Research topic: Investigation of mechanisms permitting survival of osteocytes in vascularized bone grafts, methods to revitalize cryopreserved structural bone allografts and successfully transplant living bone and joint allotransplants without long-term immune modulation.
Research results: New bone formation and remodeling within vascularized autografts, cryopreserved allografts and living allogeneic transplants are the result of circulation-derived cells rather than surviving osteoclasts and osteoblasts. Bone allotransplant viability does not require more than short-term immunosuppression if a new autogenous circulation is created by implantation of vascularized tissue from the recipient of the transplant.
Patient care application of results: Vascularized bone autografts can successfully heal recalcitrant nonunions and revascularize necrotic bone. Structural allografts can be revitalized and microsurgically-transplanted allogeneic bone and joint vitality maintained by surgical angiogenesis alone.
Simplified patient care application: Vascularized bone grafts have improved treatment of difficult carpal fracture nonunions and avascular necrosis. Future reconstruction of segmental bone loss may include use of surgical angiogenesis for revitalizing cryopreserved allografts and microsurgical transplantation of allogeneic bone and joints without need for long-term immunosuppression .
Research: A Recipe for Better Care
Early funding from OREF leads to successful research career, improved treatments
Research is vital for finding solutions. That’s the mantra of Allen T. Bishop, MD, professor of orthopaedics at the Mayo Clinic Department of Orthopedics, Rochester, Minn.
“The payoff comes as we learn something new that affects clinical practice,” Dr. Bishop said. “Orthopaedic surgeons need research to find solutions to the diverse problems they face when treating patients.”
A series of small grants and assistance from established researchers William P. Cooney III, MD, and Michael B. Wood, MD, encouraged Dr. Bishop to seek independent funding while working in Mayo Clinic’s Microsurgical Research Laboratory. He turned to the Orthopaedic Research and Education Foundation (OREF).
“OREF helped me in two very important ways,” he said. “The most significant was the Career Development Award [now called Career Development Grant]. The other was the Grant Writing Workshop [now cosponsored by the Orthopaedic Research Society and AAOS and called the New Investigator Workshop].”
The Career Development Award provided 2 years’ funding for Dr. Bishop’s fledgling laboratory effort, “Cellular Survival, Repopulation, and Neoangiogenesis in Ischemic Bone.” That research resulted in several published manuscripts, including a large-animal study that demonstrated the superiority of vascularized bone grafts in treating carpal nonunions with an avascular proximal segment (Fig. 1). The study also showed that blood flow from the interposed vascularized bone restored circulation within adjacent necrotic bone segments. This validated the use of similar grafts in scaphoid nonunions and became widely accepted in clinical practice.
Additional research showed that an arteriovenous pedicle implanted in necrotic iliac crest bone in rabbits rapidly sprouted a new intraosseous vascular bed and that the extent and speed of this process could be further stimulated by growth factor delivery.
These studies provided a framework for subsequent National Institutes of Health (NIH) funding. During the Grant Writing Workshop, which he called “eye-opening,” Dr. Bishop learned to write, revise, and refine a research proposal. Participants were required to write a grant for review by faculty and other workshop participants.
“They tore my initial grant to shreds,” Dr. Bishop recalled. Cato T. Laurencin, MD, PhD, Dr. Bishop’s assigned mentor, reviewed the grant’s strengths and weaknesses in detail and advised him on improving the proposal. Dr. Bishop took these suggestions to heart, ultimately writing 25 more drafts.
With additional guidance from his Mayo Clinic colleagues Mark E. Bolander, MD, and Kenton R. Kaufman, PhD, Dr. Bishop finally submitted the proposal to the NIH. All of the refining paid off. He received an impact/priority score of 9, resulting in R01 grant funding on his first submission—a very rare occurrence.
“It was exhausting, but I have to credit OREF for the opportunity, and the faculty at the grant writing workshop for their constructive criticism. It wasn’t pleasant at the time, but it was necessary,” he said.
The OREF funding, additional grants from the Musculoskeletal Transplant Foundation, many small grants, and two successful renewals of his R01 grant have allowed continual funding for Dr. Bishop’s work.
Problems encountered in clinical practice have been the impetus for all of Dr. Bishop’s experimental studies. “Orthopaedic surgeons are in a unique position to perform applied research directed to an identified clinical problem,” said Dr. Bishop. However, success depends upon building a team of research fellows and basic science collaborators.
