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Bone
Mechanics and Modeling
  The
Bone Mechanics and Modeling program area includes experimental and
computational modeling of the behavior of bone (including failure in
femurs, vertebrae and trabecular bone structures). This group is
actively working to validate modeling techniques to make predictions of
failure loads of these bones by laboratory testing of bone specimens
under varied conditions, high resolution imaging of bone architecture,
and experimental techniques using image-guided assesment of bone
failure. These modeling techniques are used to further understand the
factors that influence fracture such as density distribution, bone
geometry, and bone architecture. The program area also includes modeling
of several bone/implant systems as well as modeling the mechanical
consequences of space flight. This program area employs biomechanical
testing, state-of-the-art micro-imaging and visualization techniques of
connective tissue and biomaterials. As an example of the on-going
research projects, a mathematical model which allows analysis of bone
remodeling in the context of functional adaptation was recently
developed. The approach incorporates micro-tomographic imaging and
large-scale FEA modeling techniques. These techniques have exciting
potential for revealing alterations in trabecular bone structural
morphology in response to different loading and exercise regimes,
dietary supplements, pharmacological interventions, or degenerative
changes in subchondral bone associated with post- traumatic
osteoarthritis.
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  Sports
related and everyday injuries to the knee and upper extremity are very
prevalent in today's active population. This program area evaluates
joint kinematics, kinetics, muscle and ligament forces and cartilage
contact stresses when a joint is subjected to various physiological
loading conditions. This program area also studies how sports and
traumatic injuries affect the mechanics of joints and determines how
well reconstruction techniques restore joint mechanics to normal. The
objective is to provide guidelines for optimized joint ligamentous
reconstruction and arthroplasty and post-operative rehabilitation.
Cutting edge robotic technology and computational modeling techniques
are used to simulate joint motion and apply external loads. Recent and
current projects include studies of the effects of surgical intervention
on the distribution of contact pressure in the tibio-femoral and
patellfemoral joints; effect of TKA on knee joint kinematics and
ligamentous tension; investigation of the stability of the
acromioclavicular joint after reconstruction using different techniques;
and evaluation of initial strength, fatigue strength, and graft-bone
translation in anterior cruciate ligament reconstructions using a
doubled gracilis semitendinosus graft technique. We are also conducting
research to quantify and model the cartilage of children with hip
dysplasia, 3D finite element modeling of cartilage contact mechanics,
and investigations of in-vivo muscle contraction forces using 3D inverse
dynamic optimization. The effect of cartilage defects on stress
concentration is also being studied by this group.
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Age-related fractures are an
impending public health crisis, as persons over 80 represent the fastest
growing segment of the population. To prevent these fractures, it is
necessary to have a sound understanding of their etiology. While much
clinical and laboratory research has focused on the loss of bone with
age, our laboratory has adopted the view that fractures represent a
structural failure of the bone, wherein the forces that are applied to
the bone exceed its inherent strength. Therefore, we have and are
continuing to perform a series of investigations related to: 1)
developing ways of predicting bone strength non-invasively; 2)
identifying activities that lead to fractures; and 3) determining the
loads applied to the skeleton during those high-risk activities. These
investigations include case-control and observational studies of risk
factors for fracture; laboratory-based studies of the strength and
structural capacity of human cadaveric wrists, femurs, and vertebrae;
and theoretical and experimental studies of falls. In addition, we are
evaluating the ability of quantitative ultrasound to assess bone
architecture and micro-damage. We have an ongoing collaboration with a
group from the Jackson Laboratory to
evaluate the genetic determinants of peak bone mass and bone strength in
mice.
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 Pathological fractures through
osteolytic defects in the appendicular and axial skeleton occur in
patients of all ages and are often associated with pain, loss of
function and other morbidities depending on patient age, skeletal site
and underlying pathology. Bone is the most common site of metastasis
from cancer of the breast, prostate and lung. Moreover, 20-70% of
children with benign bone tumors are at risk for fracture. In managing
these patients, the orthopedist must decide if the lesion is benign or
malignant and if the tumor has weakened the bone sufficiently to cause
fracture. Currently no accurate method exists to predict fracture of
bones weakened by metastatic or benign tumors. One focus of our
laboratory is to apply structural analyses of bones with lytic defects
using composite beam theory or finite element models to better predict
the risk of pathologic fracture. Non-invasive imaging methods, such as MRI, QCT and DXA,
are used to quantify the geometric and material properties of bones with
real and simulated defects. From these images, three-dimensional
structural analyses can predict the reduced load carrying capacity of
the bone and in general the biomechanics of the metastatic bone defect
or tumor. Experimental investigation is being
conducted in parallel with clinical analysis to determine the accuracy
of the non-invasively derived structural parameters used to predict
pathologic fracture risk. In addition, Our CT
based structural analysis for predicting fracture through a skeletal
metastasis is based on the hypothesis that all bone (normal or
pathologic) follows the same constitutive relationships established for
rigid porous foams, i.e. the strength (Sy)
and modulus of elasticity (E) of bone depend on both the bone tissue
density (rho-tissue)
and the bone volume fraction (Vvb) squared.
