Sustained Stimulation by Low-dose Lipopolysaccharide Prolongs Trabecular Structure Deterioration and Immaturity of Collagen at Regenerated Bone Even After Bone Volume Recovery

Despite rapid advances in the field of bone regeneration therapy, little is known about information related to the quality of bone regenerated under chronic inflammation. This study was carried out to compare the qualities (trabecular structure and collagen maturity) of regenerated bone in rat calvaria defects under different conditions of lipopolysaccharide (LPS) stimulation. To generate different residual levels of LPS at the defect sites, two types of gelatin sponges that release the bacterial endotoxin at different rates were applied; namely, LPS sustained-release gelatin (LS-G) sponge and LPS rapid-release gelatin (LR-G) sponge. Histomorphometric analysis by micro-computed tomography showed that, compared with rapid LPS stimulation with the LR-G sponge, sustained low-dose LPS stimulation using the LS-G sponge markedly impaired bone regeneration for up to 3 weeks, but not at 6 weeks, and induced a vulnerable trabecular structure. Polarized light microscopy with picrosirius red-stained tissue samples revealed that the LS-G sponge generated immature collagen in the regenerated bone. These deleterious changes in the trabecular structure and in collagen maturation were retained even after recovery of the bone volume at 6 weeks. These findings provide insights into the quality of regenerated bone during and after chronic inflammation.