Introduction
Blunt force chest injuries with associated rib fractures are frequent in motor vehicle crashes. Injuries to the thorax can occur from impacts with the steering wheel or instrument panel in frontal crashes, from side doors in side impact crashes, or from a combination of these. Fractured ribs may lead to injury of thoracic organs, particularly with the sharp ends of the ribs having the potential to tear/puncture vessels and the lungs, or the pleura leading to pneumothorax or hemothorax [
1,
2].
Cadaveric studies and finite element models are often used to simulate and predict rib fracture patterns in injury research, and studies have used clinical data to identify patterns of rib fracture occurring with multiple causes of trauma including motor vehicle crashes [
3‐
8].
The following study evaluates the patterns of rib fracture in adult occupants involved in fatal motor vehicle crashes and combines coronial autopsy report injury descriptions with crash data information. Seat belt wearing is mandatory for all motor vehicle occupants in Australia. The aims of this study are to determine if any specific occupant characteristics, crash factors, or associated injuries identified at autopsy can predict the occurrence or number of ribs fractured.
Discussion
Increasing age was the strongest predictor of rib fractures as has been noted in previous research [
6,
14,
15]. However, the current study demonstrates that the effect of age is stronger than any other occupant or crash characteristic. This effect is clearly shown in Fig.
3 where the proportion of ribs fractured for occupants 65 years and older is far greater than for younger vehicle occupants.
Impact to the thorax results in “posterior displacement of the sternum relative to the spine”, referred to as chest deflection [
16]. The number of rib fractures is associated with the magnitude of chest deflection rather than the impact force, and the threshold of injury during chest deflection is dependent on age [
16,
17]. Increasing age results in reduced chest deflection tolerance and therefore greater injury at lower magnitudes of deflection [
16]. Age-related demineralisation and degradation of cortical bone result in bone remodelling, which causes rib deterioration, while cartilaginous sections of rib undergo calcification and hardening, which reduces rib flexibility. Thus, the ability of the aging ribcage to deflect thoracic impact is reduced and this explains why aged ribs fracture more readily [
18].
Increased risk of fatality is associated with increased number of ribs fractured for older occupants. In a study of hospitalised trauma patients, a majority of whom were motor vehicle occupants, Bulger et al. found that, for every additional rib fracture in elderly patients (> 65 years of age), the risk of pneumonia increased by 29% and mortality by 19% [
19]. Additionally, rib fractures are frequently associated with other thoracic injuries such as aortic damage and pulmonary contusions. The combined effects of rib fractures and underlying injuries are particularly detrimental to the already reduced physiological reserve and decreased ventilatory capacity of older people. Stiffening of the chest wall, reduced cardiac output, reduced lung capacity, poor blood oxygen exchange, and pain-related limitation of mechanical ventilation in older persons with rib fractures also increase associated mortality [
14].
Importantly, restraint use was not predictive of rib fracture or of a greater number of ribs fractured in either model. While seat belts load the thorax during crash impacts, the likelihood of rib fracture is not greater with restraint use compared to no restraint use. Fatalities of seat belt wearers, however, often involve significant impacts that would likely be fatal irrespective of belt wearing [
20,
21].
Liver injury was the second strongest predictor of one or more rib fractures. Lung contusion and hemothorax also appeared as significant factors in the model. Similarly, when predicting the number of ribs fractured, fractures of the right leg, splenic injury, and pelvic fracture all appeared more significant than other typical blunt injuries (e.g., aortic injury). The association with non-chest injuries most likely suggests an overall higher level of injury severity. Thoracic spinal fracture is an expected predictor of the number of ribs fractured as the thoracic spine connects to all ribs and, consequently, fracture of one of thoracic vertebrae may produce one or more rib fractures. Figure
6 demonstrates that the posterior anatomical location has a higher proportion of rib fractures for those with a thoracic spine fracture compared to no thoracic spine fracture.
Although linear regression was able to predict the number of ribs fractured, the model had a low predictive power suggesting that there are other factors affecting the number of rib fractures that are not included in this dataset such as speed of impact, vehicle intrusion and point of impact. It may also be the case that the relationship between ribs fractured and injury severity is cumulative whereby the aggregated number of ribs fractured creates distinct levels of injury severity as opposed to the incremental increase in injury severity for every additional rib fractured [
22].
While position in a vehicle and BMI were not significant predictors in any of the models, both are well-established modifiers of injury severity and patterns of injury [
23‐
27]. The extremes of BMI (Fig.
4, panels a and f) both have a higher incidence of anterior rib fractures, particularly for rib numbers two to seven. Notably, there were fewer rib fractures in rear-seated passengers (Fig.
5).
There are several limitations to this study, namely the absence of crash data for variables associated with injury severity such as speed of impact, vehicle intrusion, and point of crash impact [
28‐
30]. Other safety devices, such as airbags, are not routinely reported in TARS or coronial autopsy reports and were therefore omitted from the study. Airbags protect the head and thorax during impact, and therefore the extent to which this protective effect is contributing or preventing rib fractures or other injuries remains unclear. Additionally, rib fractures may have been missed during gross examination, predominantly in the posterior anatomical location. Although cases with rib fracture patterns attributable to were removed from the study cohort, it is possible that some of the rib fractures documented at autopsy, particularly those in the anterior anatomical location, were caused by or worsened during resuscitation attempts.
This study demonstrates the effect of increasing age on rib fractures in vehicle crashes, as well as revealing an association with liver injury, pulmonary contusion, and hemothorax. Right leg fractures, splenic injury, and pelvic fractures may reflect the severity of impact.
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