Are Rat Cancellous and Cortical Bones Comprised of the Same Material?

The aim of this study was to confirm whether rat cancellous and cortical bones are comprised of the same material and whether an analytical model can be devised to account for the compressive strength of cortical and trabecular bone based on its mineral and matrix compositions. The answer to this question will dictate whether to use a single or two distinct constitutive relationships describing the mechanical behavior of bone in general or cancellous and cortical bone separately in our computer tomography-based method to assess fracture risk in skeletal pathologies.

Two analytical models were developed. The first model considers cortical and cancellous bone mechanical properties describable by one relationship and is analyzed with a linear multiple regression analysis. The second model takes an analytic approach based on the physical relationships between bone mechanical properties and its apparent bone density analyzed by a non-linear regression model.

Cancellous- and cortical-only specimens were obtained from ten mature female rats from metaphysis and diaphysis, respectively, of the femur. Bone density was measured by Archimedes’ principle in a pycnometer and afterwards, the specimens were scanned in a micro-computed tomography system to examine bone mineral density and bone volume fraction (BV/TV). Biomechanical properties of the cancellous and cortical specimens were evaluated in a uniaxial monocyclic compression test. The structure and composition of the mineral part were analyzed by Fourier transform infrared spectroscopy (FTIR) and by ash content. The protein and collagen content of the organic matrix was examined by amino acid analysis.

There was no significant difference between cancellous and cortical bone specimens density. BV/TV for cancellous and cortical showed a mean value of 58.72 ±2.86 % and 36.63 ±4.16 %, respectively. An average Young’s modulus of 8.50 ±2.86 GPa for cortical specimen was measured, 3.4 times higher than trabecular specimen 2.47 ±1.68 GPa. The stiffness of cancellous specimen 720 N/mm was one tenth that of the cortical specimen and the post-yield behaviour of strain of the trabecular specimen was twice as much than cortical specimen. The matrix analysis showed that there is no significant difference in the content of protein and collagen between cancellous and cortical bone specimens. The mineral analysis presented a slightly higher value in ash content for trabecular bone 68.9 ±9.94% but no difference (p = 0.12) between them. FTIR determined the phosphate, carbonate, and protein/mineral content. These data showed that the phosphate content in the ν₄PO₄ domain in cancellous and cortical specimen had no difference (p = 0.25). The carbonate content in the ν₂CO₃ domain presented a weak trend that they are similar (p = 0.06). However, the protein/mineral ratio showed strong similarity (p = 0.89). From a material point of view, the p-values for the different compositional variables demonstrated no significant differences between the cancellous and cortical bone material compositions. On a structural level, the significant differences between the cancellous and cortical bone volume fractions, as expected, are based on their structural differences. The collagen content in protein demonstrated expected differences between cancellous and cortical bone specimens, since the collagen networks of the two bone structures are vastly different from one another. Linear regression analysis highlighted the importance of bone volume fraction by explaining 69% of behavior of Young’s modulus. The non-linear regression based model described 77% of the behavior of Young’s modulus based on bone’s structural and compositional parameters. While the descriptive power of this model is not as strong as the one generated by linear regression, it is however based on a well- understood relationship between bone strength and bone density.

In conclusion, this study provides insight into the compositional differences of adult rat cancellous and cortical bone, highlighting little or no differences between the compositions of the two bone types. However, a full analysis based on the entire dataset must be carried out to further confirm observation in this study.

