1. 35 CHAPTER 35 LONG BONE FRACTURE BIOMECHANICS 317 Long Bone Fracture Biomechanics Gina Bertocci, PhD, PEINTRODUCTION through the bone tissue and their load carrying capacity. Each type of bone has unique biomechanical properties andFractures in children can occur as a result of either accident therefore has a unique response to the application of force.or abuse. Clinicians are often asked to determine whether a These differences reflect their specific function in the humanfracture could have resulted from a stated cause such as a body. Cortical bone is primarily responsible for the support-fall from a sofa or bed. To determine whether a stated cause ive load-bearing function of the skeleton, while cancellousis compatible with a presenting fracture, a basic understand- bone provides cushioning to skeletal structures duringing of biomechanics as it relates to fractures can be useful. loading.Biomechanics relies upon the use of engineering or physics A more detailed description of bone anatomy is providedprinciples to study the response of biological tissue to physi- in Chapter 31.cal phenomena such as force, acceleration, or pressure. Bio-mechanical principles can provide an improved understandingof how a bone will respond to the application of a force and BIOMECHANICALits likelihood to fracture under certain conditions. CONCEPTS IMPORTANT TO The objectives of this chapter are: UNDERSTANDING FRACTURES1. To describe characteristics of bone tissue anatomy that To determine whether a bone is likely to fracture under a are related to bone strength and biomechanical response given loading condition, key biomechanical terms and con- to loading cepts must first be understood. Table 35-1 lists biomechani-2. To describe fundamental biomechanical concepts impor- cal terms and definitions. tant to fracture prediction3. To describe biomechanical factors that affect the likeli- hood of bone fracture Force The application of a force tends to cause a body or objectOVERVIEW OF LONG BONE ANATOMY with mass to accelerate, change position, or change shape. Force can be defined as the mass of an object times its accel-Long bones, such as the femur, tibia, and humerus, consist eration. Figure 35-2 illustrates the response of a spring to aof the shaft, or diaphyseal region, which consists of compact compressive force and tensile (pulling) force. A combinationor cortical bone (Figure 35-1). The segments on either end of multiple forces along with their direction of applicationof the diaphysis consist of cancellous or trabecular bone, and can be defined as the loading condition.are called the metaphyseal region. A connective tissue layer,periosteum, covers the outer surface of long bones. Moment Cortical bone tissue is dense, and is composed of haver-sian microsystems, which include concentric rings of lamel- A moment is the tendency to produce an object’s rotationlas constructed of mineralized collagen fibers and lacunae. when applied at a perpendicular distance (moment arm)The lacunae contain bone cells, or osteocytes. The number, from the axis of rotation.3 A moment can be defined assize, and distribution of the lacunae affect the cortical bone’s the force applied times the moment arm. The conceptsresponse to loading.2 related to a moment can be illustrated by the action of a Cancellous bone tissue is made up of a network of rods seesaw (Figure 35-3). When two individuals of equal massand plates that resemble a honeycomb structure. The plates are sitting on the seesaw, it is balanced, the moments onare mineralized and their thickness and direction of align- each side are equal, and no movement occurs. However,ment affects load-bearing capacity. The honeycomb struc- when an individual of larger mass (delivering a greaterture of cancellous bone tissue is made up of microstructure downward force to the seesaw) sits on one side of theunits referred to as trabeculas. Cancellous bone is less dense seesaw this will generate a larger moment, which serves tothan cortical bone and is porous in nature with a high offset the balance of the seesaw creating its downwardsurface area. motion on the side of the individual with greater mass. These differences in cancellous and cortical bone micro- This downward movement tends to rotate the seesawstructure lead to differences in the transmission of forces about is axis of rotation. 317
