Trabecular structure quantified with the MRI-based virtual bone biopsy in postmenopausal women contributes to vertebral deformity burden independent of areal vertebral BMD

Abstract

In postmenopausal women with a wide range of vertebral deformities, MRI-based structural measures of topology and scale at the distal radius are shown to account for as much as 30% of vertebral deformity, independent of integral vertebral BMD.

Introduction: Trabecular bone architecture has been postulated to contribute to overall bone strength independent of vertebral BMD measured by DXA. However, there has thus far been only sparse in vivo evidence to support this hypothesis.

Materials and methods: Postmenopausal women, 60-80 yr of age, were screened by DXA, and those with T-scores at either the hip or spine falling within the range of -2.5 +/- 1.0 were studied with the MRI-based virtual bone biopsy, along with heel broadband ultrasound absorption and pQCT of the tibia. The data from 98 subjects meeting the enrollment criteria were subjected to microMRI at the distal tibia and radius, and measures of topology and scale of the trabecular bone network were computed. A spinal deformity index (SDI) was obtained from morphometric measurements in midline sagittal MR images of the thoracic and lumbar spine to evaluate associations between structure and deformity burden.

Results: A number of structural indices obtained at the distal radius were correlated with the SDI. Among these were the topological surface density (a measure of trabecular plates) and trabecular bone volume fraction, which were inversely correlated with SDI (p < 0.0001). Combinations of two structural parameters accounted for up to 30% of the variation in SDI (p < 0.0001) independent of spinal BMD, which was not significantly correlated. pQCT trabecular BMD was also weakly associated, whereas broadband ultrasound absorption was not. No significant association between SDI and structural indices were found at the tibia.

Conclusions: Structural measures at the distal radius obtained in vivo by microMRI explained a significant portion of the variation in total spinal deformity burden in postmenopausal women independent of areal BMD.

FIG. 1

Illustration of quality scoring for the MRI exams using tibia cross sections: grade 1 = near optimum quality; grade 2 = slight blurring; grade 3 = significant blurring that might affect fine features or cause some distortion of larger features; grade 4 = uncorrected motion artifacts too severe for trabecular features to be visualized and analyzed.

FIG. 2

Cascade of image acquisition and processing steps. After data acquisition, the navigator data are used to correct for subject displacement during the scan. Subsequently, region of interest (ROI) for analysis is selected. After bone volume fraction mapping and subvoxel processing, the topological parameter densities are computed and a 3D rendering of a virtual core is computed for visual inspection.

FIG. 3

Digital representation of hypothetical skeletonized structure of trabecular network containing only surfaces and curves (the skeleton analogs of plates and rods, respectively) with their topological assignments.

FIG. 4

Midline sagittal images of the lumbar and portion of thoracic spine in three women showing the wide range of SDI values obtained by quantifying vertebral deformity: (A) SDI = 11.1, virtually all vertebrae visible show substantial deformities in this 88-yr-old woman, leading to an exceptionally high deformity burden; (B) SDI = 5.3, deformities are less pronounced in this 65-yr-old woman; (C) SDI = 2.2, no obvious deformities are discernable in this 71-yr-old woman.

FIG. 5

μMRIs of the distal radius (top row) with their respective virtual cores (bottom row) along with structural parameters from three subjects exemplifying a wide range in bone quality represented by the topological parameters that vary by over an order of magnitude between the extremes: (A) 68-yr-old woman having a well-connected bone structure; (B) 69-yr-old woman with less dense trabecular network; (C) 87-yr-old woman with sparse trabeculae and disconnected network.

FIG. 6

Associations between topological parameters in the radius and SDI showing negative and positive correlation, respectively, for voxel surface density (A) and erosion index (B) (r = −0.46, p < 0.0001 and r = 0.40, p = 0.0005).