On magnesium-modified titanium coatings and magnesium alloys for oral orthopaedic applications : in vitro investigation

Sammanfattning: In dentistry and orthopaedic surgery, research to find and developimproved biomaterials is progressing rapidly.Of specific interest is to accelerate bone formation around theimplant surface, which could improve the reliability of the implanteven in patients with compromised situations. Although the surfacemodification of the implant has been proven to certain extent topromote osseointegration, the lack of bone in the patient remains amajor issue and bone augmentation is commonly conducted priorto implant insertion. Synthetic and naturally derived resorbablematerials are commonly used. However, problems such as the lackof optimal mechanical properties or the undesirable materialresorption kinetics still exist and there still remain possibility forimprovement. Clinical approaches for orthopaedic trauma require the use ofnon-resorbable screws, plates and pins made of metallic materialssuch as titanium, cobalt-chrome and stainless steel alloys. Themajor drawback of these materials is the need of implant removalat re-entry. Therefore, the research of bioresorbable materials thatcould withstand the mechanical stresses is an ongoing topic.Based on this clinical reality, the aim of this thesis was toinvestigate the suitability of magnesium (Mg) as a biomaterial forregenerative bone applications. Namely, Mg as a doping materialfor engineered mesoporous titanium implant surfaces (Studies I, IIand III), and as a bioresorbable metal alloy for bone regenerationin bone trauma and bone defects conditions (Study IV).Study I, II, IIIMesoporous titania films produced with evaporation-induced selfassembly(EISA) technique and applied as implant surface coatingsare under investigation as a release system for the controlledadministration of several substances, such as osteoporotic drugs, toenhance early bone anchorage to the implant. Modulating the poresize of such films though the selection of EISA parameters permitsto control the adsorption of such substances into the mesoporousmatrix and their subsequent release into the peri-implant region.Studies I, II and III analysed the effect of Mg incorporation intomesoporous titania coatings towards two cellular models duringearly and later stages of cell activity.Study I characterized the morphology, chemistry, and topographyof mesoporous titania coatings and the effects of Mg-loading onsurface micro- and nano-structures. Mg release was determinedand its effect was evaluated on human foetal osteoblast populations.It was shown that mesoporous films possessed a smoothsurface with pores that faced outward. Mg adsorption did notsubstantially alter the mesoporous surface roughness both atmicro- and nano- levels. Mg was released within 24 hours ofincubation in cell culture conditions, thus its bioactive effect onlyoccurred during initial osteoblasts activity.Study II evaluated the ability of Mg-loaded mesoporous coatings tomodulate multipotent adipose-derived stromal cell differentiationtoward the osteoblast phenotype. The results demonstrated thatMg release had a strong impact on this cellular model, promotingosteoblast marker expression in standard cell culture conditions.Interestingly, Mg significantly promoted the expression of osteopontin,a protein that is essential for early biomaterial-cellosteogenic interaction.In study III, the reagents and EISA parameters in the mesoporousdeposition were varied to generate three mesoporous titaniacoatings with 2-, 6- and 7-nm average pore size, to increase Mgcontent in the interconnected porosity of the films. The effect ofvarious Mg contents released from the three mesoporous structureswas tested on human foetal osteoblasts populations with pre-designedosteogenic PCR arrays and real-time polymerase chainreaction. It was shown that Mg release affected osteogenesis andwas controlled by tuning the pore dimensions of the mesoporousfilms. Increasing pore size by 1 nm (from 6 nm to 7 nm)significantly enhanced the bioactivity of the film without alteringthe surface roughness.Study IVIn orthopaedics Mg alloys has received increasing attention asbioresorbable metals for bone regeneration. However, localizedmaterial degradation is too fast and provokes the premature loss ofmechanical properties, preventing correct cellular development andbone healing in vivo . For this reason, various alloying elements arecombined with high-purity Mg to modulate and optimize degradationbehaviour.Study IV of this thesis investigated the degradation parameters ofMg2Ag, Mg10Gd, and Mg4Y3RE alloys and how the alloysdifferently affect human umbilical cord perivascular cell adhesionand spreading. Mg4Y3RE showed the highest degradation rateand, thereby, the highest trend in increases in pH and osmolality ofthe surrounding fluid. However, both Mg4Y3RE and Mg10Gdallowed cells to better adhere and spread across their degradedsurfaces; in comparison, surface degradation of Mg2Ag was moreaggressive with weak or no visible cellular structures on it.ConclusionsIn summary, the results of the present thesis explored the potentialof Mg for its application in bone tissue regeneration. Titaniumimplant surfaces coated with mesoporous TiO2 thin films andfurther loaded with Mg enhanced bone cell activity and osteoprogenitordevelopment into mature osteoblasts. Thus, mesoporousdeposition followed by Mg loading may be a suitablealternative to existing implant surface treatments.Bioresorbable materials must degrade slowly and uniformly inorder to simulate the tissue healing process. Mg10Gd possessesreduced content of alloying element and a suitable homogenousdegradation pattern in vitro that allows proper adhesion ofundifferentiated cells. Mg10Gd thus represents a biodegradableMg-based material with promising mechanical and biologicalproperties for use in dental and orthopaedic fields.