Abstract
BACKGROUND
The National Highway Traffic Safety Administration’s (NHTSA) New Car Assessment Program (NCAP) implemented side-impact crash testing on all new vehicles since 1998 to assess the likelihood of major thoracoabdominal injuries during a side- impact crash. Higher crash test rating is intended to indicate a safer car, but the real- world applicability of these ratings is unknown. Our objective was to determine the relationship between a vehicle’s NCAP side-impact crash test rating and the risk of major thoracoabdominal injury among the vehicle’s occupants in real-world side- impact motor vehicle crashes.
METHODS
The National Automotive Sampling System (NASS) Crashworthiness Data System (CDS) contains detailed crash and injury data in a sample of major crashes in the United States. For model years 1998–2010 and crash years 1999–2010, 68,124 occupants were identified in the CDS database. Because 47% of cases were missing crash severity (ΔV), multiple imputation was used to estimate the missing values. The primary predictor of interest was the occupant vehicle’s NCAP side-impact crash test rating, and the outcome of interest was the presence of major (Abbreviated Injury Scale [AIS] ≥ 3) thoracoabdominal injury.
RESULTS
In multivariate analysis, increasing NCAP crash test rating was associated with lower likelihood of major thoracoabdominal injury at high (OR 0.8, 95% CI 0.7 – 0.9, p<0.01) and medium (OR 0.9, 95% CI 0.8 – 1.0, p<0.05) crash severity (ΔV), but not at low ΔV (OR 0.95, 95% CI 0.8 – 1.2, p=0.55). In our model, older age and absence of seatbelt use were associated with greater likelihood of major thoracoabdominal injury at low and medium ΔV (p<0.001), but not at high ΔV (p≥0.09).
CONCLUSION
Among adults in model year 1998–2010 vehicles involved in medium and high severity motor vehicle crashes, a higher NCAP side-impact crash test rating is associated with a lower likelihood of major thoracoabdominal trauma.
Keywords: Renal Trauma
INTRODUCTION
Side-impact motor vehicle crashes (MVCs) are a significant cause of injury and death. Approximately 3.18 million U.S. drivers per year are involved in police-reported side-impact crashes1, resulting in an estimated 9,400 deaths each year, or 25% of all annual crash-related fatalities2. Near-side impact crashes are unique in that the occupant is in close proximity to the impacted portion of the vehicle, resulting in a greater risk of injury compared to frontal and rear crashes3. This type of impact results predominantly in injuries to the thoracic4 and upper abdominal organs such as the kidney, liver and spleen5,6.
To improve protection during side-impact MVCs, the National Highway Traffic Safety Administration (NHTSA) New Car Assessment Program (NCAP) instituted a standardized side-impact crash test for all new vehicles in 1998 and performed this test with few modifications until it was changed for the 2011 model year. The side-impact test seeks to replicate a crash where the car is struck on the driver’s-side door at an intersection, and attempts to predict the likelihood of major (Abbreviated Injury Scale [AIS] ≥ 3) thoracic or upper abdominal organ injury (referred to herein as “major thoracoabdominal injury”). Based on results from the simulated side-impact crash, a star rating is assigned to each car, with a higher star rating indicating a lower predicted risk of major thoracoabdominal injury during side-impact MVC. We sought to validate these predictions with real world crash data. Specifically, our primary aim was to assess the efficacy of NCAP side-impact star ratings in predicting the presence of major thoracoabdominal injury following side-impact crashes in the NASS CDS dataset.
METHODS
The National Automotive Sampling System (NASS) Crashworthiness Data System (CDS) is a nationwide crash data collection program sponsored by the NHTSA. The crashes in the NASS CDS database represent a sample of all police-reported crashes in which there was a harmful event (property damage and/or personal injury) and at least one passenger car, light truck or van was towed from the scene. For each of the approximately 5,000 crashes that are investigated annually, detailed vehicle, crash, occupant and injury characteristics are recorded in a public database. Crash data is obtained by trained crash investigators from the crash site and from interviews with those involved in the crash. Injury data is obtained from medical records and then coded according to the Abbreviated Injury Scale (AIS)7 by trained coders. Each case undergoes a rigorous review by a multi-disciplinary NASS team before being entered into the database. For this study, the AIS code in the NASS-CDS database was considered “major” if it corresponded to an organ injury severity (OIS) or regional AIS score of 3 or greater. Vehicle age at the time of crash was calculated by subtracting the model year from the crash year.
