Biomedical Engineering Reference
In-Depth Information
late 50s could happen possibly for mechanical and/or other metabolic reasons
[
101
]. Surprisingly, except for a couple of histomorphometric studies on iliac crest
[
125
], where upward trend with age was noticed, it is unknown if the same age-
related patterns hold true for other bones.
Frost [
69
] developed an empirical algorithm, which was capable of estimating
missing osteons in histological sections to calculate activation frequency--infor-
mation that can normally be obtained only from in vivo labeling [
1
,
32
,
35
,
63
,
82
,
92
,
145
]. Stout et al. (human ribs), Havill et al. (macaca mulatto's femur), Frank
et al. (dog humerus) and Lees et al. (cynomolgus monkey's iliac bone), using the
algorithm developed by Frost, were able to demonstrate that activation frequency
reduced with age. However, studies using biochemical turnover markers, observed
no change in remodeling activity with age [
2
]. Remodeling is ultimately associated
with porosity, fraction of osteonal bone and to some extent collagen orientation
which are discussed hence forth.
3.1 Porosity
Haversian canals, resorption spaces, vascular channels, marrow spaces, osteocyte
lacunae and canaliculi contribute to porosity (voids) in bone [
102
]. The small
dimensions of osteocyte level porosity (lacunae and canaliculi) limited its evalu-
ation; however recent advances in staining and imaging techniques made it pos-
sible to analyze these tiny interstitial fluid spaces [
45
]. Haversian canals are the
spaces formed during remodeling periods, which contain vessels with nutrients and
other precursor cells necessary for BMUs. Osteoblasts form bone, leading from the
outer edges until they encounter vessels. Porosity increases when bone formation
lags relative to bone resorption [
102
]. Porosity in cortical bone is in the range of
5-10%, much lower than in cancellous bone (75-95%) [
49
]. Increased cortical
porosity is caused by increased number of haversian canals, greater number of
resorption spaces, or by incompletely filled osteons (and thus increased diameter
of haversian canals). Moreover, focal accumulation of resorption spaces, such as
those observed close to the endosteal surfaces, may be indicative of a diseased
state such as osteoporosis [
61
,
88
,
102
]. In addition, cortical porosity is associated
with local stress levels from habitual loading such that anterior and posterior
quadrants of femoral mid shaft, which experience lower stresses, have greater
porosity than medial and lateral quadrants [
103
,
154
].
Increased volume fraction of cortical porosity with aging has been reported by
many studies [
61
,
103
,
106
,
140
,
143
,
150
,
161
]. The rate of change in porosity
with age is reported to vary anatomically. Femoral neck and intertrochanteric
cortical porosity increase at a greater rate when compared to the diaphysis, which
was speculated to be caused by targeted remodeling [
150
]. In another study,
increased porosity with aging was found in the mid-neck region while lesser
increases were observed in the subcapital level and no age-related changes in the
trochanteric region. Moreover, no difference in porosity was observed between
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