Long Bone Fracture Biomechanics

Long Bone Fracture Biomechanics

35 CHAPTER 35  LONG BONE FRACTURE BIOMECHANICS 317 Long Bone Fracture Biomechanics Gina Bertocci, PhD, PE INTRODUCTION Fractures in children can o...

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CHAPTER 35  LONG BONE FRACTURE BIOMECHANICS

317

Long Bone Fracture Biomechanics Gina Bertocci, PhD, PE

INTRODUCTION Fractures in children can occur as a result of either accident or abuse. Clinicians are often asked to determine whether a fracture could have resulted from a stated cause such as a fall from a sofa or bed. To determine whether a stated cause is compatible with a presenting fracture, a basic understanding of biomechanics as it relates to fractures can be useful. Biomechanics relies upon the use of engineering or physics principles to study the response of biological tissue to physical phenomena such as force, acceleration, or pressure. Biomechanical principles can provide an improved understanding of how a bone will respond to the application of a force and its likelihood to fracture under certain conditions. The objectives of this chapter are: 1. To describe characteristics of bone tissue anatomy that are related to bone strength and biomechanical response to loading 2. To describe fundamental biomechanical concepts important to fracture prediction 3. To describe biomechanical factors that affect the likelihood of bone fracture

OVERVIEW OF LONG BONE ANATOMY Long bones, such as the femur, tibia, and humerus, consist of the shaft, or diaphyseal region, which consists of compact or cortical bone (Figure 35-1). The segments on either end of the diaphysis consist of cancellous or trabecular bone, and are called the metaphyseal region. A connective tissue layer, periosteum, covers the outer surface of long bones. Cortical bone tissue is dense, and is composed of haversian microsystems, which include concentric rings of lamellas constructed of mineralized collagen fibers and lacunae. The lacunae contain bone cells, or osteocytes. The number, size, and distribution of the lacunae affect the cortical bone’s response to loading.2 Cancellous bone tissue is made up of a network of rods and plates that resemble a honeycomb structure. The plates are mineralized and their thickness and direction of alignment affects load-bearing capacity. The honeycomb structure of cancellous bone tissue is made up of microstructure units referred to as trabeculas. Cancellous bone is less dense than cortical bone and is porous in nature with a high surface area. These differences in cancellous and cortical bone microstructure lead to differences in the transmission of forces

through the bone tissue and their load carrying capacity. Each type of bone has unique biomechanical properties and therefore has a unique response to the application of force. These differences reflect their specific function in the human body. Cortical bone is primarily responsible for the supportive load-bearing function of the skeleton, while cancellous bone provides cushioning to skeletal structures during loading. A more detailed description of bone anatomy is provided in Chapter 31.

BIOMECHANICAL CONCEPTS IMPORTANT TO UNDERSTANDING FRACTURES To determine whether a bone is likely to fracture under a given loading condition, key biomechanical terms and concepts must first be understood. Table 35-1 lists biomechanical terms and definitions.

Force The application of a force tends to cause a body or object with mass to accelerate, change position, or change shape. Force can be defined as the mass of an object times its acceleration. Figure 35-2 illustrates the response of a spring to a compressive force and tensile (pulling) force. A combination of multiple forces along with their direction of application can be defined as the loading condition.

Moment A moment is the tendency to produce an object’s rotation when applied at a perpendicular distance (moment arm) from the axis of rotation.3 A moment can be defined as the force applied times the moment arm. The concepts related to a moment can be illustrated by the action of a seesaw (Figure 35-3). When two individuals of equal mass are sitting on the seesaw, it is balanced, the moments on each side are equal, and no movement occurs. However, when an individual of larger mass (delivering a greater downward force to the seesaw) sits on one side of the seesaw this will generate a larger moment, which serves to offset the balance of the seesaw creating its downward motion on the side of the individual with greater mass. This downward movement tends to rotate the seesaw about is axis of rotation. 317

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SECTION V  PHYSICAL ABUSE OF CHILDREN

Metaphyseal region

Diaphyseal region

Trabecular bone with adherent periosteum and thin cortex

Thick cortical bone with relatively loose periosteum

Static

Metaphyseal region

Transition between diaphysis and metaphysis

FIGURE 35-1  Illustration of bone architecture. (From Pierce MC, Bertocci GE, Vogeley E, et al: Evaluating long bone fractures in children: a biomechanical approach with illustrative cases. Child Abuse Negl 2004;28:505-524.)

