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Hip Structure Analysis: Using DXA Scans to Measure Bone Strength

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Division of Medical Imaging ... Simulated a fall on the greater trochanter using measured geometry from HSA ... User positions laser on greater trochanter ... – PowerPoint PPT presentation

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Title: Hip Structure Analysis: Using DXA Scans to Measure Bone Strength


1
Hip Structure Analysis Using DXA Scans to
Measure Bone Strength
  • Thomas J. Beck, Sc.D.
  • Division of Medical Imaging Physics
  • Department of Radiology
  • The Johns Hopkins University School of Medicine

2
Presentation Goals
  • What strength properties can be measured?
  • What is meant by structural geometry?
  • What geometric parameters are important?
  • How can geometry be measured from DXA scans?
  • How do age trends influence geometry (and BMD)?
  • How should an effective treatment improve
    geometry?

3
Osteoporosis
  • Reduces bone mechanical strength so that
    fractures occur at certain skeletal sites with
    minimal trauma
  • Under a specific load, bone strength is a
    function of
  • Material Strength
  • Geometry (both micro and macro scales)

4
Material strength
  • Determined by composition of the tissue
  • Not accessible by any current non-invasive
    methods
  • Measuring from biopsies is technically difficult
  • CT methods claiming to measure material strength
    actually measure bone volume fractiona geometric
    effect.
  • Very useful way of treating CT data for FEA
    models but is measures of APPARENT, not actual
    material strength.

5
Structural Geometry
  • Age and osteoporosis mainly alter the amount and
    distribution of bone tissue within bones.
  • These are geometric effects
  • Geometry CAN be measured but accuracy and
    precision can be problematic.
  • Most imaging methods lack resolution to discern
    microarchitecture

6
Whole Bone Geometry
  • Linear dimensions (bending moments)

7
Stress Distributions in Cross-Sections
Compressive Stress
  • Force is uniformly distributed over bone surface
    (marrow and other soft tissue spaces do not
    contribute)

Stress is inversely proportional to bone surface
area (CSA)
8
Stress Distributions in Cross-Sections
  • Stress not uniformly distributed over bone
    surface
  • Greatest at outer margins

Stress is proportional to bone surface weighted
by distance from center of mass (cross-sectional
moment of inertia - CSMI) Maximum stress is
determined by the section modulus (calculated
from CSMI)
9
How can we measure geometry ofcross-sections?
  • Method 1 Measure from a picture of the
    cross-section acquired by CT methods
  • Requires specialized geometry software

10
Projection of bone in cross-section excludes all
soft-tissue spaces (which do not contribute to
strength)
Method 2 Measuring from a projection of the bone
tissue in the cross-section
11
Thickness profile and image of cross-section have
equal
Cross-Sectional Area (CSA)
Thickness (cm)
Distance (cm)
12
DXA only projects the mineral content and
eliminates the non contributing soft tissues
Applying The Principle to DXA
  • Get thickness profile by dividing pixel values by
    average mineral density of fully mineralized bone

Mass thickness (g/cm2)
Distance (cm)
13
DXA only projects the mineral content and
eliminates the non contributing soft tissues
Applying The Principle to DXA
  • Get thickness profile by dividing pixel values by
    average mineral density of fully mineralized bone

Thickness (cm)
Distance (cm)
14
Measuring Geometry with DXA
  • Mineral profiles are extracted from DXA images by
    HSA software.
  • Three femur cross-sections are analyzed

15
Age trends in Geometry and BMD
  • The HSA method permits the same bone mass data
    to be expressed in terms of BMD and geometry
  • HSA has been applied to gt150,000 hip scans from
    DXA studies around the world
  • Some flavor of the kinds of results obtained
    particularly what HSA tells us about bone
    homeostasis (self-preserving behavior).

16
The Expansion Paradox
  • Long bones generally expand with age by
    periosteal apposition.
  • Appears to be a homeostatic adaptation to bone
    loss that preserves section modulus.
  • Reduces BMD even without bone loss
  • Permits section modulus to be maintained with
    less material
  • May ultimately be responsible for fragility

17
As Diameter Is Increased, Same Section Modulus
Requires Less Material
Relative value
18
Simplistic Geometry of the Aging Bone
Cross-Section
20
40
60
80
100
Age
19
Implications of Homeostatic Expansion
20
40
60
80
100
Age
20
Characterizing cortical instability
Strength predicted by section modulus
Section modulus overestimates strength
21
Local Buckling
t
ro
ro
Buckling Ratio
t
In hollow tubes, strength loss due to local
buckling begins to occur with BR gt 10 BR can be
estimated from DXA data by the HSA method
22
Essential Geometric Parameters
  • CSA for resistance to axially directed loads
  • Section modulus for resistance to bending loads
  • Buckling ratio for stability of cortex under
    compressive loads (including bending).

