The Effect of Regional Variations of the Trabecular Bone Properties on the Compressive Strength of Human Vertebral Bodies DO-GYOON KIM, 1 CHRISTINE A. HUNT, 1 ROGER ZAUEL, 1 DAVID P. FYHRIE, 2 and YENER N. YENI 1 1 Bone and Joint Center, Department of Orthopaedics and Rehabilitation, Henry Ford Hospital, 2799 West Grand Boulevard, Detroit, MI 48202, USA; and 2 Orthopaedic Research Laboratories, UC Davis, School of Medicine, 4635 Second Avenue, Room 2000, Sacramento, CA 95817, USA (Received 1 June 2006; accepted 27 July 2007; published online 10 August 2007) Abstract—Cancellous centrum is a major component of the vertebral body and significantly contributes to its structural strength and fracture risk. We hypothesized that the vari- ability of cancellous bone properties in the centrum is associated with vertebral strength. Microcomputed tomog- raphy (micro-CT)-based gray level density (GLD), bone volume fraction (BV/TV), and finite element modulus (E) were examined for different regions of the trabecular centrum and correlated with vertebral body strength determined experimentally. Two sets of images in the cancellous centrum were digitally prepared from micro-CT images of eight human vertebral bodies (T10–L5). One set included a cubic volume (1 per vertebral centrum, n = 8) in which the largest amount of cancellous material from the centrum was included but all the shell materials were excluded. The other set included cylindrical volumes (6 per vertebral centrum, n = 48) from the anterior (4 regions: front, center, left, and right of the midline of vertebra) and the posterior (2 regions: left and right) regions of the centrum. Significant positive correlations of vertebral strength with GLD (r 2 = 0.57, p = 0.03) and E (r 2 = 0.63, p = 0.02) of the whole centrum and with GLD (r 2 = 0.65, p = 0.02), BV/TV (r 2 = 0.72, p = 0.01) and E (r 2 = 0.85, p = 0.001) of the central region of the vertebral centrum were found. Vertebral strength decreased with increasing coefficient of variation of GLD, BV/TV, and E calculated from subregions of the vertebral centrum. The values of GLD, BV/TV, and E in centrum were significantly smaller for the anterior region than for the posterior region. Overall, these findings supported the significant role of regional variability of centrum properties in determining the whole vertebral strength. Keywords—Vertebral centrum, Anatomic site differences, Micro-CT, Large-scale finite element modeling, BV/TV. INTRODUCTION Inhomogeneity of cancellous bone architecture and density within vertebral centrum has been consistently observed in human vertebrae. 1,9,29 This intra-centrum variation of cancellous bone properties is considered to play an important role in determining the fracture of a whole vertebral body. 8,20,28 Consistent with these considerations, it has been shown that regression models taking into account the regional variations of bone mineral density (rBMD) in the vertebral centrum are more predictive of vertebral strength compared to single point measurements. 6,21 Other studies also demonstrated that trabecular thickness can vary without changing bone quantity within a cancellous bone specimen 17 and that increases in the intraspeci- men variations of trabecular architecture can affect cancellous bone modulus independent of bone volume fraction. 32 However, a direct relationship between variability in the regional properties of cancellous tis- sue and whole vertebral body strength has not been established. Finite element (FE) analysis has been accepted as a useful tool to compute the modulus of cancellous bone. With the use of a quantitative computed tomography (QCT)-based FE model of vertebral centrum, Kop- perdahl et al. 16 found that increase in regional strain energy density of the centrum had a strong correlation with structural stiffness reduction of sagittal constructs of vertebrae. Due to the low scanning resolution, however, QCT-based models could not examine the micro-architectural variations in the cancellous cen- trum. High-resolution microcomputed tomography (micro-CT) allows for the utilization of the finer details of the microstructure in large-scale finite element (LS- FE) analyses, thus help making more mechanistic ap- proaches for understanding modulus and strength of cancellous bone. 12,19,25,30,33 Using the LS-FE analysis, it has been demonstrated that cancellous tissue strength decreases with increasing scatter of trabecular shear stresses in human vertebral cancellous bone tissue. 10 This result was attributed to the notion that structures with highly scattered tissue Address correspondence to Do-Gyoon Kim, Bone and Joint Center, Department of Orthopaedics and Rehabilitation, Henry Ford Hospital, 2799 West Grand Boulevard, Detroit, MI 48202, USA. Electronic mail: kim@bjc.hfh.edu Annals of Biomedical Engineering, Vol. 35, No. 11, November 2007 (Ó 2007) pp. 1907–1913 DOI: 10.1007/s10439-007-9363-1 0090-6964/07/1100-1907/0 Ó 2007 Biomedical Engineering Society 1907