*Edit 10/21/16: Fixed broken link to pdf, sorry. :C
tl;dr contents:
- Material Properties of Bone
- Classifying bone trauma by time of occurrence
- Types of bone trauma
- Ballistic
- Blunt force
- Sharp force
- Thermal
I’m not gonna include a lot of pictures because this is a topic that not everyone will want to see images of. You can google stuff if you want to see examples, or read my final paper where everything is cited.
Material Properties of Bone
Bone is divided into two kinds of osseous material. Cortical bone is about 80% of the mass of the exoskeleton, and forms a hollow cylinder in which the cancellous bone resides. The main purpose of cortical is for structure, while cancellous bone is spongy interior that is responsible for nutrient storage and transport.
A cross-section of a human femur, look at how cancellous bone is a lot less dense than the cortical bone.
Injuries to bone are classified based on when the injury occurs.
Ante-mortem trauma: Refers to trauma on the skeleton that occurs prior to the death of the individual. In-vivo bone exhibits healing, and edges of a fracture will be fairly smooth. As healing rates differ depending on the characteristics of the individual, the same injury may look different from case to case.
Peri-mortem trauma: Refers to trauma occurring around death. Fracture patterns observed in peri-mortem trauma can be similar to those seen in ante-mortem trauma, but have sharper edges since no healing will have occurred (in most cases). Patterns will show evidence of some terminal event, i.e fast collision with an object, but fracture edges will show characteristics of plastic deformation as the external fibers of the bone tissue begin to show micro-tears.
Post-mortem damage: Refers to damage occurring after death. Bone becomes brittle once it has dried out, and exhibits ceramic-like material properties. Fractures will be jagged and exhibit no healing, and may also be irregular. I.E: Fractures lack a common correlation to an event that caused the damage, such as those resulting from post-mortem bone shrinkage.
Types of Bone Trauma
Ballistic
Ballistic trauma on bone is usually the result of a bullet or an explosive, though this categorization may be applied to any fast traveling object that has collided with bone. Features that indicate possible ballistic trauma include the presence of a projectile that can be associated with bone, fracture patterns corresponding to a high velocity impact, or fragmentary foreign material found within the bone or the environment.
Because of physics, entry wounds tend to be smaller than exit wounds, due to the bullet losing energy as it travels within the body. Because of the varying environment inside the body, it’s difficult to predict how a bullet may travel since it may bounce off of bones or move differently due to varying densities of tissue inside the body. Thus, while there will always be an entrance wound in cases of ballistic trauma, the presence or number of exit wounds will heavily depend on the situation.
Currently, CT scanning is heavily used in this realm, since it works well on both hard and soft tissue and can thus preserve the body in a virtual state.
Blunt force
While ballistic trauma can be kind of classified as blunt force trauma, here, blunt force trauma will refer to trauma resulting from a low velocity impact from a blunt object or blunt surface. Features suggesting blunt force trauma include plastic deformation, delamination, tool marks/impressions at trauma site, or fracture patterns on the bone suggesting low velocity impact. This is because during low velocity impact, the bone will have more time to bend and deform in response to the stress before it fails.
Different kinds of applied force will exhibit itself characteristic fracture patterns as well. Knowledge of these patterns can provide additional information about the events leading up to the fracture. That’s why it’s valuable to have a fully modeled human skeleton so that anthropologists will have immediate predictions as to forces necessary to produce fracture patterns.
Currently, computational human skeleton models are not robust enough to act as a substitute to real bone, though the area is very much in active research. Researchers have also experimented with nonhuman animal models, mostly focusing on porcine and deer due to similarities with the human skeleton.
Sharp force
Sharp force trauma is characterized by trauma resulting from a tool that is edged, pointed, or beveled. Features indicating this kind of trauma include straight-lined incisions, punctures or gouges, clefs or kerfs. Considerations must be taken when examining the wound, since alterations to the bone like scrapes or score marks are not classified as sharp force trauma.
Evidence of sharp force trauma is distinctive because the tool will leave a mark that can be readily differentiated from stress based fractures. It is possible to estimate the motion of the tool based on whether it is a slash (the incision is wider than it is deep), or a stab (the incision is deeper than it is wide) through visual inspection. The handedness of an attacker, or whether a wound is self inflicted can be determined by the angle of the incision in context to the position of the body.
Micro-CT scanning allows investigators to closely examine the striations left in soft tissue or bone from the knife, allowing them to characterize with greater accuracy the properties of the knife and the traumatic incident. Scanning is also a great way of non-destructively examining the wound, prior to this technology, investigations would have cut small slices of bone/tissue to examine under a microscope.
Thermal
Thermal trauma is the result of bone that is in direct contact with high temperatures for a significant amount of time. Characteristics of bone with thermal trauma include shrinkage, discoloration, calcification, or an observable burn pattern on the surface of the bone. As soft tissue is often destroyed in cases of thermal trauma, analysis largely focuses on marks left on the bone instead.
Bone that has undergone thermal trauma is typically in a fragmentary state, making retrieval difficult. Establishing a minimum number of individuals is hard as well, thanks to possible co-mingling that may occur at the location of the fire. Bones exposed to high temperatures will shrink due to the rapid evaporation of water, though the amount of shrinking will depend on a variety of internal (age, sex, height) and external (fire temperature, oxygen content) factors. Shrinkage begins at temperatures of 700C, with augmented degrees of shrinkage until 800C when no more shrinkage occurs. Bone also undergoes structural changes during the temperature ramp, with organic material like cellulose being destroyed at 400C, recrystallization of the bone mineral at 600C, and melting of bone mineral at 1600C.
Because we have such exact knowledge of the material properties of bone, investigators have an idea of the temperature of the thermal event, and if injuries observed on the body occurred before or after the thermal trauma. A study by Herrman concluded that even with simple optical microscope analysis, it was possible to identify post-mortem sharp force trauma following incineration of bone as the evidence left on bone is distinctive enough to be recognizable even after fragmentation has occurred. Fractures resulting from blunt force or ballistic trauma required rigorous examination at high magnifications (100x) because the resulting fractures were quite similar to changes that may occur to bone during exposure to high temperatures. However, they concluded that large fragmentary bone material were more likely to be due to post-mortem trauma, while small fragments were more likely to be due to flaking of the bone from the fire.
Conclusion
The forensic anthropologist works in the space of probabilities, so everything as described can only best be held as guidelines instead of iron clad rules. However, with modern imaging technologies and advancements in the field, the forensic community as a whole has seen tremendous increases in ability to solve crimes and bring closure to grieving families. I think the work that they do is incredible, and they are deserving of much respect. It is most definitely work that I would not be able to do.
My full paper is here if you’d like to read it. I learned a great deal in my forensics class, and I’m glad that I took it. I hope you learned something too!
-Sophie