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

Surgical decision-making for Adolescent Idiopathic Scoliosis (AIS) relies on geometrical rather than biomechanical properties, such as the spine's in vivo load characteristics. While both in vivo and in vitro spinal loading experiments can provide valuable insights, a standardized method to compare motion for the Functional Spinal Unit (FSU) is lacking. This work aims to establish a systematic motion parametrization to unambiguously characterize FSU pose changes suitable for a robotic in vivo spinal loading application.

In this work, we propose an FSU motion parameterization using robotic rigid-body-tree modelling, deploying a virtual six-degree-of-freedom joint in the intervertebral space. To demonstrate the importance of the parameterization, we analysed the effect of different joint definitions on the produced displacement considering i) preoperative or intraoperative FSU reference poses, obtained from CT imaging of an AIS patient, and ii) one or both vertebral coordinate systems of the FSU. Additionally, we compared the required wrench capabilities of an actuation device to achieve pure spinal loading conditions in a pedicle screw-mounted scenario.

Applying identical virtual motions resulted in differences of up to 0.38 mm in translation and $24.{19}^{\circ }$ in rotation, depending on the joint definition. Corresponding required wrench capabilities showed maximum force and torque errors of up to 34.97 $\%$ in virtual pure translation and bending experiments.

Our findings underline the importance of a robust FSU kinematic framework, critical for ensuring safe and reliable FSU manipulation and for obtaining comparable and reproducible in vivo biomechanical data.