Changes in Cortical Bone Mineral and Microstructure with Aging and Osteoporosis - Skeletal Aging and Osteoporosis: Biomechanics and Mechanobiology - page 115

subcapital, mid-neck and trochanteric regions of femoral neck in the younger age
group, but differences were seen in the older age groups with greatest void volume
being present in the mid-neck region [ 25 ]. Bell et al. demonstrated the interesting
concept of super-osteons, which are spatially clustered remodeling osteons [ 15 ].
This merging of osteonal systems to form large cortical cavities can have dele-
terious effects on bone strength, but super-osteons were found to be independent of
age and gender [ 15 ]. Age-related increases in porosity and the differential response
over anatomical locations points out that change in mechanical milieu, particularly
age-related reduction in physical activity, may play a key role in site-specific
increase in porosity. It is also important to appreciate that such site specific
changes are likely taking place over a background of general increase in porosity
due to endocrine factors. The relative roles of endocrine and mechanical factors in
modulating age-related cortical porosity are not well understood.

3.2 Osteonal Microanatomy

During remodeling, osteoclasts dig tunnels of around 200 lm diameter and osteoblasts
fill into form cylindrical tubular structures known as secondary osteons or Haversian
systems. Primary bone starts to convert into secondary at a very young age and
becomes mostly secondary by adult age [ 68 ]. Changes in bone microanatomy during
remaining life are mixed.
Kerley reported increasing densities of osteonal fragments and secondary osteons
with age in femurs, tibias and fibulas from both genders (birth to 94 years of age)
[ 89 ]. However in human femurs, Wang et al. observed no change in the number of
secondary osteons per unit area with age, which were clustered into three age groups
between 19 and 89 years [ 155 ]. It was observed that the percentage of bony area in
each osteon decreased [ 106 , 155 ], indicating bigger central haversian canals in older
individuals. Overall size of osteons (including haversian canal) also increased in
elderly, but their number remained constant during individual's lifespan [ 106 ].
Conversely Britz et al. found reduced overall osteon size with age, measured as
osteonal area and osteonal diameter, in femurs of both males and females [ 28 ], with
female femurs consisting significantly smaller osteons than males. The authors noted
a negative relationship between weight and osteon size. Based on Martin's study
[ 103 ], Britz et al. speculated that reduced osteoclastic activity might be a reason for
reduced osteon size in older bones [ 28 ]. In a study on mid-diaphysis of male femurs,
Zioupos measured the fraction of secondary osteonal area adjusted for haversian
canal area and found increased fractional area with age even after correction for the
vascular canal size [ 161 ].
Boyce et al. found a greater number of osteons per mm 2 in the trochanteric
region than in other regions of the femoral neck. However, the osteon density did
not change with age in both trochanteric and subcapital regions, but the mid-neck
region had greater number of osteons/mm 2 in the older age group compared to the
younger age group [ 25 ]. Osteon population density (intact ? fragmented)