Documentation:FIB book/brain neck injury criteria

Introduction

Injury criteria are developed to relate the mechanical parameters applied to humans with the probability of being injured. As measuring these criteria for humans is not possible in real accident circumstances, dummies are used to simulate the situation. The idea of using dummies instead of humans comes from the fact that a structure's total mechanical response is only dependent on its geometry, material properties and forces and moments applied to its surface. Human is not an exception and this engineering assumption is the basis for using dummies instead of humans. One important step in developing realistic injury criteria is determining the injury tolerances of critical regions of the body. This is usually done using cadavers as a step between humans and dummies . Generally, when establishing an injury criteria, researchers use human volunteers, cadavers, animals and dummies each of them imposing specific limitations. Human volunteers cannot be exposed to injurious levels of loading. In cadavers, muscle tension, age, sex, tissue degradation, etc. are major shortcomings. Scaling animals' response to humans and the degree of anatomical similarity are in question. Testing dummies is the last step in validating a criteria. However, a combination of these methods can give us valuable information about human's injury tolerances.

Based on the report from the National Safety Council, accidents are the fourth leading cause of death for all age groups. 49% of the accidents are automobile accidents, which is the most common type with falls at home ranking the second (28%). Among all injuries in automobile accidents, head injuries have been estimated to occur in 71% of people injured in automobile accidents. Therefore, various quantitative methods have been proposed and developed to evaluate the severity of head injuries. One of the most common criteria is Head Injury Criterion (HIC) which relates resultant linear acceleration and time properties to probability of skull fracture. In particular, this criterion has a long-time history of being used in evaluating the head injury risk in frontal car crashes. The official HIC limit value documented in National Highway Traffic Safety Administration (NHTSA) are computed from acceleration data collected at the head of Anthromorphic Test Devices (ATD) seated in a car. Besides HIC, there exist various other head injury criteria based on other types of mechanical inputs, for example, rotational acceleration, which includes Generalized Acceleration Model for Brain Injury Threshold (GAMBIT), Head Injury Power (HIP), Rotational Injury Criterion (RIC), and stress and strain. In this article, we solely focus on HIC because it paves the way for subsequent head injury criteria development in this area, and it is one of the most influential criteria used.

According to the clinical record, drivers involved in automobile accident commonly experience severe neck injuries and there are more than three million cases of neck injuries reported every year in United States (US). Three fourths of these patients reported symptoms last six months or longer. Current estimation on whiplash injuries cost in US excesses 19 billion dollar annually. Both expense and human suffering bring attention to the public on the evaluation of various neck injury criterion. Two most common criterion are Neck Injury Criterion (NIC) and Neck Injury Criteria (Nij). NIC, which is also $NIC_{generic}$ with 15$m^{2}/s^{2}$ human tolerance, is the current standard to evaluate whiplash motion injuries (AIS1) on low speed rear end collision. $NIC_{max}$ is the maximum value of NIC(t) between the beginning of collision and the point in time where the head, relative to the neck reverses its direction of motion. $NIC_{protraction}$ is the criterion for frontal impacts. In addition, NHTSA has established a neck injury criteria called $N_{ij}$, which is the current safety standard evaluating force and momentum generated by the structure of upper neck. Other criterion such as Neck Protection Criterion( $N_{km}$), Intervertebral Neck Injury Criterion (InterIV-NIC), Neck Displacement Criterion (NDC) and Lower Neck Load (LNL Index) are not included in the article since we want to focus on the two most influential criterion among all the neck injuries evaluation systems.

Head Injury Criterion (HIC)

History

In 1943, researchers, E. S. Gurdjian and J. E. Webster, at Wayne State University did one of the first studies that look into head injury mechanisms using dogs as animal models. Starting in 1961, cadavers were used to study the head injury mechanisms by E. S. Gurdjian, H. R. Lissner, which led to the creation of the well-known Wayne State Tolerance Curve (WSTC). The curve was hand-drawn through all the data points collected from cadaver drop tests (shown in Figure 1). WSTC captures the relationship between the magnitude of linear anterior-posterior acceleration and load duration. It also served as the foundation for the Gadd Severity Index (GSI) and HIC. In 1971, Versace developed the modern version of HIC as a method to measure mean acceleration that correlates with the WSTC. Then, HIC was proposed to replace GSI in Federal Motor Vehicle Safety Standards (FMVSS) No.208 by the NHTSA. On October 18, 1986, NHTSA proposed to constrain the HIC interval to 36 ms. Later on, NHTSA also proposed to evaluate HIC performance over 15 ms interval, scale HIC to various occupant sizes, and create the injury risk curve for HIC.

