The three rollover tests of 1998–1999 Ford Explorer SUVs analyzed in the present study represent a unique dataset evaluating occupant dynamics in rollover crashes as represented by Hybrid III ATDs. To the authors’ knowledge, this study represents the first published test series of full-scale rollover crashes of a contemporary SUV with time synchronized sensor output from ATD neck transducers, roof rail accelerometers, an onboard high speed clock, and high speed external and internal video cameras.
ATD Biofidelity
The biofidelity of the Hybrid III ATD in rollover crash conditions is a significant limitation of this study as well as other published rollover studies1,15; however, ATD sensor data may be carefully evaluated within such limitations to yield objective insights. The ATDs used in this study were originally designed as a measurement tool in frontal crash tests, not rollovers. Thus, the Hybrid III ATD neck was designed and tested primarily to simulate dynamic head motion in the sagittal plane (neck flexion and extension).8,10 The Hybrid III ATD neck was not specifically designed to produce a biofidelic response under axial compression loading, typical of the neck loads resulting from the head-to-roof contacts observed in this study. Researchers have shown that with an axially directed impact to the crown of the ATD head, the ATD lower neck sensor records essentially the same magnitude axial force as the upper neck load cell, whereas the human cadaver neck lower load cell records a much reduced load magnitude compared to the upper load cell.24,29 Also, the Hybrid III ATD necks are neither frangible nor viscoelastic; therefore, they cannot accurately simulate individual vertebral displacement and/or fracture.
Notwithstanding the biomechanical differences between the necks of human cadavers and ATD mechanical necks, the F
z
measurements of the upper load cell of a restrained Hybrid III dummy may provide a reasonable estimate of the initial input load to a restrained occupant’s head when it contacts the roof in a rollover crash under similar dynamic conditions (e.g., impact velocity, impact duration). Given that the mass of a Hybrid III dummy head reasonably mimics that of the comparable human (50th percentile male in the present study), the inertial effect of the dummy head should not confound F
z
readings from its upper neck load cell.
In all three tests within the present study, F
z
values for ATD axial neck load were evaluated solely from the upper neck load cells, thereby taking into consideration, as much as possible, the significant differences in axial stiffness between the ATD and cadaver human neck (the lower neck F
z
sensor was not evaluated). These upper neck F
z
sensor readings were interpreted relative to the head impact loads in cadaver tests under similar dynamic conditions (described below). Given that the Hybrid III ATD neck was specifically designed to be biofidelic in dynamic, sagittal plane bending (flexion and extension) loads, neck flexion loads (M
y
) were evaluated in this study from the lower neck load cell of driver and passenger ATDs. Lateral bending loads (M
x
) were recorded and reported, but not interpreted in this study due to the lack of available biomechanical tolerance data for the human spine when subjected to lateral bending. Additional research in this important area, for both rollover and side impact crashes, is needed.
Injury Tolerance of the Human Cervical Spine
Semantic confusion exists in the published literature regarding cervical spine tolerance or “failure load.” Some investigators have reported cervical spine “failure loads” resulting from impacts to cadaver heads as the output force measured at the lower cervical spine, thereby neglecting the kinetic energy dissipation associated with vertebral fracture and/or subluxation above the position of the sensor as well as the viscoelastic response of the intervertebral discs.19,20 Other studies have reported both the input and output loads in cadaver tests of the cervical spine.23–26 These data provide a quantitative comparison of the significant differences between the input loads measured at the cadaver head versus what might be more appropriately referred to as the residual force magnitude that is recorded at the base of the cervical spine segment. No experimental cadaver work has measured and/or estimated the input loads at the superior aspect of the human neck, which is the site of the upper neck load cell in the Hybrid III dummy, when the head is subjected to an axial impact on the crown of the head. Additional research in this area is needed.
