Coined in French in the 1820s as a description of a pathological state of bone tissue, the term osteoporosis made its way into the English language only in the 20'*’ century. Unlike other medical concepts, which have not been substantially altered by progress in medical research, the definition of osteoporosis has constantly reflected the state of knowledge on the phenomenon itself (Schapira and Schapira, 1992). Over its 180 years of evolution, this definition has continuously sought to maintain a balance between physiological and clinical criteria. At present the definition, established by the National Institutes of Health (NIH) in 2000, describes osteoporosis as ‘a skeletal disorder characterised by compromised bone strength predisposing to an increased risk of fracture’.

It is important to note the subtle downplaying of bone mass in this most recent definition in favour of a more generic statement regarding all of the determinants of bone strength, thereby shifting the emphasis to include bone quality. This study sought to characterise the effect of bone turnover on bone quality and material properties, in an ovariectomised (OVX) ovine animal model at 12 months post-surgery. Animals were intravenously administered five fluorochrome dyes at three-monthly intervals during the study in order to label sites of bone turnover. A study of compact bone turnover and microarchitecture, using epifluorescence microscopy and microCT respectively, was carried out on the left metatarsal. Bone turnover was increased in the OVX group compared with controls. Increased turnover resulted in increased porosity in the OVX group compared to controls. The small reduction in strength resulting from these changes compared well with our calculations which were based on the reduction in load bearing area.

Analyses of fatigue-induced microdamage in control and OVX compact bone samples showed that while numerical crack density was higher in OVX, crack surface density was higher in the controls, due to the presence of more long microcracks. It was also observed that long cracks (>300|am) tended to stop at new (labelled) osteons whereas they tended to penetrate or deflect around older (unlabelled) osteons. This shows that increased turnover has a direct effect on microcrack behaviour in bone.

The lumbar vertebrae are clinically relevant sites in terms of osteoporosis because fractures are often found to occur there. Histomorphometry of the L3 vertebra revealed increased bone turnover in cortical and trabecular compartments in OVX bone compared with controls. The microarchitectural parameters;​ trabecular number, thickness and separation, relative bone volume and anisotropy were not significantly different in OVX compared with controls. However, these parameters were found to differ significantly between the cranial, mid-vertebral and caudal regions of the vertebra. Biomechanical testing showed that ultimate strength and stiffness were reduced in the OVX group. This group also displayed less plastic strain and more strain due to damage compared with controls. Although no change was found in BMD as measured by DEXA, significant changes were found in bone quality parameters 12 months post-OVX. In conclusion, this study has illustrated the importance of bone turnover in relation to bone quality.