Biomechanics of bone trauma.

To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter SPi. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. FRN2: 00015 a0010 Biomechanics of Bone Trauma Au3 DJ Wescott, Texas State University, San Marcos, TX, USA ã 2013 Elsevier Ltd. All rights reserved. dt0030 dt0035 dt0040 dt0045 dt0050 Introduction p0010 Biomechanics is the science of mechanical laws applied to biological tissue. Trauma is an extrinsic agent, force, or mechanism that causes injury or shock to a living tissue, usually due to violence or accident. Hence, the biomechanics of bone trauma is the application of mechanical laws to describe and interpret bone trauma, and involves the examination of both the intrinsic (size, geometry, and material properties such as stiffness, elasticity, and density) and extrinsic (magnitude, duration, and direction of force) factors resulting in bone injury. Understanding the biomechanics of bone trauma allows forensic scientists to use the pattern of bone fractures to deduce the type and direction of loading that caused the bone to fail. This information then often can be used to determine proximate and ultimate cause of the injury. Injuries to bone usually involve fracture (failure) or dislocation (displacement at a joint). The mechanisms of fractures are divided into direct, indirect, stress, or pathological. Fractures resulting from direct or indirect trauma are of greatest interest to forensic scientists. There are many direct and indirect agents that can cause trauma to bone including mechanical, thermal, electrical, and others. Most of the direct trauma of interest to forensic scientists results in injury at the point of impact due to blunt, sharp, or projectile forces. The differences R s0010 E L S E V IE Au2 p0015 dt0055 dt0060 dt0065 R O O F dt0025 Plastic deformation Permanent strain that is unrecoverable when the bone is unstressed. Plastic deformation occurs between the yield point and the failure point. Remodeling The removal and replacement of bone at a particular location by the coupled action of osteoclasts (bone-destructing cells) and osteoblasts (bone-building cells). Stiffness (rigidity) The ability of a material to resist deformation or the load required to cause bone to deform a given amount. It is measured as the slope of the stress–strain curve and influenced by the relative proportion of collagen and hydroxyapatite crystals. Strain The dimensional change in loaded bone. The principal strains are normal or shear. Strength The ability of bone to withstand permanent deformation (the load at the yield point) or fracture (the load at failure point). Stress The load per unit area of a bone, measured in Pascal (Pa). The stress can be normal or shear. Tension A mode of loading in which the forces act in opposite directions along the longitudinal axis of the bone. Tension results in an increase in length and decrease in width. Yield point The point where bone begins to deform plastically. dt0070 dt0075 dt0080 dt0085 S T dt0020 F IR dt0015 Anisotropic A characteristic of a material that has different mechanical properties when loaded in different directions due to its directional structure. Bending (angulation) A mode of loading such that the bone bends around its axis and experiences tension on the convex surface and compression on the opposite concave surface. Brittle Characteristic of a material that readily breaks without plastic deformation when subjected to stress. Buttressing Areas of struts or thickening of the bone. Compliance A measure of the ease with which bone deforms or the inverse of stiffness. Compression A mode of loading where the forces are acting in opposite directions along the longitudinal axis of the bone. Compression results in a decrease in length and an increase in width. Ductility The ability of a material to deform under tensile stress. Elastic deformation Deformation or strain in bone that is reversible when the stress is released. Elastic modulus (Young’s modulus) The ratio of stress to strain in the elastic region of deformation. Because of the anisotropic nature of bone, the moduli in compression and tension differ in bone or the slope of the stress–strain curve. P Glossary dt0010 Encyclopedia of Forensic Sciences, Second Edition Comp. by: MNatarajan Stage: Proof Chapter No.: 15 Date:26/4/12 Time:07:33:40 Page Number: 1 Title Name: FRN2 dt0090 among these agents are primarily determined by the size of the impact area and the magnitude and duration of the force. Indirect trauma results in injury at locations other than the point of impact or applied force. Stress and pathological injuries are associated with repetitive forces or disease processes, respectively, that weaken bone. This article will primarily focus on fractures or disruptions in the structural continuity of the bone due to direct mechanical trauma. Goals of Trauma Analysis s0015 The goals of trauma analysis in a medicolegal situation are to determine the proximate (i.e., immediate mechanism) and ultimate (e.g., manner of death) causes of the trauma. This is necessary to aid in determining the cause and manner of death by the medical examiner. In skeletonized or badly decomposed bodies, traumatic injuries of the bone may provide a major avenue for determining the cause and manner of death. However, even in fresh bodies, the direct observation of hard tissue should be conducted in all areas of suspected trauma because the gross examination of bone can provide valuable information that cannot always be obtained using other tissues or other visualizing methods (e.g., radiographs). p0020 doi:10.1016/B978-0-12-382165-2.00015-5 1 To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter SPi. