Site specific increase in heterogeneity of trabecular bone tissue mineral during oestrogen deficiency

Abstract

Heterogeneity of bone mineral during oestrogen defi ciency European Cells and Materials Vol. 21 2011 (pages 396-406) ISSN 1473-2262 AbstractAlthough osteoporosis reduces overall bone mass causing bone fragility, recent studies report that the remaining bone tissue is signifi cantly stiffer. Preliminary studies indicate that alterations in bone tissue mineral content might explain these changes, albeit that other studies report conflicting observations. The objective of this study is to quantify whether the distribution of bone tissue mineral is altered during oestrogen defi ciency. Individual trabeculae were harvested from the proximal femur of 7 ovariectomised sheep (OVX), sacrifi ced 12 months postsurgery, and 5 age-matched controls. Mineral content (wt% Ca) was determined using a quantitative backscattered scanning electron microscopy imaging approach. Mineral heterogeneity within individual trabeculae was compared by calculating the full width at half maximum (FWHM) of mineral density distributions. Mean calcium content, the spatial distribution of mineral within trabeculae and the inter-trabecular variation between regions of proximal femora were also compared. Oestrogen defi ciency increased mineral heterogeneity within individual trabeculae compared to healthy controls, as measured by FWHM (3.57 ± 0.68 vs. 3.17 ± 0.36 wt% Ca, p = 0.04). I n particular mineral variability increased between superficial and deep regions of trabeculae of OVX animals (p = 0.04). Interestingly, mineralisation variability between greater and lesser trochanters (i.e. intertrochanteric fracture line) was increased in OVX compared to CON, as indicated by a greater % difference in the standard deviation of trabecular mineral content (77.11 ± 11.70 vs. 45.64 ± 23.70 %, p = 0.03). Such changes are undetectable by evaluating the mean mineral content of bone tissue, but may contribute to changes in bone mechanical strength following osteoporotic bone loss.

A three-scale finite element investigation into the effects of tissue mineralisation and lamellar organisation in human cortical and trabecular bone

“…The mineral content of bone is known to vary with age (Augat and Schorlemmer 2006), anatomical location (Nazarian et al 2007) and the onset of osteoporosis (Brennan et al 2011a) meaning that tissue level mechanics must change accordingly. Moreover, the distribution of mineral in trabecular bone is non-uniform, with superficial regions generally being less mineralised due to the continuous remodelling of the outer surfaces of trabecular packets (Fratzl et al 2004;Brennan et al 2011a;Brennan et al 2011b). The results presented in Figure 5 highlight that it is this non-uniform distribution of mineral which contributes to the lower stiffness of trabecular bone compared to that of cortical bone.…”

“…These studies also shed light on previous discrepancies in experimental reports of tissue moduli and reveal that variations in mineral volume fraction, mineral crystal size and lamellar orientation could explain these differences. Given the dependence of tissue mineralisation and lamellar arrangement with age (Augat and Schorlemmer 2006), anatomical location (Nazarian et al 2007) and clinical disease (Brennan et al 2011a) it is clear that bone composition and organisation need to be considered before meaningful predictions of the tissue level response can be determined. With the aid of advanced imaging techniques, it is thought that this multiscale modelling approach could be used to provide a more accurate description of mechanical properties in continuum/organ level finite element models.…”

“…Again, the mesh characteristics are similar to those previously described and the constitutive behaviour is initially assumed to be similar to cortical bone, whereby the mineral content is uniformly distributed over the crosssection of the trabeculae. However, due to the continuous remodelling process, it has been reported that the mineral content in trabecular struts is non-uniformly distributed over their cross-section (Fratzl et al 2004;Brennan et al 2011a;Brennan et al 2011b). Fratzl et al (2004) showed that in some superficial regions of trabeculae, the degree of mineralisation was almost half that of the centre region, due to the presence of newly formed tissue on the outer surfaces.…”

“…The predicted effective properties of lamellar bone are considered as a function of mineral volume fraction (MVF), as this parameter has been found to vary significantly across different anatomical locations (Nazarian et al 2007) and with the onset of clinical diseases such as osteoporosis (Brennan et al 2011a). To investigate this parameter, the spacing between the mineral crystals in the 2-direction (see Figure 2 (c)) was varied to predict the effective properties for MVFs which ranged from 20% to 50%.…”

“…lamellar orientation (Hofmann et al 2006). Furthermore, significant compositional (Dickenson et al 1981;Li and Aspden 1997;Boyde et al 1998;Gadeleta et al 2000;Zioupos 2000;Brennan et al 2011a) and structural (Augat and Schorlemmer 2006) changes are known to arise during diseases such as osteoporosis. However, due to the complex hierarchical arrangement of the constituent materials, precisely how small changes in mineral/collagen composition can affect the mechanical behaviour at the level of the whole bone is not clear.…”