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The global burden of traffic injury has been reduced by innovations in vehicle safety design, but not all demographics have benefitted equally from this protection. Women (we will use this term to discuss our biological sex related study, with the understanding of and respect for the range of gender expression that does not correlate with biological sex) may be more vulnerable to risk of certain types of injury in vehicle crashes, yet safety features are largely based on testing with male-representative dummies. Atwood et al. (1) demonstrated greater relative fatality risk (on average, 2.9% higher fatality risk for female front row occupants vs. male) for females in vehicles with the newest generation of safety equipment, but the differences between male and female occupants' fatality risk fluctuate across age groups. Stigson et al. (2) report a greater risk of permanent medical impairment (PMI) in females compared to males, and countermeasures designed to mitigate this risk (specific to “neck” region injury, which is the region most associated with PMI) were not equally effective in men and women (3). Nutbeam et al. (4) found that female patients were more frequently entrapped after crash, and that entrapped male and female patients had differing injury patterns. As such, we cannot be certain that current vehicle safety standards in testing, equipment, and crashworthiness accurately reflect how women experience vehicle crashes; the current body of literature suggests that current methods in crash testing may not sufficiently account for male/female body differences.

Much of the literature evaluating sex differences in crash injury discusses relative risk and crash/occupants characteristics with significant effort toward comparing crashworthiness, crash severity, and adjusting for confounding factors that can affect estimated impact of sex on crash injury. In a recent IRCOBI conference, Brumbelow (5) asserts that “It is important to identify how non-physiological risk factors may affect injury risk estimates for females and males in order to encourage the most robust and effective countermeasures.” Brumbelow (5) also considers that investigating how differences in vehicles and crashes between men and women may reveal how these factors confound estimates for relative and fatality risk.

Acknowledging the difficulty in fully accounting for confounding factors is important in an accurate assessment of how men and women are injured in vehicle crashes. Clear representation of the problem of sex-related injury disparity is critical in prioritizing research, design, and allocating funding. The level of complexity of the issue, however, makes this clear representation challenging; true matched pairs comparisons are nearly impossible to achieve. We posit that by evaluating sex-related injury patterns using clinical data, we will demonstrate where and how male/female differences exist as real patient outcomes, regardless of how previous literature has estimated and quantified sex-related risk.

Atwood et al.'s (1) recent evaluation of the Fatality Analysis Reporting System (FARS) found that recent model year vehicles (2010–2020) with optimal occupant protection systems have reduced estimated female fatality risk relative to males to 5.8%; though this is an improvement, it still indicates disparity between sexes. Liu and Subramian (6) estimate the odds of a female occupant's severe injury likelihood as 1.25 times that of a male occupant. Males are more likely to engage in risky behaviors like speeding, driving while intoxicated, etc., increasing their overall likelihood of crash, death, and serious injury but even controlling for these factors, women are significantly more likely to suffer serious injuries due to vehicle crashes (7). Though some studies attribute differences in injury and fatality risk between sexes to driving patterns, behavior, and vehicle size, attempts to control for these factors in describing relative risk have not included physical stature, body mass, or other physiologic differences associated with sex (7).

A recent study by Brumbelow and Jermakian (7) discusses differences in injury severity between side and front crashes as well as differences in extremity injuries, and concludes that current vehicle safety testing has reduced injury risk to both sexes, perhaps more to female occupants. However, despite careful controlling for as many crash severity factors as possible, they posit that there are multiple sex-related properties as yet unknown, unmeasured, or unaccounted for within retrospective crash data analyses which may or may not be able to identify female vulnerability to (lower extremity) injury (7). Craig et al. (8) combined multiple crash-related databases to account for a broad range of crash types, crash variables, and occupant characteristics in an analysis of sex-based odds differences in crash outcomes. This study demonstrates the complexity of the issue, and concluded that:

Further, they found that in multiple models, female and male occupants were approximately equal in number of cases where each held the higher odds of injury (8). However, limitations of the study did not allow for some elements of analysis that may be relevant in comparisons of sex-related differences in crash injury outcomes—delta V, post-crash factors, or occupant characteristics like BMI, behavior, or vehicle selection (8).

