Examination of the Impact of Airbags on Renal Injury Using a National Database

Examination of the Impact of Airbags on Renal Injury Using a National Database Thomas G Smith III, MD, Hunter B Wessells, MD, FACS, Chris D Mack, MS, Robert Kaufman, BS, Eileen M Bulger, MD, FACS, Bryan B Voelzke, MD Little is known about preventative measures to lessen solid organ injury in motor vehicle collisions (MVCs). To evaluate the efficacy of airbags in reducing renal injuries in MVC, we analyzed renal injury rates in vehicles with and without airbags using the Crash Injury Research and Engineering Network (CIREN) database. STUDY DESIGN: The CIREN database was queried for MVC and renal injury from 1996 to September 2008. CIREN is weighted toward late model vehicles and selects more severely injured patients. Search fields were primary direction of force (PDOF), presence of airbags, and location of airbags (steering wheel, instrument panel, seat back, door panel, and roof-side curtain). Abdominal Abbreviated Injury Score was converted to AAST renal injury grade. Renal injury rates were compared between vehicles with and without frontal and side airbags. RESULTS: We reviewed 2,864 records and identified 139 renal injuries (28.9% AAST grade III to V). In MVCs with renal injuries, frontal impact was 54.7% of total (n ⫽ 76) and side impact was 45.3% of total (n ⫽ 63). Most occupants in frontal impact MVCs had exposure to a steering wheel airbag (74.9%); 16.6% had an instrument panel (passenger) airbags. In side impact MVCs, 32.2% of occupants had a side airbag. Compared with the non-airbags cohort, frontal airbags and side airbags were associated with a 45.3% and 52.8% reduction in renal injury, respectively. CONCLUSIONS: Passengers in automobiles with frontal and side airbags have a reduced rate of renal injury compared with those without airbags. Our data support further study of the role of airbags in reducing renal injury after MVC. (J Am Coll Surg 2010;211:355–360. © 2010 by the American College of Surgeons) BACKGROUND:

Motor vehicle collision (MVC) is the most frequent source of blunt trauma involving the genitourinary system. In 2008, almost 6 million police-reported MVCs occurred, resulting in 2.3 million injuries and 37,261 fatalities.1 Significant factors associated with both morbidity and mortality after MVC are primary direction of force (PDOF), change in velocity (⌬v), and restraint use.2,3 Several studies

have shown that lateral or side impact collisions are more lethal than head-on or frontal impact crashes.4-6 The kidneys are the most common genitourinary organs to be injured in the setting of trauma, with the vast majority secondary to blunt mechanisms.7 The incidence of renal injury after trauma is 4.89 per 100,000 persons, with a frequency of 1.2% to 3% in population-based studies.8,9 MVC as the source of renal injury ranges from 48% to 66%.8,10 Previously, we studied the sources of renal injury in the vehicle passenger compartment for both frontal and side impact collisions.11 Blunt renal injury continues to be a source of morbidity in MVC-related trauma patients. Independent of restraint use, frontal and side airbags have been shown to reduce injury severity and temper the association between ⌬v and morbidity and mortality. Airbags are designed to protect the occupant from cranial, cervical, and thoracic injury; however, we believe these restraint systems may have a protective effect for the upper urinary tract as well. To evaluate this hypothesis we used the Crash Injury Research and Engineering (CIREN) database to analyze renal injuries with and without airbag availabil-

Disclosure Information: Nothing to disclose. Work was performed for the Crash Injury Research and Engineering network (CIREN) project at the University of Washington/Harborview Injury Prevention and Research Center in cooperation with the US Department of Transportation/National Highway Traffic Safety Administration (USDOT/ NHTSA). Funding was provided by the NHTSA under Cooperative Agreement Number DTNH22-05-H-11001. Views expressed are those of the authors and do not represent the views of NHTSA. Abstract presented at the American College of Surgeons 95th Annual Clinical Congress, Surgical Forum, Chicago, IL, October 2009. Received March 15, 2010; Revised May 4, 2010; Accepted May 7, 2010. From Harborview Injury Prevention and Research Center, Harborview Medical Center at the University of Washington Medical Center, Seattle, WA. Correspondence address: Bryan B Voelzke, MD, Harborview Medical Center, 325 9th Ave, Box 359868, Seattle, WA 98104.

