Biocompatibility of calcium phosphate-coated and of geometrically structured nickel-titanium (NiTi) by in vitro testing methods

Nickel-titanium shape memory alloys (NiTi-SMA) have outstanding mechanical properties (superelasticity, shape memory effect) that make them very interesting for clinical applications. Due to the high nickel content of the material, there are concerns about possible immunological reactions of the surrounding tissue. The proliferation of osteoblast-like cells (MG-63) and the cytokine release (interleukin (IL)-6, IL-8, tumor necrosis factor (TNF)-alpha) from polymorphonuclear neutrophil (PMN) leukocytes as well as peripheral blood mononuclear cells (PBMC) in response to different NiTi specimen was studied: (a) NiTi coated with calcium phosphate (octacalcium phosphate + hydroxyapatite; OCP/HAP) from an supersaturated aqueous solution, consisting of a dense layer of sharp-edged platelets; (b) NiTi coated with calcium phosphate (hydroxyapatite) by high-temperature plasma-spraying, consisting of a rough dense layer of globular particles; (c) NiTi after geometrical structuring (microdrilling). In the first two cases, the aim was to improve the biocompatibility while retaining the mechanical properties. In the third case, the effect of mechanical treatment (damage of the passivating TiO2 surface layer) was investigated. In comparison to non-coated samples the OCP/HAP-coated SMA led to an increase in the release of cytokines, in contrast, the HAP-coated samples led to a decrease in cytokine release from PBMC. The attachment, viability, and the proliferation of MG-63-cells in the near vicinity of microdrilled structures was not different from non-strained surface areas. These results show that cell-biological in vitro techniques are suitable for the prediction of tissue reactions towards implant coatings or mechanically altered NiTi-surfaces. The surface morphology (platelets versus globular particles) appear to play a significant role that supersedes that of the chemical composition of the surface. (C) 2004 Published by Elsevier B.V..