11.1 Introduction

Concern for the validity of scientific evidence presented in court first became formalized in 1923 with Frye v. United States, where general acceptance was held as the standard for admissibility. However, it was the 1993 Daubert v. Merrell Dow Pharmaceuticals, Inc. case that forced forensic scientists to seriously examine their court‐offered opinions for scientific validity. Forensic anthropologists became concerned about their own expert testimony and whether they, if called upon, could demonstrate the scientific basis for the particulars to which they would testify.

A courtroom opinion is based on science when it is derived from evidence with a solid theoretical underpinning. A theory is considered valid only when it can be supported by numerous observations and experiments. However, it must also be understood that theories do not require proof to be accepted as fact or to be applied for their utility by the scientific community. For example, both evolution and gravity are generally accepted as theoretical facts and are used to explain and understand natural phenomena, yet neither can unequivocally be proven.

Scientific theories—whether it is gravity, evolution, or bone trauma—are continually challenged and refined by scientists who eliminate inaccurate elements so that the parts remaining solidly encapsulate whatever truths actually exist. Hypotheses are formulated relative to bone trauma theories and tested to identify conditions where the theory does not hold and where no valid judicial opinion can be formulated. In essence, exceptions to a theory are identified through this process and eliminated, strengthening the theory. Any judicial opinion derived from a theory that has remained intact after repeated scientific challenge is based on science.

Boyd and Boyd (2011) borrowed a hierarchical model from Schiffer (1988) involving archaeological theory (high‐level, middle‐range, and low‐level theory) to encompass the many diverse areas within forensic anthropology. Boyd and Boyd (2011) stated that high‐level theory is more broad‐ranging and abstract; middle‐range theory involves relating forensic observations with the varying forces responsible for them; and low‐level or methodological theory is comprised of recovery, analytic, and inferential theories. Boyd and Boyd (Chapter 1, this volume) further state that all these types of theory “…are linked together and provide necessary feedback for each other in the process of theory building.” It is considered that, because of this linkage, the hierarchical model may be revised to refer to three forms of theory—foundational, interpretive, and methodological—to more accurately depict their interaction and interdependence (Boyd and Boyd, Chapter 1, this volume). Regardless of form, theory is created through three lines of logical reasoning: abductive, deductive, and inductive (Boyd and Boyd, Chapter 1, this volume).

In this chapter, we discuss three related topics. First, the theoretical basis for bone trauma analysis is scrutinized through the three theoretical levels outlined in the first chapter of this book, coupled with the various lines of logical reasoning used in science. Next, the fundamental assumption used in bone trauma analysis is presented, and the three commonly held principles in fracture production are explored. These three principles coupled with a knowledge of intrinsic properties and extrinsic forces provide the theoretical concepts needed for the interpretation of bone trauma, as well as the fundamental building blocks for bone fracture theory and courtroom opinion. Finally, a practical approach—consisting of a trauma assessment triad and thought experiments—is promoted as a means of facilitating bone trauma evaluation and hypothesis building.

11.2 Theory

Material science and engineering is a field involving research—both theoretical and experimental—that focuses on the properties of materials and their application or use. A subspecialty of material science and engineering that deals with material failure (fractures) is biomechanics of orthopedic implants. This specialty focuses heavily on the load‐bearing capacity of various materials. Aspects of physics, chemistry, and engineering are used in this field, often to explore the magnitude of load various materials can withstand before failure. Historically, there have been many studies involving load‐bearing capacity with bone as the material in question, some dating back 30 years or more (White et al., 1973; Hartman, 1974; Carter and Hayes, 1976; Pearcy et al., 1983). Biomechanics research of orthopedic implants has examined multiple variables involving bone grafts, reparative strategies, and materials used in implant devices (Klawitter and Hulbert, 1971; Smith and Cody, 1993; Staiger et al., 2006; Ballo et al., 2007; Smith et al., 2014).

Studies such as those outlined earlier are primarily concerned with the functionality of materials or best outcomes from surgical interventions and have a theoretical basis in physics. Those involving load‐bearing capacity of test materials are designed to determine at what point the material will fail, under what conditions the material will fail, or perhaps where the failure will occur. The emphasis is on when and where, not how, failure occurs. In these fields, once a crack appears in a material, the interest in the fracture itself ends with no concern regarding how the fracture behaves. Generally, the forensic anthropologist’s interest begins where the material scientist’s interest ends. Research on bone fracture from an engineering perspective is limited, with little concern for fracture propagation or the pattern of the fracture. Forensic application of bone fracture focuses on how the fracture behaves and what influences fracture behavior.