© 2010 by the American College of Surgeons Published by Elsevier Inc.

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Crash investigation Abbreviations and Acronyms

AAST AIS CIREN ⌬v FI MVC PDOF SI

⫽ ⫽ ⫽ ⫽ ⫽ ⫽ ⫽ ⫽

American Association for the Surgery of Trauma Abbreviated Injury Score Crash Injury Research and Engineering Network change in velocity frontal impact motor vehicle collisions primary direction of force side impact

ity. The CIREN database was developed by the National Highway Traffic Safety Administration (NHTSA) to provide detailed crash site analysis and specific occupant injury data to aid in the study of prevention, treatment, and rehabilitation of MVC injuries. As such, CIREN is a multicenter research network that provides researchers with a comprehensive resource of detailed crash and injury data pertaining to vehicles and their occupants. The primary aim of our study was to evaluate the efficacy of both frontal impact (FI) and side impact (SI) airbags in reducing renal injuries due to MVC by analyzing renal injury rates in vehicles with and without airbags.

METHODS CIREN database and case selection

The National HighwayTraffic Safety Administration coordinates 8 trauma centers that enroll patients in the CIREN system. For adults (⬎13 years of age), case occupants must have sustained at least 1 injury with Abbreviated Injury Scale score (AIS) ⱖ3 or 2 injuries of medical significance with AIS ⱖ 2. In addition, the vehicle must have been a late model vehicle (ie, manufactured 6 years before the crash). For frontal crashes, occupants must have been restrained by a seat belt or have had an air bag deploy. Cases in which the occupants were fully ejected from the vehicle were excluded. For each enrolled crash, medical data on injured occupants, crash scene data, and vehicle damage information are obtained. Each crash scene and vehicle investigation uses the format established by the National Highway Traffic Safety Administration’s National Automotive Sampling System (NASS). In order to establish probable cause and injury mechanism, each case is reviewed by a multidisciplinary team. This team consists of a crash investigator, bioengineer, research nurse, and a physician team representing radiology, trauma surgery, emergency medicine, orthopaedics, and/or neurosurgery. Each injury is discussed in detail, with sources of injury (contact points) identified and classified as certain, probable, and possible.

Scaled documentation of each crash site is performed. Data pertaining to the roadway, traffic controls, road surface type, conditions, and road grade at pre- and postimpact locations are collected. Physical evidence, such as tire skid marks, is used to determine the heading angle and postimpact trajectory of the colliding vehicles. A scaled drawing of the impact and final rest positions of the vehicles is used to assist in calculation of the speed and force of the impact. Exterior vehicle inspections include detailed measurements of direct and induced damage. Using a contour gauge, a damage crush profile is obtained from the front bumper or side plane and a specific collision deformation code, which incorporates the PDOF, is assigned. In some frontal crashes, bumper frame height mismatch crushes the grill and hood area rather than at the bumper frame. For this scenario, a secondary profile above the bumper frame is taken and averaged with the bumper frame profile. These measurements are entered into a crash analysis program (Win SMASH, version 1.2.1, US Department of Transportation), which estimates the ⌬v of the vehicle during impact and the energy absorbed during the crash event. The interior of the vehicle is inspected to determine the points of contact and restraint system use. Contact points are identified by the presence of scuff patterns, dents on the vehicle interior, and skin or fabric transfers.The interior inspection also includes an assessment of the integrity of the passenger compartment and measurement of component intrusion. Injury data

