Bone Microdamage and Its Contributions to Fracture - Skeletal Aging and Osteoporosis: Biomechanics and Mechanobiology - page 91

Fig. 4 Microdamage morphology is characterized by DS/DV, which is the surface to volume
ratio of the damaged region. Higher and lower values of DS/DV ratios indicate the presence of
(left) crack-like and (right) diffuse-like microdamage morphologies, respectively. Reprinted with
permission from Elsevier [ 27 ]
Reductions in both intracortical bone remodeling in cortical bone [ 8 ] and
surface based remodeling on trabecular surfaces of cancellous bone may alter the
accumulation of microdamage with aging [ 5 , 31 ]. However, due to the inherent
differences in rates of intracortical and surface based remodeling, it is possible that
microdamage accumulation may be different between cortical and cancellous
bone. Particularly, the high surface-to-volume ratio of cancellous bone and low
surface-to-volume ratio of cortical bone paired with similar rates of bone forma-
tion results in an overall higher metabolic activity and turnover rate in cancellous
bone [ 37 ]. Consequently, many investigations show an age-related increase of
microdamage in cortical bone [ 1 , 6 ], but there are discrepancies between inves-
tigations regarding the accumulation of microdamage with age in cancellous bone.
For example, in cancellous bone, some studies found an age-related increase [ 18 ,
31 , 38 ] while others report no relationship between microdamage accumulation
and aging [ 17 , 28 , 39 ]. Because anatomical location and mean ages of populations
considered in these studies vary greatly, we suggest that the discrepancies between
these studies could result from differences in turnover rates that vary with ana-
tomical location [ 40 ] and age [ 41 ]. Consistent with the above concept, a study in
which bone turnover was suppressed using high-dose bisphosphonates showed an
increase in microcrack density in dog vertebral bone [ 42 ].
Although microdamage accumulation with aging has been observed for both
males and females (Fig. 5 ), the limited available data indicate a steeper age-related
increase in microcrack density and age in females than in males [ 1 ]. Thus,
microdamage density is significantly greater in older females than in older
males [ 6 ]. It is noteworthy that Norman and Wang's investigation was based on