Relationships among microstructural properties of bone at the human midshaft femur

Abstract

Mineralization density and collagen fibre orientation are two aspects of a bone's microstructural organization that influence its mechanical properties. Previous studies by our group have demonstrated a distinctly non-random, though highly variable, spatial distribution of these two variables in the human femoral cortex. In this study of 37 specimens, these variables are examined relative to one another in order to determine whether regions of bone demonstrating higher or lower mineralization density also demonstrate a prevalence of either transversely or longitudinally oriented collagen fibres. An analysis of rank-transformed collagen fibre orientation (as determined by circularly polarized light) and mineralization density (as determined by backscattered electron microscopy) data sets demonstrated that areas of low mineralization density (predominantly in the anterior-lateral cortex) tended to correspond to regions of higher proportions of longitudinally oriented collagen fibres. Conversely, areas of higher mineralization density (postero-medially) tended to correspond to regions of higher proportions of transversely oriented collagen fibres. High variability in the sample led to generally low correlations between the two data sets, however. A second analysis focused only on the orientation of collagen fibres within poorly mineralized bone (representing bone that was newly formed). This analysis demonstrated a lower proportion of transverse collagen fibres in newly formed bone with age, along with some significant regional differences in the prevalence of collagen fibres of either orientation. Again high variability characterized the sample. These results are discussed relative to the hypothesized forces experienced at the midshaft femur.

Fig. 1

Using a customized macro, cortex was divided into 16 radial sectors and three rings (periosteal, mid cortex and endosteal), for a total of 48 segments. Only data from the outermost 16 sectors (periosteal ring) were included in the current analysis. All sections were oriented so that medial is to the left and posterior is towards the bottom when viewed on screen or print.

Fig. 2

Illustration of image analytical steps required for creating relational images. Explanation of colour look-up table produced for relational image is provided in Fig. 3. Field width of each image = 3.75 mm.

Fig. 3

Explanation of colour look-up table used for identifying either transverse or longitudinal collagen fibre orientations within poorly mineralized bone. Field width of each image = 3.75 mm.

Fig. 4

Line plots showing the average rank by sector for collagen fibre orientation (filled squares) and mineralization (unfilled squares) for each age group (A = Younger, B = Middle, C = Older). Posterior sectors 1, 16; Postero-medial sectors 2, 3; Medial sectors 4, 5; Antero-medial sectors 6, 7; Anterior sectors 8, 9; Antero-lateral sectors 10, 11; Lateral sectors 12, 13; Postero-lateral sectors 14, 15.

Fig. 5

Examples of relational montages. See Fig. 3 for explanation of colour look-up-table.