Fracture formation in bone, especially complex fracture patterns, may seem random; however, this is not the case. There are no random fractures in bone; each fracture doggedly obeys the laws of physics. When a fracture changes direction as it propagates across the cranial vault, or when it bifurcates to form a butterfly fracture, or when it ratchets leaving secondary fractures across tubular bone or completes the fracture leaving a breakaway spur, it does so for a reason. There is nothing random in the way a fracture forms.

The foundational assumption in bone trauma analysis is the nonrandomness of fracture configuration or more specifically fracture behavior; it is this nonrandomness that allows fracture analysis and bone trauma interpretation. Intuitively, fracture patterns are repeatable when subjected to similar influences. To avoid confusion, it must be stressed that fracture behavior is highly repeatable; however, the actual fracture will be slightly different from one specimen to the next. Although the general shape of a tibia is the same between humans, its micromorphology, elasticity, density, and so on may vary considerably, resulting in variation in the subtlest aspects of fracture behavior. The greater the knowledge and experience of the practitioner, the greater their success in associating the fracture pattern presented with the force that produced it. However, analysis cannot rely on intuition, but rather knowledge based on sound theory coupled with the experience necessary to validate the theory. The theoretical basis for bone trauma analysis in a forensic setting can best be explored through an examination of foundational, interpretive, and methodological theory.

11.3 Fundamental principles in bone fracture interpretation

According to the Merriam‐Webster (2015) Online Dictionary, a principle is “a law or fact of nature that explains how something works or why something happens.” Fundamental laws or facts of nature form the foundation upon which hypotheses are constructed and theories are ultimately based. In order to understand the fundamentals of fracture production, the modes of loading must first be understood. Wescott (2013:84) stated, “Loads can be applied in tension (stretching), compression (compaction), bending (angulation), torsion (twisting), shear (sliding), or a combination of these basic components.” Although these means of loading exist in the broader, more general sense, a closer view of the fracture tip reveals only two significant types of load—tension and shear. In regard to bone fracture production, an understanding of tension and shear is foundational in trauma interpretation. As principles they can be stated as follows:

• All bone fractures initiate under tension.
• Shear forces direct fracture propagation in bone.

Collectively, these two principles make it possible to reason about the formation of a fracture and the basic influences that caused it. A cantilevered bone specimen, loaded as shown in Figure 11.1, serves as an illustration of this. Here the left side of the specimen is firmly fixed, while the right side is free to move. Upon application of a load as shown in the figure, the specimen tends to bend, placing the upper surface under tension, while the lower surface is in compression. Bone can withstand compressive forces to a greater amount than tensile forces. As the load is increased, at some point the tension in the upper surface of the specimen will exceed the strength of the material. At that point, the fracture will initiate and begin to propagate downward through the bone as shown in Figure 11.1a. The exact location where the fracture initiates may be a point of weakness in the surface of the bone and/or a point where the stress is more highly concentrated (e.g., due to local imperfections in the surface of the bone). The important thing to note is that the fracture will initiate in tension on the upper surface of the bone (as depicted in Figure 11.1) and will not form on the lower surface, which is in compression—this is the essence of the principle of tension.

Referring to Figure 11.1b, as the fracture begins to propagate, the applied load causes a shearing effect in the material. The load on the right side causes the material on the right side of the fracture to be pressed downward. At the same time, the fixed end of the bone rigidly holds the specimen in place, causing an equivalent and opposite force to push upward on the material on the left side of the fracture. As stated in the shear principle earlier, these shear forces influence the direction of the fracture as it propagates through bone. In particular, a load that applies a clockwise shearing effect (as depicted in Figure 11.1) will lead to clockwise angulation in the fracture. The fracture propagates at an angle toward the fixed end. Additionally, the greater the shear stress, the greater the corresponding fracture angulation.