Year	Entry
1991	Snyder SM, Schneider E. Estimation of mechanical properties of cortical bone by computed tomography. J Orthop Res. May 1991;9(3):422-431.
2002	Currey JD. Bones: Structure and Mechanics. Princeton, NJ: Princeton University Press; 2002.
2000	Whealan KM, Kwak SD, Tedrow JR, Inoue K, Snyder BD. Noninvasive imaging predicts failure load of the spine with simulated osteolytic defects. J Bone Joint Surg. September 2000;82A(9):1240-1251.
1997	Gibson LJ, Ashby MF. Cellular Solids: Structure and Properties. 2nd ed. Cambridge, UK: Cambridge University Press; 1997. Cambridge Solid State Science Series.
2004	Schuit SCE, van der Klift M, Weel AEAM, de Laet CEDH, Burger H, Seeman E, Hofman A, Uitterlinden AG, van Leeuwen JPTM, Pols HAP. Fracture incidence and association with bone mineral density in elderly men and women: the Rotterdam Study. Bone. January 2004;34(1):195-202.
2006	Turner CH. Bone strength: current concepts. Annals NY Acad Sci. 2006;1068(1):429-446.
1997	Guo XE, Goldstein SA. Is trabecular bone tissue different from cortical bone tissue? Forma. 1997;12(3-4):185-196.
1976	Carter DR, Hayes WC. Bone compressive strength: the influence of density and strain rate. Science. September 10, 1976;194(4270):1174-1176.
2002	Cummings SR, Karpf DB, Harris F, Genant HK, Ensrud K, LaCroix AZ, Black DM. Improvement in spine bone density and reduction in risk of vertebral fractures during treatment with antiresorptive drugs. Am J Med. March 2002;112(4):281-289.
1988	Currey JD. The effect of porosity and mineral content on the Young's modulus of elasticity of compact bone. J Biomech. 1988;21(2):131-139.
1988	Rice JC, Cowin SC, Bowman JA. On the dependence of the elasticity and strength of cancellous bone on apparent density. J Biomech. 1988;21(2):155-168.
1985	McBroom RJ, Hayes WC, Edwards WT, Goldberg RP, White AA III. Prediction of vertebral body compressive fracture using quantitative computed tomography. J Bone Joint Surg. 1985;67A(8):1206-1214.
1992	Choi K, Goldstein SA. A comparison of the fatigue behavior of human trabecular and cortical bone tissue. J Biomech. 1992;25(12):1371-1381.
1999	Bowman SM, Gibson LJ, Hayes WC, McMahon TA. Results from demineralized bone creep tests suggest that collagen is responsible for the creep behavior of bone. J Biomech Eng. April 1999;121(2):253-258.
1999	Boskey AL, Wright TM, Blank RD. Collagen and bone strength. J Bone Miner Res. March 1999;14(3):330-335.
1989	Rey C, Collins B, Goehl T, Dickson IR, Glimcher MJ. The carbonate environment in bone mineral: a resolution-enhanced fourier transform infrared spectroscopy study. Calcif Tiss Int. September 1989;45(3):157-164.
1995	Landis WJ. The strength of a calcified tissue depends in part on the molecular structure and organization of its constituent mineral crystals in their organic matrix. Bone. May 1995;16(5):533-544.
1989	Martin RB, Ishida J. The relative effects of collagen fiber orientation, porosity, density, and mineralization on bone strength. J Biomech. 1989;22(5):419-426.
2003	Heaney RP. Is the paradigm shifting? Bone. October 2003;33(4):457-465.
1990	Currey JD. Physical characteristics affecting the tensile failure properties of compact bone. J Biomech. 1990;23(8):837-844.
1975	Burstein AH, Zika JM, Heiple KG, Klein L. Contribution of collagen and mineral to the elastic-plastic properties of bone. J Bone Joint Surg. October 1975;57A(7):956-961.
2002	Riggs BL, Melton LJ III. Bone turnover matters: the raloxifene treatment paradox of dramatic decreases in vertebral fractures without commensurate increases in bone density. J Bone Miner Res. January 2002;17(1):11-14.
2001	Cowin SC, ed. Bone Mechanics Handbook. 2nd ed. Boca Raton, FL: CRC Press; 2001.
2004	Hellmich C, Ulm F-J, Dormieux L. Can the diverse elastic properties of trabecular and cortical bone be attributed to only a few tissue-independent phase properties and their interactions? arguments from a multiscale approach. Biomech Model Mechanobiol. June 2004;2(4):219-238.

Year

Entry

1991

Snyder SM, Schneider E. Estimation of mechanical properties of cortical bone by computed tomography. J Orthop Res. May 1991;9(3):422-431.

2002

Currey JD. Bones: Structure and Mechanics. Princeton, NJ: Princeton University Press; 2002.

2000

Whealan KM, Kwak SD, Tedrow JR, Inoue K, Snyder BD. Noninvasive imaging predicts failure load of the spine with simulated osteolytic defects. J Bone Joint Surg. September 2000;82A(9):1240-1251.

1997

Gibson LJ, Ashby MF. Cellular Solids: Structure and Properties. 2nd ed. Cambridge, UK: Cambridge University Press; 1997. Cambridge Solid State Science Series.

2004

Schuit SCE, van der Klift M, Weel AEAM, de Laet CEDH, Burger H, Seeman E, Hofman A, Uitterlinden AG, van Leeuwen JPTM, Pols HAP. Fracture incidence and association with bone mineral density in elderly men and women: the Rotterdam Study. Bone. January 2004;34(1):195-202.