Posterior Lumbar Fusion by Open Technique: Indications and Techniques

Spondylolisthesis

Spondylolisthesis is the anterior displacement of one vertebra relative to the one below. It is derived from the root Greek terms “spondylos” meaning vertebra and “olisthesis” meaning to slip forward. Although displacement can occur posteriorly or laterally, spondylolisthesis is synonymous with anterior displacement of the vertebral body. ^, The degree of forward translation is measured radiographically and defined as the percentage of slippage of the vertebral body relative to the adjacent one as depicted in Fig. 150.1 . The most commonly encountered degree of listhesis is less than 30%. Spondylolisthesis can be caused by several mechanisms: congenital, spondylolytic, traumatic, degenerative, and pathologic. DS is the most common etiology. Often multifactorial, DS results of a combination of arthritic and degenerative changes in the disc and facet joints that leads to spinal stenosis and vertebral body displacement ( Fig. 150.2 ). DS is considered a disease of aging as it is mostly seen in patients older than 50 years, with an increased predilection in women (2–3:1, female: male) due to hormonal effects. DS is mostly encountered at the L4–L5 level, the most mobile lumbar segment. Symptomatic patients present with radicular pain due to lateral recess compression or foraminal stenosis, or with low back pain that worsens with extension. Spondylolisthesis is best visualized on lateral, standing radiographs because supine positioning can cause the slip to reduce into normal alignment. Further evaluation with computed tomography (CT) scans or magnetic resonance imaging (MRI) is often useful to exclude other concomitant sources of pain, such as herniated discs. ^,

Conservative therapy should be considered as an initial management for most patients with DS. Surgical intervention is superior to nonoperative treatment when the pain becomes more persistent and less responsive to several conservative modalities, or if neurological deficits are present. The goals of the operation are neural decompression to alleviate symptoms from the stenosis and stabilization to prevent further slippage and instability. There is a large body of literature regarding the treatment options for patients with DS which provide some of the best evidence for lumbar spinal fusion: In appropriate candidates, surgical intervention is superior to nonoperative treatment. The highest-quality data is provided by the well-publicized Spine Patient Outcomes Research Trial (SPORT), a prospective evaluation of the effectiveness of surgical versus nonoperative treatment for patients with DS. A total of 607 patients from 13 centers in 11 states were enrolled in either a randomized cohort or an observational cohort. Inclusion criteria were persistent symptoms for greater than 12 weeks, as well as evidence of DS on lateral static radiographs. Treatment was decompression (with or without fusion) or standard nonoperative care. There was a substantial crossover from patients who were randomized to either treatment limiting the intention-to-treat analysis. For example, those who were randomized to nonoperative treatment ultimately were treated operatively and vice versa. The as-treated analysis, involving both the observational and randomized cohorts, showed a significant benefit to surgery in pain control, physical function, outcome measures (the Oswestry Disability Index [ODI]), patient satisfaction, and overall progress. These improvements were sustained 4 years after surgery. ^, Although the significant crossover of patients negated the benefits of randomization, the outcomes reported from the SPORT trial likely represent the true clinical effectiveness of spinal fusion for patients with DS in the American population.

The utility of spinal fusion to improve clinical outcomes in patients with DS has also been thoroughly investigated, showing that fusion is preferred. Laminectomy without fusion may further destabilize the decompressed level, with potential worsening of the clinical and radiographic picture. Patients who undergo decompression alone have a higher rate of reoperation than those who undergo decompression and fusion. The Herkowitz and Kurz study was the first prospective, randomized trial comparing the outcome of decompression and in situ fusion versus decompression alone in patients with degenerative lumbar spondylolisthesis. They reported significant benefit in favor of fusion: 96% the fusion patients reported a satisfactory outcome versus 44% in the decompression alone group. A number of cohorts of patients with DS treated with fusion have reported excellent outcomes, with substantial improvements in outcome measures as well as quality of life ^, ; these benefits have persisted at least 5 years after surgery.

