Abstract
The objective of this research was to delineate age-based differences in specific thoracic injury diagnoses for seat belt restrained rear seat occupants and describe the associated injury causation in order to provide insight into how the load of the seat belt is transferred to occupants of various sizes. Using data from the Crash Investigation Research and Engineering Network (CIREN), 20 cases of rear seated, lap and shoulder belt restrained occupants with AIS2+ thoracic injuries in frontal crashes were reviewed. Seven were children and adolescents age 8–15 years, 5 were 16–24 years, 3 were 25–54 years, and 5 were 55+ years. Six of the seven 8–15 year olds sustained injuries to the lung in the form of pulmonary contusion or pneumothorax. Only three of the seven sustained a skeletal (sternum or rib) fracture; only one of these three involved multiple ribs bilaterally. In contrast, four of the five 16–24 year olds sustained at least one rib fracture - often multiple and bilateral. The adult cohort (25+ years) was involved in predominantly more minor crashes; however they all sustained complex rib fractures – seven of the eight involved multiple ribs, four of the eight were also bilateral. Belt compression – either from the shoulder belt or the lap belt – was identified as the primary cause of the thoracic injuries. Often, there was clear evidence of the location of belt loading from AIS 1 chest contusions or abrasions. These findings have implications for age-based thoracic injury criteria suggesting that that different metrics may be needed for different age groups.
INTRODUCTION
For the past several years in the United States (US), 8 years of age has been the recommended transition age from a belt positioning booster seat to a lap-shoulder belt as the only form of restraint (NHTSA, 2011). This transition point is reflected in state child restraint laws where the strictest laws prohibit use of a lap shoulder belt alone until the age of 8 years (IIHS, 2011). In comparison, the Swedish law specifies delaying use of the lap-shoulder belt as the only form of restraint until approximately 10 years of age and a minimum of 135cm height (Andersson et al, 2010).
Recent revisions to the American Academy of Pediatrics Best Practice Recommendations for Child Restraint have strengthened and clarified the US recommendations by stating that most vehicle seat belts do not fit children until they are 8 to 12 years old (American Academy of Pediatrics, 2011). However, based on NHTSA’s 2009 National Survey of the Use of Booster Seats, 47% of 6 and 7 year olds had already made the transition to seat belts, and only 6% of 8–12 year olds were using some form of child restraint (NHTSA, 2010). These findings suggest that while the public health message of encouraging booster seat use well past age 8 years remains important, a detailed understanding the current crash injury risk and injury patterns for seat belt restraint adolescents age 8+ years may reveal insight that leads to the development of alternative restraint countermeasures for this age group.
While head injuries are the most common injury for all restrained occupants regardless of age (Durbin et al 2003, Bohman et al 2011, Nance et al 2011 García-España et al 2008, Viano et al 2010, Yoganandan et al 2011) thoracic injuries are seen more frequently with increasing age (Koppel et al 2011, Bansal et al 2011, Stitzel et al 2010, Ruan et al 2003). Biomechanical studies have highlighted that the stiffness of the thorax increases with increasing age (Maltese et al, 2008, Kent et al, 2009) and radiological assessment of torso shape has documented differences with age (Gayzik et al 2008, Kent et al 2005, Comeau, 2010). These variations in material and structure likely lead to differences in injury patterns and associated causation. Therefore, the objective of this research was to describe the age-based differences in specific thoracic injury diagnoses for seat belt restrained rear seat occupants and describe the associated injury causation in order to provide insight into how the load of the seat belt is transferred to occupants of various sizes. Understanding the specific details of these injuries and their injury causation provides insight into how the load of the seat belt is transferred to occupants of various sizes. By comparing the injury patterns of pre-teenagers restrained in seat belts to older teenagers and adults in seat belts, any unique needs associated with the adolescent age group may be highlighted.
METHODS
Case Selection
Crash investigation cases were selected from the Crash Injury Research and Engineering Network (CIREN) database. The CIREN database collects its data from patients admitted to a network of Level 1 trauma centers across the United States. Cases have been collected for the CIREN database from 1996 to the present. The cases included in this analysis were entered into the database in the years up to and including 2012.
