Changes in Cortical Bone Mineral and Microstructure with Aging and Osteoporosis - Skeletal Aging and Osteoporosis: Biomechanics and Mechanobiology - page 120

Table 2 Mineralization levels found in osteoporotic and fractured cases (relative to case con-
trols) and various opinions on possible causes and phases affected
Mineralization level Possible causes Affected turnover phases

Higher mineralization Lower turnover [ 142 , 157 ] Lower resorption, higher maturation

Higher turnover [ 55 ] Higher resorption, lower formation

Lower mineralization Lower turnover [ 23 ] Lower resorption, lower formation

Higher turnover [ 23 , 96 ] Higher resorption, lower maturation,

same or increased formation

and later found to correlate with mechanical properties at tissue level [ 159 ] and
was significantly associated with bone fracture in a multiple logistic regression
model evaluated for cancellous bone [ 77 ]. Crystallinity was also implicated in
crack initiation and subsequent growth due to decreased deformability before
failure [ 31 , 67 , 159 ]. Both mechanistic models [ 3 , 86 ] and experimental studies
[ 159 ] found that long crystals contribute to increased stiffness and strength, but at
the expense of ductility.
Repercussions of crystal size in osteoporotic individuals are not clear yet, but as
can be seen, specimens with large crystals potentially lack ductility and in turn they
may contribute to failure during falls. On the other hand, bone tissue with smaller
crystals lack stiffness and may fail progressively due to compromised weight bearing
capacity over prolonged periods, such as the creep observed in vertebrae. As an
example, Camacho et al. observed significantly large crystals in cancellous bone
compared to cortical bone in the vertebra of normal individuals [ 39 ].
Substitutions by ions like carbonate, fluoride, chloride and other heavy metal
ions in mineral crystals could also affect bone's quality by influencing crystal size
and/or perfection. Decreased levels of carbonate content were reported in osteo-
porotic female Eskimos compared to healthy males [ 148 ]. Carbonate molecule
plays a significant role in the dissolution of mineral and resorption of bone
[ 10 , 95 ]. In general, it is considered that new bone contains small crystals that will
have large quantities of carbonate content thus making bone imperfect and easily
removable or dissolvable during the process of resorption; as bone mineral matures
carbonate content decreases. Thompson et al. stated that in the case of osteopo-
rosis, in which bone turnover increases, younger bone was resorbed leaving behind
the mature bone that contained low carbonate content [ 148 ]. Huang et al. com-
pared site (cortical versus cancellous) and type (A, B and labile) comparisons
between normal and ovariectomized monkeys [ 85 ]. Type-A and type-B levels
increased in the cortical bone of ovariectomized sample, while type-A increased
and type-B decreased in trabecular bone and labile carbonate was found in lower
levels in both cortical and cancellous bone. Similar perception was also shared by
Boskey et al. and Gadeleta et al., that carbonate accumulation decreases with
mineral maturation, however Boskey group did not observe any differences
between normal and osteoporotic patients [ 23 ] while Gadeleta et al. observed
increased levels of type-B carbonation in osteoporotic individuals and overiec-
tomized cynomolgus monkeys [ 70 ].