In vivo retensioning of a dynamic spinal implant in a Göttingen minipig model: A proof of principle study

INTRODUCTION: The Double Spring Reduction (DSR) is a prototype dynamic scoliosis correction implant consisting of compression and torsion springs that provide continuous corrective three-dimensional forces while permitting growth. Previous in vivo studies demonstrated the feasibility of the concept. However, the corrective loads applied by the implant, decrease over time due to viscoelastic adaptation of the spine, thereby affecting the effective correction. A solution for this is to restore the initial corrective forces by retensioning the springs, but the effect of this procedure on the achieved correction remains unknown. Therefore, this study aimed to evaluate how restoring the initial preload of a dynamic scoliosis implant through retensioning affects the axial rotation in an in vivo Göttingen minipig model.

MATERIAL AND METHODS: A DSR was implanted in one 8-month old Göttingen minipig, spanning nine vertebrae (T9-L3). It contained two torsion springs connected to T9-T13 and T13-L3, each preloaded to 1 Nm torque (2 Nm in total) to induce axial rotation. Reduction surgery was performed after twelve weeks by reversing the spring preload to 1 Nm in the opposite direction. After ten days, the lost preload was restored through minimally invasive retensioning by increasing the torque an additional 0.30 Nm (0.15 Nm on each spring). CT scans were obtained after 12 weeks of induction, immediately after reduction, before retensioning, and prior to termination (20 days after reduction) to determine axial rotation. The in vivo orientation of the torsional implant was measured two times daily from reduction surgery until termination using three six-degrees-of-freedom electromagnetic attached with 3D-printed nylon clamping fixations. The implant orientation recorded over time was used to determine the torque. The change in torque over time served as proxy for the viscoelastic adaptation of the spine.

RESULTS: After 12 weeks of induction, the axial rotation increased from -4° before implantation of the DSR to 12° at maximum induction, which remained stable immediately after reduction. Prior to retensioning, the axial rotation further decreased from 12 to 3° after ten days. This corresponded with a decrease of 0.15 Nm (15%) in each spring. After retensioning, the torque decreased by 0.06 Nm (6%) at the cranial spring (T9-T13), while the force remained stable in the caudal spring. At termination, the integrity of the sensor clamping fixations was preserved, the sensors remained in their original position within the fixation, and the sensor wires remained functional.

CONCLUSION: This study showed that restoring the torque results in a substantially smaller viscoelastic effect compared to the initial viscoelastic responses observed after reduction of axial rotation. No additional axial correction was observed after retensioning. These findings provide valuable insights into the dynamic responses of the spine to altered implant loading and may contribute to optimizing implant loading strategies to improve correction.