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Research Papers

High Strain Rate Testing of Bovine Trabecular Bone

[+] Author and Article Information
A. Pilcher, X. Wang, Z. Kaltz, J. G. Garrison, G. L. Niebur, J. Mason

Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, IN 46556

B. Song, M. Cheng, W. Chen

School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN 47907

J Biomech Eng 132(8), 081012 (Jul 29, 2010) (7 pages) doi:10.1115/1.4000086 History: Received November 28, 2005; Revised May 28, 2009; Posted September 01, 2009; Published July 29, 2010; Online July 29, 2010

In spinal vertebral burst fractures, the dynamic properties of the trabecular centrum, which is the central region of porous bone inside the vertebra, can play an important role in determining the failure mode. If the failure occurs in the posterior portion of the vertebral body, spinal canal occlusion can occur and ejected trabecular bone can impact the spinal cord resulting in serious injury. About 15% of all spinal cord injuries are caused by such burst fractures. Unfortunately, due to the uniqueness of burst fracture injuries, postinjury investigation cannot always accurately assess the degree of damage caused by these fractures. This research makes an effort to begin understanding the governing effects in this important bone fracture event. Measurements of the dynamic deformation response of bovine trabecular bone with the marrow intact and marrow removed using a modified split-Hopkinson pressure bar apparatus are reported and compared with quasistatic deformation response results. Because trabecular bone is more compliant and lower in strength than cortical bone, typical Hopkinson pressure bar experimental techniques used for high strain rate testing of harder materials cannot be applied. Instead, a quartz-crystal-embedded, split-Hopkinson pressure bar developed for testing compliant, low strength materials is used. Care is taken into account for the orthotropic properties in the bone by testing only along the principle material axes, determined through microcomputed tomography. In addition, shaping of the stress wave pulse is used to ensure a constant strain rate and homogeneous specimen deformation. Results indicate that the strength of trabecular bone increases by a factor of approximately 2–3 when the strain rate increases from 103s1 to 500s1 and that the bone fractures beyond a critical strain.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Split-Hopkinson pressure bar with incident bar, transmission bar, and striker

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Figure 2

Modified, quartz-crystal-embedded split-Hopkinson pressure bar apparatus used for testing compliant, low strength materials

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Figure 3

Time history of the specimen end forces with an approximate maximum percent error (a) in excess of 31% and (b) of 5.5%. These results are for two separate pieces of bone.

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Figure 4

Enlarged view of a trabecular bone specimen (6×6×6 mm3)

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Figure 5

Dynamic stress-strain response of several bovine trabecular bone specimens with the marrow intact across a range of volume fractions (Vf=0.23,0.3,0.38,0.46)

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Figure 6

Two modes of failure were observed: (a) one that involved macroscopic fracture and (b) another that appeared to be a burst

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Figure 7

Plot of the elastic modulus versus volume fraction relationship for the quasistatic and dynamic bovine trabecular bone specimens

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Figure 8

Plot of the ultimate strength versus volume fraction relationship for the quasistatic and dynamic bovine trabecular bone specimens

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Figure 9

A nondimensional solution for pressure in the specimen for constant applied strain rate, where nondimensionalized pressure, radius, and time are shown as P¯, r¯, and t¯, respectively

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