23
The DXA Bone Mineral Mass Image
  • Each pixel measures mass of hydroxyapatite (HA)
    in g/cm2 summed along a straight-line path
    through patient from x-ray source to detector

detector
X-ray source
24
The DXA Bone Mineral Mass Image
  • Pixel values actually reflect thickness of
    hydroxyapatite (HA) after removal of any organic
    material along the path
  • For example 0.6 g/cm2 of mineral is equivalent
    to a 0.6 g layer of HA distributed over a 1 cm2
    area (there is no area in g/cm2)

25
What do we really measure with BMD?
Topographic display of pixel values in neck
region
Region length Lr fixed by software
26
What do we really measure with BMD?
Region Width depends on location of bone margins
27
What do we really measure with BMD?
BMD average mass thickness in g/cm2 BMC BMD
(g/cm2) x BA (cm2)
28
The Meaning of a Decline in BMD
  • If (and only if) BA doesnt change then loss of
    BMD means
  • Loss of bone mineral mass
  • Loss of structural strength
  • Does BA remain fixed in aging bones?

29
DXA Age Trends RevisitedAverage values per
decade from NHANES III
Relative Value
All values normalized to means of 20-29 y/o
30
Is Expansion Homeostatic? Age trends in femur
stresses in NHANES III
  • Homeostasis should keep stresses (strains) at a
    constant value independent of age, size, load, or
    other factors
  • Simulated a fall on the greater trochanter using
    measured geometry from HSA.
  • Femoral neck stresses were computed using
    engineering model with impact load a function of
    body weight

31
Age Trends in Femoral Neck Stress
White Women
White Men
Medial
Medial
Lateral
Lateral
Lateral
Medial
32
Interpretation
  • Declining BMD in aging bones doesnt necessarily
    cause stresses to increase
  • Adaptive geometric changes counter aging bone
    loss.
  • Adaptation is achieved in an expanding bone
    diameter with progressively less bone mass
  • Cortices may thin to the point of local
    instability under unaccustomed loads (falls).

33
How should effective treatments alter femur
geometry?
  • CSA should increase to reduce axial stresses
  • Section modulus should increase to reduce bending
    stresses
  • Buckling ratios should DECREASE to reduce
    susceptibility to buckling

34
Examples of HSA in Clinical Trials
  • Raloxifene vs. Placebo (Eli Lilly Co.)
  • 1365 osteoporotic women on placebo vs. 1346 on 60
    mg Raloxifene 3 year follow-up.
  • Teriparatide vs. Placebo (Eli Lilly Co.)
  • 189 osteoporotic women on placebo vs. 186 taking
    20 ?g and 183 taking 40 ?g rhPTH(1-34), 21 month
    follow-up.
  • Alendronate vs. Placebo (Merck Co.)
  • 155 osteoporotic women on Placebo vs. 164
    Alendronate 3 year follow-up.

35
Effects of Raloxifene60 mg Raloxifene vs.
Placebo at 3 year follow-up
p lt 0.05 vs. Placebo
Uusi-Rasi, K, et, al. 2006 Osteoporos. Int
36
Effects of TeriparatideTeriparatide vs. Placebo
at 21 Month Follow-up

















p lt 0.05 vs. Placebo
Uusi-Rasi, K, et al. 2005, Bone
37
Effects of Alendronate Differences From Placebo
at 3 Year Follow-up
plt0.05 vs. Placebo
(Poster 211 International Society of Clinical
Densitometry 2006)
38
Lessons
  • BMD decline with age is partially due to
    expansion of outer dimensions
  • Expansion serves to preserve bending strength but
    with less bone material
  • An expanded bone may be structurally unstable
    when subjected to unaccustomed loading forces
  • This may be the geometric explanation for why low
    density bones are fragile

39
Lessons II
  • Clinically proven treatments produce improvements
    in femur geometric strength and cortical stability

40
Measuring bone geometry from DXA
  • Specialized software is required
  • Will be commercially available from DXA
    manufacturers
  • Better scan quality control is needed especially
    in subject positioning
  • Better quality scan images will be needed
  • Also becoming available on newer scanners
  • Certification of operators?

41
Scans Unsuitable for HSA
42
Becks Hip Structure Analysis
  • Over 30 Published Studies
  • Most Recognized 2D Method of Structural Analysis
  • Integrated into Hologic Software
  • All standard HSA parameters available in Hologic
    MS-Access database.

43
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44
Hologic HSA refinements
  • BMD matches Hologic BMD with all of Hologics
    proprietary corrections
  • ROIs are in mms, not points or line sizes,
    allows for standardization between patients and
    to future scan types and models
  • Magnification effects more accurately removed
    when the new Auto Hip Positioning option is used

45
Auto Hip Positioning
  • User positions laser on greater trochanter
  • Scanner automatically moves down and then scans
    across the shaft
  • While scanning across the shaft, triangulates to
    measure height of femur off the table
  • Finds shaft for perfect positioning of femur and
    automatically starts scan
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