Fusion Surgery

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CHAPTER 102 Fusion Surgery

Andrew Perry, Choll W. Kim, Steven R. Garfin

INTRODUCTION

Fusion has been used for decades to manage a variety of spinal disorders. However, the incidence of spinal fusion surgery varies among countries and among regions within a country. During its infancy, spinal fusion surgery typically involved extensive muscle dissection, copious amounts of autogenous bone graft, bracing, and prolonged bed rest. The introduction of spinal instrumentation provided an opportunity to increase the rate of successful fusion, decrease the recovery period, minimize the cardiopulmonary and musculoskeletal deconditioning resulting from immobility, and allow surgeons to perform more complex spinal reconstructive surgeries. Initial fusion techniques were primarily posterior, but anterior column support was subsequently developed to minimize the failure rates associated with the use of posterior instrumentation. Anterior column fusion can be achieved with bone graft material secured within the disc space from either an anterior or posterior approach. Anterior column fusion has inherent advantages as it occurs along the weight bearing portion of the lumbar spine (80% anterior versus 20% posterior), has superior blood supply, and a superior ability to maintain sagittal alignment.

At present, controversy still exists concerning the indications for spinal fusion, the type of procedure to perform, the choice of graft material, and the use of instrumentation. The most widely accepted indication for spinal fusion is instability, which may arise from trauma, tumor, infection, and degenerative disease. Instability may also arise iatrogenically from the surgical treatment of these aforementioned conditions. With the improvement of surgical techniques, along with the development and marketing of spinal instrumentation, there has been a noticeable trend toward the increasing use of spinal instrumentation.1

In general, the goal of surgical fusion is to produce a solid arthrodesis through the segments considered unstable. This is true regardless of the surgical technique used or the approach taken. The end result should be a well-aligned and stable spine that is capable of protecting the neural elements and alleviating pain.

INDICATIONS

The indications for surgical fusion of the lumbar spine for instability are not clearly defined. Much of the rationale for fusion has been based on expert opinion and retrospective studies. Over the last decade, there has been increased interest in designing objective studies that use validated outcome measures of success. This is evidenced by the development of a plethora of outcome assessments tools. This differing opinion of surgeons concerning the precise indications for fusion has led to a wide discrepancy in the rate of fusion surgery in various parts of the world.

Although there is agreement regarding instability as an indication for fusion, there is a lack of consensus on the precise definition of instability. The clinical manifestations and assessment of instability of the spinal column are discussed in detail in the previous chapter. In general, the various methods for determining spinal instability are either for trauma, degenerative conditions, or tumor. The White and Panjabi ‘checklist approach’ and the Denis classification system remain the most recognized methods of determining instability in the setting of trauma.2 Recently, Gertzbein and coworkers have developed a detailed description of thoracolumbar fracture patterns that can also be helpful in determining instability.3,4 Gaines and coworkers utilize a ‘load-sharing’ system to determine the stability of the anterior column in the setting of burst fractures.5,6 This system is useful for determining when anterior reconstruction is necessary or when short-segment posterior instrumented fusion may be helpful to treat burst fractures. Whichever system is used, instability due to trauma must take into consideration the mechanism of injury, the degree of deformity, and the location of injured ligamentous and bony structures.

Instability due to degenerative conditions is typically described as relative instability. Relative instability due to these conditions is thought to lead to abnormal intervertebral motion which in turn causes pain. It is uncertain whether excessive sagittal translation on flexion–extension radiographs along with high-intensity zones seen on T2-weighted magnetic resonance imaging (MRI) images, endplate irregularities, and Modic changes on MRI, result from relative instability. This type of instability can insidiously develop into spondylolisthesis, degenerative scoliosis, and/or painful disc degeneration. As well, the surgical treatment of these disorders can lead to iatrogenic instability.

For destructive lesions, particularly tumors, the degree of vertebral body and/or pedicle involvement and the degree of deformity enter into in the determination of instability.5–7 At present, no method of systematically assessing impending instability exists for infectious processes. Other forms of instability, such as inflammatory, congenital, degenerative, and postoperative instability, are not well assessed with grading systems such as those described by Gaines and coworkers.

