Biomedical Engineering Reference
In-Depth Information
Goldman et al. performed a study of the femoral mid-shaft where the whole-bone
cross section was covered comprehensively. The amount of transverse collagen
fibers among males decreased between the younger and middle age groups, and
increased between the middle and older groups, whereas females showed this trend
only along the endosteal ring of the cortical cross-section. Similar to Vincentelli
et al., Goldman group opined that the mechanical environment in the mid-shaft of the
femur was a possible reason for the changes in collagen orientation [
76
]. Long ago,
Kuntscher [
91
] associated tensile and compressive stress and strain patterns to dif-
ferent quadrants of long bones. Using this theory, Portigliatti et al. came to the
conclusion that strain patterns have a significant role in fiber orientation in the middle
third of femoral diaphysis. Fibers were aligned more longitudinally in the lateral and
anterior quadrants (predominantly loaded in tension), compared to transversely in
the medial and posterior quadrants (predominantly loaded in compression) [
124
].
Others have reported similar observations [
131
,
146
].
4 Changes in Bone's Mineral Phase and Microstructure
During Osteoporosis
In cortical regions, osteoporosis is mainly characterized by enlargement of haver-
sian canals and cortical thinning caused by the net loss of bone from the endosteal
surface. In cancellous regions, thin and perforated trabeculae are found during
osteoporosis, caused by the removal of bone from plates and struts. DXA-based
BMD has been the single most widely accepted tool for diagnosis of osteoporosis,
despite many questions about the specificity and sensitivity of using BMD
alone. Bone strength measured ex vivo correlates moderately well to BMD
[
43
,
52
,
78
,
137
], and fracture risk increases with lower BMD [
139
,
144
]. Yet in
most studies BMD fails to fully discriminate between people suffering fractures and
those who are not [
50
,
115
,
136
]. Therefore, diagnosis solely based on T and/or Z
scores of BMD readings lacks fidelity [
115
]. Association of BMD with fracture is
site specific, for instance, fractured group had significantly lower spine BMD but not
hip BMD [
77
]. Similarly in the same study, hip BMD of cortical bone, but not
cancellous bone, had significant association with fracture risk [
77
]. However,
Holzer et al. found marginal difference between the cancellous and cortical con-
tribution to bone strength in femoral neck region [
84
]. Advances in structural level
assessment of microstructures (using microCT, high resolution CT, etc.) along with
macro (BMD using radiographs, DXA, etc.) may improve fracture risk assessment
[
71
]. In conjunction with the above concerns and shortcomings of BMD in estab-
lishing a reliable estimator for bone strength, the focus was increased on aspects like
architecture, whole bone morphology and bone matrix; all of which are unified
under the term 'bone quality'. This section will expand on mineral crystals and
micro-structural aspects of bone quality in the osteoporotic population.
BMD is a non-invasive macroscopic measure that normalizes bone mineral
content (BMC) measured over a specific area at the organ level. BMD inherently
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