Dynamic Biomechanical Examination of the Lumbar Spine with Implanted Total Spinal Segment Replacement (TSSR) Utilizing a Pendulum Testing System

Abstract

BackgroundBiomechanical investigations of spinal motion preserving implants help in the understanding of their in vivo behavior. In this study, we hypothesized that the lumbar spine with implanted total spinal segment replacement (TSSR) would exhibit decreased dynamic stiffness and more rapid energy absorption compared to native functional spinal units under simulated physiologic motion when tested with the pendulum system.MethodsFive unembalmed, frozen human lumbar functional spinal units were tested on the pendulum system with axial compressive loads of 181 N, 282 N, 385 N, and 488 N before and after Flexuspine total spinal segment replacement implantation. Testing in flexion, extension, and lateral bending began by rotating the pendulum to 5°; resulting in unconstrained oscillatory motion. The number of rotations to equilibrium was recorded and bending stiffness (N-m/°) was calculated and compared for each testing mode.ResultsThe total spinal segment replacement reached equilibrium with significantly fewer cycles to equilibrium compared to the intact functional spinal unit at all loads in flexion (p<0.011), and at loads of 385 N and 488 N in lateral bending (p<0.020). Mean bending stiffness in flexion, extension, and lateral bending increased with increasing load for both the intact functional spinal unit and total spinal segment replacement constructs (p<0.001), with no significant differences in stiffness between the intact functional spinal unit and total spinal segment replacement in any of the test modes (p>0.18).ConclusionsLumbar functional spinal units with implanted total spinal segment replacement were found to have similar dynamic bending stiffness, but absorbed energy at a more rapid rate than intact functional spinal units during cyclic loading with an unconstrained pendulum system. Although the effects on clinical performance of motion preserving devices is not fully known, these results provide further insight into the biomechanical behavior of this device under approximated physiologic loading conditions.

Convergence analysis and validation of a discrete element model of the human lumbar spine

“…Testing machines are used to study the mechanical properties of the spine (Mantell et al, 2016). The purpose of their use is to determine the stiffness of the systems (Daniels et al, 2013) and the critical force characterizing the onset of fracture in the spine (Xiang et al, 2021). The above-described methods of experimental research provide important information about the kinematic and structural parameters of the spine.…”

Development of a Computational Model of the Mechanical Behavior of the L4–L5 Lumbar Spine: Application to Disc Degeneration

“…Ex vivo studies are mainly focused on histological data and the results of magnetic resonance imaging and radiography, where it is possible to detect degeneration in the structure of the vertebrae, intervertebral discs [ 12 , 13 ], geometrical changes in the spine elements, as well as evaluate the kinematics of the system [ 14 ]. Testing machines are used to study the biomechanical properties of the spine [ 15 ], such as its stiffness [ 16 ] and the critical force corresponding to the onset of fracture in the system [ 17 ]. The above methods of experimental studies provide important information about the integral parameters of the spine, but they do not have the ability to determine the distribution of strains/stresses within the system under shock loads.…”