In order to clarify both axioms and show how they are related, consider Figure 11.2, which depicts a close‐up of Figure 11.1 just after the fracture has begun to propagate through the bone. As shown in Figure 11.2a, shear stress and tensile stress are both present at the leading end of the fracture. As shown in Figure 11.2b, it is the combination of these two stresses that pulls the material apart. On the right side of the fracture tip, the tensile stress pulls the material toward the right, while the shear stress pulls material downward. Combined, this causes the material to be pulled at an angle down and to the right. Likewise, on the left side of the fracture tip, the tensile stress pulls the material toward the left, while the shear stress pulls the material upward. Combined, this causes the material to be pulled at an angle up and to the left. The overall effect, as indicated by the dark arrows in Figure 11.2b, is that the material is pulled apart by these resulting stresses. Thus, a closer look into the actual tip of the fracture reveals that the material is still being pulled apart under tension. Specifically, the fracture will propagate in the direction where the tensile stress exceeds the tensile strength of the material. This example also demonstrates exactly how shear forces influence the fracture angulation. Figure 11.2b illustrates that increasing the shear stress (while keeping tensile stress constant) causes the combined stress to pull the material at a sharper angle, resulting in an even more angulated fracture pattern.

A generally accepted concept states that gunshot entrance wounds are internally beveled. Forensic science practitioners who deal with trauma interpretation hold this as a fact, with the principles of tension and shear presented earlier providing a solid underpinning. Tension and shear forces define the primary fracture to cantilevered tubular bone (Figure 11.3a), and the same dynamics can be seen with a gunshot entrance wound to the cranial vault (Figure 11.3b). When the bullet strikes the bone, it represents a dynamic load (a load that keeps advancing), like that exerted to the free end of a cantilevered bone (Figure 11.3a). As the bullet begins to compress the area of impact, the unaffected bone around it acts like cantilevered or fixed bone in the tubular bone example (Figure 11.3a). Tensile forces should develop first around the area of impact to produce a circular defect. As the bullet continually depresses the bone, shear forces expand the fracture continually as it propagates in an ever‐increasing, concentric fashion producing the characteristic internal bevel (Figure 11.3b). The major contributions of tensile and shear forces are undeniable, and the overall patterns seen in a fracture to tubular bone are also reflected in gunshot entrance wounds.

Another concept rooted in tension and shear involves the termination of a fracture. Fractures form as a means of absorbing energy. Each individual fracture in a complex of fractures terminates when the tensile strength of the material exceeds the tensile stress. This leads to the concept that bone fractures terminate into preexisting bone fractures. Blacksmiths and farriers have known this concept for years; a crack in metal or a split in a horse’s hoof can be terminated by drilling a hole at its tip. This hole allows the energy to be absorbed and will terminate the crack unless the energy exceeds the capability of the hole to absorb it. This same concept holds true for fractures in bones, but instead of a hole being drilled, a preexisting fracture will terminate the advance of a fracture by dissipating and absorbing its energy. If the amount of energy cannot be absorbed in the preexisting fracture, the preexisting fracture may elongate to absorb the energy, or a new fracture may open in the preexisting fracture. The point of interest here is the fact that the fracture in question terminated into a preexisting fracture. Any fracture initiating in the preexisting fracture as a result of the energy dump from the terminating fracture is a new fracture, not a continuation of the old. Fractures terminating into preexisting fractures result in the familiar T‐shaped fracture configuration that allows the sequencing of multiple gunshot wounds or multiple blunt trauma. Rooted in tension and shear, the generally accepted concept of fractures terminating into preexisting fractures—as with the concept that gunshot entrance wounds are internally beveled—provides another valuable tool when examining fracture behavior.

In fact, there are numerous “generally accepted concepts” used to interpret bone trauma. For example, the breakaway spur associated with a fracture to tubular bone is always located on the compression side of the bone and on the fixed end. Thumbnail fractures are the concentric fractures often seen on tubular bone after thermal damage. The direction the heat progressed on the bone is from the convex side of the concentric fracture to the concave side. A ring fracture to the base of the skull is caused by blunt trauma that depresses the cranium onto the cervical vertebrae or elevates the cranium away from the cervical vertebrae. Whether the ring fracture is internally beveled or externally beveled may indicate the direction the cranium was displaced to produce the fracture.

Admittedly, the complex of influences responsible for the specific shape of an internally beveled gunshot entry defect or for a complex of fractures produced in a multiple blunt trauma case is exceedingly complicated. Exactly how the principles of tension and shear affect fracture behavior varies in response to influences inside and outside of the bone. These influences are defined by the following:

• Fracture behavior is dictated by the interplay of intrinsic and extrinsic factors.

