A Review of Posterior Lumbar Fusion Techniques

Posterior lumbar fusion is a relatively common surgical procedure for pain in the low back. In this surgery two or more levels of vertebra are stabilized with bone grafts, or bone graft substitute. Then movement at those levels is limited to encourage the natural processes to grow bone in the space between the vertebra and subsequently stop all movement at this level. There are many options when planning for such a procedure and there is some debate over the best techniques. This article is a review of some of the more recent research and longer term studies in this area.

In 1991 there was a landmark study by Herkowitz and Kurz which showed that patients with degenerative spondylolisthesis had a better outcome with the combination of a fusion and a laminectomy than with the laminectomy alone. In this research it was noted that some successful outcomes were had even with pseudarthrosis, which means that the fusion was not complete and still allowed slight movement. Due to this study, for several years it was felt that full fusion was not necessary for successful outcome. In 2004 Kornblum et al detailed the long-term results of this same group of patients and found that for long term positive results (five to 14 years follow up) a solid union was more effective. Kornblum showed that the clinical outcome for those with a solid fusion was excellent in eighty-six per cent of patients compared with only fifty-six per cent excellent outcome with a pseudoarthrosis. The evidence presented in this longer term study suggests that gaining a complete fusion improves the clinical outcome for the patients in the long term.

The considerations are many when planning a surgery for a lumbar fusion. The first is whether to use instrumentation, additional hardware such as screws and rods, or not. Some surgeons argue that for older adults non instrumented fusion decreases time of procedure and loss of blood which may be preferable. However, in 1999 a Cochrane review by Gibson et al found that there was strong support in the research for better fusions when instrumentation was used. Newer studies have shown very slight benefits including decreased use of pain medication, and cost versus quality of life measures with instrumented fusions. In this authors opinion a solid fusion from instrumentation is the best choice for the positive long-term outcomes.

Some techniques also use more bone placed between the vertebral endplates, also called inter body fusion, to increase chance of a solid fusion. This technique has been shown by Ito et al to have a fusion rate as high as ninety eight per cent. However a recent systematic review by Lee et al of randomized trials using the Cochrane system of standards found that there is moderate evidence that there is no difference in complete fusion rates between posterior inter body fusions and posterior lateral fusion.

Often a bone graft is also used to improve likelihood of a solid fusion. Currently the gold standard is the use of an iliac crest bone graft (ICBG) due to evidence of between forty per cent and eighty nine per cent successful fusion rates. However with ICBG there is increased blood lost from a second surgical site as well as increased operating time and increased hospitalization time.

Each alternative has some advantages and disadvantages and these must be considered on an individual basis. When a decompression is performed in conjunction with the fusion, this removed bone from the lamina is routinely used with good results in the literature. The drawbacks may include a minimal amount of bone available, and often surgeons elect to increase the amount of bone by adding bone marrow aspirate (BMA) or ICBG. Another option is an allograft which is tissue from a different individual, however there is lots of variability in the research with this technique.

Another substance that has been hypothesized to improve fusion is the presence of bone morphogenetic proteins (BMP). In 2012 Kang et al found eighty six per cent fusion rate with use of BPM from demineralized bone matrix plus laminectomy bone compared to ninety two percent fusion rates for ICBG plus laminectomy bone. In yet another study by Schizas et al these two same procedures showed equivalent fusion rates. There are some recent studies which are shining some problems on BMP, and are focused on the safety of this substance. There has been some concern with increased rates of retrograde ejaculation in men and a recent association with recombinant human bone morphogenetic protein-2 and an increased risk for cancer.

There has been further research in the area of ceramics, platelet gels, and electrical stimulation to aid in fusion surgeries. In animal studies it has been shown that ceramic substrates can have osteopromotive (meaning they attract bone growth) properties and be a substitute for autograft in posterior lumbar fusions. In clinical studies ceramic materials have successfully been used as a bone graft extender with ICBG and fusion rates range from eighty two per cent to ninety six per cent over several different studies. However, it is interesting to note that some of these fusions were assessed with CT scan, and bone and ceramic material have the same density as viewed in a CT scan, so there may be some argument about whether a true fusion is the outcome vs unfused ceramic material. As a bone graft substitute ceramics have mixed results including longer time to achieve radiographic fusion and needing a larger area of bony surface to achieve fusion. There has been some promising animal research with platelet gels, however they have not translated into effective clinical adjuncts for fusion with two studies showing a decrease in fusion rates greater than nineteen per cent compared to fusion without platelet gel. Electrical stimulation is an adjunct to enhance fusion rates and in the research the results tend toward small percentages of benefit or no change.

There are some circumstances which put patients at risk for non union, or pseudarthrosis and they include smoking and increased motion during the healing phases. It is due to these findings that patients are counseled to cease smoking prior to a fusion procedure. A clinical study of two level fusions with laminectomy showed that there was a forty per cent pseudarthrosis rate in smokers compared to eight per cent in non-smokers.

The author of this review study makes the conclusion that the iliac crest autograft remains the best option for gaining a solid fusion, but it’s use can be reserved for long multilevel fusions. There is good evidence that for shorter fusions use of autograft of laminectomy bone in adjunct with cancellous allograft chips and simple BMA is effective without the sequela of a second surgical site. The author feels that there is not sufficient evidence to utilize foreign or man-made proteins or manufactured ceramics due to concerns of complications of unexpected immune reactions. Lastly this author recommends use of electrical stimulation be reserved for problematic cases as a safe and effective adjunct to increase likelihood of a solid fusion.