A Novel Scoliosis Instrumentation Using Special Superelastic Nickel–titanium Shape Memory Rods: a Biomechanical Analysis Using a Calibrated Computer Model and Data from a Clinical Trial

Biomechanical analysis of scoliosis instrumentation using superelastic Nickel–titanium shape memory (SNT) rods. To compare SNT with conventional Titanium (Ti) and Cobalt–chrome (Co–Cr) rods. A clinical trial has documented comparable efficacy between two adolescent idiopathic scoliosis (AIS) cohorts instrumented using SNT versus conventional Ti rods. The shape memory and superelasticity of the SNT rod are thought to allow easy rod insertion, progressive curve correction, and correction from spinal tissue relaxation, but study is yet to be done to assess the effects of the shape memory and superelasticity. Instrumentations of AIS patients from the clinical trial were computationally simulated using SNT, Ti and Co–Cr rods (5.5 or 6 mm; 30°, 50° or 60° sagittal contouring angles; 0°, 25° or 50° coronal over-contouring angles). Curve correction, its improvement from stress relaxation in the spine, and loads in the instrumentation constructs were computed and compared. The simulated main thoracic Cobb angles (MT) and thoracic kyphosis with the SNT rods were 4°–7° higher and 1°–2° lower than the Ti and Co–Cr rods, respectively. Bone–implant forces with Ti and Co–Cr rods were higher than the SNT rods by 84% and 130% at 18 °C and 35% and 65% at 37 °C, respectively (p < 0.001). Further corrections of the MT from the simulated stress relaxation in the spine were 4°–8° with the SNT rods versus 2°–5° with the Ti and Co–Cr rods (p < 0.001). This study concurs with clinical observation that the SNT rods are easier to insert and can result in similar correction to the conventional rods. The SNT rods allow significantly lower bone–implant forces and have the ability to take advantage of post-instrumentation correction as the tissues relax.