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. FRN2: 00015 While the goals of trauma analysis are to determine the mechanism and ultimate cause of the trauma, the first step of trauma analysis is adequate description of the injury, its location, and pattern. Unfortunately, there can be considerable variation in the expression of injury due to the same mechanism as well as similarities in injuries caused by different mechanisms. Proper description of the injury morphology (i.e., size, shape, location, line of fracture propagation, segment relationships, and pattern) is the basis for determining the mechanism and aids in interpreting the ultimate cause. Interpreting the ultimate cause of the injury requires additional information such as the pattern of trauma in populations, the context of the human remains, and other sources of evidence. Bone Structure and Material Properties p0030 To lay the foundation for how fractures occur, it is important to first have an understanding of the structure of the bone and its material properties. These principles guide the analysis of bone trauma and provide a framework for interpreting the proximate cause of trauma. s0025 Bone Structure p0035 The overall strength of the bone is dependent on its material composition, organization, and overall geometry (amount and overall distribution of the bone). At the most basic level, bone is a composite material composed of an organic matrix with embedded mineral crystals. The organic component consists of collagen and other noncollagen fibers, while the inorganic component is primarily hydroxyapatite crystals. The organic components of bone are primarily responsible for tension properties (i.e., elasticity and toughness), while the inorganic components are responsible for compression properties (i.e., stiffness). In addition to collagen and hydroxyapatite, living bone also contains several types of cells (i.e., osteoblasts, osteoclasts, and osteocytes), blood vessels, nerves, and a significant amount of water. The water in bone increases its resistance to fracturing by absorbing large amounts of energy. Bone generally exists in woven or lamellar form. Woven bone is unorganized and laid down quickly. It is typically found in fast-growing bones or in the callus produced during fracture repair. Lamellar bone is highly organized bone laid down in layers with the orientation of the collagen fibers at different angles in each layer. There are also two primary types of bone based on density and porosity that have different biomechanical properties. Trabecular bone (also known as cancellous or spongy bone), commonly found in the interior of a bone near the ends of long bones and in cuboidal, irregular, and flat bones, has high porosity, low density, and is organized as laminated struts called trabeculae. Cortical bone (also known as compact bone) is the dense bone that forms the thick outer wall of long bones and the thin cortex of cuboidal, irregular, and flat bones. Cortical bone is composed of lamellar bone interspersed with osteons that allow for the incorporation of blood vessels and bone maintenance cells. Osteons can either be primary osteons or secondary (Haversian systems) depending on whether they are formed new or formed by the resorption and replacement of existing bone in a process known as Material Properties of a Bone s0030 The strength of bone is generally defined as its ability to withstand loads (force or moment) without failing. Loads can be applied in tension (stretching), compression (compaction), bending (angulation), torsion (twisting), shear (sliding), or a combination of these basic components. These loads cause internal tensile, compressive, and shear stresses on the bone. The type of stress will vary with the direction of the force and the orientation of the bone. Bending loads produce tensile stress on one surface and compressive stress on the other, while torsion produces tension and compressive stress at an 45 angle to the longitudinal axis and shear force in the transverse plane. To understand how a bone behaves under loads, it is important to understand the relationship between stress and strain. Stress in its simplest definition is the intensity of load per unit area. Stress is generally measured on a small cube of bone to remove the effects of geometry. Strain, on the other hand, is the amount of dimensional change or deformation in shape that occurs due to an applied stress in one plane (Figure 1). Bone will fail when the strain becomes too great regardless of the level of stress. Strain can be either negative or