it can now be confirmed that impingement moments are unlikely to cause acetabular implant-bone interface fixation failures.

Calculation of the elastic properties of prosthetic knee components with an iterative finite element-based modal analysis: quantitative comparison of different measuring techniques

With the aging but still active population, research on total joint replacements relies increasingly on numerical methods, such as finite element analysis, to improve wear resistance of components. However, the validity of finite element models largely depends on the accuracy of their material behavior and geometrical representation. In particular, material properties are often based on manufacturer data or literature reports, but can alternatively be estimated by matching experimental measurements and structural predictions through modal analyses and identification of eigenfrequencies. The aim of the present study was to compare the accuracy of common setups used for estimating the eigenfrequencies of typical components often used in prosthetized joints. Eigenfrequencies of cobalt-chrome and ultra-high-molecular weight polyethylene components were therefore measured with four different setups, and used in modal analyses of corresponding finite element models for an iterative adjustment of their material properties. Results show that for the low-damped cobalt chromium endoprosthesis components, all common measuring setups provided accurate measurements. In the case of high-damped structures, measurements were only possible with setups including a continuously excitation system such as electrodynamic shakers. This study demonstrates that the iterative back-calculation of eigenfrequencies can be a reliable method to estimate the elastic properties for finite element models.