Xenograft-derived Bone Scaffold Shows Promise

With OREF funding, researcher seeks to develop new treatment for critical bone defects
Managing segmental bone defects can be a major challenge for orthopaedic surgeons. Of the five million-plus musculoskeletal surgeries performed in the United States every year, nearly half utilize bone grafts.

Although autologous bone grafting is considered the best approach for repairing large bone defects, the harvest of autologous bone may be painful and is associated with increased morbidity. Allografts and xenografts also have drawbacks: Foreign cellular material in these grafts may trigger adverse immune reactions in the recipients.


Daniel N. Bracey, MD, PhD, seeds bone scaffold specimens with preosteoblast cells to study their osteogenic differentiation when grown on the scaffold.
Courtesy of Daniel N. Bracey, MD, PhD

Implantation of a xenograft-derived bone scaffold that has kept its osteoconductive and osteoinductive growth factors and is free of immunogenic cellular material may address those limitations. If successfully developed, xenograft-derived bone scaffolds could be in abundant supply—perhaps even as an off-the-shelf product in the operating room.

To explore this idea further, Daniel N. Bracey, MD, PhD, received an Orthopaedic Research and Education Foundation (OREF)/Musculoskeletal Transplant Foundation (MTF)/Charles H. Herndon Resident Clinician Scientist Training Grant for a project titled, "A Novel Xenograft-Derived Bone Scaffold for Improved Treatment of Critical Bone Defects." Funding for the grant was made possible by MTF.

"If our bone scaffold is successful, it could alleviate the significant morbidity associated with autograft procedures," said Dr. Bracey, a resident-physician scientist with the department of orthopaedic surgery at Wake Forest School of Medicine in Winston-Salem, N.C.

Bone scaffold decellularization
Because their anatomy, physiology, and genotype are similar to humans, pigs are considered excellent potential donors for xenografts. Dr. Bracey is exploring the development of a bone scaffold derived from porcine cancellous bone that is biocompatible, pathogen-free, osteoconductive, and potentially osteoinductive. Previous research has shown that decellularization technology, an essential component of this work, is successful.

"I was interested in researching topics in regenerative medicine, and specifically in regenerating bone. Because decellularization technology has been used previously to make scaffolds for other biological tissues, such as tendons and meniscus, I thought I would try it with bone," Dr. Bracey explained.

To process a bone scaffold, Dr. Bracey sections a porcine femur, cubes the cancellous bone, and subjects it to a 6-step decellularization process that removes pig cells and foreign cellular material. He and his research team then identify the properties and structure of the resulting scaffold.

One aim of the study is to develop a naturally derived, biomimetic bone scaffold with appropriate micro-architecture that is decellularized, cytocompatible, and free of potential pathogens such as bacteria and viruses. Dr. Bracey and his research team use alpha-galactosyl enzyme-linked immunosorbent assay (Alpha-GAL ELISA) to determine if a significant reduction of the xenoantigen has been achieved. They assess scaffold cytocompatibility with cellular co-culture assays that determine concentrations of inflammatory cytokines. The bone samples carry selected viral and bacterial pathogens to determine if the decellularization process can eliminate them.

Another purpose of Dr. Bracey's research is to assess the scaffold's osteoinductive potential by quantifying cell osteogenic differentiation when seeded onto the scaffold using basic phenotypic and gene expression markers.

Bone scaffold potential
Results from Dr. Bracey's study show that a decellularized bone scaffold matrix can be derived from porcine cancellous bone using the decellularization/oxidation protocol he has developed. A 98 percent reduction in pig tissue DNA, for example, revealed the effectiveness of the process. The matrix is cytocompatible in vitro to exposed human cell cultures. Large quantities of the contaminating pathogens were also eliminated.

"We wanted to be sure that any pathogens present on the donor material would be removed by the processing technique," said Dr. Bracey. "We spiked donor material with large quantities of various bacteria and viruses and measured their titers at each subsequent stage of processing. Data showed that no viruses or bacteria were present on the scaffold after completion of processing. This suggests the decellularization technique could be a reliable protocol for processing materials designed for in vivo use."

Preliminary evidence also suggests the matrix provides an environment conductive to cell growth and may provide certain osteoinductive cues. A mass spectrometry analysis of the bone scaffold found that some residual host growth factors in the pig bone were retained in the decellularized bone scaffold.

"Cells attached themselves to the scaffold and proliferated, as indicated by the significant increase in DNA content over a 15-day period," said Dr. Bracey. "These cells deposited extracellular matrix components onto the underlying bone scaffold they were seeded to. Overall, these experiments show the bone scaffold has some osteoinductive potential because cells were showing evidence of differentiating into osteoblasts."

Bone scaffold treatment
Dr. Bracey and his team have already taken the next step in their work—an in vivo study using a critical defect rat model. The bone scaffold is being grafted into rat femur critical size defects to assess graft integration, remodeling, and bone healing. Rats implanted with the bone scaffold showed similar remodeling in comparison to rats implanted with demineralized bone matrix.

Alexander Jinnah, MD, a physician scientist at Wake Forest's Orthopaedic Research Laboratory, has applied for OREF funding and will continue Dr. Bracey's work as part of his doctoral dissertation over the next 2 years. "We hope that our research will lead to the successful development of a xenograft-derived scaffold that will have significant clinical and economic impact on the field of orthopaedics and greatly improve the current management of segmental bone defects," Dr. Bracey said.             

Mark Crawford is a contributing writer for OREF. He can be reached at communications@oref.org.

© Orthopaedic Research and Education Foundation (OREF)