Motorcycle injuries: a systematic review for forensic evaluation

Head and facial injuries are the most frequent injuries observed in motorcycle accidents, with brain damage being the leading cause of death among motorcyclists [5, 34, 71, 131]. These injuries can result from various mechanisms. One major cause is the impact of victims’ head with other vehicles, fixed obstacles, or the ground when they are forcefully thrown after a collision due to the high inertia involved [71]. Additionally, brain injuries are often caused by deceleration forces, as the brain is not fixed, allowing it to relative movement within the skull. This can lead to various deceleration effects, such as multifocal vascular ruptures, cerebral concussion, or diffuse axonal damage [146]. Furthermore, injuries to the brainstem and pontomedullary region can result from significant movements like hyperextension, antero-flexion, and torsion of the head, caused by either direct contact trauma or acceleration-deceleration forces [147‐149].

The most common head traumas observed in motorcycle accidents are concussions, followed by brain contusions or haemorrhages, facial and skull fractures [40, 143]. Wearing helmets prevents them: full-face helmets specifically reduce the incidence of brain contusions more effectively than the open-face type. However, the prevalence of skull fracture, subdural hematoma, and subarachnoid haemorrhage does not differ significantly between the two kinds of helmets [42]. Nonetheless, full-face integral crash helmets can also cause skull base fractures, as a portion of the impacting force is transmitted to the skull base through the chinstrap, involving the mandibular rami and condyles [15]. Moreover, heavier helmets are more likely to result in partial or complete ring fractures of the base of the skull when subjected to axial loading [25].

Brain injuries can also be linked to facial bone fractures. Studies have shown that fractures of the upper part of the face, such as the zygomatic and orbital bones, are more commonly associated with brain injuries than fractures of the lower part of the face, such as the mandibular bone [98, 150, 151].

Papers addressing skull fractures caused by direct impact have been distributed over the past 20 years [5, 34, 40, 42, 71, 98, 131]. Papers discussing severe ring fractures at the base of the skull associated with the use of full-face helmets originate from the 1990s [15, 25].

The most prevalent neck injuries due to motorcycle accidents include hemorrhage in the carotid sheath, subluxation in the occipital-atlanto-axial complex, hemorrhage in the muscles and triangles of the anterior neck, and damages along the vertebral artery [39].

The cervical spine region is the most affected part in case of fatal crashes [34]. Specifically, when the head undergoes hyperextension during a crash, the forces are transmitted through the cervical spine, leading to tissue damage [98]. The effectiveness of helmets in preventing cervical spine injuries remains a topic of debate. Some studies suggest their usefulness [116, 127], but Hitosugi et al. reported that the prevalence of cervical fractures was slightly higher in individuals wearing full-face helmets compared to those wearing the open-face type [42].

Regarding other helmet-related injuries, the helmet buckle can also cause fractures in the neck cartilage, primarily affecting the thyroid cartilage. The energy from the trauma results in the displacement of the helmet, and its buckle subsequently exerts pressure on the surrounding tissues. If the helmet buckle is positioned over the larynx at the moment of impact, it can exert enough force to potentially fracture the laryngeal cartilages [117].

Concerning neck vascular injuries, internal carotid artery dissection, though rare, has a high mortality rate after motorcycle accidents [124]. This type of injury may occur due to hyperextension and rotation of the neck, reflecting traction on the internal carotid artery as it crosses the transverse processes of the second and third cervical vertebrae. Another possible mechanism is abrupt full flexion of the neck, which can directly compress the internal carotid artery between the angle of the mandible and the upper cervical vertebrae [152]. Additionally, the pressure applied by the helmet strap on the soft tissues of the neck could also contribute to vascular damage [120]. A traumatic injury to the vertebral artery may determine death, especially in low-speed accidents, where the person may not immediately complain of specific symptoms after the accident. Instead, they may start feeling unwell hours or even days later, typically experiencing neurological symptoms like nausea, vomiting, and eventually leading to coma and death. The traumatic dissection of the vertebral artery causes cerebral infarction, followed by edema and compression of the brainstem. The vertebral artery is particularly susceptible to longitudinal stretch, and it can be hurt during sudden neck movements involving hyperextension and/or rotation. Therefore, in cases of delayed symptom presentation, forensic examination should take into account the possibility of vertebral artery dissection as a potential cause of death [61, 153].

