Evidence-Based Report on Interbody Fusions of the Lumbar Spine

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).

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.