The confusion of terms regarding cervical spine failure loads in the published literature has a significant bearing on the proper interpretation of Hybrid III ATD axial neck load sensor output in controlled rollover tests such as the present study. Bahling et al. utilized 2000 N as a neck sensor F
z
threshold for “potentially injurious impacts (PII)” in rollovers, which is in the range of the published residual force magnitude recorded at the lower cervical spine following an injurious impact to the cadaver head.1,19 In contrast, other investigators reported the actuator (input) force at the head that was required for cervical spine injury in axial compression.24 In interpreting the ATD neck sensor data in this study, an F
z
value of 7000 N was utilized as a threshold of “probable (spinal) column injury,” which is in the range of the published head impact loads in cadaver studies. A value of 150 N m was used as the threshold for probable column injury for interpretation of M
y
readings from the lower neck sensor. In reality, the true axial compressive failure load of the human cervical spine is likely slightly lower than that measured by the head input loads in cadaver tests (due to inertial effects of the cadaver head), yet substantially greater than the lower cervical spine force data reported in the same cadaver tests. Thus, the real world relevance of threshold values used for both PII and “probable column injury” as a predictor of human injury risk must be tempered, consistent with the known biofidelity limitations of the ATD neck (described previously) and the wide variation in injury tolerance of the human spine, as described below.
The biomechanical literature suggests that the tolerance of the human cervical spine to serious injury is influenced by several mechanical factors, including individual tolerance variations due to age and gender, load magnitude and loading rate, pre-alignment (initial head-to-neck position), and end conditions of the cervical spine. Pintar et al.24 reported that serious (AIS 3+) cervical spine injury under axial compression loading conditions occurred at 5856 N with a loading rate of 3 m/s (for a 67-year-old female cadaver) and 11,242 N with a loading rate of 8 m/s (for a 50-year-old male cadaver). In the present study, none of the near-side, driver ATD sensors recorded upper neck F
z
values exceeding even 2000 N neck compression, and only one far-side passenger dummy (Test B190042) exceeded a neck sensor reading of 5856 N compression. Controlling for loading rate, age, and gender in a sample of 25 human cadaver head-neck compression tests, Pintar and Yoganandan27 reported that compressive tolerance varied from 7 kN in the young (third decade) to 2 kN in the very old (ninth decade). “Failure loads” (recorded at the lower cervical spine) ranged from 2 kN at quasi-static loading rates to 5 kN at dynamic loading rates (8 m/s). Failure tolerance for the male population was 25% higher than the female population (without regard to age and rate of loading). The present study recorded roof rail-to-dummy head contacts in the range of 5 m/s (11.2 mph) in the first ground strike of the SUV rollovers.4 Thus, the roof rail-to-head impact speeds in the present study were within the range of loading rates to cadaver and Hybrid III ATD heads in the studies by Pintar et al.24,27
Pre-alignment of the head and neck complex when struck by (or when it strikes) the roof is a key variable in determining whether catastrophic injury will occur, and if so, the specific type of injury that will be sustained. Published biomechanical studies suggest that cervical spine pre-flexion yields a greater incidence of lower cervical compression and burst fractures than neutrally positioned spines.24,25 Restrained occupants in rollover collisions are typically pre-flexed, due to the initial locking of their belt restraint system, and thus are at higher risk of cervical spine injury in the presence of an intruding roof structure when compared to unrestrained occupants. This higher risk of roof impact for restrained vs. unrestrained occupants in rollovers was confirmed with U.S. field accident data.9 The degree of pre-flexion of the ATD necks prior to head-to-roof contact in the present study was not determined because the onboard cameras were positioned behind the ATDs during the rollover tests. The seat belt load cells (i.e., excluding the two sensors described previously that malfunctioned) recorded continuous tensile loads in both lap and shoulder belts throughout the test interval for both driver and passenger ATDs in all three tests. This belt load data indicated that the lap–shoulder belts locked and remained locked during the first 1000 ms test interval for all ATDs in all three tests.
Nightingale et al.20 reported on the influence of boundary or end conditions of the cervical spine relative to specific injury types. When the head was unconstrained and free to translate and/or rotate away from the applied load, no cervical injury was sustained, despite high input loads to the head. In “full constraint” conditions, with the head pre-flexed along its stiffest axis (i.e., the removal of the normal cervical lordosis), buckling and burst fractures were noted. With rotational constraint (e.g., when the neck flexes forward to the point where the chin is against the sternum), bilateral locked facets typically occurred. Using cadaver spine specimens, Pintar et al.