Although rare, severe high-energy trauma can lead to the complete separation of neck tissues, even resulting in decapitation. The latter may be caused by different mechanisms, but it is often due to an impact against an immovable object [114]. Decapitation can also be attributed to the action of the lower edge of a full-face helmet during an incident. When a full-face helmet is worn during a traumatic event, the lower edge of the helmet can exert a significantly powerful force on the surrounding tissues, potentially leading to a complete cervical spine amputation [67, 88]. Additionally, instances of decapitation have been reported in the literature due to the interaction with motorcycle components. Ihama et al. reported a case in which a motorcyclist’s neck became entangled in a rotating motorcycle chain, resulting in complete decapitation [10].

All reported articles on neck injuries have been published within the last 20 years. Notably, certain articles focusing on helmet-related injuries [116, 127] and vertebral artery dissections [120, 124] encompass publications from the last 5 years.

Injuries to the thorax and abdominal regions pose a significant risk of death due to severe blood loss, asphyxiation caused by thorax compression, and vertebral spine fractures [5, 33, 63, 121]. Common lesions in these areas include lung contusion and liver laceration, often occurring simultaneously with rib fractures [34, 81]. Impact during falls, especially between the left side of the abdomen and the end of the motorcycle handlebars, can cause splenic and pancreatic damages [44].

In high-velocity trauma, injury to the aorta is a typical and highly fatal occurrence. The ascending aorta is the most common affected segment, followed by the arch, thoracic, and abdominal sections [140]. Traumatic events in the ascending aorta or arch should be considered in cases of cardiac tamponade, aortic valve regurgitation, and myocardial contusion [84]. According to Richens et al., the aorta is subjected to various mechanical forces in anatomically vulnerable sites. Sudden deceleration can cause a stretching effect, leading to laceration of the isthmus as the ascending aorta and aortic arch are more mobile than the fixed distal descending part. Additionally, the aorta may rupture due to a sudden increase in blood pressure and entrapment between the anterior chest wall and the vertebral column [154]. The primary physiopathological mechanism of aortic laceration can be difficult to ascertain because of the diversity and complexity of crash scenarios, particularly when victims have been exposed to multiple external forces [112].

In major traumas, the thoracic spinal column may fracture. These injuries exhibit an “all or nothing” phenomenon, irrespective of the cause of the trauma, and tend to be more severe compared to similar lesions in the cervical region [127]. The thoracic spinal column may extensively suffer the trauma, or it remains intact with no damage. This effect is caused by the restraining effect of the ribs and sternum on the thoracic spine, making it less mobile. Consequently, if a force is powerful enough to break through a segment, the adjacent mobile segments are unable to absorb the remaining force, leading to displacement and significant injury to the spinal cord. Moreover, the thoracic canal provides slightly less space per segment compared to the cervical spine, favouring even slight displacement more dangerous [155].

Complete trunk severance cases are rare and typically associated with accidents involving high impact speeds. Muggenthaler et al. [76] documented a case of complete trunk severance resulting from a collision with a road signpost.

The articles mentioned in this paragraph have all been published in the last 20 years. Specifically, the articles addressing injuries to the aorta [112, 140] and spine [127] are from the last 5 years.

These are often attributed to contact with the motorcycle fuel tank during the collision [101]. These injuries can include fractures of the pelvic ring bones, damage to internal organs within the pelvic cavity, associated with pelvic haemorrhages of varying degrees, as well as lesions to the soft tissues of the lower abdomen, perineum, groin, or testicular area [9, 52].

Fuel tank injuries are typically experienced by the motorcycle driver, particularly in frontal collisions, when after impact, a sudden deceleration can propel the driver forward, colliding with the tank [71, 101, 156]. Interestingly, drivers who attempt to avoid a collision and topple over immediately before the impact are less likely to experience fuel tank injuries [52].

Even passengers can suffer groin injuries in accidents involving two riders on a motorcycle. In fact, passengers who are seated behind the driver often slide on the saddle, sometimes even onto the fuel tank, and then may hit the driver’s buttocks and back. However, the protection of the driver’s buttocks and back, can reduce the passengers’ risk of underbelly injuries [104].