The inclusion criteria were as follows:
Frontal crashes identified by Collision Deformation Classification (CDC) code (general area of damage = F)
Principal direction of force from 11 to 1 o’clock
Occupant age 8+ years seated in the rear row(s) of the vehicle
Restrained by a lap and shoulder belt
Abbreviated Injury Scale (AIS) score 2 or greater injury to the thorax
Crash Investigation Data Collection
Each CIREN team uses similar methods to collect data. Crash investigation teams are dispatched to the crash scene to measure and document the crash environment, assess the damage to the vehicles involved in the accident, and determine occupant contact points according to a standardized protocol. Crash investigators investigate the interior and exterior of the vehicles involved, looking for evidence of occupant contacts, including scuff marks, interior vehicle deformation or rips, transfer of tissue, hair, bodily fluids and clothing. Occupant contact points on the interior of the vehicle are documented by photograph and are included in the detailed crash report. These on-scene investigations are supplemented by information provided by witnesses, crash victims, police reports, emergency medical service personnel, physicians, and hospital medical records. This information allows investigators to generate reports that estimate the vehicle dynamics, occupant kinematics during the crash and detailed descriptions of the injuries sustained in the crash by body region, type of injury and severity of injury. The crush of the vehicles involved in the accident is quantified and the instantaneous change in velocity (delta V), an accepted measure of crash severity, is calculated.
Case Review Process
As part of the CIREN process, a detailed case review was conducted by a team consisting of emergency medicine physicians, engineers, crash investigation specialists and database analysts. The case review process included a review of crash conditions, restraint and occupant characteristics, occupant injuries and occupant contact points within the vehicle.
For crashes within the CIREN database after 2005, injuries with AIS 2 and greater are analyzed using the CIREN Biomechanics Tab (BioTab) methodology to describe the injury causation scenario (ICS) (Schneider et al. 2011). BioTab methodology documents the details of the factors involved in injury causation, specifically identifying the physical components (or occupants) within the vehicle that contributed to the injury, regional and organ mechanism of injury (such as bending, shear, compression, or tension) and any additional contributing factors (such as the occupants pre-crash position, intrusion into the occupant space or past medical history). Each ICS included “involved physical components” (IPC) (structures external to the occupant) that represent those structure that contacted the occupants and through the contact, caused the injury. Because it is not always possible to know for sure what caused an injury, the researcher must also assign confidence levels of “Certain”, “Probable” and “Possible” to each ICS and IPC. The BioTab method also allows for the documentation of multiple scenarios in cases where a single scenario cannot be determined with certainty.
For this study, each case was further reviewed by the authors to confirm the ICS and identify specific evidence of shoulder belt presence in front of the torso (i.e. through evidence of clavicle loading through fracture or abrasion). For those cases that met the inclusion criteria but were not previously analyzed via the BioTab method, a full BioTab analysis was conducted.
In addition, since the data collection included years in which CIREN coded injuries according to multiple versions of the AIS, all injuries using older versions were recoded according to the 2005 (2008 Update) protocol (Association for the Advancement of Automotive Medicine, 2008).
Standard summary statistics (frequencies, means, proportions) were calculated for the occupant and crash characteristics. The relationship between age and delta-v was calculated using Pearson’s coefficient of correlation.
RESULTS
Thirty-two cases met the inclusion criteria; 12 were subsequently excluded from the study due to 1) seat belt misuse such as shoulder belt placement under the occupant’s arm (n=3), 2) subsequent rollover or other impacts after the frontal crash (n=4), 3) dorsal loading of the occupant due to cargo in the storage area of the vehicle (n=4), and 4) use of seat belt extender (n=1). A total of 20 cases were included in the final analysis. Crash, vehicle, occupant, restraint and injury characteristics of the reviewed cases can be found in Table A1 in the appendix. Seven were children and adolescents age 8–15 years, five were 16–24 years, three were 25–54 years, and five were 55+ years.
The crash partner in four of the seven 8–15 year olds was a high mass, high profile vehicle such as a large sport utility vehicle, tractor trailer, or fire truck; the remaining three were impacts with a tree or a pole. Both scenarios resulted in a large change of velocity for the case occupant.