To ultimately make cars that are safe for all bodies, we must isolate the risk factors which are truly due to male/female physiological differences and evaluate which elements need to be represented in crash testing. Though vehicle safety has improved overall in the past decades, this improved protection may not apply to all occupants equally. Abrams and Bass (9) posit that “there may be unobserved trends in the injury patterns, and therefore outcomes, between male and female occupants.” The objective of this study is to evaluate the disparities in abdominal and pelvic male/female injury patterns through a clinical lens; this novel approach evaluating the NTDB allows for analysis of injury patterns in trauma patients after vehicle crash. By reviewing patient injury data, this study demonstrates how injuries correlate with sex, how male and female patients are affected by these injuries, and how injury patterns demonstrate the clinical picture of known sex-related disparities in crash injury.

In the 56,839 patients meeting study criteria, women and men were represented fairly evenly in overall proportion of crashes. For all patients, age group 16–25 was 23.2% of the total and the next highest group was 26–34 at 16.9%. These two groups combined were 40.1%, with age groups 35–44, 45–54, 55–64, and 65–74 represented nearly equally at 13.6, 12.4, 13.7, and 11.7%, respectively. There was a sharp drop off in crash numbers from ages 75 and above, with those groups comprising only 9.1% of the total number of crash patients. Within age groups, men were more highly represented from ages 16–44, but this equalized from ages 45–54 and then reversed, with women as the greater number of crash victims above age 55 (increasing proportions with each jump in age group). Injuries (by number or total counts) were similarly evenly distributed across male/female patients, with some exceptions: women had greater number of liver injuries in all AIS categories 2–5, greater number of bony pelvis injuries in all AIS categories 2–5, greater number of pelvis organ injuries in AIS category 5, and greater number of stomach injuries in AIS categories 3 and 4.

A critical piece of information uncovered by this group is the presence of elevated shock index in female crash victims at rates greater than in male crash victims. Women crossed the shock threshold (SI > 1.0) at a lower ISS score in all injury regions. Women also crossed the shock threshold at fewer number of total injuries than men. However, the difference in SI converged as number of total injuries increased. This difference was most pronounced in patients under the age of 30 and in patients with pelvis injury.

Shock index is a useful tool in the rapid assessment of a trauma patient (14). A quick look at HR and SBP and simple calculation (e.g., is the ratio HR:SBP > 1?) can orient the clinician to the hemodynamic status of the patient, even in an austere environment with minimal equipment. Since hemorrhagic shock is a leading cause of death during initial trauma intervention, early recognition of shock is key to timely treatment (14). Because patients can appear “normal” despite significant hemorrhagic loss due to physiologic compensation, an SI > 1.0 can provide an early alert for those patients likely to need mass transfusion, ICU admission, or other interventions to prevent morbidity and mortality (14).

The normal ratio of heart rate to systolic blood pressure ranges between 0.5 and 0.7, with some sources accepting up to < 0.9 as within the range of normal. SI is a better predictor of shock than HR and SBP separately, and since not all blood loss is visible it is critical to identify hemorrhagic shock quickly (15). With hypovolemia (less than normal amount of blood and fluid in the body's circulatory system) caused by blood loss due to injury, the initial physiological response in trauma patients is increased heart rate, which compensates for the reduction in stroke volume (how much blood the heart pumps out to the body with each heartbeat). Heart rate (beats per minute) multiplied by stroke volume (mL) equals cardiac output, which is expressed in liters per minute. This cardiac output is what supplies the body's tissues with oxygen, and perfusion of these tissues with oxygen is the normal state of a healthy patient (16).

As a traumatically injured patient's cardiac output declines due to blood loss, the body's tissues receive less oxygen than needed. This mismatch leads to multiple other compensatory mechanisms (peripheral vasoconstriction, anaerobic respiration, diversion of blood from non-critical organs to heart and brain) (16). A patient in early or class I shock (blood loss up to 15% of circulating blood volume) may demonstrate very few clinical signs that they are in trouble; elevation in HR is usually the first clue (17). As the HR rises, the SI will increase; this will continue and become more pronounced as systolic blood pressure begins to drop (which first happens consistently at 30% or greater blood volume loss) (17). It is dangerous to wait until a patient falls into a precise category of shock before taking action (hemorrhage control, transfusion, operative intervention); hemorrhagic shock is a clinical emergency which requires immediate treatment as soon as it is detected (17).

