Biocompatibility and mechanical stability of Nitinol as biomaterial for intra-articular prosthetic devices

Abstract

Anterior Cruciate Ligament (ACL) disruption is a common injury in human and veterinary medicine. Currently used surgical implants do not possess the same tensile properties as intact ACL. Nitinol, a superelastic alloy of nickel and titanium, could be a suitable intra-articular prosthesis. However, its behavior in synovial fluid is unknown. The objectives of this project are to evaluate the corrosion and mechanical properties of Nitinol strands after incubation in synovial fluid.For comparative analysis, Nitinol (Ti-55.97Ni, wt. %) strands fully annealed (n=192) with hard black oxide surface finish were incubated in synovial fluid, physiologic saline (NaCl), and phosphate-buffered saline (PBS) for 1, 3 and 6 months. Corrosion resistance was evaluated by measuring the dissolved content of metallic ions using inductive coupled plasma mass spectroscopy and by characterizing the material surface using scanning electron microscopy and electron dispersive spectroscopy. The tensile mechanical properties were evaluated before and after each incubation period. A two-way ANOVA and a Tukey post-hoc tests were used to compare the mean differences between groups. SEM showed mild changes such as increased porosity, with deeper holes and corrosive pits, after 6 months of incubation in synovial fluid. EDS showed similar titanium, nickel and oxide layer distributions and intensities on all surfaces. Ultimate tensile strengths and total elongations were not affected by incubation (p>0.5). However, lower and upper plateau strengths, along with residual elongations, increased slightly following incubation in synovial fluid (p=0.001). These results showed that the incubation of Nitinol in synovial fluid does not significantly affect its surface composition, but induces mild changes in its surface morphology and its mechanical properties. Therefore, Nitinol appears to be safe for intra-articular use.