A radial truck tyre is constructed from rubber components and layers of cord reinforcements made of polyester, steel, and nylon. These cords are embedded in a rubber matrix with specific spatial orientations. The tyre is mounted on a rim made of steel or aluminium (Michel et al. 2011).
A tyre burst refers to a rupture of the tyre structure when the latter cannot maintain the pressure contained inside. Schematically, the cause can be mechanical, chemical, or a combination of both (Michel et al. 2011). The primary cause of chemical bursts is the application of heat to the tyre or the development of heat within the tyre. The thermal expansion of the air inside the tyre leads to the reduction of the mechanical resistance properties of the tyre structure and thus to its bursting or even explosion in certain circumstances. For example, a welding operation on the rim can create enough heat to initiate an internal chemical reaction (Dolez et al. 2008).
Mechanical tyre bursts can result from an over-pressurization, tyre demounting, or “zipper” failure in tyres in poor condition or with a structural weakness (Michel et al. 2011). Possible causes of over-pressurization of the tyre are an inadequately adjusted compressor pressure, a malfunctioning pressure gauge or valve, or deliberate over-pressurization when mounting the tyre to the rim.
The name zipper failure comes from the zipper-like appearance of the mid to upper sidewall area of the tyre after bursting, exposing an even line of severed casing steel cords (Hefny et al. 2009; Murty 2009; Pomara et al. 2013). Possible mechanisms include casing damage exposing the tyre's inner liner to air or moisture contamination, overloading, the mechanical impact that has damaged the tyre's structure, and driving with an under or over-pressurized tyre (Murty 2009; Pomara et al. 2013).
Handling a deflated tyre has no risk of bursting, the real danger exists when air enters the tyre during inflation. Summer heat can be an additional factor in tyre bursting due to the increased pressure inside the tyre (Hefny et al. 2010). This phenomenon commonly occurs when the vehicle is in motion, but it can also occur when the vehicle is stationary and when the tyre is being inflated or secured to the axle (Hefny et al. 2009; Obafunwa et al. 1997). According to Hefny’s series combining cases of non-fatal and fatal large tyre blast injuries, the majority of injuries occur in service stations during tyre mounting and servicing. Young men working as mechanics are by far the most affected (Hefny et al. 2009).
An experimental study that focused on establishing the strength limits of a tyre during inflation, showed that the tyre burst pressure obtained for a 11R22.5 truck tyre in a hydrostatic burst test was 1.826 MPa (18.26 bar or 264.85 psi) which corresponds to a pressure of 2.5 times the maximum inflation pressure recommended by the tyre manufacturer (Michel et al. 2011). The main danger results from the brief evacuation of the air contained, which can propel the tyre and cause very serious injuries to people nearby. The severity of the injuries depends on the size of the tyre, the contained air pressure, and the distance between the tyre and the victim (Hefny et al. 2009). Blast injuries in the case of bursting tyres can be caused by the single or more commonly, the combined effect of three main entities which are the pressure wave of the blast, the impact of the tyre rim or other fragments, and the displacement of the body being thrown against the ground or other surfaces or objects. Schematically, these constitute respectively the well know primary, secondary and tertiary classification of blast injuries described in explosions caused mostly by armed conflict, terrorist bombings, industrial disasters, aeroplane crashes, and domestic gas leaks (Galante et al. 2021). The main difference is the absence of thermal or chemical effects that belong to the quaternary blast injuries (Blechner and Seiler 1995; Larkins et al. 2020)
A study (Hefny et al. 2009) reported that the most commonly injured body parts are the head and facial bones, which is explained by the fact that the victims were facing the tyre when it burst (Hefny et al. 2010), and about 25% of the injured had multi-trauma. The reported overall mortality was 19% (Hefny et al. 2009).
When a large tyre bursts, an increase in atmospheric air pressure is created, known as an overpressure or blast wave, and can induce severe barotrauma. Sudden excess of pressure, even if small, causes significant physical effects on the body (Kumar et al. 2016; Pomara et al. 2013). The pressure wave travels through media of different densities, provoking acceleration and deceleration as it passes through the tissue (Housden 2012). The absorbed part of the shock wave has variable effects depending on the tissue. Solid organs are not very sensitive, while hollow viscera are very vulnerable, as well as pulmonary alveolae and eardrums. This is well demonstrated in the studies reporting this phenomenon, where autopsy findings related to the primary blast injuries showed haemopneumothorax, partially collapsed lungs with foci of parenchymal contusions, and haemorrhagic suffusion of the mesentery, stomach, and intestines (Hefny et al. 2009; Murty 2009; Pomara et al. 2013). Intestinal perforation or rupture is rare (Larkins et al. 2020). Injuries to solid viscera are related to very high blast loading (Housden 2012). The reported cases above have shown that the first victim had lacerations of the proximal aorta and the pulmonary artery. It is explained by the rapid deceleration that produces shear forces resulting in tearing injuries to semi-mobile body structures at their attachment sites (Parmley et al. 1958; Pomara et al. 2013). We also observed the presence of hyoid bone fracture in the second case. There was no similar finding in the previously reported tyre burst cases, but it has been documented that hyoid bone and thyroid cartilage fractures can be seen in explosion-related primary blast injuries with high levels of blast overpressure (Galante et al. 2021). As shown, the majority of primary blast injuries are internal and the shock wave can cause death without obvious visible external injury (Pomara et al. 2013). A safety distance of 2.5 m from the tyre is recommended when inflated (Yu et al. 2017).
Secondary blast injuries are the consequence of the projection of the tyre or one of its components and surrounding debris energized by the blast and propelled outward, causing non-penetrating and penetrating injuries (Murty 2009; Pomara et al. 2013). It can be particularly dangerous and explains the variety of sustained injuries. These projectiles can cause superficial erosions, ecchymosis, haematomas, contused wounds, and even bone fractures. Direct impact injuries typically cause focal damage at the site of impact. Cases of foreign objects penetrating the eyes (Choi et al. 2009; Žiak et al. 2017) and the maxillary sinus are reported (Malachovský et al. 2016). In one reported case, Lau (Lau 1995) described a penetrating mandibular injury from the handle of a sledgehammer when the inner tube of a military truck burst associated with an atlanto-occipital dislocation. We reported a similar finding in the first case. The victim got hit in the face by the tyre and upon internal examination, an atlanto-occipital dislocation was discovered. Atlanto-occipital dislocation is primarily an injury of the ligaments between the occiput and upper cervical spine (Hall 2015). The most common mechanism of this lesion is sudden hyperextension of the atlanto-occipital joint. This induces disruption of the tectorial membrane and alar ligaments, allowing dislocation of the craniovertebral junction to occur (Fard et al. 2016; McKenna et al. 2006). In both cases, the projectile striking the face had the effect of a violent blow throwing the head backwards, resulting in posterior hyperextension and dislocation of the cervical spine (Lau 1995).
Tertiary blast injuries are explained by the propulsion of the body and subsequent impact against a surface or other environmental structures, resulting in blunt trauma (Hefny et al. 2010; Obafunwa et al. 1997). Head injuries are major tertiary blast injuries and lead in most cases to instantaneous death. They are the most common findings in fatalities (Pomara et al. 2013). The displacement of the body can result in scalp contused laceration, subarachnoid haemorrhage, subdural haemorrhages, cerebral contusion, and cranium fractures. Rib fractures disrupting the chest wall are also seen and can alter the mechanics of pulmonary ventilation (Marro et al. 2019).