Background:Researchers initially proposed the substitution of apoptotic chondrocytes in the superficial cartilage by injecting mesenchymal stem cells (MSCs) intraarticularly.This effect was termed as bio-resurfacing.Little evidence supporting the treatment ofosteoarthritis (OA) by the delivery ofa MSC suspension exists.The aim of this study was to investigate the effects of injecting allogenic MSCs intraarticularly in a rat OA model and to evaluate the influence of immobility on the effects of this treatment.Methods:We established a rat knee OA model after 4 and 6 weeks and cultured primary bone marrow MSCs.A MSC suspension was injected into the articular space once per week for 3 weeks.A subgroup of knee joints was immobilized for 3 days after each injection,while the remaining joints were nonimmobilized.We used toluidine blue staining,Mankin scores,and TdT-mediated dUTP-biotin nick end labeling staining to evaluate the therapeutic effect of the injections.Comparisons between the therapy side and the control side of the knee joint were made using paired t-test,and comparisons between the immobilized and nonimmobilized subgroups were made using the unpaired t-test.A P value ＜ 0.05 was considered significant.Results:The three investigative approaches revealed less degeneration on the therapy sides of the knee joints than the control sides in both the 4-and 6-week groups (P ＜ 0.05),regardless of immobilization.No significant differences were observed between the immobilized and nonimmobilized subgroups (P ＞ 0.05).Conclusions:Therapy involving the intraarticular injection of allogenic MSCs promoted cartilage repair in a rat arthritis model,and 3-day immobility after injection had little effect on this therapy.