Compressed

Stretched

FIGURE 35-2  Response of spring to compression (center) and tension (right). Application of these forces alters the shape of the spring.

BIOMECHANICAL MATERIAL PROPERTIES Properties inherent to a specific biological material that influence how it will respond when exposed to physical phenomena (e.g., force, acceleration) are referred to as biomechanical 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

Elasticity

Force normalized over the area to which it is applied is referred to as stress. Stress can be determined by dividing the force by the cross-sectional area of force application. Therefore, for a given force, as the cross-sectional area decreases, stress increases. That is, a force applied to a small crosssectional area will yield a higher stress than when that same force is applied to a large cross-sectional area (Figure 35-4). The stress that develops within a bone under force application is an important factor in determining how the bone will respond and whether the bone will fracture. Stress also can be described based upon the direction or type force that is applied to an object. Terms that define specific types of stress depend upon the characteristics of force application; these include compression, tension, bending, torsion, and shear (Table 35-1). Figure 35-5 demonstrates graphical depictions of stress applications.

A material is said to be elastic if it deforms under external force, but returns to its original shape when the force is removed. A biomechanical material property that describes the elastic nature of a material is elasticity. Elasticity can be thought of as defining the stiffness of a material, and it is often described through a parameter known as the elastic modulus (abbreviated as E1). The elastic modulus, E, of a material can be derived for a material by knowing the ratio of stress to strain. E is independent of the size and shape of a material. The elastic modulus is a commonly reported material property in engineering handbooks for biological materials and for common materials such as metals, wood, plastics, etc. Often engineers will perform an experiment that entails applying a force or stress to a material, while measuring the corresponding deformation or strain. These quantities are then plotted against 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 load (or stress) as compared with a material with a high elastic modulus (E2 in Figure 35-7).

Strain The change in length of an object (e.g., bone) normalized to its original length is referred to as strain. Strain can be determined by dividing the change in length of an object by its original length (Figure 35-6). Strain is also an important factor in determining whether or not a bone fractures under certain loading conditions.

Yield Strength From a stress : strain curve, additional material properties can be defined and are important to predicting a bone’s

CHAPTER 35  LONG BONE FRACTURE BIOMECHANICS

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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 material properties

Characterizes a material and defines how a material will respond to exposure to physical phenomena (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 defined as the stress beyond which an object or structure will undergo permanent or plastic deformation. (Note: Although the term “plastic” is often used clinically to indicate a reversible situation, in contrast the terminology “plastic” behavior or deformation in reference to material response indicates a permanent or irreversible response.) In other words, a material responds elastically below the yield strength (material returns to its original shape when the load

is removed) and deformation is reversible in this region. In contrast, stresses greater than the yield strength will lead to plastic, irreversible, deformation. Figure 35-8 illustrates the point on the stress : strain curve that identifies a material’s yield strength. The stress : strain curve illustrates 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.

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SECTION V  PHYSICAL ABUSE OF CHILDREN

Moment

Moment

Arm

Arm

Smaller mass Larger mass

Force

Force

Axis of rotation Moment small < Moment large

Moment = Moment

FIGURE 35-3  Individuals with equal mass balance the seesaw creating equal moments (left). The individual with greater mass generates a larger moment causing the seesaw to rotate (right).

F

Stress

F

Stress

>

E = Elastic modulus E = Stress/strain E1 stiffer than E2

E1

Stress E2

FIGURE 35-4  A force applied to a smaller cross-sectional area generates a higher level of stress as compared to when this same force 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

Compression

F

F Tension

Plastic region Fracture strength

F F

Elastic region

F

F

F

Bending

F

F Torsion

Stress

F

Ultimate strength

E1

Shear

Yield strength

FIGURE 35-5  Type of stress depends upon the characteristics of force 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

Strain =

∆L 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 FIGURE 35-6  Force applied to an object can stretch or deform the object. The change in length as compared with an object’s original length is defined as strain.

