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Vascularized Bone Grafts in Spine Surgery


Summary of Key Points

  • Vascularized bone grafts combine structural properties with biological enhancement through vascularization.

  • Common indications for vascularized bone grafts include:

    • Bone graft greater than 5 cm

    • Strut graft that will be more than 4 cm from the anterior border of the spine and thus more prone to fracture

    • A pseudoarthrosis after a nonvascularized bone graft

    • An area of the spine that will require radiation postoperatively, as in the setting of a malignancy

    • Cases of infection where placing instrumentation or avascular bone would propagate the infection

    • Surgeries in which fusion is expected to be difficult to achieve, as with operations for neurofibromatosis.

  • Harvesting and implantation of vascularized bone grafts requires technical precision, including stabilization of the graft in the recipient site and microvascular anastomosis.

  • The donor site morbidity and complications associated with vascularized bone grafting are significant and important to understand when considering use of vascularized bone grafts.

Acknowledgment

The authors would like to acknowledge Ashwini D. Sharan, MD for previous work on this book chapter.

Bone grafting has had an important role in orthopedic surgery since Barth first introduced bone-grafting techniques in the late 19th century. Bone grafts typically have been used in fracture nonunion treatment, joint arthrodesis, bone cavity filling, replacement of bone lost because of infection, trauma, tumor, augmentation of fracture healing, and spinal fusion. The different types of bone grafts used today include autogenous cancellous, nonvascularized autogenous cortical, vascularized autogenous cortical, allogeneic cancellous, allogeneic cortical, allogeneic demineralized bone matrix, and allogeneic inductive proteins.

Structural bone grafts commonly are used in spine surgery to provide stability in an area where a defect has been created. Currently, the gold standard for bone grafts is the autograft, which has the best biological compatibility and leads to fewer nonunions. The most common complications associated with its use include pain at the donor site and a lack of incorporation of the graft. The advent of the use of vascularized bone grafts has provided the spine surgeon with a potentially powerful tool for treating difficult spine problems. The proposed advantage of the vascularized bone graft is the provision of nutrients to the deep structures of the graft. The transfer of living bone results in less bone remodeling and greater structural integrity of the graft.

The purpose of this chapter is to discuss bone grafts and the basic biology of graft incorporation, along with causes of nonunion. The history of vascularized bone grafts is presented, as is a surgical technique for the donor site. The indications in spine surgery will be discussed, along with a review of the results of its use in this field.

Bone Grafts in Spine Surgery

Albee first described utilizing a bone graft for spinal fusion in 1911 as a treatment for Pott disease. Many advances have been made since that time, and fusion is now the standard treatment for a variety of spinal disorders. Achieving a proper fusion involves two key components: (1) preparation of the site to be fused and (2) stimulation of bone formation with the use of a bone graft. The most effective graft material currently available is autologous cancellous bone. This graft has a large surface area that allows for vascularization of the graft and incorporation with the host bone. In cases in which the fusion must span several segments, the amount of autogenous cancellous bone that is available may not be sufficient.

Autologous cortical bone is another commonly used graft in spine surgery. Unfortunately, this graft has fewer osteoblasts that survive and is associated with a slower rate of revascularization. This slower revascularization results in a slower rate of incorporation of the graft, thus limiting its use. The advantages of a cortical graft, however, are that it can provide immediate structural support and that it is available in larger sizes. Over time, during a process called creeping substitution , the strength of the graft decreases. During this process, the avascular nature of the graft causes resorption by osteoclasts, while new bone is laid down by osteogenic cells originating from the recipient bed rather than the graft, a phenomenon first observed and described by Phemister in 1914. This is why a cortical graft (such as a strut graft used in the treatment of kyphosis) may take up to 2 years to incorporate completely. During the process of creeping substitution, the bone graft is found to be weakest at 6 months, increasing the risk of fracture at the graft site. By retaining its vascular supply and the viability of the osteocytes, a vascularized bone graft provides a mechanically stronger support than a nonvascularized graft.

Vascularized Bone Grafts

The use of vascularized bone grafts parallels the developments associated with the history of vascular surgery. The beginnings of vascular surgery can be traced back to Carrell’s classic paper published in 1908, “Results of the Transplantation of Blood Vessels, Organs and Limbs,” in which he describes a technique whereby blood vessels can be anastomosed. Various tools for anastomosing small vessels were designed and tested in the years following that publication. Androsov designed the first vascular stapling machine, Jacobsen and Suarez demonstrated the utility of the microscope in the operating room, and Buncke and Schulz improved microsurgical instrumentation and performed much of the early experimental work in the field. Strauch and colleagues used a canine model to transpose a rib to the mandible on its internal mammary pedicle in 1971, and in 1973 a free vascularized rib graft was performed in a dog by McCullough and Fredrickson. The first free skin flap using microvascular anastomoses was reported by Taylor in 1973, and in 1975 Taylor transferred a fibula to a tibial defect as the first free vascularized bone graft in a human.

The vascularized bone graft traditionally has been used in refractory nonunions or in areas where there is a large segmental defect. The need to use a more structurally supportive bone graft for kyphosis surgery resulted in the use of the first vascularized bone graft in spine surgery. Surgery for severe kyphosis secondary to infection, trauma, or deformity requires spanning multiple levels. When a nonvascular rib or fibula was used to span such a defect, the length of the graft and the slow rate of incorporation resulted in a high rate of nonunion. Bradford encountered fatigue fractures in four of 23 patients when a nonvascularized fibula was used for spinal kyphosis surgery. These results encouraged spine surgeons to seek out alternatives to traditional bone grafts. In two separate reports, Rose and associates and Bradford described successful techniques of using a rib graft with a vascular pedicle. , Because the cross-sectional area of the rib was too small and could not provide the structural support needed for certain areas of spinal fusion, surgeons began to explore the use of the fibula as a vascularized graft. , Surgeons also explored the use of pedicle iliac crest bone graft for procedures such as sacral reconstruction and lumbosacral fusion. , Currently, the rib, fibula, and iliac crest are used as primary donor sites for a vascularized graft.

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