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Vol. 28. Num. 1.January - February 2017Pages 1-50
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Vol. 28. Num. 1.January - February 2017Pages 1-50
Special article
DOI: 10.1016/j.neucie.2016.02.001
Pedestrian head injury biomechanics and damage mechanism. Pedestrian protection automotive regulation assessment
Biomecánica y mecanismo de producción del traumatismo cráneo-encefálico en el peatón atropellado. Evaluación de la normativa actual en la automoción
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Carlos Arregui-Dalmasesa,b,
Corresponding author
carlos.arregui@upc.edu

Corresponding author.
, M. Carmen Rebollo-Soriac, David Sanchez-Molinad, Juan Velazquez-Ameijided, Teijeira Alvareze
a Departamento de Ingeniería Mecánica, Universidad Politécnica de Cataluña, Barcelona-Tech, Barcelona, Spain
b Center for Applied Biomechanics, University of Virginia, Charlottesville, VA, United States
c Servicio de Patología Forense, Instituto de Medicina Legal de Catalunya, IMLC, Barcelona, Spain
d Departamento de Resistencia de Materiales y estructuras en Ingeniería, Universidad Politécnica de Cataluña, Barcelona-Tech, Barcelona, Spain
e Instituto Navarro de Medina Legal, Gobierno de Navarra, Facultad de Medicina, Universidad de Navarra, Pamplona, Navarra, Spain
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Article information
Abstract
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Figures (3)
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Tables (3)
Table 1. Classification of the damaged anatomical region, source: PCDS, n=4500.
Table 2. Classification of injuries of the knocked down pedestrian by anatomical region and AIS code, n=4500.
Table 3. Source of TBI, n=674.
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Abstract
Introduction

Pedestrian–vehicle collisions are a leading cause of death among motor vehicle accidents. Recently, pedestrian injury research has been increased, mostly due to the implementation of European and Japanese regulations. This research presents an analysis of the main head injury vehicle sources and injury mechanisms observed in the field, posteriorly the data are compared with the current pedestrian regulations.

Methods

The analysis has been performed through an epidemiologic transversal and descriptive study, using the Pedestrian Crash Data Study (PCDS) involving 552 pedestrians, sustaining a total of 4500 documented injuries.

Results

According to this research, the hood surface is responsible for only 15.1% of all the head injuries. On the other hand, the windshield glazing is responsible for 41.8%. In case of sedan vehicles the head impact location exceeds what is expected in the current regulation, and therefore no countermeasures are applied. From all the head injuries sustained by the pedestrians just 20% have the linear acceleration as isolated injury mechanism, 40% of the injuries are due to rotational acceleration.

Conclusions

In this research, the importance of the rotational acceleration as injury mechanism, in case of pedestrian–vehicle collision is highlighted. In the current pedestrian regulation just the linear acceleration is addressed in the main injury criteria used for head injury prediction.

Keywords:
Biomechanics
Pedestrian protection
Head injury
Pedestrian collision
Rotational acceleration
Resumen
Introducción

Los atropellos son una de las principales causas de muerte entre los accidentes de tráfico. Recientemente, ha aumentado el estudio de los atropellos, principalmente debido a la aplicación de la normativa europea y japonesa en protección de peatones. Esta investigación presenta un análisis del traumatismo cráneo-encefálico del peatón atropellado, asociándolo con la estructura del vehículo responsable de la lesión, su mecanismo de daño y comparando el resultado con la normativa existente.

Métodos

La metodología empleada ha consistido en un estudio epidemiológico descriptivo y transversal, mediante el estudio de datos de peatones atropellados recogidos en la base de datos americana (PCDS) que analiza a un total de 552 peatones atropellados y un total de 4.500 lesiones documentadas.

Resultados

De acuerdo con este estudio, el capó es el causante del 15,1% de las lesiones de la cabeza del peatón, mientras que el parabrisas es responsable de 41,8% de todas las lesiones. En el caso de los vehículos tipo utilitario la ubicación del impacto de la cabeza se produce por encima de lo que se espera en la regulación actual y, por lo tanto, no se aplican las contramedidas necesarias. De todas las lesiones en la cabeza sufridas por los peatones solo el 20% tiene la aceleración lineal como mecanismo de lesión, el 40% de las lesiones se deben a la aceleración rotacional.

Conclusiones

En esta investigación se pone de manifiesto la importancia de la aceleración rotacional como mecanismo de daño en la cabeza del peatón atropellado. En la normativa actual solo la aceleración lineal está contemplada en la formulación del principal criterio biomecánico utilizado para predecir el traumatismo cráneo-encefálico.

