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TECHNICAL BRIEF

# Evaluation of Filler Materials Used for Uniform Load Distribution at Boundaries During Structural Biomechanical Testing of Whole Vertebrae

[+] Author and Article Information
Do-Gyoon Kim, X. Neil Dong, Ting Cao, Kevin C. Baker, Richard R. Shaffer, David P. Fyhrie

Bone and Joint Center,  Henry Ford Hospital, Detroit, MI, 48202

Yener N. Yeni1

Bone and Joint Center,  Henry Ford Hospital, Detroit, MI, 48202yeni@bjc.hfh.edu

http:∕∕isb.ri.ccf.org∕biomch-l∕archives

1

To whom correspondence should be addressed.

J Biomech Eng 128(1), 161-165 (Sep 21, 2005) (5 pages) doi:10.1115/1.2133770 History: Received June 21, 2004; Revised September 21, 2005

## Abstract

This study was designed to compare the compressive mechanical properties of filler materials, Wood’s metal, dental stone, and polymethylmethacrylate (PMMA), which are widely used for performing structural testing of whole vertebrae. The effect of strain rate and specimen size on the mechanical properties of the filler materials was examined using standardized specimens and mechanical testing. Because Wood’s metal can be reused after remelting, the effect of remelting on the mechanical properties was tested by comparing them before and after remelting. Finite element (FE) models were built to simulate the effect of filler material size and properties on the stiffness of vertebral body construct in compression. Modulus, yield strain, and yield strength were not different between batches (melt-remelt) of Wood’s metal. Strain rate had no effect on the modulus of Wood’s metal, however, Young’s modulus decreased with increasing strain rate in dental stone whereas increased in PMMA. Both Wood’s metal and dental stone were significantly stiffer than PMMA ($12.7±1.8GPa$, $10.4±3.4GPa$, and $2.9±0.4GPa$, respectively). PMMA had greater yield strength than Wood’s metal ($62.9±8.7MPa$ and $26.2±2.6MPa$). All materials exhibited size-dependent modulus values. The FE results indicated that filler materials, if not accounted for, could cause more than 9% variation in vertebral body stiffness. We conclude that Wood’s metal is a superior moldable bonding material for biomechanical testing of whole bones, especially whole vertebrae, compared to the other candidate materials.

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Copyright © 2006 by American Society of Mechanical Engineers
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## Figures

Figure 1

Loading surfaces of whole bones such as human vertebrae are very nonuniform and need to be covered with a layer of suitable material for uniform load transmission during the mechanical testing (left). An actual test fixture with the specimen molded into Wood’s metal is shown (right). Micro-CT-based FE simulations were based on this configuration.

Figure 2

Typical stress-strain plots from compressive testing of 10mm long Wood’s metal, dental stone, and PMMA specimens at 0.01s−1.

Figure 3

(a) Mean compressive modulus of dental stone, Wood’s metal, and PMMA. * indicates the significantly lower modulus of PMMA. (b) Mean yield stress of dental stone, Wood’s metal, and PMMA. Because failure of dental stone was brittle, failure stress is plotted instead. * indicates the significant difference between Wood’s metal, and PMMA. Failure stress of dental stone was not included in the statistical analysis. Error bars represent standard deviation.

Figure 4

(a) Percent difference in the FE-estimated stiffness of a vertebral body between simulations with rigid (modulus: 200GPa) and nonrigid fillers (Dental Stone, Wood’s metal, and PMMA). Simulations with minimal filler material (enough to flatten the end surfaces) are shown as “thin” and those additional layers of filler material (about 0.6mm at both ends) are shown as “thick.” (b) Percent difference between the FE-estimated stiffness of the vertebral body as calculated from the rigid-filler model and that of the construct with rigid and nonrigid fillers.

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