Degenerative lumbar spondylolisthesis with symptomatic spinal stenosis is often treated with decompression and fusion. In April 2016, the New England Journal of Medicine published results of two conflicting trials on the topic: The Swedish Spinal Stenosis Study1 and the Spinal Laminectomy versus Instrumented Pedicle Screw (SLIP) study.2 The Swedish study concluded that fusion was not beneficial for patients with lumbar stenosis with or without spondylolisthesis. The SLIP study found significant benefit to adding a fusion when performing lumbar laminectomy for treating grade I degenerative lumbar spondylolisthesis. Both studies provided relatively long-term follow-up data.

The two RCTs differed in two important ways: Study Population and Primary Outcome Measure. The Swedish study randomized

247 patients with 1- or 2-level lumbar spinal stenosis with or without spondylolisthesis. The Swedish study did not characterize the radiographic stability of patients. No flexion-extension radiographs were performed. Conversely, the SLIP study contained a homogenous population of non-mobile single-level Grade I spondylolisthesis patients. It is therefore not surprising that the Swedish study did not identify any benefit from fusion based on a heterogeneous study population, while the SLIP study, which randomized a homogeneous group of 66 patients with single-level grade I spondylolisthesis did identify a significant benefit from fusion.

Not only were the study populations different, the primary outcomes of the studies were also different – The Swedish study used Oswestry Disability Index (ODI) as the primary outcome measure, while SLIP used the SF-36 physical component summary (SF-36 PCS). The SLIP study did use ODI as a secondary outcome measure and although the study was underpowered for ODI, patients treated with fusion had less disability over time. At 4 years the difference in ODI scores between groups was 9 points favoring fusion (p=0.05). Using a minimum clinically important difference (MCID) of 10 points for ODI, 85% of patients treated with both fusion and laminectomy achieved the MCID, while only 61% of patients treated with laminectomy alone achieved the MCID (p=0.04).