The role of refractory element based coatings on the tribological and biological behaviour of orthopaedic implants

In the area of orthopaedic implants, particularly hip joint replacements, only three groups of metals and alloys are considered suitable as load - bearing implant materials, namely Ti  Ti alloys, Co  Cr alloys and 316L stainless steel. As a result of problems, largely associated with failure of the anchoring poly methyl methacrylate (PMMA) bone cement, the survival of present-day load - bearing prostheses cannot be guaranteed beyond 10 to 12 years. This limited lifespan has essentially limited joint replacements to the elderly. However, the increased number of younger patients requiring replacements has accentuated the need for joint prostheses with improved performance and longevity. The introduction of joint prostheses however with significantly enhanced durability in turn accentuates problems associated with implant loosening, wear, corrosion and, in particular, biological incompatibility. At present, very few monolithic materials are capable of satisfying all service requirements. The solution may lie in the use of coating technology, creating new materials / surfaces with unique combinations of properties that cannot be achieved from the use of monolithic materials alone. The first part of this paper reviews the types of coatings that have been used, or are currently being considered as implant materials, to include porous metallic coatings, “bio-active” (hydroxyapatite, bioglasses) and “bio-inert” (alumina) ceramic coatings. Their advantages and limitations will be discussed. The second part will focus on recent developments in the use of refractory element based coatings, with specific reference to work conducted on sputtered niobium and tantalum. Possible avenues for research to include advanced coating (multilayered / multicomponent) systems, surface treatments and duplex engineering will be reviewed.