Assessment of the Dynamic Response of a Lumbar Spine Functional Unit Under Axial Compressive High Loading Rate: Outcome on the Axial Disc Bulge and its Relation to the Load Magnitude

Human lumbar spine tolerance to the compressive impact loading is less when compared to its tolerance to the perpendicular dynamic load. The dynamic response of the functional spinal unit in compressive loading is governed by the viscoelastic behavior of the IVD (Intervertebral disc). The axial bulge of the disc is the result of viscoelastic nature of the nucleus which tends to swell under high loading rate. This characteristic causes the end-plate to bow into the cancellous bone as it is supported by the strong cortical bone on its periphery. The end-plate is one of the important elements in the functional spinal unit if failed results disc material to progress into the vertebral body beneath it. This paper quantifies the axial bulge of the end-plate under dynamic compressive load using Finite Element Method. A simple validated axis symmetry FE model is employed to identify the most vulnerable lumbar spine level using the sensitivity analysis. This is followed by the development of more detailed FE model with viscoelastic modeling of the nucleus and the annulus. The dynamic load is applied on the superior vertebral body which follows triangular loading profile with 50ms rise time. The axial bulge is quantified at the center of the disc as this is the location of maximum deflection and local stress in the end-plate. The ratio of axial bulge and the total FSU deflection is plotted against magnitude of load applied to gain insight regarding the relation between load magnitude and axial bulge. This study will complement the research on end-plate fracture mechanism and its role in causing the burst fracture based on the magnitude of load.