Compositional and mechanical properties of growing cortical bone tissue: a study of the human fibula

Abstract

Human cortical bone contains two types of tissue: osteonal and interstitial tissue. Growing bone is not well-known in terms of its intrinsic material properties. To date, distinctions between the mechanical properties of osteonal and interstitial regions have not been investigated in juvenile bone and compared to adult bone in a combined dataset. In this work, cortical bone samples obtained from fibulae of 13 juveniles patients (4 to 18 years old) during corrective surgery and from 17 adult donors (50 to 95 years old) were analyzed. Microindentation was used to assess the mechanical properties of the extracellular matrix, quantitative microradiography was used to measure the degree of bone mineralization (DMB), and Fourier transform infrared microspectroscopy was used to evaluate the physicochemical modifications of bone composition (organic versus mineral matrix). Juvenile and adult osteonal and interstitial regions were analyzed for DMB, crystallinity, mineral to organic matrix ratio, mineral maturity, collagen maturity, carbonation, indentation modulus, indicators of yield strain and tissue ductility using a mixed model. We found that the intrinsic properties of the juvenile bone were not all inferior to those of the adult bone. Mechanical properties were also differently explained in juvenile and adult groups. The study shows that different intrinsic properties should be used in case of juvenile bone investigation.

Figure 1

Top left to bottom right: Evolution of transverse section of fibula with age (age and sex are indicated above each section), taken at the same location. Illustrations come from DMB measurement by X-rays. Note the evolution of the cortex and trabeculae. Scale bars represent 1.5 mm. The right bottom graph shows the DMB distribution in juveniles (Solid line) and adults (Dot line) in osteonal and interstitial bone.

Figure 2

Transverse fibulae section at low magnification at two ages (x1). (a–c) Quantitative digitized microradiography was used to measure the degree of mineralization of bone (DMB, g/cm³), and (b–d) identical sections observed in polarized light. Rectangles show the location of the high magnification illustrations in Figs. 3 and 4. Illustrations will be available at high resolution.

Figure 3

At high magnification (x2.5) from left Fig. 2, top image shows the drifting osteons present in juvenile bone. Note the heterogeneity of the mineralization, with dark (low mineralization) and white osteons (higher mineralization). Red arrow and blue lines point of drifted osteons as shown in Robling et al. 1999. Middle image shows the corresponding section seen in polarized light. Drifting osteons exhibit a variation in the direction of transverse drift along their longitudinal axes, intermitent regions of concentric morphology and change in drift direction over time. Bottom image illustrates at 8 µm thin-section of May-Grünwald Giemsa staining, from endosteal (E) to periosteal (P) area, at high magnification (x10). Bone modeling is characterized by a formation (F) and resorption (R) at different location. Formation is mostly oriented throught the perioste and resorption in endoste, allowing the bone growth. Illustrations will be available at high resolution

Figure 4

At high magnification (x2.5) from right Fig. 2, Top image shows the circular osteons present in adult bone. Note the heterogeneity of the mineralization, with dark (low mineralization) and white osteons (higher mineralization). Red arrows and blue lines point of the circular and smaller osteons. Middle image shows the corresponding section seen in polarized light. Bottom image illustrates at 8 µm thin-section of May-Grünwald Giemsa staining, from endosteal (E) to periosteal (P) area, at high magnification (x10). Black arrows shows a limited bone cellular activity, with cavity filled with giant adipocytes. Illustrations will be available at high resolution.

Figure 5

Evolution of the parameters with age for both osteonal and interstitial bone regions. Juvenile data in triangle and adult data in circle, sharp symbol for osteonal region and plane symbol for interstitial region. Solid lines represent significant interactions with age in juvenile or adult group, using the pooled osteonal and interstitial regions.

Figure 6

Box Plot of Juvenile data in light grey, Adults data in dark grey, for osteonal and interstitial regions. P-value comparing groups are indicated. The difference (p) between Osteonal and Interstitial is shown using a Wilcoxon paired test and the difference between Juvenile/Adult is shown using a Mann-Whitney unpaired test.