The effect of the scalp on the effectiveness of bicycle helmets' anti-rotational acceleration technologies

Abstract

Objective: Medical data has lead to the common understanding that bicycle helmets need to be improved to better protect against brain injuries resulting from rotational acceleration. Although many different technologies exist for reducing rotational acceleration during impacts, the lack of an official testing standard means that their evaluation is based on customised set-ups that may differ and not represent real accident conditions. Previously, the authors have shown that scalp tissue plays an important role during helmet testing by absorbing energy and creating a low friction interface between head and helmet, thus reducing rotational accelerations and velocities. However, no published study has yet examined the effectiveness of anti-rotational helmet technologies in the presence of a biofidelic scalp layer. The objective of this study is to address this gap.Methods: Three different commercially available helmet models, each one equipped with a different technology, were tested in the presence of scalp tissue, in two different scenarios; with and without the technology present. The effectiveness of each of these technologies is already documented in other studies, but only in the absence of a biofidelic scalp layer. Tests were carried out using HIII headform with porcine scalp attached to the outmost layer.Motion tracking was used to compare the impact kinematics of each helmet model in both scenarios.Results: Results showed that when a biofidelic scalp layer is present, there is no statistical difference between helmet models with and without the anti-rotational technology in terms of rotational acceleration, velocity, relative rotation, impact duration and injury risk. Conclusions:Results suggest that the presence of the scalp can obscure the functionality of anti-rotational acceleration technologies. This could indicate that the effectiveness of technologies tested in previous studies, which have not tested anti-rotational acceleration technologies in the presence of a realistic scalp layer, may exaggerate the contribution of such technologies if compared with a more biofidelic set-up. The study supports the fact that headforms should be better designed by incorporating artificial skin layers that can better imitate scalp's behaviour and, in addition, provides insights for the design of technologies against rotational acceleration.

Oblique impact responses of Hybrid III and a new headform with more biofidelic coefficient of friction and moments of inertia

“…The difference between the response of headforms under the same oblique impact conditions and using the same helmet can stem from the large difference in the coefficient of friction (CoF) at the headform/liner interface. Previous studies have shown that this CoF has significant effects on the head kinematics during oblique impact (Trotta et al, 2018b;Zouzias et al, 2021). Human skin at different regions has very different surface frictions, depending on the skin surface condition and hydration (Derler et al, 2015).…”

“…In contrast to this headform, the MoIs and CoF of the Cellbond headform are in better agreement with those obtained from measurements on live and postmortem human subjects ( Trotta et al, 2018a ; Connor et al, 2020 ). Previous research has studied helmet response with headforms that have more biofidelic MoIs ( Kendall et al, 2012 ; Connor et al, 2020 ) or CoF ( Trotta et al, 2018b ; Juste-Lorente et al, 2021 ; Zouzias et al, 2021 ) but not a headform that has both. For the first time, this study used a headform that has both biofidelic CoF and MoIs.…”

“…Future works should use the current test condition and a motorcycle helmet to test EN960 and other commonly used headforms, such as NOCSAE. We used an isolated headform without the neck, similar to previous studies on helmet impact ( Halldin et al, 2001 ; Stigson et al, 2017 ; Trotta et al, 2018b ; Connor et al, 2018 ; Zouzias et al, 2021 ). It has been shown that the neck has an effect on the head response, and this effect varies depending on the loading direction ( Nightingale et al, 1996 ; Ghajari et al, 2011a ; Ghajari et al, 2011b , 2013 ; Abayazid et al, 2021 ).…”

“…The results showed that the porcine scalp reduced both translational and rotational accelerations through energy absorption and sliding. Another recent study showed that the animal scalp also affects the performance of helmet technologies designed for head rotation mitigation ( Zouzias et al, 2021 ).…”

“…They found that the 3D-printed headform produced much higher rotational acceleration and velocity than the EN960-M headform under the oblique impact, but it is not clear whether this difference is due to their different MoIs or CoFs, or both. Similarly, other studies have used headforms with realistic CoF but not MoIs ( Trotta et al, 2018b ; Juste-Lorente et al, 2021 ; Zouzias et al, 2021 ). The rotational kinematics of a headform that has both biofidelic MoIs and CoF remains unknown.…”