Effect of in vivo implantation sites on the graft-to-bone osteointegration induced by gradient nanofibrous scaffolds

Gradient nanofibrous scaffolds hold great promise as viable ligament grafts to enhance graft-to-bone integration for long-term reconstruction of damaged anterior cruciate ligaments. However, the effect of in vivo implantation sites on graft-to-bone osteointegration induced by gradient scaffolds remains largely unexplored. Here, we systematically investigated the effects of nanofibrous scaffolds with fiber organization and nanoHA/BMP-2 gradients on graft-to-bone osteointegration by implanting them into tibial and femoral bone tunnels, respectively. The nanofibrous scaffolds were fabricated to possess gradient regions and uniform regions at two ends, and the gradient regions were implanted into femoral tunnels and tibial tunnels as two experimental groups. The biomechanical test showed that the ligament grafts broke in the intraarticular zone at 12 and 24 weeks of implantation, confirming a good biofixation between the grafts and bone tissues at both implantation sites of the tibial and femoral tunnels. Additionally, the gradient region of the ligament graft was found to enhance osteointegration and promote the formation of more secure graft-to-bone interfaces, as compared to uniform regions. In femoral bone tunnels, a relatively slow osteointegration process was observed even in the gradient region of the ligament grafts, which showed the formation of distinct fibrocartilage interfaces at 12 weeks and further evolved into mature bone tissues at 24 weeks. In tibial bone tunnels, higher nanoHA/BMP-2 content led to rapid penetration of newly-regenerated bone tissues in the gradient regions of the grafts at 12 weeks, followed by bone remodeling and complete integration with native tibial plateau at 24 weeks. These findings might provide new insights to develop biomimetic gradient scaffolds for matching the osteointegration process and speed in tibial and femoral bone tunnels.