BACKGROUND:Cartilage degeneration and subchondral bone damage are the main pathological features of osteoarthritis,and treatment based on this pathological feature will be a promising improvement for osteoarthritis. OBJECTIVE:To design and study an annotated strontium ranelate-loaded drug delivery system and to observe its therapeutic effect on promoting cartilage repair and improving subchondral bone structure in osteoarthritis. METHODS:(1)In vitro experiment:Strontium ranelate was loaded into sodium alginate/collagen hydrogel matrix to construct in situ drug delivery system,and the in vitro slow release performance of the system was characterized.Strontium ranelate-loaded sodium alginate/collagen hydrogel(experimental group)and alginate sodium/collagen hydrogel(control group)were co-cultured with bone marrow mesenchymal stem cells,respectively,and cultured cells were used as a blank control group to detect cell proliferative activity.After chondroblast-induced differentiation,saffron O staining,Alcian blue staining and RT-qPCR were performed respectively.The two hydrogels were co-cultured with osteoblasts,and the cultured cells were used as a blank control group for immunofluorescence staining and RT-qPCR.(2)In vivo experiment:A total of 18 adult SD rats were selected and the model of right posterior knee osteoarthritis was established by the method of medial meniscectomy.After 1 week,the rats were divided into three groups by the random number table method:The blank group did not receive any treatment.The control group was injected with sodium alginate/collagen hydrogel in the knee,and the experimental group was injected with strontium ranelate-loaded sodium alginate/collagen hydrogel,with 6 rats in each group.After 6 weeks,the samples were subjected to Micro-CT scanning,hematoxylin-eosin staining,saffron O-solid green staining and immunofluorescence staining. RESULTS AND CONCLUSION:(1)In vitro experiment:Strontium ranelate-loaded sodium alginate/collagen hydrogel had porous microstructure and sustainable release of strontium ranelate.At 21 days,the cumulative release reached(60.89±0.58)%.Bone marrow mesenchymal stem cell staining showed that both hydrogels had good cytocompatibility.The results of the CCK-8 assay demonstrated that strontium ranelate-loaded sodium alginate/collagen hydrogel could promote the proliferation of bone marrow mesenchymal stem cells.The results of Safranin O staining,Alcian blue staining,immunofluorescence staining and RT-qPCR exhibited that strontium ranelate-loaded sodium alginate/collagen hydrogel could promote chondrogenic differentiation of bone marrow mesenchymal stem cells.Immunofluorescence staining and RT-qPCR revealed that strontium ranelate-loaded sodium alginate/collagen hydrogel could decrease bone resorptivity by increasing the ratio of osteophosphorin/nuclear factor κB receptor activator ligand.(2)In vivo experiment:Micro-CT scan verified that compared with the blank group and control group,the subchondral bone volume fraction and bone mineral density of the knee of rats were increased in the experimental group(P<0.05,P<0.01).Histological staining displayed that compared with the blank group and control group,the knee cartilage injury was significantly reduced;the expression of type II collagen was promoted,and the expression of matrix metalloproteinase 2 protein was inhibited in the experimental group(P<0.05,P<0.01).(3)These results confirm that the strontium ranelate-loaded sodium alginate/collagen hydrogel can promote the repair of cartilage defects in osteoarthritis and reconstruct the complex interface between cartilage and subchondral bone.