STRUCTURE AND FUNCTION OF VERTEBRAL TRABECULAR BONE

Study Design. A combined morphologic and finite- element study on vert ebral trabecular bone. Objective. To relate the form and function of v ertebral trabecular bone, in an attempt to better understand the mecha nical function of a lumbar vertebra. Summary of Background Data. The a rchitecture of bone is closely related to its mechanical function (Wol ff's Law). In the human spine, vertebrae are subjected to a large vari ety of loads. Yet, these bones show a typical architecture, which mean s that they carry typical loads. Methods. Five trabecular bone cubes f rom specific sites of a lumbar vertebra were ED-reconstructed for comp uterized analysis. The architecture of the specimens was quantified by the bone volume fraction and a measure of anisotropy, the mean bone l ength. A finite element model was used to calculate internal stresses within a homogeneous vertebral body under basic loads. For each load c ase, bone volume fraction of the specimens was compared with the equiv alent von Mises stress, and mean bone length was compared with the pri ncipal stress directions. Results: Bone volume fraction poorly related to the von Mises stress in the physiologic load case of axial compres sion. However, high bone volume fractions exist at locations where mul tiple load situations occur (e.g., near the pedicles and endplates). R emarkably, these sites also show finer architectures. Comparison of me an bone length with principal stresses revealed that the vertebral tra becular bone architecture particularly, but not entirely, corresponds to the stress field under axial compression. The horizontal struts nea r the endplates were found to be due to the function of the healthy in tervertebral disc, and facetal joint loads introduce stress components that relate well with the bone structures near the pedicle bases. Con clusions. The trabecular bone architecture and the vertical orientatio n of the facet joints suggest that walking may be the main activity th at determines the lumbar vertebral bone architecture.