Instrumented Fusion

Fusion is widely accepted for symptomatic patients with DS; however, the debate is centered on the need for instrumentation in achieving fusion. In the first major prospective study, Fischgrund and colleagues randomized 68 patients with DS to posterolateral fusion with or without supplemental instrumentation. The use of fixation significantly improved fusion rates, with 82% in the instrumented cohort versus 45%, but no differences were seen in clinical outcome at 2 years follow-up. Another prospective study with a longer follow-up time of 5 years randomized 129 patients to either supplemental pedicle screw fixation or no pedicle screw instrumentation PLF. The results showed no significant difference between the two groups in terms of work capacity and pain measures. The use of transpedicular instrumentation has shown to improve activities of daily living with no significant effect on overall outcomes at short- and long-term follow-up. ^, A meta-analysis confirmed an increased rate of solid fusion in patients with adjunctive instrumentation without similar improvement in function. No differences in fusion rates were seen in a prospective study with 82 patients randomized to unilateral versus bilateral instrumentations at 3 years, leading the authors to suggest that unilateral fusion decreases operative time and reduces complications related to misplaced screws.

Kornblum and colleagues assessed the outcomes of patients who received a fusion without instrumentation for DS with an average follow-up time of 8 years. Their results revealed that patients who had radiographic pseudoarthrosis were significantly less likely to have a clinical outcome rated as excellent or good and performed significantly worse in symptom severity and physical function compared to those who had achieved successful radiographic arthrodesis. Many surgeons have used this study to advocate for the use of adjunct instrumentation in posterolateral fusion. ^, The use of supplemental transpedicular instrumentation leads to higher rates of radiographic arthrodesis, a finding that has been associated with better clinical outcomes. Although there is only limited evidence that instrumentation directly improves outcomes in patients with DS, a survey of surgeon members of the North American Spine Society showed that the majority would recommend instrumented fusion, and the most recent guidelines suggest the addition of instrumentation to improve fusion rates. ^{, ,}

Interbody fusion has also shown good clinical outcomes in patients with DS, with successful union rates as high as 98%. Theoretically, the addition of an interbody graft increases the fusion surface area, potentially improving fusion rates. Studies that have directly compared posterolateral fusion with or without additional interbody fusion have not shown a clear benefit to one approach in comparison to the other. Campbell and colleagues performed a systematic review of articles published up to July 2016 to compare the clinical outcomes of patients with DS who underwent interbody fusion to those who underwent posterolateral fusion alone. They found no differences in patient-reported outcomes or fusion rates between the two groups. To date, no clear data exist favoring the addition of interbody fusion to posterolateral fusion alone, and thus the decision is left up to the surgeon’s expertise and technical preference.

Interspinous process devices (IPDs) have been developed as less invasive alternatives to spinal fusion with the goal of decompressing the spinal canal and preserving segmental motion. The IPDs act as a distracting force between the spinous processes with the goal of increasing the diameter of the spinal canal. ^, IPDs were originally developed to treat patients with symptomatic spinal stenosis and neurogenic claudication that improves in flexion. Within the past decade, the indications of IPDs usage have expanded to include lumbar spondylolisthesis, degenerative scoliosis, and facet joint arthropathy. Nevertheless, the utilization of IPDs remains extremely controversial for two main reasons: the lack of randomized clinical trials with long-term clinical and radiological follow-up, and, more importantly, the reports of reoperation, recurrence of symptoms, and progression of degenerative changes after IPD implantation. The use of IPDs to treat the symptoms of DS is highly debatable as it has been associated with high rates of further progression necessitating reoperation. ^,

High-grade spondylolisthesis is defined as the displacement of a spinal vertebral body relative to an adjacent level of greater than 50% ( Table 150.3 ). It most commonly affects the L5–S1 level. The treatment of adult patients with high-grade spondylolisthesis is challenging; the degree of listhesis can cause technical difficulties in the operating room, and postoperative complication rates may be high. A trial of conservative treatment is reasonable in the absence of significant neurological deficits. Operative intervention should be considered in patients with intractable back pain, radiculopathy, weakness, or progressive deformity. The average of listhesis progression is greatest in adolescents and declines with aging as autofusion progresses. Posterior reduction and fusion without decompression has been reported with a low rate of pseudoarthrosis (11.4%). Others have used adjunctive fixation with iliac screws and/or transvertebral screws. The most commonly performed operation includes posterior instrumented fixation and fusion, an attempt at partial deformity reduction, and interbody structural support. However, there is no definitive consensus proving the superiority of one approach over another.