Given the inclusion criteria, 100% of the occupants in the cases sustained an AIS 2+ thoracic injury. 65% sustained an AIS 3+ thoracic injury and 10% sustained an AIS 4+ thoracic injury. The distribution of all injuries sustained by these occupants by body region and age group is shown in Figure 1 (AIS 2+) and Figure 2 (AIS 3+). In these figures, every injury is represented once. For example, if an occupant had three thoracic injuries each individual injury is counted in the figure.
Figure 1:
Distribution of AIS 2+ injuries by body region and age group.
Figure 2:
Distribution of AIS 3+ injuries by body region and age group.
The pattern of thoracic injury varied by age. Six of the seven 8- to 15-year olds sustained injuries to the lung in the form of pulmonary contusion or pneumothorax (Table 1). Only three of the seven sustained a skeletal (sternum or rib) fracture; only one involving multiple and bilateral ribs. In contrast, four of the five 16- to 24-year olds sustained at least one rib fracture - often multiple and bilateral. The adult cohort (25+ years) was involved in predominantly more minor crashes (Table 2); however they all sustained complex rib fractures. Seven of the eight involved multiple ribs, and four were bilateral.
Table 1:
Pattern of Thoracic Injuries by Age Group
| Age group (yrs) | AIS 2+ Thoracic Injuries |
|---|---|
| 8–15 (n=7) | n=13
|
| 16–24 (n=5) | n=10
|
| 25–54 (n=3) | n=10
|
| 55+ (n=5) | n=13
|
Table 2:
Delta v (average and interquartile range) by Age Group
| Age group (years) | Average delta v (km/hr) | IQR delta v (km/hr) |
|---|---|---|
| 8–24 | 54 | 48–60 |
| 25+ | 32 | 26–35 |
The ratio of skeletal to non-skeletal thoracic injuries increased with age, however this trend was not significant (p=0.3) likely due to the small sample size for such an analysis (Figure 3). There was a significant negative correlation between age and delta-v (p = 0.034).
Figure 3:
Ratio of skeletal versus non-skeletal thoracic injuries by age group.
For these seat belt-restrained occupants, concomitant abdominal injuries were common regardless of age. 65% of the occupants sustained an AIS 3+ non-thoracic injury and 50% sustained an AIS 4+ non-thoracic injury (Table 3). The number of associated non-thoracic injuries decreased as age increased. However, such injuries were often intra-abdominal solid organ and vascular injuries that, when combined with the thoracic injury led to poor outcomes. Also notable is the absence of head injuries in the adult cohort.
Table 3:
Pattern of Associated Injuries by Age Group
| Age group (yrs) | Associated AIS3+ injuries in other body regions |
|---|---|
| 8–15 (n=7) | n=16
|
| 16–24 (n=5) | n=17
|
| 25–54 (n=3) | n=4
|
| 55+ (n=5) | n=4
|
Belt compression – either from the shoulder belt or the lap belt – was identified as the primary cause of the AIS 2+ thoracic injuries. Often (16/20), there was clear evidence of the location of belt loading from AIS 1 chest contusions or abrasions.
DISCUSSION
This analysis revealed important age-based differences exist in thoracic injury causation for rear seated, lap and shoulder belt restrained occupants. The focus on rear seated occupants was unique in that most previous thoracic injury causation studies have focused on drivers or right front passengers. A dedicated analysis for rear seat occupants was necessary as the geometry of the occupant space differs greatly compared to front seat occupants and drivers whose thoracic injury causation is dominated by contact with the frontal air bags and steering wheel.
Our results revealed that, in comparison to adult and elderly occupants, adolescents and young adults who sustain thoracic injuries were in crashes with a higher change in velocity and their crash partners were more often higher mass or higher profile vehicles or a narrow object such as a tree or pole. These types of crash partners likely lead to both high magnitude of intrusion as well as higher delta-v. Their injuries were predominantly lung contusions or pneumothoraces whose causation has previously been described as dependent on the rate of loading thus suggesting that a large change in velocity is needed for such injuries to occur (Gayzik et al 2007; Danelson et al 2011). The importance of younger age and substantial change in velocity was previously highlighted in a review of pulmonary contusions in front seat occupants, age 15 years and older (O’Connor et al, 2009). In that study, younger age (<25 years) and higher delta v (>45 km/hr) were identified as important predictors of this injury.
