Prolongation of the average life expectancy and an active life-style in old age are related to the constant increase in the number of joint diseases which eventually require a surgical procedure. The diseased joint is replaced with a joint prosthesis, the functionality of the joint is recovered, and pain is reduced. In the last decade the number of joint replacement operations has increased several times over and is expected to increase further. In order to enable patients a painless and active life-style, it is necessary to develop materials which are long lasting in vivo. Metallic biomaterials must exhibit high corrosion and wear resistance. In vitro research on materials under simulated physiological conditions is presented. These experiments are complemented by examples from clinical practice performed in collaboration with orthopedic surgeons. Morphological and chemical changes in the material during the course of in vivo performance are related to processes of wear and corrosion. The local and systemic consequences of these processes in the human body are presented.
Metals and alloys used in orthopaedics and dentistry are exposed in vivo to various agents and environmental conditions. One of the important factors that determine the corrosion behaviour of biomaterials is the pH of the environment. The corrosion resistance of stainless steel 316L, titanium and titanium alloy Ti-6Al-4V was studied in terms of their electrochemical properties and biodegradation in simulated physiological solutions of different pH values (4.5, 6.5, 7.5 and 8). The electrochemical characteristics of individual metal components were also investigated. The concentration of dissolved metal ions released during 32 days immersion under simulated physiological conditions was determined by inductively coupled plasma mass spectroscopy. The corrosion behaviour of stainless steel 316L is strongly affected by the pH of the physiological solution in the range from 4.5 to 8.0. The corrosion resistance was greater at higher pH and the concentrations of released metal ions less. The behaviour of titanium and its alloy however is almost independent of the pH.
The corrosion behaviour of Nitinol alloy containing nearly equi-atomic composition of nickel and titanium and its constituent metals (nickel and titanium) was investigated in simulated Hanks physiological solution (pH value 7.5) and pH modified simulated Hanks physiological solution (pH values 4.5 and 6.5) and by electrochemical method of anodic potentiodynamic polarization at 37ºC. In this chloride-rich medium the corrosion stability of Nitinol is limited by the susceptibility to localized corrosion and is in that sense more similar to nickel than to titanium. The corrosion stability of Nitinol is strongly dependent on the surface preparation – grinding, polishing or chemical etching. Whereas a ground surface is not resistant to localized corrosion, polished and chemically etched surfaces are resistant to this type of corrosion attack. The reasons for this behaviour were investigated through metallurgical, topographical and chemical properties of the surface as a function of surface preparation. For that purpose, scanning electron microscopy combined with chemical analysis, confocal microscopy and X-ray photoelectron spectroscopy were used. The surface roughness decreased in the following order: chemically etched ) ground ) polished surface. Besides differences in topography, distinct differences in chemical composition of the outermost surface are observed. Ground, rough surfaces comprised mainly titanium oxides and small amounts of nickel metal. Chemically etched and, especially, polished surfaces are composed of a mixture of titanium, nickel and titanium oxides, as studied by angle resolved X-ray photoelectron spectroscopy. These results emphasize the importance of detailed investigation of the metal surface since small differences in surface preparation may induce large differences in corrosion stability of material when exposed to corrosive environments.
CoCrMo orthopaedic alloy was oxidized potentiostatically in various simulated physiological solutions in order to reveal differences in the composition, thickness and structure of the surface layers formed as a function of solution composition. X-ray photoelectron spectroscopy, combined with angle-resolved measurements and depth profiling, was used for the purpose. The following simulated physiological solutions were used: (1) 0.14 M NaCl, (2) simulated Hanks physiological solution containing various inorganic salts, (3) simulated Hanks physiological solution containing an aliquot of synovial fluid retrieved at a revision operation, and (4) minimum essential medium containing various inorganic salts, amino acids and vitamins. No significant differences between alloy treated in these solutions were observed after oxidation in the passive region; the oxide films are a few nanometres thick and, except in NaCl solution, contain a small amount of calcium phosphate. After oxidation at a potential in the transpassive range however, the oxide thickness increases considerably due to incorporation of cobalt and molybdenum oxides. Further, the concentration of calcium phosphate increases. The layers formed in minimum essential medium and Hanks solution containing synovial fluid comprise nitrogen and carbon containing species. The addition of synovial fluid significantly affects the behaviour in Hanks solution.
Bearing surface represents a critical point of the hip prosthesis. The development of metallic, polymer and ceramic materials used in the total hip arthroplasty is reviewed. Materials which are in use today are described in more detail. Furthermore, the history of different bearing combinations is described – metal-on-polyethylene (MOP), ceramic-on-polyethylene (COP) metal-on-metal (MOM) and ceramic-on-ceramic (COC). Several issues are described in detail for each bearing: wear, wear debris particles, tissue response to wear debris particles and current long-term results. Today surgeons are confronted with a variety of bearings options for hip arthroplasty. Currect clinical outcomes of these bearings are reviewed and patient selection is discussed as well. Finally, some new trends in hip arthroplasty are presented including resurfacing arthroplasty and ceramic-on-metal bearings.