Anisotropy Some materials respond differently when loading is applied from different directions. These materials are defined as anisotropic. Materials responding similarly under loading

CHAPTER 35  LONG BONE FRACTURE BIOMECHANICS

conditions applied from varying directions are referred to as isotropic. The material will respond elastically at stresses below the yield strength, and plastically when stresses exceed the yield strength.

FACTORS AFFECTING LIKELIHOOD OF FRACTURE The likelihood of a fracture occurring in a specific bone is dependent upon a number of factors that can be categorized as extrinsic or intrinsic factors. Models capable of predicting fractures are dependent upon accurate representation of both intrinsic and extrinsic factors. Whether or not a fracture occurs depends upon the internal stresses developed within the bone, and whether these internal stresses exceed the fracture threshold.

Intrinsic Factors The response of bone to loading is dependent upon intrinsic factors that include both material and structural or geometric characteristics of the bone.

Elastic Modulus. Table 35-2 provides elastic moduli for cortical bone and trabecular bone in comparison with other common materials. Table 35-2 illustrates that cortical bone is stiffer than trabecular bone (Ecortical > Etrabecular). Therefore, for a given loading condition (below the yield strength), trabecular bone will undergo greater deformation as compared with cortical bone. Trabecular bone tissue found at the epiphysis and metaphysis of long bones has a high level of porosity, providing the ability to deform without failure. The openings in trabecular bone tissue are filled with marrow and fat, which help to provide a degree of energy absorption under loading.4,5

Table 35-3

Anisotropy and Strength. As previously stated, bone tissue is considered an anisotropic material, responding differently under varying directions of loading. For example, Table 35-3 shows that femoral cortical bone tissue loaded longitudinally (parallel to the long axis of the bone) in tension has a higher ultimate strength (133 MPa) as compared to when the same tissue is loaded in a transverse direction (perpendicular to the long axis of the bone) under tension (51 MPa).6 Long bones also typically have increased strength under compressive loading conditions, and exhibit the lowest strength under application of shear loading. Table 35-3 also illustrates this concept for femoral cortical bone tissue.6 When the femur is loaded longitudinally (parallel to the long axis of the bone) under compression, the ultimate strength 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 compression and tension loading in the transverse direction.

Table 35-2

Bone Material Properties

321

Elastic Moduli for Bone Tissue and Other Materials3

Material

E (GPa)*

Cortical bone

12-24†

Trabecular bone

0.005-1.5†

Stainless steel

190

Polyethylene‡ (UHMWPE)

1.2

*Gigapascal = 109 pascal. † Depends upon density and direction of loading. ‡ Ultra high molecular weight polyethylene, used in joint replacements.

Ultimate Strength and Elastic Modules for Femoral Cortical Bone Under Varying Loading Conditions and Directions Type of Bone

Direction

Cortical bone, femur

Longitudinal

Cortical bone, femur

Loading Condition

Ultimate Strength

Elastic Modulus

Tension

133 MPa

17.0 GPa

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.

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SECTION V  PHYSICAL ABUSE OF CHILDREN

Strength (MPa)

30

ri

20 ro FIGURE 35-10  Approximation of long bone cross-sectional geometry (ri = inner radius; ro = outer radius).

10

0 0

200

400

600

Bone mineral density (mg/cc) FIGURE 35-9  Compressive strength of femoral trabecular bone tissue increases with increasing bone mineral density.8

Femoral Geometric Characteristics Table 35-4 and Moment of Inertia for Newborn vs. 6-Month-Old Child Moment of Inertia

Outer Diameter

Cortical Thickness

Newborn

63 mm4

6.0 mm

2.15 mm

6 months

291 mm4

9.0 mm

2.0 mm

Age

Density and Strength. Bone tissue strength is also dependent upon bone mineral density.7 In general, the strength of bone tissue increases with increasing bone mineral density as illustrated in Figure 35-9. Trabecular bone mineral density ranges from 0.1 to 1.0 g/cc, whereas cortical bone tissue density is approximately 1.8 g/cc. Since bone mineral density is directly related to strength, it is important when assessing the likelihood of fracture to consider those conditions that can alter bone mineral density (see Chapter 31). Bone Geometric Characteristics. The geometric characteristics of the bone structure will also affect its response to loading. Depending on whether the bone is subjected to bending, axial loading or torsion determines which aspects of the bone geometry relates to its resistance to fracture. Important geometric characteristics can include the inner and outer diaphysis radii, along with the cross-sectional area, or cortical wall thickness, of the bone structure. The Importance of Intrinsic Factors Related to Pediatric Bone Tissue Curry8 compared age-related bone tissue from younger children to adults. Bone tissue from younger children dissipated more energy before fracture. With larger haversian canals, a child’s bone is more porous and thus tolerates a greater level of strain as compared with adult bones prior to fracture.

