Objective:
Critical-sized craniofacial bone defect in osteoporosis remains a clinical challenge for surgeons due to the limitation of natural bone healing. Contemporary methods of treatment are not always satisfactory. The introduction of bone tissue engineering provides new strategy for craniofacial bone defects in osteoporosis. Recent studies have demonstrated the osteogenesis effect of strontium (Sr), which plays a distinct role on promoting bone regeneration and inhibiting bone resorption. The purpose of this study is to provide a novel approach using adipose-derived stem cells (ASCs) and strontium-based (Sr) scaffolds for promoting craniofacial bone regeneration in rats with osteoporosis.

Materials and Methods:
The porous Sr substituted hydroxyapatite (SrHAP) cryogel scaffolds combining the functions of stem cells and Sr elements were developed with the goals to promote osteoporotic bone regeneration in craniofacial bone defects. We fabricated cryogel scaffolds with optimal composition and physico-chemical property. To study the effect of strontium, we divided our cryogel scaffolds into three groups, including the control group, Sr negative group, and Sr positive group. Further, we evaluated the in vitro osteogenic effects of Sr-containing scaffold. Finally, we applied our ASCs/cryogel construct to the nude mice and ovariectomized rats, to evaluate the in vivo bone regeneration ability.

Results:
The SEM images of cryogel scaffolds showed open interconnected pores and microporous morphology. The in vitro results showed that Sr released from SrHAP enhanced cell viability, alkaline phosphatase activity, and mRNA expression levels of osteoblast related genes. For in vivo study, we conducted subdermal implantation of ASCs/cryogel construct in nude mice. The staining intensity was much higher for OCN and COL I in the Sr-positive group, which confirmed better osteogenic differentiation and osteogenesis in vivo. In further, we implanted our ASC/cryogel construct in the alveolar bone defect model. The result showed better osteogenesis in Sr-positive group at 4W and 8W after implantation, judging from less bone defect size on micro-CT images.

Conclusions:
The osteoporotic bone regeneration in critically craniofacial bone defects of osteoporotic rat model showed that the porous SrHAP scaffolds not only exhibited superior osteo-inductivity to enhance early bone formation, but could also stimulate angiogenesis comparing with HAP scaffolds. In conclusion, our study demonstrated that the SrHAP cryogel combined with ASCs might be a new strategy for the regeneration of craniofacial bone defects in osteoporosis.

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