FAErb_2025_Importance of a Systematic Intervertebral Motion Parametrization for in vivo Assessment of Spine Biomechanics

Surgical decision-making for Adolescent Idiopathic Scoliosis (AIS) is based on geometrical but not biomechanic properties, such as the spine\u2019s in vivo load-deformation characteristics. While both in vivo and in vitro spinal loading experiments could provide valuable insights, the field lacks a standardized method to parameterize and compare motion at the joint level of the Functional Spinal Unit (FSU).In this work, we propose an FSU motion parameterization convention using robotic rigid-body tree modeling, because it allows an unambiguous motion parametrization between vertebrae as rigid bodies and a virtual 6-DoF rigid body joint in the intervertebral space.We employed a mesh-mapping approach to quantify the relative vertebral shifts (up to 3.19 mm) and rotations (up to 28.34\u00b0) between high-resolution preoperative (PO) and low-resolution intraoperative (IO) CT scans of two scoliotic FSUs from an AIS patient. Applying identical virtual motions to PO versus IO-defined coordinates following our convention resulted in up to 0.38 mm displacement and a 24.19\u00b0 angular difference in the final position of the deflected vertebra. Additionally, we compared the required wrench to use pure force or pure torque to the virtual FSU motions of a pedicle screw-mounted spinal loading device. Corresponding spinal loading variations showed a maximum force and torque error of up to 34.97$\\%$ in pure translation and bending experiments, respectively.Our findings underline the benefits and importance of an FSU kinematic framework in ensuring safe and reliable manipulation or loading experiments, which are critical for obtaining comparable and reproducible in vivo data.