Injury data are coded by the multidisciplinary team after review of individual hospital reports, radiographs, and, when appropriate, autopsy reports. Individual injuries are coded using AIS and an overall Injury Severity Score (ISS) is calculated.12 Patient interviews are also conducted to supplement crash and injury data. The study is approved by the Institutional Review Boards at all participating trauma centers. Case selection criteria

The CIREN database was used to identify patients for this investigation.The database was established in 1996, and as of September 2008, data on 3,616 occupants were available for analysis. For the purposes of our study, only front seat occupants ⱖ16 years old involved in frontal or side collisions were included, leaving 2,864 occupants. Search fields included PDOF, presence of airbags, and type of airbag (steering wheel, instrument panel, seat back, door panel, and roof-side curtain). Rear collisions were excluded because their small number precluded statistical analysis.To evaluate renal injury severity, abdominal AIS assigned to a specific blunt renal injury was converted to an AAST renal injury grade. Conversion from abdominal AIS to renal AAST grade has been pre-

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Table 1. Distribution of Renal Injury by Direction of Force Direction of force Frontal Renal injury (AIS)

Renal injury AAST-OIS

No injury Contusion NFS* Contusion, minor Contusion, major Laceration NFS* Laceration, minor Laceration, moderate Laceration, major Hilar avulsion Rupture NFS Total

1 2 2 3 4 5

Side

n

%

n

%

Total, n

1,929 3 20 4 5 21 10 8 5 0 2,005

96.21 0.15 1 0.2 0.25 1.05 0.5 0.4 0.25 0

796 8 20 5 1 16 4 5 3 1 859

92.67 0.93 2.33 0.58 0.12 1.86 0.47 0.58 0.35 0.12

2,725 11 40 9 6 37 14 13 8 1 2,864

AAST-OIS, American Association for the Surgery of Trauma-Organ Injury Scale; AIS, Abbreviated Injury Score; NFS, not further specified.

viously validated.13-15 Finally, renal injury rates were compared between vehicles with and without frontal and side airbags to determine the effect of airbag deployment.

RESULTS Demographics

A total of 2,864 records were reviewed as a population base for the study. The average age of the study population was 40.77 years old and of the subjects with renal injury, 51% were male and 49% were female. A total of 139 renal injuries were identified with FI MVC the source of injury in 54.7% (76 injuries) and SI MVC the source in 45.3% (63 injuries). The majority of occupants involved in FI MVCs had steering wheel airbags (74.9%); however, only 16.6% had instrument panel (passenger) airbags. In SI MVCs, side airbags were available to protect 32.2% of occupants (inclusion of all side airbags types regardless of location). The ⌬␯ of the FI MVC in the nonairbags group was 42 kph; in the airbags group it was 43 kph. In the SI MVC cohort, the ⌬␯ in the nonairbags group was 38 kph; in the airbags group it was 34 kph. Additionally, 95% of frontal airbags deployed in FI crashes, but only 58% of side airbags deployed in SI crashes. Injury results

The mean renal AIS scores among occupants with renal injuries in frontal collisions were 2.53 in the airbag group

and 2.86 in the nonairbag group (p ⫽ 0.10 per WilcoxonMann-Whitney test). In side impacts, the means were 2.44 in the airbag group and 2.67 in the nonairbag group (p ⫽ 0.32). The distribution of injury by PDOF is listed in Table 1. Interestingly, 28.9% of injuries were intermediate to high grade (renal AAST grade III to V). Occupants involved in SI MVCs were significantly more likely to experience renal injuries than those in FI collisions (7.3% vs 3.8%, p ⬍ 0.0001), though the AIS score was similar among those with renal injuries (2.5 vs 2.6, p ⫽ 0.20). Seatbelt use was noted in 33% of crashes overall, with 33% of crashes in the FI group, and 34% of crashes in the SI group. In the FI group (n ⫽ 2,005), 62 injuries occurred in vehicles with airbag deployment (3.4%) and 14 injuries occurred in vehicles without airbag deployment (7.53%). Although the absolute number of injuries was greater in the airbag groups, frontal airbags conferred a 45.3% reduction in the frequency of injury (Table 2). A Wilcoxon-MannWhitney test, equivalent to a Student’s t-test but for nonnormal data, showed a p value of 0.0026. In the SI group (n ⫽ 859), 6 injuries occurred in vehicles with side airbags (4.2%) and 57 injuries occurred in vehicles without side airbags (8.0%) (Table 3). Among the side airbags cohort, the absolute number of injuries was lower and the frequency of injury was reduced by 52.8% (p ⫽ 0.058). Airbag deployment correlated with a lower renal injury rate in

