Understanding how bone breaks and analysing fracture patterns are important principals of orthopedics because it alludes the clinician to the areas of maximal trauma and associated injuries as well as allowing the clinician to predict the areas of residual weakness which determines the optimal management of the fracture.
Ballistic fractures are relatively common injuries with unique fracture patterns, but the mechanism of their formation remains unknown. Previous studies have focused on analyzing the fractures sustained from higher velocity projectile injuries (minimum velocity 200 ft/s) to theorize how fractures develop in direct ballistic injuries (
4, 6). However, this approach involves analyzing the patterns of complete and often comminuted fractures to predict how the fracture started, rather than analyzing slower range velocities to include the critical range of injuries from no permanent skeletal injury to complete fracture. To our knowledge, this is the first study utilizing this slower range of projectile velocities (10-200 ft/s) to analyze the initiation and propagation of direct ballistic fractures.
This study employed a reductionist approach to this complex injury by using slow-velocity, spherical, non-deforming, steel projectiles to analyze the initiation and propagation of ballistic fractures. This was to limit the number of variables, particularly projectile deformation, and reproduce impact surface areas with multiple samples. The resultant fractures show constant repeatability between sample groups and samples impacted at similar pre-impact velocities (
Table 1). The fractures produced in this experiment were consistent with those previously reported in higher velocity injuries ( 4- 6) and therefore, the extrapolation of these observations to higher velocity projectiles may help to explain the mechanism of injury in ballistic fractures.
There appears to be a critical impact kinetic energy required to initiate fracture of bone with the 9 mm diameter steel projectile used. At very low kinetic energies (< 0.21 J), the projectile may temporarily indent the cortex, however, this study design can only assess the permanent damage rather than dynamic changes in the bone. The presence of the thin soft periosteum may well have cushioned the initial impact minimizing the formation of permanent deformation. As the pre-impact energy increases (0.21-1.08 J) the periosteum is observed as having been cut and the bone is progressively indented and permanently compressed, but not fractured. This indentation depth increases with increased projectile velocity.
Beyond a critical energy a fracture cascade is initiated. The rather coarse pixel size (17 um) of the micro-CT limits the resolution with which the 3D pattern of the fracture events can be clearly resolved. For impact kinetic energy of 1.08 J the micro-CT images show the presence of a cone-like crack that initiates at the edge of the contact area and extends into the bone. The extent of this is more evident parallel to the axis of the bone. With increasing pre-impact energy, the depth of the residual impression and the extent of the radial cracks become more readily evident. The crack is more readily seen as it is more open than many of the other cracks. This occurs because the plastic deformation associated with the permanent impression generated by the impact is now wedging open the cracks. Initially a small longitudinal fracture occurs (1.08 J) and then enlarges with increasing velocity to form part of the classic double butterfly fracture (1.34 J). These fractures initiate from the impact site and are almost parallel to the bone axis before extending at approximately 45o to the long axis of the bone. Prior to these fractures extending as far as the opposite side to the bone fracture develops beneath the impact site oriented at 90° to the projectiles trajectory. At the contact site there are multiple cone cracks that develop, as observed by Knight et al. (
14) for glass, as well as axial cracks initiating from the internal surface of the bone as it is pressed into the subsurface cavity space. This combination of cone and axial cracks results in the comminution of the underlying bone at the point of impact ( Figure 7).
Figure 7. The Cone Crack (Arrowed) Fracture Pattern and Additional Subsurface Comminution as Observed Beneath the Impact site at Impact Velocities Slightly Above the Threshold for Crack Initiation
The red zone is the plastic or permanently deformed bone beneath the impact site.
With increased energy the superior and inferior butterfly fractures of each side extend to the opposite side of the bone (1.72 J), ultimately coming in contact with one another to form a longitudinal fracture on the opposite cortex, completing the double butterfly fracture pattern.
The first evidence of periosteal damage in our study was a longitudinal split mirroring the underlying longitudinal fracture. This occurred at the same critical energy required for fracture formation (1.08 J). With increased kinetic energy the periosteum was seen to progress from a single split to a stellate tear. It is likely however, that periosteal bruising will be seen below the threshold velocity for fracture, but our model of non-living, non-perfused tissue prevents such observations.