positive depending on the type of load applied. For example, if the two ends of a cube of bone are pulled apart during tensile stress, the bone will undergo an increase in length (longitudinal strain) and a corresponding decrease in breadth (transverse strain). In this case, there is positive strain in length and negative strain in breadth. The ratio of transverse strain to the longitudinal strain is Poisson’s ratio, which is 0.3 for bone. If the load is applied in a manner that causes the angles of the bone cube sides to become distorted or slide, the bone is said to be undergoing shear strain. p0040 p0045 x y0 x0 x y y E L S E V IE R F IR S T s0020 remodeling. Cortical bone responds differently to loads depending on the number and size of osteons. Due to its structure, cortical bone is stronger along the longitudinal axis than along the transverse axis. Finally, the macrostructure of a bone is influenced by its cross-sectional shape, areas of buttressing (thickening), and differential density. During life, the architecture and material properties of a bone adapt to meet the functional demands it experiences. R O O F p0025 Anthropology/Odontology | Biomechanics of Bone Trauma P 2 Comp. by: MNatarajan Stage: Proof Chapter No.: 15 Date:26/4/12 Time:07:33:40 Page Number: 2 Title Name: FRN2 Normal strain Shear strain Figure 1 Schematic illustration of normal (tension) strain and shear strain. Strain is the fractional change in dimension of loaded material. Normal strain along the longitudinal axis is the difference between the deformed length (y) and the original length (y0) divided by the original length ((y y0)/y0). The normal strain in the transverse axis is the deformed breadth (x) divided by the original breadth (x0) divided by x0. Poisson’s ratio is the ratio of the transverse strain (x) to the longitudinal strain (y). f0010 To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter SPi. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. FRN2: 00015 3 Anthropology/Odontology | Biomechanics of Bone Trauma Fail Elastic energy YP ile ct Du ne bo f0020 P Figure 3 Idealized schematic illustrating the principles of a load–deformation curve for different bone conditions and quality. Brittle bone is stiffer and therefore has a steeper slope but fractures with little or no deformation in the plastic region and requires less work for failure than normal bone. Ductile bone undergoes greater deformation before failing and requires greater work for failure than normal bone. YP, yield point; FP, fracture point. deformed before breaking under tension. Brittle bone is more resistant to compression stress, while ductile bone is more resistant to tensile stress. The energy absorbed by a bone per volume of area is equivalent to the area under the stress–strain curve and is sometimes referred to as the modulus of toughness. The area under the elastic region is the amount of energy absorbed elastically, while the area under the plastic region is equivalent to the energy absorbed plastically. The stress–strain curve provides a basic understanding of the intrinsic nature of how bone responds to mechanical loads, but it must be kept in mind that bone is a dynamic composite tissue that exhibits anisotropic and viscoelastic characteristics and has a unique geometry. During life, the mechanical characteristics of bone must combine to meet the need for a stiffness and compliance while minimizing skeletal weight. Stiffness of bone reduces strain so it does not deform significantly under load and allows muscles to function more efficiently. Compliance, on the other hand, allows bone to absorb energy and deform to avoid failure during direct or indirect dynamic loading from a fall or blow. The anisotropic nature of bone causes it to behave differently depending on the direction of the force. As a result, the strain on bone is not necessarily the same as the direction of the applied force. That is, the value of the modulus in the stress–strain curve can be different depending on the orientation of the bone. The viscoelastic nature suggests that bone can deform like an elastic material. However, like a viscous material, bone will continue to deform or creep under constant pressure and its stiffness depends on the rate at which the load is applied. Bone becomes more brittle under rapid loading. Therefore, the ultimate or fracture strain is decreased. Also, as a result of its viscoelastic properties, bone is nearly twice as resistant to failure in compression as in tension. This is why bone will normally fail under tension before compression in adults. The actual forces required to break a living bone are frequently different from the intrinsic strength of bone material, S T Figure 2 Idealized schematic illustrating the principles of load–deformation and stress–strain curves. The ultimate force (or stress) is denoted by the height of the curve, while the ultimate deformation (or strain) is denoted by the point of failure. The area under the curve represents the work to failure. The area between the origin and the vertical line dropping from the yield point equals the amount of energy absorbed elastically. The