Although the literature regarding decompression with and without fusion is fairly supportive on the side of fusion, the use of instrumentation to augment fusion is less concrete. The driving force behind the use of instrumentation is an attempt to create a more rigid environment affording a greater potential for fusion.

Zdeblick et al. compared noninstrumented posterolateral fusion and two different types of instrumentation. These results revealed that the use of semi-rigid instrumentation afforded a fusion rate of 77 %, significantly greater than the 65 % fusion rate seen with noninstrumented fusions. Rigid fixation improved fusion rates to 95 %. Clinical outcomes demonstrated 89 and 95 % good or excellent results for semi-rigid and rigid instrumentation although no statistical analyses were performed on these results. However, a limitation of this study is its relatively short follow-up of 1 year [16].

One of the largest retrospective reviews to date was accomplished through the pooled data of more than 314 spine surgeons performing fusion surgery over a 1 year period from January 1991 through December of that year. The results of 2,684 patients demonstrated improved fusion rates with instrumentation over noninstrumented fusion, 89 % vs. 71 %, respectively. However, due to its retrospective nature, numerous baseline differences among treatment groups existed (i.e., age at time of surgery, previous operations, workers compensation status) which make inferences on clinical outcomes difficult [20]. In the meta-analysis by Mardetko et al., 239 patients across nine studies were reviewed which received various methods of instrumentation. Posterolateral fusion with instrumentation yielded between 93 and 95 % fusion rates. This did not significantly increase the rate of satisfactory outcomes compared to noninstrumented fusions [8].

A prospective randomized study of 68 patients with degenerative spondylolisthesis by Fischgrund et al. analyzed outcomes comparing decompression and noninstrumented posterolateral fusion with decompression and an instrumented fusion at 2-year follow-up [21]. Fusion rates were significantly higher in instrumented patients compared to noninstrumented fusions, 83 % vs. 45 %, respectively. Clinical outcomes in terms of pain relief and activity level were not significantly different between the two groups (76 % vs. 85 % excellent/good results).

A more recent meta-analysis was reviewed which analyzed randomized control trials and comparative observation studies between 1966 and 2005. Six studies were included in the review, including three observational and three randomized studies. Data from these studies revealed a significantly improved relative risk (RR) of achieving a solid fusion using instrumentation over noninstrumented fusions. The randomized studies demonstrated a greater RR of achieving solid fusion than did the observational studies (1.96 vs. 1.20) highlighting the importance of randomized control studies. The authors went on to conclude that although clear evidence exists that fusion rates are improved by the addition of instrumentation, however, current literature has been unable to show a definitive clinical improvement using an instrumented fusion over a noninstrumented fusion [22].

As mentioned previously, the SPORT trial’s surgical intervention arm in the treatment of degenerative spondylolisthesis was decompression with or without fusion, with or without instrumentation, and with or with anterior column support. A 4-year analysis of those treatments was reviewed [23]. Clinical outcomes demonstrated significant improvements in pain with an instrumented fusion over noninstrumented fusion at short-term follow-up (1 and 2 year) but this difference was no longer evident at 3- and 4-year follow-up. Physical function scores were not significantly different at 3 and 4 years follow-up but some small differences in favor of instrumentation with anterior support were seen at 2 years. Fusion rates (assessed mostly with X-ray) demonstrated at 67 % rate for noninstrumented fusion, 85 % for a posterolateral instrumented fusion, and 87 % for an instrumented fusion with anterior support. However, the follow-up in Fischgrund et al. and the SPORT trial is 2 and 4 years, respectively. Longer term follow-up may be the deciding factor in a determination of whether an instrumented solid fusion will offer improved clinical outcomes over a noninstrumented solid fusion.

The use of anterior interbody support (Fig. 22.2) is rooted in several theoretical advantages: additional surface area for fusion, biomechanical stability, improved sagittal alignment, and indirect reduction of neuroforaminal and central stenosis by restoration of disc height. There are a number of techniques utilized to obtain anterior support including: anterior lumbar interbody fusion (ALIF), posterior lumbar interbody fusion (PLIF), transforaminal lumbar interbody fusion (TLIF), and the direct lateral or transpsoas interbody fusion (DLIF). Each technique is associated with its own set of challenges and morbidity which is beyond the scope of this chapter. Most studies to date comparing the utilization of anterior support versus instrumented posterolateral fusion are retrospective case series of a mixed cohort of patients making interpretations to its effectiveness difficult. No high quality randomized controlled trials in the setting of degenerative spondylolisthesis have been done to assess whether the additional of anterior support improves clinical outcomes over those seen with a solid posterolateral fusion.

