Seat Belt Injury

Etiology

The use of restraints in vehicles prevents severe head injuries and reduces the risk of ejection during collisions. Seat belts save lives by securing individuals in their seats, but they also introduce specific injury patterns associated with their use. The 3-point adult harness rests across the shoulder, chest, and abdomen, where it can transmit significant force depending on the mechanics of the MVC. Seat belt injuries occur when these forces are transmitted to the passenger, causing blunt trauma to underlying structures, most commonly affecting the neck, chest, and abdomen.

Epidemiology

According to 2015 data, approximately 35,000 deaths occur annually in the United States as a result of MVCs.[4] The estimated fatality rate from MVCs has decreased from 16 deaths per 100 million miles driven in 1930 to 1.8 deaths per 100 million miles in recent years.[5] NHTSA estimates that seat belts reduce the risk of fatal injury by approximately 43%.

According to the National Safety Council, motor vehicle deaths in the United States increased by 996% from 1913 to 2022 across all types of motor vehicles. However, the role of automobiles in everyday life has changed significantly since tracking began. In 1913, there were approximately 1.3 million vehicles and 2 million licensed drivers in the United States, with no available estimates for total miles driven. By 2022, these numbers had risen to 283.4 million vehicles, 235 million licensed drivers, and 3.2 trillion miles driven annually. The National Safety Council also notes that in 1923—the first year for which mileage estimates were available—the motor vehicle death rate was 18.65 deaths per 100 million miles driven. Since then, the mileage-based death rate has decreased by 93%, reaching 1.33 deaths per 100 million miles driven.

The literature provides varying estimates on the incidence of seat belt–related injuries. Visible seat belt marks on the neck and chest are generally less predictive of severe underlying injuries and are present in fewer than 5% of patients. In contrast, seat belt injuries to the abdomen and lumbar spine are more commonly observed in patients with external signs of trauma, although the data on this correlation remain inconsistent.

Pathophysiology

Injuries sustained in MVCs are primarily related to changes in velocity and the dissipation of kinetic energy. Factors such as high vehicle speed, rollover events, passenger ejection, and loss of consciousness, particularly when accompanied by a low Glasgow Coma Scale (GCS) score upon emergency department arrival, are predictive of greater force transmission and, consequently, more severe injury.[6]

Seat belts are designed to transmit energy across the clavicle, chest wall, and superior pelvis, particularly the anterior superior iliac spine (ASIS). When a seat belt rests on the neck or above the ASIS, the force is transmitted to the softer tissues of the neck and the more vulnerable abdominal viscera. The transmission of physical force may explain various injury patterns. The seat belt can act as a fulcrum, with the point of contact remaining fixed while the areas above and below continue to move forward.

The "seat belt sign" was first described by Garrett and Braunstein in 1962 as an area of ecchymosis across the abdominal wall corresponding to the location of the lap belt. "Seat belt syndrome" refers to the combination of musculoskeletal and visceral injuries associated with the forces transmitted through seat belt use.[7]

The Morel-Lavallée lesion (MLL) is a closed traumatic soft tissue degloving injury caused by shearing forces that separate the dermis from the underlying fascia.[8] MLL most commonly occurs in association with pelvic trauma (30%) and thigh trauma (20%), typically over the greater trochanter. These injuries may result from sports-related incidents, falls, or MVCs.

Skin abrasions and bruising may be visible on the surface, while internal injuries, such as bowel and mesenteric trauma, are common. Lumbar spine fractures may also occur. Solid organ injuries, including lacerations of the liver and spleen, as well as injuries to other visceral organs such as the pancreas and kidneys, are also possible. When a seat belt impacts the neck, clinicians may diagnose potential injuries such as cervical vascular injury, cervical spine fractures, and clavicle fractures. Thoracic injuries caused by seat belt force may include sternal and rib fractures, pulmonary contusions, and, less commonly, myocardial contusions.

The flexion-distraction fracture of the spine, first described by GQ Chance in 1948, was not recognized before the introduction of lap seat belts. With the widespread adoption of seat belts in North America during the late 1960s, cases reporting transverse fractures began to increase. Recent studies report a 40% incidence of intra-abdominal injury, with some suggesting this condition may be underreported, especially in pediatric populations.[9][10] Please see StatPearls' companion resource, "Chance Fractures," for more information.

Histopathology

Damage to internal structures can occur in seat belt injuries. The histopathology of bony fractures is similar to that caused by blunt force trauma. Similarly, injuries to the mesentery or bowel are typically characterized by tearing, many of which may be undetectable on computed tomography (CT) scans and could be very small at the histological level.

History and Physical

A comprehensive history should be obtained from all patients involved in MVCs as soon as possible. Key information includes the use of restraints, airbag deployment, vehicle speed, position in the vehicle, collision type, time of extrication, loss of consciousness, and the injury levels of other individuals involved in the accident. These details are essential for a comprehensive evaluation of the patient.