area between the two vertical lines (yield and ultimate strain) represents the amount of energy absorbed plastically. The slope in the elastic region of the curve represents the modulus of elasticity or stiffness. The yield point represents the point when bone stops behaving elastically and begins to behave plastically. R O O F Deformation To understand the relationship between stress and strain, imagine a cube of bone loaded in tension until it breaks. The load can be measured as a function of deformation to form a load–deformation curve (Figure 2). As bone is loaded, it will begin to deform. For a while, the deformation of bone is linearly proportional to the load placed on it. If the deformation is less than 3%, the bone will return to its original shape once unloaded. However, as the load continues to the yield point, the curve begins to flatten. As can be seen in Figure 2, less loading is required to cause increasing deformation. If the load continues to increase, the bone will eventually reach a point of failure and will fracture. The load–deformation curve can be mathematically converted to a strain–stress curve to eliminate the effects of geometry. In the linear portion, the stress and strain are proportional to each other. The relationship of tensile stress to tensile strain is known as the modulus of elasticity or Young’s modulus, while the ratio of shear stress to shear strain is known as the shear modulus. The greater the resistance of bone to stress (steepness of stress–strain curve slopes), the greater is its stiffness. If the bone is unloaded in the elastic region, the stress falls to zero when the strain returns to zero and the bone returns to its original shape. If the load is continued and deformation reaches the yield point, slippage occurs between layers of atoms and molecules at the cement lines, and the stress will return to zero but the strain will not. In this case, the bone is behaving plastically and will remain deformed without healing. If the load continues to increase, the bone will eventually reach its ultimate deformation or fracture point. Brittle bone will fracture under tension before or slightly after reaching the yield point and normally does not show any significant plastic deformation (Figure 3). Tough or ductile bone, on the other hand, will become plastically E L S E V IE R F IR p0050 FP YP Plastic energy Deformation (strain) f0015 Britt le bo ne bo ne No rm al Area under curve = work force to failure Load reg Slope Ela sti c Load (stress) io n Yield point FP FP eg ic r st Pla YP ion Ultimate deformation (strain) Ultimate force (stress) Comp. by: MNatarajan Stage: Proof Chapter No.: 15 Date:26/4/12 Time:07:33:40 Page Number: 3 Title Name: FRN2 p0055 p0060 To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter SPi. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. FRN2: 00015 Anthropology/Odontology | Biomechanics of Bone Trauma Effects of Age and Disease on Bone Material Properties p0065 The age of the individual and some diseases may affect the quality and geometry of the bone, and therefore can significantly influence the fracture risk. Bone’s resistance to fracturing can be influenced by any process that changes its material composition and geometry. The ratio of inorganic and organic components of bone (bone mineral density, BMD) affects stiffness (Figure 3). Highly mineralized bone is stiff but also brittle. As a result, less energy is required to break brittle bone than more compliant, less mineralized bone (Figure 3). Immature bone has a lower BMD than adult bone. As a result, immature bones have greater elasticity but less stiffness than adult bones and can absorb more energy and deformation before fracturing. This is why children are less likely to experience bone fractures from trauma than adults. Incomplete fractures are more common in the immature skeleton because the bone undergoes large deformation due to the ductile properties of the bone (Figure 3). In older adults, on the other hand, the bones become more mineralized, making them stiffer and more brittle. As a result, the bones of older individuals are stronger under compression loads, but less energy is required to cause them to break under tension. In addition, the strength of bone in older individuals may be reduced because of the greater number of secondary osteons, reduction of water content, and the loss of bone, especially trabeculae. Secondary osteons reduce bone density and increase cement lines, while the loss of water decreases the amount of energy that can be absorbed from trauma. Similar to age, pathological changes in bone can affect its quality and geometry. Individuals with diseases such as osteoporosis, osteogenesis imperfecta, osteomyelitis, diabetes, Paget’s disease, Cushing’s disease, rickets, scurvy, tumors, rheumatoid arthritis, and others may be at increased risk of bone fracture. Osteogenesis imperfecta, for example, reduces bone quality and causes long bone cortices to thin. Any apparent disease affecting bone should be noted and its role in the observed fractures should be discussed. s0040 Fracture Propagation s0045 When more energy is