For the adolescents (age 8–15 years), often these lung injuries occurred in the absence of skeletal fractures to the rib or sternum. When those fractures did occur, they were commonly single rib fractures with little clinical importance. It is important to note that younger age does not necessarily predispose someone to a lung contusion. Rather, we hypothesize that the flexibility of a younger occupant’s ribs is greater than that of older occupants resulting in less injury at lower delta-v. At higher delta-v, the increased flexibility of the younger thoracic cage imparts deformation to the underlying tissue (without fracturing) and lung injury occurs.
As age increased, the injury pattern shifted to multiple, bilateral rib fractures in less severe crashes. All adults age 25 years and up sustained complex rib fracture patterns – seven of eight had multiple rib fractures and four of the eight were bilateral. The high prevalence of rib fractures in elderly front seat occupants has been previously highlighted by several studies (Bansal et al 2011, Koppel et al 2011, Kent et al 2008). While the AIS severity for rib fractures does not modulate with age, rib fractures are associated with increased mortality for older occupants compared to adolescents and young adults. Bulger et al (2000) in a review of CIREN data identified that those 65+ years sustaining rib fracture had twice the mortality compared to younger occupants sustaining similar fractures. This elevated mortality is likely due to an increased incidence of secondary pneumonia. Kent et al (2008) used National Trauma Databank data to reinforce this concept. In their analysis, rib fractures were the only injuries for over half of occupants 60+ years who sustained a fatal chest injury. Stitzel and colleagues (2010) used sophisticated statistical modeling techniques to identify a threshold age above which age becomes an important contributor to mortality. For rib fractures, this threshold was 67–70 years.
Age-based differences in whole body thoracic response have been noted as well. Maltese et al (2008) calculated pediatric thoracic stiffness from force-deflection data from the chest of patients receiving CPR at a pediatric hospital. The elastic force generated by the chest at 15% normalized sternal displacement (Fel15) was calculated for subjects age 8 to 22 years and compared to similar data from adults (Tsitlik et al 1983). The Fel15 for those 51 to 74 years decreased at 2.4 N/year, whereas Fel15 for those 8 to 22 years increased at 16.7 N/year (Maltese et al, 2010). These stiffness differences likely contribute to the variation in thoracic injury with age described herein.
In addition to age-based differences in thoracic injury patterns, the pattern of other associated injuries demonstrated interesting findings. For the adolescents, their thoracic injuries were associated with abdominal injuries – to both solid and hollow organs – that are typical of the constellation of injuries known as ‘seatbelt syndrome’ (Durbin et al 2001). This finding highlights that seatbelt syndrome is not limited to occupants who are restrained only by a lap belt but can also be sustained by those restrained by a lap and shoulder belt (Arbogast et al 2007). It is conceivable that the flexibility of the adolescent thorax contributes to additional loads being placed on abdomen. This is further exacerbated by the inability of rear seats to provide optimal geometry for this age child leading to the occupant scooting forward on the seat cushion (Bilston and Sagar 2007; Huang et al 2006) and essentially raising the lap belt higher on the abdomen. In contrast, for seat belt restrained adult occupants with thoracic injuries, associated injuries to other body regions are rare suggesting that the thorax is the main body region at risk for injury. Countermeasure strategies implemented in the front seat such a load limiters which are designed to manage the load transmitted to thorax may be appropriate for wider diffusion into the rear seat fleet (Foret-Bruno et al 2001; Forman et al 2009).
The absence of head injuries in the adult cohort was notable. This may be primarily due to the lower delta v of the crashes involving adult occupants. In addition, literature suggests that there are possible kinematics that may lead to the absence of these injuries. In sled tests of post mortem human subjects (PMHS) restrained in a sled buck replicating a rear seat environment, substantial rearward rotation of the torso coupled with forward translation of the pelvis was demonstrated (Michaelson et al. 2008). This motion would keep the head away from potential contact with the vehicle interior. Adolescents who have been suggested to have a higher center of gravity may not have this rearward rotation or it might be less pronounced.