Spondylolysis and Isthmic Spondylolisthesis

Spondylolysis is a fracture in the pars interarticularis and can be found in 5% to 6% of the population. Isthmic spondylolisthesis results from elongation or traumatic fractures of the pars interarticularis. In time, the anterior shear forces at the level of the listhesis eventually cause degeneration of the disc at the slipped level ( Figs. 150.3 and 150.4 ) . The pediatric population is more likely to be affected by isthmic spondylolisthesis. Nevertheless, symptoms most commonly develop between the third and fifth decades of life. High-grade isthmic spondylolisthesis in adolescents tends to have an unfavorable natural history, with a high risk of progression and low likelihood of improvement with conservative management. ^, Fusion is the preferred operation for patients with isthmic spondylolisthesis. Decompression alone is rarely used because it has been associated with poorer outcomes, up to a 27% slippage progression rate, and accelerated disc degeneration. A prospective randomized trial followed patients (aged 18 to 55 years) with lumbar isthmic spondylolisthesis (of any grade) with persistent symptoms for at least 1 year. These patients were randomized to posterolateral fusion or to an intensive exercise program; those randomized to posterolateral fusion were additionally randomized to include adjunct transpedicular instrumentation versus autologous iliac crest bone graft alone. At both 1 year and 2 years, those treated surgically had significantly improved pain scores and disability indices compared to those in the exercise group. In a long-term follow-up of the same patients (9 years after surgery), surgical patients had significantly better global assessment and self-satisfaction. Many studies have reported excellent results in patients with isthmic spondylolisthesis who underwent a fusion. ^{, ,} A 2005 meta-analysis showed a significant benefit to instrumentation; fusion rates were higher in patients who underwent instrumented fixation (90% vs. 77%), as was clinical success (85% vs. 64%). Instrumentation can also aid in listhesis reduction, when attempted, which has also been argued to improve clinical outcomes in patients with isthmic spondylolisthesis. However, the lack of statistical significance between the approaches in terms of fusion rates and outcomes has been reported, even in the long-term follow-up. ^,

The addition of interbody fusion to posterolateral fusion is also debated. Good outcomes, including higher fusion rates, have been reported with PLIF. A systematic review of 1538 patients from six randomized controlled trials suggested no significant difference among patients who underwent PLIF versus PLF in terms of outcomes of pain, function, and complication rates at up to 3 years follow-up. However, PLIF cases were associated with better outcomes in the same measures at longer follow-up points. A limitation of this review was that only four studies included patients with slips greater than grade II. A prospective trial found better clinical outcomes at 6 months and 1 year after surgery in those treated with a combination of anterior lumbar interbody fusion (ALIF) and posterior instrumentation compared to those treated with posterolateral fusion alone, although the benefit was not present at 2 years. Reviews have disagreed on the potential benefit of circumferential fusion, ^{, ,} and thus the addition of an interbody fusion is left to the surgeon’s experience and the patient’s pathology.

Spinal Stenosis Without Spondylolisthesis

Spinal stenosis describes a narrowed spinal canal with neural compression and is mainly classified into degenerative and congenital. Degenerative spinal stenosis typically occurs in middle-aged and older adults as a result of ligamentum flavum hypertrophy or buckling and/or facet joints arthropathy. Anatomically, spinal stenosis is classified into central stenosis, lateral stenosis, and foraminal stenosis. Depending on the location and the level of stenosis, patients may describe back pain, radicular pain, numbness, occasional weakness, and neurogenic claudication. Symptoms may improve with flexion (shopping cart sign); symptomatic patients should be evaluated by MRI, with or without additional CT scans. ^, The natural history is characterized by gradual onset and slow progression. Differential diagnoses include peripheral vascular disease, peripheral neuropathy, and degenerative conditions of the hip.

