A radiographic and physical analysis of factors affecting seat belt position in sitting car seat

It has been reported that the risk of seat belt injury is higher in cases where the SBS is located higher-up 8. We analyzed differences in personal body shape affect the position of the SBS and investigated the relationship between body shape and the initial lap belt position when sitting in a car seat.

The prototype of the three-point seatbelt was first installed in production vehicles in 1959, and its basic structure has remained unchanged for over 60 years. It is specified as standard equipment in the seat belt laws of many countries, and most of the major risks associated with traffic accidents are lower for seat belt users. 14. However, as seat belts have become more widely used, there have been more reports of belt-induced injuries. Garrett and Braunstein were the first to report various organ injuries associated with seat belts, and called it seat belt syndrome. 6. Chance fracture, a flexion distraction fracture of the thoracolumbar spine, is famous for its high complication rate in traffic accidents due to the strong flexion force at the lap belt, and is now also called “seat belt” fracture 15,16. When treating patients with seat belt syndrome, it is important to be aware of complications such as intestinal and mesenteric injuries in the intra-abdominal organs 4 and arterial injuries in the retroperitoneal organs 5. Doersch and Dozier described a characteristic compression scar as a sign to suspect seat belt syndrome, and called it the “SBS” 7. SBS refers to subcutaneous hemorrhagic spots consistent with the belt site and is associated with intra-abdominal organ damage 17although there may be no associated injury even when SBS is present 18. Johnson and Eastridge reported that if the SBS is superior to the ASIS, there is an increased risk of intra-abdominal injury and abdominal surgery, and if the SBS is inferior to the ASIS, there is an increased risk of pelvic fracture, which is an indication and predictor of abdominal surgery 19. Assuming that the same energy is applied, the abdominal organs are more likely to be damaged than the pelvis, and therefore the presence of SBS above ASIS is considered to be a risk for seat belt syndrome. However, the mechanism by which bleeding spots caused by seat belts, which should be on the ASIS, occur above the ASIS is still unclear, and therefore no preventive measures have been established.

We assume that the position of the lap belt, related to seatbelt injury at the time of the crash, can be divided into a static factor, the initial lap belt position to sit on car seat, and a dynamic factor, for example upward shift of seatbelt and rotation of pelvis. And SBS can be determined by adding these two factors. This study identified physical characteristics that affect the initial lap belt position. There were few reports that analyzed physical characteristics were related to injuries in car accidents.

First, it has been reported that higher BMI is associated with higher risk of damage and mortality in occupants 20,21,22. Zarzaur et al. hypothesized that this was due to inappropriate use of seat belts and investigated the effects of seat belts and obesity on the presence of intra-abdominal injuries 23. They found that intra-abdominal organ injury and mortality were not associated with seat belt use in obese occupants, but the risk of abdominal injury with AIS 1 or higher was. With regard to mortality, Elkbuli et al. also reported that seat belt use reduces mortality regardless of BMI 24, so there is no disagreement that seat belt use is desirable even in obese occupants. However, these reports focused only on obesity and did not take into account age, sex, and height as in the present study.

About SBS location, Hartka et al. measured the position of the SBS in actual car accident patients on CT, and reported that the X-value was greater in obese people while the Z-value did not change much 25. In contrast, at the laboratory level, Reed et al. measured the lap belt position, and reported that a 10 kg/cm2 An increase in BMI resulted in a 43 mm anterior and 21 mm superior position 26. They also mentioned that the distance from the pelvis, and thus the possibility that the belt will not properly rest on the pelvis, will increase.

In this study, weight, BMI, and WC all influenced the X-value, Z-value, and lap belt angle. These variables are all factors associated with obesity. BMI was found to be most strongly correlated with X-value and Z-value, and the higher the BMI, the higher the Z-value. In males, BMI was most strongly correlated with lap belt angle, and the higher the BMI, the smaller the lap-belt angle. A small lap belt angle may make it easier for the pelvis to submarine, increasing the upward shift in an accident. Obese people may therefore have a higher risk of seatbelt injury due to higher SBS caused by higher Z-value and larger upward shift (Fig. 6).

Figure 6

The relationship between the pelvis and seat belt position of a lower BMI occupant (a) and the pelvis and seat belt position of a higher BMI occupant (b) are shown. In comparison, the X and Z values ​​are larger, and the lap belt angle is smaller for the higher BMI occupant (c). This may facilitate the submarine phenomenon where the pelvis goes under the lap belt.

The reason for the difference between our results and Hartka et al. 25 may be the difference in posture when measuring the X-value and Z-value. Their CT study would have been measured in the supine position, but our study used a coordinate system in the sitting position. It is known that the total spinal alignment in the seat sitting position is significantly different from that in the supine position, especially the posterior tilt of the pelvis 27,28,29. Therefore, there may be differences in the X- and Z-values ​​calculated from CT taken in the supine position.

Second, height had no effect on the initial lap belt position in most cases. Only the X- and Z-values ​​in females were significantly correlated with height. There are no reports that mention height and SBS, nor are there any reports that height is related to spinopelvic alignment in the sitting position. Compared to the results for weight and BMI mentioned above, the relevance is extremely low, so we consider height to be negligible. However, since women were on average about 12 cm shorter than men in this study, it is possible that height and initial position may be correlated only in the height range below a certain level.

Third, LL and SS were not correlated with X-value, Z-value, or lap belt angle. There have been no reports mentioning the relationship between spinopelvic alignment and SBS. Buckland et al. reported that the pelvis tends to tilt posteriorly in obese individuals 30. Nishida et al. investigated changes in spinal alignment in the sitting position while seated in a car seat, and reported that there were differences depending on body shape even in the same seat 31. In this study, there were also no significant correlations between weight or BMI and LL or SS. Therefore, lumbar pelvic alignment is not considered to be involved in the change of initial lap belt position in obese subjects. However, lumbopelvic alignment is related to sitting posture, and sitting in an inappropriate posture may be associated with seat belt injury. Richardson et al. conducted crash tests using a cadaver on a car seat that was reclined 50 degrees, and reported several damage patterns 32. Further research is needed on the relationship between sitting posture and traffic trauma including seat belt injury.


The first limitation was that the X-ray images were taken immediately after sitting. The sitting posture may change over time due to differences in muscle fatigue or comfort. Therefore, no matter when an image was taken, it could not be considered a permanent alignment.

Secondly, the radiography was performed at a single, predetermined seat angle. A different angle or height of the seat or height of the steering wheel may have led to different results. Since the optimal seat position differs from person to person, it would be better if the subject could choose the seat position, etc., to get closer to real-world results. However, since complex instruments can interfere with radiography, a single seat position was used in this study. In addition, to minimize exposure, only one X-ray image was taken.

Third, this study included subjects between 150 and 187 cm in height, but different results may have been obtained if subjects of lesser height (for example, children) were included. In other words, the X- and Z-values ​​may have increased in subjects shorter than 150 cm.

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