“I find making a research plan to be similar to cooking,” he said. “I know what I want to make but need to find the best recipe or technique to address a specific hypothesis and/or aim. Collaborations with biomechanical engineers, molecular biologists, and immunologists are necessary for this purpose. Often, an initial plan does not work, and requires modification, much as a chef modifies an unsuccessful recipe.”
Dissatisfaction with results in treating carpal osteonecrosis and scaphoid nonunions provided the basis for Dr. Bishop to investigate angiogenesis in bone. “The scaphoid has always been a difficult bone. It’s surrounded by joint fluid, covered in articular cartilage over most of its surface and has a problem with its blood supply. Yet it is the most commonly fractured wrist bone,” he explained.
With his OREF funding, Dr. Bishop studied better methods for treating scaphoid fractures using vascularized bone grafting. His findings led to changes in clinical practice, followed by subsequent re-evaluation of clinical outcomes. The transfer of research findings to clinical practice has led to improvements in treating scaphoid nonunions and Kienbock’s disease, an interruption in the blood supply to the lunate bone in the wrist.
One treatment—transferring a pedicle of blood vessels from the end of the radius into the scaphoid to vascularize the transplanted bone—had had some success. Yet when Dr. Bishop applied this method in practice, he found that it was only 70 percent effective, especially in cases in which the scaphoid length was not restored to its original dimensions or when the proximal pole was small and fragmented.
He hypothesized that the amount and quality of distal radius bone was insufficient to adequately restore scaphoid length and correct the resultant carpal instability. He tried an alternative—a distal femoral medial condylar flap, using an anatomic study to identify the best harvest site. Results, which have been analyzed and published, were better than the radius pedicle grafts.
Although some issues still need resolving, the use of vascularized bone grafting is now widely accepted in cases in which a fracture is difficult to heal due to osteonecrosis of the proximal scaphoid pole or in cases in which prior surgery has failed.
Continuing to advance
The NIH grant enabled Dr. Bishop to take the research he’d begun with OREF funding to the next level.
Dr. Bishop and his research team have investigated ways to maintain viability of allogeneic bone and joint transplants without need for life-long immunosuppression, which is expensive and, with prolonged use, may result in metabolic problems, osteonecrosis, opportunistic infections, and neoplasm.
“In our alternative method, we replace or switch the circulation from the original allogenic bone vasculature with a newly developed, or ‘neoangiogenic,’ autologous circulation provided by implantation of vascularized tissue from the recipient at the time of microsurgical transplantation,” he explained.
Dr. Bishop accomplishes this by reconstructing a segment of missing femur or tibia with a size- and shape-matched fresh allogenic bone, transplanted with microsurgical repair of the bone nutrient blood supply in a fashion identical to a standard vascularized fibular autograft. In addition, however, adjacent autogenous vascularized tissue—an arteriovenous bundle or fascial flap, depending upon the specific animal model—is placed within the bone.
So far, Dr. Bishop has shown that angiogenesis from the implanted tissue will quickly grow into the transplanted bone, maintaining measurable bone blood flow and allowing maintenance of osteocyte viability and active bone remodeling. Thus, only short-term immunosuppression—generally 2 weeks—is needed. Thereafter, even though the allogeneic nutrient vessels are rejected, the new blood supply maintains bone circulation.
Through sex-mismatched transplants, Dr. Bishop has shown that the bone remodeling and healing that occur following bone allotransplantation are the result of circulation-derived cells from the recipient, not from the surviving cells of the transplanted organ.
“I think my research team has contributed to our understanding of vascularized bone grafts, the benefits of angiogenesis to treat avascular necrosis and revitalize structural allografts, and the future potential of living bone allotransplantation in orthopaedic practice,” Dr. Bishop said. His close collaboration with Alexander Y. Shin, MD, has resulted in a new direction of investigation, focused on improving the results of peripheral nerve repair, paralleling their clinical practice of treating brachial plexus injury.
Dr. Bishop is currently is investigating graft and systemic immunologic responses to bone allotransplantation in a Yucatan mini-pig model. He will compare the results of bone allotransplantation to those from conventional cryopreserved allograft reconstruction. If successful, these results will provide a basis for ethical application in clinic, and show how research begun with funding from OREF can improve clinical practice.
“OREF can and does support research that changes clinical practice and improve outcomes for patients,” Dr. Bishop said. “But without financial support of orthopaedic clinicians, those of us who do the research probably wouldn’t get started. OREF funding gives us the chance to develop a substantial, long-lasting research career.”