Spinal arthrodesis, graft materials, and graft techniques

1

What is spinal arthrodesis?

Spinal arthrodesis is defined as osseous union (fusion) between adjacent vertebrae due to surgical intervention. During surgery, adjacent bone surfaces are decorticated and graft material is applied to promote bone growth between adjacent vertebrae. Spinal instrumentation or external immobilization may be utilized to limit motion at the surgical site and enhance fusion. Spinal arthrodesis procedures are broadly categorized by location as anterior spinal column fusions, posterior spinal column fusions, or circumferential spinal fusions (also called 360° fusions, global fusions, or combined anterior and posterior column fusions).

2

Describe some of the functions of spinal instrumentation in relation to spinal arthrodesis procedures.

Spinal instrumentation is used to limit intersegmental motion and create a favorable mechanical environment that promotes spinal fusion. Other functions of spinal instrumentation include correction of spinal deformities and restoration of immediate mechanical stability to spinal segments whose structural integrity has been compromised by spinal pathologies such as fracture, tumor, infection, and degenerative disorders.

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What factors influence successful healing of a spinal fusion?

• (1)

  Type of graft material (e.g., autograft, allograft, synthetic biomaterials)

• (2)

  Local factors:

  • Quality of the soft tissue bed into which bone graft is placed

  • Method of preparation of the graft recipient site

  • Mechanical stability of the spine segment(s) to be fused

  • Graft location (anterior vs. posterior spinal column)

  • Spinal region (the cervical region is considered a more favorable environment for fusion versus the thoracic or lumbar regions)

• (3)

  Systemic host factors :

  • Patient age

  • Presence of metabolic bone disease

  • Nutritional status

  • Perioperative medications

  • Tobacco use

4

How does patient age and medical history influence healing of a spinal fusion and selection of an appropriate graft material?

The bones of skeletally immature patients have an inherent osteogenic potential, and high rates of arthrodesis are reported regardless of whether autograft, allograft, composite grafts, or ceramics are utilized as graft material. Lower fusion rates are encountered in the adult population and union rates have been shown to decline with increasing age. Additional factors that negatively impact fusion rates in adults include endocrine disorders (e.g., diabetes), medications, and tobacco use.

Certain medications have the potential to impair fusion if used in the perioperative period because they inhibit or delay bone formation. Examples include nonsteroidal antiinflammatory drugs (NSAIDs, e.g., ibuprofen, ketorolac tromethamine), cytotoxic drugs (e.g., methotrexate, doxorubicin), certain antibiotics (e.g., ciprofloxacin), corticosteroids, and anticoagulants (e.g., warfarin [Coumadin]). Recent evidence suggests that the adverse effects of NSAIDs on spinal fusion healing are related to dose and duration of use, and that incorporation of normal-dose NSAIDs into postoperative pain management protocols for spine fusion patients is reasonable and is associated with improved pain control and reduction in adverse events associated with use of opioids. In contrast, other medications such as teriparatide have been reported to enhance healing of spinal fusions and are under investigation for this purpose.

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How does tobacco use interfere with spinal fusion?

The rate of successful spinal arthrodesis in smokers is lower than in nonsmokers. Cigarette smoking has been shown to interfere with bone metabolism and inhibit bone formation. Nicotine is considered the agent responsible for these adverse effects. The precise mechanisms responsible remain under investigation and include inhibition of graft revascularization and neovascularization, as well as osteoblast suppression. These effects are mediated by inhibition of cytokines.

6

What are some of the common types of graft materials used in spinal arthrodesis procedures?

Common graft material options for use in spinal fusion include:

• Autograft : Autograft bone is obtained from the patient undergoing surgery. Sources include the patient’s ilium, fibula, or ribs. Autograft bone may also be obtained from the operative site and is termed local bone graft . Vascularized autografts (e.g., rib, fibula) are an option for use in complex reconstruction scenarios. Bone marrow aspirate (BMA) is an autologous source of stem cells, osteoblasts, and growth factors and may be used in conjunction with other graft materials.

• Allograft : Allograft bone is human cadaveric bone, which is available in a variety of shapes and compositions.

• Demineralized Bone Matrix (DBM) : Allograft bone may undergo processing by acid extraction to remove bone mineral while retaining collagen and noncollagenous proteins. The end product is DBM, an allograft form with osteoinductive activity.

• Bone Morphogenetic Proteins (BMPs) : BMPs are osteoinductive proteins that are members of the transforming growth factor beta (TGF-β) superfamily of growth factors. As BMPs are soluble proteins, use of a carrier is required to maintain an effective local concentration and prevent diffusion away from the operative site.

• Ceramics : Various ceramics are available for use as bone graft material, including beta-tricalcium phosphate (β-TCP), hydroxyapatite (HA), calcium sulfate, silicate-substituted calcium phosphate (Si-CaP), and beta-calcium pyrophosphate (β-CPP).

