Bone Regeneration Studied by μMRI and Solid-State NMR Spectroscopy

Bone substitutes are increasingly used in orthopedic interventions. Currently, there is a high interest to optimize the bone scaffolding materials for optimal healing. Using a tibial head defect we investigated bone regeneration using biodegradable poly(lactic-co-glycolic acid) (PLGA) scaffolds, providing a macro-porous three-dimensional carrier. Cylindrical scaffolds with similar porosity but different pore sizes of 100-300, 300-500, or 500-710 μm were implanted into a tibial defect of a rat model. Two or four weeks after implantation, the scaffolds were monitored by μMRI and solid-state NMR. In particular, the molecules of the regenerated extracellular matrix (collagen and apatite) were quantitatively studied. Using μMRI, the implanted PLGA scaffolds were clearly visible and a homogeneous generation of ECM was obvious. The regeneration of the collagen moiety was studied by 13C CPMAS NMR. The total amount of collagen synthesized in the scaffolds depended on the pore size of the scaffolds, best results were obtained for the matrix with 300-500 μm pores. Order parameter measurements of the collagen amino acids showed already very good agreement with those from the natural bone. The inorganic ECM component of the de novo formed bone was investigated by 31P CPMAS NMR. It could be shown that hydroxyapatite was synthesized in the implant by the chondrocytes. The amount of hydroxyapatite increased significantly towards the end of the 4 week animal study indicative of progressed biomineralization. In all experiments, a pore size of 300 to 500 μm turned out to be most effective. From our molecular assessment, both concentration and molecular dynamics of the de novo formed ECM was already very close to that of native bone. However, as the μMR images revealed, the macroscopic trabecular bone structure in the implants was isotropic as oppose to the anisotropic structure in healthy bone.