transferred to a bone than it can absorb, the bone will fracture because the strain will surpass the ultimate strain. Energy absorbed by a traumatic load builds up in the bone and is released by forming cracks. The greater the energy absorbed by the bone, the greater is the number of cracks that will form. As a result, high-energy trauma will cause bone to fragment, while low-energy loads will usually cause fracture without fragmentation. Bone is weakest in shear followed by tension and strongest in compression. Therefore, fractures will normally propagate in the bone along tension and shear planes. The shear planes run at 45 angles from compressive and tensile stresses. Fractures also follow the path of least resistance. In the skull, for example, there are regions of buttressing (greater thickness) that impede horizontal bending of the skull bones. Therefore, fractures are more likely to occur between the areas of buttressing because the bone can be more easily bent. Fracture lines or cracks will often be diverted toward less buttressed areas. Likewise, fractures may be terminated at suture lines or preexisting cracks since the energy is more efficiently dissipated through these structures. p0075 Fracture Types E L S E V IE R F IR S T s0035 Fracture Propagation and Fracture Types R O O F and numerous other factors can affect the force required to fracture a living bone. In a living person, bone is protected from fracturing through energy-absorbing mechanisms including muscle contractions and deformation of soft tissues. During a fall, for example, muscles contract and reduce the bending of bones by lessening the tensile forces, and soft tissue will absorb much of the energy. However, if the energyabsorbing mechanisms that protect the bone are impaired by surprise, restriction, or incapacity, bones are more likely to fracture under less force. The size and geometry of bones also affect its ability to resist fracture. The geometry of bones allows them to effectively withstand normal loads while remaining light. Intuitively, large bones distribute forces over a larger area and are therefore more resistant to fracture than smaller bones of similar shape. Bones with a larger cross-sectional second moment of inertia and polar moment of inertia are also more resistant to bending and torsional fracture, respectively. Bones that are further from the neutral axis are more efficient at resisting strain. If two bones have the same cortical thickness, the bone with the larger diameter will have greater resistance to fracturing. P 4 p0070 Comp. by: MNatarajan Stage: Proof Chapter No.: 15 Date:26/4/12 Time:07:33:40 Page Number: 4 Title Name: FRN2 The type of fracture produced in bone depends on the amount and location of force applied and the area of impact. Proper description of bone injuries can provide information regarding the type and direction of forces and aid in the mechanism and ultimate cause of trauma. Unfortunately there is very little standardization in the procedures or terminology for documenting bone injuries. Furthermore, most bone trauma classifications are derived from the medical literature that are not necessarily appropriate for forensic scientists. Bone fractures are commonly classified based on general fracture types that occur in all bones, fractures that occur in specific bones, and fractures that cause soft tissue damage. Detailed descriptions of fracture types for each major bones of the body can be found in many references (see section ‘Further Reading’). The intent here is not to develop a standard system or provide an exhaustive list, but to give a biomechanical description of some of the fracture types commonly found in the forensic literature and typical forensic cases. In all cases, bone injuries should be documented mentioning the bone or bones involved, specific location of injury on each bone, type of injury (fracture, dislocation, etc.), appearance of the injury, patterning of fracture lines, apparent direction of the force, length of the fractures, presence of deformation, evidence of the timing (antemortem, perimortem, postmortem) of the injury, and any evidence of complications. In addition, when possible, whether the fracture is open or closed, the percentage and direction of apposition (amount of contact between fragments in fresh or healed injuries), direction of rotation (internal or external rotation of the distal end), and degree and direction of angulation should be documented. The first major distinction in fracture type is whether the break involves complete or incomplete discontinuity. s0050 p0080 p0085 p0090 To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter SPi. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. FRN2: 00015 Anthropology/Odontology | Biomechanics of Bone Trauma Transverse fractures result from forces applied perpendicular to the longitudinal axis of the bone, often as the result of bending forces on brittle bones and bones that are not under compression due to weight-bearing functions. When bent, the tensile stress is applied to the convex surface opposite the direction of force and compression stress on the concave surface forming a neutral axis (point at which the tensile and compressive stresses cancel each other out) near the center of the bone shaft. Since bone is more resistant to compression than tensile stress, the bone will fail on the tensile side. As the fracture propagates across the shaft toward the compression side, the crosssectional area of the bone is reduced and the neutral axis shifts toward the compressive side until the fracture is complete. If the bone is under compressive forces as well, a transverse fracture will begin on the tensile side, but as the fracture crack moves toward the compression side, shear forces become greater and the fracture will begin to travel along the shear plane at approximately a 45 angle in one or both directions because bone is weaker in shear than in compression. If the fracture is propagated in both directions, it will result in a butterfly fracture characterized by a wedge-shaped fragment of bone on the side opposite of the force. Whether a butterfly fragment is produced probably depends on the duration and magnitude of the bending and compression loads. Bending forces may result in an incomplete transverse fracture (beginning on the tensile side), called a greensick fracture, that may or may not deviate at right angles. In greenstick fractures, the unfractured portion of the bone often remains permanently bent. Greenstick fractures are more common in immature bones that have greater compliance, but may also occur in adult bones such as the ribs. Under high loads, crushing or comminution of the bone may also occur. Avulsion fractures, which are usually of less forensic significance because they are self-inflicted, also result in a transverse fracture line. However, an avulsion fracture results when a muscle tendon, ligament, or joint capsule is pulled creating significant tensile stress that caused the bone to fracture. Oblique fractures result from a combination of moderate bending and compressive forces or bending and torsion that cause the bone to break diagonally (often at a 45 angle) to the R O O F General fracture types p0095 E L S E V IE R F IR S T s0055 long axis. The angle of an oblique fracture depends on whether the compressive or bending force is greatest. If the compression forces are greater, the fracture will be more oblique. If the bending forces are greater, the fracture will be more transverse. Long oblique fractures, which are often difficult to differentiate from spiral fractures, occur when the predominant force is torsion. Oblique fractures often begin as transverse fractures but quickly follow the shear plane, resulting in a break with a short transverse section and a longer oblique section. Spiral fractures occur due to torsion or twisting force that produces a fracture that circles or spirals around the shaft. When a long bone shaft is twisted, the compressive and tensile stresses are at approximately a 45 angle to the shaft. Tensile stresses produce a fracture that winds around the surface and breaks completely when a longitudinal fissure occurs or the beginning and ending edges of the crack connect. The fracture originates where the tension stress is greatest and follows the angle of rotation until the fracture ends are approximately parallel or above one another. The direction of the twisting can be determined by the direction of the spiral. Comminuted fractures are those that result in more than two fragments, and often result from large direct or indirect forces with high energy absorption. Indirect forces frequently result in a ‘T’ or ‘Y’ pattern fracture. Comminuted fractures may also result in direct blunt force trauma or penetration from a high-velocity projectile. Crush fractures occur due to direct force to the bone which results in depression (forces originating on one side) or compression (forces originating on two sides) fractures. Depression fractures can result in incomplete penetration of projectiles or blunt trauma produced by an object striking the bone. The degree of fracturing is affected by the size of the impact area and the velocity of the force. If the area of impact is small and the velocity is great, the resulting fracture may be a penetrating injury. Torus, buckling, or impact fractures are due to compression force and occur when the ends of a long bone are driven toward each other. The resulting fracture is an outward displacement of the cortical bone around the circumference of the bone, usually near the end of the long bone shaft. Examples of how this type of fracture occurs include fracturing of the proximal humerus when falling onto outstretched upper limbs or fracturing of the metacarpals when punching or striking an object with the fist. Because the diaphysis is composed of thicker, denser cortical bone and the metaphysis is primarily trabecular bone surrounded by a thin layer of cortical bone, compression forces are more likely to result in buckling of the metaphysis than of the diaphysis. Buckling is also more likely to occur in immature bones than in adult bones. p0100 Comp. by: MNatarajan Stage: Proof Chapter No.: 15 Date:26/4/12 Time:07:33:40 Page Number: 5 Title