Biomechanical studies have shown improved stability with the use of anterior support. A study on calf specimens that underwent nondestructive flexion-extension testing was reviewed. In order to examine the effects of different types of reconstruction on adjacent levels, the L5-S1 level received pedicle screw fixation with and without anterior support. The contours of the rods were bent in order to duplicate restoration of sagittal alignment providing the theoretical advantage of anterior support. This was compared to kyphotically placed rods with and without anterior support. Restoration of sagittal balance with a lordotic PLIF/posterior instrumentation construct decreased motion across the operative segment but also yielded the highest strain at the adjacent level disc space. Compared with a kyphotic posterolateral fusion, PLIF may lead to even higher load at the superior adjacent level because of the increased stiffness of the fixed segments even if local kyphosis is corrected by PLIF [24]. This increased strain at adjacent levels will be discussed later in this chapter during the discussion of dynamic stabilization devices.

Yashiro et al. retrospectively reviewed 58 patients with degenerative spine disease (31 with degenerative spondylolisthesis) treated with instrumented posterolateral fusion or a PLIF. They reported a 60 % union rate in instrumented posterolateral fusion versus 91 % union rate in PLIF group. This lower union rate in the posterolateral fusion group is lower than other reported series for instrumented posterolateral fusion and was not explained in the article. The authors did report improved sagittal alignment and maintenance of disc height in the PLIF group. No clinical outcomes, however, were reported in this study [19].

A retrospective review of 85 patients with degenerative lumbar spine disease, not isolated degenerative spondylolisthesis, and radiologic evidence of instability was reviewed. Fifty-five patients were treated with an instrumented posterolateral fusion alone and 30 received PLIF/posterolateral fusion. The patients were followed for a mean period of 32 months. Of these patients, 86 % improved with respect to their pain symptoms, but only 46 % had a good to excellent overall result. Patients treated with a posterolateral fusion plus PLIF did not demonstrate superior clinical outcomes compared to those with a posterolateral fusion alone [25]. A study of 35 isthmic spondylolisthesis patients receiving either a posterior lumbar fusion (n = 18) or a posterior lumbar fusion and PLIF (n = 17) demonstrated correction of subluxation, disc height, and foraminal area in the group in which a PLIF procedure was performed, but not in the posterolateral fusion-only group. Again, no statistical differences were demonstrated in terms of neurological or functional improvement or in terms of fusion rate; follow-up was only 2 years [26].

Lauber et al. performed a prospective study on a mixed cohort of patients suffering from degenerative (n = 19), isthmic (n = 19), and dysplastic (n = 1) spondylolisthesis. Follow-up at 2- and 4-year intervals was presented. Overall fusion rate was 94.8 % and this study did not separate out the different pathologies. The functional outcomes and pain scores showed rapid deterioration to baseline pre-operative levels at 4 years. However, in subset analysis, those patients with an isthmic spondylolisthesis had significantly better clinical improvement, highlighting the importance of well-designed studies, as not all types of spondylolisthesis are similar [27].

A prospective randomized trial investigating instrumented posterolateral fusion, PLIF, and posterolateral fusion/PLIF was performed on mixed cohort of degenerative spine patients (42 with degenerative spondylolisthesis). No differences in union rates were found amongst the groups at 2 years (92 %, 95 %, and 96 %, respectively). Again, although the groups undergoing a PLIF demonstrated improved sagittal alignment and disc height, this did not result in improved clinical outcomes [28].

With the lack of randomized, prospective studies comparing anterior support to posterolateral fusion in degenerative spondylolisthesis, its undertaking should be done with caution. Although posterolateral fusion compared to decompression alone has been associated with increased morbidity [29–31], the addition of anterior support further adds to this risk. Reported rates of complications with addition of anterior support have ranged between 8 and 80 % [32]. Although many of the same complications exist with decompression and instrumented posterolateral fusion (i.e., dural tear, hardware breakage, etc.), the increased technical demands associated with the addition of anterior support lend itself to an increased risk of these complications. Again, an examination of the risks associated with these procedures is difficult to account for due to low quality evidence available and differences in reporting major and minor complications.

In the current health care setting, increasing attention is being placed on cost utility of additional procedures, devices, and technologies. In a 10-year study, Kim et al. demonstrated that fusion with instrumentation significantly increases cost per quality adjusted life year (QALY). Compared with decompression alone, decompression plus instrumented fusion was associated with an improvement in quality of life at a cost of $185,878 per QALY [33]. In a health care system with limited resources, this additive cost may be prohibitive especially in the setting of limited clinical improvement afforded by these additional devices.

