ASM 275 UNIT 3 LABS. Skeletal Trauma and Timing of Bone Injury (Lab 12)
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Course
ASM 275
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ASM 275
Learning Goals
By the end of this lab, the student will be able to:
· Identify key aspects of the anatomy of the human pelvis.
· Assess the sex of a decedent using features of pelvic anatomy.
· Apply ordinal data standards to a forensic problem.
· Describe the range of human variation in t...
asm 275 unit 3 labs skeletal trauma and timing of bone injury lab 12
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Skeletal Trauma and Timing of Bone Injury (Lab 12)
Learning Goals
By the end of this lab, the student will be able to:
· Identify key aspects of the anatomy of the human pelvis.
· Assess the sex of a decedent using features of pelvic anatomy.
· Apply ordinal data standards to a forensic problem.
· Describe the range of human variation in the human pelvis.
· Apply trait weighting standards to more ambiguous cases.
Introduction
Trauma is defined as an injury or disruption caused to living tissue by any external force. It was only recently that
forensic anthropologists began to consult on the interpretation of trauma visible on the skeletons of victims of violent
crimes. However, this aspect of the forensic analysis has become increasingly important over the last few decades.
With improved understanding of fracture formation processes and a growing number of experimental studies
documenting how fractures propagate in the skeleton it is now expected that the forensic anthropologist provides a
description of trauma during their investigation. The goal is to provide an accurate and complete description of the
wounds, but not necessarily to offer a specific interpretation that may overreach beyond what the data can support in
a court of law. The Daubert standards (lab 2) must always be considered.
The ability to recognize and interpret trauma-related signatures contributes to the death investigation in several ways.
First, trauma analysis can provide information that is useful to the medical examiner for declaring the manner of death
(e.g., homicide, suicide, accidental). Second, trauma analysis can help identify the nature of the force that caused the
injury, which may lead to the murder weapon. Third, by identifying the number of wounds and their relationship to
each other, the investigator can determine whether one or more implements was used, which possibly speaks to the
number of individuals involved in committing a crime. Fourth, ascertaining the sequence of injuries speaks to the
circumstances of the crime scene and addresses the fundamental question - what happened? Fifth, trauma analysis
can be used to reconstruct the spatial relationship between the attacker(s) and the decedent, which can corroborate
or falsify suspect or witness testimony. And sixth, the ability to recognize healed trauma, i.e., trauma not directly
related to the moment of death, can provide information on the decedent's medical history that could be used to
positively identify them. Repeated, healed or healing injuries may also speak to patterns of domestic violence and
child abuse.
Forensic anthropologists classify trauma with respect to three broad classes of weapons: projectile gunshots
(handguns), blunt force objects (bats), and sharp force objects (knives). However, these broad categories may fail to
categorize all traumatic injuries. For example, a bullet that impacts the skull after losing much of its velocity will create
a wound more similar to blunt force trauma than a typical penetrating gunshot wound. For this reason, forensic
anthropologists also recognize the importance of velocity in determining the extent of a traumatic injury. Two broad
classes of weapons are identified: high velocity projectiles and low velocity objects.
The area of direct impact also matters. Sharp force trauma physically impacts the body with low velocity over a very
small area, thus making a cut in the bone but not shattering it. Blunt force trauma physically impacts the body with
low velocity over a relatively wide area, often shattering the bone but not penetrating it. High velocity projectile trauma
often combines penetrating wounds (bullet holes) and more significant destruction of the surrounding tissues.
Whether a bone fractures when put under stress depends on the energy it must absorb from the foreign object.
Objects moving with higher velocity impart more energy onto the skeletal tissues, resulting in more damage.
Identifying the nature of the force (projectile, blunt force, sharp force) impacting the body is one of the primary goals
of the forensic analysis.
In addition to determining the type of force impacting the body the forensic anthropologist is also interested in
determining the timing of impact with respect to the individual's death. Here again, three timelines are typically
discussed: antemortem trauma is that which occurred prior to death, perimortem trauma is that which occurred
around the time of death, and a postmortem alteration is bone damage that occurred after the time of death. These
are not clearly distinguishable categories (as discussed below) and it is often very difficult to determine whether a
wound is perimortem or postmortem.
