Medical implants
There exist different aspects how nanometer-sized structures can control the mechanical properties and thereby the biocompatibility of medical implant materials. The orthopaedic surgery is often confronted with an early revision of hip prostheses due to the phenomenon of aspetic loosenig. This phenomenon occurs preferentially, but not exclusively, on Ti based hip stems fixed in the femoral bone by means of bone cement (fig. A). Thus, presently there exists a great insecurity about the application of distinct stem models. Several critical factors are considered to be responsible for the early failure of these hip prostheses. In particular nanometer-sized metallic wear debris generated in the frictionally loaded interface hip stem - cement may affect the cell response of the ambient tissue (fig. B). Main focus of the research project is the determination of significant factors favouring the generation of harmfull wear debris with the aim to avoid generation of such particles.
As the generation of metallic wear debris is restricted to implant surface regions of a few micrometers thickness, the knowledge of material surface properties of the contact partners such as topography, chemical composition and mechanical behaviour under clinical relevant loads is essential for an understanding of the mechanisms of particle generation. Implant surface processing routines as well as medical applications of hip prostheses is taken into account.
For this purpose, mico- and nanoscopic implant surface damages and surface damaging processes due to processing routines and clinical relevant loadings are characterized. Nanometer-sized cement reinforcing oxide particles and residual blasting media were found after wear simulation of commercially manufactured cemented stems (fig. C). They initiate most probably implant surface degradation by nanoscopic abrasive wear. In the next step the implant surface topography and chemistry will be mechanically and thermochemically modified to target an optimized implant surface design. By a thermochemical treatment nanoscopic precipitations of high harndess will be generated in the outer implant surface resulting in an improved wear resistance (fig. D). A new implant surface treatment offers a promising surface design for an improved strength of the interface stem - cement by a combined mechanical – thermochemical treatment. It can comply with the need of primary implant stability due to surface topography and with the need of drastical reduction of harmfull nanometer-sized particles due to nanoscopic chemical surface modifications.
