DEVELOPMENT OF CERVICAL SPINE MODEL FOR BIOMECHANICAL TESTING OPTIMIZATION

Background: Anatomical differences between samples, tissue fatigue, tissue decomposition with prolonged testing materials, and the implications of incorporating biohazardous materials are a few of the many limitations associated with laboratory spinal biomechanical tests that utilize cadaveric specimens. A biomimetic substitute would be preferable to a cadaver specimen in some cases. To our knowledge, a mechanical analog of the cervical spine does not exist. The objective of this project was to design a surrogate cervical spine model, which could be 3D printed, that would replicate the natural range of motion (ROM) and stiffness of the neck under simulated physiological loads. Methods: An average-sized disc and functional spinal unit (FSU) were created with SolidWorks, and lateral bending (LB), flexion (F), and extension (E) were simulated. The disc properties and the load applied to the FSU were modified, and the resulting ROM was determined. A compressive load of 100-600N was applied, in addition to a 1 Nm moment, to determine the compressive load where ROM aligned with average cervical ROM. Fixtures were added to the C2 and C7 vertebrae, and discs were created to prevent contact between the facet joints. The spine assembly was compressed by using a threaded end stud on each end to tension a metal cable through the intervertebral bodies and discs. Elastic cables were also affixed through the posterior spinous processes and the left and right facet joints to reduce flexibility. Results: When using a disc with an elastic modulus of 8 MPa, 300 N was the optimal compressive force that resulted in a ROM most similar to an average of 4 ̊ in LB and F/E. Under this compressive load, simulations revealed that maximum compression of the disc was 2 mm. This data was used to design the intervertebral discs such that they would not be thinner than 3.5 mm during compression. Intervertebral discs were designed to specifically fit the faces between each vertebral level of the cervical spine. A surrogate spine model has been completely designed in SolidWorks. Conclusions: A novel cervical spine mechanical analog has been developed which will aid in spinal biomechanics R&D. The kinematics of this model linearly replicate the ROM and stiffness of a cadaveric cervical spine under simulated physiological loads. The model will be used to optimize robotic trajectories and experimental methodology prior to experimentation with a cadaveric spine. Future testing will use 3D tracking to directly compare the ROM of the proposed model with that of a cervical spine specimen.