Wound ballistics of firearm-related injuries—Part 2: Mechanisms of skeletal injury and characteristics of maxillofacial ballistic trauma

Abstract

Maxillofacial firearm-related injuries vary in extent and severity because of the characteristics and behaviour of the projectile(s), and the complexity of the anatomical structures involved, whereas the degree of tissue disruption is also affected by the distance of the shot. In low-energy injuries there is limited damage to the underlying skeleton, which usually dominates the clinical picture, dictating a more straightforward therapeutic approach. High-energy injuries are associated with extensive hard and soft tissue disruption, and are characterized by a surrounding zone of damaged tissue that is prone to progressive necrosis as a result of compromised blood supply and wound sepsis. Current treatment protocols for these injuries emphasize the importance of serial debridement for effective wound control while favouring early definitive reconstruction.

Although firearm-related injuries inflicted to the maxillofacial region frequently affect adjacent structures of the neurocranium or neck, by current criteria the head, face, and neck are considered separately in the context of ballistic trauma. This is justified by the complex anatomy and articulation of the maxillofacial structures resulting in different injury patterns, which are also more difficult to reproduce in ballistic models. As a result of these difficulties, there is a limited number of experimental studies investigating the mechanisms of maxillofacial missile injuries, by contrast to the extensive literature dealing with their treatment.

In this second part of a review article on wound ballistics, specific mechanisms of ballistic bone penetration are described as a basis for understanding the pathophysiology of maxillofacial ballistic trauma. Maxillofacial gunshot (bullet) and shotgun (pellet) injuries are then presented, with respect to injury patterns commonly encountered and their surgical implications.

Mechanisms of ballistic bone injury

Bone tissue offers increased resistance to penetration compared to soft tissue due to its hardness, in addition to its greater density and strength. With bone impacts, both the retardant effect on the penetrating missile and the potential for energy transfer are marked. Under these circumstances, the critical factors for injury are the limited capacity of osseous tissue to absorb the energy of impact without fracturing and the toughness of cortical bone, which determines the extent of crack propagation. Furthermore, recent evidence suggests that there are similarities between ballistic fractures in bone and glass, indicating that under the energy transfer associated with ballistic injuries, bone behaves as a brittle material.

In a classic series of experiments, Huelke et al., using human cadaveric femurs as targets, showed that the degree of bone injury produced by spherical projectiles increased with progressively higher velocities, ranging in severity from incomplete penetration or simple ‘drill-hole’ defects, to comminuted fractures with complete separation of the bone ends. These authors demonstrated mathematically that the energy expended by the projectile penetrating normal and mildly osteoporotic femurs may actually be a linear rather than a quadratic function of the impact velocity, due to the resistance of bone. This relationship was depicted by a drop in the percentage of energy loss during penetration as the impact velocity was increased, because velocity affects the kinetic energy of the projectile raised to the second power, much more than it does with the amount of the energy transferred to the bone. In these series, impacts to the dense cortical bone of the femoral shaft caused significantly greater energy expenditure than those directed to the metaphyseal region where cancellous bone predominates. Also, comminuted fractures were more common in the shaft, which was related to the narrow tubular configuration of the cortex in this area, the latter feature effectively distributing the loading generated by the impact around the entire periphery of the bone.

Bone marrow has fluid properties allowing cavity formation within it by high-velocity projectiles, also suggested by Huelke et al. following penetration of the distal metaphyseal regions of femurs. In those experiments, defects of explosive character at the exit site were observed as a manifestation of cavitational effect by projectiles penetrating at velocities above 300–500 m/s, in extreme cases resulting in complete separation of the femoral condyles from the shaft. Contrary to soft tissue, cavitation in bone is not followed by collapse of the cavity walls due to lack of elasticity, but rather the hydraulic pressure built-up results in immediate pulverization of the surrounding bone structure. According to Kneubuehl, this mechanism is primarily responsible for ballistic bone fractures, whereas in the absence of bone marrow, as in flat bones, bullets tend to create drill-hole defects. Cavitation was not prominent with shaft impacts in the series of Huelke et al., due to the limited bone marrow contained in these parts.