The NCAP side-impact crash test used during the study period8 was intended to simulate a severe right angle collision. The test was performed for model years 1998 through 2009, and involved a 3,015-pound moving deformable barrier striking the driver door of the stationary vehicle at 38.5 miles per hour. A belted SID-H3 dummy (Side-impact dummy, Humanetics, Plymouth, MI), which is designed to represent the height, weight and habitus of a 50th-percentile male, is positioned in the driver’s seat and in the left rear seat. Sensors attached to the dummies record the peak lateral acceleration at points representing the upper ribs, lower ribs and T12. All test results are publicly available at http://www.nhtsa.gov/Research/Databases+and+Software.
The NCAP side-impact star rating is based solely on the thoracic trauma index (TTI), which is the average of the peak lateral acceleration of the rib accelerometers and the accelerometer at T12. The estimated relationship between TTI and OIS ≥ 3 thoracic and upper abdominal organ injury is based on injuries sustained by cadavers that were subjected to lateral loads meant to simulate a side-impact MVC. Based on these data, the likelihood of major thoracic or upper abdominal organ injury is estimated to be <5% for a 5-star rating, 5–10% for a 4-star rating, 11–20% for a 3-star rating, 21–25% for a 2-star rating, and >25% for a 1-star rating9–11.
In order to compare NCAP side-impact crash star ratings to CDS injury data, each automobile in the CDS database was matched to a crash test based on the make, model and year of the car. Since NHTSA crash testing is only performed when there is a major vehicle design change, each automobile from the CDS database was matched to the most recent previous crash test for the respective make and model. Though NCAP side-impact crash testing was not performed for model year 2010, these vehicles were included in the analysis if there were no major changes to the vehicle design after the most recent crash test for that make and model.
NCAP side-impact crash star ratings are available for the driver and left rear passenger. The ratings for the driver were applied to occupants who were seated in the front-left and front-right positions, and ratings for the left rear passenger were applied to occupants in the left rear and right rear positions.
Crash severity is difficult to quantify, but is related to the change in velocity that a vehicle experiences during a crash. This value, known as ΔV, is often not known, but can be estimated by crash investigators. Depending on how ΔV is estimated, it is coded as total ΔV, barrier equivalent speed, or investigator-estimated ΔV. For analysis purposes, these variables were combined into a single variable. Despite this, ΔV was missing in 47% of cases. Another important predictor of outcome after MVC, seatbelt use, was missing in 21% of cases.
Since it is unlikely that ΔV and seatbelt use were missing at random, the missing values were imputed by chained equations as described elsewhere12,13. This imputation method treats the dataset as a simple random sample. The predictors used were presence of rollover, airbag deployment, impact location, occupant ejection, entrapment, seat position, age, height, weight, gender, Injury Severity Score, number of injures, trauma center level, duration of hospital stay, occurrence of death, occurrence of major abdominal injury, presence of airbag, presence of narrow crash impact (e.g., collision with pole), TTI, and NCAP weighting factor. The imputation process was duplicated twenty times, allowing us to model both the point estimates and the variance of our imputed data.
During the study period, there were a total of 68,124 occupant records in CDS. Following multiple imputation, there were 1,430,604 occupant records. Occupants were excluded if the vehicle could not be matched to an NCAP crash test (157,605, 11%); if the vehicle rolled (120,223, 8%); if the collision was purely frontal or rear and did not involve the passenger compartment of the vehicle (440,070, 31%); if the crash occurred on the far side of the vehicle relative to occupant position (315,157, 22%); and if the occupant was less than age 16 (49,305, 3%), was ejected from the vehicle (9,149, 1%) or not seated on the outside of the first or second row of the vehicle (9,308, 1%). Following exclusions, 329,787 records were available for analysis, corresponding to 23% of total CDS records.
