Pathogenesis of Fragility Fractures: A Biomechanical View .1 Pathogenesis of Fragility Fractures:

download Pathogenesis of Fragility Fractures: A Biomechanical View .1 Pathogenesis of Fragility Fractures:

of 47

  • date post

    19-Aug-2018
  • Category

    Documents

  • view

    212
  • download

    0

Embed Size (px)

Transcript of Pathogenesis of Fragility Fractures: A Biomechanical View .1 Pathogenesis of Fragility Fractures:

  • 1

    Pathogenesis of Fragility Fractures:A Biomechanical View

    Mary L. BouxseinOrthopaedic Biomechanics LaboratoryBeth Israel Deaconess Medical CenterBoston, MAUSA

  • 2

    Outline

    Pathogenesis of fractures and determinants of bone strength

    Age-related changes that contribute to skeletal fragility

    Interaction between skeletal loading and bone strength

    Non-invasive assessment of bone strength

  • 3

    Design of a structure

    Consider what loads it must sustain

    Design options to achieve desired function Overall geometry

    Building materials

    Architectural details

  • 4

    Determinants of whole bone strength

    Geometry Gross morphology (size & shape)

    Microarchitecture

    Properties of Bone Material / Bone Matrix Mineralization

    Collagen characteristics

    Microdamage

  • 5

    Hierarchical nature of bone structure

    Seeman & DelmasN Engl J Med 2006; 354:2250-61

    Macrostructure

    Microstructure

    Matrix Properties

    Cellular Composition and Activity

  • 6

    FRACTURE?

    Loads applied to the bone

    Bone strength

    Hayes et al. Radiol Clin N Amer. 1991; 29:85-96Bouxsein et al. J Bone Miner Res. 2006; 21:1475-82

    Applied loadBone strength

    > 1 fractureFactor ofrisk

    Bone MassGeometry

    Material properties

    Fall traitsProtective responses

    Bending, lifting

    Biomechanical approach to fractures

  • 7

    Pathogenesis of fragility fractures

    Age-related changes that contribute to fragility fractures:

    1) Decreased bone strength

    2) Increased propensity to fall

  • 8

    Assessing bone biomechanical properties

    Deformation

    Force

    StructuralProperties

    MaterialProperties

  • 9

    Biomechanical testingKey properties

    Displacement

    Force

    Failure load

    Energy absorbed

    Stiffness

  • 10Review of the relevant material & geometric properties vs loading mode

    compression

    bending

    Cross-sect area, A r2Axial rigidity, EA

    Moment of inertia, I r4Section modulus, Z r3Bending rigidity, EI

    Material: Elastic modulus E (density)aUltimate strength S (density)b

  • 11

    Effect of cross-sectional geometry on long bone strength

    aBMD (by DXA) = =

    Compressive strength

    Bending strength

    =

    =

    Bouxsein, Osteoporos Int, 2001

  • 12

    Bone strength

    SIZE & SHAPE macroarchitecturemicroarchitecture

    MATERIAL tissue compositionmatrix properties

    BONE REMODELINGformation / resorption

    OSTEOPOROSIS DRUGS

    Bouxsein. Best Practice Res Clin Rheumatol, 2005; 19:897-911

  • 13

    Outline

    Determinants of bone strength

    Age-related changes that contribute to skeletal fragility

  • 14

    Age-related changes in mechanical properties of bone tissueCortical bone

    % loss 30-80 yrs

    -8%

    -11%

    -34%

    Elastic modulus, E

    Ultimate strength, S

    Toughness

    Cancellous bone% loss 30-80 yrs

    -64%

    -68%

    -70%

    Bouxsein & Jepsen, Atlas of Osteoporosis, 2003

  • 15Whole bone strength declines

    dramatically with age

    0

    2000

    4000

    6000

    8000

    10000

    young

    old

    Femoral neck(sideways fall)

    Courtney et al. J Bone Joint Surg Am. 1995; 77:387-95 Mosekilde. Technology and Health Care 1998; 6:287-97

    Lumbar vertebrae(compression)

    Who

    le b

    one

    stre

    ngth

    (New

    tons

    )

    0

    2000

    4000

    6000

    8000

    10000

    young old

  • 16Age-related changes in geometry

    Adaptation to maintain whole bone strength

    Seeman. Lancet 2002; 359: 1841-50 Seeman. N Engl J Med. 2003; 349:320-3

  • 17

    Age-related changes in femoral neck vBMD, geometry, and strength indices

    Trabecular vBMDCortical vBMD

    Total areaMOIapAxial rigidityBending rigidity

    WomenF vs M

    (P-value)Men% Change, ages 20-90 yrs

    - 56**- 24**

    13**- 1- 39**- 25**

    - 45**- 13**

    7*- 21**- 31**- 24**

  • 18

    Age-related changes in trabecular microarchitecture

    Decreased bone volume, trabecular thickness and number

    Decreased connectivity

    Decreased mechanical strength

    Image courtesy of David Dempster

  • 19

    Microarchitectural changes that influence bone strength

    Force required to cause a slender column to buckle:

