Nitinol Memory Rods Versus Titanium Rods: A Biomechanical Comparison of Posterior Spinal Instrumentation in a Synthetic Corpectomy Model

Abstract

Study Design: Biomechanical investigation. Objective: To compare the biomechanical performance of nitinol memory metal rods and titanium rods when used as posterior spinal instrumentation in a synthetic model. Methods: Biomechanical testing was performed using ultra-high-molecular-weight polyethylene blocks. Nineteen spinal constructs were created to allow comparison of 5.5-mm nitinol rods with 5.5-mm titanium rods. Static compression and rotational testing were performed on an Instron 8874 and Instron 4202 at 37°C to simulate body temperature. Results: The average titanium construct stiffness under static compression or bending was 47.2 ± 9.1 N/mm while nitinol’s stiffness averaged 48.9 ± 12.4 N/mm ( P = .83). During axial rotation testing, the nitinol rod system showed no torsional stiffness difference from the titanium system: 0.95 ± 0.03 Nm/deg versus 0.96 ± 0.17 Nm/deg, respectively ( P = 0.91). There was a statistically significant difference between the average torsional yield point for the titanium constructs (14.4 ± 1.6 Nm/deg) and nitinol constructs (21.3 ± 0.8 Nm/deg) ( P = .004). The torsional toughness of the nitinol constructs was also statistically greater than the titanium rods: 473 GN/m3 versus 784 GN/m3 ( P = .0006). There was no statistically significant difference between the nitinol group sustaining a higher number of fatigue cycles until failure and the titanium group (181 660 cycles vs 64 104 cycles, respectively, P = .22). Conclusions: This study provides biomechanical evidence that nitinol rods used in a posterior construct are comparable to titanium rods with regard to compression and have increased torsional failure load and torsional toughness. While nitinol trended toward superior fatigue resistance, there was no significant difference in nitinol versus titanium construct fatigue resistance.