Lumbar fusion is a commonly performed procedure for various spine conditions, including spondylolisthesis, degenerative disk disease, dynamic instability, or lumbar spondylosis. Despite numerous studies, significant debate remains regarding the most effective surgical technique for lumbar fusion. Specifically, use of an interbody graft in lumbar fusion is an area of controversy. Although an interbody graft potentially provides the benefits of improved fusion rate and restoration of foraminal height, it is also associated with increased complications and costs. In this chapter, key studies are reviewed which compare posterolateral fusion with an interbody graft versus posterolateral fusion alone in regard to effect on fusion, clinical outcomes, and cost-effectiveness. Together, the current evidence generally supports the use of an interbody graft in lumbar fusion as a cost-effective means to achieve solid fusion and improved clinical outcome. However, further studies are required to confirm the use of an interbody graft as superior to methods that rely on posterolateral fusion alone.
Rationale for Using Interbody Grafts
The use of an interbody graft was first adopted after evidence suggested that persistent back pain may be related to diskogenic disease, and interbody placement may reduce motion-related pain by adding structural stability. Posterolateral fusion has historically been used to reduce segmental motion. However, stabilization of the load-bearing portion of the spine, namely the anterior column, may be more effective in achieving fusion than fusion across the facet joints alone. Interbody graft use in lumbar fusion is supported by a number of class II and III studies which demonstrate increased fusion rate, restoration of foraminal height, preservation of biomechanical properties, reduced reoperation rate, and cost-effectiveness. High quality class I evidence for or against interbody fusion is lacking. Conversely, a number of analyses demonstrate no difference in clinical outcome and emphasize negative effects of interbody grafts, such as increased intraoperative duration, length of stay, and complications. In addition, there are multiple variables of interbody grafts, such as graft composition, bone material within the interbody, and different types of interbody grafts including expandable cages, that have yet to be rigorously evaluated ( Fig. 23.1 ).
Increased Rate of Fusion
The main rationale for using an interbody graft in lumbar fusion is the potential to improve fusion rate. Physiologically, interbody placement provides increased surface area for fusion as well as ample vascular supply following adequate decortication of the endplates. There are two randomized controlled clinical trials and a meta-analysis that support improved fusion with interbody compared with posterolateral fusion alone ( Table 23.1 ). The two key randomized studies supporting interbody fusion over posterolateral fusion are those by Christensen et al. and Fritzell et al. In 2002, Christensen et al. demonstrated a significant increase in fusion rate in circumferential versus posterolateral fusion alone. This randomized clinical trial evaluated 148 patients with severe chronic low back pain who underwent either posterolateral fusion with Cotrel-Dubousset instrumentation (posterolateral fusion [PLF]) or circumferential fusion with posterolateral fusion plus retroperitoneal insertion of a Brantigan cage (anterior lumbar interbody fusion [ALIF]). Patients were followed over the course of 8 years and 1 year postoperative static radiographs were evaluated. For patients who underwent PLF, continuous intertransverse bony bridges on at least one side were assessed, whereas ALIF was evaluated by continuous bone structure surrounding Brantigan cages and absence of migration of the cage. Overall, patients who underwent additional interbody placement for severe chronic back pain demonstrated significantly increased fusion rate (92%) over those with posterolateral fusion alone (80%) ( P <.04). In a similar comparison, Fritzell et al. demonstrated improved fusion with internal fixation when comparing anterior lumbar interbody fusion or posterior lumbar interbody fusion (ALIF/PLIF) versus PLF or PLF with variable screw placement and internal fixation (PLF+PSF). This prospective randomized multicenter study evaluated 294 patients who underwent one of the above treatments (or no surgery) for chronic low back pain. Patients were diverse and collected from 19 different centers. At 2 years after surgery, radiologists evaluated bony fusion on a 3-point scale, ultimately demonstrating that use of interbody graft improved fusion rate (91% and 87%) over posterolateral fusion alone (72%) ( P <.004).