Extrinsic factors involve forces outside the bone, such as the magnitude of the load, the area and rate of application, the mass of the object impacting the bone, the duration of the load, and the direction of the load. Intrinsic factors are those inside the bone that contribute to the way a bone behaves when subjected to a load. Bone, as a material, is far more complicated than many of the homogeneous materials tested for load‐bearing capacity in biomechanics studies. Intrinsic factors include a bone’s viscoelastic and anisotropic properties. Viscoelasticity is the capability of bone to possess both viscous (a quality between a solid and a liquid) and elastic qualities (the ability to return to its original shape after a load is removed). Anisotropicity is the condition where the physical properties of bone are not constant, but vary depending upon the direction of the force. For example, the shaft of a femur can accommodate much greater load when it is applied axially as opposed to laterally.

Another intrinsic factor of bone is morphology, both macro morphology and micromorphology. Long bones are not simple cylinders; rather, their macro‐morphology or shape varies according to the specific bone. For example, a tibia shaft possesses a much more complicated shape than a femur shaft. However, a specific bone from one individual, such as a humerus, is very similar in its macromorphology to that of another individual, but may vary more in its micromorphology (e.g., haversian system, cortical thickness and shape, trabeculae configuration, etc.). It is these micro‐ and macromorphology variations—variations that are often difficult to measure—that can result in different outcomes in the finest details of bone fracture pattern. However, the finest details in a fracture pattern usually hold less value in fracture interpretation than the more conspicuous fractures.

Lastly, biological factors (e.g., age, sex, pathology, drugs, alcohol, medications, etc.) will influence the viscoelasticity of bone in ways that can make it more elastic or more brittle, affecting its susceptibility to fracture. Although bone is exceedingly complicated in its composition and morphology, and it is susceptible to many intrinsic influences, it does exhibit behavior that can be demonstrated using simple material models. Models that use diverse materials can be tested so that the intrinsic quantities are known and can be varied, and the failure characteristics can be studied by altering the extrinsic factors. Predicting a specific fracture pattern, void of any consideration of the extrinsic and intrinsic factors involved, is not feasible.

In a general sense, fracture patterns are predictable at a macroscopic level, but not at the smallest level of detail. The overall behavior is predictable because it adheres to simple principles. This predictable behavior lends itself well to understanding circumstances under which a fracture might occur in a given bone. As previously demonstrated, when one end of a tubular bone is cantilevered and a load is continuously applied to the free end, an angular fracture will form that propagates toward the cantilevered end (Figure 11.4a). However, in this same example, it is impossible to predict how many secondary fractures (Figure 11.4c) there will be or even if a secondary fracture will occur at all. Secondary fractures appear to form as the primary fracture moves across the bone and encounters resistance to its advance due to subtitles in bone morphology, composition, microstructure, or any number of factors (Figure 11.4a). When this happens, the primary fracture will terminate; however, the dynamic load will continue to force the free end of the bone down. Tensile forces now build on the newly exposed surface of the primary fracture (i.e., the surface of the new fracture) until stress risers build at its weakest point (Figure 11.4b). When this occurs, a new fracture will open under tension, and shear forces will direct it toward the cantilevered end. This new fracture becomes a continuation of the primary fracture, and the short portion of the primary fracture that terminated (i.e., remnant of the initial primary fracture) will remain as the secondary fracture (Figure 11.4c).

The absence, existence, or number of secondary fractures is dictated by the intrinsic (e.g., elastic component, density, morphology, micromorphology, etc.) and extrinsic (e.g., area and rate of application, direction, etc.) factors present during the fracture formation. When examining bone fractures in the absence of a strictly controlled laboratory experiment, there is no way of accounting for all of the internal and external factors involved in their production. However, with an examination of the fracture pattern and a basic comprehension of the intrinsic factors at play, an anthropologist can make a reasonable estimation of the extrinsic factors required to cause the traumatic event. This determination, when founded on sound and accepted scientific principles, can represent a powerful analytical tool. Although the fracture is not predictable at small scales, the overall behavior is predictable and lends itself well to understanding circumstances leading to the fracture.

Anne Kroman (2007, p. vii) was correct when she stated that:

…bone trauma is best viewed as a continuum (rather than discrete independent categories), with the variables of force, acceleration/deceleration, and surface area of impacting interface governing the appearance of the resulting fractures.