2. 318 SECTION V PHYSICAL ABUSE OF CHILDRENMetaphyseal Trabecular bone region with adherent periosteum and thin cortex Thick cortical bone Diaphyseal with relatively region loose periosteum Static Compressed Stretched Transition FIGURE 35-2 Response of spring to compression (center) and between tension (right). Application of these forces alters the shape of the diaphysis and spring.Metaphyseal metaphysis region BIOMECHANICAL MATERIAL PROPERTIESFIGURE 35-1 Illustration of bone architecture. (From Pierce MC, Properties inherent to a specific biological material thatBertocci GE, Vogeley E, et al: Evaluating long bone fractures in children: a influence how it will respond when exposed to physical phe-biomechanical approach with illustrative cases. Child Abuse Negl nomena (e.g., force, acceleration) are referred to as biome-2004;28:505-524.) chanical material properties. These properties characterize biological tissues and are not dependent on the size or shape (geometry) of the material. An example of a biomechanical material property is elasticity.Stress ElasticityForce normalized over the area to which it is applied is A material is said to be elastic if it deforms under external force,referred to as stress. Stress can be determined by dividing the but returns to its original shape when the force is removed. Aforce by the cross-sectional area of force application. There- biomechanical material property that describes the elasticfore, for a given force, as the cross-sectional area decreases, nature of a material is elasticity. Elasticity can be thought of asstress increases. That is, a force applied to a small cross- defining the stiffness of a material, and it is often describedsectional area will yield a higher stress than when that same through a parameter known as the elastic modulus (abbreviatedforce is applied to a large cross-sectional area (Figure 35-4). as E1). The elastic modulus, E, of a material can be derivedThe stress that develops within a bone under force applica- for a material by knowing the ratio of stress to strain. E istion is an important factor in determining how the bone will independent of the size and shape of a material. The elasticrespond and whether the bone will fracture. Stress also can modulus is a commonly reported material property in engi-be described based upon the direction or type force that is neering handbooks for biological materials and for commonapplied to an object. Terms that define specific types of stress materials such as metals, wood, plastics, etc. Often engineersdepend upon the characteristics of force application; these will perform an experiment that entails applying a force orinclude compression, tension, bending, torsion, and shear stress to a material, while measuring the corresponding defor-(Table 35-1). Figure 35-5 demonstrates graphical depictions mation or strain. These quantities are then plotted againstof stress applications. each other and the slope of the line formed by the plot can be determined to yield the elastic modulus (Figure 35-7). A material with a low elastic modulus (E2 in Figure 35-7) would have greater deformation (greater strain) under a given loadStrain (or stress) as compared with a material with a high elasticThe change in length of an object (e.g., bone) normalized to modulus (E2 in Figure 35-7).its original length is referred to as strain. Strain can be deter-mined by dividing the change in length of an object by its Yield Strengthoriginal length (Figure 35-6). Strain is also an importantfactor in determining whether or not a bone fractures under From a stress : strain curve, additional material propertiescertain loading conditions. can be defined and are important to predicting a bone’s
3. CHAPTER 35 LONG BONE FRACTURE BIOMECHANICS 319 Table 35-1 Biomechanical Terms and Definitions Term Definition Anisotropy Material that displays different material properties and responds differently to loading in different directions. Long bones are typically strongest in compression and weakest in shear loading. Bending stress Occurs when a force is applied perpendicular to the long axis of a structure or object causing tension on one side and compression on the other. Biomechanical Characterizes a material and defines how a material will respond to exposure to physical phenomena material properties (e.g., force, acceleration). Modulus of elasticity is an example of a material property. Biomechanics Study of response of biological tissue to