Documentation:FIB book/Helmet

Helmets are used in multiple sports to prevent head injury. Sports include:

• Snowboarding
• Cycling

Introduction

Cycling is a popular form of active transportation used for day-to-day commuting and leisure activities. Despite its popularity, there are injury risks associated with cycling. Head injuries are recorded as the main reason for two-third of hospital admissions and three-quarter of deaths in cycling related accidents . Therefore, researchers are engaged in developing novel head-protection mechanisms to reduce the risks of head injuries in cycling accidents. Helmets are special head gear that are designed to protect the head from the injuries occurring in accidents. Traumatic brain injuries (TBI), morbidity, and mortality are considered as main factors in designing helmets . These helmet designs have been focused to distribute and reduce the linear acceleration of the head upon impact, typically using a hard shell and deformable materials which absorbs the impact energy. Rigid expanded polystyrene (EPS) has used in these helmet designs to reduce the risk of skull fractures, penetrating injuries, and brain injuries . Previous studies have been revealed that the linear acceleration causes skull fracture and rotational acceleration causes axonal shear strain in the brain that causes concussion . The impact studies and epidemiology data have shown that these helmets require more design effort, experimentation, and standardization to improve the safety.

Injury criteria are used to evaluate the efficacy of helmet designs in preventing head and brain injuries. The Head Injury Criterion (HIC) is an empirical formula used in conjunction with Injury Assessment Reference Values (IARV) to predict skull fractures. HIC is based on the resultant linear acceleration measured at the center of gravity of the head . Risk of concussion is calculated using peak angular velocity and the brain injury risk criterion. A brain injury of severity 2 on AIS (abbreviated injury scale) is defined as a mild-to-moderate concussion . New helmet designs are focused on various energy dissipation techniques that are strategically incorporated into helmet designs to reduce the risks of injury. Multi-directional Impact Protection System (MIPS) is one of the main designs that uses a low-friction layer to allow the shell to rotate up to 10-15 mm during impacts. This design approach is intended to dissipate the energy, which is normally transmitted to the head during rotational movements, thereby it reduces the strain and shear experienced by the brain during the impact .  

The main objective of this section is to review the biomechanical literature to describe the methods and the experimentations related to testing of helmets. The section is mainly focused on the strengths and shortcomings of current helmet testing methods.

Helmet Testing Methods  

Standard testing protocols are important to provide a baseline safety requirement for different helmet designs. The experiments and studies leading to improve the safety in designs are mainly following these baseline standards and recommendations in their testing methods. However, some researchers have tested alternative approaches to study various safety aspects that are not covered by these standard safety protocols.

Standardized Testing Methods

The most common testing method uses a monorail (or a guidewire) to freely drop a model of a head with a helmet on a flat, domed, and angled anvil . The recorded linear and rotational accelerations, and velocities are used to calculate the risk of head and brain injury. These calculated relative injury risks, are used to evaluate effectiveness of the particular helmet design. Monorail drop tests are highly controlled and repeatable, hence their popularity in testing for helmet attenuation. US Consumer Product Safety Commission (CPSC), Snell Memorial Foundation, American Society for Testing and Materials (ASTM), and Canadian Standards Association (CSA) are regulatory bodies that provide safety standards and recommendations for helmet designs . These standards provide specific guidelines to test helmets; a magnesium head form is used to drop from 1.5 - 2.2 m (including height adjustments to account for friction) to cause an impact velocity of 6.2 m/s on to differently shaped surfaces . These requirements specify that the helmet must reduce the linear acceleration to below a threshold value which, depending on the initial velocity and test protocol, is between 200 m/s² and 300 m/s². This acceleration is recommended to be measured using a single uniaxial accelerometer aligned with the primary axis of the impact . The recorded peak acceleration is used to calculate risk of skull fracture. However, as only measurement of uniaxial linear acceleration is required, this approach is insensitive in identifying the risk of concussion, as it fails to account for the injury contributions of rotational acceleration, impact duration and impact location .

Experimental helmet testing methods evaluate a broad range of parameters that are not mentioned in the present protocols and guidelines. Moreover, small deviations from the standardized procedure, such as using different types of head forms, neck designs, impact points, impact angles, drop heights, and surface friction conditions are evaluated in these experimental studies.

Non-Standardized Testing Methods  

Figure 1: An illustration of a setup of a drop test that is used to study head impacts similar to head injuries caused by bicycle accidents

Figure 2: An example of a vertical drop test. The Hybrid III head and neck model is attached to different helmets and different angles in the anvil are studied

These non-standardized experimental testing methods are focused on evaluating different parameters in different helmet designs that are not covered in general guidelines and protocols. These tests usually focused on reconstructing more realistic collision conditions based on epidemiology data. The recent developments in instrumentation technology have helped the researchers to study different parameters associated with helmet designs and injuries using novel approaches. Mainly, this has used to develop a common framework to predict concussion based on linear acceleration, rotational acceleration, impact duration, and impact location data .  

Several biomechanical studies used the Hybrid III head and neck (the standards recommend magnesium and ISO head forms without a neck model) to evaluate the risks of injuries . With its viscoelastic "skin" covering, the Hybrid III head and neck are more biofidelic in this impact modality than the magnesium recommended model and they are more representative of anthropometric inertial characteristics .  The standardized tests only specify flat and domed impact surfaces; however, recent studies have shown oblique impacts have increased the risks of TBI . Bliven et. al.  experimentally tested the helmets on oblique impacts surfaces based on previous study of Willinger et. al. on advanced bicycle helmet test methods . Bliven et al. made use of the experimental setup pictured in figures 1 and 2 . Fig. 1 shows a drop test setup using a Hybrid III head and neck that is used to evaluate the performance of helmets in dynamics oblique impact loading conditions . The helmeted or un-helmeted head and neck model is dropped on to the flat or angled anvil and the resulted impact is measured according to SAE J211 standard .