The articles mentioned in this paragraph have all been published in the last 20 years. Specifically, some articles addressing fuel tank injuries [52, 101, 104] are from the last 5 years.

Motorcycle riders are particularly susceptible to limb injuries due to their heightened exposure to direct impacts [5]. Non-fatal limb injuries are the most frequent, encompassing ligamentous lesions, fractures, and dislocations [34, 38, 93]. These lesions occur when limbs become entrapped between the motorcycle and the ground or impact with fixed road signs or poles [48, 59, 97].

Regarding the upper limbs, the most common limb injuries are fractures of the shoulder, forearm, and hand [59, 97], reflecting the motorcyclist’s position, with flexed elbows being farthest from the impact point [132].

Forearm and hand injuries have been associated with lower mortality rates, because upper extremities act like a “crumple zone” when crash at highway speeds, protecting the head and neck region from direct impact in head-first hit damages in frontal crashes [95]. The distal portion of the upper extremity absorbs the energy of the collision, potentially reducing severe proximal trauma to the head and neck [12].

Hand lesions are more common in motorcycle drivers than in passengers, because during a collision, the driver instinctively locks their elbows and firmly holds onto the handlebars, redistributing the resulting force into the palm and metacarpal base. Thumb carpometacarpal joint injuries are particularly common due to the thumb’s position onto the handlebar grip making it more vulnerable to trauma [5, 103].

Concerning the lower limbs, the tibia is the most common site, followed by the proximal femur, particularly in lateral impact, patella and foot [20, 34, 125]. The heel is particularly susceptible with calcaneus fractures, Achilles tendon ruptures, and defects. The motorcycle’s lack of spoke guards or poorly designed guards favours the entrapment of the pillion passenger’s heel between the spokes and the frames, resulting in crushing and grinding injuries from the continuously rotating wheels [30, 74].

The articles cited in this paragraph cover a wide range of time periods, from the 1970s to the 1990s [12, 26, 30] to the 2020s [125, 132]. This temporal variety is evident in articles discussing both upper and lower limb injuries.

The topic of injury prevention strategies and technologies has gained increasing interest in recent years. In fact, the articles mentioned in this paragraph have all been published since 2009, and a substantial number of them are very recent, with publication dates within the last five years [113, 115, 119, 122, 123, 128, 129, 133, 136, 137, 141, 145].

Injury prevention strategies and technologies for motorcyclists have focused on two main aspects. Firstly, helmets are used to primarily protect the head, an area highly correlated with increased mortality rates. Secondly, protective clothing and wearable devices are employed to shield the remaining parts of the body.

The mandatory use of motorcycle helmets for both drivers and adult passengers has become a global legal requirement, with few discrepancies in regulations across the world. However, variations persist, particularly regarding restrictions on transporting child passengers in Asia and Africa, despite the prevalence of motorcycle use in these continents [129, 157].

It is well known that motorcyclists wearing helmets tend to experience less frequent and severe episodes of head and facial injuries. Helmets have demonstrated effectiveness in mitigating and preventing traumatic brain injuries in motorcycle accidents, particularly during impacts with large energy-absorbing surfaces, for example when the head collides with the ground or the surface of a car body. However, the protective capacity of helmets is constrained when the materials they are made of surpass their tolerance thresholds, as in high-speed impacts or direct trauma with objects presenting limited surface areas, such as light poles, trees, or angular parts of the vehicle [131, 133, 139, 143].

Modern helmets are designed to have a rigid polycarbonate outer shell with a firmly placed energy-absorbing liner, while a soft expanded polystyrene or polyurethane foam padding forms the innermost layer.

Helmet damage can range from subtle blemishes to obvious defects, and their careful examination can provide valuable information about the accident dynamics and cranial injuries sustained by the motorcyclist. Conventional helmet damage assessment involves the manual removal of each layer, a process prone to oversight and additional damaging. To reduce these risks, the use of computed tomography scanning of the helmet has been suggested. This method allows precise delineation of damage within each layer, including breaks in the outer shell or compression in the inner layers. The radiological examination can also facilitate measurements of the thickness and density of the foam and can detect bloodstains, confirming that the helmet was worn by the motorcyclist at the time of impact [158].