Sharp trauma, blunt trauma, and gunshot trauma are discrete categories assigned according to their apparent form of delivery; however, the fractures produced in each of the categories are again dictated by the interplay of intrinsic and extrinsic factors. Kroman’s comment may be exemplified by comparing blunt trauma characteristics to a cranial vault from a ball‐peen hammer versus a .50 caliber bullet. A ball‐peen hammer has two striking surfaces. The first is a circular flat surface consistent with most hammers, but the second is a spherical‐shaped surface, close in size to a .50 caliber bullet. If the ball‐peen striking surface of the hammer impacts the cranial vault at a slow rate of application (~30 feet/second), it will produce a fracture pattern typical of blunt force trauma. However, if a steel ball .50 inch in diameter impacts the cranium at a rate of application consistent with a sniper rifle (~2800 feet/second), the fracture pattern is totally different even though the impact area is the same. On the Kroman bone trauma continuum, the only variable in this example is the rate of application, but, with the lower and rapid rates of application, the impact site is typical of blunt trauma and gunshot trauma, respectively. At what point along this rate of application continuum does blunt trauma begin to look like gunshot trauma?

This also applies to sharp trauma injury. The area of impact is an exceedingly thin surface, much thinner than the striking surface of a 2 × 4 board, and both may be applied at a slow rate (~30 feet/second), but the end results are very different. To borrow a statement from blood pattern analysis, “…stabbing events are considered a beating with a sharp object” (James et al., 2005:123). This statement aligns somewhat with Kroman’s view of a bone trauma continuum. To be more specific, with all other intrinsic and extrinsic factors held constant, and the knife‐edge striking surface is slowly increased to that of a 2 × 4, at what point does sharp trauma begin to look like blunt trauma? Clearly, the variation of any one of the intrinsic properties or extrinsic forces can have a profound impact on fracture behavior.

11.4 A practical approach to bone trauma evaluation and hypothesis building

The authors propose a two‐step approach that facilitates fracture analysis and hypothesis building by using a fracture assessment triad in conjunction with thought experiments. A fracture assessment triad is composed of (i) fracture behavior (i.e., principles of tension and shear form the basis for evaluation as guided by various generally accepted concepts regarding bone trauma), (ii) intrinsic properties, and (iii) extrinsic circumstances, which represent the totality of information needed to evaluate bone trauma. Given any two items in the triad, the remaining item can be determined (Figure 11.5) through deductive reasoning. It must be noted that, outside of a laboratory setting, it is impossible to “know” every intrinsic and extrinsic variable that influences fracture production. Nonetheless, if the dominant extrinsic causes and the intrinsic properties can be estimated, the resultant fracture behavior can be postulated. In like manner, when fracture behavior and the extrinsic cause are known, the intrinsic properties of the bone that influence fracture behavior can be surmised. However, in a medical examiner’s morgue and ultimately a judicial setting, the most pertinent component of the triad is the cause of the trauma (i.e., extrinsic forces). Determining the cause of bone trauma requires that fracture behavior be evaluated and intrinsic properties be surmised. The anthropologist must first evaluate the fracture by using principles of tension and shear along with the shape of fracture lines as they traverse a bone, angle of each fracture relative to the bone surface, angle of the fracture surface relative to the bone surface, secondary fracture orientation, and so on along with generally accepted concepts (e.g., bevel direction in gunshot wounds, preexisting fractures, breakaway spurs, etc.). Locating where each fracture initiated is exceedingly useful and should be routinely undertaken in fracture analysis. Next, every attempt should be made to understand the intrinsic properties of the bone containing the fracture. For example, a very old adult will likely have bones that are much more brittle than those of a very young adult. Deductive reasoning of these two areas (fracture behavior and intrinsic characteristics) will illuminate the extrinsic properties responsible for the fracture pattern. From these extrinsic properties, the source of the trauma (e.g., hammer, gunshot wound, ball bat, stab wound, etc.) may be deduced or eliminated.

Given any one item in the triad, elements of the remaining two items can be determined (Figure 11.5) through inductive reasoning. The fracture assessment triad, through inductive reasoning, facilitates the development of hypotheses that can be tested under controlled settings. For example, fracture shape may be of interest. Since the remaining two areas are responsible for the fracture shape, inductive reasoning can be used to identify specific elements from the intrinsic properties and the extrinsic forces to hold constant or to vary in an attempt to experimentally reproduce the fracture shape.

