Investigation on Age-related Changes of Swine Cortical Bone
Feng, L; +Jasiuk, I
+University of Illinois Urbana Champaign, Urbana, IL
Senior author [email protected]
INTRODUCTION
As a biological material, bone is constantly under development.
These biological processes such as remodeling and aging are known to
increase the susceptibility of bone to fracture. The age-related changes
in the fracture toughness and other mechanical properties are highly
influenced by the alteration of the bone structure and chemical
compositions. Various studies have been conducted to study the changes
in the bone structure, composition and mechanical properties due to
aging. However, these studies are generally focused on only one or two
aspects. It is still an open question that how the age-related changes in
bone’s structure and composition affect its mechanical properties. To
answer this question, a systematic experimental work was designed and
carried out in this study to measure the effects of aging on structure,
composition and mechanical properties of bone at different structural
scales. A comprehensive analysis was conducted to address the
correlations between each other.
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METHODS:
Femurs from pigs of different ages (6-month, 12-month and 42month) were obtained from the Imported Swine Research Lab at the
University of Illinois at Urbana-Champaign. Only the mid-diaphysis
parts of the femurs were used for samples preparation.
Structure
SEM imaging of swine femoral cortical bone from these three age
groups demonstrated big differences among these three types of
specimens. Woven bones together with large number of 200 μm in
diameter large absorption sites (locations for osteon bone formation)
were the main structural components in a 6 month specimen. In a 12
month specimen, lamellar bone was the dominant cortical bone type
while the major cortical bone type in a 42 month specimen was
secondary osteon bone. 6 month specimens had highest porosity due to
the large number of absorption sites, followed by 12 month specimens.
42 month specimens had least porosity.
Mechanical properties
The elastic modulus and hardness of bright lines, and thick and thin
lamella of circumferential lamella and the inner wall and thick and thin
lamella of osteon were measured by nanoindentation and are
summarized in Figure 2. It is seen that, in general, there was a strong age
effect on the mechanical properties of these bone structure components,
while the effect on individual structure components was varied.
Fourier Transform Infrared (FTIR) microspectroscopy was used to
detect the changes in the mineral and organic contents, mineral to
organic ratio, carbonate content, crystallinity, and other parameters
affecting bone’s characteristics. 5 μm thick thin bone slices were
prepared and placed between two barium fluoride windows for FTIR
microscopy (Perkin-Elmer Spotlight 400) study. Software ENVI 4.5 was
used to analyze the spectrum data obtained by the FTIR.
Scanning electron microscopy (SEM) was used to obtain the
information on bone types, porosity and osteon volume fraction.
Specimens with both longitudinal and transverse cross-sections were
polished and examined by the SEM. The coordinate system is defined in
such a way that the long-axis of long bone is longitudinal and
circumferential is the transverse direction.
Nanoindentation was employed to measure the local mechanical
properties (elastic modulus and hardness) of the following bone’s
microstructures: single osteon layers, lamellar layers, interstitial lamellar
layers and woven bone. A Hysitron TI 900 TriboIndenter® with a
diamond fluid cell Berkovich tips was used to perform nanoindentation
tests on these swine femur cortical bone samples under a hydrated
condition. The macro-scale mechanical properties were measured by
using tensile testing on dumbbell shape samples. The tensile properties
such as Young’s modulus and ultimate tensile strength were obtained
and the results were analyzed by statistical methods.
RESULTS:
Composition
By calculating the peak areas and the ratio between two peak areas, the
information on the chemical compositions was obtained as shown in
Figure 1. The mineral content and the mineral to organic ratio increased
with age while crystallinity decreased with age.
Figure 1. Color contour plot of chemical composition information of an
osteon measured by FTIR
Figure 2 Indentation results for age effect. The bars show three groups
samples in different indent locations with standard error as the error bars
The results from the tensile testing showed that the elastic modulus
increased with age from 15.2 GPa to 18.8 GPa to 21.9 GPa. The ultimate
tensile strength of the 42-month samples is 119.8 MPa. Interestingly, the
6-month and 12-month samples yielded similar ultimate tensile strength
(99.5 MPa and 97.1 MPa).
DISCUSSION:
We presented a comprehensive and systematic study on these swine
femoral cortical bone tissues as described in methods sections to address
how aging affect the following three aspects: structure, composition and
mechanical properties. Our experimental results indicate there is a big
difference among these three age groups in structure, chemical
composition and mechanical properties of cortical bone. Cortical bone
tissues in swine femur are continually undergoing alterations with age.
These changes could be well connected. For instance, the increasing
trend in mechanical properties of the micro structural components could
be due to the alteration of the chemical compositions such as mineral to
organic ratio. There are several open questions centered on the tensile
test results such as why 6-month and 12-month samples share the similar
range for ultimate tensile strength? Our next step is to carefully study
these questions and more importantly, to look at the actual correlation
between the age-related changes in structure, composition and
mechanical properties of swine femoral cortical bone. These
experimental data will serve as inputs for the multi-scale
micromechanics models to predict the elastic modulus, fracture
toughness and ultimate tensile strength of bone as a function of age. This
methodology can be applied to assess bone’s properties due to diseases.
Poster No. 579 • 56th Annual Meeting of the Orthopaedic Research Society