physical phenomena such as force, acceleration, pressure, etc. Compression Stress created by compressing or “squeezing” an object or structure. Deformation Change in size or shape of an object due to application of force. Deformation can be elastic or permanent. Elasticity Material is said to be elastic if it deforms under stress (e.g., external forces), but then returns to its original shape when the stress is removed. Often described through modulus of elasticity, E, which is the ratio of stress to strain and can be thought of as defining the stiffness of a material. Force Application of which tends to cause a body or object with mass to accelerate, change position, or child shape. (Force = Mass × Acceleration) Fracture Failure of structure such that it is unable to support or withstand the applied load. Fracture threshold Level of force or stress above which a fracture will occur. Load Describes the application of forces or moments to a body or object. Moment The tendency of a force to produce body or object rotation when applied at a perpendicular distance (moment arm) from the axis of rotation.3 (Moment = Force × Moment Arm) Shear stress Stress produced when force application is aligned with the surface of a body or object. Strain Change in length normalized to the original length of a body or object.3 (Strain = Change in Length/Original Length) Stress Force normalized to the area over which it is applied.3 The same force applied to a smaller cross section will yield a higher stress. (Stress = Force/Cross-Sectional Area) Tension Stress created by extending or “pulling” an object or structure. Torsional stress Results from twisting an object or structure about its longitudinal axis. Ultimate strength Stress beyond which an object or structures will fail or fracture. Viscoelasticity Material is said to exhibit viscoelastic behavior if its response is dependent upon rate of strain application. A viscoelastic material will appear stiffer at higher rates of strain application. Yield strength Also known as elastic limit. Stress beyond which an object or structure will undergo permanent deformation. Material responds elastically below the yield strength.response to loading. The yield strength of a material can be is removed) and deformation is reversible in this region. Indefined as the stress beyond which an object or structure will contrast, stresses greater than the yield strength will lead toundergo permanent or plastic deformation. (Note: Although the plastic, irreversible, deformation. Figure 35-8 illustrates theterm “plastic” is often used clinically to indicate a reversible situation, point on the stress : strain curve that identifies a material’sin contrast the terminology “plastic” behavior or deformation in reference yield strength. The stress : strain curve illustrates key proper-to material response indicates a permanent or irreversible response.) In ties related to a material’s response to loading. The materialother words, a material responds elastically below the yield will respond elastically at stresses below the yield strength,strength (material returns to its original shape when the load and plastically when stresses exceed the yield strength.
4. 320 SECTION V PHYSICAL ABUSE OF CHILDREN Smaller Moment Moment mass Arm Arm Larger mass Force Force Axis of rotation Moment = Moment Moment small < Moment largeFIGURE 35-3 Individuals with equal mass balance the seesaw creating equal moments (left). The individual with greater mass generates a largermoment causing the seesaw to rotate (right). F F E = Elastic modulus Stress E = Stress/strain E1 E1 stiffer than E2 Stress > StressFIGURE 35-4 A force applied to a smaller cross-sectional area E2generates a higher level of stress as compared to when this sameforce is applied to a larger cross-sectional area. Strain FIGURE 35-7 Elastic modulus is the slope of the stress : strain curve. It is often referred to as the stiffness of a material. In this diagram, the material defined by E1 is stiffer than the material defined by E2. F F F Elastic Plastic F F region region F Fracture strength Stress F F F Ultimate strength F E1 FCompression Tension Bending Torsion Shear Yield strengthFIGURE 35-5 Type of stress depends upon the characteristics offorce application. Strain at fracture Strain FIGURE 35-8 Stress : strain curve illustrating key properties related to a material’s response to loading. The material