Bone Biology and the Biomechanics of Bone Fracture
The microscopic structure of bone was discussed previously in lab 3. However, these details are directly relevant for
,understanding how a bone fractures and a brief review is warranted. Bone is a composite material, being comprised
of an organic component and an inorganic component. The organic component mostly consists of collagen fibers, a
connective tissue that makes bones elastic and pliable. The inorganic component consists mostly of calcium
phosphate salts (hydroxyapatite) that are crystalline in structure and embedded along the collagen fibers.
Hydroxyapatite gives bones rigidity. In healthy adults, the collagen fibers are oriented parallel to the shaft of a long
bone, which creates structural rigidity. This is called lamellar bone (see lab 3). Bone that is diseased is composed of
woven bone, in which the collagen fibers are oriented haphazardly (Figure 2). The initial healing of a fracture also
uses woven bone to stabilize the break.
Bone disease in which new bone has been laid down in a quick and disorganized manner. [2]
Although all bone is composed of collagen and hydroxyapatite this fact alone does not explain or predict how any
specific traumatic impact will result in a fracture. Bone is heterogeneous, meaning the bones in the body vary
considerably in shape, thickness and the distribution of cortical and trabecular bone (see lab 3 and 4). As described
by Symes et al. (2012), bone is also anisotropic, which means it responds to external forces differently depending
on the direction the force is applied. In other words, for any given element of the human skeleton the same magnitude
of force applied from different directions will result in different fracture patterns due to the intrinsic qualities of the
bone. For example, the ends of long bones have a thin outer layer and are mostly composed of spongy trabecular
bone. The long bone ends, then, will respond differently to external stress than the middle of the shaft, which is
mostly thick, cortical bone. The most common directional descriptions of force are presented in Figure 3.
Directional forces impacting the body. [3]
Under tension the ends of the bones are pulled apart, which stretches the collagen fibers along their length. Fractures
caused by tension are often clean breaks that have few secondary fracture lines. Under compression the ends of the
bone are pushed together causing multiple breaks and secondary fracture lines that radiate from the point of impact.
Under shearing the force impacts the body at an oblique angle, which often causes the two fracture ends to be
misaligned because they are pushed in opposite directions. Under torsion the force is also directed obliquely but with
one end of the bone being stationary. This causes fracture lines to spiral down the axis of the bone shaft. In real life,
most fractures result from a combination of these directional forces. However, it is useful to consider tension and
compression as complementary fracture mechanisms, which leads to the first axiom of interpreting skeletal trauma
(Currey, 1970; Zephro and Galloway, 2014).
The first axiom for understanding bone fracturing patterns is: bone is weaker under tension than under compression.
This means that bone will fracture first at areas where tensile forces are experienced. The ability of bone to withstand
compression is nearly twice that of the ability of bone to withstand tension.
Consider the diagram of a long bone shaft shown in Figure 4. A force is applied from the top of the bone striking it at a
90-degree angle. The side of the bone being struck is under compression because the bone is bending under the
force of the blow. The small black arrows on the top of the long bone shaft indicate the direction of bone movement
under compression. The bone at the immediate point of impact is physically being squeezed together. The opposite
side of the bone is under tension as the collagen fibers are being pulled apart. In other words, the bone is being
stretched on the opposite side that the force is applied. Because bone is stronger under compression than tension it
will fracture first on the opposite side of the bone from which the force was applied. This fact alone allows the forensic
anthropologist to determine the direction of force.
, Response of a long bone shaft to perpendicular force. [4]
The bending of the bone in the example above indicates another quality that is critical for interpreting trauma
(Berryman et al., 2012, 2013; Symes et al., 2012; Zephro and Galloway, 2014). Bone is a viscoelastic material, which
means it responds to force differently depending on the speed at which it impacts the body. Static forces impact the
body at low velocity for a relatively long period time (a blow to the head with a brick). Dynamic forces impact the body
suddenly and quickly, but also dissipate rapidly (a bullet wound). Bone is better able to resist dynamic forces but once
the bone fails it shatters without bending or deforming. In other words, bones act like a brittle material (like a pane of
glass) under a dynamic force. Comminuted fractures and secondary fractures (see below) are often the result of
dynamic stress. To the contrary, bone reacts in a more pliable fashion under static stress. This means the bone will
bend and deform prior to fracturing (like a piece of plastic). Static stress more often results in single fracture lines;
comminuted fractures (see below) are uncommon.