Name: FRN2 p0105 p0110 P These two classes of fractures are then subdivided based on the type of force, pattern of cracking, degree of fracturing, and bone type. Incomplete fracture types are commonly described as bow injuries (the force is dissipated between the yield and failure points), torus, greenstick, toddler, vertical, and depression fractures. Complete fractures are classified based on their shape and location and include transverse, oblique, spiral, comminuted, butterfly, segmental, and epiphyseal fractures. Cranial fractures include linear (simple linear, diastatic, and stellate), crush (depressed), and penetrating. While discussions of the type of fracture are useful for understanding the mechanism of force, the different types of fractures are not mutually exclusive. Linear fractures, for example, may arise from a depression fracture associated with blunt trauma or from a penetrating fracture associated with gunshot projectile trauma. In general, the type of fracture that occurs is often associated with the velocity and mass of the striking object and the area of the impact forces. 5 p0115 p0120 Cranial fractures s0060 Fractures of the skull can occur from direct or indirect trauma. Linear, crush, and penetrating fractures are common with direct trauma. As with more general fracture types, these fractures are not mutually exclusive. Under direct trauma, the skull behaves similar to a semielastic ball. The curve of the cranial vault at the impact site will flatten or bend internally while the surrounding bone will bend outward. Fracturing will occur on the tensile side in areas of bending. Whether the fracture will begin at the impact site and radiate away or begin away from p0125 To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter SPi. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. FRN2: 00015 Odontology: Post Mortem Interval (00006); Anthropology/ Odontology: Bone Pathology and Ante Mortem Trauma (00014); Anthropology/Odontology: Bone Trauma (00016). Further Reading Berryman HE and Symes SA (1998) Recognizing gunshot and blunt cranial trauma through fracture interpretation. In: Reichs KJ (ed.) Forensic Osteology: Advances in the Identification of Human Remains, 2nd edn., pp. 333–352. Springfield, IL: Charles C Thomas. Bilo RAC, Robben SGF, and Rijn RR (2010) Forensic Aspects of Pediatric Fractures: Differentiating Accidental Trauma from Child Abuse. New York: Springer. Currey JD and Butler G (1975) The mechanical properties of bone tissue in children. Journal of Bone and Joint Surgery 57A: 810–814. Elstrom JA, Virkus WW, and Pankovich A (2006) Handbook of Fractures, 3rd edn. New York: McGraw-Hill. Galloway A (ed.) (1999) Broken Bones: Anthropological Analysis of Blunt Force Trauma. Springfield, IL: Charles C Thomas. Gurdjian S, Webster JE, and Lissner HR (1950) The mechanism of skull fracture. Journal of Neurosurgery 7: 106–114. Johnson KD and Tencer AF (1994) Biomechanics in Orthopaedic Trauma: Bone Fracture and Fixation. London: Informa Healthcare. Kimmerle EH and Baraybar JP (2008) Skeletal Trauma: Identification of Injuries Resulting from Human Rights Abuse and Armed Conflict. Boca Raton, FL: CRC Press. Lovell NC (1997) Trauma analysis in paleopathology. Yearbook of Physical Anthropology 40: 139–170. Moraitis K and Spiliopoulou C (2006) Identification and differential diagnosis of perimortem blunt force trauma in tubular long bones. Forensic Science, Medicine, and Pathology 2: 221–229. Pierce MC, Bertocci GE, Vogeley E, and Moreland MS (2004) Evaluating long bone fractures in children: A biomechanical approach with illustrative cases. Child Abuse and Neglect 28: 505–524. Spitz WU and Spitz DJ (2006) Medicolegal Investigation of Death: Guidelines for the Application of Pathology to Crime Investigation, 4th edn. Springfield, IL: Charles C Thomas. Turner CH (2006) Bone strength: Current concepts. Annuals of the New York Academy of Sciences 1068: 429–446. Turner CH and Burr DB (1993) Basic biomechanical measurements of bone: A tutorial. Bone 14: 595–608. Wieberg DAM and Wescott DJ (2008) Estimating the timing of long bone fractures: Correlation between the postmortem interval, bone moisture content, and blunt force trauma fracture characteristics. Journal of Forensic Sciences 53: 1028–1034. R O O F the impact site and radiate in both directions probably depends on the elasticity of the skull, the size of the impact area, the size and shape of the object impacting the skull, the architecture of the skull at the impact area, the magnitude of the force, and other factors. Linear fractures often occur when the skull is struck or impacts an object of high mass. Low-velocity direct forces often result in linear or depressed (crush) fractures of the cranial vault. High-energy loads with a small impact area or pointed objects striking the skull will result in depression or penetrating injuries. Stellate or starshaped fractures are formed by multiple linear fractures radiating from the point of impact. If the damage is more extensive, comminuted fractures may occur. Concentric fractures are produced when a blunt or penetrating object causes inward bending of