Cortical Bone Microstructure And Anisotropy Of Mechanical Properties

1
Cortical Bone Microstructure And Anisotropy Of
Mechanical Properties
Alejandro A. Espinoza Department of Aerospace
Mechanical Engineering University of Notre Dame
2
Hierarchical Structure of Bone

• Whole bone.
• Cortical and trabecular bone.
• Osteonal Architecture.
• Collagen fibril arrangements.
• Fibril arrays by themselves.
• Mineralized collagen.
• Basic components collagen, mineral and water.

Giraud-Guille (1988), and Weiner-Wagner (1998)
3
Bone Mineralization

• Bone has 75 of its mineral in the interstitial
  sites between collagen fibrils. Hasegawa et
  al., 1994
• Varying degrees of mineralization define bone
  tissue strength.
• Mature secondary bone is generally more
  mineralized than younger primary woven bone.
• Mineralization and mineral orientation can be
  quantitatively studied by electron or X-Ray
  Diffraction patterns. Sasaki et al.
• Ongoing research tries to link mineral
  orientation to overall elastic anisotropy.

4
Research Objectives

• Structure-property understanding to make possible
  the development of a biomimetic synthetic bone
  graft substitute.
• Increase in the detail and resolution of
  anisotropy characterization studies.
• Anisotropy in composites is described by the type
  of elastic symmetry exhibited by the material and
  determined by the orientation of bone mineral.
• Describe and correlate the role of the preferred
  orientation of bone mineral to anatomical
  position and microstructural composition of bone
  tissue.

5
Anisotropic Symmetries

• Orthotropic materials have three orthogonal
  planes of elastic symmetry (Longitudinal, radial
  and circumferential).
• The six main diagonal coefficients of the
  stiffness matrix Cij are measured experimentally.
  
• From these cij values, E in each direction can be
  obtained.

6
Experimental Techniques

• Elucidation of structure-function relationship
  attempted by different routes.
• Acoustic techniques
• Ultrasonic stiffness measurements.
• Acoustic Scanning Microscopy.
• Microhardness Experiments
• Nano and micro indentation stiffness surveys.

7
Sample Preparation

• Slices are cut in 5 intervals from the cortical
  bone shaft of a femur.

8
Experimental Procedure

• Final sample shape is 5 mm side cubes whenever
  possible.
• Dry and saturated weights recorded according to
  Archimedes principle and used to compute
  density.
• Measurement of thickness sample in each
  orthogonal direction.
• Ultrasound measurement of longitudinal and shear
  velocities.
• Computation of Cij coefficients cij v2r

9
Experimental Measurements

• Time of Flight ultrasound technique

10
Experimental Measurements

• Calibration with 5 mm thick steel gage block.
• Time delay Dt is used to calculate velocity.

11
Stiffness Coefficients
12
Anisotropy Ratios
13
Anisotropy Ratios
14
Results

• Average density values measured 1.905 g/cm3
• Test specimen data Male/78 yrs. old.
• Coefficients per quadrant (average) GPa

15
Discussion

• Observed behavior in ratios is L/R gt L/C gt R/C.
• Difference in L/R, L/C and R/C ratios suggests
  Transverse Isotropy rather than Orthotropy.
• This fact was noticed also also by Lang (1970),
  and Yoon and Katz (1976).
• However, orthotropy is proved in this particular
  sample by the ANOVA test.
• 15 to 30 and 75 to 85 sections more
  anisotropic than the rest of the shaft due to
  less presence of osteonal bone.
• Overall, results for stiffness values agree with
  work by other investigators.

16
ANOVA Results

• Hypothesis crad ccirc
• Significance level a 0.05
• ANOVA by quadrants
• Lateral p 1.082E-2 Anterior p 8.330E-3
• Medial p 3.247E-2 Posterior p 1.892E-3
• ANOVA for all data
• p 2.634E-7
• Hence, orthotropy is the preferred symmetry.

17
Future Work

• Correlate preferred orientation of bone mineral
  to anisotropic directions using XRD.
• Perform stereology studies to characterize
  morphologic features of cortical bone from 2D
  samples.
• Perform these experimental analyses on more human
  bone specimens.

18
Acknowledgements

• 21st Century Grant State of Indiana
• Faculty Advisors
• Thank You