In a final series, Harger and Huelke also showed that, at higher impact velocities, the diameter of the projectile has greater influence than its mass on the energy expenditure and the resultant bone damage, which is consistent with the magnitude of cavitational effects as related to the presenting area of the penetrating body. They concluded that the bone damage produced as a result of cavitation depends primarily on projectile velocity and size, whereas at lower velocities, cavitation is not a prominent feature and the mass of the projectile becomes relatively more important.

It follows that the energy transfer in ballistic bone injuries is a more complex phenomenon than in soft tissue; admittedly it also remains less well understood. The drag force opposing the motion of the projectile within bone has different characteristics than in soft tissue, being independent of the projectile velocity according to Harvey et al. Actually, because the amount of energy transferred during ballistic penetration is influenced by the time spent by the bullet in contact with the bone, which is inversely proportional to its velocity, it is possible for a relatively slow non-deforming handgun bullet to cause more damage than a stable rifle bullet. Microfractures created by the penetrating projectile within the cortical bone substance can partly explain this intricate response. These microfractures tend to radiate around the wound channel and beneath the impact site, creating an area of lesser resistance ahead of the advancing projectile so that it makes its way through the bone more easily. A high-velocity bullet upon impact is expected to produce such defects more extensively, thereafter requiring relatively lower amounts of energy for the penetration process.

Military and hunting rifles, as well as Magnum handguns, produce high-energy injuries with extensive bone comminution, documented both in experimental studies and retrospective reviews. It has also been observed that maxillofacial injuries by military rifle bullets at close range show greater comminution than those inflicted from a long distance with much of the bullet’s energy used up. However, Clasper and Hodgetts have reported an unusual case of accidental point-blank wounding by an M16 rifle bullet of current (NATO) design, resulting in a drill-hole defect in the humeral head, despite an apparently oblique course of the projectile through bone; the low-energy transfer in this case was explained by the bullet penetrating mostly cancellous bone, and the short wound track through soft tissue due to the low muscle bulk of the area. Undoubtedly, an important contributing factor for such a low-energy bone injury despite high impact velocity is the streamlined shape of military rifle bullets eliciting lower drag forces. This could be validated in correlation with a recently published finite element analysis of mandibular ballistic injuries, which revealed significantly less energy loss by 7.62-mm military rifle bullets compared with 6.3-mm steel spheres, when the former penetrated at high velocities perpendicular to the bone surface.

High-velocity missiles penetrating into soft tissue are capable of causing indirect fractures of adjacent long bones by the expansion of the temporary cavity in their wake. These fractures represent a definite feature of high-energy transfer, notwithstanding they are simple rather than comminuted. Indirect fractures of the skull base occur with high-energy penetrating head trauma, but because of the unyielding conditions within the cranial cavity, even handgun bullets penetrating intracranially can create enough hydraulic pressure to cause linear fractures of the thin orbital plates, manifesting as peri-orbital haematoma. The autopsy on President Lincoln showed shattered orbits, supposedly from this mechanism.

Ballistic fractures are almost always accompanied by damage to the surrounding soft tissues, which may be augmented by bone fragmentation, especially in the skull or pelvis. Bone fragments created by high-velocity penetration are dispersed in all directions. Harvey et al. suggested that fragments driven out into the adjacent temporary cavity are forced back with the collapse of the cavity, retaining a connection with the parent bone possibly by periosteal attachments. Beyer pointed out that in battle wounds, bone fragments were not always retained in close approximation to the shattered bone, although this did not indicate their importance as wounding agents. Other experts have also stated that bone fragments do not receive enough energy to produce further wounding. However, experimental studies with high-velocity spherical projectiles have demonstrated that bone fragments are hurled forward as secondary missiles and may exit the wound in the direction of the bullet. Contact with bone may also cause the projectile to tumble, deform, or fragment, resulting in further soft tissue injury. Depending on the angle of impact and the projectile velocity, the bullet can ricochet off the bone surface and follow an altered trajectory at reduced velocity.

General features of maxillofacial ballistic injuries

Maxillofacial firearm-related injuries are customarily classified as either penetrating or perforating ; each of these categories is determined by the terminal location of the projectile and its wounding effects. Penetrating wounds are caused by missiles of low impact velocity, such as handgun bullets, with a small point of entry leading to the missile embedded in tissue. Perforating wounds are typically produced by higher velocity bullets, which create an exit wound that is often larger than the entrance.