will respond elastically at stresses below the yield strength, and plastically when stresses exceed the yield strength. ∆L L Strain = L Ultimate Strength The ultimate strength of a material is the stress at which a material or structure will fail. In the case of bone tissue, failure would be defined as fracture. ∆L Force AnisotropyFIGURE 35-6 Force applied to an object can stretch or deform the Some materials respond differently when loading is appliedobject. The change in length as compared with an object’s original from different directions. These materials are defined aslength is defined as strain. anisotropic. Materials responding similarly under loading
5. CHAPTER 35 LONG BONE FRACTURE BIOMECHANICS 321conditions applied from varying directions are referred to as Anisotropy and Strength. As previously stated, boneisotropic. The material will respond elastically at stresses tissue is considered an anisotropic material, responding differ-below the yield strength, and plastically when stresses exceed ently under varying directions of loading. For example,the yield strength. Table 35-3 shows that femoral cortical bone tissue loaded longitudinally (parallel to the long axis of the bone) in tensionFACTORS AFFECTING has a higher ultimate strength (133 MPa) as compared to when the same tissue is loaded in a transverse directionLIKELIHOOD OF FRACTURE (perpendicular to the long axis of the bone) under tensionThe likelihood of a fracture occurring in a specific bone is (51 MPa).6dependent upon a number of factors that can be categorized Long bones also typically have increased strength underas extrinsic or intrinsic factors. Models capable of predicting compressive loading conditions, and exhibit the lowestfractures are dependent upon accurate representation of strength under application of shear loading. Table 35-3 alsoboth intrinsic and extrinsic factors. Whether or not a frac- illustrates this concept for femoral cortical bone tissue.6ture occurs depends upon the internal stresses developed When the femur is loaded longitudinally (parallel to the longwithin the bone, and whether these internal stresses exceed axis of the bone) under compression, the ultimate strengththe fracture threshold. is 193 MPa, whereas when it is loaded longitudinally in tension, the ultimate strength decreases to 133 MPa. This difference in strength is also seen when the comparing com-Intrinsic Factors pression and tension loading in the transverse direction.The response of bone to loading is dependent upon intrinsicfactors that include both material and structural or geomet-ric characteristics of the bone. Elastic Moduli for Bone Tissue Table 35-2 and Other Materials3Bone Material Properties Elastic Modulus. Table 35-2 provides elastic moduli for Material E (GPa)*cortical bone and trabecular bone in comparison with other Cortical bone 12-24†common materials. Table 35-2 illustrates that cortical boneis stiffer than trabecular bone (Ecortical > Etrabecular). Therefore, Trabecular bone 0.005-1.5†for a given loading condition (below the yield strength),trabecular bone will undergo greater deformation as com- Stainless steel 190pared with cortical bone. Trabecular bone tissue found at Polyethylene‡ (UHMWPE) 1.2the epiphysis and metaphysis of long bones has a high levelof porosity, providing the ability to deform without failure. *Gigapascal = 109 pascal.The openings in trabecular bone tissue are filled with marrow † Depends upon density and direction of loading.and fat, which help to provide a degree of energy absorption ‡ Ultra high molecular weight polyethylene, used in joint replacements.under loading.4,5 Ultimate Strength and Elastic Modules for Femoral Cortical Bone Under Varying Loading Conditions and Table 35-3 Directions Type of Bone Direction Loading Condition Ultimate Strength Elastic Modulus Cortical bone, femur Longitudinal Tension 133 MPa 17.0 GPa Cortical bone, femur Longitudinal Compression 193 MPa* 17.0 GPa Cortical bone, femur Transverse Tension 51 MPa 11.5 GPa Cortical bone, femur Transverse Compression 133 MPa 11.5 GPa *Strength of femoral cortical bone tissue varies depending upon loading direction and characteristics of the load. The red box designates the conditions under which the femoral cortical bone has the greatest strength.7 (The specimens evaluated in this study were harvested from cadavers ranging in age from 19-80 years.) MPa, Millipascal; GPa, gigapascal.
