Contribution of Trabecular and Cortical Components to the Mechanical Properties of Bone and Their Regulating Parameters M. ITO, 1 A. NISHIDA, 1 A. KOGA, 2 S. IKEDA, 3 A. SHIRAISHI, 4 M. UETANI, 1 K. HAYASHI, 1 and T. NAKAMURA 3 1 Departments of Radiology and 2 Structural Engineering, Nagasaki University, Nagasaki, Japan 3 Department of Orthopaedic Surgery, University of Occupational and Environmental Health, Kitakyushu, Japan 4 Chugai Pharmaceutical Co., Ltd., Tokyo, Japan To evaluate the mechanical contributions of the spongiosa and cortex to the whole rat vertebra, we developed a finite element analysis (FEA) system linked to three-dimensional data from microcomputed tomography (micro-CT). Twen- ty-eight fifth lumbar vertebrae (L-5) were obtained from 10-month-old female rats, comprised of ovariectomized (ovx, n  6), sham operated (n  7), and alfacalcidol- treated after ovx (0.1 g/kg [n  8] and 0.2 g/kg [n  7]) groups. The trabecular microstructure of L-5 was mea- sured by micro-CT. Yield strength at the tissue level (YS), defined as the value at which 0.034% of all elements reached yield stress, was calculated by the FEA. Then, the ultimate compressive load of each specimen was measured by mechanical testing. The YS of the whole bone (YSw) showed a significant correlation with ultimate load (r  0.91, p < 0.0001). The YS values of the isolated spongiosa (YSs) and cortex (YSc) were calculated in models with varying amounts of trabecular or cortical bone mass. The mechanical contribution of the spongiosa showed a non- linear relationship with bone mass, and ovx reduced the mean mechanical contribution of the spongiosa to the whole bone by 13% in comparison to the sham group. YSs had a strong relationship with trabecular microstructure, especially with trabecular bone pattern factor (TBPf) and structure model index (SMI), and YSc had a strong rela- tionship with cortical bone volume. The structural param- eters most strongly related to YSw were BV/TV and TBPf. Our micro-FEA system was validated to assess the me- chanical properties of bone, including the individual prop- erties of the spongiosa and cortex, in the osteoporotic rat model. We found that the mechanical property of each component had a significant relationship with the respec- tive bone mass, volume, or structure. Although trabecular microstructure has a significant relationship with bone strength, in ovx bone with deteriorated trabecular micro- structure, the strength depended mainly on the cortical component. (Bone 31:351–358; 2002) © 2002 by Elsevier Science Inc. All rights reserved. Key Words: Finite element analysis (FEA); Microcomputed tomography (micro-CT); Mechanical testing; Three-dimensional (3D) structure. Introduction The strength of the vertebral body has been shown to be related to the apparent bone density. However, the relative mechanical contribution of the trabecular and cortical bone components to the strength of the whole vertebral bone is unclear and the results of previous studies have not been consistent. In human vertebral bone, Rockoff et al. 22 found that 45%–75% of the axial load was transferred by the cortex, whereas Hayes et al. 7 found only a 6%–12% reduction in the load-bearing capacity after removal of the cortical bone. This difference was consid- ered due to the different methods used in evaluating the load distribution and the relative amounts of trabecular bone in the specimens. Finite element analysis (FEA) is a simulation method that can reveal the stress distribution when a bone is loaded. Muller et al. 18 applied FEA to the images of three-dimensional (3D) human vertebral bone, showing that simulated age- and disease- dependent trabecular bone loss affected the strength of whole bone. However, selective analyses of the stress distribution and strength of the vertebral bone were not performed in their study, perhaps due to the limitation of the FEA system used. We have developed a new 3D FEA system to evaluate the mechanical properties of whole vertebral bone that accounts for the mechan- ical interactions of the spongiosa and the cortex. The FEA system was linked to micro-CT data, enabling the calculation of bone strength and stress distribution of the spongiosa and cortex using 3D images. We hypothesized that the FEA system would be able to predict the ultimate mechanical compressive strength of the whole lumbar bone of rats and, furthermore, to calculate indi- vidually the strengths of the isolated spongiosa and cortex. The purposes of our study were to validate our FEA system for the assessment of the mechanical properties of whole bone, to evaluate the relationship between the individual strength of spongiosa or cortex and bone mass or structure and, finally, to evaluate the mechanical contribution of spongiosa to whole bone using an osteoporotic rat model. Address for correspondence and reprints: Dr. Masako Ito, Department of Radiology, Nagasaki University, 1-7-1 Sakamoto, Nagasaki 852-8501, Japan. E-mail: masako@net.nagasaki-u.ac.jp Bone Vol. 31, No. 3 September 2002:351–358 351 © 2002 by Elsevier Science Inc. 8756-3282/02/$22.00 All rights reserved. PII S8756-3282(02)00830-X