|Level of Evidence||Study Type||Reference||Description||Conclusion|
|I||Prospective randomized trial||Christensen et al., 2002||Prospective randomized clinical study analyzing ALIF vs. instrumented PLF in severe chronic low back pain.||Circumferential fusion demonstrated higher fusion rate with decreased reoperation rate.|
|II||Prospective comparison||Fritzell et al., 2002||Multicenter randomized study comparing PLF vs. PLF with variable screw placement and internal fixation (PLF+PSF) vs. ALIF/PLIF for severe chronic low back pain.||Internal fixation improved fusion rate compared with posterolateral fusion only.|
|I||Prospective randomized study||Kim et al., 2006||Prospective randomized study compared PLF vs. PLIF vs. PLF+PLIF.||No difference in fusion rate between the groups. PLIF demonstrated better sagittal balance than PLF alone.|
|I||Randomized clinical trial||Videbaek et al., 2011||Randomized controlled clinical trial compared ALIF + instrumented PLF vs. instrumented PLF for severe back pain.||No difference in sagittal balance.|
|I||Randomized clinical trial||Thalgott et al., 2009||Randomized clinical trial which compared ALIF+instrumented PLG vs. instrumented PLF for severe back pain.||Freeze-dried graft was inferior to frozen graft with a higher potential for pseudoarthrosis.|
|III||Retrospective review||Hsieh et al., 2007||Retrospective overview of medical records and radiographs to evaluate ALIF vs. TLIF.||ALIF is superior to TLIF in restoration of foraminal height, local disk angle, and lumbar lordosis.|
|II||Prospective nonrandomized clinical study||Oliviera et al., 2010||Prospective study evaluating XLIF for degenerative lumbar conditions with stenosis.||XLIF is an effective method to decompress the spine and increase foraminal height.|
|I||Randomized controlled trial||Soegaard et al., 2007||Cost-utility evaluation of a randomized trial comparing ALIF vs. PLF with instrumentation.||ALIF is significantly more cost-effective over a long-term analysis.|
|III||Retrospective review||Macki et al., 2015||Retrospective review of patients who underwent first-time lumbar fusion.||PLF +PLIF/TLIF was associated with greater spondylolisthesis correction as well as reduced reoperation compared to PLF alone.|
|III||Retrospective review||Bydon et al., 2015||Retrospective cost analysis.||PLIF demonstrated decreased reoperation rate, improved long-term cost savings, and better functional outcomes compared with PLF.|
In addition to these randomized studies, a recent meta-analysis also supports improved fusion with interbody placement. Luo et al. evaluated nine different studies, focusing on the clinical effectiveness of PLIF versus PLF for isthmic spondylolisthesis. In this study, fusion rate was evaluated in 421 patients and found to be significantly lower for PLF than PLIF with an odds ratio of 0.29 (0.14,0.58; P = .005).
Foraminal Height Restoration
Another argument for interbody use is that it restores foraminal height ( Fig. 23.2 ). Hsieh et al. compared the change in foraminal height between two different interbody placement methods. When comparing ALIF versus transforaminal lumbar interbody fusion (TLIF), the authors demonstrated that an anterior approach was superior, restoring foraminal height by 18.5%, whereas TLIF decreased foraminal height by 0.4%. One possible explanation for this suboptimal increase in foraminal height with TLIF may be compression along the pedicle screws with a dorsal approach. Several studies report a significant increase in foraminal height when a direct lateral approach is used. In a small nonrandomized study, patients who underwent eXtreme lateral interbody fusion (XLIF) for central or lateral lumbar stenosis experienced an increase (13.5%) in foraminal height. Similarly, a recent retrospective study by Alimi et al. demonstrated that single-level XLIF was effective for unilateral foraminal stenosis with a significant increase in foraminal height after placement of the graft (18 + 0.5 vs. 11 + 0.5 mm, P <.001).