26 determined that the average peak M
y
magnitude resulting in “major” neck injury was 97 N m at the specific site of injury in the cervical spine (i.e., not positioned as inferior as the lower neck load cell of the Hybrid III ATDs in the present study). The peak axial head impact force ranged from 3000 to 9700 N, with the peak flexion bending moment at the injured level ranging from 19 to 169 N m. Thus, the degree of constraint imposed by the contacting surface, such as an intruding roof, can be a major determinant for cervical spine injury. In all three tests of the present study, the far-side, passenger ATDs recorded lower neck flexion (M
y
) loads that exceeded 175 N m. None of the near-side, driver ATD lower neck sensors recorded peak M
y
loads exceeding 110 N m. No dynamic test data for cervical spine tolerance in lateral bending was identified; therefore, no interpretation of the peak M
x
sensor outputs for any of the ATDs was made.
The biomechanical impact environment to which the restrained ATDs were subjected in the present study was shown to be comparable to that of published cadaver studies on the basis of the following: head impact load magnitude and direction, loading rate, pre-alignment of the ATD head and neck, and degree of rotational head constraint. Thus, a comparison of the ATD sensor outputs for upper load cell F
z
and lower load cell M
y
values to probable column injury values of 7000 N and 150 N m, respectively appears scientifically reasonable, subject to the limitations described above.
Roof Crush as a Correlate or Cause of Injury
Two opposing views exist in the published literature regarding whether roof crush causes serious injury or whether it is simply associated or correlated with serious injury. One group of investigators has concluded that restrained occupants receive catastrophic head and neck injury from diving into the roof and making head-to-roof contact while the top of the vehicle/roof is striking/hitting the ground during a rollover.1,3,5,15,16 Other investigators have concluded that serious neck injuries to restrained occupants are directly caused by the dynamic intrusion of the roof structure into the occupant’s survival space during a rollover crash.12,13,28 The present study reports quantitative evidence of the temporal relationship between dynamic roof deformation, lap–shoulder seat belt loads, and restrained ATD neck loads, which provides further clarification in this scientific debate.
Objective roof/pillar deformation occurred prior to the occurrence of Peak neck loads (F
z
, M
y
, M
x
) for both driver and passenger ATDs in all three rollover tests. Prior to the occurrence of Peak neck loads, the driver and passenger ATD heads contacted the roof one or more times in all three tests; the magnitude of neck sensor responses ranged from 3 to 57% of the respective dummies’ Peak neck loads. Thus, the restrained driver and passenger ATDs did, indeed, “dive” into the roof while the roof was in contact with the ground, resulting in Local Maximum neck load values, which in no instance exceeded 57% of the Absolute Maximum (Peak) neck load. In particular, the “diving” neck loads (Local Maxima) for the far-side, passenger ATDs were only 2–13% of the Peak neck loads (F
z
and M
y
) for all three tests (Table 2, Fig. 4).
In each test, the Peak neck loads of the far-side, passenger ATD occurred within 10 ms following the time of maximum objective roof crush for all three tests, which is consistent with published inertial effects of the ATD head in transmitting an axial force to the lower neck sensors.24 In contrast, Peak neck loads for the near-side, driver ATD occurred up to 65 ms following the time of occurrence of objective roof crush for all three tests. The onboard cameras also revealed observable roof crush into the far-side versus the near-side occupant compartment during the time of the peak neck loads, which was completely consistent with the timing and higher neck load magnitudes of the passenger, compared to driver, ATD for all three tests.
The lap and shoulder belt load profiles for both driver and passenger ATDs provide further insights in the scientific debate regarding roof crush as a correlate versus a cause of serious injury. During the time interval of the Peak neck loads in the passenger ATDs for all three tests, the shoulder belt load, which should increase in magnitude as it resists torso augmentation, instead consistently decreased by 65–85% to only 44 N in some cases. The lap belt showed the same reduction in load during the time period that Peak neck load was recorded; it was reduced by 58% to approximately 156 N (Table 4, Figs. 8a and 8b). The belt load reductions occurred at the same time that on-board cameras clearly recorded the passenger ATD hanging upside down in the lap–shoulder belt and the roof crushing into the occupant space. This time of “observable” roof crush also occurred contemporaneously with the “objective” roof crush predicted by the accelerometers mounted on the SUV roof rails. The belt load that was recorded during each Peak neck load event for the far-side, passenger ATD was less than 50% of the Peak belt load in all three tests (Table 4). The shoulder and lap belt load cells indicated that the belts were off-loaded (i.e., loading decreased) as the roof crushed down on the ATD head and pushed the ATD back toward the seat cushion, away from the shoulder and lap belts, at the time of Peak neck loads (Fig. 9). Additionally, the deformation of the passenger (far-side) B-pillar lowered the upper attachment point (i.e., D-ring) for the belt, causing slack in the shoulder belt with a concomitant reduction in shoulder belt load.
