An investigation of nitinol and titanium alloys for the correction of spinal scoliosis

Surgery for early onset scoliosis (EOS) often requires instrumentation which enables the pediatric patient's spine to grow. The drawback of growing rods (Ellipse Technologies, USA) is the requirement of periodical extension (at least twice a year). In contrast, in growth- guidance sliding devices such as the Shilla (Medtronic, USA) or LSZ-4D (Conmet, Russia) the extension procedure is not necessary since only one or few fixtures are locked whilst other fixtures make it possible for rods to slide allowing growth of the spine. However, due to the absence of fusion in sliding devices increased rate of fractures of rods is possible. Additionally, movement of rods against fixtures may result in the generation of wear debris. Among traditional metallic materials used for such instrumentation titanium alloys are the most biocompatible and are widely used as fixtures material, while Nitinol, an alloy with Shape memory effect, is a promising material for rods due to having potential for more gradual and sustained correction associated with its shape memory effect and potentially reducing adjacent segment degeneration due to its super-elastic properties. The aim of this thesis was to investigate complications associated with implantation of growth-guidance sliding LSZ-4D in pediatric patients and to investigate the use of Nitinol and titanium alloy (Ti6Al4V) for this application. Analysis of retrieved components of LSZ growth-guidance sliding device (LSZ-4D) made from titanium alloy (Ti6Al4V) implanted in paediatric patients together with associated tissues showed metallosis which in some cases led to the development of complications including seromas and fistula formation. These complications occurred in 5 out of the 25 patients investigated. Another 4 patients encountered fatigue fracture of rods.The volume wear rate measured for the retrieved components of LSZ-4D was found to be 12.5 mm3 per year. In these 25 patients with implanted LSZ-4D devices there was a statistically significant increase in titanium and vanadium ions in the whole blood. With a view to replacing titanium alloy rods with Nitinol an investigation on the fatigue behaviour of Nitinol revealed that creating a microstructure which increased deformation due to a martensitic phase transformation improved high strain amplitude fatigue resistance of Nitinol, whereas precipitation of nanosize Ni-rich particles and decreasing of Ti4Ni2ox inclusions resulted in an increase in low strain fatigue resistance. The wear behaviour of Nitinol in comparison to titanium alloy was also investigated and showed that the wear resistance of Nitinol tested against titanium alloy (Ti6Al4V) in the simulated body environment is approximately 100 fold greater compared to titanium alloy (Ti6Al4V) and is similar to that of CoCr alloy. Nevertheless, Nitinol - Ti6Al4V combination still generates high volumetric wear associated with debris release from the Ti6Al4V component. Coating and treating the surface with TiN (titanium nitride), DLC (diamond like carbon) coatings and ion implantation of nitrogen ions in some instances reduced the wear. The deposition of a TiN coating only on the titanium component in a Nitinol - Ti6Al4V combination significantly improved the wear performance and this was associated with the protection of titanium component whereas the application of DLC improved the wear performance of both titanium and Nitinol. [on implantation or deposition of TiN or DLC coatings on both counterparts was found to be less effective. The current study indicates the importance and potential of reducing the reliance on titanium alloy by using Nitinol for patients with scoliosis and especially in those cases where relative moment of the fixtures is required, such as in growing patients.