Figure 1. Wayne State Tolerance Curve (WSTC) DOI: 10.1242/dmm.011320

Mathematical Definition

HIC is formulated analytically as the following:

$HIC=\max _{t_{1},t_{2}}\left({\frac {1}{t_{2}-t_{1}}}\int _{t_{1}}^{t_{2}}a(t)dt)\right)^{2.5}(t_{2}-t_{1})$

where $a(t)$ is the linear resultant acceleration (described in gravity unit, $g$). Assume the entire time duration of the acceleration is $T$ (in seconds), $t_{1}$ and $t_{2}$ are two time points within in the impact ($0\leq t_{1}\leq t_{2}\leq T$). The optimal interval $[t_{1},t_{2}]$ that maximizes HIC value is called the HIC interval and they are not necessarily unique. In frontal car crashes, time duration of the frontal impact usually ranges from 80 - 150 ms.

Major Limitation

One of the biggest concerns of using HIC is that it solely uses linear acceleration measurements to compute its value, lacking a consideration on other important factors that contribute to skull fracture injury such as rotational acceleration, location and area of the impact, and etc.

Applications

HIC is used extensively in vehicle safety as a major performance index to evaluate the risk of head injury, especially in frontal impacts. In FMVSS No.208 and FMVSS No.213, the upper limit HIC value is set to 1000 for a maximum of 36 ms interval and 700 for a maximum of 15 ms interval, under the condition that a Hybrid III dummy head moving at 15 mph to impact the interior of a vehicle. HIC36 is the old standard whereas HIC15 is the current standard.

HIC is also referenced in the New Car Assessment Program , which is a car safety program that evaluates new automobile designs against different types of safety threats. Not only the automobile industry, but the aircraft industry also regulates certifications according to certain HIC requirements. In the Head Injury Risk Curve (Figure 2), which was developed from cadaver drop tests, the probability of causing a maximum abbreviated injury scale (MAIS) >= 2 injury is mapped to a HIC value ranging from 0 to 3000. From this curve, HIC value at 1000 implicates a risk at around 50% of MAIS >= 2 injury. It is also mentioned in the report that HIC value at 700 represents a 31% probability of MAIS >= 2 injury.

Figure 2. Head Injury Risk Curve, indicating the probability of MAIS >= 2 injuries. NHTSA report

Besides vehicle safety, HIC is actively used in different sports. The major one is contact sports such as hockey, soccer, American football, lacrosse, as well as many others to help better design helmets and protect athletes from head injuries. Other types of sports include motorsports, cycling, and horse riding, which all tend to bear the possibility of resulting in head injuries.

Neck Injury Criteria (Nij)

History

The prior versions of FMVSS No. 208 were based on a number of studies form 1874 to 1996. These studies used dummies, volunteers, cadavers, and isolated and intact specimens. For children, stillborn infants, a two-week dead infant and pigs were used. These studies investigated the tolerance values for neck in different loading conditions, namely, compression, tension, flexion and extension. In most of these studies, small sample sizes were used. It was shown that tension tolerance of the neck structure decreases when it is under a combined loading condition of tension and extension.

Based on the previous version of FMVSS, if a dummy's axial and rotational response had fallen in the shaded box (Figure 3), it would have passed the requirements. However, this criteria does not consider the effect of combined loading on the neck. A study by Prasad and Daniel (1984) on porcine subjects showed that tension forces and extension moments should be linearly combined to create a reliable criteria.

Later on, the idea was extended to other combined loading conditions which may occur for the neck: tension-extension, tension-flexion, compression-extension, compression-flexion.

Figure 3. Previous FMVSS neck tolerance values. NHTSA report

Definition

Based on what have been said, the $N_{ij}$criteria was introduced, where i and j indices represent axial load and bending load conditions, respectively . Therefore, four values can be calculated: $N_{TE}$, $N_{TF}$,$N_{CE}$,$N_{CF}$. So, a new shaded box was developed (Figure 4). Any dummy response inside the shaded box passed the test.

Injury tolerances are dependent on geometric differences, material properties of tissues and the degree of skeletal maturity. All of these factors might vary between age groups.Therefore, different intercepts are assigned to different age groups and sizes, and the intercepts have seen some changes through the development of the $N_{ij}$criteria, which will be discussed later.

As the intercept values are different between dummy types, each axis of the box is divided by its critical value. So, all the resulting $N_{ij}$ values will be between 0 and 1 and a global comparison can be done even between different dummy types. The normalized shaded box will be a square with intercept values of one.

Figure 4. The first release of $N_{ij}$for the 50th percentile male HIII dummy (Table 2). The shaded box represents the area in which requirements are met. NHTSA report

In Hybrid III dummies, the 6-axis upper neck standard load cell measures forces and moments in all three directions. In frontal crash tests, the significant motion occurs in the sagittal plane. Movements out of this plane are not of primary importance. Therefore, the three crucial variables to be extracted from the load cell are: axial force ($F_{z}$), shear force ($F_{x}$) and bending moment ($M_{y}$). Shear force value is used to calculate the moment at occipital condyles. To do so, the height of the load cell above the condyles is measured and multiplied by $F_{x}$. The result is subtracted from $M_{y}$ to yield the occipital condyles moments.