Table 2. Frontal Impact and Renal Injury Stratified by Airbag Deployment

Table 3. Side Impact and Renal Injury Stratified by Airbag Deployment

Renal injury No Frequency n

Renal injury No Frequency n

Air bag: front

No Yes Total

n

172 1,757 1,929

92.5 96.6

14 62 76

Yes Frequency

7.5 3.4

Total, n

186 1,819 2,005

Air bag: side

No Yes Total

n

659 137 796

92.0 95.8

57 6 63

Yes Frequency

8.0 4.2

Total, n

716 143 859

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Table 4. Front Impact Renal Injury Stratified by Change in Velocity ⌬␯ < 40 kph Air bag: front No Yes Frequency n Frequency

Renal injury

n

No Yes Total

66 10 76

86.8 13.2

624 17 641

97.4 2.7

Total, n

Renal injury

n

690 27 717

No Yes Total

64 4 68

⌬␯ > 40 kph Air bag: front No Yes Frequency n Frequency

94.1 5.9

679 27 706

96.2 3.8

Total, n

743 31 774

⌬␯, change in velocity.

both the FI and SI cohorts at lower and higher crash ⌬␯ (Tables 4, 5).

DISCUSSION We previously examined sources of renal injury after MVCs using a large national database, the National Auto Sampling System.11 In this study, the restraint system and steering wheel correlated with renal injury in FI MVC; intrusion of the vehicle side panel was associated with renal injury in SI MVC. The next logical step in our study of crash injury analysis of renal injuries is identification of preventive measures to decrease these sources of injury. In the driver’s side of the passenger compartment during FI collisions, the steering wheel and seat belt are injury sources. The mechanism of steering wheel injury is blunt force due to impact with the chest and abdomen of the occupant. So, prevention of this injury source would intuitively result in a lower incidence of injury. Although the seat belt is designed as a restraint mechanism, it also doubles as an injury source in FI collisions, as previously shown.11 No relationship between PDOF, ⌬␯, restraint use, and severity of renal injury has been definitively correlated to this point. Despite this, our hypothesis that airbags reduce the frequency of renal injury is supported by a 45.3% renal injury reduction among vehicles in frontal collisions with airbags (p ⫽ 0.0026). Similarly, in SI collisions, specific components of the passenger compartment are identified as injury sources, namely, the armrest and door panel. We initially hypothesized that airbags in SI collisions may have a less dramatic effect on renal injury due to difference in the designed

purpose of side airbags. This is supported by data indicating that presence of side airbags increases abdominal injuries.16 Side airbags are designed to reduce more fatal and debilitating injuries involving the thoracic vasculature, brain, and spine. Because the kidneys are located in the retroperitoneum inferior to the chest cavity, side airbags, by design, should not ensure protection. Despite this limitation, the frequency of renal injury was reduced by 52% with side airbag deployment, although this reduction was not statistically significant. A concern with airbags mitigating injury is the uniformity in deployment. This has been studied in SI airbags. Others have noted that side airbags fail to deploy in a significant number of cases, 34% to 57%.17,18 Noted factors contributing to failure of SI airbag deployment included initial plane of contact (despite PDOF) and ⌬␯ of impact. This likely is due to the location of sensors within the vehicles because there is no industry standard for sensor location. The ex vivo forces necessary for renal injury have been described experimentally.19,20 The most recent model notes that speed of impact on the kidney is an important factor, and an impact speed of greater than 6.7 m/second or 15 mph is needed to create a renal injury of AAST grade III.21 In these experiments, impact velocity below this speed results in injuries that do not require intervention or follow-up (AAST grades I, II). Based on these data, airbags could have a protective effect if the impact threshold for airbag deployment was less than 6.7 m/second or 15 mph. There are several limitations to this study that must be addressed before drawing conclusions from our data. The main limitation of the CIREN dataset is that patients are