Biomechanical Principles of Interbody Grafts
Spinal biomechanics are important to consider during lumbar fusion. Use of interbody cages and placing the graft under compression may ultimately improve fusion and healing rate based on Wolff’s law. In addition, regional and global sagittal balance must be either preserved or restored to achieve good outcome following lumbar fusion and reduce the likelihood of adverse complications. Macki et al. demonstrated lumbar fusion with interbody graft (PLIF or TLIF) was more effective in reducing spondylolisthesis (13.06%) compared with traditional posterolateral fusion alone (5.67%) ( P <.001). Kim et al. compared the use of 1- or 2- level PLIF, PLF, and PLIF+PLF in the management of degenerative lumbar disease. After 3 years of follow-up, patients who underwent PLIF or PLIF+PLF demonstrated postoperative improvement in lumbar lordosis and segmental angle ( P <.05, P <.05); however, those who underwent PLF alone did not demonstrate any significant improvement. Similarly, a meta-analysis by Zhou et al. also demonstrated better restoration of spinal alignment with interbody use. Despite these results, there are conflicting studies which evaluated spinopelvic parameters of pelvic tilt and lumbar lordosis, and demonstrated no difference in sagittal balance between PLF and ALIF.
A number of studies have attempted to quantify the level of clinical improvement interbody fusion provides over traditional posterolateral lumbar fusion. In 2007, Soegaard et al. analyzed data from the 2002 Christensen study, demonstrating that interbody fusion was clinically superior to posterolateral fusion alone. In this analysis, the authors included the Dallas pain questionnaire (DPQ), the Oswestry Disability Index (ODI) and overall general health measured by Short Form-36 (SF-36). Those who underwent interbody fusion had a postoperative mean ODI score of 28 compared with 40 for those who underwent posterolateral fusion, suggesting significantly less disability after interbody fusion ( P <.004). SF-36 scores confirmed greater postoperative physical health in those who underwent ALIF compared with PLF (39 vs. 33, P = .005). In addition, at the 5- to 9-year follow-up, those who underwent interbody fusion had significantly less back pain measured by VAS pain scale (3 vs. 6, P <.05). Most recently, a study by Glassman et al. utilized the National Neurosurgical Quality and Outcomes Database (N 2 QOD) to evaluate interbody use (TLIF) versus PLF for isthmic or degenerative spondylolisthesis. Cases within each group were propensity-matched and change in ODI evaluated, demonstrating those who underwent interbody fusion had significant reduction in disability at 3 (19.4 vs. 26.0, P = .009) and 12 months (20.8 vs. 29.3, P = .001) postoperatively. In a conflicting study, Hoy et al. conducted a randomized clinical trial of 100 patients comparing TLIF and PLF for low back pain with radiculopathy. At 2 years after surgery, no statistically significant difference was seen in daily activity, work ability, mental health, or back pain/radiculopathy when using ODI, DPQ, and SF-36.
Use of an interbody graft adds significantly to the implant costs of lumbar fusion, and several studies have evaluated whether this additional cost is associated with improved outcomes. These studies also account for the costs of reoperation. In 2007, Soegaard et al. evaluated the data generated in the 2002 Christensen et al. study from a long-term economic perspective, and concluded that interbody fusion is less expensive from a societal perspective. Specifically, the reoperation rate was significantly reduced with interbody use ( P <.009), with a 7% reoperation rate in circumferential fusion versus 22% in instrumented posterolateral fusion. The lower reoperation rate for circumferential fusion also significantly contributed to its lower cost compared with posterolateral fusion alone ( P <.001). In addition, loss of productivity costs were also greater in patients who underwent posterolateral fusion only, with a reduced rate of return to work (11.1% vs. 44.4%) and reduced workforce productivity ($175,640 vs. $145,883). Additional cost variables favored interbody use, including outpatient visits ( P <.023), comorbidity-related surgeries ( P <.025), and transportation ( P <.008). Similarly, a retrospective review of 103 patients by Bydon et al. demonstrated that interbody use was associated with reduced repeat surgery. This study evaluated patients at 2 years postoperative and compared instrumented PLF versus PLIF/TLIF for spondylolisthesis. The authors demonstrated that PLF was associated with increased (26.51%) reoperation rate compared with lumbar fusion with interbody graft (9.26%) for 137 patients undergoing first-time surgery for lumbar spondylolisthesis ( P = .13).