Development of human lumbar spine FE models for adult and the elderly

For occupant protection in vehicle crash, several kinds of ATDs (Anthropomorphic Test Devices) and associated injury criteria have been used to evaluate the performance of a vehicle body, restraint systems and other safety devices. Because of the lack of sufficiently validated injury criteria for the lumbar spine, it has been a concern that the effectiveness of some safety features for injury reduction based on the dummy and associated injury criteria may not be reasonably assessed. Therefore, in this study, a human FE model capable of evaluating lumbar spine skeletal injuries was developed. Considering an increasing percentage of the traffic accidents relating to elderly people due to extending span of human life and decreasing birthrate, not only an adult model but also a model that represents lowered tolerance of the elderly was developed. From traffic accident statistics, 35 and 75 years old (y.o.) were defined as the representative ages of adult and elderly populations. An existing human FE model for an adult male was adopted for the baseline. The material properties of the trabecular bone of the lumbar vertebra for 35 and 75 y.o. were determined using the compression characteristics from the literature. Because of the lack of published data, those of the cortical bone were estimated so that the maximum (fracture) forces of the whole vertebrae under static compression predicted by the models agree to those of the tests from the literature. The isolated vertebra FE models were validated against dynamic compression and endplate impact tests for respective generations from the literature, and the result showed good agreement in maximum (fracture) forces. Then the whole lumbar spine model was validated against dynamic flexion, extension and lateral bending tests from the literature for the overall kinematics.