Table 1. Summary of the Relationship Between Pre-impact Kinetic Energy and the Resultant Bone and Periosteal Injury
Preimpact Kinetic Energy, J Effect Bone Periosteum < 0.21 Non-permanent injury Non-permanent injury 0.21-1.08 Indentation Linear split 1.08-1.34 Longitudinal split Stellate tear 1.34-1.72 Double butterfly and cone crack initiates Progressive enlargement of stellate tear > 1.72 Fractures extend to opposite side of bone Progressive enlargement of stellate tear
This proposed cascade of fracture initiation and propagation is consistent with observations of fracture events of brittle materials (
14, 15). In both the latter studies the extent of the cracking and comminution increased with pre-impact energy. These observations are also the precursors to the observations suggested by Harger et al. who proposed, that as the projectile penetrates the bone it expands the wound tract at high velocity, forming ‘shock waves’ which magnify the damage far beyond the simple drilling effect ( 16). We do suggest that in the current study, the role of shock waves is minimal and rather the expanding plug of bone that is displaced and extensively comminuted is a result of multiple cone cracks and the radial and vertical displacement of the bone beneath the impact site. Additionally, the fractures seen closely mirror those observed in higher velocity projectile injuries ( 4- 6).
Previous reports have suggested that a forced flexion occurs on the opposite side to impact when the projectile impacts the bone, resulting in indirect failure from tension forces on that side of the bone (
4) ( Figure 8). Accepting that bone is weaker in tension than compression, one would therefore expect to see a transverse fracture pattern commencing on the opposite side to impact ( 4). In this study, no such fracture was identified. We therefore believe that the projectile itself does not deform the bone sufficiently for tension fracture on the opposite side of the bone to occur, unless it occurs by retardation of the temporary cavity in higher velocity injuries.
Figure 8. The Previously Proposed Double Butterfly Fracture Mechanism
Similarly, as the sample without bone marrow fractured in a similar fashion to the whole bone samples, we believe it unlikely that pressure within the marrow cavity is responsible for the formation of the observed fracture patterns. This contrasts with the view of Sellier et al. who discusses increased intra-medullary pressure as a mechanism for ballistic fracture (
As this study examined only low velocity projectiles, it is limited in its ability to assess the effect of the temporary cavity and violence of higher velocity injuries, which may impart different forces on the bone (
17). However, due to the remarkable similarity between the fracture patterns seen in high velocity ballistic trauma and those observed in this study, it is likely that fractures produced by high velocity ballistic impacts follow a similar fracture cascade. Further research should extend to include higher velocity projectiles into samples with the soft tissue envelope preserved to confirm this likelihood.
Understanding this cascade will not only allow the clinician to better predict the severity of injury in lower velocity projectile trauma, but also, in higher velocity projectile trauma, allow the clinician to predict how the fracture fragments arose and what happened to them during the insult to better understand the dissipation of energy and optimal skeletal management.
This study presents only the pre-impact kinetic energies rather than the energy transfer. While it can be noted that the energy transfer will be less than these reported, because no projectiles became embedded in the bone, the exact energy transfer was not determined. Pre-impact kinetic energy has been used as an indication of a projectiles potential to cause damage (
18) and because the energy values we were studying were so low (0.013-5.35 J) we used these values to assess boney injury. However, energy transfer is more accurate at denoting the injury severity ( 19) and as such future studies should aim to use energy transfer as opposed to pre-impact kinetic energy. Furthermore, a better understanding of the dynamic response could be achieved with the application of biosensors on the samples; further research should consider this modality.
To our knowledge, this is also the first study to utilize micro-CT to analyze fracture in ballistic skeletal injuries. This study has demonstrated the value of micro-CT as a powerful tool for imaging the bullet impact site. It was found to offer a more accurate measure of the resultant fracture morphology than the conventional basic fuchsin staining process, where artefactual damage from the dehydration, sawing and grinding process are likely and difficult to differentiate from injury incurred during impact (
Conclusions: Low velocity projectile impact fractures of the anterior mid-diaphysis of the femur follow a reproducible fracture cascade from indentation to the initiation of cone cracks followed by the development of radial cracks that propagate to form butterfly fractures. The development of radial fractures, along with subsurface tensional damage below the impact site, result in the extensive comminution observed. At much higher impact velocities shock wave effects may have made an additional contribution to the observed damage.