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Investigation of the force associated with the formation of lacerations and skull fractures

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Abstract

Post-mortem examination is often relied upon in order to determine whether a suspicious death was natural, accidental, suicidal or homicidal. However, in many cases the mechanism by which a single injury has been inflicted cannot be determined with certainty based on pathological examination alone. Furthermore the current method of assessing applied force relating to injury is restricted to an arbitrary and subjective scale (mild, moderate, considerable, or severe). This study investigates the pathophysiological nature of head injuries caused by blunt force trauma, specifically in relation to the incidence and formation of a laceration. An experimental model was devised to assess the force required to cause damage to the scalp and underlying skull of porcine specimens following a single fronto-parietal impact. This was achieved using a drop tower equipped with adapted instrumentation for data acquisition. The applied force and implement used could be correlated with resultant injuries and as such aid pathological investigation in the differentiation between falls and blows. Experimentation revealed prevalent patterns of injury specific to the reconstructed mechanism involved. It was found that the minimum force for the occurrence of a laceration was 4,000 N.

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Author information

Authors and Affiliations

  1. Centre for Forensic Science, WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, UK

    E. J. Sharkey & N. NicDaeid

  2. State Pathologist’s Office, Marino, Dublin 1, Ireland

    E. J. Sharkey & M. Cassidy

  3. Royal College of Surgeons in Ireland, 123 St Stephens Green, Dublin 2, Ireland

    M. Cassidy

  4. School of Mechanical & Materials Engineering, University College Dublin, Belfield, Dublin 4, Ireland

    J. Brady & M. D. Gilchrist

  5. School of Human Kinetics, University of Ottawa, Ottawa, Ontario, Canada, K1N 6N5

    M. D. Gilchrist

Authors
  1. E. J. Sharkey
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  2. M. Cassidy
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  3. J. Brady
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  4. M. D. Gilchrist
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  5. N. NicDaeid
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Corresponding author

Correspondence to N. NicDaeid.

Appendix

Appendix

The acceleration calculations used took various contributing factors into account, including energy and free fall acceleration due to gravity. Studies have shown that air resistance has a negligible effect on impact velocity during freefalls from less than 50 ft [39], although in guided drop weight experiments where the impactor travels along guide rail tracks, friction effects will reduce the actual impact speed below that predicted by theory. Assuming constant acceleration due to gravity and ignoring friction, the velocity at impact is directly related to the fall height by Newton’s equation of motion, where the initial velocity (m/s) equals zero.

$$ {\text{Newton}}'{\text{s equations of motion}}:\quad \quad \quad \quad \begin{array}{*{20}{c}} {V = U + AT} \hfill \\ {{V^2} = 2AH} \hfill \\ {S = UT + 1/2{A^2}} \hfill \\ \end{array} $$

where V denotes velocity at impact (m/s), U is the initial velocity (=0 m/s when falling from rest), A is acceleration (=9.81 m/s2 when in freefall due to gravity), T is time (s) and S is distance travelled (m).

The principle of conservation of energy allows us to calculate the impact energy transmitted by an impacting implement to a target.

$$ {\text{Conservation of energy}}:\quad \quad \quad \quad \begin{array}{*{20}{c}} {E = {\text{PE}} + {\text{KE}}} \hfill \\ {{\text{PE}} = MAH} \hfill \\ {{\text{KE}} = \frac{1}{2}M{V^{{2}}}} \hfill \\ \end{array} $$

where E is the total energy (J), PE is the potential energy (J) (=0 at the point of impact since H = 0 m), KE is the kinetic energy (J) and M is mass of impacting implement and drop carriage (kg).

The work-energy principle can be used in cases where an object in motion has been brought to rest, e.g., when an impactor penetrates an object and stops after a certain depth of penetration. Work done by a force is calculated by multiplying the force acting on the body by the distance it has been moved (assuming that the force and displacement are parallel, as in this case). In order to obtain an average estimate of the impact force, the distance travelled after impact is used with the work-energy principle. The total work done by the force during the impact event is equal to the initial kinetic energy immediately prior to impact.

$$ {\text{Work - energy principle:}}\quad \quad \quad \quad {W_{\text{net}}} = {F_{\text{avg}}}D = \frac{1}{2}M{V^{{2}}} $$

where W net is the net work energy (J), D is the displacement during impact (m) and F avg is the average impact force (N).

The accelerometer used in the drop weight experiments allowed us to calculate actual acceleration values during impact; these were used instead of theoretical values, which ensured that friction losses were properly considered in our data. The acceleration values allowed us to calculate velocity, energy, and force using the equations outlined above.

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Sharkey, E.J., Cassidy, M., Brady, J. et al. Investigation of the force associated with the formation of lacerations and skull fractures. Int J Legal Med 126, 835–844 (2012). https://doi.org/10.1007/s00414-011-0608-z

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