Biomedical Engineering Reference
In-Depth Information
3 Aging and Microstructure of Cortical Bone
Modeling of bone results in changes in the size and shape of bone, while
remodeling turns over the internal bone volume. The rate of modeling is largely
reduced after skeletal maturation whereas remodeling is a life-long process [
101
].
Remodeling occurs in three stages (activation, resorption and formation) that
replaces old with new bone and serves three main functions; first, to balance
essential minerals within the serum, second to build skeletal strength by adapting
to mechanical needs, and finally to repair microdamage formed during daily
activities that prevents fracture risk [
33
,
101
]. The first function can be accom-
plished by removing bone indiscriminately from any location. On the other hand,
the latter two functions require site-specific remodeling [
33
]. Therefore, remod-
eling is categorized into targeted (site-specific) and non-targeted forms [
33
,
103
,
112
,
116
]. A haversian canal is formed at the center of newly formed secondary
osteons, and a reversal line, also called as cement line, is created at the transition
region between old and newly formed bone, which is often used to differentiate
secondary osteons from primary osteons [
101
].
Remodeling has been shown to increase following menopause [
53
,
123
].
Remodeling rate is believed to decrease during aging [
68
]. The most reliable means
to infer remodeling is histomorphometry; however, biopsy requirement and inac-
cessibility of sample collection from points of interest (femoral neck, vertebral
bodies) limit this approach. Nonetheless, earlier mathematical methods [
68
] predict
remodeling rate using resorption spaces and refilling basic multicellular units
(BMUs) identified by osteoid seams. Net remodeling representing the average
remodeling that occurred during the life-time of the individual is termed as previous
remodeling in this chapter and it was quantified by examining the amount of section
occupied by secondary osteons and their remnants [
82
,
104
,
114
,
145
]. Remodeling
rate is also assessed based on biochemical bone turnover markers (alkaline phos-
phatase, osteocalcin, tartrate-resistant acid phosphate, collagen telopeptide markers)
from serum or urine [
2
,
17
]. The limitation of this method is that it is a systemic
measure. Remodeling estimated but such tests reflect current remodeling, i.e., at a
particular instant in the individual's life.
Frost used biopsies from human ribs [
68
] to estimate remodeling dynamics via
the activation frequency variable (a.k.a. bone turnover rate-number of osteons/
mm
2
/year). Ribs were chosen because of the ease in obtaining biopsy, and they are
subjected to high remodeling rates under continuous cyclic loading due to
breathing. Frost administered tetracycline stains twice within a period of 10 days,
which labeled mineralizing bone twice [
68
]. The author observed that activation
frequency declined to a lifetime minimum level at 35 years of age which slightly
increased in late 50s, and declined again towards 90s [
68
]. These data indicated
that bone was rapidly converting into secondary osteonal form and maintained at
relatively undermineralized levels at younger ages, followed by increased
mineralization during adulthood, when systemic factors like hormonal changes
have reduced remodeling to its minimum level. The increase in remodeling in the
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