Age-dependent microdamage removal following mechanically induced microdamage in trabecular bone in vivo

In order to examine the potential age-related response of trabecular bone to microdamage, a novel animal model utilizing a bone chamber to load existing distal femoral trabecular bone of rats was developed. Fifteen 8-month-old (mature) and fifteen 24-month-old (old) Fischer Brown Norway rats underwent bilateral insertion of the bone chamber. After a 3-week recovery period, one leg per animal underwent damage-inducing loading. Double fluorochrome labeling was used to identify microcracks induced by loading. A greater crack density was found in loaded trabecular bone than in corresponding unloaded control bone at day 0 in both age groups (mature n = 5, old n = 4). At day 35 post loading, older rats (n = 3) had greater crack density (suggesting little removal of microcracks), whereas younger rats (n = 5) had no difference between loaded and unloaded limbs, suggesting induced microcracks were removed. The difference in bone volume fraction between the loaded and unloaded limb were significantly different at 21 and 35 days post loading when comparing the old with the mature rats. The data suggest a reduced ability of bone to recover after damage in the older rats. The damage-inducing capabilities of the animal model were demonstrated using double fluorochrome labeling in vivo for detection of microcracks. The results indicate that removal of microdamage is altered with age.