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TECHNICAL PAPERS: Bone/Orthopedic

# Sensitivity of Multiple Damage Parameters to Compressive Overload in Cortical Bone

[+] Author and Article Information
Elise F. Morgan1

Orthopedic Biomechanics Laboratory, Department of Mechanical Engineering,  University of California, Berkeley, CA 94720efmorgan@bu.edu

John J. Lee

Orthopedic Biomechanics Laboratory, Department of Mechanical Engineering,  University of California, Berkeley, CA 94720

Tony M. Keaveny

Orthopedic Biomechanics Laboratory, Department of Mechanical Engineering,  University of California, Berkeley, CA 94720 and Department of Bioengineering,  University of California, Berkeley, CA 94720tmk@me.berkeley.edu

1

Corresponding author. Present address: Department of Aerospace and Mechanical Engineering, Boston University, Boston, MA 02215.

J Biomech Eng 127(4), 557-562 (Mar 17, 2005) (6 pages) doi:10.1115/1.1933916 History: Received December 18, 2003; Revised March 17, 2005

## Abstract

Damage accumulation plays a key role in weakening bones prior to complete fracture and in stimulating bone remodeling. The goal of this study was to characterize the degradation in the mechanical properties of cortical bone following a compressive overload. Longitudinally oriented, low-aspect ratio specimens $(n=24)$ of bovine cortical bone were mechanically tested using an overload-hold-reload protocol. No modulus reductions greater than 5% were observed following overload magnitudes less than 0.73% strain. For each specimen, changes in strength and Poisson’s ratio were greater $(p=0.02)$ than that in modulus by 10.8- and 26.6-fold, respectively, indicating that, for the specimen configuration used in this study, longitudinal elastic modulus is one of the least sensitive properties to a compressive overload. Residual strains were also proportionately greater by 6.4-fold $(p=0.01)$ in the transverse than axial direction. These results suggest that efforts to relate microcrack density and morphology to changes in compressive mechanical properties of cortical bone may benefit from considering alternative parameters to modulus reductions.

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## Figures

Figure 1

(a) Schematic of loading protocol: following a one-minute preload, specimens were loaded at a constant strain rate (0.1%∕s) to a predetermined peak strain, −εo, unloaded to the preload and held for 3min, and finally loaded to failure. (b) Representative stress–stress strain traces showing both axial and transverse strains.

Figure 2

Both modulus (a) and strength (b) reductions exhibited increasing variability with increasing applied strain magnitude. No reduction in modulus greater than 5% was observed prior to 0.73% strain. Strength reductions occurred earlier and were greater than modulus reductions (p=0.02) for each specimen.

Figure 3

The magnitude of axial (closed circles) and transverse (open diamonds) residual strains increased with increasing applied strain magnitude. Residual strains were defined as the value of strain at the end of the hold period. The magnitudes of the transverse residual strains were greater than the product of the initial Poisson’s ratio and the axial residual strain (p=0.01).

Figure 4

(a) The change in Poisson’s ratio increased with applied strain magnitude (p=0.01), and displayed a large amount of variability at large applied strains. Change in Poisson’s ratio was greater than that in modulus for each specimen (p=0.02). (b) Representative axial strain, transverse strain, and Poisson’s ratio as functions of time during the initial loading cycle for a specimen loaded to εo=0.80%. As the applied strain increased on the initial loading cycle, the transverse strain increased proportionately more than the axial strain, causing an increase in Poisson’s ratio with loading.

Figure 5

Representative fracture patterns consist of both obliquely oriented (a , b ) and vertically oriented (c , d ) fractures. Specimens often exhibited multiple, parallel fractures (b , d ).

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