Trauma Mechanisms
This section aims to give an insight into the 4 main trauma mechanisms (blunt force, sharp force, thermal and ballistic) and how they can be distinguished from one another.
Blunt Force Trauma
Blunt force trauma injuries are those typically sustained from low-energy impacts resulting from a broad instrument delivered over a relatively large surface area. It is the result of dynamic forces such as compression, bending, and shearing forces.
Depending on the force, simple fractures or comminuted fracturing can be present. This includes radiating and concentric fractures. Resulting fracture morphology of blunt force trauma encompasses a wide variety of fracture patterns which are determined by both intrinsic and extrinsic factors.
Intrinsic factors:
Biomechanical properties of bone including morphology, density, buttressing, microstructure, age etc.
Viscoelastic properties of bone.
Extrinsic factors:
Nature of the applied force and weapon characteristics including velocity, weight, distance, object shape and loading duration.
Examples of a tool which may deliver a blunt force:
Organic tool – human fist, head butt.
Portable tool – baseball bat, brick, hammer.
Contact surface – floor, wall, stairs.
The severity, extent, and appearance of blunt force trauma injuries depends on:
The amount of force delivered to the body. Force is dependent on kinetic energy (KE=1/2MV²).
The time over which the force is delivered.
The region struck.
The extent of the body surface over which the force is delivered.
The nature of the weapon.
Depending on the force, simple fractures or comminuted fracturing can be present. This includes radiating and concentric fractures. Resulting fracture morphology of blunt force trauma encompasses a wide variety of fracture patterns which are determined by both intrinsic and extrinsic factors.
Intrinsic factors:
Biomechanical properties of bone including morphology, density, buttressing, microstructure, age etc.
Viscoelastic properties of bone.
Extrinsic factors:
Nature of the applied force and weapon characteristics including velocity, weight, distance, object shape and loading duration.
Examples of a tool which may deliver a blunt force:
Organic tool – human fist, head butt.
Portable tool – baseball bat, brick, hammer.
Contact surface – floor, wall, stairs.
The severity, extent, and appearance of blunt force trauma injuries depends on:
The amount of force delivered to the body. Force is dependent on kinetic energy (KE=1/2MV²).
The time over which the force is delivered.
The region struck.
The extent of the body surface over which the force is delivered.
The nature of the weapon.
Sharp Force Trauma
Sharp force trauma injuries are those typically sustained from low velocity impacts resulting from a narrow/sharp instrument delivered over a relatively small surface area. It is the result of narrowly focused, dynamic compression forces which lacerates or cuts through soft and hard tissues. Actions may include slashing, stabbing, cutting, sawing, and hacking.
Injury types resulting from sharp force trauma:
Puncture – Cone shaped focus perpendicular to bone surface.
Incision – Force applied over long, narrow surface area.
Cleft/notch – Dynamic force from a long, sharp-edged implement applied perpendicular to bone surface.
Examples of a tool which may deliver a sharp force:
Variety of implements with narrow focussed point or edge - e.g. knife, axe, saw, syringe.
Fracture lines (rare):
Fracture lines spread outward in radiating lines from punctures and clefts; however, concentric fractures are rare with sharp force trauma.
Multiple weapon and blade types based upon characteristics:
Cutting edge (straight, serrated etc.)
Blade mass (1/2MV²) and dimensions (length, breadth etc.)
Blade pattern (single edge, double edge etc.)
Injury types resulting from sharp force trauma:
Puncture – Cone shaped focus perpendicular to bone surface.
Incision – Force applied over long, narrow surface area.
Cleft/notch – Dynamic force from a long, sharp-edged implement applied perpendicular to bone surface.
Examples of a tool which may deliver a sharp force:
Variety of implements with narrow focussed point or edge - e.g. knife, axe, saw, syringe.
Fracture lines (rare):
Fracture lines spread outward in radiating lines from punctures and clefts; however, concentric fractures are rare with sharp force trauma.
Multiple weapon and blade types based upon characteristics:
Cutting edge (straight, serrated etc.)
Blade mass (1/2MV²) and dimensions (length, breadth etc.)
Blade pattern (single edge, double edge etc.)
Thermal/Burning Trauma
X-ray diffraction has been used to identify the key stages in the burning process sustained during thermal trauma:
Colour changes in burned bone have been described in five stages:
- Dehydration: The hydroxyl-bonds break and both the loosely bound water (physiosorbed) and bonded water (chemisorbed) are lost.
- Decomposition: The organic components of the bone are removed by pyrolysis.
- Inversion: The loss of the carbonates likely associated with the conversion of hydroxyapatite crystal structure to beta-tricalcium phosphate.
- Fusion: Characterised by the warping and coalescence of the crystal matrix.
Colour changes in burned bone have been described in five stages:
- Unaltered fresh bone (normal bone colour): Protected by soft tissue.
- Heat line (white line or translucent bone): Initial line of contact and heat destruction to bone.
- Heat border (brown to white band of variable width): Location where organic material (collagen) is permanently altered and destroyed by heat, which distinguishes it from green bone. This feature follows contours of the preceding heat line.
- Charred (black) Advanced stage of burning: Bone is thought to be directly in contact with fire and heat, hence the colour resulting from a reduction atmosphere. Complete loss of organic material and moisture, which compromises the bone structure, resulting in tensile shrinkage fractures that run both parallel and perpendicular to the heat border.
- Calcined (grey to white): Post organic destruction and modification of bone mineral content (crystallization of hydroxyapatite in bone). Structures exhibit deformation and distortion along with heat-induced fractures and shrinkage.
Ballistic Trauma
Ballistic trauma is the result of the transfer of kinetic energy from the projectile. Severity of injury is dependant on projectile characteristics and tissue characteristics. Common fracture patterns include radiating, concentric and fragmentation. Ballistic trauma is the result of a compressive force, including bending. It is the result of dynamic loading at high velocity. This can result in catastrophic bone shattering where the rate of energy transfer between the projectile and the bone is such that the bone will behave in a brittle fashion and pass straight through to failure point, rather than going through the normal elastic/plastic deformation phases.
The effect of a bullet on a bone can be considered in four distinct categories:
Wound bevelling, Wound shape, Wound size, and Fracture lines.
Types of ballistic wounds include:
Penetrating = has an entry wound, but no exit wound.
Perforating = has an entry wound and an exit wound.
Primary Fractures:
Plug and spall produced by the penetration of the projectile through the bone. Plug formation seen when thick bone fails in shear due to large radial tensile forces. Spalling produces the characteristic internal bevel. The result is a cone shaped defect.
Secondary Fractures:
Radial fractures at the point of origin of impact may form very quickly. The material fails in tension and large pie-shaped fragments are produced by the radial fracture lines.
The effect of a bullet on a bone can be considered in four distinct categories:
Wound bevelling, Wound shape, Wound size, and Fracture lines.
Types of ballistic wounds include:
Penetrating = has an entry wound, but no exit wound.
Perforating = has an entry wound and an exit wound.
Primary Fractures:
Plug and spall produced by the penetration of the projectile through the bone. Plug formation seen when thick bone fails in shear due to large radial tensile forces. Spalling produces the characteristic internal bevel. The result is a cone shaped defect.
Secondary Fractures:
Radial fractures at the point of origin of impact may form very quickly. The material fails in tension and large pie-shaped fragments are produced by the radial fracture lines.