The stress shielding effect often degrades the quality and quantity of bone near implants. Thus, the shape and structure of metallic biomaterials should be optimally designed. A dominant inorganic substance in bone is biological apatite (BAp) nanocrystal, which basically crystallizes in an anisotropic hexagonal lattice. The BAp c-axis is parallel to elongated collagen fibers. Because the BAp orientation of bone is a possible parameter of bone quality near implants, we used a microbeam X-ray diffractometer system with a beam spot, which had a diameter of 50 μmφ or 100 μmφ, to evaluate it.
Two animal models were prepared: (1) a nail model (φ: 3.0 mm, SUS316L), which was used to understand the stress shielding effect in a rabbit tibial marrow cavity; and (2) a model of a lotus-type porous implant (φ: 3.4 mm, mean pore diameter: 170 μm, SUS304L), which was used to understand the effect of the unidirectional-elongated pore direction in anisotropic bone tissue of a beagle mandible. The porous implants were implanted so that the pore direction was parallel or perpendicular to the mesiodistal axis of the mandible.
For the porous implant model, new bone formation strongly depended on the elongated pore direction and the time after implantation. For example, four weeks after implantation, new bone formed in pores of the implants, but the BAp orientation degree in the new bone was more similar to that in the original bone in the elongated pores parallel to the mesiodistal direction than that in the perpendicular pores. These differences in bone formation inside the parallel and perpendicular pores may be closely related to the anisotropy of original bone tissue such as the orientations of collagen fiber, BAp, and blood vessels. The orientation degree of the BAp also changed in the nail model. The stress shielding effect decreased the orientation degree of the BAp c-axis in the tibia along the longitudinal axis.
Thus, the optimal design of metallic biomaterials, including such characteristics as implant shape, pore size, and elongated pore direction should be based on the anisotropy of the bone microstructure.
