Biocompatilibity-related surface characteristics of oxidized NiTi.

In the present study, we examined the effect of NiTi oxidation on material surface characteristics related to biocompatibility. Correspondence between electron work function (EWF) and adhesive force predicted by electron theory of adsorption as well as the effect of surface mechanical stress on the adhesive force were studied on the nonoxidized and oxidized at 350, 450, and 600 degrees C NiTi alloy for medical application. The adhesive force generated by the material surface towards the drops of alpha-minimal essential medium (alpha-MEM) was used as a characteristic of NiTi adsorption properties. The study showed that variations in EWF and mechanical stress caused by surface treatment were accompanied by variations in adhesive force. NiTi oxidation at all temperatures used gave rise to decrease in adhesive force and surface stress values in comparison to the nonoxidized state. In contrary, the EWF value revealed increase under the same condition. Variations in surface oxide layer thickness and its phase composition were also followed. The important role of oxide crystallite size in EWF values within the range of crystallite dimensions typical for NiTi surface oxide as an instrument for the fine regulation of NiTi adsorption properties was demonstrated. The comparative oxidation of pure titanium and NiTi showed that the effect of Ni on the EWF value of NiTi surface oxide is negligible.

Effect of porosity on the osteointegration and bone ingrowth of a weight-bearing nickel-titanium bone graft substitute.

Porous nickel-titanium (NiTi) alloy is a promising new material for a bone graft substitute with good strength properties and an elastic modulus closer to that of bone than any other metallic material. The purpose of this study was to evaluate the effect of porosity on the osteointegration of NiTi implants in rat bone. The porosities (average void volume) and the mean pore size (MPS) were 66.1% and 259+/-30 microm (group 1, n=14), 59.2% and 272+/-17 microm (group 2, n=4) and 46.6% and 505+/-136 microm (group 3, n=15), respectively. The implants were implanted in the distal femoral metaphysis of the rats for 30 weeks. The proportional bone-implant contact was best in group 1 (51%) without a significant difference compared to group 3 (39%). Group 2 had lower contact values (29%) than group 1 (p=0.038). Fibrotic tissue within the porous implant was found more often in group 1 than in group 3 (p=0.021), in which 12 samples out of 15 showed no signs of fibrosis. In conclusion, porosity of 66.1% (MPS 259+/-30 microm) showed best bone contact (51%) of the porosities tested here. However, the porosity of 46.6% (MPS 505+/-136 microm) with bone contact of 39% was not significantly inferior in this respect and showed lower incidence of fibrosis within the porous implant.

Effect of metal alloy surface stresses on the viability of ROS-17/2.8 osteoblastic cells.

In this study we compared the effect of structural stresses and surface roughness on biocompatibility of NiTi- and Ti-alloy for ROS-17/2.8 osteoblastic cells. We suggest here that cell viability and cell attachment are linear functions of internal (structural) stress and subgrain size of the implant alloy. However, this is not the case with surface roughness. The two-phase state in these materials is characterized by different mean values of structural stresses (sigma) in alpha-martensite and beta-phase. We found a straight correlation between cell viability and sigma(beta)/sigma(alpha) ratio. Atomic force microscopy revealed that, even after equal surface polishing treatments, roughness varied significantly between the different alloys. The effect of the surface structure of the alloy on the osteoblastic ROS-17/2.8 cell survival rate was studied with combined calcein-ethidium-homodimer fluorescence labeling. The possible effects on cell attachment to substrate were studied by staining the focal contacts with paxillin antibody. All the NiTi surfaces were tolerated well and the cells attached most abundantly to the roughest NiTi surface but the smoothest Ti-alloy surface. However, other parameters of the material state, such as the surface stresses created by hot rolling seem to be responsible for some of the attachment and cell survival features observed in this study.

Bone modeling controlled by a nickel-titanium shape memory alloy intramedullary nail.

Nitinol (NiTi) shape memory metal alloy makes it possible to prepare functional implants that apply a continuous bending force to the bone. The purpose of this study was to find out if bone modeling can be controlled with a functional intramedullary NiTi nail. Pre-shaped intramedullary NiTi nails (length 26 mm, thickness 1.0-1.4 mm) with a curvature radius of 25-37 mm were implanted in the cooled martensite form in the medullary cavity of the right femur in eight rats, where they restored their austenite form, causing a bending force. After 12 weeks, the operated femurs were compared with their non-operated contralateral counterpairs. Anteroposterior radiographs demonstrated significant bowing, as indicated by the angle between the distal articular surface and the long axis of the femur (p = 0.003). Significant retardation of longitudinal growth and thickening of operated femurs were also seen. Quantitative densitometry showed a significant increase in the average cross-sectional cortical area (p = 0.001) and cortical thickness (p = 0.002), which were most obvious in the mid-diaphyseal area. Cortical bone mineral density increased in the proximal part of the bone and decreased in the distal part. Polarized light microscopy of the histological samples revealed that the new bone induced by the functional intramedullary nail was mainly woven bone. In conclusion, this study showed that bone modeling can be controlled with a functional intramedullary nail made of nickel-titanium shape memory alloy.