Principal Investigator: David Dean, PhD
Organization: The Ohio State University
Craniomaxillofacial (CMF) trauma may result in a gap in the bones making up the braincase or the face. These are among the most common skeletal deficits suffered by trauma patients. Indeed, facial fractures, make up about 10% of civilian emergency room trauma, whereas CMF trauma made up 26% of battlefield injuries sustained in Operation Iraqi Freedom and Operation Enduring Freedom (Afghanistan). While these injuries are 90% survivable, the surgical repair of the resulting skeletal defects often requires multiple revision surgeries. The standard-of-care source of bone to fill these defects, if they are an inch or larger in size, is a bone graft taken from elsewhere in the patient’s body, such as the top of the pelvis bone near the hip or the middle portion of the fibula bone in the calf. In an AFIRM II (Armed Forces Institute of Regenerative Medicine) project, we studied the use of 3D printed, artificial bone implants prepared from a resorbable polymer, poly(propylene fumarate), also known as PPF. These implants had been seeded with bone progenitor cells and cultured in growth factors that lead to cells proliferating and coating the implant as well as maturing into strong bone. To save time and expense bone grafts can be directly implanted with growth factors. However, growth factors in solution cannot be restricted to the healing site. The diffusion of one commonly used growth factor, bone morphogenetic protein 2 (BMP2), away from the healing site is associated with inflammation and an increased risk of cancer. While it avoids that risk, our process of maturing an artificial bone graft outside the body proved limiting. The bone that we grew in the lab did not fill in and strengthen as quickly as bone healing in the body that is under a load. That load drives a process referred to as remodeling, literally a reworking of the cells in the bone and their surrounding mineralized matrix that provides strength. During our AFIRM II project we found that we were able to have more uniform, faster, and higher quality production of bone using 3D printed PPF scaffolds if we switched the source of growth factors from whole growth factors (i.e., large, freely circulating proteins) to attaching only the active site of the growth factor, referred to as its ligand (i.e., a very small protein), prior to seeding cells. By tethering ligands to our implant, we can both limit the growth factor’s effects on cells to the implant’s surface and better control the ligand dose’s relationship with the bone healing response. Also, during our AFIRM II project, we developed a technique that is expected to enhance the healing of our artificial bone grafts. That is the inclusion of thin membranes containing bioprinted microvascular channels which provide a blood supply rich in nutrients and oxygen. Indeed, the rate of healing is known to be dependent on the availability of a well perfusing blood supply. The goal of this study is to test our approach in large animals (sheep) to see if PPF scaffolds covered with tethered ligands and surrounded by a well-perfused microvasculature will speed healing following a definitive (i.e., not requiring revision) CMF trauma reconstruction surgery and will result in a sustained structural repair and functional competence.