Extrinsic Factors Bone tissue response to loading is also dependent upon extrinsic factors such as loading characteristics. Types and Characteristics of Loads The characteristics of the loading applied to a bone are key to understanding its resistance to fracture. The magnitude, distribution, and direction of loading are important to whether or not a bone will fracture. Various combinations of geometric characteristics (intrinsic factors) are key to a bone’s ability to resist fracture depending upon the char­ acteristics of loading. Two common loading conditions, bending and torsion, are discussed in greater detail next.

Bending. When a bending load, or moment, is applied to a bone, the maximum internal stress (σ) developed within that bone is dependent upon the magnitude of the applied moment and the geometric characteristics of the bone. The internal stress is directly related to the bending moment applied and indirectly related to the moment of inertia (described later). In a simplified representation of a long bone cross-sectional geometry as a hollow tube, the inner radius (ri, center of bone to medullary canal wall) and outer radius (ro, center of bone to outer cortical wall) affect the level of stress developed within the bone (Figure 35-10). When a bending load is applied, the structure is in tension on the side of load application, and in compression on the opposite side. The ability of a bone to resist bending stress is dependent upon its moment of inertia, I. When a long bone is approximated as a hollow tube, the moment of inertia can be defined as; I = Π 4 ( ro 4 − ri 4 ) Where Π = 3.14159265.

As an example, Table 35-4 compares geometric characteristics that are used to determine the moment of inertia for the femur of a newborn and 6 month-old-infant to illustrate this concept. As shown, the 6-month-old child has a moment of inertia, which is 4.6 times that of a newborn. The maximum internal stress, σ, associated with a application of a bending moment, M, can then be estimated as; σ = My I ,

where y = distance from the neutral axis, or ro when estimating the maximum internal stress. Using the previous comparison of the newborn and 6-month-old child’s femur geometry and moment of inertia, it can be shown that for a given bending moment application, the internal femoral stress, σ, experienced by the 6-month-old is one third of that experienced by the newborn.

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Once the maximum internal bending stress, σ, has been estimated, this value is then compared with the ultimate strength of the bone tissue to determine the response of the bone. If σ exceeds the ultimate bending strength of the bone tissue, then the bone will fail or fracture. Torsion. A similar type of analysis can be undertaken as it relates to torsional loading conditions applied to a long bone. In torsional loading, internal bone stress is directly related to the torque (force times moment arm) or twisting applied to the bone. The bone’s ability to resist fracture under these conditions is dependent upon the polar moment of inertia, J. Assuming again that the cross-section of a long bone can be approximated as a hollow tube (Figure 35-10), the polar moment of inertia, J, can be defined as;

Response to Rate (Speed) of Loading Application Bone tissue response is also dependent upon the rate at which the loading is applied. Materials that are time- or rate-dependent are referred to as viscoelastic. Figure 35-11 illustrates the influence of strain rate (rate of deformation) on cortical bone ultimate strength and elastic modulus.9 This figure shows that the ultimate strength and elastic modulus increase with rapid loading or deformation. The ultimate strength increases by roughly a factor of 3, while the elastic modulus increases by a factor of approximately 2 over the strain rate range. (Note that normal activities typically occur at a strain rate of <0.01/sec.)

J = Π 2 ( ro 4 − ri 4 )

where r represents the radius of the bone, T represents the torque applied, and J represents the polar moment of inertia. Using the geometric data provided and polar moment of inertia determined (Table 35-5) for the newborn and 6-month-old child, the maximum internal torsional stress for a given applied torque can be expressed as: T6 mo = T (4.5) 582 Tnewborn = T (3) 126 T6 mo : Tnewborn = 0.33

Therefore, it can be shown that for a given torque application to the femur, the internal torsional stress experienced by the 6-month-old child would be one third of that experienced by a newborn. Again, once the maximum internal torsional stress has been determined, this value can be compared with the ultimate torsional strength of the bone tissue to determine whether or not a fracture would occur. Internal stress values that exceed the strength of the bone will lead to failure or fracture of the bone.

