ABSTRACT
Three different regions of interest (ROIs) were defined in pQCT scans (XCT-3000 machine, Stratec, Germany) taken at the tibial mid-diaphyses of 12 pre-menopausal (pre-MP) and 12 post-menopausal (post-MP) women who were otherwise normal, according to the volumetric bone mineral density (vBMD) value of their corresponding pixels (voxels) as assessed by their respective attenuation values. They were classified as "low-vBMD" (LD-ROI, with a vBMD of 200-400 mg/cm(3)), corresponding chiefly to trabecular-subcortical bone; "medium-vBMD" (MD-ROI, vBMD = 400-800 mg/cm(3)), corresponding mainly to porous cortical bone or cortical-subcortical bone, and "high-vBMD" (HD-ROI, vBMD higher than 800 mg/cm(3)), corresponding to dense cortical bone. The fraction of the total cross-sectional bone area covered by the HD-ROI was 16% higher, and that covered by the MD-ROI 20% lower, in pre-MP than post-MP women. No differences concerning the LDROIs were found. A close, linearly negative relationship was found between the MD- and HD-ROI fractions in all the women together, with no inter-group differences in slope. The Stress-Strain Index (an indicator of the torsional stiffness and strength of the whole bones that involved both the vBMD and the spatial disposition of the HD bone in the cross-section - torsional moment of inertia -) correlated linearly and positively with the cross-sectional area of the HD-ROI, with a higher slope for pre-MP than post-MP women. Qualitatively, a. post-MP women showed a significantly more prevalent discontinuity of the voxels in the HD-ROI than pre-MP women, and b. the tendency of LD-ROIs to accumulate along the mechanically lesseffective (antero-posterior) axis of the image - a characteristic of pre-MP bones - was visually less evident in post-MP bones. These features describe non-invasively some changes induced by menopause in the human tibia that may be critical for defining the skeletal condition and to monitor the effects of treatments addressed either to protect or to improve mechanically the bone structure, beyond the possibilities of standard densitometry.