Once instability is determined, surgical treatment generally requires adequate correction of deformity, reconstruction of bony defects, and fusion of the affected segments. The choice of method by which surgical correction is performed remains a challenging task.

TECHNIQUE

The method by which surgery is performed depends on the cause and character of the instability. The available methods of treatment include anterior, posterior, or combined anterior–posterior surgical approaches. General guidelines have some degree of support either in the literature or by consensus (Fig. 102.1).

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Fig. 102.1 Algorithm for the surgical treatment of instability via fusion. Instability due to trauma, tumor, infection, as well as iatrogenic induced instability are discussed. Instability due to degenerative conditions such as spondylolisthesis, scoliosis, and disc disease are discussed in previous chapters.

Anterior lesions

When instability is due to a lesion that is primarily anterior, surgical stabilization can be performed from an anterior approach. This anterior approach generally involves discectomy, perhaps corpectomy, structural grafting and anterior instrumentation. Anterior surgery is indicated if there is canal compromise due to the anterior lesion, if anterior column stability is deficient, or a previous posterior procedure has failed to stabilize the segment(s). The load-sharing classification system of Gaines and coworkers is one method of determining the integrity of the anterior column.5,6 The load-sharing classification system describes the integrity of the anterior column in the setting of trauma. However, this concept can be applied to other lesions of the anterior column. If there is no significant osteoporosis or deformity that requires correction, then anterior strut grafting with anterior instrumentation will provide sufficient stabilization (Fig. 102.2).

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Fig. 102.2 L2 pathologic fracture due to metastatic adenocarcinoma. (A) T2-weighted sagittal MRI showing pathologic fracture of L2. (B) T1 axial image with contrast showing canal stenosis. (C) Postoperative radiographs after L2 corpectomy, anterior cage strut, and anterior instrumentation.

However, certain conditions warrant posterior surgery, either together with the anterior surgery or posterior surgery alone. Posterior surgery alone can be used to treat lesions that are otherwise anterior if the anterior column has sufficient stability and there is no need for decompression. In some cases, such as when there is no neurologic deficit, posterior-only surgery can be used to indirectly reduce retropulsed fragments of bone. Posterior surgery may be further entertained if there are anteriorly exposure risks, such as previous abdominal surgery or severe obesity. Posterior fusion may also be necessary if there is significant osteoporosis. In the setting of osteoporosis, anterior surgery alone may be inadequate in achieving spinal stability (Fig. 102.3).

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Fig. 102.3 Construct failure after anterior-only treatment of a osteoporotic burst fracture. (A) Lateral radiograph immediately after vertebral corpectomy, allograft strut fusion, and anterior instrumentation. Black dotted lines highlight endplates above and below the fusion site. (B) Lateral radiograph 4 weeks after surgery. Black dotted lines highlighting the endplates show interim kyphosis above the construct. The vertebral body above has collapsed into the superior screws.

Posterior lesions

When instability stems from a posterior lesion, posterior surgery via instrumented fusion is usually the best option (Fig. 102.4). Unfortunately, instability due to a purely posterior lesion is relatively uncommon. The most common such lesion is the Chance fracture involving a flexion–distraction injury. If there is severe osteoporosis, additional levels may be added to the construct. The length of posterior instrumentation can be extended as necessary to achieve stability without much added difficulty. If necessary, pedicle screws can also be reinforced with bone cement to achieve better fixation in osteoporotic bone. Interbody fusion can be performed posteriorly via transforaminal lumbar interbody fusion (TLIF) or posterior lumbar interbody fusion (PLIF) to improve fusion rates, especially in long fusions or in fusions to the sacrum.

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Fig. 102.4 Metastatic erosion of posterior elements. (A) Lateral radiograph showing posterior element bony destruction. (B) Sagittal MRI showing canal compromise. (C) Axial CT image showing bone erosion of facets, right pedicles, lamina, and spinous process. (D) Lateral radiograph after resection, decompression, and posterior instrumented fusion.

