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Introduction
Osteoporosis occurs most frequently in post-menopausal women and the
elderly. It is defined as a systemic skeletal disease characterized by
low bone mass and micro-architectural deterioration of bone tissue, with
a concomitant increase in bone fragility and fracture risk. Until
recently the structural analysis of these fractures has been limited to
two-dimensional sections. Due to the inherent destructiveness of this
method, dynamic assessment of fracture progression has not been
possible. An image-guided technique, utilizing micro-compression in
combination with micro-computed tomography (µCT),
has been developed, which allows for the first time, the direct
three-dimensional visualization and quantification of fracture
progression on the microscopic level. This technique allows for the
definition of a quantitative method to relate the global failure
properties of trabecular bone to those of the individual trabeculae.
The goal of this project was first, to design a micro-mechanical testing
system, composed of the micro-compression device (MCD) and the material
testing and data acquisition system (MTDAQ), and second, to validate the
testing system to perform step-wise testing of trabecular bone specimens
using image-guided failure analysis (IGFA) technique and µCT
imaging. This technique has been implemented before by using a standard
mechanical testing machine and aluminum foam specimens. However, in
order to obtain better results on bone specimens, the design and
manufacture of the abovementioned MTDAQ was deemed necessary.
Methods
IGFA is based on the step-wise or time-lapsed compression and
imaging of bone specimens. It is a three-dimensional and dynamic
technique that allows for the study of bone failure beyond the elastic
region. It is also capable of studying the influence of local variation
of bone tissue and structure on the mechanical behavior of bone. IGFA
allows simoultaneous µCT
imaging as bone tissue is progressively compressed beyond yield. The
testing protocol consists of applying sequential compression steps of
0%, 2%, 4%, 8%, 12%, 16% and 20% global strain, while simultaneously
measuring nominal stress. The specimen is imaged after each strain step
to observe microstructural deformation.
The MCD is designed to house the test specimens and act as a
transportable link between the mechanical testing and micro-CT imaging
steps. Its duties are to hold the specimen, lock the applied strain to
the specimen, record the applied load via an onboard load-cell, and
provide a radiolucent window for scanning. Previously, the
micro-compression device (MCD) was loaded axially via a standard
mechanical testing machine. The axial strain was applied through a
pushpin on the MCD cap and locked manually. This combination introduced
errors in the form of strain application fluctuation, due to reduced
sensitivity of the testing machine at small strains, and the application
of unknown manual torque in the strain locking process. In order to
alleviate these problems, the MCD is instead interfaced with the MTDAQ.
This system is designed to introduce axial strain to the specimen using
a custom screw drive micro-stepping compression actuator. Also, an
onboard LVDT measures the applied strain to the specimen.
Specimens from whale vertebral bodies (n=10) and ERG aluminum foam
specimens (n=30) were divided into two groups with nearly identical mass
and apparent densities. One group was tested from 0-20% strain based on
conventional continuous mechanical testing method, while the other  
was tested from 0-20% strain using IGFA testing method. All specimens
were cored along the principal trabecular axis and cut into cylinders of
8 mm diameter and 16 mm
height taking into consideration protocols to minimize end effect
artifacts such as attachment of brass end caps to both ends of the
specimens using cyanoacrylate. All biological specimens were wrapped in
saline soaked gauze and stored at –4°C
to preserve their mechanical properties. The specimens were tested at a
0.01s-1 strain rate and preconditioned to 0.3% strain at a
rate of 0.005 s-1 for 10 cycles. Whale specimens were chosen
for biological samples due to the large size of the vertebral bodies
with large regions of homogenous, uniformly oriented trabeculae. The
aluminum foam specimens were selected due to their predictable
mechanical characteristics, and similar architecture to human lumbar
spine trabecular network. Statistical analyses were performed by ANOVA.
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Results
Mechanical properties obtained from the continuous and step-wise
methods were not significantly different for both aluminum and whale
trabecular bone specimens (p>0.05). Both testing methods yielded very
similar stress-strain graphs with almost identical elastic regions, and
plastic regions with overlaying standard error bars for both whale bone
specimens and aluminum foam specimens. This was further concurred by
performing regression analyzes between the stress data from both testing
methods (r2=0.98 for whale specimens, and r2=0.98
for aluminum foam specimens).
In aluminum specimens a general trend of buckling was observed within
the structure due to imposed strain (Figure 1-a), where the individual
trabeculae throughout the entire specimen tended to bend and buckle.
Failure was observed differently in whale trabecular bone specimens
(Figure 1-b). Over the imposed strain range of 20%, the top one third
of the specimen crumbled significantly with the individual trabecular
elements experiencing high levels of deformations such as bending and
buckling, whereas the remaining lower part of the specimen was
relatively intact. It is observed that bone fails in a brittle manner
due to the presence of minerals, but then it show s
a very ductile post failure behavior. The collagen network within the
trabeculae is responsible for keeping the mineral phase together over a
long range of strain. |
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