An advanced human surrogate neck for sports impact injury research

Sport participants are exposed to a unique environment in which traumatic brain injury (TBI) risk is elevated through deliberate and/or accidental impacts. Despite the increased awareness of the detrimental effects of TBI in high profile sports, the causal mechanisms for different TBI types are not universally well understood. Research to better understand, and so mitigate these causes with protective equipment, frequently relies on the use of anthropometric test devices (human-like surrogates representing human body parts) to ethically conduct impact experiments. The degree to which the measurement outcomes (i.e. impact phenomena, injury correlations and so risk assessments) are affected by the properties of the neck is disputed, but the biofidelity of the current surrogate necks (e.g. the Hybrid III, H3SN) is generally accepted as less than ideal.

The central premise of this research is that the neck has a significant effect on head impact event outcomes, that the limitations of currently available surrogate necks adversely influence the simulation and measurement of these outcomes, and that the creation of a superior surrogate neck without these limitations is both feasible and would provide more representative and insightful experimental data for the sports research community. The research presented evaluates this hypothesis through the development and utilisation of two improved surrogate necks in two sport specific impact studies.

A sagittal plane only surrogate neck (novel surrogate neck, NSN) was developed to more closely approximate human passive resistance to motion with characteristics otherwise similar to the H3SN. During experimentally simulated backward falls in judo, an advanced surrogate head constrained by the NSN experienced a 300% increase in angular velocity change, compared to the H3SN. The reduced stiffness of the NSN permitted greater angular acceleration of the skull with subsequent occipital contact with the mat reversing this motion and causing a potentially dangerous relative displacement of approximately 13 mm between the surface of the brain and the skull. In contrast, the high stiffness of the H3SN prevented observation of an injury mechanism that potentially explains the incidents of lethal acute subdural hematoma observed amongst novice judo practitioners.

Secondly, a more advanced Loughborough University Surrogate Neck (LUSN), was then successfully developed to represent the 50th percentile human neck, maintaining the head’s neutral posture with appropriate range, distribution, and resistance to motion in each of the principal anatomical planes, separately and in combination, using three appropriately positioned encapsulated ball joints and a rotational bearing assembly. The LUSN showed excellent biofidelity and closer agreement to reported post-mortem human subject, human volunteer, and computational simulation responses, under quasi-static and low speed dynamic load cases, compared to the H3SN and the more expensive Thor-M surrogate neck.

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High-speed helmeted baseball impacts were investigated in the laboratory using the LUSN and H3SN to support an advanced instrumented surrogate head. Peak resultant linear acceleration magnitudes were compared for the skull and for density- matched 6DOF sensors in the brain. The mean magnitudes of the first and second resultant linear acceleration brain peaks were found to be significantly higher for the LUSN constraint versus the H3SN (+15.9% and +8.6%). The resonant response of the skull, matched to human cadaveric data, significantly contributed to the magnitude of the resultant linear acceleration of the brain and the skull; both were found to be influenced by the neck constraint.

The research concludes that the biofidelity of a surrogate neck can significantly influence the measured phenomena for a surrogate head impact during both high- speed/low-mass and low-speed/high-mass sport impacts. By implication, properties of the human neck, and the surrogates used to represent it, can significantly influence the phenomena associated with predicted TBI severity from similar experiments, though further research is required to understand the medical implications of such phenomena and confirm or eliminate a causal link with TBI risk.