A novel scoliosis instrumentation using special super elastic nickel-titanium shape memory alloy spinal rods can result in equivalent correction as conventional rods but with less stress at bone-implant interface: A biomechanical evaluation through simulations

Abstract

Introduction: A nickel-titanium (SNT) rod was engineered to be highly malleable below 20°C and maximally superelastic after austenite phase transformation at 37°C. Its shape-memory properties enable progressive correction during austenite phase transformation. Its strain-stress curve has a wide superelastic plateau enabling relatively constant corrective forces and post-instrumentation correction improvement from tissues relaxation. A pilot clinical trial using SNT rod in AIS instrumentation has been completed with comparable efficacy results to conventional titanium rods, but little biomechanical investigation has been done on how to maximally utilize the shape-memory and superelastic properties for deformity correction. Objectives: To investigate the correction and stress level at the bone-implant interface and in the shape-memory SNT rod in AIS instrumentation. Methods: The computational study was performed using biomechanical models of 7 thoracic AIS patients. The main thoracic Cobb angles (MT) were 43°–76° and thoracic kyphosis (TK) 7°–12°. We alternately simulated instrumentations using pedicle screws and SNT (at 18°C and 37°C), titanium (Ti) and cobalt-chrome (CoCr) rods. The independent variables were the rod diameter (5.5 and 6 mm), predesigned rod bending angle (30° and 40°) and rod derotation angle (90° and 110°). The simulated correction maneuvers were rod reduction and rod derotation. For a total of 224 virtual instrumentations, the resulting MT and forces in the rods and at bone-screw interfaces were computed and compared. Results: Simulated MT correction (curve reduction) by SNT rod was on average -12° (before the austenite phase transformation) and improved to -26° (after the phase transformation), vs. -31° (Ti rod) and -34° (CoCr rod). The associated bending moments throughout the rods were 1.4, 3.4, 4.6, and 6 Nm on average, while the average bonescrew forces were 19, 45, 62 and 74 N. Larger SNT rod diameter or pre-designed bending angle increased TK by about 5° without changing MT correction, but larger SNT rod derotation improved the MT correction by -5°. Peak bending moments in SNT rods were 7–16 Nm; only in SNT rods of larger diameter, rod bending and derotation angles, were peak bending moments (11–16 Nm) above the threshold bending (8 and 10 Nm for 5.5 and 6 mm diameter SNT rods) for the SNT rod to work in the superelastic domain. Conclusions and significance: This study elucidated scoliosis correction mechanisms using SNT rods and demonstrated that SNT rods allowed lower forces during initial rod reduction and derotation, and provided in situ progressive correction through the subsequent austenite phase transformation. With the selection of certain SNT rod parameters, it is possible to achieve similar MT correction as with Ni and CoCr rods, but with the advantage to work in the superelastic plateau of their strain-stress curve, which could possibly provide further post-instrumentation correction after tissue relaxation.