A third category is the avulsive or ablative injuries, characterized by significant bone and soft tissue loss. These are caused either by close-range shotgun blasts, with avulsion created by multiple pellets close to each other, or by high-velocity rifle bullets which may produce massive exit wounds as a result of bullet tumbling, bone fragmentation, or both ; in the latter case, the avulsive wound may be considered as part of a perforating injury.

Maxillofacial firearm injuries vary in their clinical presentation depending on the anatomical structures involved. In the upper face, injury to the orbital or cranial contents is the overriding concern, whereas in the lower face, damage to the intraoral lining almost invariably complicates ballistic fractures. Mandibular injuries often result in bone comminution, occurring with little relation to the projectile calibre or velocity, particularly in the anterior mandible, which is supported by little soft tissue envelope and behaves like a contoured long bone. Contrary to previous models viewing an articulated long bone at the moment of bullet impact as a beam loaded in bending, Kieser et al. have suggested that the projectile does not deform the bone sufficiently to create opposite areas of tension and compression preceding non-ballistic fractures, because of the enormous forces exerted over a small area at a high rate. This seems also appropriate to the mandibular body, representing a fundamental difference from blunt trauma. Tangential bullet trajectories through the anterior mandible can cause avulsive injuries. The bones of the midface are also prone to comminution due to their thin construction and honeycomb pattern, but because of these very characteristics they are capable of absorbing limited amounts of energy and the resultant fractures are generally less severe than those affecting the mandible. Despite the potentially destructive effect of ballistic forces, between 15% and 40% of facial wounds involve only soft tissue.

General features of maxillofacial ballistic injuries

Maxillofacial firearm-related injuries are customarily classified as either penetrating or perforating ; each of these categories is determined by the terminal location of the projectile and its wounding effects. Penetrating wounds are caused by missiles of low impact velocity, such as handgun bullets, with a small point of entry leading to the missile embedded in tissue. Perforating wounds are typically produced by higher velocity bullets, which create an exit wound that is often larger than the entrance.

A third category is the avulsive or ablative injuries, characterized by significant bone and soft tissue loss. These are caused either by close-range shotgun blasts, with avulsion created by multiple pellets close to each other, or by high-velocity rifle bullets which may produce massive exit wounds as a result of bullet tumbling, bone fragmentation, or both ; in the latter case, the avulsive wound may be considered as part of a perforating injury.

Maxillofacial firearm injuries vary in their clinical presentation depending on the anatomical structures involved. In the upper face, injury to the orbital or cranial contents is the overriding concern, whereas in the lower face, damage to the intraoral lining almost invariably complicates ballistic fractures. Mandibular injuries often result in bone comminution, occurring with little relation to the projectile calibre or velocity, particularly in the anterior mandible, which is supported by little soft tissue envelope and behaves like a contoured long bone. Contrary to previous models viewing an articulated long bone at the moment of bullet impact as a beam loaded in bending, Kieser et al. have suggested that the projectile does not deform the bone sufficiently to create opposite areas of tension and compression preceding non-ballistic fractures, because of the enormous forces exerted over a small area at a high rate. This seems also appropriate to the mandibular body, representing a fundamental difference from blunt trauma. Tangential bullet trajectories through the anterior mandible can cause avulsive injuries. The bones of the midface are also prone to comminution due to their thin construction and honeycomb pattern, but because of these very characteristics they are capable of absorbing limited amounts of energy and the resultant fractures are generally less severe than those affecting the mandible. Despite the potentially destructive effect of ballistic forces, between 15% and 40% of facial wounds involve only soft tissue.