Palabras clave:
Biomecánica
Protección de peatones
Traumatismo cráneo-encefálico
Atropello
Aceleración rotacional
Full Text
Introduction

Traumatic brain injuries (TBIs) are presented in scientific literature as one of the main injuries to pedestrians in accidents.1–4 Additional studies have shown that head and neck injuries suffered by pedestrians represent almost 60% of all injuries to pedestrians in accidents.5

In an effort to reduce the risk of head injuries to pedestrians who are hit by cars, researchers have developed several tools, such as head impactors, pedestrian dummies and computer models. These tools have helped to increase the biomechanical knowledge of a collision during an accident. It has been observed that the local stiffness of the structures of vehicles is a primary concern in reducing the risk of a head injury and that simulations of impacts with mannequin pedestrians and computer models enable us to test other factors that affect the risks of TBI, such as the geometry of the vehicle and its influence on the angle and speed of the impact to the head with the vehicle.6

At present, it is a requirement to pass experimental tests in order to sell a new vehicle in Europe and Japan (EC 78/2009, TRIAS 63-2004). Other very influential consumer tests, such as EuroNCAP include the pedestrian test in order to evaluate vehicles’ overall safety. The purpose of this study is to examine this regulation; evaluating the pedestrian's head protection in the event of a collision. This research presents an analysis of primary head injury mechanisms, sources of injury observed in field studies and their comparison with current regulations. The location of the impact to the pedestrian's head on the vehicle and the collision distance is measured by wrap around distance (WAD). The head injury mechanism for pedestrians will be contrasted with the biomechanical criteria used in current regulations.

Material and methodsAnalysis of head trauma and head impact location based on the Pedestrian Crash Data Study database

The analysis was performed through a descriptive epidemiological and transversal study of the American Pedestrian Crash Data Study (PCDS) database. The PCDS database has 552 investigated cases in the years 1994–1998, collected in various locations of the United States of America, with a total of 4500 injuries. The database contains more than 300 variables for each case, including age, weight, height, speed of the vehicle at the moment of collision, category of the vehicle, classification of the injury according to the Abbreviated Injury Scale (AIS), source of damage for each anatomical area, etc. In this study, No. will refer to the pedestrians and n to the injuries.

In the PCDS database, pedestrian is defined as any person who is in contract with the ground, road, or public or private pavement. Motorists, skateboarders and cyclists are excluded. The pedestrian cannot be lying down or seated, the vehicle must have the original equipment, the first point of impact between the pedestrian and the automobile must be in front of the upper part of pillar A. All investigations of collisions were performed at the scene of the accident. If the knocked down pedestrian could not be interviewed or the vehicle could not be located in the 24h after the accident, the case was dismissed. The inclusion criterion for this particular study was pedestrians aged 12 years or older and taller than 150cm

The STATA program was used for the statistical analysis. The version used was STATA 7.0 (Stata Corporation, 4905 Lakeway Drive, College Station, Texas, USA).

The first objective to investigate in this database was to study the importance of head injuries in the case of accidents. A second objective was to determine the source of the TBI. A total of 674 head injuries have been counted in the database with all of the previously listed criteria.

Rotational acceleration as a mechanism of head injury in case of an accident

Martin and Eppinger7 in 2003 analysed the AIS code of each TBI and associated each code with its injury mechanism. Each injury was classified in 3 potential damage mechanisms according to the principal acceleration component that this injury may cause: rotational acceleration, translational acceleration and combined acceleration, in the event that the injury is caused by both rotational and translational acceleration.

In order to evaluate the influence of rotational acceleration in injuries contained in the PCDS database, a new variable was included in the analysis, which was the Martin transformation matrix. It has been implemented in the database to assess the presence of different injury mechanisms. In 440 of the 674 head injuries, the transformation was performed successfully. All undefined/doubtful injuries were classified as minor injuries (AIS 1), so there was no loss of information for moderate or severe injuries (AIS 2+).

ResultsSource of head injury in the knocked down pedestrian

The analysis of the PCDS database revealed the significance of head injuries by evaluating the anatomical region injured in a collision. Representing the total number of injuries available in the database, a total (n=4500) and pedestrians (No.=552) was obtained. Table 1 shows that the head is the third most frequently damaged anatomical region, behind the lower and upper limbs.

Table 1.

Classification of the damaged anatomical region, source: PCDS, n=4500.

Anatomical region  Frequency  Percentage  Cumulative 
1. Head  763  17.02  17.02 
2. Face  724  16.02  33.04 
3. Neck  18  0.4  33.44 
4. Chest  217  4.82  38.27 
5. Abdomen  233  5.18  43.44 
6. Spine  208  4.62  48.07 
7. Upper limbs  865  19.22  67.29 
8. Lower limbs  1469  32.64  99.93 
9. Unknown  0.07  100.00 
Total  4500  100.00   

After a more in-depth analysis was performed and the injuries were classified according to the probability of survival as described by the AIS code, Table 2 was obtained. It shows the percentage of injuries by anatomical region damaged and their respective AIS code.