Repeatability and Reliability
The FMVSS 208 dolly rollover test methodology has been criticized for its alleged lack of reliability and/or repeatability.15 The results of this study, however, demonstrated that the 208 rollover tests are very reliable when viewed from a vehicle-based, occupant protection frame of reference. Test-to-test comparisons of the FMVSS 208 rollover results revealed remarkable similarity in predicting the time of occurrence of Peak neck loads for both driver and passenger ATDs. The Peak neck F
z
occurred in all three tests at 530 ± 15 ms for the driver ATD (Fig. 10 and Table 3) and 730 ± 15 ms for the passenger ATD (Table 3). The Peak neck M
y
occurred in all three tests at 530 ± 18 ms for the driver ATD (Fig. 11 and Table 3) and 750 ± 21 ms for the passenger ATD (Table 3). The Peak value for M
x
occurred in all three tests at 530 ± 18 ms for the driver ATD and 770 ± 13 ms for the passenger ATD. These small variations in time of occurrence of roof/pillar deformation and Peak neck loads are particularly remarkable given that (1) the data came from six different dummies and three different vehicles tested on three different days, (2) the differences in the time of occurrence of the Peak neck loads was ≤20 ms in an overall time interval of 1000 ms, and (3) each 1000 ms time interval included either a sampling rate of 20,000 data points (B190043 and B190042) or 12,500 (B180220).
Quantitative analyses were undertaken to determine the significance probability of each pairwise comparison. Restated, the probability that the similarity of the maximum and minimum values of any two tests were related to something other than chance was analyzed (e.g., B190042-F
z
vs. B190043-F
z
, B190042-F
z
vs. B190220-F
z
, and B190043-F
z
vs. B190220-F
z
). Appendix 1 examines this issue and computes the probability for each pair of test metrics. This analysis revealed that there is a 93.1–98.6% probability that the differences in time occurrence of Peak neck loads between these results are NOT due to random chance alone. Stated more simply, there is a less than 7% probability that these differences are coincidental.
Validity
The constellation of driver vs. passenger neck loads measured in this test series of SUV rollovers was consistent with certain incidence trends of catastrophic head and neck injuries observed in real-world rollovers with restrained front seat occupants.9,21,28 None of the peak neck loads recorded in the near-side, driver ATD neck sensors exceeded the threshold values for probable column injury, which were used in this study. In contrast, the far-side, passenger ATDs recorded Peak neck loads that consistently exceeded the threshold for probable column injury due to neck flexion, M
y
. These findings are consistent with 1992–1998 data obtained from NHTSAs National Accident Sampling System (NASS) database, which showed serious spinal injuries were more frequent for the far-side occupants (compared to the near-side), where the source was most often coded as roof, windshield, and interior.21
A probability of two common catastrophic spinal column injuries was predicted by the passenger ATD neck sensors in all three rollover tests. A combination load of compression and of flexion is associated with burst fractures and wedge compression fractures. This load profile was recorded by the upper and lower neck sensors in the far-side passenger ATD in B190042. Bilateral facet dislocation injuries have been associated with sagittal plane flexion.17 The flexion moments recorded by the passenger ATD lower neck sensors in tests B180220 and B190043 exceeded the threshold values for probable spinal column injury by 40–240%. Thus, the results of this rollover test series suggest that the FMVSS 208 dolly rollover test may be a valid predictor of serious spinal injuries to restrained occupants in real-world rollovers.
The results of this study provide a unique data set that furthers understanding of probable spinal column injury mechanisms within a rollover crash environment. Such information may assist the scientific community and automotive engineers in recommending and designing appropriate intervention strategies to mitigate morbidity and mortality in rollover crashes. Moreover, these data may inform government agencies in formulating appropriate public safety policy to improve rollover crash protection for restrained occupants.