Relationship between mechanical properties and distribution of lamellae in the human femoral shaft

Abstract

The correlation between microstructure and stress distribution along a bone has always been one of the main concerns of skeletal biomechanics. By applying an original technique for isolating osteon samples and loading them under tension or compression along their axis, Ascenzi and co-workers (1, 2, 3, 4) stated that the lamellae best able to support tensile stress are those with longitudinally oriented fibre bundles (dark in cross sections observed under the polarizing microscope), while the lamellae best able to support compressive stress are those with transversely oriented fibre bundles (bright in cross sections observed under the polarizing microscope). From this result it appears clear that an analysis of the distribution and thickness of the two types of lamellae would make it possible to provide informations about the distribution of tensile vs. compressive strenght within compact bone. Such a type of analysis can be carried out using either a semiautomatic (9). or an automatic apparatus for acquisition and computation of geometric data. According to our experience the two methods give comparable results. However, an automatic apparatus is most convenient because it allows to reduce dramatically the v/orking time. In fact, the time required for analysing a cross section of a femoral diaphysis drops from two months to two hours when a semi-automatic apparatus is replaced by an automatic apparatus. So the proposal of one of us, Prof. A. Boyde, to further develop this research in his laboratory, using a Quantimet 720 image analysing computer, was accepted with enthusiasm (6, 10). The material used in this investigation was a serie. of undecalcified cross sections, 100 micra thick, each located 1 cm. from the next, and prepared using a Leitz annular blade microtome. The distribution of osteon. es and interstitial bone in terms of the proportion of their cross-sectional area taken up by bright lamellae as assessed by polarized light microscopy was done by performing a series of three operations. The first was the calculation of the surface area taken up by bright lamellae with respect to the dark background. The purpose of the second operation was to calculate the proportion of the surface actually occupied by the bone section, that is, excluding the discontinuities in the section. The aim of the third operation was to calculate what proportion of the surface of the bone section was occupied by bright lamellae through computer processing of the results of the first two operations.