Design Automation Lab. – ND Biomechanics Group Cortical Bone Microstructure And Anisotropy Of Mechanical Properties Alejandro A. Espinoza Department of

Design Automation Lab. – ND Biomechanics Group

Cortical Bone Microstructure Cortical Bone Microstructure And Anisotropy Of And Anisotropy Of Mechanical PropertiesMechanical Properties

Alejandro A. Espinoza Department of Aerospace & Mechanical Engineering

University of Notre Dame


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)


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.


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.

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C cOrthotropiij

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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.


Sample Preparation

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


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 = v2·


Experimental Measurements

“Time of Flight” ultrasound technique


Experimental Measurements

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


Stiffness Coefficients

Average Stiffness Coefficients by Longitudinal Section

Specimen 5098

5

10

15

20

25

30

35

15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

Longitudinal Section [%]

Sti

fnes

s C

oeffi

cien

ts [

GPa]

C long C rad C circ


Anisotropy Ratios

Anisotropy Ratios by Longitudinal Section

Specimen 5098

0.0

0.5

1.0

1.5

2.0

2.5

3.0

15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

Longitudinal Section [%]

Ani

sotr

opy

Rat

ios

[uni

tles

s]

Cl/Cr Cl/Cc Cr/Cc


Discussion

•Observed behavior in ratios is: L/R > L/C > 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.


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.


Acknowledgements

•21st Century Grant – State of Indiana

•Faculty Advisors

Thank You

Documents

Design Automation Lab. – ND Biomechanics Group Cortical Bone Microstructure And Anisotropy Of Mechanical Properties Alejandro A. Espinoza Department of