6. 322 SECTION V PHYSICAL ABUSE OF CHILDREN 30 ri Strength (MPa) 20 ro FIGURE 35-10 Approximation of long bone cross-sectional geometry 10 (ri = inner radius; ro = outer radius). 0 0 200 400 600 Femoral Geometric Characteristics Bone mineral density (mg/cc) Table 35-4 and Moment of Inertia for Newborn vs. 6-Month-Old ChildFIGURE 35-9 Compressive strength of femoral trabecular bonetissue increases with increasing bone mineral density.8 Moment of Outer Cortical Age Inertia Diameter Thickness Newborn 63 mm4 6.0 mm 2.15 mm Density and Strength. Bone tissue strength is also depen-dent upon bone mineral density.7 In general, the strength of 6 months 291 mm4 9.0 mm 2.0 mmbone tissue increases with increasing bone mineral densityas illustrated in Figure 35-9. Trabecular bone mineraldensity ranges from 0.1 to 1.0 g/cc, whereas cortical bonetissue density is approximately 1.8 g/cc. Since bone mineraldensity is directly related to strength, it is important when Bending. When a bending load, or moment, is appliedassessing the likelihood of fracture to consider those condi- to a bone, the maximum internal stress (σ) developed withintions that can alter bone mineral density (see Chapter 31). that bone is dependent upon the magnitude of the applied Bone Geometric Characteristics. The geometric charac- moment and the geometric characteristics of the bone. Theteristics of the bone structure will also affect its response to internal stress is directly related to the bending momentloading. Depending on whether the bone is subjected to applied and indirectly related to the moment of inertia (describedbending, axial loading or torsion determines which aspects later).of the bone geometry relates to its resistance to fracture. In a simplified representation of a long bone cross-sec-Important geometric characteristics can include the inner tional geometry as a hollow tube, the inner radius (ri, centerand outer diaphysis radii, along with the cross-sectional area, of bone to medullary canal wall) and outer radius (ro, centeror cortical wall thickness, of the bone structure. of bone to outer cortical wall) affect the level of stress devel- oped within the bone (Figure 35-10). When a bending load is applied, the structure is in tension on the side of loadThe Importance of Intrinsic Factors application, and in compression on the opposite side. The ability of a bone to resist bending stress is dependent uponRelated to Pediatric Bone Tissue its moment of inertia, I. When a long bone is approximatedCurry8 compared age-related bone tissue from younger chil- as a hollow tube, the moment of inertia can be defined as;dren to adults. Bone tissue from younger children dissipated I = Π 4 ( ro 4 − ri 4 )more energy before fracture. With larger haversian canals, a Where Π = 3.14159265.child’s bone is more porous and thus tolerates a greater levelof strain as compared with adult bones prior to fracture. As an example, Table 35-4 compares geometric charac- teristics that are used to determine the moment of inertia for the femur of a newborn and 6 month-old-infant to illustrateExtrinsic Factors this concept. As shown, the 6-month-old child has a momentBone tissue response to loading is also dependent upon extrin- of inertia, which is 4.6 times that of a newborn.sic factors such as loading characteristics. The maximum internal stress, σ, associated with a appli- cation of a bending moment, M, can then be estimated as;Types and Characteristics of Loads σ = My I ,The characteristics of the loading applied to a bone are key where y = distance from the neutral axis, or ro when estimat-to understanding its resistance to fracture. The magnitude, ing the maximum internal stress.distribution, and direction of loading are important to Using the previous comparison of the newbornwhether or not a bone will fracture. Various combinations and 6-month-old child’s femur geometry and moment ofof geometric characteristics (intrinsic factors) are key to a inertia, it can be shown that for a given bending momentbone’s ability to resist fracture depending upon the char- application, the internal femoral stress, σ, experienced byacteristics of loading. Two common loading conditions, the 6-month-old is one third of that experienced by thebending and torsion, are discussed in greater detail next. newborn.