The fracture assessment triad provides the necessary information to identify the cause of perimortem trauma as well as postmortem damage and taphonomic influences. This information can then be subjected to thought experiments, which can be accomplished without the cost of equipment and laboratory space and in much less time. Admittedly, an exercise termed “thought experiments” seems trivial or frivolous; however, they have long been a powerful analytical tool used by philosophers and scientists from Galileo to Albert Einstein (Brown, 1993). In fact, Einstein’s theory of special relativity derived from his thought experiments as a youth. According to the Brown and Yiftach (2016):

Thought experiments are devices of the imagination used to investigate the nature of things. They are used for diverse reasons in a variety of areas, including economics, history, mathematics, philosophy, and the sciences, especially physics. Most often thought experiments are communicated in narrative form, frequently with diagrams.

The paramount findings needed by medical examiners and the courts involve extrinsic factors, specifically the identification or physical explanations regarding the weapon. Thought experiments used in conjunction with the fracture analysis triad would pair the observed findings (i.e., fracture appearance) with the likely intrinsic influences (i.e., bone morphology, elasticity, plasticity, brittleness, anatomic features affecting fracture behavior, etc.) and subject them to the influence of potential extrinsic factors (e.g., area of impact, direction of force, rate of impact, mass of object impacting bone, etc.). This deductive exercise will not only facilitate interpretation but also produce hypotheses where inductive reasoning can provide answers regarding extrinsic factors that influence fracture patterns. This represents interpretive theory as described by Boyd and Boyd (2011; Chapter 1, this volume), which is observation‐based and clearly more applied. Thought experiments serve as a tool to mentally examine how the fracture(s) in question could have originated, as well as effects of various intrinsic and extrinsic influences on fracture behavior. Thought experiments frequently use extreme, even farcical experiments to more clearly delineate the salient elements responsible for the fracture. As a cautionary note, it must be emphasized that mental laboratory conclusions are not sufficient for the courts, but these thought experiments can lead to accepted physical explanations or research designs and testing that can be used in a judicial setting.

The utility of the proposed two‐step approach can be demonstrated through the following examples taken from blunt force cranial trauma research. Gurdjian et al. (1947, 1950a, b) proposed a theory that, with blunt force cranial trauma, the fracture can initiate at a distance from the impact site. However, Smith et al. (1991) stated that cranial fractures initiate at and radiate from the impact site. Kroman et al. (2011) tested this theory using five cadaver heads and high‐speed photography and found that fractures radiated from the impact site in all five cases, thus refuting Gurdjian et al. (1947, 1950a, b). However, Fenton et al. (2015) used high‐speed photography to examine fractures to fresh crania when struck by an object and demonstrated that fractures can originate distantly to the impact site. To further complicate matters, both researchers produced video evidence clearly supporting their conclusions. In essence, it can be clearly seen that these two seemingly inconsistent conclusions are both apparently true. How can well‐structured research conducted by highly capable researchers (Kroman et al., 2011; Fenton et al., 2015) produce opposite findings? The specific force that produced the disparate results can be viewed using the fracture assessment triad and thought experiments.

For this example, inductive reasoning may be used to examine each of the fracture patterns independently. The pattern with fractures radiating away from the impact site may be examined first. Something in either the intrinsic or extrinsic components of the triad must be responsible and that “something” must be different between the two research protocols. In the intrinsic component, bone brittleness is a concern; however, for the purpose of the thought experiment, it can be assumed that the heads used are basically similar with some degree of elasticity. In the extrinsic component, many factors can vary. Since the original experiments involved blunt trauma, both used slow loading, which can be held as a constant in the thought experiment. Of the extrinsic factors that could have made a difference, abductive reasoning suggests the size of the impact area as a factor. For the thought experiment, extremes can be used; two hammers are visualized—one with a striking surface 1 foot in diameter and the other with a striking surface of 1/8th inch in diameter and each with the same mass. The visual image of each striking the skull at the same slow rate produces two distinctly different visual results. The smaller striking surface penetrates the skull immediately, with the possibility of fractures radiating from the site of penetration, whereas the larger striking surface does not penetrate the skull, but rather deforms it. No logical means exist that enables fractures to initiate remotely from the penetrating impact site produced by a small striking surface.

