Spinal Fusion: Posterior Approaches

E. Andrew Stevens and Charles L. Branch Jr.

Damage and degeneration of the lumbar disc can be the result of aging, activity, and trauma. Biochemical degradation of the disc matrix and disc space narrowing can lead to segmental instability and pain.1 Laboratory and clinical studies have identified the nucleus pulposus as a chemical irritant, able to incite an inflammatory response in surrounding tissues.² Intraoperative tissue stimulation has implicated the outer disc anulus as the primary generator of low back pain, with less of a contribution from facet joints and muscle.³ Physiologic stress on an unstable joint sustains a cascade of inflammation, joint destruction, deformity, and scar formation with tethering and compression of neural elements. Lumbar fusion is used to try and halt this painful, progressive condition.

Biomechanical and Biologic Rationale for Treatment

Arthrodesis

The primary goal of fusion is to decrease movement and thereby decrease symptoms associated with pathologic movement. Fusion can alleviate pain that emanates from damaged or inflamed annular fibers as well as symptoms of neural compression related to positional instability, spondylolisthesis, and stenosis. A solid arthrodesis halts the progression of microinstability and associated tropism and stenosis.⁴

The posterior approaches to lumbar fusion are the most direct and familiar to most surgeons. Historically, posterolateral fusion (PLF), also known as transverse process fusion or dorsolateral fusion, has been the most commonly used method of lumbar arthrodesis. PLF generally involves the fusing of transverse processes and lateral facets through the onlay of autologous bone or allograft, with or without the addition of instrumentation or bracing. Paraspinous musculature permits vascular ingrowth, cellular migration, and delivery of growth factors necessary for fusion; however, muscular interposition may also prevent solid fusion.⁵ PLF reduces motion in the posterior spinal column, but biomechanical studies have shown that some degree of motion is maintained through the disc space and may account for residual pain following solid PLF.¹ Circumferential fusion, through the addition of an interbody graft, is a more biomechanically rigid arthrodesis. Biomechanical advantages of interbody fusion include fusing the motion segment in the center of its axis of rotation, compressive forces on the graft, and an excellent local blood supply and bony surface for fusion in the vertebral body endplates.⁴

Deformity Correction and Prevention

Another biomechanical challenge faced by surgeons treating lumbar instability is the correction and prevention of spinal deformity. The idea of sagittal balance is becoming popular in the literature and is purported to influence the later effects of fusion on adjacent level degeneration.⁶ Interbody fusion permits correction and maintenance of disc space height, foraminal patency, and restoration of lordosis. However, these advantages must be weighed against the additional adjacent-level stress created by these rigid constructs.

Neural Decompression

High-grade spondylolisthesis and gross instability can cause spinal stenosis, but microinstability can also cause stenosis through tissue inflammation and hypertrophy. The posterior approaches to lumbar fusion all allow for the examination and decompression of neural structures through laminotomy, foraminotomy, facetectomy, and discectomy as needed. Discectomy not only permits mechanical decompression of neural structures, it may also improve the local biochemical environment by removing a nidus of inflammation. Chemical irritation from disc material can cause a tethering of the thecal sac and nerve roots. A solid fusion can limit traction on tethered neural elements but should not replace attempts to decompress and release neural structures and remove inflammatory debris.

Indications

Recent literature supports the role of fusion surgery as part of a multidisciplinary approach to lumbar disc pathology.⁷ Advantages of surgical over nonsurgical treatments have been demonstrated for chronic back pain,⁸ lumbar stenosis,⁹ and spondylolisthesis.¹⁰ Exact criteria for lumbar fusion have not been established, but the broad indication for fusion is symptomatic or progressive instability of a spinal motion segment. Fusion has also been advocated in certain cases of recurrent disc herniation, massive midline or bilateral disc herniation, painful pseudarthrosis, traumatic or iatrogenic instability, and as an adjunct to anterior fusion.

Both PLF and posterior lumbar interbody fusion (PLIF) have been used successfully to treat any of the above diagnoses. However, because PLIF typically requires as least some retraction of the thecal sac and nerve roots, its use is relatively contraindicated above L3. PLIF is also commonly avoided in reoperations as a dorsal scar and neural tethering can increase the risk of dural and neural injury. Transforaminal lumbar interbody fusion (TLIF) was developed in an effort to avoid the neurologic complications associated with PLIF, while still permitting circumferential fusion through a single incision. TLIF may be used higher in the lumbar spine as the complete hemifacetectomy allows a more lateral trajectory to the disc space, avoiding retraction of the thecal sac. TLIF may also be advantageous in reoperations as the more lateral bone removal provides a fresh working channel to the disc space and avoids dorsal scar and retraction against tethered neural elements.4

Work-up

Before any patient is considered for lumbar fusion, it is mandatory that they have tried and failed an adequate nonoperative treatment attempt. Conservative management strategies vary, but should generally contain some combination of the following: nonsteroidal antiinflammatory medications, judicious use of oral corticosteroids or epidural injections, strengthening and stretching exercises, physical therapy, activity modification, low-velocity chiropractic or massage manipulation, and possibly bracing.¹¹ Furthermore, a thorough evaluation of any potential psychiatric, social, and medicolegal influence in a patient’s life is strongly recommended as these factors have been shown to have a tremendous impact on outcome and disability.¹² The history, signs, symptoms, and exam findings suggesting lumbar instability or discogenic low back pain should be concordant with radiographic and/or provocative test results.

