Background  Adiponectin, a common adipocytokine in serum, can affect bone formation and bone resorption. However, whether it can promote fracture healing still remains uncertain.

  Objective  To study the effect of recombinant adiponectin on the tibial fractures in SD rats.

  Methods  Twenty four 2-month-old SD male rats with a body weight of 200g were collected. After the fracture model was established, the rats were randomly divided into three groups and immediately injected 0.5 mL adiponectin or 0.9% NaCl once locally. Adiponectin (1 mg/kg) was given to rats in the treatment group 1 (n=8), 2 mg/kg for treatment group 2 (n=8), and control group (n=8) was given 0.9% NaCl. At 6 weeks after modeling, the rats were sacrificed and bilateral tibias were collected. Callus structure was observed by HE and Masson staining. Osteogenic related factors were observed by immunofluorescence staining. Callus microstructure was detected by Micro-CT to evaluate the effect of adiponectin on tibial fracture.

  Results  HE staining results showed that the callus of the treatment group was significantly thickened and lamellar bone formation was better than the control group. Masson staining results showed that the treatment group had more collagen in the tibial callus, and the callus maturity was better than the control group. Immunofluorescence staining results showed that in periosteum, bone lacunae and hair layer, the adiponectin 1 mg/kg group and the adiponectin 2 mg/kg group had significantly higher OCN and ALP expressions than those of the control group (OCN: 0.61% ± 0.21% and 0.44% ± 0.17% vs 0.27% ± 0.17%; ALP: 0.85% ± 0.62% and 0.70% ± 0.34% vs 0.35% ± 0.15%; P＜0.05, respectively). Results of Micro-CT showed that the bone callus volume fraction (BV/TV: 53.76% ± 7.89% vs 26.37% ± 4.32%), trabecular thickness (Tb.Th: 363.9 ± 51.45 μm vs 269.6 ± 36.80 μm), trabecular number (Tb.N: 15.68 ± 2.11/cm vs 10.51 ± 1.49/cm) and degree of separation (Tb.Sp: 344.6 ± 98.79 μm vs 604.3 ± 78.27 μm) in adiponectin 1 mg/kg group were better than those of the control group (all P＜0.05). The same trends were observed in adiponectin 2 mg/kg group (BV/TV: 45.10%±5.45%, Tb.N: 14.4±1.83/cm and Tb.Sp: 436.1±63.75µm) and the control group (all P＜0.05), but there was no significant difference in the thickness of bone trabecular. Adiponectin 1 mg/kg group and 2 mg/kg group had no significant difference in these parameters.

  Conclusion  Adiponectin can promote callus growth, bone microstructural reconstruction and the expression of osteogenic factors, so as to shorten the time of fracture healing in SD rats.