Rib and sternum fracture risks for restrained occupants in frontal car crashes

OBJECTIVE

Most car occupant fatalities occur in frontal crashes and the thorax is the most frequently injured body region. The objectives of the study were, firstly, to quantify the relation between risk factors (such as speed and occupant age) and rib and sternum fracture injury probability in frontal car crashes, and, secondly, to evaluate whether rib fracture occurrence can predict sternum fractures.

METHODS

Weighted German data from 1999-2021 were used to create the injury risk curves to predict both, at least moderate and at least serious, rib and sternum fracture risks. A contingency table for rib and sternum fractures allowed the calculation of sensitivity, specificity, and precision, as well as testing for the association.

RESULTS

Elderly occupants (≥65 years old) had increased rib and sternum fracture risk compared to mid aged occupants (18-64 years old). Besides occupant age, delta-V was always and sex sometimes a significant predictor for skeletal thoracic injury. Sternum fractures were more common than rib fractures and more likely to occur at any given delta-V. Sternum fractures occurred often in isolation. Female occupants were at higher risk than males to sustain at least moderate rib and sternum fractures together and sternum fractures in isolation. Rib and sternum fractures were associated, but low sensitivity and precision show that rib fractures do not predict sternum fractures well.

CONCLUSIONS

Elderly and female occupants were at the highest risk and should be targeted by thoracic injury criteria and thresholds for frontal crash occupant protection. Rib and sternum fractures were not associated. Therefore, sternum fractures need to be predicted and evaluated separately from rib fractures.

Comparison of the effectiveness of rotating seats in autonomous vehicles and airbags in traditional vehicles to minimize head injuries in frontal crash

We analyzed and compared the effectiveness of the rotating seat of autonomous vehicles and that of an airbag in traditional vehicles as head protection measures in a frontal crash. Driver frontal crash models of traditional and autonomous vehicles and a head finite element model were established. Four evaluation indexes were used for comparison. The airbag proved more effective than seat rotation in terms of injury criteria and brain tissue injury risk under frontal crash conditions with and without brake involvement. A segmented protection measure based on crash acceleration values is proposed to improve crash safety in future autonomous vehicles.

Adapting load limiter deployment for frontal crash diversity

Objective: Current European restraint systems may not realize their full protection potential in real-world frontal crashes because they are highly optimized for specific conditions. This research sought to quantify the potential benefit of adapting seat belt load limit thresholds to a wider range of occupant and crash characteristics.Methods: Numerical simulations using Hybrid III dummies were conducted to determine how varying load limiter thresholds could affect occupant kinematics and injury outcome in frontal impacts. Occupant-compartment models were developed with a restraint system consisting of a frontal airbag and a 3-point belt with retractor, buckle pretensioner, and load limiting at the shoulder. Load limiting threshold was varied in 5 frontal impact scenarios, covering as wide a range of real frontal crash conditions as possible. The simulated thoracic injury risks were converted into injury probability values using Abbreviated Injury Scale (AIS) 2+ age-dependent thoracic risk curves. These values were then applied to a British real-world frontal impact sample to determine the injury reduction potential of optimized load limiting, taking into account occupant seating position, impact scenario, occupant size, and occupant age and assuming that an appropriate adaptive system was fitted to all cars.Results: In low-severity impacts, a low load limit provided the best chest protection, without increasing risk to other body regions, for both the 50th and 95th percentile dummies in both front seating positions. In high-severity impacts, the low limit was not recommended because it allowed the driver dummy to move into close proximity with the vehicle interior, although there appeared to be some benefit of lower load limiting for the 50th percentile front passenger dummy, due to the increased ride down space in that seating position. Adapting the load limit showed no injury reduction potential for 5th percentile drivers. Utilizing the best load limit threshold in real-world crashes could reduce the number of occupants with AIS 2+ chest injuries from belt loading from 377 to 251 (a 33% reduction), correspondingly reducing the number of occupants with AIS 2+ chest injuries (from all sources) in the whole frontal impact population from 496 to 370. This is a reduction in injury rate from 6.4% to 4.8%.Conclusions: The concept of an adaptive load limiter shows most promise in low-speed frontal crashes where it could lower the AIS 2+ chest injury risk for most front seat occupants, except the smallest of drivers. Generally, adaptive limiters show less potential effectiveness with increased crash severities. Overall, an intelligent adjustment of load limiting threshold could result in a reduction of at least a third of front seat occupants with AIS 2+ chest injuries associated with restraining loads and an overall reduction in AIS 2+ chest injury rate in frontal crashes from 6.4% to 4.8.