Gunshot injury patterns

Gunshot wounds to the face, previously classified as ‘low-velocity’ or ‘high-velocity’, are now categorized according to the energy transfer characteristics along the missile path, which correlate with the magnitude of tissue injury and tissue loss ( Table 1 ). Injuries involving low energy transfer typically cause non-avulsive, penetrating or perforating wounds, usually with some comminution at the point of bone penetration. High-energy ballistic injuries, commonly produced by rifle bullets, are recognized by their extensive, often avulsive nature, involving hard and soft tissues, although after re-approximation of the wound edges they may prove to have little actual soft tissue loss. Their distinctive feature, however, is the extent of non-viable tissue, which may be greater than first apparent, as a result of the damage produced beyond the grossly evident wound. This constitutes the zone of injury, which is an area of evolving tissue deterioration resulting from inflammation and disturbed blood circulation. Above all, potentially lethal or disabling effects depend more upon the anatomical track of the missile rather than its energy transfer characteristics.

Table 1
General features of maxillofacial gunshot and close-range shotgun wounds according to energy transfer characteristics.
Wound characteristics Gunshot (bullet) wounds Shotgun (pellet) wounds
Low-energy High-energy
Type of injury Usually penetrating, with limited bone comminution Usually perforating, potentially avulsive; associated with grossly comminuted fractures Avulsive (penetrating or perforating); associated with grossly comminuted fractures
Tissue loss Little Variable Massive
Zone of injury Limited Extensive Extensive

Clark et al. analyzed 250 facial gunshot wounds treated during two periods, based on the predicted bullet path, with respect to the four anatomical units of the facial skeleton, namely the mandible, lower midface with tooth-bearing portions of the jaws, orbital region, and frontal cranium. The frontal cranium was involved in the majority of cases attributed to a large number of assaults. Extensive injuries involving multiple anatomical areas of the face, comprising 43 cases (24%) of the more recent subgroup in the above study, did not correspond to these anatomical patterns. Multiple involvement is a common feature among high-energy gunshot injuries.

In several other studies, the classical division into upper, middle, and lower facial thirds is widely used for describing the location of gunshot wounds to the face, but there is a tendency to exclude those injuries primarily affecting the frontal region as representing intracranial injuries. A more detailed method of classification proposed by Bénateau et al., distinguishes central from lateral facial areas, each divided into lower (mandible), middle (maxilla), and upper units ( Fig. 1 ).

Fig. 1
Classification scheme for maxillofacial ballistic injuries, as described by Bénateau et al. M: medial face; L: lateral face; 1: mandible; 2: lower midface (maxilla); 3: upper midface (orbits and nasoethmoidal complex). Injuries are designated by their subunit components, each recorded as medial (M) or lateral–right (R) or left (L), followed by the number indicating the level.

The site of entry and the estimated trajectory of the projectile allow a general impression of the structures disrupted and may predict, with a certain safety margin, possible life-threatening complications, although the unpredictable path of bullets due to the ricocheting effect of bone surfaces reduces the diagnostic value of classifications based on such features. In a retrospective review of 100 patients, Dolin et al. demonstrated transfacial bullet trajectories in three entry zones, lateral and posterior face (A), anterior midface (B), and anterior mandible (C), and found that the need for airway control, commonly with orotracheal intubation, most often arose with trajectories through zones B and C. The need for tracheostomy in the acute setting is an uncommon scenario, affecting 8–13% of the patients in recent studies. However, among 75 surviving firearm victims reviewed by Hollier et al., 16 required tracheostomy, all of whom had suffered injuries to the lower third of the face. High-energy rifle injuries of the mandible are especially likely to cause significant airway compromise requiring intervention. Angiographic evaluation for possible major vascular injury may also be indicated depending on the path of the projectile.

Anterior face

Severe gunshot and shotgun injuries of the face are commonly related to suicide attempts, with the gun fired under the chin while the neck is often hyper-extended, resulting in a non-lethal wound. When applied in this manner, the weapon produces a typical pattern of injury involving the midportion of the mandible, with a variable extent into the central or lateral midfacial region. Bullets that deform or fragment early by impact with the dense mandibular bone, produce greater tissue disruption. With a more vertically directed aim, penetration of the frontal sinus or anterior skull base occurs. When this type of injury is produced by a rifle bullet, a large exit wound is created ( Fig. 2 ). On the other hand, it is not unusual to find a low-velocity bullet retained subcutaneously at its point of exit through the frontal bone, owing to the toughness and elastic properties of the scalp. Alternatively, the muzzle may be inserted into the mouth, sparing the mandible.

