HIGH-RESOLUTION MAGNETIC-RESONANCE-IMAGING - 3-DIMENSIONAL TRABECULARBONE ARCHITECTURE AND BIOMECHANICAL PROPERTIES

The purpose of this study was to use high-resolution magnetic resonanc e (MR) imaging combined with image analysis to investigate the three-d imensional (3D) trabecular structure, anisotropy, and connectivity of human vertebral, femoral, and calcaneal specimens. The goal was to det ermine whether: (a) MR-derived measures depict known skeletal-site-spe cific differences in architecture and orientation of trabeculae; (b) 3 D architectural parameters combined with bone mineral density (BMD) im prove the prediction of the elastic modulus using a fabric tensor form ulation; (c) MR-derived 3D architectural parameters combined with BMD improve the prediction of strength using a multiple regression model, and whether these results corresponded to the results obtained using h igher resolution depictions of trabecular architecture. A total of 94 specimens (12 x 12 x 12 mm cubes) consisting of trabecular bone only w ere obtained, of which there were 7 from the calcaneus, 15 from distal femur, 47 from the proximal femur, and 25 from the vertebral bodies. MR images were obtained using a 1.5 Tesla MR scanner at a spatial reso lution of 117 x 117 x 300 mu m. Additionally, BMD was determined using quantitative computed tomography (QCT), and the specimens were nondes tructively tested and the elastic modulus (YM) was measured along thre e orthogonal axes corresponding to the anatomic superior-inferior (axi al), medial-lateral (sagittal), and anterior-posterior (coronal) direc tions, ii subset of the specimens (n = 67) was then destructively test ed in the superior-inferior (axial) direction to measure the ultimate compressive strength. The MR images were segmented into bone and marro w phases and then analyzed in 3D. Ellipsoids were fitted to the mean i ntercept lengths, using single value decomposition and the primary ori entation of the trabeculae and used to calculate the anisotropy of tra becular architecture. Stereological measures were derived using a prev iously developed model and measures such as mean trabecular width, spa cing, and number were derived. Because the spatial resolution of NBR i mages is comparable to trabecular bone dimensions, these measures may be subject to partial volume effects and were thus treated as apparent measures, such as BV/TV, Tb.Sp, Tb.N, and Tb.Th rather than absolute measures, as would be derived from histomorphometry. In addition, in a subset of specimens, the Euler number per unit volume was determined to characterize the connectivity of the trabecular network. There were significant differences in the BMD, trabecular architectural measures , elastic modulus, and strength at the different skeletal sites. The p rimary orientation axes for most of the specimens was the anatomic sup erior-inferior (axial) direction. Using the fabric tensor formulation, in addition to BMD, improved the prediction of YM (SI), while includi ng some of the architectural parameters significantly improved the pre diction of strength. In comparing MR-derived 3D measures with those ob tained from 20 mu m optical images (n = 18; 9 vertebrae, 9 femur speci mens), good correlations were found for the apparent Tb.Sp and Tb.N, m oderate correlation was seen for the apparent BV/TV, and poor correlat ion was found for the apparent Tb.Th. Using these higher resolution im ages, the fabric tensor formulation for predicting the elastic modulus also showed improved correlation between the measured and calculated modulus in the axial (SI) direction. In summary, high-resolution MR im ages may be used to assess 3D architecture of trabecular bone, and the inclusion of some of the 3D architectural measures provides an improv ed assessment of biomechanical properties. Further studies are clearly warranted to establish the role of architecture in predicting overall bone quality, and the role of trabecular architecture measures in cli nical practice. Currently, MR techniques may not be used instead of hi stomorphometry; however, they may provide an ideal platform for assess ing trabecular architecture in vivo, at multiple skeletal sites longit udinally, and assist in understanding the etiology of osteoporotic and aging changes, for studying osteoporosis progression and therapeutic efficacy. (C) 1998 by Elsevier Science Inc. All rights reserved.