Historically, spinal fusions were augmented with autogenous bone graft, either local bone from the decompression or from the iliac crest. However, for longer spinal fusions or in revision situations, autogenous grafting can be inadequate. Addition-ally, harvest site morbidity has always been a source of debate. Although various bone graft substitutes and extenders exist, none has made more of an impact than the use of bone morphogenetic proteins (BMP). Recombinant human derivatives of BMP 2 and 7 (rhBMP 2 and rhBMP7) have both received limited FDA approval for the use in spinal fusion, of which rhBMP2 is most widely used. Controversy exists regarding the complications that exist, which may have been under-reported in the original studies [34, 35].

There have been several well-designed studies that highlight the success of BMP2 in achieving posterolateral fusion in the setting of degenerative spondylolisthesis. In the study by Boden et al., groups were compared utilizing rhBMP2 with and without posterior pedicle instrumentation as well as a group receiving autograft and instrumentation. Although a small study of only 25 patients, it demonstrated improved rates of fusion with the use of BMP as well as quicker improvements in clinical outcomes. The group undergoing posterolateral fusion without instrumentation showed the quickest improvement in clinical outcomes. Interestingly, this group also had a 100 % fusion rate while affording the shortest operative time by avoiding the instrumentation [36]. In 2009, a study of 463 patients with degenerative spondylolisthesis was reported in which one group received instrumented posterolateral fusion with rhBMP2 (n = 239) and another group received an instrumented posterolateral fusion with iliac crest bone graft (n = 224). At 2-year follow-up, the rhBMP2 group had a 96 % fusion rate compared to 89 % for the iliac crest group, which was statistically significant; however, clinical outcomes did not show superiority for either group. Both operative time and blood loss were lower in the rhBMP groups and in this study, 60 % of patients reported continued pain at the donor site at final follow-up. The authors concluded that given the improved fusion rates and equivalent clinical outcomes, the use of rhBMP2 is advantageous by avoiding the donor site morbidity of iliac crest bone graft [37].

The use of rhBMP7 has also been shown advantageous in augmenting a posterolateral fusion. In a prospective, randomized, multicenter study, Vaccaro et al. compared the use of rhBMP7 to autogenous iliac crest bone grafting in 36 patients undergoing posterolateral fusion without instrumentation. At 2-year follow-up, 55 % of patients receiving rhBMP7 achieved a solid fusion compared to 40 % of autograft patients. Improvements in clinical outcome measures were found to be similar between groups [38]. In a 4-year follow-up of the same patients, fusion rates in the rhBMP7 and autograft groups improved to 68 % and 50 % respectively, with similar improvements observed in clinical outcomes. Unfortunately, small numbers of patients available at final follow-up precluded the formation of statistically significant results [39].

The original studies leading to FDA approval of both rhBMP 2 and 7 reported almost no adverse events steaming from the use of BMP during surgeries involving either anterior support [40–42] or posterolateral fusion [37, 43, 44]. Although these studies were of high quality design (randomized, prospective), they were biased by their industry sponsorship. Since these original studies, there have been numerous complications reported with the use of BMP. A recent review was performed which highlights some of the major adverse events associated with the use of rhBMP2 [34]. The authors reviewed the original peer-reviewed literature as well as publicly available FDA databases and summaries associated with rhBMP2. In those studies involving posterolateral fusion, there appeared to be increased morbidity with the use of BMP in the early post-operative period relating to increased pain scores, functional outcomes, and wound complications compared to those patients undergoing iliac crest bone graft harvest. This result is counterintuitive in that the treatment effect of not performing the additional surgery associated with iliac crest bone harvest should be highest in the early post-operative period in favor of those patients receiving rhBMP. Published FDA documents also show higher rates of adverse events related to back and leg pain in rhBMP2 patients compared to controls (16 % vs. 4.8 %, respectively) which was not discussed in any of the peer-reviewed articles. Other reported adverse events include end-plate osteolysis and graft subsidence with anterior support devices, radiculitis, bone overgrowth, and increased rates of delayed infection.

Despite its popularity for the treatment of a wide range of spine disorders, lumbar fusion is not without its pitfalls. Biomechanical studies have demonstrated that spinal fusion increases intradiscal pressures in levels above and below the fusion and that the pressure increases with the number of levels fused [24, 45]. This additional strain brings the added concern for development “adjacent segment degeneration” (Fig. 22.3). However, clinical trials to date have not definitively determined whether this adjacent segment degeneration is a result of the fusion [46, 47] or if it represents the natural history of degenerative disease [48]. There is also added concern that should revision surgery be necessary for adjacent segment disease, a previous fusion may lead to higher rates of pseudoarthrosis [49]. These concerns have led to development of motion preserving technologies in hopes of controlling motion, as opposed to a fusion which attempts to completely inhibit motion, thereby theoretically decreasing the incidence of adjacent segment disease.

Stay updated, free articles. Join our Telegram channel

Join