A Multi-Scale Approach to Implications of the Preferred Vertebral Trabecular Orientation on Spine Biomechanics

Abstract

Knowledge of the influence of loading directions on trabecular bone remodeling in spine is of significant value in understanding the development of spine deformities and vertebral bone quality across different scales.
Information on the constitution of a preferred trabecular orientation and mechanical properties of trabecular bone are important indicators in this respect. The current thesis aimed at exploring these aspects across multiple length scales in the spine. The thesis is divided in two parts. The influence of loadings less dominant than compression, i.e. shear, on the constitution of a preferred trabecular orientation in the spine on the macro-tissue level (>10 mm) was investigated in the first part (Part I). This influence was related to mechanical characteristics of trabecular structures on the micro-tissue scale (1-10 mm) in the second part (Part II).
In Part I, primary trabecular orientations (PTOsmacro) near the superior and inferior vertebral endplates of L1 and L5 of 6 human spine cadavers were determined on the macro level using micro computed tomography imaging (voxel size = 120 m3), by calculating the dominant fabric principal vector. Their relative deviations to the axial compression vectors in the spines, quantified by the normals to the endplate (NEs), were determined afterwards. The average deviation between the PTOmacro and NEs was 6.24⁰ (±4.34⁰). The PTOsmacro did not show a preference towards the anterior or posterior direction relative to the NE. From the deviations, it was concluded that trabecular bone in the spine predominantly adapts to compression loads. However, secondary loading directions, such as shear, are of additional influence.
In Part II, 13 small cubes (6.0x6.0 mm) from the volumes of interest in Part I were analysed on the micro level with regard to elasticity. Components, component ratios and primary elastic orientations (PEOmicro) of elasticity tensors, computed by the simulation of mechanical tests in finite element (FE) models, were calculated. PTOs of the cubes (PTOsmicro) were compared to the PEOsmicro and related to the PTOsmacro and NEs (Part I) qualitatively. Elasticity tensor components were within a reasonable range (approximately 1-250 MPa, excluding outliers) and no material symmetry was found, i.e. the structures were mechanically anisotropic. PTOsmicro deviated 13.90⁰ (±8.04⁰) with respect to the PEOsmicro on average. 10 out of 13 PEOsmicro had similar anterior or posterior tendencies as the PTOsmacro with respect to the NEs. 11 out of 13 PTOsmicro had similar anterior or posterior tendencies as PTOsmacro with respect to the NEs.
Elastic properties of typical trabecular structures in the vertebral bodies were successfully determined. Due to a relatively low resolution, PEOsmicro deviated strongly with the PTOsmicro. Such deviations could function as indicators for bone quality in skeletal disease diagnostics using low resolution imaging. PTOsmicro and PEOsmicro agreed relatively well to the PTOsmacro on the macro-tissue level, in terms of anteriorly or posterior tendencies relative to axial loading in the spine. This outcome shows promise for multi-scalar biomechanical analysis of trabecular bone.