Fig. 2
Reformatted three-dimensional computed tomography (CT) image (postoperative) of a self-inflicted craniofacial injury from a military rifle bullet treated 15 years ago. The high-energy nature of the injury is indicated by the extent of frontal bone debridement performed by the neurosurgical team around the exit site, as well as by the associated fracture in the orbito-nasoethmoidal complex and lower midface. A concomitant large palatal defect, which was intractable to initial attempts at closing, was eventually closed with a tongue flap (courtesy of I. Michaelides, DMD, COL, Dental Corps, Hellenic Army).

When a handgun is fired submentally in hard contact against the skin, the propellant gases expelled from the muzzle expand within the tissues, creating an explosive effect which may resemble in severity wounds from rifles and shotguns. Massive injuries of this type, which are more common with high-power loadings of handgun cartridges, may involve destruction of the chin and lower lip, and loss of the symphysis. Although the damage inflicted indicates a high-energy injury, it is not the result of the striking energy of the projectile.

Lateral and posterior mandible

Low-velocity bullets penetrating the lower face become rapidly destabilized upon encounter with the mandible and can easily be retained within the tissues. The resultant fracture may be accompanied by more extensive soft tissue trauma than expected, resulting from bullet tumbling and also bone and tooth fragments driven deeply into the floor of the mouth and tongue. Injuries to these areas cause significant bleeding and can evolve into gross swelling and haematoma, with an immediate threat for the airway.

In the case of high-energy fractures, extensive comminution is seen in addition, with splinters from shattered bone and teeth bursting outwards and creating an exit wound of explosive type. Whitlock and Kendrick reported a case of assault rifle injury with an exit wound just above the entrance of the bullet, apparently created by a piece of mandibular bone expelled in a direction opposite to that of the bullet. On the other extreme, a high-velocity bullet may traverse the soft tissues of the cheeks through the open mouth without encountering bony structures, resulting in low-energy transfer.

Midface

In large studies excluding or containing limited numbers of self-inflicted injuries, the midface rather than the lower face presents as the most frequently damaged facial area. Depending on the direction of the shot, midfacial entry sites are more likely to be associated with intracranial penetration, and because of this risk in the case of rifle injuries, only victims receiving a tangential or sideways hit usually survive. Military rifle bullets completely traverse the facial skeleton remaining largely stable throughout their path, except when hitting from a long distance.

Lower midfacial injuries involve the upper alveolus, palate, and maxillary sinuses, although the typical bilateral patterns of Le Fort fractures are unusual. In high-energy injuries, there may be wide exposure of the maxillary sinus to the outside or destruction of the nasal pyramid. Damage to the globe is most common with gunshot injuries affecting the orbital region and nasoethmoidal complex, and occasionally a bullet penetrating the orbit will exit through the contralateral temporomandibular joint (TMJ) region. Other structures that may be involved creating long-term surgical problems are the lacrimal apparatus, the facial nerve, and the parotid gland.

Mechanisms of indirect injury

The air cavities interspersed among the delicate bony structures of the midface and the absence of bulky muscles generally tend to mitigate cavitational effects produced by high-velocity missiles in this region. It should be noted, however, that the posterior floor of the mouth with the surrounding bone forms a compact area, which can effectively accommodate cavitational changes. Also, ballistic penetration into the posterior mandible is generally associated with large amounts of energy transfer compared to the midface, due to the greater overall tissue volume and density resulting in longer wound tracks. Formation of a large temporary cavity in this region has been demonstrated by Chinese researchers, produced by a 5.56-mm steel sphere, which was fired at 1500 m/s into the masseteric area of a dog. Using the same canine model, with spherical projectiles fired at 1300 m/s, they also detected associated vascular damage extending beyond the wound edges, as a result of blunt injury induced by the expanding cavity. High-velocity projectiles have been shown to cause extensive injury to the endothelial surface of blood vessels locally and at some distance from the wound, presumably as a result of the temporary cavity and stress waves produced ; endothelial injury, albeit largely reversible, appears to be an important component in the pathology of the zone of injury.