Table 2.

Classification of injuries of the knocked down pedestrian by anatomical region and AIS code, n=4500.

Anatomical region  AIS 1  AIS 2  AIS 3  AIS 4  AIS 5  AIS 6 
1. Head  278  89  187  107  90  11 
2. Face  670  40  11 
3. Neck  18 
4. Chest  77  23  67  34  11 
5. Abdomen  115  82  13  16 
6. Spine  133  53 
7. Upper limbs  746  76  43 
8. Lower limbs  964  293  204 
9. Unknown  11 
Total  3012  656  534  161  113  24 

From these data, it is possible to conclude that as the severity of the injuries increases, the presence of TBI in the knocked down pedestrian increases, since the TBI is responsible for 79% of all injuries classified as AIS 5.

By analysing the PCDS database for pedestrians who fulfilled this study's inclusion criteria and categorising the structures of the vehicle responsible for TBIs, it was found that the windscreen was responsible for 41.8% of all TBIs, while the bonnet was responsible for only 15.1% of all TBIs, surpassed even by the ground in the second impact of the pedestrian, with a value of 15.6%.

The consequences of TBIs caused by windscreens are normally considered to be injuries of lesser importance, but the analysis of the PCDS database shows that only 33% of them are considered TBIs of lesser importance (AIS 1) and nearly 58.2% are considered severe TBIs (AIS 3+) (Table 3).

Table 3.

Source of TBI, n=674.

PCDS code  Source of injury  AIS 1  AIS 2  AIS 3  AIS 4  AIS 5  AIS 6 
722  Pillar A  15  20  12  62 
770  Bonnet  33  11  30  14  13  102 
773  Bonnet-windscreen transfer  14  11  44 
775  Windscreen  94  24  83  41  37  282 
776  Front end  21 
947  Ground  54  11  16  15  105 
Other  21  13  58 
              Total  674 

For dual symmetrical structures, such as pillar A, the values have been added together.

Following the strategy outlined in the methods section to evaluate the influence of the different types of acceleration as an injury mechanism, a new variable was created in the PCDS database implementing the Martin transformation matrix. Of the 674 head injuries that fulfilled the inclusion criterion, 440 could be compiled with the new variable. Fig. 1 shows the results of applying the transformation matrix to the database. We see that only 20% of all TBIs analysed have translational acceleration as the sole injury producer mechanism. In 40% of TBI cases, rotational acceleration is the sole injury mechanism. In the remaining 40%, TBIs may be caused by rotation or translation mechanisms.

Fig. 1.
(0.06MB).

TBIs classified by production mechanism from the Martin transformation matrix.

By representing the physical mechanisms producing TBIs in knocked down pedestrians and classifying them by AIS type, we see that these mechanisms are present in all grades of AIS, either as an isolated or combined mechanism (Fig. 2).

Fig. 2.
(0.1MB).

Production mechanisms of TBIs represented according to AIS classification from the Martin transformation matrix.

Discussion

Analysis of the PCDS database has shown that the windscreen is responsible for 41.8% of all TBIs fulfilling the inclusion criteria of this study. Similar conclusions have been reported by Otte and Tohlemann8 in 2001, since in their research 23.7% of all injuries were caused by the structure of the windscreen.

In this study the windscreen was responsible for the TBI of the knocked down pedestrian on twice as many occasions as the bonnet. Current Japanese and European regulations (EC 78/2009, TRIAS 63-2004) do not include any head impact in the windscreen area in their protocols, restricting the bonnet as the only area likely to be hit. Currently, only EuroNCAP evaluates the windscreen when the pedestrian's head hits it. It is easy to observe in the results published on its website9 that almost all vehicles have significant deficiencies in the lower area of the windscreen, since this area of probable collision is an unresolved area for the safety of pedestrians.

For the EuroNCAP protocol, the maximum WAD likely to be impacted is 2100mm This value has been shown to be insufficient according to the experiments performed by Kerrigan et al.10–12Fig. 3 summarises the research of 13 accidents at 40km/h performed by Kerrigan at the University of Virginia with a new model utility vehicle (7 tests) and a new model large sport utility vehicle (SUV) (6 tests), using 5 post mortem human surrogates (PMHSs) and 8 tests with the Polar-II dummy. The location of impact of the head on the vehicle in wrap around distance (WAD) and its comparison with the height of the pedestrian is shown in this figure. In all cases where the crashed vehicle was a utility vehicle, the average WAD value was 2106mm When the vehicle was an SUV, the average value was 1736mm, a value very close to the height of the pedestrian.

Fig. 3.
(0.11MB).

