Apatite c-axis Orientation Tracks Disc-Mediated Local Principal Stress Directions in the Human Lumbar Vertebra

Bone mechanical competence depends not only on bone mass but also on the anisotropic orientation of the collagen/biological apatite (BAp) composite, yet its relationship to the local mechanical environment in human vertebrae remains unclear. Here, we examined whether region-specific BAp c-axis orientation corresponds to predominant principal stress direction estimated by µCT-based finite element analysis. The anterior region showed predominantly uniaxial alignment along the craniocaudal axis, corresponding to a predominant principal stress direction oriented primarily along the craniocaudal axis. In contrast, the cranial and caudal subchondral regions exhibited a two-dimensional in-plane orientation pattern corresponding to predominant principal stress directions within the anteroposterior-mediolateral plane, consistent with load transfer through the intervertebral disc, which imposes transverse in-plane loading on the endplate region. At the corticotrabecular junction, the preferred BAp orientation changed markedly within a spatial range of 100 μm or less. Overall, these findings demonstrate region-specific stress-orientation correspondence in human vertebrae and support crystallographic orientation as a quantitative bone-quality indicator of vertebral mechanical adaptation.