Higher SI was associated with worse injuries and with youth—the ability to compensate physiologically by increasing heart rate is stronger in younger patients. In addition, many patients above the age of 65 use medications inhibiting their ability to increase HR regardless of need (i.e., beta blockers), a variable not available in this database. Of note, even with lower injury severity scores, females had significantly higher shock indices regardless of injury region, except for stomach. This indicates either an inaccuracy in widely held standards in vital signs due to inattention to sex differences or a greater physiologic response to trauma in females, or another factor as yet unknown.

If female crash injury victims are entering shock states with lesser injury severity than male crash injury victims, this may have serious clinical implications. It suggests that female patients are somehow more susceptible to compromised hemodynamics and elevated SI after injury, which indicates a higher likelihood of transfusion, ICU stay, and mortality (14). In trauma care, clinicians consider the mechanism of injury and injury severity as context for expected patient hemodynamic status; if sex is not considered in this calculus, clinicians could be delaying identification of shock in female patients. No current trauma assessment uses sex as a data element or directive, except in injuries of pregnant people (which is focused on pregnant physiology and complications, not sex specifically). It is possible that clinical practice may need to adapt to sex-related hemodynamic differences, which will become clearer with further research.

The implications of evidence showing greater physiologic distress in women with equivalent or lesser injuries from vehicle crash than men are potentially meaningful across a number of domains. From a clinician standpoint, increasing use of shock index as a tool to assess impending worsening of clinical status may result in earlier identification of those individuals in need of intervention or may signal a need to EMS personnel for greater haste, higher level of care, or preparation of a trauma center for their arrival (16). Evaluating crash injury with the knowledge that a female patient may have higher likelihood of shock could allow trauma clinicians to risk stratify and recognize and treat shock more aggressively, potentially resulting in fewer complications. As we develop our understanding of the relationship between sex and shock index, it is possible that clinicians caring for patients in the ICU or inpatient unit may need to consider the impact of sex on hemodynamics in their treatment plans.

From a vehicle safety and design standpoint, the implication of either lesser physiologic reserve or lower resilience in female occupants, greater vulnerability to injury, and increased likelihood of increasing shock index in certain injuries/injury patterns may require a re-evaluation of current practice in how safety and design are conceived and developed. Aside from the clear imperative to move forward in average-sized female dummy evolution and use, current standards in vehicle safety must include review of the impact of current equipment on those injuries for which females have increased risk of shock. Transparent, equity-focused research and design will require a commitment to eliminating disparities in crash injury from both manufacturing and legislation, areas which have, until recently, allowed these disparities to remain unaddressed.

With greater numbers of pelvis (bony, AIS 2–5), pelvis (organ, AIS 5), liver (AIS 2–5), and stomach injuries (AIS 3–4) in women, it is possible that some element of anatomical difference between male and female bodies is interacting with safety equipment in a way that increases these injuries. Since current vehicle safety equipment is designed for a standard male figure, the question of whether a female occupant may somehow be under-protected, either due to equipment fit (i.e., seat belt positioning) or because female occupants make out-of-standard adjustments to accommodate their size, proportions, or weight distribution (i.e., distance to steering wheel) needs further investigation. Female drivers are often considered “out-of-position,” but this designation of women as non-standard is the primary root of multiple inequities in research and design of daily-use, safety-related, or health-affecting equipment. The absence of female bodies as their own standard clearly has consequences, which in the case of vehicle crashes, can be serious and life-altering.

By elucidating the differences between male/female injury patterns and connecting them to male-preferential safety equipment, we may clear a path for research to pursue any number of vehicle occupant variations in injury and vehicle design; whether this will require improved dummy technology, computer modeling, or a combination of the two remains to be seen. Findings from the clinical approach described in this study can be used to address sex-based discrepancies in a critical area of injury-related public health, and can be used to prioritize future directions for sex-related crash injury research.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

The studies involving humans were approved by IRB Medical College of Wisconsin. The studies were conducted in accordance with the local legislation and institutional requirements. Written informed consent for participation was not required from the participants or the participants' legal guardians/next of kin in accordance with the national legislation and institutional requirements.

SC: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Writing—original draft, Resources, Supervision, Writing—review & editing. KS: Formal analysis, Validation, Writing—review & editing. KD: Data curation, Formal analysis, Methodology, Writing—review & editing. CT: Data curation, Formal analysis, Methodology, Validation, Writing—original draft, Writing—review & editing. FP: Conceptualization, Formal analysis, Methodology, Supervision, Validation, Writing—review & editing.