How does implant distribution affect 3D correction and bone-screw forces in thoracic adolescent idiopathic scoliosis spinal instrumentation?

•Increasing the implant density has a modest effect on the 3D correction of the scoliotic spine•Only the implant density on the concave side of the curvature is associated with increased Cobb angle correction•Higher implant density tends to overconstrain the instrumented spine and results in higher forces at bone-screw interface•If fewer screws are to be used, convex screw drop-out appears to be the most suitable.

BackgroundOptimal implant densities and configurations for thoracic spine instrumentation to treat adolescent idiopathic scoliosis remain unknown. The objective was to computationally assess the biomechanical effects of implant distribution on 3D curve correction and bone-implant forces.Methods3D patient-specific biomechanical spine models based on a multibody dynamic approach were created for 9 Lenke 1 patients who underwent posterior instrumentation (main thoracic Cobb: 43°-70°). For each case, a factorial design of experiments was used to generate 128 virtual implant configurations representative of existing implant patterns used in clinical practice. All instances except implant configuration were the same for each surgical scenario simulation.FindingsSimulation of the 128 implant configurations scenarios (mean implant density = 1.32, range: 0.73-2) revealed differences of 2° to 10° in Cobb angle correction, 2° to 7° in thoracic kyphosis and 2° to 7° in apical vertebral rotation. The use of more implants, at the concave side only, was associated with higher Cobb angle correction (r = − 0.41 to − 0.90). Increased implant density was associated with higher apical vertebral rotation correction for seven cases (r = − 0.20 to − 0.48). It was also associated with higher bone-screw forces (r = 0.22 to 0.64), with an average difference between the least and most constrained instrumentation constructs of 107 N per implant at the end of simulated instrumentation.InterpretationLow-density constructs, with implants mainly placed on the concave side, resulted in similar simulated curve correction as the higher-density patterns. Increasing the number of implants allows for only limited improvement of 3D correction and overconstrains the instrumentation construct, resulting in increased forces on the implants.