Effect of estrogen deficiency on regional variation of a viscoelastic tissue property of bone

Abstract

Estrogen deficiency changes the regional distribution of tissue mineral density leading to alteration of the mechanical properties of bone at the tissue level. Direct measurement of the regional variation of elastic modulus and viscosity, which is the capacity to resist time-dependent viscoelastic deformation, will aid in our understanding of how estrogen deficiency alters bone quality. It was observed that, compared to bone from other anatomical sites, the jaw bone is less sensitive to estrogen deficiency. Thus, the objective of this study was to examine the effect of estrogen deficiency on (1) the regional variations of tissue modulus and viscosity of bone using nanoindentation, and (2) the modulus-viscosity relationships in jaw and vertebral bones for comparison between different anatomical sites. Mandibular and vertebral bone specimens of sham surgery and ovariectomized (OVX) rat groups were subject to nanoindentation in hydration. Indentation modulus and viscosity were measured at relatively new (less mineralized) tissue regions and at the corresponding pre-existing old (more mineralized) tissue regions of mandibular and vertebral bones. In the mandibular bones, significant regional variations of indentation modulus and viscosity were observed (p<0.039) and OVX increased the indentation viscosity. While significant positive correlations were found between indentation modulus and viscosity (p<0.001), the correlation slopes for the mandibular and vertebral bones were significant different (p<0.001). The current results indicated that changes in viscoelastic property and its regional variation should be examined to obtain a better understanding of estrogen deficiency-dependent alteration of bone quality.

Fig. 1

Descriptive regions for nanoindentation in 3D micro-CT images and under an indenter microscope; (a) Four mandibular regions (AB, alveolar bone (black dots); CB, cortical basal bone (black squares); TBM, marginal region of trabecular bone (black dots); TB, inner region of trabecular bone (black squares)) and (b) two vertebral regions (VBM, marginal region of vertebral bone (black dots) and VB, inner region of vertebral bone (black squares)).

Fig. 2

(a) A typical nanoindentation creep curve (b) with an excellent curve fitting.

Fig. 3

Comparisons of indentation (a) modulus and (b) viscosity between sham and OVX groups. The indentation modulus was marginally different at the inner regions of vertebral trabecular bone (VB, *; p=0.055) between sham and OVX groups while the indentation viscosity was significantly different at the mandibular cortical basal bone (CB, #; p<0.036) and at the marginal region of mandibular trabecular bone (TBM, ##; p<0.012), and marginally different at the inner regions of mandibular trabecular bone (TB, **; p=0.065). No significant difference was found at all other regions (p>0.1).

Fig. 4

(a) Strong positive correlations between indentation modulus and viscosity were found for mandibles (η=1221.307E-3520.293, r²=0.698, p<0.001, n=363) and vertebrae (η=557.625E-1629.957, r²=0.743, p<0.001, n=216) while the slopes of correlation were significantly different between the two bones (ANCOVA, p<0.001) and (b) significantly different slopes of correlation between sham and OVX groups were found for mandibular bones (ANCOVA, p=0.05) while no significantly different correlation was found for the vertebral bones (ANCOVA, p=0.667).