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Spine Fusion: Biology and Biomechanics


Summary of Key Points

  • Successful integration and mechanical performance of a bone graft in spinal fusion is a function of graft properties and the graft–host interface.

  • The principles of osteoinduction, osteoconduction, and osteogenesis dictate bone graft incorporation, the process of graft tissue resorption and replacement with new host bone.

  • Autograft, allograft, demineralized bone matrix, xenograft, ceramics, bone morphogenetic proteins, cellular bone matrices, mesenchymal stem cells, and other combinations have been used with varying success to promote healing, stabilization, and bony fusion.

  • Nicotine, drugs, steroids, osteoporosis, radiation, and other systemic and/or local factors may negatively impact bone growth.

  • Remodeling occurs in response to load bearing.

  • Cortical graft has the greatest compromise in mechanical strength at 12 weeks because of the need for initial osteoclastic resorption, and returns to normal between 1 and 2 years.

  • Excessive motion at the fusion mass lead to pseudarthrosis, or failure of fusion. Rigid internal fixation reduces pseudarthrosis rates in most clinical applications.

Acknowledgment

Our thanks to Drs. Fanor M. Saavedra-Pozo, Miguel Mayol del Valle, Ian P. Côté, and Michael Y. Wang who authored prior editions of this chapter. Their work laid the foundation for this rendition.

In the late 19th century, Sir William Macewen firmly established bone grafting as a treatment option for replacing missing bone and enhancing bone formation. His interest in bone grafting led him to attempt allograft and autograft fusion in his patients. In the United States, spine fusion was first reported in the early 1900s by Albee for the treatment of Pott disease and by Hibbs, who used fusion surgery to halt the progression of scoliotic deformity. , Since that time the indications for and number of spine fusions have increased dramatically. Spine arthrodesis is one of the most common surgical procedures performed in the United States today. Unfortunately, a number of complications have been associated with spine fusion. Pseudarthrosis can occur in as many as 35% to 40% of multilevel lumbar intertransverse fusions. A large, prospective review showed that 23.6% of revision surgeries following lumbar spinal fusion were performed because of pseudarthrosis. Historically, donor site morbidity was also a considerable concern with autograft. To achieve successful bony fusion, minimize complications, and achieve a good functional outcome, it is important to understand the various structural, biologic, and biomechanical aspects of bone fusion. Bone grafting involves transplanting bone tissue from one site to another to achieve bony union. The terms used for describing them are usually derived from the bone’s origin, anatomic placement, or composition. Autograft is a transplanted tissue within the same individual; allograft is tissue coming from a genetically different individual of the same species; xenograft is tissue transplanted from one species to a member of a different species; isograft is tissue obtained from a monozygotic twin. A graft transplanted to an anatomically appropriate site is defined as orthotopic, whereas if it is transplanted to an anatomically dissimilar site, it is termed heterotopic. Bone grafts are also categorized by composition as cortical, cancellous, corticocancellous, or osteochondral .

