S-REX - SPINAL REMOVABLE EXPANDABLE IMPLANT: PRECLINICAL IMPLANT DESIGN OPTIMIZATION USING PARTICLE-BASED BONE MODELS

Loosening and migration of spinal pedicle screws, is a feared clinical complication of spinal fusion treatments in patients with poor bone quality. The authors have developed a novel removable expanding screw technology (1) that was adapted for a pedicle screw application, to reduce the risk of loosening. The implant features two wings deployed from the screw after implant insertion. After extensive voice of customer input from a spinal surgeon expert group, three concepts with unique design features were developed. The aim of this study is evaluate these concepts using a novel particle-based simulation to understand the implant design parameters that influence performance. This simulation models the failure of individual particles (2) which overcomes the limitations of Finite Element simulations with bone-implant failure analysis. The result of the simulation demonstrated 36% less bone damage on a single load cycle for one of the designs that had the most similarity to conventional surgical technique, compared to a standard screw of the same thread geometry. The surgeons were interested in investigating the optimal wing position and wing geometry for this lead design, as an extension to the initial particle-based simulation. The second simulation based the loading scenario on an established study where the screw-implant construct is fixed and the loading applied through the anterior vertebral body (3). To understand where S-REX fits against conventional implants a number of screw implant scenarios were evaluated: non-augmented, small wings, larger wings and PMMA cement augmented. In a 10 cycle simulation vs no augmentation the reduction in subsidence was 28%, 58% and 72% respectively. Based on these results the wings were moved closer to anterior cortex of the vertebral body and their expansion was maximised. Clinician feedback on this final version was positive and this version is currently being manufactured for biomechanical testing in the AO's laboratories. The S-REX project is a collaboration between REX Ortho and the AO Foundation.