Validation of a Lumbar Spine Fracture Injury Criterion for Finite Element Human Body Model Simulations

Abstract

Despite improvements in vehicle safety, leading to reduced moderate and severe injuries, in-depth studies on real-world vehicle crash data reveal that the risk of thoracic and lumbar spine fractures has not experienced a similar decline. This discrepancy may be attributed to the fact that thoracic and lumbar spinal injuries are not adequately assessed in vehicle safety regulations and consumer testing programs, potentially due to insufficient information or a limited understanding of the injury mechanism. In this thesis, the aim was to validate a newly developed model of the human lumbar spine with an associated strain-based fracture injury risk function (IRF). To reach this aim, the new lumbar spine model was used in reconstructions of tests on isolated lumbar spines, the lumbar spine model was integrated in the SAFER Human Body Model (HBM) and the updated model was used in reconstructions of sled tests with Post Mortem Human Subjects (PMHSs) in reclined positions and of a real accident. The risks of lumbar spine injury fractures were estimated using the proposed strain-based injury criterion and compared with the injury scores from the original tests and in the accident and with other those from recently proposed force based lumbar spine fracture injury risk functions. In addition to the HBM simulations, an FE model of the Test device for Human Occupant Restraint (THOR) used in reconstructions of the sled tests to assess the comparability of the proposed IRF and the measurements obtained from the THOR FE model. For the isolated lumbar spine validations, the results indicated that the first lumbar vertebra (L1) had the highest risk of fracture, which was consistent with the experimental findings, and in the non-injurious case, the predicted risks were low at all vertebral levels. In the whole-body validation, the reconstruction indicated a high risk of L1 fracture. This was consistent with the findings from the laboratory tests, in which 3 out of 5 PMHSs experiencing fractures at L1 during the experiments. In the accident reconstruction, for which the occupant experienced a fracture at L5, there was an estimated fracture risk in the reconstruction of with the highest value of approximately 70% at L5. However, the force-based injury criteria showed low fracture risk in both the reconstructions of the sled tests and the accident, indicating a lower ability to predict risk of fractures. The new lumbar spine model and strain-based IRF appear to appropriately predict the risk of lumbar spine fractures when used isolated and when used as an integrated unit in a complete HBM. The result of the THOR simulation revealed a significant dependence on variations in H-point position on the THOR-sled FE model.