7. CHAPTER 35 LONG BONE FRACTURE BIOMECHANICS 323 Once the maximum internal bending stress, σ, has been Response to Rate (Speed)estimated, this value is then compared with the ultimate of Loading Applicationstrength of the bone tissue to determine the response of thebone. If σ exceeds the ultimate bending strength of the bone Bone tissue response is also dependent upon the rate attissue, then the bone will fail or fracture. which the loading is applied. Materials that are time- or Torsion. A similar type of analysis can be undertaken as rate-dependent are referred to as viscoelastic. Figure 35-11it relates to torsional loading conditions applied to a long illustrates the influence of strain rate (rate of deformation)bone. In torsional loading, internal bone stress is directly on cortical bone ultimate strength and elastic modulus.9 Thisrelated to the torque (force times moment arm) or twisting figure shows that the ultimate strength and elastic modulusapplied to the bone. The bone’s ability to resist fracture increase with rapid loading or deformation. The ultimateunder these conditions is dependent upon the polar moment of strength increases by roughly a factor of 3, while the elasticinertia, J. Assuming again that the cross-section of a long bone modulus increases by a factor of approximately 2 over thecan be approximated as a hollow tube (Figure 35-10), the strain rate range. (Note that normal activities typically occurpolar moment of inertia, J, can be defined as; at a strain rate of <0.01/sec.) J = Π 2 ( ro 4 − ri 4 ) Table 35-5 compares the femoral geometric characteris- Combining Intrinsictics and polar moment of inertia of a newborn and 6-month- and Extrinsic Factorsold child. Table 35-5 shows that the 6-month-old child hasa femoral polar moment of inertia that is 4.6 times that of a The process by which one determines whether a fracturenewborn. This difference is important because it relates to occurs in a laboratory setting is obviously somewhat differentthe bone’s ability to resist fracture. than that which can be used in a clinical setting. Nonethe- When torque is applied to a bone, the associated less, it is of value to understand the idealized steps that onemaximum internal torsional stress, τ, can be determined would take assuming that all information relevant to anusing the following equation: incident, the associated loading conditions, and the child’s bone structure and properties could be obtained. Figure τ = Tr J 35-12 provides an overview of the idealized conceptualwhere r represents the radius of the bone, T represents the process that would be used to determine whether a fracturetorque applied, and J represents the polar moment of inertia. would occur for a given incident.Using the geometric data provided and polar moment of In this idealized approach, the loading characteristicsinertia determined (Table 35-5) for the newborn and (direction of application, location of application, and mag-6-month-old child, the maximum internal torsional stress for nitude) associated with an incident are extracted or deter-a given applied torque can be expressed as: mined through close examination of the event. These loading characteristics are used to estimate the internal stress devel- T6 mo = T (4.5) 582 Tnewborn = T (3) 126 oped within the bone structure. The internal stress devel- T6 mo : Tnewborn = 0.33 oped within the bone is then compared with the strength of the bone tissue to determine the resultant response. Therefore, it can be shown that for a given torque appli- Although the approach presented in Figure 35-12 pro-cation to the femur, the internal torsional stress experienced vides an “ideal” method for assessing fractures, manyby the 6-month-old child would be one third of that experi- unknowns usually exist, preventing clinical application ofenced by a newborn. this approach. Unknowns might include complex loading Again, once the maximum internal torsional stress hasbeen determined, this value can be compared with the ulti-mate torsional strength of the bone tissue to determine 300 70whether or not a fracture would occur. Internal stress valuesthat exceed the strength of the bone will lead to failure or Cortical bonefracture of the bone. Ultimate strength (MPa) Elastic modulus (GPa) 150 40 Femoral Geometric Characteristics Table 35-5 and Polar Moment of Inertia for a Newborn vs. 6-Month-Old Child Polar Moment Outer Cortical 0 10 Age of Inertia Diameter Thickness .0001 1 1000 Strain rate (I/sec) Newborn 126 mm4 6.0 mm 2.15 mm FIGURE 35-11 Ultimate cortical bone strength and elastic modulus in tension versus strain rate. Both properties increase with increasing 6 Months 582 mm 9.0 mm 2.0 mm strain rate. * = Strain rate of typical normal activities.
