Intrapopulation variability in mineralization density at the human femoral mid-shaft

Abstract

One of several microstructural variables known to affect the mechanical properties of bone is the degree of mineralization of bone matrix. The aim of this study was to examine mineralization density, and its variability with age and sex, from a biomechanical perspective. Histological sections, prepared from mid-shaft femora obtained at autopsy from 40 individuals, were imaged using quantitative backscattered electron microscopy. Each cross-section montage was divided into 48 segments according to anatomical position. Mean grey-level values were quantified for each segment. One-way ANOVA with Tukey HSD post hoc tests were used to test for differences in mineralization between segments, age groups and sexes. Results showed a decrease in overall degree of mineralization density with adult age, but an increase in its coefficient of variation. Degree of mineralization was significantly lower in the periosteal third of the cortex, particularly in the antero-lateral aspect. This pattern was most prevalent amongst the youngest individuals in the sample. Whereas males between ages 45-64 years had a higher average degree of mineralization than females, the opposite was true of the older age group. Mineralization significantly decreased between middle and older age groups in males, but not in females. Despite limited consistencies in the location of high and low average mineralization bone through the cortex, the degree of interindividual variation, even within a single age and sex group, overwhelmed population level trends. The patterns of variability identified in this study are consistent with results of an analysis of collagen fibre orientation using the same sample material.

Fig. 1

Individual images obtained with BSE-SEM to determine mineralization density were each standardized and then montaged into a single image of the entire cross-section using an automated program. Cracks are artefacts of preparation and imaging techniques.

Fig. 2

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. All sections were orientated so that medial is to the left and posterior is towards the bottom when viewed on screen or print.

Fig. 3

Comparison between circumferential rings by sector, young age group, sexes pooled. Abbreviations for sectors: Posterior = sectors 1,16; Postero-medial = 2,3; Medial = 4,5; Antero-medial = 6,7; Anterior = 8,9; Antero-lateral = 10,11; Lateral = 12,13; Postero-lateral = 14,15.

Fig. 4

Examples of montages from each age group, with colour look-up table applied. Top row = younger age, middle row = middle age, bottom row = older age.