While full-face helmets provide comprehensive protection, open-face and half-cover helmets, although less secure, are still in use [42, 145]. Recent studies suggest that full-face helmets offer better protection against head and face injuries, but may be less effective in preventing neck injuries and skull base fractures [136, 145]. Notably, motorcycle full-face helmet visors, while not designed for energy absorption, redirect mid-face impact forces to the upper and lower face, enhancing protection [145]. Despite the overall recommendation for full-face helmets due to their reduced mortality and injury probability, their usage is often limited due to discomfort, particularly in subtropical and tropical climates where most motorcyclists reside [141].

Despite the universally recognized head protection helmets offer, a debated issue remains concerning a potential increase in the risk of cervical spine injuries due to increased hyperflexion-hyperextension movements induced by the helmet’s lower edge [15, 117, 133]. Some studies suggest that helmet use reduces the risk of cervical spinal cord injuries during motorcycle crashes [116, 127]. However, Hitosugi et al. [42] reported a prevalence of cervical fractures wearing full-face helmets. The literature is inconclusive, suggesting that helmet use does not directly increase spinal injuries, but may be less effective in preventing severe cervical injuries compared to cranial injuries [94, 108]. Nevertheless, the present review indicates that fatal skull base ring fractures associated with the use of full-face helmets have been reported in articles dating back to the 1990s [15, 25]. This reinforces the notion that contemporary helmets are effective in preventing more severe spinal injuries. On this point, advancements in helmet technology have recently introduced airbag-equipped helmets, representing a novel class of protective headgear. These helmets, equipped with inflatable structures, have demonstrated a reduction in concentrated impact force to the lower- or mid-face region, resulting in decreased head rotation and brain strain. Contrary to conventional helmets, these devices only deploy when necessary, preserving the field of vision and ventilation during normal use [145].

In injury prevention, wearing a helmet is not only crucial but equally important is the correct method of wearing it. Proper helmet fixation and correct strap positioning are essential for effective prevention of head and neck injuries. Loose helmet straps can cause ejection or compromise anterior neck structures in high-impact crashes [136]. Moreover, helmeted individuals are significantly less likely to sustain shoulder fractures, suggesting a protective effect of helmets. A possible explanation is that energy transmission through the helmet and neck flexion during the impact may shield the proximal humerus and shoulder girdle [132].

The current regulation from January 2021 is ECE 22.06, which introduces new test procedures for the production and assessment of next-generation helmets. These include a rotational acceleration test to assess oblique impacts, linear impact tests for both high and low energy impacts, testing of retention systems and new assessments for helmet visors to measure impact resistance against small objects thrown at high speed. In addition, ECE 22.06 requires a test to assess helmet stability on the head. While the previous standard, ECE 22.05, simulated the risk of “roll-off”, where the helmet rotates forward and disengages from the rider’s head on impact, ECE 22.06 extends the assessment to the possibility of backward rotation, potentially exposing the rider’s forehead or neck after impact. This evaluation includes the open chin guard tear test, which ensures complete detachment of the chin guard to reduce the risk of neck injury. ECE regulations also assess the point of impact on the helmet during a crash. While ECE 22.05 identified six primary impact points (front, top, back, sides, and chin), ECE 22.06 extends the evaluation to include 12 additional intermediate impact points along the mid-lines [159].

Concerning protective clothing for motorcyclists, epidemiological observations highlight the vulnerability of light motorcycle drivers compared to heavy motorcycle users. Light motorcycle drivers, often not wearing protective clothing, are statistically more involved in road accidents, as these motorcycles are commonly used for work and transport [83]. Technical clothing such as jackets, pants, shoes, and gloves has shown effectiveness in preventing soft-tissue injuries, particularly open wounds, but has no significant impact on systemic injuries or fractures [75, 83].

The usage of protective clothing reduces the probability of upper (shoulder and elbow) and lower limb (buttock and thigh) skin injuries, while providing less effective protection for the chest, abdomen, back, and groin [91]. However, the use of motorcycle protective clothing in warm seasons may impair cognitive and psychophysical functions, potentially affecting riding performance and safety due to body overheating [82, 91].