Statistical analysis was performed in Stata SE 11.2. P values reflect t-test for comparison of means and chi-square test or logistic regression for comparison of categorical values. Multivariate analyses were performed using stepwise backward elimination.
RESULTS
Occupant, vehicle and crash characteristics are presented in Table 1. The cohort is relatively young, with most occupants being belted, front-seat drivers. The majority of injuries in this cohort were minor, with a median ISS of 1. Major thoracoabdominal injuries occurred in 5.8% of occupants and most commonly resulted from thoracic rather than upper abdominal injuries.
Table 1.
Occupant, vehicle and crash characteristics for the study cohort.
| All Occupants | |
|---|---|
| Occupant Characteristics | |
| Mean Age, years (SD) | 37.8 (17.6) |
| Mean Height, cm (SD) | 170.0 (12.1) |
| Mean Weight, kg (SD) | 76.5 (19.2) |
| Female Sex (%) | 50.1% |
| Driver (%) | 75.5% |
| Front-Seat (%) | 93.9% |
| Seatbelt Use (%) | 85.6% |
| Injury Characteristics | |
| Median ISS, range | 1 (1–75) |
| Major (AIS/OIS ≥ 3) | |
| Thoracoabdominal injury (%) | 5.8% |
| Thoracic injury (%) | 5.8% |
| Spleen injury (%) | 0.6% |
| Liver injury (%) | 0.4% |
| Kidney injury (%) | 0.1% |
| Died | 3.1% |
| Vehicle/Crash Characteristics | |
| Median Model Year | 2002 |
| Median Crash Year | 2006 |
| Mean Vehicle Age, Years (SD) Vehicle Type (%) |
3.6 (3.0) |
| Passenger Vehicle | 66.9% |
| Light truck/SUV | 33.1% |
| Mean Curb weight, kg (SD) | 1566.0 (385.7) |
| Star Rating (%) | |
| 5-Star | 29.0% |
| 4-Star | 27.5% |
| 3-Star | 33.9% |
| 2-star | 4.5% |
| 1-star | 5.1% |
| Side airbag present (%) | 20.8% |
| Front airbag present (%) | 77.9% |
| ΔV | |
| 25kmph (Low severity) | 69.4% |
| ≥25kmph; <40kmph (Medium severity) | 22.7% |
| ≥40kmph (High severity) | 8.0% |
Death occurred in 3% of all occupants. Among occupants who died, 68% suffered major thoracoabdominal trauma. Among the occupants who died and did not sustain major thoracoabdominal trauma, the most commonly injured region was the head, which was injured in 43% of occupants.
Median model year of the occupant’s vehicle was 2002, and median crash year was 2006. The mean vehicle age was 3.6 years. Most occupants (67%) were in passenger-vehicles, and the vehicle crash star rating was 3 or greater for the vast majority (90%) of occupants. Only 21% of occupants were in a vehicle with side-impact airbags, while 78% of occupants were in a vehicle with front-impact airbags. ΔV was <40kmph for the vast majority (92%) of occupants.
Occupant and vehicle information, stratified by the presence of a major thoracoabdominal injury, is presented in Table 2. Occupants with major thoracoabdominal injury were older, taller, heavier, less likely to be female and less likely to wear seatbelts (p ≤ 0.001). Vehicles with occupants sustaining major thoracoabdominal injuries were older, more likely to be passenger-vehicles versus light trucks/SUVs, had a lower curb weight, and a lower side-impact star rating (p < 0.001). There was no difference between the groups in terms of the presence of side-impact airbag in the vehicle (p=0.54). Higher ΔV was associated with a significantly greater likelihood of major thoracoabdominal trauma. Compared to a low severity crash (ΔV<25kmph), the odds of sustaining a major thoracoabdominal injury in a medium severity crash (ΔV ≥25kmph and <40kmph) were 6.02 (95% CI 4.92 – 7.37, p<0.001), and the odds of sustaining a major thoracoabdominal injury in a high severity crash (ΔV ≥40kmph) were 24.80 (95% CI 20.36 – 30.22, p<0.001).