Experimental results: relationship between WAD and the height of the knocked down pedestrian for utility vehicles and SUVs. The diamonds correspond to impacts with PMHSs.

The sliding up of a pedestrian when contact is made with the vehicle is greater in the event of PMHSs. In cases where the impacted vehicle was a utility vehicle, the average WAD value was 2310mm and where it was an SUV, the average value was 1852mm This is due to the greater flexibility of the PMHSs during the folding of the vehicle compared to the Polar-II dummy and the subsequent different interaction of the pelvis with the front end of the vehicle. This result highlights the need to apply pedestrian protection measures to the windscreen area as stated in the previous epidemiological study and, therefore, the need for it to be included in the existing regulations. This need is further highlighted in utility vehicles, and it might even be necessary to increase the area likely to be tested above 2100mm of WAD for consumer trial protocols.

In 1971, Versace13 proposed a biomechanical criterion that was later accepted by the National Highway Traffic Safety Administration (NHTSA) and included in United States regulation FMVSS 208. This biomechanical criterion is known as head injury criterion (HIC) and is formulated in the following way:

where ar is the translational acceleration resulting from the three vector components measured in the head's centre of gravity, expressed in units of gravity acceleration g (1g=9.81m/s2), t1 and t2 are arbitrary times that maximise HIC function, expressed in seconds.

The HIC has shown to be a reasonable indicator in predicting head injuries and is a good initial tool for developing vehicle countermeasures but this criterion cannot predict all injuries suffered by a pedestrian, since some major injuries are related to rotational acceleration, such as subdural haematomas or diffuse axonal injury.1,14 In this analysis it was observed that rotational acceleration is two times more frequent than linear acceleration as a TBI mechanism for knocked down pedestrians.

Conclusions

The PCDS database was analysed by focusing on TBIs suffered by pedestrians in the event of a collision with a vehicle and the significance of head injuries in the knocked down pedestrian was highlighted.

The source of the injury was investigated to determine which structures of the vehicle were responsible for these head injuries. The windscreen was responsible for 41.8% of all TBIs and the bonnet was responsible for 15.1% of TBIs.

The different AIS codes associated with the TBIs presented by pedestrians were cross referenced with the transformation matrix developed by Martin, and this transformation resulted in the rotational acceleration having a presence twice as frequent as linear acceleration and a greater importance in severe injuries, AIS 3+.

The existing and mandatory regulations in Europe and Japan (EC 78/2009, TRIAS 63-2004) only includes impacts on the bonnet, i.e. it does not take into account the windscreen for the test and, therefore, these laws do not promote the countermeasures in the windscreen. Additionally, consumer trials such as EuroNCP should increase the area likely to be impacted above a WAD of 2100mm The HIC is the primary biomechanical criterion accepted and applied by the automobile industry to design a vehicle in terms of its potential head impact against the bonnet. The HIC has been shown in scientific literature to be a good indicator of TBI and it is a good initial tool for developing vehicle countermeasures. However, this criterion cannot predict all injuries suffered by a pedestrian. Linear acceleration has currently been adopted as the primary production mechanism of TBI but the rotational kinematic components of acceleration should be included in future biomechanical pedestrian protection criteria. Modifying the area of the windscreen also involves modifications to the dashboard and the support under the windscreen. Therefore, the pedestrian protection requirements are difficult to fulfil due to the high requirements applied in this area. However, this must be a priority for the automobile industry and should be included in future regulations.

Funding

No funding.

Authorship

Carlos Arregui-Dalmases: Conception of and designing the study, writing the manuscript. M. Carmen Rebollo-Soria: Analysis and interpretation of the PCDS database, revision of the article in its different versions. Juan Velazquez-Ameijide: Revision and characterisation of the transformation matrix, AIS classification. David Sanchez-Molina: Analysis of pedestrian protection regulations, extraction of biomechanical criteria and differentiation of protocols. Critical revision of the final manuscript with important contributions in the methodology section. Rafael Teijeira: Data processing, revision of the literature, approval of final version for publication.

Conflicts of interest

The authors do not report any conflict of interest in this manuscript.

Acknowledgements

The authors would like to express our gratitude to Dr Basem Henry for his expert use of the PCDS database. We are also indebted to Dr Rodney Rudd for all of his comments on the correct behaviour of the kinematics of the pedestrian's head and, finally, to Dr Johan Ivarsson for all of his conversations regarding head injury production mechanisms.

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Please cite this article as: Arregui-Dalmases C, Rebollo-Soria MC, Sanchez-Molina D, Velazquez-Ameijide J, Alvarez T. Biomecánica y mecanismo de producción del traumatismo cráneo-encefálico en el peatón atropellado. Evaluación de la normativa actual en la automoción. Neurocirugia. 2017;28:41–46.

Copyright © 2016. Sociedad Española de Neurocirugía
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