In areas where skin is firmly attached to bone, unique cavitational effects have been observed with high-velocity missile impact. In the face and head, distant and intermediate-range entrance wounds created by powerful rifle bullets may have a stellate appearance similar to that seen with contact gunshot injuries. This occurs because of the sudden expansion of a temporary cavity as the bullet encounters bone after penetration of the scalp or facial skin, which causes large tears to radiate from the initially round entrance hole, producing the resultant gaping wound. Lethal wounds of this type in the face may appear 25–30 mm in diameter. Ragsdale and Josselson, using high-speed films of experimental shots at gelatin-encased target bones, demonstrated apparently explosive decompression of an expanding temporary cavity by dissection along the proximal gelatin–bone interface. It has been suggested that the resultant stripping of the investing soft tissues around the site of the missile wound is likely to devascularize an underlying area of bone comminution.

There is convincing evidence that high-energy maxillofacial gunshot injuries may be associated with indirect brain damage. In a case reported by Treib et al., a World War II veteran presented with epileptic seizures, which reappeared after an asymptomatic period of nearly 40 years, related to a wartime facial gunshot wound. That injury had been inflicted by a powerful military pistol at close range; the bullet entered the face at the nasion without penetrating the cranial cavity, to be found lodged near the second cervical vertebra. The patient had also suffered Le Fort II and III fractures from blunt trauma. The authors suggested that the hydrodynamic effect of the bullet caused indirect damage to the brain, which presumably resulted in a slow degenerative process eventually reactivating the epileptogenic focus. In another case, a soldier seriously wounded to the face by a military rifle bullet, which passed inferior to the base of the skull, suffered traumatic brain injury (TBI) with headaches, vomiting, dizziness, and post-concussional ‘frenzies’ 1 year after the incident.

The occurrence of indirect cerebral damage in maxillofacial gunshot injuries has been studied extensively by Tan et al., using steel spheres fired at 1400 m/s and 800 m/s through the left masseteric area in two respective groups of dogs. The faster projectiles produced larger maxillofacial wounds, also associated with a significantly higher incidence of gross cerebral injury, which affected 71.7% of the animals as compared to only 7.1% of those in the lower velocity group.

In a subsequent, sophisticated study, the same group of authors investigated the mechanism of the cerebral injury using high-velocity spherical projectiles as well as assault rifle bullets targeted at the mandible and maxilla of pig heads. Based on the peak values of the acceleration forces generated, they concluded that the predominant causative mechanism for the associated cerebral damage was the strong vibration of the head due to the sustained momentary acceleration. This was supported by the more severe pathological changes in the basal regions of the brains observed previously, as these areas are subjected to shearing forces by the rough interior surface of the skull base. A relevant finding was the high peak values of acceleration particularly in the case of mandibular injuries incurred by the M16 assault rifle, not only in the direction of the shot line but also perpendicular to that ; this could be the result of the radial expansion of large temporary cavities produced by M16 rifle bullets.

Tan et al. felt that the pressure changes transmitted to the brain due to the ballistic pressure wave of the impact, as well as the bony stress sustained by the cranium, both participate to a lesser extent than the vibration mechanism in the development of cerebral damage. However, the peak values of ballistic pressure waves recorded during wounding by rifle bullets especially to the mandibular area were comparable to pressure ranges known to cause TBI in laboratory animals (approximately 103–207 kPa) . Since these recordings were obtained by pressure wave transducers inserted into the brain substance, they may actually have reflected more accurately the ensuing changes within the cranial cavity.

Banks previously noted that bullets striking the mandible may cause indirect subgingival fractures of teeth at a distance from the impact site, which he attributed to shock waves transmitted through the dense bone of the mandible. The impact to the mandible may produce a stress concentration effect, and possibly hydrodynamic pressure transmission through the periodontal space, which presumably could cause either fracture or even extrusion of a tooth ( Fig. 3 ). Stress distribution patterns following ballistic penetration of the mandibular angle have been demonstrated extending from the point of impact to the ipsilateral condylar neck and to the mandibular body anteriorly.

Jan 17, 2018 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Wound ballistics of firearm-related injuries—Part 2: Mechanisms of skeletal injury and characteristics of maxillofacial ballistic trauma
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