Imaging

Segmental instability is dynamic pathology by definition; therefore, it is essential that imaging studies include dynamic views. Flexion and extension as well as supine and standing x-rays in both AP and lateral planes should be routine. The absence of overt kyphotic or translational deformity on radiography does not completely rule out pathologic instability, however. Microinstability, characteristic of degenerative disease, manifests as disc narrowing, articular sclerosis, osteophytosis, synovial cyst formation, and other sequelae of inflammation. Magnetic resonance imaging (MRI) best demonstrates soft tissues including discs, ligaments, and synovium; it may also be helpful in identifying subtle fractures and potential neoplasia. Characteristic MRI findings in an unstable, degenerated motion segment may include dark narrowing of a desiccated disc and bright, hyperintense widening of the facet joint spaces on T2-weighted sequences (Fig. 26.1). Computed tomography (CT) scans best reveal bony architecture and bone quality, and they permit valuable measurements of pedicle diameter and vertebral body depth for instrumentation planning. It may also be helpful in preoperative planning to obtain complete spine x-rays to assess overall alignment and sagittal balance.

Discography

Although the above studies are sensitive in demonstrating disc degeneration and segmental instability, it is often difficult to determine which disc is symptomatic and most clinically relevant. Discography through the injection of an opaque medium into multiple lumbar discs may help to localize a symptomatic disc by eliciting the patient’s symptoms. It is deemed most helpful if a patient has adjacent discs that do not elicit pain with injections that serve as controls. Information about adjacent levels may be helpful in predicting which patients will be likely to have persistent pain or accelerated degeneration at a transitional level after a fusion.¹³ Although the accuracy of discography is debated, and currently there is no gold standard by which to test its validity, any information obtained while considering a patient for fusion should be considered potentially useful.

Fig. 26.1 T2-weighted bright MRI showing degenerated facet joints (arrows) in a hypermobile lumbar segment.

Surgical Technique

Patient Preparation and Positioning

Each of the posterior approaches to lumbar fusion discussed in this chapter are prepared and positioned similarly. Placement of a Foley catheter should be considered based on the anticipated surgical time and blood loss. Bilateral lower extremity sequential compression devices are used for deep venous thrombosis prophylaxis. A dose of prophylactic antibiotics is usually given during positioning. The patient is positioned prone on chest rolls to lower intraabdominal pressure and improve venous drainage. Typically, chest rolls are placed parallel to the operating table. However, occasionally for the morbidly obese we turn the rolls perpendicular to the table, placing one roll beneath the upper chest and shoulders and one roll beneath the anterior iliac crests, leaving the patient’s breasts and abdominal pannus hanging between. Arms are placed on arm boards with abduction limited to 80 degrees as to prevent brachial plexus injury. Careful attention is paid to padding all appendages and pressure points. There should be no pressure on electrocardiogram leads, a pillow should be placed under the lower legs to protect the knees and toes, and padding should guard the spiral groove and ulnar nerve. Goggles and/or a ProneView (Dupaco, Oceanside, CA) or similar device should be used to protect the face, eyes, and endotracheal tube. A surgical “time-out” is executed prior to incision.

Transverse Process Fusion

A dorsal midline incision is made and subcutaneous tissues are dissected sharply or with monopolar coagulation until the deep fascia is encountered. The fascia is incised adjacent to the spinous processes bilaterally, preserving a midline ligamentous tension band. The paraspinous muscles are released from the laminae in a subperiosteal fashion, and the dissection is taken out to the facets bilaterally. Self-retaining retractors used for this exposure should be relaxed and repositioned intermittently to prevent inadvertent soft-tissue traction injury or ischemia. Lateral radiographs should be obtained to confirm the operative levels prior to arthrodesis.

Bleeding may be anticipated as the soft tissues around the facets are removed and the transverse processes are exposed. The terminal branches of the segmental lumbar vessels form a rich vascular network rostral, caudal, and lateral to the facet joints, overlying the pars interarticularis, and ventral to the transverse process (Fig. 26.2). Bipolar electrocautery is used for hemostasis to limit the transmission of thermal energy to surrounding tissues. To prevent injury to the nerve root or anterior transverse artery, the plane formed by the transverse process and intertransverse ligament should serve as the ventral boundary of the dissection.

Soft tissue and coagulation debris should be aggressively removed on and around the lamina, pars, facet joint, and dorsal transverse process, including the facet capsule and intrafacet synovium. Laminotomy and foraminotomy can be performed as needed for neural decompression, and the facet and dorsal transverse process should be decorticated. All bone removed should be kept and further picked of any remaining soft tissue before being morselized. Pedicle screw and plate/rod fixation may be employed at this point as desired, and the fixation apparatus can help buttress the graft material in a dorsolateral position.