This leads to a second thought experiment that focuses on the remote fracture in Fenton’s experiment. In this thought experiment, the remote fracture initiation point can be set to open in the squamosal suture, and the fracture can be set to advance to the impact site on the superior aspect of the parietal bone. The bone will be assumed to possess a more elastic than brittle character. The fracture characteristics and the various intrinsic factors will serve as constants and each of the extrinsic factors can be varied individually to allow the mind’s eye to observe the effects on the fracture. Abductive reasoning points to the slow rate of loading that allows the bone to deform before fracturing.

For this second thought experiment, a malleable cylinder (e.g., a marshmallow provides a farcical, but useful, mental image) will serve as a suitable model (Figure 11.6). The purpose of this experiment is to examine how surface tension behaves when the cylinder is deformed. First, the mind’s eye may allow ink dots to be placed around the equator of the cylinder, and then the cylinder may be held between thumb and index finger (Figure 11.6a) and compressed (Figure 11.6b). With axial loading, the circumference expands, resulting in tensile forces increasing on the external surface perpendicular to the long axis of the cylinder (Figure 11.6c), and the dots placed around the imaginary equator will begin to separate. This simple thought experiment becomes much more complex when applied to a cranial vault, but the same basic dynamics exist. When a cranium (Figure 11.7a) with a relatively high elastic component is impacted by a blunt object with a broad striking surface (e.g., a 2‐by‐4 board) and at a slow rate of application, the vault will slowly bend outward with tension being exerted on the external surface as seen in Figure 11.6c. As the compressive load continues to increase, tensile stress will continue to rise, and a fracture will initiate at the weakest site (the squamosal suture in this example) and may advance along areas defined by bone morphology, bone composition, bone thickness, and so on, toward the impact site (Figure 11.7a). As the load continues, tensile forces will increase on the external surface parallel to the long axis of the cylinder (Figure 11.6d). As the plates of bone defined by these fractures bend outward, fractures appear across the plates, perpendicular to the radial fractures. Figure 11.6 clearly illustrates how radiating (Figure 11.6c) and concentric fractures (Figure 11.6d) can occur with blunt trauma to a cranial vault.

The thought experiment has allowed the realization that bone elasticity and the ability for the bone to deform during the impact can explain how a fracture can radiate from the impact site and also form at a site remote to the impact site. When a cranium (Figure 11.7b) with a relatively low elastic component is impacted by a blunt object (high mass, like a hammer) with a constricted impact surface and at a slow rate of application, the blunt object will tend to fracture the vault at the impact site and fractures will tend to radiate away from the impact site (Figure 11.7b). On the other hand, when a cranium (Figure 11.7a) with a relatively high elastic component is impacted by a blunt object (moderate mass, like a 2 × 4 board) with a broad impact surface and a slow rate of application, the blunt object will tend to deform the skull, increasing surface tension perpendicular to the axis of impact and causing fractures to initiate remotely.

The fracture assessment triad serves as a means of facilitating research design and the thought experiment allows hypotheses to be constructed through inductive reasoning, where the goal might be to produce specific fracture characteristics (e.g., fractures occurring remote to the impact site) by varying intrinsic qualities of bone (e.g., elasticity, brittleness) with extrinsic factors (e.g., surface area of impact, rate of application). Thought experiments and the fracture assessment triad are tools for the forensic anthropologist to use in case assessment, as a guide in hypothesis formulation and research design and as an aid in the classroom.

11.5 Conclusion

The foundational theoretical assumption required to validate bone trauma analysis as a functional specialty within forensic anthropology is the nonrandomness of fracture configuration. The laws of physics dictate fracture behavior, which allows a measure of predictability and permits interpretation. Within interpretive theory, forensic anthropologists apply abductive reasoning during the initial examination of a trauma case as a means of suggesting the force that may have produced the trauma (blunt, sharp, gunshot, etc.). Inductive reasoning is used to identify intrinsic and extrinsic factors that influence fracture patterns and to construct and test hypotheses. Deductive reasoning can then be used to further narrow the possibilities and ultimately identify the precise source of the trauma.

A fracture assessment triad used in conjunction with thought experiments is proposed as a valuable tool for fracture analysis, hypothesis formulation, and research design. If any two of the three areas of the triad are known, deductive reasoning can be used to determine the third. The fracture assessment triad, along with inductive reasoning, can be used to develop hypotheses and construct research designs to test them. Thought experiments are proposed as an expedient, cost‐free means of testing these hypotheses using either deductive or inductive reasoning. Thought experiments, though often absurd, can lead to a deeper understanding of proposed questions and, in so doing, can lead to more appropriate research design and testing.

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