It is important to note that a number of the excluded cases were scenarios in which cargo in the trunk of the vehicle deformed the rear seat back and loaded the posterior torso of the occupant resulting in AIS2+ thoracic injuries. These cases were excluded as part of this analysis as the relative contribution of the seat belt and cargo in injury causation could not be delineated. These cases do, however, highlight an important injury mechanism that may suggest current regulations that govern seat back latch integrity (FMVSS 207) may not be adequately protecting against real world circumstances where the trunk of a vehicle may be substantially loaded with cargo. This injury mechanism has been described previously (Mandell et al 2010).
The data presented here is subject to certain limitations. First, all cases were taken from the CIREN database, which is a sample of cases seen at a series of Level 1 trauma centers in the United States. As a result, no statistical sampling was performed to ensure that the sample cases represent the population. The current study does not attempt to calculate risk or likelihood but rather reports injury patterns, given an AIS 2+ thoracic injury. In addition, the small sample size limits the ability to detect any differences in injury patterns for the various occupant demographics and crash characteristics. However, the descriptive case series is important to generate hypotheses for further epidemiological, biomechanical and simulation-based comparisons of thoracic injury patterns for various occupant groups.
These findings have implications for age-based thoracic injury criteria suggesting that that different metrics may be needed for different age groups. The differences in thoracic injury patterns between children and adults influence how pediatric injury criteria scaled from adult biomechanical data should be interpreted. Due, in part to the differences in material properties described above, children frequently receive injury to the lung tissue, in the form of pulmonary contusions without accompanying rib fracture and therefore injury criteria should be directed towards predicting these injuries. Injury criteria developed from adult PMHS impact data will likely predict best the injuries present in those experiments – rib fractures with the occasional soft tissue injury (Eppinger et al. 1999, Kallieris et al. 1981, Kroell et al. 1974). As a result, scaling thoracic injury criteria for children from adult criteria that are based on number or severity of rib fractures may not produce a criteria that can discriminate appropriately between children with thoracic injury and those without.
CONCLUSIONS
Important age-based differences exist in thoracic injury causation for rear seated, lap and shoulder belt restrained occupants. In particular, injured adolescents appear to be involved in higher change in velocity crashes with a higher mass or profile crash partner or impacts with narrow objects such as trees or poles. Their thoracic injuries were predominantly lung contusions caused by high velocity loading. As age increased, the injury pattern shifted to multiple, bilateral rib fractures in less severe crashes. These findings have implications for age-based thoracic injury criteria suggesting that that different metrics may be needed for different age groups.
Acknowledgments
The authors would like to acknowledge the National Science Foundation (NSF) Center for Child Injury Prevention Studies at The Children’s Hospital of Philadelphia (CHOP) for sponsoring this study and its Industry Advisory Board (IAB) for their support, valuable input and advice. The views presented are those of the authors and not necessarily the views of CHOP, the NSF or the IAB members.
APPENDIX
Table A1:
Crash characteristics of cases included in the study.