Submarining sensitivity across varied anthropometry in an autonomous driving system environment

Objective: Self-driving technology will bring novelty in occupant seating choices and vehicle interior design. Thus, vehicle safety systems may be challenged to protect occupants over a wider range of potential postures and seating choices. This study aims to investigate the effects of occupant size, seat recline, and knee bolster position on submarining risk and injury prediction metrics for reclined occupants in frontal crashes.Methods: Frontal crash finite element (FE) simulations were performed with the 3 simplified Global Human Body Model Consortium (GHBMC) occupant models: small female, midsize male, and large male. Additionally, a detailed GHBMC midsize male model was used to compare with selected simplified cases. For each simulation, parameters including seatback recline angle (0.9°, 10.9°, 20.9°, 30.9°) and knee bolster position relative to the occupant (baseline, close, far, and no knee bolster) were varied. Impacts were simulated with the U.S. New Car Assessment Program 56 km/h frontal crash pulse. Occupant kinematics data were extracted from each simulation in a full-factorial sensitivity study to investigate how changes in anthropometry, seating position, and knee bolster position would affect submarining across all simulated cases.Results: Overall, increasing the occupant-to-knee bolster distance resulted in more submarining cases. The threshold for submarining was also affected by the seat recline angle. The lowest threshold observed occurred with 10.9° of recline with the small female model. Submarining was observed at recline angles at and above 20.9° for the midsize male model and 30° for the large male model. The initial lap belt position, pelvis orientation, and their relationship were good predictors of submarining. Increased lumbar flexion moment was observed with increased seat recline angle as well as occupant-to-knee bolster distance. The detailed GHBMC model was more prone to submarining than the simplified model.Conclusions: Submarining may be a major challenge to overcome for reclined occupants, which may become more prevalent with autonomous driving systems. This study shows that the angle of recline, anthropometric variation, and position of the knee bolster affect the risk of submarining. To our knowledge, this is the first study to computationally evaluate the occupant protection implications of seatback recline for multiple body sizes, postures, and positions relative to the vehicle interior.

Small female rib cage fracture in frontal sled tests

OBJECTIVES

The 2 objectives of this study are to (1) examine the rib and sternal fractures sustained by small stature elderly females in simulated frontal crashes and (2) determine how the findings are characterized by prior knowledge and field data.

METHODS

A test series was conducted to evaluate the response of 5 elderly (average age 76 years) female postmortem human subjects (PMHS), similar in mass and size to a 5th percentile female, in 30 km/h frontal sled tests. The subjects were restrained on a rigid planar seat by bilateral rigid knee bolsters, pelvic blocks, and a custom force-limited 3-point shoulder and lap belt. Posttest subject injury assessment included identifying rib cage fractures by means of a radiologist read of a posttest computed tomography (CT) and an autopsy. The data from a motion capture camera system were processed to provide chest deflection, defined as the movement of the sternum relative to the spine at the level of T8.  A complementary field data investigation involved querying the NASS-CDS database over the years 1997-2012. The targeted cases involved belted front seat small female passenger vehicle occupants over 40 years old who were injured in 25 to 35 km/h delta-V frontal crashes (11 to 1 o'clock).

RESULTS

Peak upper shoulder belt tension averaged 1,970 N (SD = 140 N) in the sled tests. For all subjects, the peak x-axis deflection was recorded at the sternum with an average of -44.5 mm or 25% of chest depth. The thoracic injury severity based on the number and distribution of rib fractures yielded 4 subjects coded as Abbreviated Injury Scale (AIS) 3 (serious) and one as AIS 5 (critical). The NASS-CDS field data investigation of small females identified 205 occupants who met the search criteria. Rib fractures were reported for 2.7% of the female occupants.

CONCLUSIONS

The small elderly test subjects sustained a higher number of rib cage fractures than expected in what was intended to be a minimally injurious frontal crash test condition. Neither field studies nor prior laboratory frontal sled tests conducted with 50th percentile male PMHS predicted the injury severity observed. Although this was a limited study, the results justify further exploration of the risk of rib cage injury for small elderly female occupants.