Trauma

Stress is the force applied to a bone, strain is the bone’s response to stress. This strain can be divided into parts, elastic and plastic deformation. In elastic deformation, the bone is able to bend in response to an applied force, yet when that force is removed it returns back to its original shape. However, at some point due to the intrinsic properties of that bone, if force is not removed, a yield point will be reached in which a bone is so weakened from bending that it starts to permanently deform its shape. This is known as plastic deformation. Without removal of the force, the bone will continue to weaken until its failure point is reached, it can withstand no more force, and it fractures. 


Material properties of bone: 

Bone is heterogeneous, anisotropic and viscoelastic. 

  • The heterogeneity of bone means that it appears in different sizes and shapes throughout the body, which means that bone reacts differently to stress based on these factors. 

  • The anisotropy of bone means that it will react differently based on the direction and location that the load is applied

  • The viscoelasticity of bone has to do with how ductile the material is. Because bone is composed of both organic and inorganic components, it has elasticity and strength. This means that with a slow loaded force, bone will bend before it breaks. This also means that wet versus dry bone will respond differently to force because the viscoelasticity is different between them. 


Structural properties of bone: 

  • Cross sectional shape of bone will determine where bone is most likely to break based on the direction of the force

  • The ratio of cortical bone to trabecular bone along with the shape of that bone will determine how force is dissipated through the bone. Trabecular bone is able to absorb and dissipate force more evenly than cortical bone because of the structure of the bone, being spongy and porous whereas cortical bone is compact and doesn’t have the open spaces to dissipate force. Cortical bone is better at absorbing transverse loads due to its ability to bend. 

Shape matters! The shape of the bone will determine how it responds to force. For example cylindrical shaped bones like your metacarpals and phalanges are able to resist torsional (twisting) forces the best. Square shaped bones like the carpals are able to resist bending forces the best. Tubular shapes like our long bones are good at resisting both torsional and bending forces due to their medullary cavity that helps dissipate force along with a strong cortical covering that provides strength and rigidity. 


Composition also matters. The inorganic component of bone, hydroxyapatite, comprises 60% of cortical bone. This material is good at resisting compressive forces, contributing to the fact that bone tends to break in tension first. 


Blunt Force Trauma: Wide or narrow area of impact, elastic and plastic deformation, breaks in tension and compression, leaves radiating and concentric fractures. From BFT we can say the MNI, the order of impacts, the type of fracture and the direction of impact. In plastic deformation seen with BFT to the cranium, the bone breaks in a weeble-wobble pattern. The force of the impact causes the bone to break in tension and compression, which causes those forces to act on the areas of bone next to the AOI. In determining order of fractures, Puppe’s law states that fracture lines terminate into pre-existing fracture lines. 


In BFT to long bones, a wedge fracture is the most characteristic though bone does not always break in this way. When it does however, as the bone moves through tension to shear to compression, there appears a straight edge break at tension. Moving from shear to compression you see a dog ear pattern. 



Sharp Force Trauma


AO/OTA looks at the bigger picture, telling us fracture classification used by medical professionals, such as greenstick, transverse, or comminuted. Fractography looks at fracture surface morphology and can tell us the area of the bone in tension or compression and the direction of the load. The nuances in fractography include identifying the areas of bone mirror arrest ridges, bone hackle, and cantilever curl. Bone mirror is the site of fracture initiation which is where the bone broke in tension. Arrest ridges are opposite of the bone mirror and are raised ridges and valleys that run perpendicular to the direction of impact. This is where the bone broke in compression. The bone hackle is the angular or rounded ridges aligned in the direction of propagation from the bone mirror to the arrest ridges. It looks like little termite paths moving towards compression. The cantilever curl is the final ridge on the compressive side of the bone before complete fracture. 


Characteristics of knives

  • Blade bevel: single or double

  • Edge bevel: single or double

  • Serrated vs nonserrated

  • Action of tool: stabbing, sawing, chopping, slashing

  • Blade dimensions

  • Side of edge bevel

  • Direction and orientation of wounds

  • Depth of penetration

In cuts, knives produce V shaped kerf floors with a breakaway spur or area of lipping on the side of the edge bevel. Knives produce thin, incised marks on the bone without fracturing bone and include weapons such as macheters, box cutters, razor blades and axes. If a serrated knife is used, the teeth will be on the side of the cut mark that has stair stepped striations whereas the straight beveled edge will have smoother undulating striations. A knife cut wound is produced by a sawing motion across bone and is often associated with these serrated knives. Puncture or gouging wounds are a penetrating wound from the end of a sharp object that impacts bone and is caused by compression of the outer table. 


Chopping wounds are technically SFT with characteristics of BFT. These wounds are caused by a heavy object with a sharp edge that does result in the fracturing of bone. With chopping trauma you will have squared off edges and blunt trauma in the middle of the incised wound. 


Characteristics of saws: 

  • Saw size: teeth or points per inch

  • Saw set: alternating, wavy or raker

  • Saw shape: rip or crosscut

  • Saw power; hand or power

  • Direction of saw motion

The objective when looking at saw mark analysis is to document the characteristics evident of the kerf walls and kerf floor to find the class characteristics of the saw. In a saw cut you will have a kerf floor, kerf wall, false starts, and breakaway spur. The breakaway spur can tell you the direction that the saw was being used in. In a hand saw, you will actually have fewer false starts than with a power saw, which will have more aggressive and deep false starts. In addition, the consistency of striations will be more irregular in a hand saw than with a power saw. 


