The currently used worldwide regulatory criteria for controlling head injuries is commonly known as the HIC. Ever since its adoption in the FMVSS208 in 1972, this criteria has been controversial [4]. It is interesting to trace the development of HIC to evaluate its biomechanical basis. The earliest tolerance criterion for head injuries was introduced by Lissner et al. [5] in 1960 and is known as the Wayne State Tolerance Curve (WSTC). This tolerance curve was based on human cadaver skull fracture data, pressure application directly to the exposed brain of animals, and impacts to animals, and human volunteer data. Further examination of the WSTC was conducted in Japan over several years of research with impact experiments on animals and human cadaveric skulls [6]. Once again, scaling techniques were used to derive threshold of concussion curve, and turned out to be very similar to that originally proposed by Lissner et al. [5] and modified in the long duration regime by Patrick [7]. Theoretical justification of this curve was provided by finite element analysis of the skull and the brain by Ruan and Prasad [8]. In this study, iso-stress curves for the skull and the brain were developed as a function of average head acceleration and its time duration.

As early as in 1966, Gadd analyzed the basic biomechanical data supporting the WSTC and other animal human exposure data to propose the concept of severity index for evaluating head injury potential. The severity index was calculated by integrating the head acceleration raised to the power 2.5 (a ^2.5) over the entire duration of the pulse with 1,000 being the critical value. Early evaluation of the severity index was conducted by Hodgson et al. [9] who concluded that a good correlation existed between the severity index and degree of concussion in 29 stumptail monkeys subjected to impact, and with frontal bone fractures in cadaveric specimens. It was soon recognized that time duration of the head acceleration pulse affects the S.I., to the point that under a 1G environment, SI of 1,000 is exceeded every 1,000 s. Therefore, limitation of the pulse duration for calculation of S.I. was important. Versace [10] proposed an alternative formulation of SI which subsequently was adopted in the FMVSS208 in 1972. The new formulation is commonly known as the Head Injury Criteria (HIC). Ever since its introduction, HIC has been controversial [4].

Within the deliberations of the International Standards Organization (ISO) working group on biomechanics, it was suggested that the critical value of HIC be raised to 1,500 in frontal impact regulatory tests at 48 kph. The U.S. delegation rejected this proposal based on existing cadaveric data in which HIC’s were measured along with skull fractures and brain injuries. The rationale supporting the position taken by the U.S. delegation was explained in a paper by Prasad and Mertz [11]. The investigation also showed that human volunteers had undergone HIC exposures in the 1,000 range with airbags without any brain injuries. These HIC durations were long (30–36 ms) suggesting that limitations of the HIC duration were essential to evaluate the risk of head injuries. They suggested that calculation of HIC should be restricted to a maximum of 15 ms. They also developed a head injury risk curve that associated the probability of head injury versus HIC. A HIC level around 1,450 corresponded to a 50 % risk of skull fracture, while a HIC level of 700 corresponded to a 5 % risk of skull fracture. The head injury risk curve was later shown to predict the efficacy of reconditioned helmets against injury producing head impact in High School football [12].

Instead of limiting HIC durations to 15 ms, NHTSA chose to adopt 36 ms duration in the FMVSS208. However, Transport Canada adopted 15 ms HIC duration, but set the HIC limit at 700. This level of HIC corresponded to a 5 % risk of skull fracture or serious head injury as predicted by the Prasad and Mertz risk curve [11]. Recognizing that a 700, 15 ms HIC was more stringent than the 1,000, 36 ms HIC, NHTSA also adopted the Canadian Regulation in the 2003 phase-in of the Advanced Airbag Rule (FMVSS208). Even though, scaling techniques showed that the HIC for the 5th percentile dummy head should be 779, a limit of 700 was adopted for the 5th percentile dummy and the 6 years old dummy. Scaling was used to establish the limit of 500 for the 3 years old dummy and 390 for the CRABI dummy. Details of the scaling techniques can be found in an Alliance Submission [13] and NHTSA’s report by Eppinger et al. [14].

In the European regulation the HIC (renamed as the Head Performance Criteria (HPC)) is calculated only if contact between the occupant head and a vehicle component occurs. In these situations, HIC calculation is limited to the duration of contact with a maximum of 36 ms. An HPC of 1,000 is the limit. Additionally, the resultant head acceleration cannot exceed 80 g’s for more than 3 ms cumulatively. The HPC is considered to be satisfied if during the test no contact occurs between the head and any vehicle component. This last requirement is consistent with field data observations in the past– non-contact head injuries are extremely rare if any- and supported by more recent analysis of real world data by Yoganandan et al. [15].

As far as any theoretical basis for HIC is concerned, controversy continues. Many reasons are advanced, the main one being that HIC is based solely on the measurement of linear accelerations at the center of gravity of the head and its unit is “seconds”. Obviously, angular accelerations also cause shear deformations in the brain. However, limits of angular accelerations have not yet been introduced in any regulation in the world. If the limit is near 13,600 rad/s² [16], such high angular accelerations are hardly ever seen in regulatory testing of restraint systems in frontal impacts, and may be the reason why angular acceleration limits have not been specified in any regulation so far. It is currently believed that the effect of combined linear and angular acceleration on brain responses can be accounted for through detailed mathematical models of the human head.

The European frontal impact regulation has adopted neck injury criteria proposed by Mertz [1] in 1984. Mertz’s proposal consisted of specifying limits on peak extension and flexion moments at the occipital condyles of the dummy (head/neck junction) and time varying tension, compression and shear forces as measured by load cells in the Hybrid III dummy head. The peak extension and flexion moment limits were based on cadaver and human volunteer tests and accident reconstructions. The time varying tension forces were derived from accident reconstruction data using Volvo vehicles and the Hybrid III dummy for which real world injury data were available. The long duration end of the force-time curve (>45 ms) is based on muscle strength of a male volunteer. In terms of peak tensile force, the limit was suggested to be 3.3 kN which is in close agreement, to 3.1 kN, which was proposed by McElhaney and Myers [17] who based this conclusion on available biomechanical data. Compressive tolerance of the neck proposed by Mertz was based on reconstruction of injury-producing high school American football “spearing maneuvers” against a mechanical tackling device with the Hybrid III dummy [18]. It was an indirect way of determining dynamic axial compression tolerance limit. The peak tolerable load was 4.0 kN that dropped down to 1.1 kN if the duration of loading was 30 ms or longer. The peak load of 4.0 kN compares to 4.8 kN ± 1.3 kN mean axial load to failure in six cadaveric rotation constrained specimens reported-by McElhaney and Myers [17]. It should be noted that Mertz’s proposal called the neck injury tolerance levels as Injury Assessment Reference Values. Further, it was stated that being below these IARV’s meant that significant neck injury, due to the loading condition considered, was unlikely. With further biomechanical research conducted since the introduction of the Hybrid III 50th percentile male dummy and the development of a family of Hybrid III dummies, the determination of Injury Assessment Reference Values for these new dummies took on heightened importance. The Advanced Airbag Rule required changes in the FMVSS208 to further include child and 5th percentile dummies, NHTSA [19].