Biocompatibility of NiTi implants: generated stresses and their experimental measurement

Abstract

The stresses generated by implants strongly affect the surrounding tissues’ reaction, which determines the biocompatibility of a construction. Some examples of osteoblasts’ development under compressive macro-stresses typical for the clamping slit-rings are given in this paper. A simplified X-ray diffraction method for measuring stresses generated by such constructions is considered. The method is based on the approximation of the dependence of the relative strain of an experimental crystal lattice on the diffraction angle θ by the Nelson-Riley linear function with extrapolation to θ=90° and allows determination of an average strain value in the direction normal to a construction surface. Using the example of a clamping slit-ring used for oblique bone fracture treatment, the paper shows that despite the peculiarities of macro-stress generation caused by incomplete restoration of such a construction's initial shape (incomplete reverse martensitic transformation), the stress state of a construction’s surface can be considered as a plane-stress state characterized by the typical relationship between a strain perpendicular to a surface and the principal stresses. The advantages of the method are discussed: the increase in the number of experimental diffraction peaks (including weak-intensity peaks) used for an average relative strain measurement, the feasibility of using Young’s modulus and Poisson’s coefficient determined by mechanical tests; the possibility of controlling macro-stresses generated by constructions of any complicated shape and of any phase composition. The macro-stresses measured by the elaborated method are shown to correspond with high precision to the stresses obtained by engineering calculations.