Spinal fusion of biodegradable poly(propylene fumarate) and poly(propylene fumarate-co-caprolactone) copolymers in rabbits

Background: Autologous bone grafts are currently the standard in orthopedic surgery despite limited donor sources and the prevalence of donor site morbidity. Other alternatives such as allografts are more readily available than autografts but have lower rates of graft incorporation. Methods: Here, we propose a novel graft alternative consisting of an injectable poly(propylene fumarate) (PPF) and poly(propylene fumarate-co-caprolactone) P(PF-co-CL) copolymer with a recombinant human bone morphogenetic protein-2 (rhBMP-2)/vascular epithelial growth factor (VEGF) release system accompanied by hydroxyapatite (HA). The efficacy of scaffold formulations was studied using a standard, bilateral, L-level (L5-L6) posterolateral transverse spinal fusion using New Zealand white rabbits. Rabbits were divided into 4 experimental groups: group I, negative control; group II, autograft (positive control); group III, injectable PPF scaffold with rhBMP-2/VEGF release system and HA; group IV, injectable P(PF-co-CL)scaffold with rhBMP-2/VEGF release system and HA. Spines were harvested at 6 weeks and 12 weeks after surgery, and spinal fusions were assessed using manual palpation, radiographic analysis, micro-computed tomography (μCT) assessment, and histologic analysis. Results: Of the 4 experimental groups, the injectable P(PF-co-CL) scaffold displayed superior initial strength and faster degradation than scaffolds constructed from PPF alone and facilitated the fusion of lateral processes in the rabbit standard posterolateral spinal fusion model. The results obtained from manual palpation, radiology, and μCT showed no difference between the P(PF-co-CL) group and the PPF group. However, histologic sections showed more osteogenesis with the new injectable P(PF-co-CL) scaffold. Conclusion: Injectable P(PF-co-CL) polymers showed promising spine fusion abilities in rabbits after 12 weeks of posterolateral implantation.