A Kinematics-Based Testing Protocol to Study the Mechanics of the Human Lumbar Spine
The objective of this work was to develop a new kinematics-based testing protocol to quantify the axial and shear force components and rotational moment properties of the human cadaveric lumbar motion segment unit (MSU) in response to specific kinematic inputs. Modern, non-fusion spinal devices claim to treat degenerative disc disease better than traditional fusion surgery. Though there have been many biomechanical studies completed on these devices, there is still a debate over their efficacy. Conventional testing methods provide insight into the rotational properties of the MSU but lack the sensitivity or capacity to quantify lumbar MSU’s mechanical properties including shear and axial force components. There is a need for a new testing protocol capable of measuring the segmental properties of the lumbar MSU and to study the influence of non-fusion spinal devices on them. Seven human cadaveric lumbar MSUs were mounted in a custom designed spine robot and flexed or extended about six unique points of rotation located ¼, ½, and ¾ of the anterior-posterior depth of the intervertebral disc and at similar points located 5mm below the endplate of the subjacent body. The MSUs were rotated until a target bending moment of 8Nm was reached. Measurements of the shear force along the disc plane, axial force normal to the disc plane, bending moment, and sagittal rotation were used to determine the effects of different prescribed kinematic conditions on MSU mechanics. MSU rotations differed significantly between each rotational point. As the point of rotation moved from anterior to posterior, and from the disc midline to the end plate of the subjacent body, rotation increased. For flexion, significant differences in shear and axial forces occurred between rotational points at ¼ and ¾ depth of disc. During extension, shear and axial forces were significantly different between all rotational points along the disc plane. The results of this study show that the new kinematics-based testing protocol described herein has the capacity to detect significant differences in the segmental rotation and shear and axial forces for small perturbations in the MSU location of the center of rotation for both flexion and extension.
MSU mechanics, biomechanical testing, spine robot, kinematics, shear and axial force components, Analytical, Diagnostic and Therapeutic Techniques and Equipment, Investigative Techniques, Medicine and Health Sciences
University of Tennessee Health Science Center
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