    Directly proportional to Column material

    Cross-sectional geometry

    Inversely proportional to (Length of column)2 www.du.edu/~jcalvert/tech/machines/buckling.htm

    http://www.du.edu/~jcalvert/tech/machines/buckling.htm

  • 20

    Theoretical effect of cross-struts on buckling strength

    # Horizontal Effective Buckling Trabeculae Length Strength

    0 L S

    1 1/2 L 4 x S

    Bouxsein. Best Practice Res Clin Rheumatol, 2005; 19:897-911

  • 21

    Cortical porosity increases with age(41 iliac biopsies, age 19-90)

    Age (years)

    0

    3

    6

    9

    12

    15

    0 20 40 60 80

    r = 0.78 P < 0.001

    (%)

    4-fold increase incortical porosity from

    age 20 to 80

    Increased heterogeneity with age

    Brockstedt et al. Bone 1993; 14:681-91

  • 22

    20-year-old 80-year-oldMayhew et al, Lancet 2005

    Age-related changes in femoral neck cortex and association with hip fracture

    Those with hip fractures have:

    Preferential thinning of the inferior anterior cortex Increased cortical porosity

    Bell et al. Osteoporos Int 1999; 10:248-57Jordan et al. Bone, 2000; 6:305-13

  • 23

    Age-related changes in bone properties associated with fracture risk

    Decreased bone mass and BMD

    Altered geometry

    Altered architecture Cortical thinning

    Cortical porosity

    Trabecular deterioration

    Images from L. Mosekilde, Technology and Health Care. 1998

    young

    elderly

  • 24

    Outline

    Determinants of bone strength

    Age-related changes that contribute to femoral fragility

    Interaction between skeletal loading and bone strength

  • 25

    Etiology of age-related fractures

    FRACTURE?

    Loads applied to the bone

    Bone strength

  • 26

    > 1, Frx < 1, no Frx

    Applied loadBone strength

    The factor of risk concept

    Identify activities associated with fracture

    Use biomechanical models to determine loads applied to bone for those activities

    Estimate bone failure load for those activities

    =

    Hayes et al. Radiol Clin N Amer. 1991; 29:85-96 Bouxsein et al. J Bone Miner Res. 2006; 21:1475-82

  • 27

    Falls and hip fracture

    Over 90% of hip fxs associated with a fall

    Less than 2% of falls result in hip fracture

    Fall is necessary but not sufficient condition

    Sideways falls are most dangerous

    What factors dominate fracture risk?

  • 28

    Factor Adjusted Odds Ratio

    Fall to side 5.7 (2.3 - 14)Femoral BMD 2.7 (1.6 - 4.6)*Fall energy 2.8 (1.5 - 5.2)**Body mass index 2.2 (1.2 - 3.8)*

    * calculated for a decrease of 1 SD** calculated for an increase of 1 SD

    Greenspan et al, JAMA, 1994; 271(2):128-33

    Independent risk factors for hip fracture

  • 29Estimating loads applied to the hip during a

    sideways fall

    Human cadavers

    Human volunteers

    Crash dummy

    Mathematical models and simulations

    Peak impact forces applied to greater trochanter: 270 - 730 kg (2400 - 6400 N)

    (for 5th to 95th percentile woman)

    Robinovitch et al. 1991; Biomech Eng. 1991; 113:366-74Robinovitch et al.1997; Ann Biomed Eng. 1997; 25:499-508

    van den Kroonenberg et al J BiomechanicEngl.1995; 117:309-18 van den Kroonenberg et al J Biomech.

    1996; 29:807-11

  • 30

    Femur is strongest in habitual loading conditions

    Keyak et al. J Biomech. 1998; 31:125-33

    0123456789

    Stance Sideways fall

    Failu

    re lo

    ad (k

    N)

    P < 0.0012318 300 N7978 700 N

  • 31

    Femur is strongest in habitual loading conditions

    Keyak et al. J Biomech. 1998; 31:125-33 van den Kroonenberg et al. J Biomech. 1996; 29:807-11

    0123456789

    Stance Sideways fall

    Failu

    re lo

    ad (k

    N)

    P < 0.0012318 300 N7978 700 N

    =Applied loadFailure load

    Thus, > 1 for sideways fall in elderly persons

    Fall load

  • 32

    Vertebral fractures

    Difficult to study Definition is controversial Many do not come to clinical attention Slow vs. acute onset The event that causes the fracture is often

    unknown

    Poor understanding of the relationship between spinal loading and vertebral fragility

  • 33

    Estimating loads on the lumbar spine

    Schultz et al.1991; Spine. 1991; 16:1211-6 Wilson et al.1994; Radiology. 1994 ; 193:419-22 Bouxsein et al. J Bone Miner Res. 2006; 21:1475-82

  • 34

    Factor of risk for vertebral fracture (L2)Bending forwards 90o with 10 kg weight in hands

    Age

    0.00.20.40.60.81.01.21.41.61.8

    10 20 30 40 50 60 70 80 90 100

    Men

    0.00.20.40.60.81.01.21.41.61.8

    10 20 30 40 50 60 70 80 90 100Age

    Women

    11.9% 30.1%

    +92% over life** +28% over life**

    ** P

  • 35

    Proportion of individuals with

    > 1, per decade

    0102030405060

    20-29 30-39 40-49 50-59 60-69 70-79 80+

    %

    4%4% 11