Femoral Geometric Characteristics Table 35-5 and Polar Moment of Inertia for a Newborn vs. 6-Month-Old Child Polar Moment of Inertia

Outer Diameter

Cortical Thickness

Newborn

126 mm4

6.0 mm

2.15 mm

6 Months

582 mm

9.0 mm

2.0 mm

Age

The process by which one determines whether a fracture occurs in a laboratory setting is obviously somewhat different than that which can be used in a clinical setting. Nonetheless, it is of value to understand the idealized steps that one would take assuming that all information relevant to an incident, the associated loading conditions, and the child’s bone structure and properties could be obtained. Figure 35-12 provides an overview of the idealized conceptual process that would be used to determine whether a fracture would occur for a given incident. In this idealized approach, the loading characteristics (direction of application, location of application, and magnitude) associated with an incident are extracted or determined through close examination of the event. These loading characteristics are used to estimate the internal stress developed within the bone structure. The internal stress developed within the bone is then compared with the strength of the bone tissue to determine the resultant response. Although the approach presented in Figure 35-12 provides an “ideal” method for assessing fractures, many unknowns usually exist, preventing clinical application of this approach. Unknowns might include complex loading

300

70 Cortical bone

150

0 .0001

40

1 Strain rate (I/sec)

Elastic modulus (GPa)

τ = Tr J

Combining Intrinsic and Extrinsic Factors

Ultimate strength (MPa)

Table 35-5 compares the femoral geometric characteristics and polar moment of inertia of a newborn and 6-monthold child. Table 35-5 shows that the 6-month-old child has a femoral polar moment of inertia that is 4.6 times that of a newborn. This difference is important because it relates to the bone’s ability to resist fracture. When torque is applied to a bone, the associated maximum internal torsional stress, τ, can be determined using the following equation:

10 1000

FIGURE 35-11  Ultimate cortical bone strength and elastic modulus in tension versus strain rate. Both properties increase with increasing strain rate. * = Strain rate of typical normal activities.

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SECTION V  PHYSICAL ABUSE OF CHILDREN

Loading conditions

Internal bone stress

Bone response

Bending F

F

σbending = My/I

Stress

b

σbending E1

F

Moment = F(b)

Strain

Incident assessment FIGURE 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, and unknown biomechanical properties of the specific child’s bone tissue. Because such a quantitative approach often falls short in a clinical setting, a qualitative, modified approach to fracture assessment (described below), which is based in principle upon the idealized quantitative approach, can usually be implemented.

QUALITATIVE FRACTURE ASSESSMENT MODEL In the absence of quantitative data regarding the specific event and the child’s bone tissue properties, the determination of whether a fracture is biomechanically compatible with a stated cause can be aided by using the components described in Figure 35-13. The proposed qualitative Fracture Assessment Model attempts to convey the interrelationship of injury causation, injury mechanism, and fracture type. The components of this model can be defined as follows.

Injury Causation Injury causation is a detailed description of the event that leads to a specific fracture or injury. Often the stated cause of the injury is presented by the caregiver. The description should include as many details as possible, such as the child’s initial position and posture, dynamics during the fall, landing position, and surface upon which the child fell. An example of injury causation is a child rolling from an 18 inch high bed from a horizontal posture onto a carpeted floor landing onto an outstretched arm.

Injury Mechanism The injury mechanism describes how forces or accelerations associated with the injury causation (i.e., event) can be

Injury causation

Fracture type

Fracture assessment Injury mechanism FIGURE 35-13  Fracture Assessment Model. A model to assess fractures and their biomechanical compatibility with a stated cause is dependent upon three components: injury causation, injury mechanism, and fracture type. There should be continuity between these three components.

transmitted to the bone structure (or region of the body) where a fracture is present. The description should include the direction of force(s), the planes of the body where the force(s) are applied, and an evaluation of the transmission of these forces to the bone. An example of an injury mechanism is a torsional load applied to the tibia when a toddler’s foot becomes entangled with a toy or carpet while running or walking in the forward direction. A typical resulting fracture from this type of torsional load application would be a nondisplaced spiral fracture of the tibia.