In the absence of discrete weakness and/or bowel or bladder dysfunction, the initial treatment is conservative in a stepwise approach. Surgery is reserved for those who have intractable pain or severely reduced quality of life and who have failed conservative therapy. The goals of surgery include pain relief, improved mobility, and prevention of neurologic deterioration. This is often accomplished by decompressive procedures, including laminectomies or laminotomies, partial facetectomies, foraminotomies, with or without discectomies. A fusion is added if there is concern for spinal instability or likely iatrogenic instability from extensive laminectomy or facetectomy ( Figs. 150.5 and 150.6 ) . ^,

The SPORT study included a trial of patients with lumbar spinal stenosis in the absence of spondylolisthesis who were randomized to decompression surgery without fusion or standard nonoperative care. In this trial, 289 patients were enrolled from 13 centers across the United States. The trial also had significant crossover between the randomized cohorts: the as-treated analysis, which negated the randomization but may be more indicative of outcomes in the American surgical candidates, showed a benefit to surgery in all primary outcomes that was sustained at 2 years. At 8 years, an analysis of 56% of patients who remained enrolled in the study showed that those who received surgery had significantly greater improvements in pain and function compared to the nonoperative treatment group. Intriguingly, the type of the techniques used for fusion (noninstrumented posterolateral fusion, instrumented posterolateral fusion, and 360-degree fusion) did not affect outcomes.

In the Maine Lumbar Spine Study, 148 patients with lumbar stenosis were followed at 4 and 8 to 10 years to compare outcomes of surgical treatment to conservative management. Those treated surgically (81 patients) had significantly better back or leg pain and overall satisfaction when followed up to 4 years, although the improvement seen with surgery was greatest initially. At 8- to 10-year follow-up, improvement in leg pain and greater back-related functional status were seen in those treated surgically; however, by this time, improvements in low back pain and patient satisfaction were similar in those with and without surgical intervention. Predictors of good outcome after decompression may include greater walking capacity, milder symptoms, and fewer comorbidities. The improvement of radiculopathy symptoms after decompression in patients with lumbar stenosis is well supported in the literature. There is a lack of high-level evidence for the benefits of decompression only in patients with low back pain.

Fusion or stabilization is not routinely recommended in the absence of spondylolisthesis, scoliosis, or sagittal malalignment. Supplemental fusion can increase operative times, blood loss, and complication rates as well as costs. One prospective study evaluating the benefit of laminectomy with or without arthrodesis in the treatment of lumbar spinal stenosis found that those with a fusion had a significant improvement in back pain at 2 months compared to those treated with decompression alone; however, in all other outcome measures there was no benefit to the addition of a fusion. Another study with longer follow-up found no significant benefit to the addition of either noninstrumented or instrumented fusion to decompression. Thus, the most recent guidelines on the management of patients with spinal stenosis without spondylolisthesis recommend decompression without additional fusion in patients without evidence of instability.

The National Neurosurgery Quality and Outcomes Database (N2QOD) group investigated outcomes of patients with lumbar stenosis and a significant low back pain, in the absence of spondylolisthesis or scoliosis, who underwent decompression without fusion or stabilization. They found that back pain improved significantly based on functional outcome measures (EQ-5D and ODI). The authors concluded that the presence of back pain in patients with lumbar stenosis should not be “a contraindication” to decompression-only surgery. One major limitation of the study is the short-term follow-up time of 1 year. Pearson and colleagues analyzed the data of SPORT trial to determine the differences between surgical and nonoperative outcomes in patients with spinal stenosis. Based on the results of multivariate analysis, the following factors were associated with a more improved outcome with surgery than with nonoperative treatment: baseline ODI ≤56, not smoking, neuroforaminal stenosis, not lifting at work, and the presence of a neurological deficit, regardless of other specific characteristics.

Herniated Lumbar Disc

Patients with persistent sciatica due to disc herniation amenable to conservative treatment should be considered for surgery. Several randomized controlled trials, including the Maine Lumbar Spine Study and the SPORT trial, showed that surgical intervention is associated with outcomes superior to nonoperative treatment. In SPORT trial, pain and function improvement following surgical intervention persisted over 8 years. Moreover, pain improvement within 3 months after decompressive surgery was associated with similar improvements in disability, which in turn correlated with improvement in the overall physical health at 12 months.