You can characterize the teeth per inch or points per inch of a saw. Rip saws have a cutting edge on each tooth while crosscut saws have alternating cutting edges on every other tooth. Because of this, rip saws produce a characteristic U or square shaped kerf floor while crosscut saws produce a W shaped kerf floor. The saw set refers to the direction of the teeth, and you can have alternating, wavy or raker sets. 


In ballistic trauma, bone acts like glass. It is caused by a rapid loaded narrow focus of entrance impact in which the elastic component of bone immediately absorbs the kinetic energy, is unable to compensate, and immediately turns rigid and shatters without any deformation. The fracture morphology of GSWs are plug and spall wounds. 


An entrance wound will have internal beveling due to the bullet pushing all of the bone towards the inside. The exit defect will often be larger and more irregular with external beveling. In this type of trauma to the cranium, we see heaving concentric fractures due to the intracranial pressure causing all concentric fractures to fail externally. There is no weeble-wobble pattern like seen in BFT. However, if the bullet exits through the face then this pattern is not observed as the whole face will essentially blow apart due to the thin bones of this region. 


Other characteristics of fracture patterns are: there are more radiating fractures at the entrance than the exit and entrance radiating fractures can travel faster than the bullet. This means they can reach the other side of the cranium before the bullet physically exits that side, causing the bone to weaken prior to the exit wound appearing. 


Keyhole fractures are a type of entrance wound in which the bullet strikes tangentially at a shallow angle. This causes the outer table to be lifted off causing external beveling. The bullet enters through the round part of the keyhole defect. 


Intraoral GSWs behave uniquely due to the intraoral pressure that causes a compression fracture on the outside of the mandible, called a collusion fracture. This pressure leads to abnormal beginning of the ascending rami outwards, causing compression at the mental eminence, like breaking a wishbone. 


In GSWs of long bones, there may never be an exit defect. This could be due to the velocity of the bullet not being enough to fully penetrate the bone or due to the location of the GSW. There are less fractures observed when a bullet hits the cortical bone near the epiphyses. On a long bone we see an X marks the spot pattern. 



Thermal Alterations and trauma

There are three diagnostics of thermal trauma: body position, color change, and heat induced fracturing. Body position can be inferred due to tissue shielding. In a normal burn situation, the body enters into pugilistic posture due to the moisture content causing flexor muscles to contract. Because of this posture, we expect certain areas of the body to burn first and certain areas to be shielded by joint articulations or other tissue shielding. 


We can identify these burn patterns by the color changes of burned bone. Bone goes from unaltered to having a heat border, to being charred and then to calcined. The heat border is an off-whitish area that is protected from direct contact by receding tissues. It is dehydrated and has experienced molecular alteration, and it will glow if backlit. The presence of this white border is indicative of a wet bone burn. Sometimes there will also be a narrow heat line at the junction between heat altered and unaltered bone. 


Charred bone is carbonized bone that has come into direct contact with heat and flames. It is still recognizable and diagnostic, yet metrics should not be taken from these bones. Calcined bone is bone that has lost all organic components and moisture due to heat and flames. It is usually highly fragmented or very fragile, it has experienced shrinkage and deformation, and it should never be measured. Bones with lots of spongy bone like the vertebrae can generally survive calcination. 


These color changes can also tell use whether a burn was abnormal or not because we can tell whether wet or dry bone was burned. In a dry bone burn, the heat border is usually brown in color and there is no heat line. We can also observe abnormal burn patterns due to the lack of tissue shielding and the body not entering int pugilistic posture. 


Thermally induced fractures include the bullseye, curved transverse, and step fractures. The curved transverse fracture is indicative of soft tissues and periosteum retracting and pulling down on the bone. The curves indicate the direction that the fire is moving. The bullseye fracture is similar to the curved transverse but it appears in joint areas with high trabecular bone and appears as concentric rings. Step fractures are specific to calcined bone failure. We expect this to occur in long bones that are the first areas to burn in a normal bone scenario. 


We expect certain locations to fracture, such as the wrist, ankle and elbow due to the nature of pugilistic posture and tissue shielding. Therefore these fractures should not be misinterpreted as other types of trauma. 


One important considering with thermal fracturing is that they are not caused by kinetic energy, therefore they cannot extend into unburned bone. Any fracture that crosses from burned into unburned bone must have occurred before the body was burned. In addition, thermal trauma actually preserves sharp force trauma and other forms of trauma unless fully calcined, though it will modify metrics. 


In BFT to the cranium with thermal fractures, we expect the BFT fracture lines to be less burned than the surrounding bone due to the brain juices escaping from the fractures and protecting the bone surface at those locations. 


Peri vs post mortem fractures

In dry bone fractures, the fracture outline will be longitudinal or perpendicular to the grain of the bone. In addition, the fracture angles will be right angled and perpendicular. In wet bone, the fracture outlines will be curved or V shaped with angled or obtuse fracture angles.