Table 2.
Occupant and vehicle information, stratified by the presence of a major thoracoabdominal injury.
| No/Minor Thoracoabdominal Injury | Major Thoracoabdominal Injury | OR (95% CI) | P Value | |
|---|---|---|---|---|
| Occupant Characteristics | ||||
| Mean Age, years (SD) | 37.4 (17.0) | 41.7 (20.5) | - | <0.001 |
| Mean Height, cm (SD) | 170.2 (11.7) | 171.6 (10.8) | - | 0.001 |
| Mean Weight, kg (SD) | 76.4 (18.9) | 79.8 (20.0) | - | <0.001 |
| Female Sex, % | 49.7% | 44.7% | 0.82 (0.73 – 0.93) | 0.001 |
| Driver, % | 75.2% | 74.1% | 0.95 (0.82 – 1.09) | 0.44 |
| Front-Seat, % | 93.6% | 94.4% | 1.14 (0.88 – 1.49) | 0.32 |
| Seatbelt Use, % | 87.6% | 71.5% | 0.35 (0.30 – 0.41) | <0.001 |
| Vehicle/Crash Characteristics | ||||
| Mean Vehicle Age, years (SD) | 3.6 (3.0) | 3.9 (2.7) | - | <0.001 |
| Light truck/SUV, % | 34.1% | 23.0% | 0.58 (0.50 – 0.67) | <0.001 |
| Mean Curb weight, kg (SD) | 1577.7 (391.2) | 1499.4 (336.7) | - | <0.001 |
| Mean Star Rating (SD) | 3.8 (1.1) | 3.6 (1.1) | - | <0.001 |
| Side airbag present, % | 18.3% | 17.5% | 0.95 (0.81 – 1.12) | 0.54 |
| ΔV | ||||
| 25kmph (Low severity) | 72.3% | 20.6% | - | |
| ≥25kmph; <40kmph (Medium severity) | 21.7% | 37.2% | 6.02 (4.92 – 7.37) | <0.001 |
| ≥40kmph (High severity) | 6.0% | 42.3% | 24.80 (20.36 – 30.22) | <0.001 |
ΔV = Change in velocity (crash severity).
Preliminary analysis revealed evidence of effect modification with ΔV, so multivariate logistic regression was stratified according to ΔV. Accounting for age and seatbelt use, a lower NCAP side-impact crash test rating was associated with an increased likelihood of major thoracoabdominal injury at medium and high ΔV, but not at low ΔV (Table 3). Compared to an occupant of a 5-star vehicle, the odds of a major thoracoabdominal injury for an occupant of a 4-star vehicle was 2.0 (95% CI 1.2 – 3.3, p=0.01) at high ΔV, 1.8 (95 CI 1.1–2.9, p=0.01) at medium ΔV, and 1.2 (95% CI 0.8–2.0, p=0.41) at low ΔV. Compared to an occupant of a 5-star vehicle, the odds of a major thoracoabdominal injury for an occupant of a 3-star vehicle was 2.1 (95% CI 1.3–3.4, p<0.01) at high ΔV, 2.0 (95% CI 1.3–3.1, p<0.01) at medium ΔV, and 1.3 (95% CI 0.8–2.0, p=0.33) at low ΔV. The odds of major thoracoabdominal injury were not demonstrably higher for occupants of 1- and 2-star vehicles, but this may reflect the small number of occupants in each of these groups. While NCAP side-impact crash test rating was an important predictor of major thoracoabdominal injury at medium and high ΔV, age and seatbelt use were significant predictors only at low and medium ΔV. This suggests that structural factors, which are reflected in the NCAP side-impact crash test rating, are most important at high crash severity, whereas occupant factors such as age and seatbelt use are most important at low crash severity.
