Poster AA9 - A Multiplanar Kinematic Comparison of Cervical Spine Decompression Surgery Approaches

Introduction: Posterior surgical approaches for spinal decompression to treat cervical spine pain impact spinal stability differently depending on how much disruption is caused to the surrounding paraspinal musculature, facet joints, and lamina (the part of the vertebra connecting the spinous and transverse processes). These surgical approaches include: 1) laminectomy, involving complete removal of the lamina, 2) laminotomy, removing only part of the lamina, and 3) muscle-preserving selective laminectomies (SL), which maintain the extensor musculature and facet joints. Experimental testing typically evaluates spinal range of motion (ROM) and anatomical changes from these surgical approaches by applying known pure moment loads to the spine and recording the resulting segmental motion in anatomical directions of Flexion-Extension (FE), Lateral Bending (LB), and Axial Rotation (AR). The relationship between input load and output response is the passive spinal stiffness, which is a major contributor to spinal segment stability. However, the spine’s complex joint structure and multiple degrees of freedom (DOF) make it challenging to capture spinal behavior in combinations of loading in multiple anatomical directions simultaneously. We developed a novel testing protocol using complex, geometric loading trajectories to map the spine’s multiplanar behavior under combined anatomical loading directions. Additionally, we created a visualization method to display the spine’s multidimensional bending ROM and stiffness in all load combinations within its physiological limits. This methodology will be used to compare differences in segmental biomechanical behavior between posterior spinal decompression surgical approaches.

Materials and

Methods: One C2-T1 cadaveric cervical spine specimen was mounted to a six-DOF KUKA robot, and its moment-displacement behavior was recorded with an ATI load cell, NDI Optotrak sensors, and simVITRO software. First, the specimen will be loaded in combined FE and LB circles about the craniocaudal axis up to 2 Nm to capture the shape of the spine’s physiologic ROM under maximal combined FE and LB bending (Fig 1a). This boundary is used to crop a custom, displacement-controlled, geometric loading scheme, such that loading only occurs within the expected organic ROM of the spinal segment (Fig 1d). The geometric combined loading scheme is a combination of FE and LB bending (Fig 1b) in three different trajectories applied about the craniocaudal axis (viewed in the spine’s transverse plane): 1) clockwise and counter-clockwise circles of changing diameter from the neutral zone to the extrema, 2) medially and distally moving “rays” passing through the spine’s neutral zone, and 3) angled “petals” which move both clockwise/counter-clockwise and medially/distally simultaneously. These trajectories are designed to share the same several hundred kinematic poses throughout the spine’s ROM. This results in eight distinct angles at which each pose can be approached by the different loading trajectories, and at which the spine’s stiffness behavior can be sampled (Fig 1c). The specimen will be loaded by each of the three trajectories in four states of the C4-C5 FSU: 1) in its intact form, 2) with an SL, 3) with a laminotomy, and 4) with a laminectomy.

Results, Conclusions, and Discussions: The applied displacements and measured spinal loads are combined into a resultant stiffness value of the spinal segment throughout the loading trajectories, which is represented as a 3D surface, where peaks, valleys, and changes surface slope illustrate instantaneous spinal stiffness behavior at each pose in response to the bending applied to the specimen. The surface offers visualization of the biomechanical behavior in previously uncharacterized combinations of loading. With removal of stabilizing musculature and bone structure, we expect to see maximum spinal stiffness decrease across the course of the surgical approaches, with the intact specimen stiffness being the highest and laminectomy being the lowest. As the posterior musculature becomes more disturbed, we expect to see decreased stiffness in flexion and increased stiffness in extension, which may change the location within the specimen’s ROM where peak stiffness occurs, presenting potential new insights into areas of pain or discomfort in a living patient who has had these surgical procedures. In the laminotomy case, where the lamina is removed unilaterally, we expect to see new asymmetries in specimen’s behavior along the LB axis that were not present in the intact state. Characterizing differences in expected stiffness between surgical approaches can define potential cutoff values for spinal instability in the nontraumatic degenerated cervical spine. These data can be used to develop predictive algorithms between pre-surgical biomechanics and desired post-surgical biomechanics, aiding clinicians in selecting surgical approaches for future spinal decompression patients and reducing the need for revision surgeries.

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