A novel modelling methodology which predicts the structural behaviour of vertebral bodies under axial impact loading: a finite element and DIC study

Cervical Spine Injuries (CSIs) arising from collisions are rare in contact sports, such as rugby union, but their consequences can be devastating. Finite Element (FE) models have been widely used to give a more in-depth understanding of the biomechanics of injury. Several FE modelling approaches are available in the literature, but a fully calibrated and validated FE modelling framework for cervical spines under compressive dynamic-impact loading is still lacking and material properties are not adequately calibrated for such events. This study aimed to develop and validate a methodology for specimen-specific FE modelling of vertebral bodies under impact loading. Thirty-five (n=35) individual vertebral bodies (VBs) were dissected from porcine spine segments, potted in bone cement and μCT scanned. A speckle pattern was applied to the anterior aspect of the bones to allow Digital Image Correlation (DIC), which monitored the surface displacements. Twenty-seven (n=27) VBs were quasi-statically compressively tested to a load up to 10kN applied at a rate of 1kN/min from the cranial side. Specimen-specific FE models were developed for fourteen (n=14) of the samples in this group. The material properties were optimised based on the experimental load-displacement data using a specimen-specific factor (kGSstatic) to calibrate a density to Young’s modulus relationship. The average calibration factor arising from this group was calculated (K¯GSstatic) and applied to a control group of thirteen (n=13) samples. The resulting VB stiffnesses was compared to experimental findings. The final eight (n=8) VBs were subjected to an impact load applied via a falling mass of 7.4kg at a velocity of 3.1m/s. Surface displacements and strains were acquired from the anterior VB surface via DIC, and the impact load was monitored with two load cells. Specimen-specific FE models were created for this dynamic group and material properties were assigned again based on the density-Young’s modulus relationship previously validated for static experiments, supplemented by an additional factor (KGSdynamic) derived from an iterative comparison between numerical stiffness predictions and experimental findings. The optimised conversion factor for quasi-static loading, K¯GSstatic, had an average of 0.033. Using this factor, the validation models presented average numerical stiffness value 3.72 greater than experimental. From the dynamic loading experiments, the value for KGSdynamic was found to be 0.14, 4.2 times greater than K¯GSstatic. Average numerical stiffness was 2.3 greater than experimental. Almost all models presented similar stiffness variation and regions of maximum displacement to what observed via DIC. The developed FE modelling methodology allowed the creation of models which predicted both static and dynamic behaviour of VBs. Deformation patterns on the VB surface were acquired from the FE models and compared to DIC data, achieving high agreement. This methodology is now validated to be fully applied to create whole cervical spine models to simulate axial impact scenarios replicating rugby collisions events.