The rho-tissue
accounts for changes in tissue mineralization, and the Vvb accounts for
changes in trabecular morphology. To the best of our
knowledge, this hypothesis has never been validated for metastatic
cancer bone tissue. Therefore, it was
our objective to establish that the mechanical properties of metastatic
cancer bone tissue were governed by the power law functions.
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Mechanisms of bone fracture and optimization of
fracture treatment are important areas of orthopaedic research. Although
with proper treatment most fractures heal without significant
complications, some fractures do not heal. Moreover, among various
treatment options, the optimal method of treatment remains
controversial. One area of active research at our laboratory is the
evaluation of various methods of fracture fixation. We use an optical
method to capture fracture gap motion produced by the application of
loads simulating activities of daily living or phases of gait after
fracture fixation with various devices. From the motion data, we perform
rigid body kinematic analysis to quantify the direction and magnitude of
the motion and evaluate the efficacy of the fixation methods. We have
also provided an objective measure of fracture instability by defining
this as the structural compliance (i.e., the amount of fracture
displacement as a function of applied load). By applying loads to the
flexor and extensor tendons crossing the wrist joint and monitoring
fracture fragment displacement with serial CT scans, we demonstrated
that fracture classification systems based on fracture pattern in
Colles' fractures were unrelated to fracture instability. However, using
these same techniques we have demonstrated that using more rigid
fixation is not necessarily better fixation for minimizing fracture
motion when treating Colles' fractures. We are also involved in the
biomechanical evaluation of bioresorbable plates for metacarpal fracture
fixation, and internal fixation methods for femoral shaft fractures in
children.
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  This group
performs imaging, densitometric and mechanical evaluation of bone
specimens from animal studies conducted by outside collaborators to
evaluate the safety and efficacy of new osteodynamic agents. Studies are
carried out with adherence to strict Good Laboratory Practices (GLP)
regulations set out by the Food and Drug Administration (FDA). The group
members are trained in densitometric, imaging and mechanical testing
techniques required by GLP study regulations. Animal models that have
been evaluated include transgenic mice, rats, dogs, rabbits and
non-human primates. Data have been generated for bone mineral content,
apparent bone density, cross-sectional geometric properties and
microstructural morphology; and variously related to material and
structural mechanical properties including compressive strength of
trabecular core specimens and whole vertebral bodies, bending and
torsional strength of cortical bone beam specimens and whole bones
including the proximal femur, hip and long bones of the appendicular
skeleton. The group is constantly improving imaging protocols, analysis
methods, mechanical testing techniques and the efficiency of data
acquisition.
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This program area
employs mechanical testing, animal models and several imaging modalities
to investigate various aspects of the biomechanics of spinal fusion,
spinal instrumentation and trauma and the use of novel biomaterials in
spinal surgery.
The mechanisms underlying the initiation and
progression of spinal fusion processes, the use of novel methods and
materials, and the optimization of internal spinal fixation devices
employed to promote these processes, are important areas in spinal
orthopedics. This group, employing a lateral fusion rabbit animal model,
is actively involved in investigating the effects of electrical
stimulation in conjunction with several artificial bone substitutes and
the use of genetic engineering derived products, in promoting the
process of spinal fusion. The success of fusion is being assessed using
mechanical (three- and four-point bending), histological, and imaging
methods (DXA, micro-CT). A multi-axial spine testing device has been
developed in our lab which, when combined with non-contacting optical 3D
motion measurement system, allows for the investigation and
quantification of the performance of several internal spinal fixation
systems under complex dynamic loads. In particular, quantifying the
ability of such systems to restrict motions across the injured segments
to a minimum, thought to be critical for the initiation and progression
of the fusion process, is greatly facilitated by the use of this
device.
A second major area is the use of novel biomaterials for
spinal surgery. In an ongoing collaboration with several companies, we
have demonstrated the efficacy of a novel family of elastic-biopolymers
in greatly reducing epidural adhesions (the adhesion and consequent
tethering of the dura mater and nerve roots by fibrotic tissue, a common
complication of decompressive laminectomy or discectomy surgical
procedures). This family of materials is also under investigation for
the reconstitution (i.e. restoration of mechanical function) of early
degeneration of the intervertebral disc. Such materials hold great
promise due to our ability to tailor-make these materials both with
respect to their mechanical and material properties, and as carriers for
biological agents.
A third area of research is aimed at
elucidating the underlying causes for vertebral compression fractures
that are very prevalent in the elderly population. Although they cause
substantial pain, deformity, and disability, there is no widely accepted
treatment. The effects of high load rate in a simulated fall
configuration and the vertebral material properties on the ultimate
fracture load and consequent loss in the stability of the spinal
segment, are being investigated. Furthermore, as part of the use of
novel biomaterials, the use of several biocompatible cements and
hydroxyapatite composites for augmentation of the failed vertebral body,
a surgical procedure known as percutaneous veterbroplasty, is actively
being assessed. For this purpose, a specially designed high rate
vertebral fracture jig, used to create a simulated compression-flexion
fracture, and a 5DOF stability test jig, used to assess the 3D stability
of the fractured and augmented spinal segment, are employed.
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