Given the limitations of protective clothing, specific impact protection technologies have been developed for areas of the body where protection is limited, such as the back and abdomen [91]. Nevertheless, their adoption remains uncommon among on-road motorcyclists, and there are no laws governing the usage of such protective measures [139].

Protective clothing cannot entirely prevent limb fractures during falls, especially severe ones affecting the proximal femur in lateral impacts. Hard hip protectors, designed to cover the greater trochanter, effectively absorb and redirect impact energy away from vulnerable areas, preventing direct hip fractures [64]. Studies on limb injuries cover several decades, from the 1970s through the 1990s [12, 26, 30] to the 2020s [125, 132]. During this wide timeframe, to the best of our knowledge, no effective protective device suitable to prevent limb bone or soft tissue injuries was developed. Indeed, while current protective equipment is effective in preventing skin injuries, it still has significant limitations in preventing more severe damage [75, 83].

Back protectors, initially designed for racing sports, have shown effectiveness in reducing back cutaneous injuries but are limited in protecting against spinal injuries caused by bending and torsional forces [102, 128]. The integration of hard-shell or airbag technologies into back protectors may enhance their effectiveness in preventing serious spinal injuries [92].

Recent developments include rider-worn pelvic protection devices designed to reduce the risk of injury from contact with the motorcycle fuel tank during a crash. Simulation studies suggest potential benefits in the absorption and distribution of impact energy, but the understanding of pelvic biomechanics under anteroposterior loading is currently limited [137].

The effective prevention of trunk injuries remains a subject of ongoing study. The substantial number of recently published studies examining these injuries, particularly in relation to aorta laceration [112, 140], spinal fractures [127], and fuel tank injuries [52, 101, 104], underscores the persistent challenge posed by these injuries.

The evaluation of injuries for analyzing the dynamics of the accident is of forensic significance in criminal, civil, or insurance cases, since the results of medical examinations or autopsies enhance the information gathered from on-site inspections and circumstantial data. Fatal injuries typically result from direct impacts on the body during accidents involving other vehicles, the ground, or fixed obstacles. In many cases, multiple impacts usually involve an initial collision with objects, followed by a secondary impact with the ground. Furthermore, the subjects may sustain tertiary injuries due to being struck by vehicles or colliding with fixed obstacles such as road poles, walls, or barriers [81, 131].

In fatal cases from direct impacts the primary cause of death is a neurogenic shock due to a direct head impact [40, 71]. The site of cranial blunt trauma can be determined by combining autopsy findings, typically skull fractures and intracranial hemorrhages, with data collected from the accident scene. Therefore, it is crucial to establish whether the victim was wearing a helmet and, in such instances, to have the ability to examine it. Locating the break points on the surface of the helmet can provide crucial information regarding the precise point of impact.

It is also paramount to assess the helmet’s fit on the subject, by measuring the length of the buckle to assess its suitability for the subject’s facial and neck proportions, given that excessive helmet excursion during a crash can potentially led to laryngeal cartilage fracture which can be better identified by radiological studies such as computed tomography.

Direct trauma to the thorax and abdomen can result in internal thoracoabdominal organ lacerations, leading to fatal hemorrhagic shock. Such trauma can occur due to contact with the ground or components of the motorcycle, such as its handlebars [5, 33, 44, 63, 121]. At autopsy, if that kind of vehicle collision dynamics is suspected, bruising patterns that mimic the shape of handlebar components should externally be sought. During the examination of the body, consideration should be given to whether the victim was wearing protective motorcycle clothing. This factor can potentially mitigate skin injuries, particularly on the limbs, making them less apparent on external inspection.

Fatal injuries can be attributed also to the forces of acceleration and deceleration resulting from sudden, unrestricted body movements, which induce compressive, tensile, and shear strains of the tissue. Specifically, neck hyperextension can damage the brainstem and cervical region, presenting at autopsy as brain hemorrhages without cranial fractures, fractures of the cervical vertebrae, soft tissue hemorrhages, or carotid artery dissection. This vessel is particularly susceptible to hyperextension movements as the vascular wall can undergo stretching and tearing [34, 98, 124]. Even in cases of low-energy trauma where the subject deceases some days after the accident, suspicion should fall on vertebral artery dissection. This vessel is notably vulnerable to longitudinal stretching and can rupture even in mild hyperextension or neck rotation [61, 153]. A meticulous macroscopic and microscopic examination of the vertebral artery is mandatory when symptoms such as nausea and vomit were reported in clinical record.