• Composite grafts : Various types of scaffolds may be combined with biologic elements (e.g., BMA, growth factors) to promote fusion.

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Which three properties are desirable in an ideal graft material for use in spinal fusion?

• 1.

  Osteoinduction: The graft should contain growth factors (noncollagenous bone matrix proteins) that can induce osteoblast precursors to differentiate into mature bone-forming cells.

• 2.

  Osteoconduction: The graft should provide a framework or scaffold (bone mineral and collagen) onto which new bone can form.

• 3.

  Osteogenesis: The graft should contain viable progenitor stem cells that can form new bone matrix and remodel bone as needed.

Autograft bone has been considered the gold standard for spinal fusion bone graft materials. Autograft bone possesses all of the properties required to achieve spinal fusion as it is osteoinductive, osteoconductive, and osteogenic.

8

Compare the properties of current graft materials in relation to an ideal graft material for use in spinal fusion.

See Table 25.1 .

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How are graft materials classified according to their intended use?

Graft materials may be compared to autograft bone and classified according to their intended use:

• Extender: This type of graft material is indicated for use in combination with autograft bone. Such graft material permits use of a lesser volume of autograft without compromising fusion rates. A bone graft extender can also permit a finite amount of autograft to be utilized over a greater number of spinal levels without compromising the fusion rate when compared with fusion using autograft alone.

• Enhancer: This type of graft material is used in conjunction with autograft bone to increase the rate of successful arthrodesis above the rate of fusion achieved with use of autograft alone.

• Substitute: This type of graft material is used as an alternative to autograft bone and is intended to provide equivalent or superior fusion success compared with autograft bone.

10

What are some differences between cortical and cancellous bone graft in relation to use of each type of graft in spinal fusion procedures?

Cortical bone comprises the outer portion of skeletal structures. It is compact and exhibits high resistance to bending and torsional forces. These properties allow cortical bone to provide structural support when used as a graft material. Cortical bone is incorporated by creeping substitution, which occurs slowly over years. Cancellous bone is less dense than cortical bone and provides a porous matrix essential for osteogenesis in areas not requiring immediate structural support. Cancellous bone is incorporated more rapidly than cortical bone because of direct bone apposition onto the scaffold provided by its bony trabeculae. Bone graft used in spinal fusion procedures may be comprised entirely of cancellous or cortical bone, or a combination of both. The ratio of cancellous to cortical bone varies depending on the bone graft donor site and technique used for graft procurement and graft preparation.

11

Explain the difference between nonstructural and structural graft material.

Nonstructural grafts (also termed morselized grafts) consist of particles of bone (e.g., cancellous bone from the iliac crest). This graft type does not provide structural stability when used by itself. Adjunctive spinal instrumentation is generally used to facilitate bony union. Structural grafts provide mechanical support during the process of fusion consolidation.

12

Compare and contrast the healing potential of the anterior spinal column and posterior spinal column with respect to spinal fusion.

Biologic factors and biomechanical factors are different in the anterior and posterior spinal columns. Graft materials placed in the anterior column are subject to compressive loading, which promotes fusion. Structural grafts are commonly used to restore the load bearing capacity of the anterior spinal column. In the anterior spinal column, the wide bony surface area combined with excellent vascularity of the fusion bed, which is composed primarily of cancellous bone, creates a superior biologic milieu for fusion. High fusion success rates in the anterior column may be achieved using autograft bone, allograft bone, as well as specific bone graft substitutes. In contrast, the posterior spinal column is composed primarily of cortical bone and subject to tensile forces, which provide a less favorable healing environment for spinal fusion. Posterior column graft materials are generally not required to provide structural support. Posterior column fusion is highly dependent on biologic factors, including the presence of osteogenic cells, osteoinductive factors, as well as the quality of the soft tissue and osseous bed into which the graft material is placed. In view of this more challenging healing environment, autogenous iliac bone graft has traditionally been the gold standard for achieving posterior spinal fusion. Fusion rates also vary by spinal region, with the highest fusion rates associated with cervical and thoracic fusions and the lowest rates with lumbar fusions, especially posterolateral lumbar fusions.

13

What anatomic structures provide potential sites for posterior spinal arthrodesis?

In the cervical region , posterior spinal fusions are achieved by applying bone graft to the lamina, facet joints, and spinous processes. In the thoracic and lumbar regions , the lamina, facet joints, spinous processes, and transverse processes are available sites for arthrodesis. These bone surfaces require meticulous preparation including removal of all overlying soft tissue prior to graft application. In addition, it is critical to remove the outer cortical bone surface (decortication) to expose underlying cancellous bone and provide access to the pluripotent stem cells within the patient’s bone marrow to achieve a consistent and high likelihood of successful fusion ( Fig. 25.1 ).

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