8. 324 SECTION V PHYSICAL ABUSE OF CHILDREN Loading Internal Bone conditions bone stress response Bending F σbending Stress b σbending = My/I E1 F F Moment = F(b) Strain Incident assessmentFIGURE 35-12 Idealized conceptual approach to determining bone response to loading for a given incident.conditions that consist of a combination of loading types,unknown magnitude and direction of the loading, andunknown biomechanical properties of the specific child’s Injury Fracturebone tissue. Because such a quantitative approach often falls causation typeshort in a clinical setting, a qualitative, modified approach Fractureto fracture assessment (described below), which is based in assessmentprinciple upon the idealized quantitative approach, canusually be implemented. Injury mechanismQUALITATIVE FRACTURE FIGURE 35-13 Fracture Assessment Model. A model to assessASSESSMENT MODEL fractures and their biomechanical compatibility with a stated cause is dependent upon three components: injury causation, injuryIn the absence of quantitative data regarding the specific mechanism, and fracture type. There should be continuity betweenevent and the child’s bone tissue properties, the determina- these three components.tion of whether a fracture is biomechanically compatiblewith a stated cause can be aided by using the componentsdescribed in Figure 35-13. The proposed qualitative Frac- transmitted to the bone structure (or region of the body)ture Assessment Model attempts to convey the interrelation- where a fracture is present. The description should includeship of injury causation, injury mechanism, and fracture the direction of force(s), the planes of the body where thetype. The components of this model can be defined as force(s) are applied, and an evaluation of the transmission offollows. these forces to the bone. An example of an injury mechanism is a torsional load applied to the tibia when a toddler’s footInjury Causation becomes entangled with a toy or carpet while running or walking in the forward direction. A typical resulting fractureInjury causation is a detailed description of the event that from this type of torsional load application would be a non-leads to a specific fracture or injury. Often the stated cause displaced spiral fracture of the tibia.of the injury is presented by the caregiver. The descriptionshould include as many details as possible, such as the child’s Fracture Typeinitial position and posture, dynamics during the fall, landingposition, and surface upon which the child fell. An example The fracture type is the morphological description of theof injury causation is a child rolling from an 18 inch high resulting fracture pattern and its location on the bone. Anbed from a horizontal posture onto a carpeted floor landing example of fracture type is a spiral fracture to the midshaftonto an outstretched arm. of the tibia. Pierce et al1 provides a detailed overview of fracture types associated with various loading conditions. Loading conditions and resulting fracture types are coveredInjury Mechanism in Chapter 32.The injury mechanism describes how forces or accelerations This model should be applied in a stepwise progressionassociated with the injury causation (i.e., event) can be starting with the injury causation moving through the injury
9. CHAPTER 35 LONG BONE FRACTURE BIOMECHANICS 325mechanism and finally onto the resulting fracture type. There Fracture Type: Comminuted spiral fracture of the femur.should be continuity of flow from one component of the Fracture Assessment: In this case the injury mechanism necessarymodel to the next. Evaluating each component of this model to create a comminuted spiral femur fracture cannot bewill help the clinician to qualitatively “reconstruct” the event ascertained from the injury causation. A comminutedand evaluate the compatibility of the fracture and the stated spiral fracture of the femur would require exposure to highcause. That is, the injury causation must be capable of generat- level of torsional loading. Therefore the continuity of flowing specific forces of a magnitude and direction that can lead from one component of the model to the next is broken,to an injury mechanism that is capable of generating a specific and it can be concluded that the stated cause of injury andfracture type. If the injury causation can lead to fall dynamics that presenting fracture are biomechanically incompatible.are capable of generating a loading pattern that can createa specific fracture type, then the clinician has an improved level KEY POINTS IN FRACTUREof confidence that the stated cause could have resulted in the ASSESSMENTfracture. (This same assessment model can also be used forother types of injuries.) The simplified