The standard operation for herniated disk is open discectomy, which is discussed more extensively in the chapter covering lumbar discectomy. The role of a fusion operation in patients with disc herniation is debated. Most commonly, patients are treated with discectomy alone, and good outcomes with a long period of follow-up have been described. , Some have advocated that a fusion also be performed in conjunction with discectomy for the primary treatment of patients with radiculopathy due to a herniated disc, although this is rarely performed today and is not our practice. Prospective trials that have compared discectomy to discectomy and posterolateral fusion in patients with radiculopathy (both primary and recurrent) have found no benefit to routine fusion in patients without instability, deformity, or chronic low back pain. ^{, ,} Patients who have a large, centrally herniated disc, causing bilateral radiculopathy or bowel and bladder dysfunction might also require a fusion ( Fig. 150.7 ).

The overall reoperation rate following discectomy for herniated disc ranges from 9% to 15%, with the majority of reoperations occurring in the first postoperative year. Disc re-herniation is the most common reported cause for revision. There are two main surgical options after re-herniation: a revision discectomy, or discectomy combined with fusion. In a retrospective analysis, the N2QOD group assessed outcomes of patients who underwent repeat discectomy (25 patients) versus instrumented fusion (12 patients) at a single institution. They found that both treatment cohorts exhibited similar pain and functional outcome scores (ODI and VAS) and quality-adjusted life year measurements at 3 and 12 months. Another retrospective study with a longer follow-up time (60 to 134 months) found that “the differences in outcomes between disc excision alone and posterolateral fusion groups were insignificant.” Additional fusion is associated with increased intraoperative blood loss, operative time, length of hospital stay, and total cost of the procedure with no clear benefit or superiority to decompression.

Chronic Low Back Pain

About two-thirds of adults have low back pain at some point in their lives. The lifetime prevalence of low back pain has been reported to be between 70% and 85%. The prevalence is expected to rise with aging demography. In the vast majority of patients with low back pain, their pain is alleviated within weeks. A much smaller minority of patients—but still an important segment of the population—have chronic debilitating pain, hindering employment and producing a very poor quality of life.

Low back pain is a symptom, not a disease. The data evaluating surgery, particularly fusion, for patients with chronic low back pain of an unknown etiology are reviewed here. The vast majority of these patients have no accompanying neurologic symptom or radiologic evidence of spondylolisthesis, spondylolysis, spinal canal stenosis, foraminal stenosis, or nerve root impingement by a herniated intervertebral disc. Instead, they present with chronic, debilitating, and unremitting low back pain, and imaging, if notable, is only remarkable for nonspecific degenerative changes.

Management of such patients can be frustrating. Analgesics might only provide moderate pain relief, their side effects notwithstanding. There is some evidence that patients with chronic low back pain improve with rehabilitation including cognitive and behavioral components. There is far less evidence in favor of a plethora of other interventions, including local injections, intraspinal steroid injections, epidural injections, facet joint or intradisc injections, radiofrequency denervation, or spinal cord stimulation. ^, The benefit of a fusion surgery for patients with chronic nonradicular low back pain has been the subject of three randomized, controlled trials ( Table 150.4 ). The evidence, however, raises more questions than it answers.

The first major randomized, controlled clinical trial evaluating the benefit of surgery in patients with chronic low back pain was by the Swedish Lumbar Spine Study Group, which enrolled 294 patients from 19 centers. Inclusion and exclusion criteria are listed in Table 150.4 . Within the limited and generally very precisely defined population (although a surgeon’s interpretation of the anatomic location responsible for pain is quite vague), patients were randomized to nonoperative treatment—equivalent to being on a waiting list for surgery, with no precise intervention—versus one of three operative approaches. The results of the trial highly favored surgery. There was a statistically significant reduction in back pain in those treated operatively, although the clinical benefit was perhaps less impressive because only one-third of patients improved, compared to 7% of the controls. Outcome measures and return-to-work rate also both significantly favored surgery.

