Noninvasive patient-specific fracture risk assessment
The
diagnosis of fracture risk and osteoporosis has been
traditionally done based on bone mass measurements. However,
recent studies show that fracture incidence cannot be
predicted by bone mass alone and factors such as bone
geometry, microstructure, and bone’s material properties
affect an individual’s fracture risk. Therefore, new
fracture assessment tools that include other factors in
addition to bone mass are needed. The overall goal of this
research project is to develop a new and improved
noninvasive patient-specific fracture risk assessment tool
that utilizes computed tomography and fracture
mechanics-based finite element modeling. Specifically, the
project focuses on the fracture risk assessment of Colles’
fracture which is shown to be an early indicator of
increased risk of future spine and hip fractures.
Our initial
research efforts in this area focused on using idealized
geometries of human radius bone to validate the
computational method that we developed. This approach . The
results obtained from this work identified the significant
influence of the cortical bone geometry on Colles’ fracture
load (Ural, 2009). In addition, the simulations showed that
best fracture risk prediction can be obtained through
combined evaluation of intrinsic properties of the bone and
external factors during a fall (Buchanan and Ural, 2010).
Contour images of the fracture plane of
the radius bone during Colles’ fracture
Following the establishment of the
modeling approach, we are now applying this approach to
actual human radius bone images obtained using HR-pQCT to
establish a patient-specific fracture risk assessment. This
project is in collaboration with Columbia University.
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A sample HR-pQCT bone
image of distal radius bone from a 63-year-old subject and
the corresponding finite element model. |
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Evaluation of microscale fracture mechanisms in bone
The effect of microstructural features on the toughness and
crack propagation behavior in human cortical bone has been demonstrated
in the literature by experimental studies. However, despite
a general understanding of the effect of bone microstructure
and its properties on toughness and crack growth, a thorough
assessment of the relationship between the bone microstructure,
its mechanical properties, and microcrack formation and growth
is not present in the literature. The aim of this research project
is to provide a mechanical understanding that explains the effect
of each microstructural component including osteons, cement
lines, pores, and interstitial bone on cortical bone fracture.
Our research efforts in this area first
focused on the finite element evaluation of the influence of
cement lines on crack propagation behavior using a single
microstructural unit. The new modelling approach that we
developed utilizing a single microstructural unit was the
first study in the literature that included both toughness
and strength considerations in the assessment of the effect
of cement line on bone fracture. This study demonstrated the
significant role that the strength of the cement line plays
in the crack deflection characteristics (Mischinski and
Ural, 2011).
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A sample finite element mesh
demonstrating crack penetration and an crack deflection case
(a), (c). Stress contours for crack deflection into a cement line and crack penetration into an osteon
(b), (d).
Building up on this new approach and new
findings, our subsequent study utilized actual human bone
microstructure images and focused on developing more
advanced modelling techniques such as arbitrary crack growth
to assess bone fracture behavior. The results of this study
provided additional insight into the influence of cement
lines on crack propagation trajectory and fracture response
and identified the effect of different microstructural
arrangements on the microscale fracture processes.
(Mischinski and Ural, 2011).
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A microscopy image of human cortical bone, the corresponding finite element mesh, and crack growth trajectory based on variation in material properties
of the cement line. |
Modeling of strain rate effects on bone fracture
Bone is subject to a wide range of strain rates during daily
activities or traumatic fracture events such as accidents or
falls. Previous studies showed that the mechanical response
of bone, including its modulus of elasticity, yield stress and
strain, and ultimate stress and strain vary with the loading
rate. A comprehensive understanding of traumatic fractures requires
an investigation of bone’s resistance to fracture initiation
and propagation under a variety of low and high strain rates.
Most of the fracture toughness measurements reported in the
literature under varying strain rates, however, corresponded
to quasi-static conditions. In this study, we aim to evaluate
the effect of strain rate on fracture toughness of human cortical
bone during crack propagation using a finite element approach.
The simulation results showed that bone’s resistance against
the propagation of fracture decreased sharply with increase
in strain rates up to a threshold level and attained an almost
constant value for strain rates larger than this threshold (Ural
et al, 2011). We also evaluated the effect of loading rate
on distal forearm fracture. The simulations showed that the
most drastic reduction in fracture load occurs at strain
rates corresponding to the transition from controlled to
impact falling. These results are particularly important for
the improvement of fracture risk assessment in the elderly
because they identify a critical range of loading rates that
can dramatically increase the risk of distal forearm
fracture (Ural et al., 2012). This project is in collaboration with RPI and
Cranfield University.
 
Normalized R-curve slope
vs. strain rate and normalized Kinit vs.
strain rate for 2D compact tension specimens. |