Table 3.
Multivariate logistic regression models for predicting major thoracoabdominal injury at low, medium and high ΔV.
| OR (95% CI) | P Value | |
|---|---|---|
| Low ΔV (<25kmph) | ||
| Star Rating | ||
| 5 | Referent | |
| 4 | 1.2 (0.8 – 2.0) | 0.41 |
| 3 | 1.3 (0.8 – 2.0) | 0.33 |
| 2 | 0.9 (0.3 – 2.8) | 0.81 |
| 1 | 1.2 (0.5 – 3.1) | 0.70 |
| Age | 1.04 (1.03 – 1.05) | <0.001 |
| Seatbelt use | 0.3 (0.2 – 0.5) | <0.001 |
| Medium ΔV (≥25kmph; <40kmph) | ||
| Star Rating | ||
| 5 | Referent | |
| 4 | 1.8 (1.1 – 2.9) | 0.01 |
| 3 | 2.0 (1.3 – 3.1) | <0.01 |
| 2 | 1.8 (0.8 – 4.0) | 0.13 |
| 1 | 1.3 (0.6 – 2.7) | 0.47 |
| Age | 1.03 (1.02 – 1.04) | <0.001 |
| Seatbelt use | 0.45 (0.3 – 0.6) | <0.001 |
| High ΔV (≥40kmph) | ||
| Star Rating | ||
| 5 | Star Rating | |
| 4 | 2.0 (1.2 – 3.3) | 0.01 |
| 3 | 2.1 (1.3 – 3.4) | <0.01 |
| 2 | 2.3 (1.0 – 5.4) | 0.06 |
| 1 | 2.3 (1.1 – 4.9) | 0.03 |
| Age | 1.01 (1.00 – 1.02) | 0.15 |
| Seatbelt use | 0.73 (0.5 – 1.1) | 0.09 |
ΔV = Change in velocity (crash severity).
Treating crash star rating as a linear variable, and accounting for age and seatbelt use, increasing crash star rating was associated with lower likelihood of major thoracoabdominal injury at high (OR 0.8, 95% CI 0.7 – 0.9, p<0.01) and medium (OR 0.9, 95% CI 0.8 – 1.0, p<0.05) ΔV, but not at low ΔV (OR 0.95, 95% CI 0.8 – 1.2, p=0.55). In this model, age was a significant covariate at low (OR 1.04, 95% CI 1.03 – 1.05, p<0.001) and medium (OR 1.03, 95% CI 1.02 – 1.03, p<0.001) ΔV, but not high ΔV (OR 1.01, 95% CI 1.00 – 1.02, p = 0.16). Seatbelt use was a significant covariate at low (0.29, 95% CI 0.19 – 0.45, p<0.001) and medium (OR 0.46, 95% CI 0.33 – 0.63, p<0.001) ΔV, but not at high ΔV (OR 0.73, 95% CI 0.51–1.05, p=0.09).
DISCUSSION
We compared crash test results with real-world injury data to determine if, in fact, the NCAP side-impact crash test is predictive of major thoracoabdominal injury. We found that, at medium and high ΔV, occupants of 3-star and 4-star vehicles are at more likely to suffer major thoracoabdominal injuries than occupants of 5-star vehicles. At low ΔV, NCAP side-impact crash test rating was not predictive of major thoracoabdominal injury.
Occupant age and seatbelt use were important predictors of major thoracoabdominal injury in low and medium severity crashes, but not in high severity crashes. We hypothesize that at high crash severity, the predominant mechanism of injury is the impact itself, and the factors mitigating force transmission during impact are the predominant determinants of injury. This is the most analogous to the NCAP side-impact crash test, and may explain the strong relationship between crash test ratings and major thoracoabdominal injury at higher crash severities. At low crash severity, injuries are more likely to be caused by secondary impacts, and may be mitigated by occupant factors such as belt use14 and age15,16.