Combined anterior–posterior lesions

In combined lesions of both the anterior and posterior columns, the surgical approach is dictated by several factors. The first issue is whether anterior surgery is necessary. Anterior surgery is necessary if there is isolated or predominantly anterior canal compromise requiring decompression or if anterior column stability is compromised. Anterior surgery is usually in the form of strut or structural grafting. If there is good bone stock and no significant deformity, then anterior instrumentation may be all that is needed. If there is no canal compromise and the anterior column is stable, posterior instrumented fusion alone is sufficient to obtain stability. If there is osteoporosis, the levels of posterior fusion can be increased for improved fixation. When the anterior column is deficient and it is combined with severe deformity, osteoporosis, or infection, then anterior strut graft fusion combined with posterior instrumentation is necessary (Fig. 102.5).

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Fig. 102.5 Spinal osteomyelitis and epidural abscess. (A) Contrast-enhanced sagittal MRI showing osteomyelitis of L5 and S1 along with anterior and posterior epidural abscesses. (B) Contrast-enhanced axial MRI showing circumferential infection. There are also abscesses in the right paraspinal muscles. (C) Lateral radiograph after surgical treatment. Treatment involved L5 and partial S1 laminectomies and right L5–S1 facetectomy. (D) Second stage partial L5–S1 corpectomies with cage strut graft were followed by third stage posterior instrumented fusion.

FACTORS AFFECTING THE OUTCOME OF FUSION SURGERY

Achieving arthrodesis and determining when this occurs are still problems that confront the spine surgeon. In general, spinal fusion is a nonphysiological operation that attempts to achieve bone formation in a site where this does not normally occur. In addition to the local and systemic factors that influence typical fracture healing, mechanical factors also influence the healing process during spinal fusion. These mechanical factors include the distance to be bridged by the fusion mass, the magnitude and planes of motion, and the structural properties of the graft. For convenience, the factors that affect successful spinal arthrodesis can be grouped into local, biologic, and surgical factors.

Local factors

Location

The physiologic and biomechanical forces acting on the healing of anterior interbody and posterolateral fusions are quite different. Anterior interbody fusions are revascularized through the vertebral bodies themselves and the graft material used in the interspace is under compressive loading. Bone healing is favored under conditions of axial loading and stability. As such, interbody fusion tends to occur more readily than posterolateral spinal fusion. In a posterolateral fusion, revascularization is primarily derived from the bony surface of the transverse process and surrounding muscle tissue. In this region, there are few or no compressive forces acting on the graft material. Consequently, the posterolateral spinal fusion environment generally does not tolerate the use of purely osteoconductive materials as stand-alone substitutes. At this site, osteoinductive substitutes are more likely to be successful as extenders or enhancers for spinal fusion. In the anterior spine, purely osteoconductive substitutes may be suitable when they are rigidly immobilized.

Fusion site

Arthrodesis depends on ingress of osteoprogenitor and inflammatory cells from the fusion bed and from the surviving bone cells located in the bone graft. Bone healing is greatly affected by the local blood supply along with the availability of osteoprogenitor cells. The fusion bed vascular supply is a source of supportive nutrients, a vehicle for endocrine signals, and a pathway for the recruitment of osteoprogenitor and inflammatory cells. The preparation of the bony surfaces where the graft material is to be placed also plays an important role in determining the outcome of fusion. Decortication allows vascularization of the fusion bed. In addition, it is important to remove avascular tissue such as scar tissue during surgery since a fusion bed with excessive scar tissue is less likely to achieve successful fusion.

Biologic factors

Nutrition

Malnutrition is associated with reduced cognitive function, poor wound healing, impaired muscle function, decreased bone mass, immune dysfunction, anemia, delayed recovery from surgery, and ultimately increased morbidity and mortality.7 Nutritional deficiencies can be confirmed using various studies such as total white blood cell count, serum albumin and total protein, transferrin levels, nitrogen balance, and anthropometric measurements.

Hormones

Thyroid hormones have a direct stimulatory effect on cartilage growth and maturation, and promote bone healing.8 These hormones are required for the synthesis of somatomedins by the liver.9 Growth hormone promotes bone healing by increasing calcium absorption from the intestine, and promoting bone formation and mineralization.8 Androgens and estrogens are considered important for skeletal development and preventing age-related bone loss. Estrogens may increase bone mineralization through their effect on increasing serum parathyroid hormone (PTH).10 Excess corticosteroids can negatively affect bone healing by decreasing synthesis of the major components of the bone matrix. These steroids have also been shown to inhibit the differentiation of osteoblasts from mesenchymal cells.11

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