The following formula is used to calculate $N_{ij}$:

$N_{ij}={\frac {F_{z}}{F_{int}}}+{\frac {M_{y}}{M_{int}}}$

$F_{z}$: Axial load (tension or compression based on the condition of loading)

$M_{y}$: Moment (flexion or extension)

$F_{int}$: Critical value used for normalization of the load

$M_{int}$: Critical value used for normalization of the moment

Development of critical intercepts

The idea of developing a new neck injury criteria came from a study by Mertz/Prasad which was done on porcine subjects aimed to the 3-year-old age group. They used a formulation to calculate critical intercepts for a 3-year-old dummy. 30 Nm for tension and 2500 N for extension were their proposed critical values. Later on, the data were reanalyzed using the multivariate logistic regression method. Results indicated 2000 N and 34 Nm as the best critical values for tension and extension, respectively, for the 3-year-old dummy.

Tension and extension critical values for other dummy sizes were determined by scaling the values for 3-year-old dummy using scaling techniques (Table 1).

Table 1. The scaled tension and extension critical values of different dummy sizes based on the 3 year old dummy. *adult dummy tension intercepts are replaced by the results from experimental data. NHTSA report

The critical tolerance values of the neck in flexion and extension were previously determined by Mertz et al (1971) as shown in Figure 3 using human cadavers (57 Nm for extension and 190 Nm for flexion). The same ratio (flexion = 3.33* extension) between amounts for flexion and extension was used to set the $N_{ij}$critical intercept values for the flexion (Table 2).

As mentioned before, the tension critical intercept values for all dummies, except the adult dummies, were calculated by scaling the value for 3-year-old dummy. The reason is that there are plenty of experimental data to get a better estimate of these values for adult dummies. The study done by Nightingale (1997) on cadavers combined with results from Yoganandan and Pintar experiments yielded a proposed amount of 3600 N for tension intercept value. For females, a value between the values for mid-sized male and 6-year-old dummy was chosen.

NHTSA-sponsored studies demonstrated that the compression tolerance of the neck is not significantly different from the tension tolerance. So, the same compression intercept values as extension intercept values were suggested. Table 2 summarizes what has been said.

After the publication of the first version of NHTSA injury criteria in 1998, comments were received and the criteria were revised and a new version was published in 2000. Critical intercept values were changed and NHTSA added some independent critical peak values for tension and compression forces. Also, additional critical intercept values were incorporated for in- (IP) and out-of position (OOP) situations.

One particular important comment to the previous version was that in case of in-position situation, the occupant is aware of the incoming accident and it is very likely that the neck muscles get tensed to anticipate the crash. As a result, the tensed muscles can carry a portion of the load applied to the neck by the head. Therefore, the tension and extension intercept limits were raised for in-position situation by 10 percent. Furthermore, the peak tension limit was increased by 25 percent for in-position tests.

The final $N_{ij}$intercept limits and peak limits are shown in table 3. These are the values which are currently used to calculate $N_{ij}$.

Table 2. The very first version of intercept values for $N_{ij}$published by NHTSA in 1998. NHTSA report

Table 3. The latest version of $N_{ij}$critical intercept limits and peak force limits. NHTSA report.

Limitation

$N_{ij}$ criteria is developed to predict the risk of AIS 3+ injuries to the neck. A large proportion of neck injuries are not severe. For example, whiplash injuries which usually occur in rear-end low-speed collisions are classified as long-term AIS 1 injuries. However, $N_{ij}$ is widely used in literature for quantifying the risk of whiplash.

One study was done to investigate the accuracy of different neck injury criteria in predicting AIS 1 injuries. Human volunteers did five different tasks designed to simulate the conditions of a rear-end car accident. As an example, they were hit in forehead by soccer balls at different speeds. Video recordings, bite block accelerometer and inverse dynamic methods were used to calculate accelerations, forces and moments in the head and cervical spine. As none of the volunteers experienced an AIS 1 neck injury, it was expected that different neck injury criteria yield a consistent result with AIS 1 injury. $N_{ij}$ was at most 0.31 in all the experiments. On the other hand, NIC overestimated the risk of AIS 1 neck injury. It was concluded that load-based injury criteria ($N_{ij}$, $N_{km}$, etc.) can predict AIS 1 injuries more effectively than acceleration-based ones such as NIC.

Another limitation of the $N_{ij}$ criteria is that it only considers the axial forces. The reason can be the fact that, in frontal crashes, shear force ($F_{x}$) is negligible comparing to axial ones. However, in rollover crashes the shear force can play an important role. The $N_{km}$ criteria was introduced to incorporate the shear force effect.