Fracture Type The fracture type is the morphological description of the resulting fracture pattern and its location on the bone. An example of fracture type is a spiral fracture to the midshaft 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 covered in Chapter 32. This model should be applied in a stepwise progression starting with the injury causation moving through the injury

CHAPTER 35  LONG BONE FRACTURE BIOMECHANICS

mechanism and finally onto the resulting fracture type. There should be continuity of flow from one component of the model to the next. Evaluating each component of this model will help the clinician to qualitatively “reconstruct” the event and evaluate the compatibility of the fracture and the stated cause. That is, the injury causation must be capable of generating specific forces of a magnitude and direction that can lead to an injury mechanism that is capable of generating a specific fracture type. If the injury causation can lead to fall dynamics that are capable of generating a loading pattern that can create a specific fracture type, then the clinician has an improved level of confidence that the stated cause could have resulted in the fracture. (This same assessment model can also be used for other types of injuries.) The simplified examples provided below will help to illustrate application of the Qualitative Fracture Assessment Model.

Fracture Assessment Case 1: Skiing Incident Stated Cause: 5-year-old child involved in skiing incident. Injury Causation: Child was downhill skiing and the tip of her ski caught on a tree trunk. The child continued to move forward down the slope, abruptly falling to the ground, facing supine with her leg folded beneath her. The child was subsequently unable to bear weight on one lower extremity. Injury Mechanism: The tree trunk retained the tip of the ski such that the ski rotated relative to the child’s lower extremity, introducing a torsional (twisting) load on the tibia. Fracture Type: Spiral fracture to the diaphysis of the tibia. Fracture Assessment: In this case the injury causation can lead to an injury mechanism that can cause the presenting fracture. As previously stated, torsional loading will lead to spiral fractures and given the injury causation in this case, it is reasonable that torsional type loading will be present. The assessment of this case is that the stated cause is compatible with and supports the resulting fracture.

Fracture Assessment Case 2: Sofa Fall Stated Cause: 6-month-old child fell from sofa. Injury Causation: The caregiver stated that the child was lying on a sofa 18 inches above a padded carpeted floor and then rolled off the sofa. The caregiver also stated that the child fell free to the floor, did not impact any object during the fall, and no limbs were retained by the sofa. Injury Mechanism: When evaluating the fracture type (spiral comminuted), one would evaluate the injury causation to determine if torsional loading (necessary to generate a spiral fracture) could result. Given that the fall was a free fall to the floor, however, and no limbs were impinged within the sofa, it is difficult to envision how a torsional load could be introduced during this event. The most likely loading pattern resulting from a free fall from a sofa is a bending or compression pattern. Also of interest in this case, the presence of bone fragments suggests a high level of energy was imparted to the child’s femur. A fall from a sofa onto a carpeted floor would not be classified as a high-energy event.

325

Fracture Type: Comminuted spiral fracture of the femur. Fracture Assessment: In this case the injury mechanism necessary to create a comminuted spiral femur fracture cannot be ascertained from the injury causation. A comminuted spiral fracture of the femur would require exposure to high level of torsional loading. Therefore the continuity of flow from one component of the model to the next is broken, and it can be concluded that the stated cause of injury and presenting fracture are biomechanically incompatible.

KEY POINTS IN FRACTURE ASSESSMENT ●

Knowledge of biomechanics is important when attempting to determine whether a bone will fracture under given loading conditions. l Both intrinsic and extrinsic factors must be considered when attempting to determine whether a bone will fracture under given loading conditions. j  Intrinsic factors important to determining likelihood of fracture:   Bone biomechanical or material properties (elastic modulus, yield strength, ultimate strength, etc.)  Bone geometry (cortical wall thickness, inner and outer radii)  Extrinsic factors important to determining likelihood of fracture:  Characteristics of loading (type, direction and rate of application) l Internal bone stress is dependent upon loading conditions and geometrical characteristics of the bone. l Bone tissue response to loading is dependent upon internal bone stress as well as the material or biomechanical properties of the bone. l When assessing fractures, injury causation, injury mechanism and fracture morphology must be considered. l Fracture morphology must be compatible with a specific injury mechanism which can be derived from a specific injury causation. There must be continuity between injury causation, injury mechanism and fracture type.

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