Table 5. Side Impact Renal Injury Stratified by Change in Velocity

Renal injury

No Yes Total

n

356 28 384

⌬␯, change in velocity.

⌬␯< 40 kph Air bag: side No Yes Frequency n Frequency

92.7 7.3

84 4 88

95.5 4.6

Total, n

Renal injury

n

440 32 472

No Yes Total

165 14 179

⌬␯> 40 kph Air bag: side No Yes Frequency n Frequency

92.2 7.8

22 1 23

95.65 4.35

Total, n

187 15 202

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not selected in a statistically random method, but rather are selected based on the enrollment criteria for patients presenting to the 8 involved trauma centers. The patients therefore represent a case series rather than a populationbased sample, which limits the ability to make inferences to nationwide rates. For this reason, a nonparametric method, the Wilcoxon-Mann-Whitney test, was used to assess statistical significance. Additionally, our dataset was small because the patient population of CIREN reflects data collected from only 8 centers. As such, we view this as a hypothesis-generating study, rather than a definitive evaluation of the protective characteristics of passenger compartment airbags in renal injury. Furthermore, because patients must give their consent to be included in the CIREN database, there is likely some selection bias inherent in the use of this and other convenience sampling databases. Despite this limitation, the CIREN methods offer considerable strengths that offset the above limitation. CIREN provides more in-depth injury and medical data than any other nationwide crash database by confirmation of injuries by the treating physicians and documentation of injuries by high-quality medical data. Additionally, crash analysis and assignment of sources of injury are conducted in a consistent, rigorous manner, resulting in a more in-depth biomechanical understanding of the injuries. Our study is the first to evaluate the protective effect of FI and SI airbags on a specific organ system. Others have shown that restraint systems, including airbags, significantly reduce occupant mortality; however, no mention is made of specific organ systems.2 We found a significant difference in the renal injury rate from FI MVC in collisions with and without airbags, although the difference in SI collisions was not significant. This degree reduction in injury rate was surprising, especially in the SI group, given the design of SI airbags and their intended protective effect for the thoracic and cervical spine. We also report a discrepancy in the deployment rates of FI and SI airbags; 95% of frontal airbags deployed in FI crashes, and only 58% of side airbags deployed in SI crashes. In conclusion, despite the limitations noted earlier, the CIREN data allow us to make the following conclusions and inferences. Passengers in automobiles with frontal airbags have a reduced rate of renal injury compared with those without airbags. Although the study is limited to renal injury, an inference can be made with solid organ abdominal injury and a decrease in injury. Further study is needed in the area of solid abdominal organ injury and airbag protective effect to solidify this inference. Although solid organ injury is a less common source of mortality, it is a significant source of morbidity and increases the duration of hospital stay. As such, recognizing the protective effect of

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airbags and improving airbag design could decrease solid organ injury severity and frequency. Improvements such as this could translate to a reduction in physical, emotional, and financial tolls that adverse events like MVCs place on greater society. Our data support further study of the role of airbags in reducing renal injury after vehicle collision. Author Contributions Study conception and design: Smith, Voelzke, Wessells Acquisition of data: Mack Analysis and interpretation of data: Mack, Smith, Voelzke Drafting of manuscript: Smith, Voelzke Critical revision: Bulger, Kaufman, Wessells, Voelzke

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