Compressive axial mechanical properties of rat bone as functions of bone volume fraction, apparent density and micro-ct based mineral density

Abstract

Mechanical testing has been regarded as the gold standard to investigate the effects of pathologies on the structure-function properties of the skeleton. With recent advances in computing power of personal computers, virtual alternatives to mechanical testing are gaining acceptance and use. We have previously introduced such a technique called structural rigidity analysis to assess mechanical strength of skeletal tissue with defects. The application of this technique is predicated upon the use of relationships defining the strength of bone as a function of its density for a given loading mode. We are to apply this technique in rat models to assess their compressive skeletal response subjected to a host of biological and pharmaceutical stimulations. Therefore, the aim of this study is to derive a relationship expressing axial compressive mechanical properties of rat cortical and cancellous bone as a function of equivalent bone mineral density, bone volume fraction or apparent density over a range of normal and pathologic bones. We used bones from normal, ovariectomized and partially nephrectomized animals. All specimens underwent micro-computed tomographic imaging to assess bone morphometric and densitometric indices and uniaxial compression to failure. We obtained univariate relationships describing 71-78% of the mechanical properties of rat cortical and cancellous bone based on equivalent mineral density, bone volume fraction or apparent density over a wide range of density and common skeletal pathologies. The relationships reported in this study can be used in the structural rigidity analysis introduced by the authors to provide a non-invasive method to assess the compressive strength of bones affected by pathology and/or treatment options.

Fig. 1

(a) An illustration of cortical and trabecular specimen preparations for conventional mechanical testing and nano-indentation through a series of frames: (I) whole bone; (II) cut at condyles; (III) distal metaphyseal cut for trabecular nano-indentation; (IV) specimen ready for cortex shaving; (V) distal metaphyseal cortex shaved; (VI) trabecular bone specimen cut; (VII) cortical bone specimen cut; (VIII) two diaphyseal cuts for cortical nano-indentation. (b) Representative distal metaphyseal cancellous bone samples with the cortex shaves from control (left) and ovariectomized (right) animals.

Fig. 2

A depiction of axial and transverse cortical bone samples embedded bone in resin for nano-indentation.

Fig. 3

Univariate exponential relationships describing the mechanical behavior of rat cortical and trabecular bone. (a), (c), and (e) describe modulus of elasticity based on the independent variable of equivalent mineral density, bone volume fraction or apparent density, respectively. Likewise, (b), (d), and (f) describe yield strength based on the independent variable of equivalent mineral density, bone volume fraction or apparent density, respectively.

Fig. 4

Representative µCT cross-sections from the mid-diaphysis and from the distal metaphysis illustrate cortical and trabecular bone morphologies, respectively, for CON, OVX and NFR groups. More significant cortical changes are observed in the NFR specimen, yet significant trabecular changes are observed in both the OVX and NFR specimen.