Though data on real-world crashes are limited17, thoracic and upper abdominal injuries are known to be a major cause of morbidity and death after MVC18,19. The NCAP side-impact crash test, which was performed on all new or redesigned vehicles from model year 1998 through 2009, was intended to predict the likelihood of major thoracoabdominal injury when the occupant is subjected to a side-impact crash. Many consumers purchase vehicles with the expectation that a crash star rating accurately reflects the safety of the vehicle they are purchasing. Our results suggest that this expectation is reasonable, and a consumer can expect that a vehicle with a higher side-impact crash test rating does offer improved protection of the thorax and upper abdomen following side-impact crashes, particularly at medium and high crash severity. Additionally, our analysis suggests that the NCAP side-impact crash test rating may be combined with vehicle telemetry data to identify a high-risk auto crash, which is an important component of recent Centers for Disease Control (CDC) guidelines on field triage for injured patients20.
This study was limited by a number of factors. In an attempt to confine our analysis to occupants involved in side-impact MVCs, we deviated from the NASS sampling scheme, which was designed to be representative of police-reported crashes in the U.S. with a harmful event, and does not attempt to represent side-impact crashes specifically. Therefore, we cannot conclusively state that our findings are representative of side-impact crashes in the United States.
The United States NCAP side-impact crash test was performed on the left side of the vehicle, and dummies are positioned exclusively on this side of the vehicle. In order to extrapolate to passengers on the right side of the vehicle, we assumed left to right symmetry of crash test results. However, other studies have demonstrated distinct injury patterns for occupants on the right and left side of the vehicle in near-side impacts17. This is related to asymmetry of the vehicle as well as the abdomen (the liver, which is on the right side of the body, is larger than the spleen, which is on the left). Another reason for unique injury pattern among right-sided occupants is the potential for loading by an unbelted driver moving toward the passenger during a right-sided impact. Because right front-passengers are present only 20% of the time, loading of the driver by the unbelted right-front passenger is much less likely21.
A frequent criticism of the United States NCAP side-impact crash test is the use of a barrier that resembles the height and weight of a passenger car, as opposed to a light truck. Though we did not extensively explore the effect of the striking vehicle on our results, other studies have demonstrated an increased risk of severe injury when struck by a light truck as opposed to a passenger vehicle22 and future studies should assess whether the crash-test ratings apply in such cases of vehicle height mismatch.
In 2011, the NHTSA instituted major changes to the side-impact crash test. Among other changes, the test utilizes new dummies meant to represent a typical adult male as well as a small adult female, and relies on data from the head, pelvis and chest for calculating the crash test rating. These are all welcome changes to the side-impact crash test, and future studies should assess the efficacy of the new ratings system in predicting major injury and death.
Acknowledgments
Dr. Voelzke received Grant Number KL2 TR000421 from the National Center for Research Resources (NCRR), a component of the National Institutes of Health (NIH). Its contents are solely the responsibility of the authors and do not necessarily represent the official view of NCRR or NIH.
Footnotes
Conflict of Interest Statement:
None of the authors have conflicts of interest to report.
Select data from this study were presented at the American Urological Association Annual Meeting in 2012 (Atlanta, Georgia).
Conflict of Interest Disclosures: None of the authors have conflicts of interest to report.
Contributor Information
Bradley Figler, University of Washington Department of Urology, Harborview Medical Center, Harborview Injury Prevention and Research Center.
Christopher D. Mack, Group Health Research Institute.
Thomas G. Smith, III, Baylor College of Medicine, Scott Department of Urology, Ben Taub General Hospital.
Hunter Wessells, University of Washington Department of Urology, Harborview Medical Center, Harborview Injury Prevention and Research Center.
Robert Kaufman, University of Washington Department of Surgery Harborview Injury Prevention and Research Center.
Eileen Bulger, Universityof Washington Department of Surgery, Harborview Medical Center, Harborview Injury Prevention and Research Center.
Bryan Voelzke, University of Washington Department of Urology, Harborview Medical Center, Harborview Injury Prevention and Research Center.
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