Furthermore, $N_{ij}$ ignores any motions out of the sagittal plane. Again, based on the type of the crash, the dominant movement can vary. So, $N_{ij}$ does not seem to be reliable in general conditions.

Moreover, the severity of an impact strongly depends on the time period in which it is exerted, and $N_{ij}$ neglects this fact.

Application

$N_{ij}$ criteria is widely used in crash tests to verify the security of the cars in preventing potential neck injuries. None of the four values of $N_{ij}$ should exceed 1.0 for the car to pass the test. This threshold is defined by FMVSS standards.

Another application of the $N_{ij}$ criteria is in designing and testing protection devices. For instance, it is used to compare various types of headrests and car seatbacks to find which type is more effective in preventing neck injuries.

Neck Injury Criterion (NIC)

History

The rapid extension-flexion movement of cervical spine neck injury in automobile collision results in patients suffering and medical cost everyday. In order to evaluate this injury prevention measures, it is necessary to establish and verify the cervical neck injury criterion to predict the severity of neck injury by using impact test dummies.

Most neck injuries in rear-end collisions occur at low collision speeds, generally less than 20 km/h. Additionally, Swedish team found a high proportion of whiplash related to this low velocity impact, and they proposed a hypothesis that whiplash is related to the pressure effect in the cervical spinal canal. In order to investigate this hypothesis, Örtengren, et al. (1996) established a series of experiments to detect possible plasma membrane leakage in a pig model system by simulating whiplash. Many typical symptoms can be explained by the result and it is suggested that the pressure change is the cause of ganglion injuries.

Based on the experiment of pig, the following research by Boström established a mathematical model, which was verified by the experimental data and validated against real world crash data.

The model predicts the pressure change in the spinal canal and uses it as a function of the volume change in the spinal canal when the cervical spine is bent. Based on these findings, NIC is proposed with the relative acceleration between the top and the bottom of the cervical spine.

Definition

NIC is a mathematical model that predicts neck injuries in low speed rear-end automobile collisions. The proposed equation was:

Figure 5. The possible direction fluid flow in the cervical spine (C1-T1) link.

$NIC(t)=L*a_{rel}(t)+v_{rel}(t)^{2}$

Where $a_{rel}$and $v_{rel}$ are the relative horizontal acceleration and velocity between bottom (T1) and top (C1) of the cervical spine (T1-C1). The term $L$ represents the length of the cervical spine, which was set at 0.2 m by measurement of the pig model and assumed to be similar in humans.

$NIC(t)_{pig}=0.2*a_{rel}(t)+v_{rel}(t)^{2}$

After that, a preliminary estimate of human tolerance level of NIC < 15$m^{2}/s^{2}$was proposed.

This NIC is designed to be used in rear impacts and potentially in frontal impact for AIS 1 injury criteria. The current standard to indicate whiplash neck injury criterion in rear end impacts is ${NIC}_{max}$,the maximum value during the first 150 milliseconds of the test.

${NIC}_{max}=\max(0.2*a_{rel}(t)+(v_{rel}(t))^{2})$

Due to the non-related velocity sign in the formula, a generic formula for extreme NIC values called $NIC_{generic}$ can be expressed as:

$NIC_{generic}=0.2*a_{rel}(t)+v_{rel}(t)*|v_{rel}(t)|$

Another criteria $NIC_{protraction}$with threshold of 15$m^{2}/s^{2}$ can be used in frontal-impact criterion and analysis:

$NIC_{protraction}=|\min NIC_{generic}|$

Limitation

NIC is based on pig model experiment on fluid flow and pressure gradients in spinal column. Since the similarity of fluid pulse wave between human and animal model is unknown, the accuracy of criterion remains questionable. Moreover, the mechanism behind neck injuries and whiplash injury is still under debate. Therefore, whether NIC is a significant factor or not needs further research.

In addition, Wheeler et al. (1998) investigated the low speed human volunteer crash test at speeds of 4 km/h and 8 km/h. 38% of total 40 volunteers reported minor complaints after the crash test. However, none of the experiments exceeded the human tolerance level of NIC < 15$m^{2}/s^{2}$. This experiment provided the evidence that current NIC is not able to calculate and predict some types of short term neck injuries such as the one described in the above scenario.

Lastly, when investigating neck injuries with NIC, especially whiplash injury, volunteers sitting positions and awareness of the impact can bring inconsistencies to the results.

Application

NIC is widely used for improving vehicle safety design and crash prevention design. It sets up the effectiveness especially for seat back and head restraint system with the threshold 15$m^{2}/s^{2}$ when applied feedback data from test dummies in vehicle collision tests. Not only for vehicle safety, NIC also plays the role in the design of sports gears, amusement park facilities, and etc. It can be applied to the cases where the risk and prevention of whiplash injuries are considered.