Title: Development of Biofunctional Porous Coatings for Bone Implants (Ontwikkeling van biofunctionele poreuze deklagen voor botimplantaten)
Other Titles: Development of Biofunctional Porous Coatings for Bone Implants
Authors: Braem, Annabel
Issue Date: 25-Sep-2012
Abstract: The application of orthopaedic and dental implants has revolutionised th e medical treatment of degenerative bone diseases, restoring compromised body functions and quality of life of many. However, despite the vast i mprovements realised during the past decades, implant failure still occu rs, leading to painful and costly revision surgery. The most prominent c auses of failure are implant loosening and infection. Therefore, current implantology research is focusing on strategies to improve implant fixa tion and/or reduce the incidence of infections. A key requirement for a long-term implant fixation is osseointegration, i.e. a direct implant/bo ne contact (without an intervening fibrous tissue layer) maintaining a r igid fixation of the implant during functional loading. Regarding implan t-related infections, which are usually of bacterial etiology, the impla nt should ideally be resistant to bacterial colonisation. From a mat erial¬ís perspective, the events at the implant/bone interface are mainly being mediated by the implant surface topography and chemistry and it i s generally hypothesised that by tuning the surface properties, it is fe asible to control the biological response. Titanium and its alloys, whic h are widely applied as implant materials for load-bearing applications, because of an excellent biocompatibility in combination with a high str ength, are capable of osseointegration under optimised conditions. To fu rther improve the integration in the host bone, porous Ti coatings are o f special interest, because these allow establishing a firm interlocking with the surrounding tissue by ingrowth of bone into the porous structu re. Alternatively, chemical modification or functionalisation of the oth erwise inert Ti surface in order to improve its osteoconductive an d osteoinductive behaviour is also widely investigated. Bioactive glasse s or glass-ceramics are often proposed as coating materials, because of their ability to chemically bond to bone, strengthening the interface be tween implant and host bone, and an osteostimulatory effect enabling fas ter regeneration of bone tissue surrounding the implant. This doctor al research focuses on the development of porous Ti coatings on Ti or Ti -6Al-4V substrates whether or not in combination with a bioactive glass- ceramic surface layer on the internal pore walls. The main objective of this thesis is a qualitative investigation of the relationship between m icrostructure and biological outcome regarding osseointegration and bact erial colonisation. Therefore, after establishing the various coating pr ocesses, a biological screening in comparison to several clinically rele vant reference surfaces is performed, addressing in vitro bacteria l attachment, in vitro cytocompatibility and in vivo bone re sponse. A new powder metallurgical processing route for the production of porous pure Ti coatings on Ti or Ti-6Al-4V is established. By the electrophore tic deposition of various grades of TiH2 powders followed by dehydrogena tion and sintering in high vacuum, porous Ti coatings with varying thick ness, pore morphology, and surface roughness can be obtained. Adhesion s trengths over 100% higher than for a state-of-the-art vacuum plasma spra yed porous Ti reference coating can be obtained. Moreover, in comparison to more conventional processing routes applying pure Ti powders, the us e of hydrides allows lowering the sintering temperature below that of th e alfa-ß transition of the Ti-6Al-4V substr ate, preserving the original microstructure and concomitant mechanical p roperties.Electrophoretic deposition in water-free electrolytes is a lso shown to be a valuable tool in the application of melt derived bioac tive glass-ceramic coatings on dense Ti or Ti-6Al-4V substrates. By coup ling the Ti alloys as cathode and in combination with a high vacuum sint ering atmosphere, uncontrolled oxidation of the Ti at the substrate/glas s interface can be avoided, resulting in a good mechanical strength and strong adhesion of the coatings to the substrate. Both sodium free a nd sodium containing bioactive glass-ceramic coatings inside porous Ti c oatings are synthesised following an all-alkoxide sol-gel approach. Thro ugh vacuum assisted impregnation of the porous coatings with the sol fol lowed by spinning and heat treatment in vacuum, the internal pore surfac e is coated with a fragmented micrometre thin layer of bioactive glass-c eramic, while the original open pore structure is preserved. An in vitro investigation of the bacterial surface colonisation, w hich can be related with an implant¬ís susceptibility to infections, is p erformed for Staphylococcus aureus and Staphylococcus epidermid is, the two bacteria strains most commonly associated with prosthetic joint infections. Bacterial colonisation is found to linearly correlate with the average surface roughness (Sa) for pure Ti or Ti alloy s urfaces. Hence, experimental porous Ti coatings exhibit a strongly reduc ed bacterial colonisation in comparison to a state-of-the-art porous vac uum plasma sprayed Ti coating. Increasing the porosity while maintaining the average roughness level favours again the strong tendency of bacter ia to accumulate in the open surface pores. On the other hand, for chemi cally modified Ti surfaces with increased hydrophilicity, a trend toward s a decreased bacterial colonisation is seen. So, as an enhanced osseoin tegration capacity (increased porosity, pore size,. . . ) inherently inc reases the bacterial colonisation, surface modification of the porous Ti to reduce bacterial adhesion is essential. In vitro cytocompatibility testing reveals an advanced osteoblast differentiation for the experimental porous Ti coatings when compared to various state-of-the-art dense and porous Ti surfaces, highlighting a p otential advantage when envisaging faster osseointegration.In viv o experimentation focusing on the early peri-implant bone formation i n a rabbit model demonstrates bone ingrowth into the porous Ti coatings, shifting the generally accepted lower threshold value for bone ingrowth from 50 μm to the microporosity range below 10 μm . Moreover, ingrown bone displays well-established interconnections to t he surrounding cortical bone, emphasising the potential of these porous coatings for the micro-interlocking of an implant into the host bone. In addition, the in vivo results confirm the osteostimulatory effect of bioactive glass-ceramic coatings. Both a melt derived bioactive glas s-ceramic coating on a dense cp Ti substrate and a sol-gel derived bioac tive glass-ceramic coating in a porous Ti coating significantly enhance bone regeneration with 30% and 70% respectively when compared to the ori ginal Ti substrates.
ISBN: 978-94-6018-542-7
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Surface and Interface Engineered Materials
Department of Materials Engineering - miscellaneous
Physical Metallurgy and Materials Engineering Section (-)

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