Patients should be asked about any pain, with particular attention to the potentially distracting nature of injuries. A comprehensive medical and surgical history and details on medications and drug allergies are essential, as many of these patients may require surgery.

Evaluation

Evaluation should begin with assessment of the airway (with cervical spine protection), breathing, and circulation. A brief neurological evaluation should follow, and the patient should be fully exposed and examined to ensure that injuries are not missed. Vital signs and the shock index should be closely monitored for any signs of deterioration. The GCS score should be documented. Ongoing monitoring of vital signs is essential, with particular attention to trends such as an elevated heart rate with stable blood pressure or an increased respiratory rate. Central venous access may be considered for measuring central venous pressure and to facilitate rapid fluid infusion if necessary.

Laboratory testing in patients who have survived an MVC varies in sensitivity and specificity. Leukocytosis, low hemoglobin, elevated liver transaminases, elevated amylase and lipase, increased lactate levels, and a high base deficit may indicate severe injury.[11] However, some laboratory values may be normal within the first few hours of severe trauma. Human chorionic gonadotropin and urinalysis can help detect injuries to the genitourinary system and pregnancy. Blood type and crossmatch should be ordered early.

Plain x-rays of the chest and pelvis are typically performed in patients with a significant mechanism of trauma. Ultrasound of the lungs, heart, and viscera should be conducted in all unstable patients using Focused Assessment with Sonography for Trauma (FAST) or its extended version. In patients classified as “transient responders” under the Advanced Trauma Life Support (ATLS) guidelines, this response should be documented, and management plans should be tailored accordingly, considering the facility’s available human and technical resources.[12]

CT imaging plays a vital role in the evaluation of patients with blunt trauma. The pan-scan approach—which includes CT of the head, cervical spine, chest, abdomen, and pelvis, with reconstitution of the thoracic and lumbar spine—has been shown to save time, reduce diagnostic delays, and contribute minimally to cost and radiation exposure. However, patients must be hemodynamically stable to undergo CT. In critically ill patients or those with suspected occult injuries, reassessment using physical examination, laboratory results, and targeted imaging should be considered.

Treatment / Management

The treatment of patients with seat belt–related injuries depends entirely on the type and severity of the injury.[13] Patients with vascular, visceral, or spinal injuries should be admitted to the hospital, preferably to a trauma center. In contrast, injuries involving the ribs, sternum, and soft tissues are generally managed conservatively, often on an outpatient basis, with a focus on pain management. The management of these injuries should follow local practice patterns and adhere to established standard-of-care guidelines.

Differential Diagnosis

The differential diagnosis for individuals involved in MVCs with a significant mechanism is broad, as any area of the body may be injured, sometimes in an occult manner. A key consideration is that clinicians should maintain a high index of suspicion for injuries involving the spine, hollow organs, and vasculature, especially in cases with obvious external signs of seat belt injury.

Treatment Planning

Treatment planning often hinges on the fundamental emergency department axiom: "Can we take care of the patient here, or do they require transfer?” The approach should also incorporate specialty care considerations, including pregnancy, preexisting malignancies, or prior spinal injuries and surgeries.

A designated trauma surgeon, bypass capability, blood bank, and operating room staff with trauma expertise may not always be available. In small community hospitals, few general surgeons are equipped to treat critically unstable patients whom they have managed to stabilize. While a patient may survive an emergency thoracotomy and have chest tubes in place, decisions regarding transfer should be guided by staffing considerations, including nursing, respiratory, and pharmacy support.

Transportation itself presents significant challenges. Weather conditions may prevent air transport, as most rotary-wing flights are limited to visual flight rules conditions, with strict policies governing wind and altitude. While cross-coverage is always planned, aviation is considered a fickle science. Helicopters and planes may be grounded for maintenance, experience issues after startup or during flight, and may either return to base or fail to take off entirely. Additionally, road conditions can cause delays, even for the shortest ground transports.

Toxicity and Adverse Effect Management

The toxicological risks of medications used during trauma care are generally outweighed by their benefits, as prompt pharmacological intervention can be lifesaving. In critical situations, delaying treatment due to concerns about toxicity may lead to worse outcomes.

Prognosis

The prognosis of seat belt injuries depends largely on the specific type of injury sustained. Damage to the mesentery or bowel can cause severe morbidity if not detected early. However, with timely surgical intervention, most patients recover without long-term complications. Fractures of the cervical, thoracic, or lumbar spine carry prognostic significance, depending on the severity of the bony injury and the presence of neurological deficits. For vascular injuries, early diagnosis is crucial, although some carotid or vertebral injuries can result in irreversible neurological impairment despite prompt recognition. Early identification of severe underlying injuries, beyond surface abrasions, is essential for ensuring appropriate treatment, management, and transfer.[14]

Complications

Early treatment and intervention for spinal, visceral, and vascular injuries significantly reduce the risk of complications. Timely recognition improves survival, minimizes long-term disability, and reduces the need for more extensive interventions.