Evaluation of commercial implants with nanoindentation defines future development needs for hydroxyapatite coatings

The performance of biomedical implants relies on the ability to assess and refine the microstructure of biomaterials. Instrumented nanoindentation was applied to determine the mechanical properties of plasma sprayed hydroxyapatite-coated implants from different commercial vendors. All biomedical devices contained both amorphous and crystalline phases. Nanoindentation of the amorphous phase revealed a hardness of 1.5 ± 0.3 GPa and an elastic modulus of 48 ± 6 GPa. The crystalline phase revealed a range in hardness of 3.0-7.7 GPa. The large range is attributed to the presence of porosity, surrounding amorphous areas, and hydroxyapatite (unmolten particle cores and recrystallized hydroxyapatite). A selection of the powder type (spray-dried or sintered) leads to different mechanical properties within the coating. A spray-dried powder provides a lower hardness and elastic modulus when unmolten particle cores are included in the coating. Meanwhile recrystallized areas are intermediate in hardness. The combination of a polished cross-section and nanoindentation offers the ability to determine a range of quality control tests including hardness, elastic modulus, bond strength, fracture toughness, substrate and coating roughness, crystallinity, and coating thickness. Property maps determined from nanoindentation will provide a graphical representation of the mechanical property distribution within the coating and provide a basis for coating property refinements. The assessment of commercial coatings is used a basis for discussion of future developments for hydroxyapatite coatings