Biomechanical analysis of plasma treated NiTi materials for spinal deformity correction

Abstract

Scoliosis is the abnormal lateral curvature of spine and may lead to early back pain and cardiopulmonary compromise as condition deteriorates. Surgery will be considered in severe cases. However, the current techniques can only achieve 60-70% correction due to the visco-elastic properties of spinal tissues. Hence, nitinol has been developed to overcome these problems by using the shape memory and superelastic properties. Nevertheless, its nickel concentration is a major clinical concern. Previously, we successfully demonstrated the Ni suppression on superficial layer by plasma surface treatment. This study aims at determining the biomechanical properties of the plasma-treated nitinol spinal rods. The samples were treated at 800°C 1 hour with furnace-cooling, 450 °C 30 minutes with water-quenching and followed by nitrogen plasma immersion ion implantation at 40kV for 4 hours. The rods embedded with spinal functional unit were undergone static and dynamic compressive bending tests under temperature control according to ASTM F1717-01. Five models were used in the static test. For the dynamic test, two models were used for each of the three loading levels (75%, 50% and 25% of ultimate load). The fatigue strength was equal to the loading level with two models endured five million cycles without failure. All samples were plastically deformed without failure after the static test. The average ultimate load was equal to 516.2 N (SD = 18.1 N). For the dynamic test at 75%, there was unilateral fracture of nitinol rod in each model after half million cycles. Bilateral fracture near the rod and screw junction in both models was occurred at 50% after million cycles. The last two models were loaded for over five million cycles without failure at 25%. The fatigue strength was equal to 25% ultimate load, i.e. 129.0 N. Our results are comparable to the existing implants and considerable for clinical trial. SUMMARY: Scoliosis is the abnormal lateral curvature of spine and may lead to early back pain and cardiopulmonary compromise as condition deteriorates. Surgery will be considered in severe cases. However, the current techniques can only achieve 60-70% correction due to the visco-elastic properties of spinal tissues. Hence, nitinol has been developed to overcome these problems by using the shape memory and superelastic properties. Nevertheless, its nickel concentration is a major clinical concern. Previously, we successfully demonstrated the Ni suppression on superficial layer by plasma surface treatment. This study aims at determining the biomechanical properties of the plasma-treated nitinol spinal rods.