In the thoracoabdominal region acceleration and deceleration forces can cause fatal injuries especially in high-energy trauma with the involvement of the aorta, that is highly susceptible to stretching and tearing, primarily in its more mobile segments, such as the isthmus and arch [140, 154], resulting in hemorrhagic shock without significant direct trauma indicators. A violent hyperextension of the thoracic spine can lead to severe vertebral fractures and spinal cord damage due to the limited mobility of this portion, which is firmly anchored to the ribs and sternum [127, 155].

In the forensic medical evaluation of a motorcycle accident, one of the most challenging and pertinent issues is distinguishing the driver from the passenger. In literature, it is reported that drivers are predominantly male, while passengers are female [139]. When analyzing motorcyclist fatalities by age, a notable spike is observed among subjects aged between 21 and 30 years. This trend can likely be attributed to a lack of riding experience and impulsive behavior among teens and younger adults [131]. Additionally, a higher prevalence of fatal crashes occurring at night and on weekends has been reported, with positive toxicology tests for alcohol and drugs for both drivers and passengers [139]. In most cases, it has been observed that drivers were wearing helmets at the time of the accident, whereas passengers more commonly did not wear helmets, thereby increasing the likelihood of sustaining severe traumatic brain injuries [143]. Nevertheless, these are general epidemiological data obtained from literature review, and cannot be used to discriminate between the passenger and the driver in individual cases.

The injuries sustained can provide some useful information. The ejection dynamics differ for drivers and passengers due to variations in their initial positions and postures.

The rider’s distinctive attitude is to hold the handlebars while driving, thus establishing a direct connection between himself and the motorcycle. In case of a collision, this connection can prevent the rider from being ejected from the motorcycle, thus increasing the risk of sustaining crushing or burning injuries as a result of impact with various parts of the motorcycle. Such incidents are more likely to occur in low energy collisions, as in high energy collisions the rider is typically thrown and ejected at a distance due to the significant inertial forces, preventing them from remaining anchored to the vehicle [71].

Conversely, pillion passengers have different options for support. They can either grasp the bar attached to the tail of the motorcycle or hold onto the driver’s haunches in front of them. However, due to their less stable position, pillion passengers are typically propelled higher into the air during the accident, resulting in a longer fall to the ground from a greater distance [71].

Some specific lesions may serve as distinguishing factors between the driver and the passenger involved. Passengers can suffer groin injuries, usually mild and caused by impact with the driver’s buttocks and back [104]. Furthermore, the passenger’s heel can become entrapped between the spokes and the rear wheel frame, resulting in crushing and grinding injuries [30, 74]. In drivers, lesions to the palm of the hand and the thumb carpometacarpal joints are more prevalent. This is because, during a collision, drivers tend to instinctively lock their elbows and grip the handlebars tightly [5, 71, 103]. Pelvic damage and fractures, called fuel tank injuries, tend to be associated with the driver’s impact against the fuel tank in frontal crashes where the sudden deceleration leads to the body sliding forward making contact with the fuel tank [71, 101, 156].

Nevertheless, the reconstruction remains a challenging task due to the intrinsic complexity of accident dynamics. This procedure entails a multidisciplinary approach, as demonstrated in many specific cases, mostly for the identification of the manner of death, consisting in conducting interviews with individuals directly involved in or witnessing the incident, performing mechanical and engineering examinations (i.e., a kinematic analysis) and procuring documented visual evidence of the occurrence [160, 161]. In cases involving motorcycle accidents, recorded images of the event can be sourced from various means, including surveillance cameras, helmet-mounted cameras worn by riders, and dashboard-mounted cameras in the vehicles participating in the accident.

A critical analysis of findings collected at accident scenes, including skid marks and the resting positions of vehicles, combined with the forensic evaluation of injuries in the light of developments in safety devices, addresses medical-legal responses based on scientific evidence.