In this specific experiment, the drop tests were carried out for 4.8 m/s impacts on 30-, 45-, and 60-degree anvil-angles from horizontal plane and a 6.2 m/s impact on a 45-degree surface .  The accelerometers are placed on the center of gravity (CG) and, a linear accelerometer and a rotational accelerometer are used to measure the accelerations during the experiments. The rotational acceleration α_y and rotational velocity ω_y are recorded along the transverse y-axis. Impact velocity is measured with a time gate. The accelerometer data is captured at a sampling rate of 20 kHz and a low-pass filtered at Channel Frequency Class (CFC) 1000 (complying to the SAE J211 guidelines). Finally, HIC and brain injury criterion (BrIC) values are calculated and used to transform the measured values into injury criterion. This is used to assess the helmet’s role in reducing of risks of injuries during an impact.

MIPS Helmet Testing Methods

The testing procedure used for MIPS is different from the US and Canadian standard drop-rail methods. In this procedure, head forms with MIPS helmets are dropped unconstrained onto an oblique surface at 6.5 m/s and to measure the impacts and movements (6 DoF) to estimate brain injury risks. Different arguments have been raised regarding the use of head forms in these tests. The studies have found that the rotations upon impact are within the upper neck’s neutral zone, therefore, they acknowledged that the introducing of neck models have a positive impact on head kinematics of the overall study . Other research groups argue that the use of  a neck model would make it more biofidelic . Therefore, it is clear that the neck representation influences the kinematics, but its biofidelity is an open question required more studies .  

MIPs vs. Traditional Helmet Designs  

A study showed traditional helmet designs reduce the risks of brain injury in all drop conditions; 90.6% in a 1.5m drop and 60.3% in a 2.0m drop) . The performance of traditional and MIPS helmets is evaluated and compared using oblique impact in most studies. Recent studies have shown that MIPS helmets reduce AIS 2 brain injury risks by an average of 24.8% compared to tradition helmets dropped at a rotational acceleration of 7.2 ±0.6 krads/s² . In another study, finite element models are used to evaluate and compare the performance of different helmets under more loading conditions. A detailed finite element head model and three different helmet designs were used for modelling and analysis. Each helmet was modeled to be impacted at 4 locations (crown, front, rear and side) with a rigid hard surface. The results showed the helmet with an extra sliding layer added between the scalp and the traditional helmet has the lowest peak strain and reduced rotational acceleration in most loading conditions . This study concluded that finite element analysis is useful to evaluate more helmet designs in various loading conditions.

The previous experiment has recorded a maximum AIS of 2 during its trials. Accelerations are maintained below 90g and MIPS helmets had been able to reduce accelerations by 22%. Therefore, traditional (or EPS) helmets has showed a higher risk of AIS 2 in the brain injury risk criterion . In another study, MIPS helmets and conventional helmets are compared using an instrumented HIII head and neck model. This study has focused on drop tests onto an adjustable anvil to simulate oblique impacts. MIPS helmets were found to be reducing rotational acceleration in all impact conditions, by 21% to 44% relative to the conventional EPS helmets. This has reduced the probability of the AIS 2 head injury risks by 32% to 91% .

Discussion

It is important to study helmet designs and their performance to standardize different helmet designs considering their ability to reduce head risks injuries due to various cycling accidents.   A novel head form with biofidelic inertial properties was compared to HIII and EN960:2006 head forms to determine the effects of moment of inertia on oblique impact tests. Helmeted oblique impact tests resulted in differences of 71, 68, and 29% in the peak rotational acceleration response to impact in the x-, y-, and z- axes of rotation, respectively. This study confirmed the need to develop more biofidelic head forms, particularly for certification tests of oblique impacts .  

A study of the impact performance of 12 different bicycle helmets via drop tests addressed multiple shortcomings. Firstly, helmets were not tested at every impact location and configuration. The second concern addressed by this study related to test lines, representing the boundaries within which helmets must meet regulatory requirements for impact performance. Test lines typically do not cover the head, meaning that helmets can fail to meet criteria below the test lines and still meet certification requirements. As a result, helmets are not required to demonstrate the same degree of effectiveness across their whole surface .

A study of oblique impact drop tests created a crude approximation of a scalp to simulate movement of the scalp relative to the cranium. The artificial scalp consisted of an inner rubber bathing cap, a thin layer of oil, a medicine ball section, and an outer rubber bathing cap (in the outward direction). The artificial scalp was validated against tests on a volunteer. Tests performed on heads fitted with movable artificial scalps experienced reduced rotational accelerations by up to 56% relative to their fixed-scalp counterparts. While this test was a crude approximation, it suggests that scalp movement may need to be accounted for in the calculation of injury thresholds, particularly in oblique impact tests .

Oblique impact tests have highlighted the need to develop inertially biofidelic headforms, as opposed to continuing the use of HIII headforms, which maintain utility in linear acceleration tests. Oblique impact tests also highlighted the need to effectively simulate the human scalp, due to its role in attenuating rotational acceleration. While the established monorail drop-tests have proved effective in simulating linear impacts, it is clear that a modified or entirely novel method is required to accurately recreate oblique impacts.

Future Research

Helmet testing procedures can be improved using simple and repeatable standardized helmet testing procedures to assess concussion and brain injury risks. Linear impact tests, finite element studies can be further improved to investigate more realistic impacts similar to real accidents complying to the helmet testing protocols. Additionally, helmets are required to be tested with more impact locations and novel acceleration reduction standards should be investigated to improve their safety features .