The surgically treated patients in the Swedish Lumbar Spine Study were randomized to one of three operations: posterolateral fusion with or without instrumentation or posterolateral fusion with interbody fusion. No significant difference was seen among the three groups in outcome measures. With longer follow-up, patients treated with instrumentation or interbody fusion had significantly higher rates of reoperation.

Given the potential implications of this study, it is not surprising that it has been thoroughly evaluated. ^, Although the trial did not compare surgery to any structured intervention, this might mimic clinical reality. The inclusion and exclusion criteria may be one of the main factors limiting how this study relates to the general population. Their definition of low back pain was, in fact, quite nonspecific ; other criteria, however, were quite specific—such as the requirement that patients be out of work for at least 1 year—and might explain some of the differences seen compared with other trials.

Two other prospective randomized, controlled trials have investigated the benefit of surgery in the same population of patients, finding minimal or no benefit to surgery. ^, The first trial was from Norway and enrolled 64 patients; inclusion and exclusion criteria are listed in Table 150.4 . Patients were randomized to either instrumented posterolateral lumbar fusion or an intensive rehabilitation program. Although there was a significant improvement in outcome measures postoperatively in the group treated with surgery compared with preoperative function, there was no difference between surgery and the rehabilitation group at 1-year follow-up.

The MRC spine stabilization trial compared 349 patients who were randomized to fusion or an intensive rehabilitation program. The minimal inclusion criteria they used were age between 18 and 55 years and at least 1 year of chronic low back pain. There was a significant improvement in the primary functional outcome (ODI) in the patients treated with surgery compared to the rehabilitation group, but the improvement was of questionable clinical significance.

Clearly, the best treatment for patients with nonradicular low back pain with nonspecific degenerative changes found on imaging has yet to be precisely defined. There are many possible reasons the data have been so contradictory. First, it is likely that in the absence of a clear pathologic etiology, such as spondylolisthesis, patients grouped into the category of “low back pain” probably have different foci responsible for their symptoms. If surgery is considered, patient selection is key, as well as understanding that the best scenario is likely only a moderate improvement in pain and function. Another reason the data are so divergent may be the wide variability in inclusion and exclusion criteria (see Table 150.4 ): The MRC spine stabilization trial included most patients who had back pain of unknown etiology for longer than 1 year; the criteria for the Swedish Lumbar Spine Study Group trial were more stringent. Because any benefit to surgery is likely to be only partial, some of their criteria—including disability preventing patients from working for at least 1 year, failure of conservative management, and persistent pain for 2 years—may in fact be quite critical to appropriate selection.

Smoking and Fusion

Smoking may negatively affect the spinal fusion outcomes. Studies have shown that smoking significantly increases the rates of nonunion and perioperative complications such as infection and adjacent segment disease. The ability of cigarette smoke to hinder fusion has been seen in a number of animal models and may be related to alterations in gene expression, disruption of angiogenesis during the healing process, or due to the antiinflammatory effects of smoking. Glassman and colleagues showed a significantly increased rate of pseudoarthrosis in patients who continued to smoke after surgery (14.2% in nonsmokers vs. 26.5% in smokers); lower return-to-work rates were also seen in that population. However, this difference was not present in patients who quit smoking for more than 6 months after their operation compared to nonsmokers. In fact, smoking was identified as an independent indicator for a higher probability of undergoing a revision spinal fusion surgery, based on the United States Nationwide Inpatient Sample (NIS). A retrospective review of 581 lumbar surgery cases showed that smoking is significantly associated with an increased estimated blood loss (with a mean of 328 mL more for each pack per day smoked) and perioperative transfusion rates. Osteoinductive biological bone graft alternatives might partially overcome the inhibitory effects of smoking. A study evaluating the fusion rates using rhBMP-2 versus iliac crest bone graft in smokers and nonsmokers showed an increased rate of fusion in smokers treated with rhBMP-2 over iliac crest bone graft (95.2% vs. 76.2% respectively), but smokers still had consistently poorer clinical outcomes, even if treated with rhBMP-2. Although these results are promising, smoking cessation is the best way to improve fusion rates.