| Case ID | Occupant’s Vehicle Type | Object Impacted | PDOF (deg) | CDC | Delta V (km/h) | Age (yrs) | Gender | Height (cm) | Weight (kg) | Seat | Evidence of Shoulder Belt Loading | MAIS | Injured Body Regions | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| H | F | T | A | S | UE | LE | |||||||||||||
| 1 | Sedan | Tree | 0 | 12FZEN4 | 58 | 13 | M | Unk. | 72 | 21 | Chest abrasion following belt path | 3 | x | x | x | ||||
| 2 | Hatchback | Sedan | 350 | 12FDEW1 | 35 | 70 | F | 160 | 1 22 | 21 | Chest contusion NFS | 5 | x | x | |||||
| 3 | Minivan | Tree | 350 | 12FDEW5 | 60 | 10 | F | 109 | 34 | 21 | Upper chest abrasion; loading marks on belt webbing | 4 | x | x | x | ||||
| 4 | Sedan | Tree | 0 | 12FZAW7 | 95 | 18 | M | 175 | 64 | 23 | R shoulder abrasion and contusion | 5 | x | x | x | x | x | ||
| 5 | Sedan | Sedan | 350 | 12FDEW3 | Unk. | 62 | F | 168 | 67 | 21 | R breast contusion; loading marks on belt webbing | 3 | x | x | |||||
| 6 | Hatchback | Pickup | 10 | 12FDEW3 | 51 | 17 | F | 153 | 51 | 21 | Blood and loading marks on belt webbing | 6 | x | x | x | ||||
| 7 | Sedan | Tree | 0 | 12FLEE9 | 20 | 60 | F | 158 | 87 | 22 | R shoulder, neck, and upper chest abrasions | 3 | x | ||||||
| 8 | SUV | SUV | 340 | 11FDEW3 | 36 | 8 | M | 152 | 40 | 23 | Chest contusion following belt path; R clavicle fracture | 2 | x | x | x | ||||
| 9 | Sedan | Minivan | 20 | 01FDEW3 | 48 | 21 | M | 173 | 70 | 21 | R rib 11 fracture; loading marks on belt webbing | 2 | x | x | x | ||||
| 10 | Minivan | Large truck | 350 | 12FDAW6 | Unk. | 29 | M | 185 | 73 | 21 | L shoulder and central chest abrasions following belt path; loading marks on belt webbing, D-ring, and latchplate | 3 | x | x | |||||
| 11 | Minivan | Large truck | 350 | 12FDAW6 | Unk. | 29 | M | 201 | 153 | 23 | Chest contusion following belt path; loading marks on belt webbing, D-ring, and latchplate; rips in occupant’s clothing underlying belt | 4 | x | x | |||||
| 12 | Sedan | Pickup | 350 | 12FYEW4 | 29 | 15 | M | 183 | 109 | 23 | R shoulder abrasion; L Rib 12 fracture; fabric transfer on belt | 2 | x | x | |||||
| 13 | Van | Utility pole | 0 | 12FYAN7 | 50 | 14 | F | 160 | 58 | 31 | R clavicle fracture; chest abrasions; loading marks on belt webbing and D-ring | 5 | x | x | x | ||||
| 14 | Station Wagon | Sedan | 0 | 12FDEW2 | 26 | 82 | F | 165 | 57 | 23 | L and central chest contusions; mid chest abrasion | 4 | x | x | |||||
| 15 | Sedan | Large truck | 350 | 12FDMW | Unk. | 12 | M | 143 | 40 | 23 | Central chest contusion and abrasion | 5 | x | x | |||||
| 16 | SUV | Tree | 0 | 12FLEW4 | 61 | 20 | F | 163 | 56 | 21 | L upper and R lower chest abrasions | 5 | x | x | x | x | |||
| 17 | Sedan | Sedan | 340 | 11FDEW3 | Unk. | 19 | M | 168 | 54 | 23 | Loading marks on belt webbing | 4 | x | x | x | x | |||
| 18 | Station Wagon | Large truck | 0 | 12FDAW9 | Unk. | 11 | F | 155 | 42 | 21 | Cervical spine dislocation; loading marks on belt webbing and latchplate | 5 | x | x | x | x | |||
| 19 | Pickup | Hatchback | 0 | 12FDEW5 | 46 | 39 | M | 180 | 77 | 21 | L shoulder contusion; loading marks on belt webbing, D-ring, and latchplate | 4 | x | x | |||||
| 20 | Sedan | Concrete pole | 0 | 12FREW3 | 34 | 60 | F | 165 | 53 | 23 | Chest contusion following belt path; scuff mark on belt webbing | 5 | x | x | |||||
Injured body region abbreviations are as follows: H – head, F – face, T – thorax, A – abdomen, S – spine, UE – upper extremity and LE – lower extremity. Seat position is defined as follows: first digit is the row in the vehicle – 2 is second row, 3 is third row, and the second digit is the seat position, 1 is left, 2 is center and 3 is right
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