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Threshold of the skull injury for blunt force impacts under free and constraint boundary conditions

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Abstract

The formation of skull fractures is an important topic in legal medicine. In particular, the influence of boundary conditions is controversially discussed in the literature. A study focusing solely on this aspect was missing. This study aimed to investigate the influence of boundary conditions on the energy threshold for head fractures. Because of the great variability of biological tissue of real skulls, we opted for a head model made from a polyurethane sphere filled with gelatin. Furthermore, we decided to investigate two opposite situations: A fixed configuration where a model was placed on a rigid surface and a (quasi) free boundary configuration where the head model was held at a force of 5 N compensating for gravity. For both configurations, we determined the acceleration signal of the impactor, the force, and the energy threshold for head fracture. It turned out that the fracture forces for both configurations were the same whereas the energy threshold was 11.0 J for the fixed and 13.6 J for the free boundary. The difference seems to be negligible if compared to the effect of varying structural mechanical properties of real human heads. This means that in a forensic case, the two situations most probably cannot be distinguished. To investigate the influence of the impactor mass, we developed a mathematical model and fitted the experimental data. As a result, we found that in the free configuration, a larger mass increases the energy threshold for head fracture. So that in principle, the two configurations are distinguishable.

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Notes

  1. In comparison to real long bones (e.g., hemurus) Frankenberg et al. [18] found a lower fracture force for polyurethane cylinders.

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Authors and Affiliations

  1. Institute of Forensic Medicine, University of Bern, Bern, Switzerland

    Lea Siegenthaler & Alessio Vaghi

  2. Institute of Applied Physics, University of Bern, Bern, Switzerland

    Michael Strehl & Martin Frenz

  3. Institute for Surgical Technology and Biomechanics, University of Bern, Bern, Switzerland

    Philippe Zysset

  4. Bpk Consultancy GmbH, Thun, Switzerland

    Beat P. Kneubuehl

Authors
  1. Lea Siegenthaler
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  2. Michael Strehl
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  3. Alessio Vaghi
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  4. Philippe Zysset
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  5. Beat P. Kneubuehl
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  6. Martin Frenz
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Correspondence to Lea Siegenthaler.

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Siegenthaler, L., Strehl, M., Vaghi, A. et al. Threshold of the skull injury for blunt force impacts under free and constraint boundary conditions. Int J Legal Med 134, 553–563 (2020). https://doi.org/10.1007/s00414-019-02023-2

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  • DOI: https://doi.org/10.1007/s00414-019-02023-2

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