Structure and function of vertebral trabecular bone.

Abstract

STUDY DESIGN: A combined morphologic and finite-element study on vertebral trabecular bone. OBJECTIVE: To relate the form and function of vertebral trabecular bone, in an attempt to better understand the mechanical function of a lumbar vertebra. SUMMARY OF BACKGROUND DATA: The architecture of bone is closely related to its mechanical function (Wolff's Law). In the human spine, vertebrae are subjected to a large variety of loads. Yet, these bones show a typical architecture, which means that they carry typical loads. METHODS: Five trabecular bone cubes from specific sites of a lumbar vertebra were 3D-reconstructed for computerized analysis. The architecture of the specimens was quantified by the bone volume fraction and a measure of anisotropy, the mean bone length. A finite element model was used to calculate internal stresses within a homogeneous vertebral body under basic loads. For each load case, bone volume fraction of the specimens was compared with the equivalent von Mises stress, and mean bone length was compared with the principal stress directions. RESULTS: Bone volume fraction poorly related to the von Mises stress in the physiologic load case of axial compression. However, high bone volume fractions exist at locations where multiple load situations occur (e.g., near the pedicles and endplates). Remarkably, these sites also show finer architectures. Comparison of mean bone length with principal stresses revealed that the vertebral trabecular bone architecture particularly, but not entirely, corresponds to the stress field under axial compression. The horizontal struts near the end-plates were found to be due to the function of the healthy intervertebral disc, and facetal joint loads introduce stress components that relate well with the bone structures near the pedicle bases. CONCLUSIONS: The trabecular bone architecture and the vertical orientation of the facet joints suggest that walking may be the main activity that determines the lumbar vertebral bone architecture.