When a static force is applied, bone undergoes three phases (Currey, 1970). During the first phase the bone will bend
in response to a force but will not be permanently deformed once that force is removed. This is called the elastic
phase - the bone bends, absorbs the energy, and returns to its original state without any permanent change to its
structure. Most minor blows to the body (a kick to the shin) fall into this category. As the magnitude or duration of
force increases the bone will continue to bend and deform to absorb the energy imparted. However, during this phase
when the force is removed the bone will not return to its original state. In other words, the bone is permanently
deformed. This is called the plastic phase. The third and final phase is when the bone has been stressed beyond its
capacity to absorb energy. The force exceeds the structural strength of the bone and it gives way. The bone has
fractured.
The second axiom for understanding bone fracturing patterns is: bone responds as a brittle material under dynamic
forces and as an elastic material under static forces. Dynamic forces do not deform the bone but simply shatter it.
Static forces deform the bone prior to causing a fracture.
Classifying Trauma
When enough force is applied to bone, a discontinuity (i.e., break) will form. If the discontinuity travels through the
bone physically separating it into two pieces, it is called a fracture. A displaced fracture occurs when the two
continuous bone surfaces no longer meet. In Figure 5, the broken ends of the femur are displaced in the Open wound
example. If the bone is not "set" by a doctor it will heal but be permanently misshapen. Comminuted fractures are
those in which multiple fractures occur at a site of injury. In comminuted fractures the bone has fragmented into 2 or
more pieces indicating a high level of energy was impacting the body, consistent with dynamic loading. A butterfly
fracture is a special type of comminuted fracture of long bones typically resulting from blunt force trauma. Fractures
can also be characterized with respect to soft tissue injury. A fracture in which the bone does not break the outer
surface of the skin is called closed, while a fracture that breaks the skin is called open. Open fractures have a much
higher chance of secondary infection.
, Different classifications of trauma. [5]
If the discontinuity does not completely separates the bone it is called an infraction (or an incomplete fracture). All
else being equal, fractures represent more severe injuries than infractions and suggest more energy has impacted
the body. Infractions are more common in children because of the high collagen content of growing bones; whereas
elderly individuals with low bone mineral density (osteoporosis) are more likely to experience a complete fracture.
Infractions may also suggest that the energy impacting the body was spread over a relatively wide area (such as
being hit by a car), or that the amount of energy impacting the body was limited (being hit by a car at a low speed)
(Galloway et al., 2014). A greenstick fractures results when the bone is impacted at an angle or is bent resulting in
tension on one side of the bone and compression on the other side. An incomplete fracture with form transversely
(see below) and often travel longitudinally up and down the shaft of the long bone. These types of fractures are more
common in children. A depression fracture is specific to the cranial vault. It involves an injury that usually crushes the
trabecular bone and some of the outer cortical bone but does not completely separate the fragments. An example of
a depression fracture is presented in Figure 6. There are dozens of additional clinically-defined fracture categories;
however, many of these are not relevant to forensic anthropology (see Galloway et al., 2014).
Depressed cranial fracture. [6]
Secondary Fracture Lines
The preceding discussion focused on the formation of a primary fracture associated with the direct area of impact.
However, discontinuities are often associated with secondary fracture lines, which usually originate from the point of
impact or circumscribe the impact site. Secondary fracture lines result when the bone directly at the point of contact
(bullet entry point) cannot absorb all of the energy imparted into the body through the formation of the primary
fracture (bullet hole). There are two types of secondary fracture lines.
Radiating Fractures disperse outward from the site of impact and can usually be traced back to it. This fact is useful
for identifying the site of impact if the damage to the body has result in extreme fragmentation. Radiating fractures
represent the energy traveling through the bone along the lines of least resistance - area of low bone density, low
bone thickness, high proportions of trabecular bone, cranial sutures, and the numerous foramina located throughout
the body. Concentric Fractures or Hoop Fractures form concentric fracture lines that surround the site of impact.
These lines are usually associated with high velocity projectile and low velocity blunt force trauma. Concentric
fractures are associated with high levels of energy impacting the body.
Figure 11 shows both types of fractures resulting from a gunshot wound to the distal femur. Note the single large
vertical radiating fracture (blue arrows) that begins at the gunshot entry point (red arrow) and proceeds parallel along
the shaft of the long bone, and the smaller concentric fracture (green arrow) located distal to the entry wound that
affects the superior portion of the joint surface. The concentric fracture is curvilinear and has a contour that follows
that of the primary impact site.
Gunshot to the distal femur. [11]
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