In vitro and in vivo characterization of novel plasma treated nickel titanium shape memory alloy for orthopedic implantation

Nickel–titanium (NiTi) shape memory alloys are potentially very useful in orthopedic and dental implantation, since the materials possess super-elasticity and shape memory effects that other current medical metallic materials do not have. Possible nickel ion release, however, hampers their medical applications, particularly in orthopedic implants where fretting is always expected at the articulating surface. We have utilized plasma immersion ion implantation (PIII) to alter the surface chemistry of the materials in order to reduce nickel release. The enhanced corrosion resistance and surface mechanical properties have been reported previously. This paper describes the cytocompatibility and in vivo performance of the PIII treated and untreated samples. NiTi discs with 50.8% Ni were treated by nitrogen and oxygen PIII at 40 kV. After PIII, titanium nitride (TiN) and packed titanium oxide (TiO) are formed on the surface. The degree of cell proliferation on the untreated and oxygen treated samples is slightly inferior to that on the nitrogen treated sample. However, in vivo bone formation on the nitrogen plasma treated samples is better compared to the untreated control and oxygen treated one at all time points. Our work thus provides the evidence that nitrogen plasma modified NiTi alloys are potentially suitable for orthopedics without inducing harmful biological effects.