examples provided ● Knowledge of biomechanics is important when attempt-below will help to illustrate application of the Qualitative ing to determine whether a bone will fracture under givenFracture Assessment Model. loading conditions. ● Both intrinsic and extrinsic factors must be considered when attempting to determine whether a bone will frac-Fracture Assessment ture under given loading conditions. j Intrinsic factors important to determining likelihood ofCase 1: Skiing Incident fracture:Stated Cause: 5-year-old child involved in skiing incident. Bone biomechanical or material properties (elasticInjury Causation: Child was downhill skiing and the tip of her modulus, yield strength, ultimate strength, etc.) ski caught on a tree trunk. The child continued to move Bone geometry (cortical wall thickness, inner and forward down the slope, abruptly falling to the ground, outer radii) facing supine with her leg folded beneath her. The child j Extrinsic factors important to determining likelihood of was subsequently unable to bear weight on one lower fracture: extremity. Characteristics of loading (type, direction and rateInjury Mechanism: The tree trunk retained the tip of the ski such of application) that the ski rotated relative to the child’s lower extremity, ● Internal bone stress is dependent upon loading conditions introducing a torsional (twisting) load on the tibia. and geometrical characteristics of the bone.Fracture Type: Spiral fracture to the diaphysis of the tibia. ● Bone tissue response to loading is dependent upon inter-Fracture Assessment: In this case the injury causation can lead nal bone stress as well as the material or biomechanical to an injury mechanism that can cause the presenting properties of the bone. fracture. As previously stated, torsional loading will lead ● When assessing fractures, injury causation, injury mecha- to spiral fractures and given the injury causation in this nism and fracture morphology must be considered. case, it is reasonable that torsional type loading will be ● Fracture morphology must be compatible with a specific present. The assessment of this case is that the stated cause injury mechanism which can be derived from a specific is compatible with and supports the resulting fracture. injury causation. There must be continuity between injury causation, injury mechanism and fracture type.Fracture Assessment Case 2: Sofa Fall ReferencesStated Cause: 6-month-old child fell from sofa.Injury Causation: The caregiver stated that the child was lying 1. Pierce MC, Bertocci GE, Vogeley E, et al: Evaluating long bone frac- tures in children: a biomechanical approach with illustrative cases. Child on a sofa 18 inches above a padded carpeted floor and Abuse Negl 2004;28:505-524. then rolled off the sofa. The caregiver also stated that the 2. Mullender MG, Huiskes R, Versleyen H, et al: Osteocyte density and child fell free to the floor, did not impact any object histomorphometric parameters in cancellous bone of the proximal femur during the fall, and no limbs were retained by the sofa. in five mammalian species. J Orthop Res 1996;14:972-979.Injury Mechanism: When evaluating the fracture type (spiral 3. Lucas GL, Francis CW, Friis EA: A primer of biomechanics. Springer, New York, 1999. comminuted), one would evaluate the injury causation to 4. Hall SJ: Basic biomechanics, ed 3, WCB McGraw-Hill, Boston, 1999. determine if torsional loading (necessary to generate a 5. Gomez MA, Nahum AM: Biomechanics of bone. In: Nahum A, Melvin spiral fracture) could result. Given that the fall was a free J (eds): Accidental Injury: Biomechanics and Prevention, ed 2, Springer-Verlag, fall to the floor, however, and no limbs were impinged New York, 2002, pp 206-227. 6. Reilly DT, Burstein AH: The elastic and ultimate properties of compact within the sofa, it is difficult to envision how a torsional bone tissue. J Biomech 1975;8:393-405. load could be introduced during this event. The most 7. Lotz JC, Gerhart TN, Hayes WC: Mechanical properties of trabecular likely loading pattern resulting from a free fall from a sofa bone for the proximal femur: a quantitative QCT study. J Comput Assist is a bending or compression pattern. Also of interest in Tomogr 1990;14:107-114. this case, the presence of bone fragments suggests a high 8. Currey JD, Butler G: The mechanical properties of bone tissue in chil- dren. J Bone Joint Surg 1975;57:810-814. level of energy was imparted to the child’s femur. A fall 9. Wright TM, Hayes WC: Tensile testing of bone over a wide range of from a sofa onto a carpeted floor would not be classified strain rates: effects of strain rate, microstructure and density. Med Biol as a high-energy event. Eng Comput 1976;14:671-680.