Optimization of mimetic periosteum autografts for the treatment of nonunions

Bone presents truly regenerative capacity being able to regenerate into a native state in response to injuries. Despite this self-renewal potential, bone healing is not absent of complications and different conditions can interfere with the regenerative process, leading to delayed fracture and in some cases fracture nonunion. Fracture nonunion is a major cause of chronic pain and disability and, despite the low incidence of nonunion and delayed union fractures (5-10%), the numerous fractures that take place globally (~180 million every year) emphasizes the huge economic burden that fracture nonunion represents. Once detected, fracture nonunion requires a surgical approach, and the use of bone autografts that provide and osteoinductive, osteogenic and osteoconductive environment for a successful repair. However, the availability of bone grafts is limited. The scarcity of bone tissue that can be used for autografts have consolidated the need for novel tissue engineering approaches as potential candidates for the treatment of nonunion and for long bone defects, prone to evolve to nonunions. Tissue engineering strategies allow for the combination of novel tunable materials along with different biological adjuvants, including growth factors and cells. During the bone regenerative response, the periosteum, a fibrous layer surrounding the bone, plays a key role delivering osteochondroprogenitor cells and crucial growth factors into the injured tissue. Thus, we developed a tissue engineering strategy where biocompatible, 3D melt-electro-written polycaprolactone membrane would act as a mimetic periosteum. The engineered mimetic periosteum allows vascularization of the construct either when implanted ectopically or orthotopically. Additionally, we demonstrated its capacity to be functionalized with rhBMP-2, the most important morphogen for bone regeneration, both exposed on the membrane surface attached through PEA-hFN or encapsulated in microparticles covalently bound to the PCL membrane. When functionalized with low doses of rhBMP-2 the mimetic periosteum demonstrated great osteogenic potential in vitro, inducing human MSCs differentiation into osteoblasts. More importantly, in vivo results indicate that the functionalization of the mimetic periosteum with rhBMP-2 allows regenerative properties able to heal critical size femoral defects in SD rats with high efficiency and reproducibility using unpreceded low doses of rhBMP-2. Ultimately, the mimetic periosteum demonstrated its ability to deliver key mesenchymal progenitor cells into the injured site. All these results indicate that our engineered mimetic periosteum represents an efficient system for rhBMP-2 and progenitor cells delivery with important translational potential.