the bone between radiating fractures. They generally circumscribe the impact area and are roughly perpendicular to the radiating fractures. As the plates of bone between radiating fractures bend inward, tensile stress occurs on the external surface of the outer table and progresses like transverse fractures from the outer to the inner table. Bullet wounds are usually characterized by internally beveling entrance and externally beveling exit fractures. The shape of the wound depends on the angle at which the bullet strikes the bone. Because of the high energy and velocity associated with projectile force, the bone is unable to absorb the energy and fractures will commonly radiate from the initial perforating fracture. Concentric fracturing may also occur. However, unlike radiating and concentric fractures caused by blunt trauma, these fractures will initiate due to tension on the inner table of the vault due to intracranial pressure. In addition, trauma resulting from a gunshot will not display the inward deformation of the bone often seen in fractures occurring from blunt trauma. F IR S T p0130 Anthropology/Odontology | Biomechanics of Bone Trauma P 6 E L S E V IE R See also: Anthropology/Odontology: Animal Effects on Bone (00001); Anthropology/Odontology: History of Forensic Anthropology (00002); Anthropology/Odontology: Recovery and Retrieval: Forensic Archaeology (00005); Anthropology/ Comp. by: MNatarajan Stage: Proof Chapter No.: 15 Date:26/4/12 Time:07:33:41 Page Number: 6 Title Name: FRN2 To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter SPi. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. FRN2: 00015 Non-Print Items Abstract: The biomechanics of bone trauma is the application of mechanical laws to describe and interpret damage that occurs to bone. It involves the examination of both the intrinsic and extrinsic factors resulting in bone injury. Knowledge of the biomechanics of bone trauma allows forensic scientists to use bone fractures to deduce the type and direction of loading that caused the bone to fail. The strength of the bone is its ability to withstand loads without failing, and is dependent on its material composition, organization, and overall geometry. Loads can be applied in tension, compression, bending, torsion, shear, or a combination of these basic components. These loads cause internal tensile, compressive, and shear stresses on the bone. However, bone is a dynamic composite tissue that exhibits anisotropic and viscoelastic characteristics and will react differently to loads depending on its orientation and the rate at which the force is applied. When more energy is transferred to a bone than it can absorb, the bone will fracture. The type of fracture produced in the bone depends on the magnitude, direction, rate, and area of force applied and the structural and material properties of the bone at and near the location of the force. Keywords: Biomechanics; Bone; Bone injury; Brittle; Ductility; Elasticity; Forensic anthropology; Fracture; Plasticity; Stiffness; Strain; Stress; Trauma Author and Co-author Contact Information: Au1 Daniel J. Wescott Department of Anthropology Texas State University 601 University Drive San Marcos TX 78666 USA Tel.: +1-573-424-9663 E-mail: [email protected] Biographical Sketch Daniel J. Wescott is the director of the Forensic Anthropology Center at Texas State University in San Marcos, Texas. He received his BA and MA in anthropology at Wichita State University in 1994 and 1996, respectively. In 2001, he earned his Ph.D. in anthropology from the University of Tennessee. He has an active research program focusing on testing hypotheses and answering questions related to forensic anthropological methods and skeletal biology, especially bone biomechanics. Wescott’s research has been disseminated in leading professional journals. He has served on the editorial board of the Journal of Forensic Sciences, and as a member of the Scientific Working Group for Forensic Anthropology. To protect the rights of the author(s) and publisher we inform you that this PDF is an uncorrected proof for internal business use only by the author(s), editor(s), reviewer(s), Elsevier and typesetter SPi. It is not allowed to publish this proof online or in print. This proof copy is the copyright property of the publisher and is confidential until formal publication. FRN2: 00015 Author Query Form Book: Encyclopedia of Forensic Sciences, Second Edition (FRN2) Article No.: 00015 Dear Author, During the preparation of your manuscript for typesetting some questions have arisen. These are listed below. Please check your typeset proof carefully and mark any corrections in the margin of the proof or compile them as a separate list. Your responses to these questions should be returned within seven days, by email, to MRW Production, email: [email protected] Query Details Required AU1 Please check the full affiliations for accuracy. These are for Elsevier’s records and will not appear in the printed work. AU2 Please check whether the section heading levels are correct as typeset. AU3 Please provide relevant websites for this article. Author’s response