**1. Introduction**

Tendon healing in the bone tunnel is the one of the most critical points for tendon repair [1]. Various types of tendon grafts such as peroneus longus, tibialis, gracilis, semitendinosus, and Achilles tendons can be transplanted for replacing the ruptured ligament/tendon tissues [2]. However, the failure rate after surgery has still remained the cause of poor recovery capacity, so more strategic studies are necessary [3]. One of the main reasons for poor healing is because the mechanical stresses keep affecting repaired tendon-bone tunnel junction [4]. The unique transitional tissue, the enthesis which is a fibrocartilage tissue, is generated with a connected region between tendon and bone tissue [5]. Thus, for successful tendon/ ligament reconstruction, osteointegration of inserted tendon grafts in the bone tunnel is strongly recommended [6].

Recombinant human bone morphogenetic protein-2 (rhBMP-2) is a well-known growth factor for new bone formation [7–9]. This promotes the differentiation of undifferentiated mesenchymal cells into chondrogenic or osteogenic lineages that support new bone formation [10, 11]. This differentiation also needs in the reconstructed tendon-bone tunnel region for postnatal enthesis formation. There have been many studies about an effective rhBMP-2 delivery system using collagen-based materials such as sponge and gel for sustained release [12–15]. In the previous studies, rhBMP-2 was used for bone-tendon interface healing using a collagen sponge [16, 17]. However, the collagen sponge system had a limitation of rhBMP-2 localization to the only targeted region and minimizing leakage from the bone tunnel [18].For the enhanced delivery of rhBMP-2 into the targeted surgical sites, a viscous collagen gel may be useful. Another previous study had performed a comparative study of the osteogenic effects using two different rhBMP-2 delivery systems such as collagen sponge and collagen gel in a rat spinal fusion model [19]. The results showed that rhBMP-2 containing a collagen sponge had side effects of leakage BMP. To overcome this limitation, viscous collagen gel was applied to rabbit tendon-bone tunnel regions in this study.

The purpose of this translational study is to investigate whether the rhBMP-2-containing collagen gel can localize in the surgical site and improve new enthesis formation within reconstructed tendon-bone tunnel after the surgery.