Anatomy of the Bone–Bone Interface

Histological Components

On a gross level, all bones are composed of two basic components: cortical (compact) bone and cancellous (trabecular) bone. Cortical bone is a dense, solid mass, except for its microscopic channels, and contains parallel stacks of curved sheets called lamellae, which are separated by bands of interlamellar cement. Regularly spaced throughout the lamellae are small cavities, or lacunae. Lacunae are interconnected by thin, tubular channels called canaliculi . Entrapped bone cells (osteocytes) are located in the lacunae, and their long, cytoplasmic processes occupy canaliculi. The cell processes within canaliculi communicate by gap junctions, with processes of osteocytes lying in adjacent lacunae. Canaliculi open to extracellular fluid at bone surfaces, thus forming an anastomosing network for the nutrition and metabolic activity of the osteocytes. Cortical bone possesses a volume fraction of pores less than 30% and has an apparent density of up to about 2 g/mL. Its compressive strength is approximately tenfold that for a similar volume of cancellous bone. Cancellous bone is porous and appears as a lattice of rods, plates, and arches individually known as trabeculae. It has a greater surface area and can be readily influenced by adjacent bone marrow cells. Because of this structural difference, cancellous bone has a higher metabolic activity and responds more readily to changes in mechanical loads. Cortical and cancellous bone may consist of woven (primary) or lamellar (secondary) bone. Woven bone forms the embryonic skeleton and is then resorbed and replaced by mature bone as the skeleton develops. In the adult, woven bone is found only in pathological conditions, such as fracture healing and tumors. Woven and lamellar bones differ in formation, composition, organization, and mechanical properties. Woven bone has an irregular pattern of collagen fibers, contains approximately four times as many osteocytes per unit volume, and has a rapid rate of deposition and turnover. The osteocytes of woven bone vary in orientation, and the mineralization of woven bone follows an irregular pattern in which mineral deposits vary in size and in their relationship to collagen fibrils. In contrast, the osteocytes of lamellar bone are relatively uniform, with their principle axis oriented parallel to that of other cells and to the collagen fibrils of the matrix. The collagen fibrils of lamellar bone lie in tightly organized, parallel sheets, with uniform distribution of mineral within the matrix. The irregular structure of woven bone makes it more flexible, more easily deformed, and weaker than lamellar bone. For these reasons, the restoration of normal mechanical properties to bone tissue at the site of a healing fracture requires eventual replacement of the woven bone of the fracture callus with mature lamellar bone.

Biology of Bone Graft Incorporation

Osteoinduction, Osteoconduction, and Osteogenicity

The principles of osteoinduction, osteoconduction, and osteogenesis dictate bone graft incorporation. Osteoinduction refers to the ability of a material to induce stem cells down a bone-forming lineage by utilizing factors that stimulate bone growth. Induction requires an inducing stimulus and an environment favorable for bone formation. Osteoconduction refers to the ability of a material to act as a structural framework for bone growth. The three-dimensional (3D) architecture of the graft dictates its osteoconductivity. Osteogenesis is the process by which a material provides cells that will ultimately produce bone, including mesenchymal stem cells, osteoblasts, and osteocytes. Whereas cortical bone grafts provide more structural support, cancellous bone has more osteogenic potential because of the presence of bone marrow and its larger surface area.

Graft Material

Autograft

Autogenous iliac crest bone was once considered the gold standard of graft material. Historically, it has been the most successful graft source in spine fusion aside from bone harvested from concurrent spinal decompression. Cancellous autograft has the requisite matrix proteins, mineral, and collagen for the ideals of osteogenicity, osteoinductivity, and osteoconductivity. Its large trabecular surface makes it highly connective as well. Donor site complication rates as high as 25% to 30% have been reported, although a rate of 8% seems more realistic and is more commonly cited. , Morbidity may be associated with an increased incidence of blood loss, chronic donor site pain, operative time, infection, and nerve injury. Furthermore, the quantity of bone available is limited and may be insufficient for long-segment fusions or in patients who have had previous graft harvests. Autogenous cortical bone is useful when structural support is needed at the graft site. Otherwise, it is less desirable than cancellous bone because of the absence of robust bone marrow and, as a result, fewer osteoprogenitor cells. Additionally, these cells are less likely to survive because they are embedded in a compact matrix where the diffusion of nutrients essential for cell proliferation is impeded compared with the cancellous environment. Cortical bone also has less surface area per unit weight with matrix proteins exposed, and connectivity is therefore marginal. Vascular ingrowth into cortical bone is slow. Mechanical strength lags because incorporation takes longer. Although cancellous bone is incorporated fairly rapidly and remodeled, portions of cortical graft may remain necrotic for extended periods. When the likelihood of avascular graft healing is low, as in previously irradiated tissue beds, vascularized grafts may be more desirable because of the presence of greater numbers of osteogenic cells.

Demineralized Bone Matrix

Demineralized bone matrix (DBM) is present in a variety of forms, each of which has a variable degree of osteoinductivity. Current data support its use as a bone graft extender but not as a pure substitute or enhancer. Autogenous and allograft DBMs are osteoinductive because of the presence of low doses of bone morphogenetic proteins (BMPs; ∼0.1% by weight). Collagenous and noncollagenous proteins also serve as osteoconductive material that is left after the demineralization process. As such, DBM has become a common adjunct in high-risk spinal fusions, including revision cases after prior laminectomies (limited bone graft) and those patients with comorbidities.

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