The effects of increased intracortical remodeling on microcrack behaviour in compact bone.

The behaviour of microdamage in bone is related to its microstructural features and thus has an important role in tissue structural properties. However, it is not known how cracks behave in areas of increased intracortical remodeling. More remodeling creates wider variation in the properties of the primary microstructural features of cortical bone, namely osteons. This situation may occur after treatment involving parathyroid hormone or events such as menopause/ovariectomy. High turnover was modeled in this study by using ovariectomy (OVX) to induce surgical menopause in sheep. We hypothesized that osteon age would influence microcrack behaviour during propagation. Five fluorochrome dyes were administered intravenously at different time-points over 12 months post-OVX to label remodeling sites and all animals were then euthanized. Compact bone specimens (2x2x36 mm) were harvested from the right metatarsal. Samples were cyclically loaded to failure and then histological analyses were carried out. Cracks were categorized by length into three groups; short (<100 mum), intermediate (100-300 mum) and long (>300 mum). Numerical crack density (Cr.Dn) of long cracks was greater in controls compared with OVX. Controls also displayed a higher crack surface density (Cr.S.Dn) compared with OVX (p<0.05). The behaviour of short cracks did not differ between old and new osteons, but intermediate and long cracks preferentially stopped at newer osteons compared with older ones (p<0.05). This mechanism may have an important role in terms of prolonging fatigue life. We conclude that recently formed secondary osteons have a unique influence on propagating microcracks compared with older osteons. Therefore localized remodeling levels should be considered when studying microcrack behaviour in bone.