A systematic review and meta-analysis of cohort studies revealed that the risk of any major injuries was significantly lower in seatbelt users than non-users with a relative risk of 0.47 . Additionally, although statistical significance was obtained in the analyses of injuries in each body region, the relative risk for abdominal injuries (0.87) was higher than that for head (0.49), neck (0.69), and lower limb injuries (0.77) . This fact is substantially due to seatbelt-induced abdominal injuries. A recent study based on nationwide hospital data suggested that the AIS score of the abdomen was the single significant influencing factor for fatalities of motor vehicle passengers . The authors also suggested that seatbelt compression might somewhat contribute to the occurrence of abdominal organ injuries. Accordingly, that study was the first to show the actual situation of moderate and severe abdominal injuries due to compression by seatbelts.
In the present study, spleen, kidney, and liver injuries were the three most common injuries induced by seatbelts. This finding is in accordance with previous reports showing that the liver, spleen, and digestive system are the most frequently injured among all abdominal organs in frontal collisions [7, 8]; additionally, research has shown that seatbelts are associated with renal injuries and high odds of liver injuries [9, 10]. Our study also revealed that the severity of all liver injuries was moderate and not severe. According to a large retrospective study, MVC-related liver injuries in patients who used a seatbelt with airbag deployment were the least likely to sustain severe injuries and/or death and had fewer complications, whereas patients with no protective device had the highest risk of these outcomes . Our analysis also confirmed that seatbelt-induced liver injuries were not severe.
In this study, we confirmed that force applied to the lower abdomen was a positive predictor and that airbag deployment was a negative predictor of seatbelt-induced severe injuries. Forces to the lower abdomen were due to compression by lap belts with an incorrect belt path. Even if passengers basically use seatbelts correctly, errors sometimes occur. A recent study of seatbelt position among rear-seat passengers showed that the lap belt was placed above the correct position (on the anterior superior iliac spine) in 40% of male passengers . Therefore, health-care professionals must advise patients on the proper use of seatbelts, with a lap belt fitting the ilium under the theory that inertial loads from the body interacting with the lap belt are distributed to a greater degree over the skeleton. Airbag deployment mitigates the seatbelt-induced forces by reducing forward movement. Although the frequency of mild injuries to the face or extremities has increased with the use of airbags, the forces to the abdomen have decreased by the distribution of blunt force energy away from the abdomen [21–24]. The purpose of a restraint system is to gradually decelerate the occupant over a longer period of time as well as distribute collision forces over a larger portion of the bony skeleton . Therefore, airbag deployment in conjunction with correct seatbelt use can protect against severe abdominal injuries.
Collision velocity was not a predictive factor for seatbelt-induced severe injuries in the present study. However, according to a previous study, the vehicle velocity at impact was directly proportional to the severity of intra-abdominal organ injuries . The study showed that a rise in the impact velocity from 40 to 50 km/h led to a 2.8% higher rate of abdominal trauma and that a rise in the impact velocity from 40 to 70–79 km/h led to a 24.6% higher rate of abdominal trauma . The difference between this result and present result may be due to the difference in the collision velocity; our data showed lower collision velocities (the mean EBV in the moderate and severe injury groups was 36.2 and 43.9 km/h, respectively). Additionally, the BMI was not a predictive factor for severe injuries. According to a previous study based on a huge collision database, obese occupants had a higher incidence rate of belt placement superior to the anterior superior iliac spine than occupants with a normal BMI; however, there was no significant difference between the occupants’ BMI and the incidence of abdominopelvic organ injuries . Therefore, our result is in accordance with this previous study.
Seatbelt use during severe MVCs may result in the transmission of significant forces to the abdomen and chest instead of hard collision with the steering or instrument panel. A seatbelt sign is a linear skin discoloration suggesting subcutaneous hemorrhage by seatbelt compression during the collision. The presence of a seatbelt sign on the abdomen implies a significant impact and transfer of kinetic energy to the abdominal wall. One study suggested that up to 10% of MVC passengers with a seatbelt sign had an underlying hollow viscus injury . Another study showed that among patients with a seatbelt sign, two-thirds had an underlying bowel injury . However, a recent retrospective cohort study revealed no association between a seatbelt sign and the occurrence of intra-abdominopelvic injuries . Therefore, in spite of the seatbelt sign, physicians should still suspect seatbelt-induced injuries especially in the lower abdomen of restrained passengers without deployment of an airbag.
This study had some limitations. First, the data used in this study were extracted from collisions that occurred from 1995 to 2011; more recent data were not obtained. Therefore, the safety systems of the vehicles in our study may have differed from those currently in use. However, the results show the basic trends and information concerning seatbelt-induced severe abdominal injuries. When similar studies using recently obtained collision data are performed in future, they may reveal changes in the prevalence and characteristics of seatbelt-induced abdominal injuries based on the present results. Second, we used a United States vehicle collision database in this study. Because of the worldwide variations in traffic situations and collision characteristics, such as passenger body size, vehicle size, and speed limits, the present findings may not be generalizable to all countries. Additional research using data from international sources is required. Third, because we chose vehicle passengers who had injuries with AIS scores of > 1, we did not identify mild injuries induced by seatbelts. However, because the objective of this study was to elucidate the characteristics and predictive factors of seatbelt-induced severe injuries, this issue might not have influenced the present results. Further research including all seatbelt-induced injuries is required to improve safety for vehicle passengers.