The overall mean percent change of listhesis per year was 1.30% (95% confidence interval − 0.17 to 2.78). Using the upper limit of the confidence interval, we observed a percent advancement of listhesis of up to 2.78% per year. This included patients who saw no change or a slight improvement in listhesis. One large observational study calculated the percent advancement of degenerative spondylolisthesis to be 2.6% per year (12%/4.6 years), but included those with greater than 5% slippage (13). This was observed in 12% of patients out of those with initial spondylolisthesis at the beginning of the observation period. We saw a progression in 5 patients out of 14 with baseline spondylolisthesis, with an assumption of a meaningful advancement occurring beyond 1.25% per year. Nine patients did not meet this level of progression, despite having had the RFA intervention. Looking at just the 5 patients with meaningful progression, we calculated a median progression of 3.48% (Range: 2.99–6.50%). Using a similar crude calculation for the Matsunaga study, their observed slip progression rate was 0.99% (15.6%/15.8 years) in 34% of patients with degenerative spondylolisthesis managed non-operatively (14). We conclude that our findings are sufficiently similar to the natural slip progression of degenerative spondylolisthesis ranging from 0.99%-2.6% as best as we could define it. The neuroablative procedure, lumbar medial branch nerve RFA does not destabilize the spine by advancing spondylolisthesis in this patient population. Some paraspinal muscle atrophy likely occurs post-procedure as previously shown, but is unlikely to contribute to further lumbar spine instability in the form of progressing degenerative spondylolisthesis (18).
We did observe a slight decrease in spondylolisthesis over time in 6/14 (43%) of patients. We believe that this may have been mostly due to the small margin of error in measurement of listhesis that is on the order of millimeters. Several sources of error may occur to produce this: body position effects such as recumbency or rotational, variability in imaging resolution, or calibration across the different imaging modalities used. Improvement in spondylolisthesis may theoretically occur with strengthening of lumbar paraspinal and abdominal muscles as studies associate weakening or atrophy of these muscles to developing degenerative spondylolisthesis (23,24).
Most patients in our study had greatest listhesis over L4-5 level which is consistent with previous epidemiological studies (2,7,21). Median BMI in our study was 33.5 which is categorized as obese. BMI as a risk factor for degenerative spondylolisthesis remains controversial (7,8). It has been theorized that excessive weight may exacerbate load and shearing forces on the spine and contribute to degenerative spondylolisthesis. Conversely, BMI may also contribute to degenerative disc disease, facet overlap with osteophytosis, and ossification of ligaments which may facilitate spondylolisthesis stabilization (1,22). The ratio of female: male in our study was 9:5. Though our sample size was too small to confirm the prevalence gender ratios reported in other studies, it is in accord with epidemiological studies reporting degenerative spondylolisthesis having a greater prevalence in women (4,5,7).
Study Limitations
This study has several limitations. First, the sample size is small. Hundreds of patients were screened through our center to identify patients with lumbar pain originating from facets undergoing RFA with coexisting spondylolisthesis and good quality baseline imaging within 4 months of RFA. Several international studies report a low prevalence of degenerative spondylolisthesis ranging from 12% up to 30%. This may be observed in the general American population as well and a small percentage will have pain, specifically of facet etiology. Second, our study is from a single institution. Current findings thus reflect on the clinical practices of the institution, which may limit the generalizability of the results. Third, in some patients, the pre- and post-RFA imaging was in different recumbency positioning which may affect observed spondylolisthesis to a small degree. This still remains controversial in the medical literature (8, 9). Fourth, there remains limited data on the accepted rate of advancement of degenerative spondylolisthesis per year as a baseline comparison. We found a single study which analyzed such data and used a similar method for calculation applied to another, which can obfuscate comparison studies such as ours (13). Also, we are implicitly assuming a continuous linear progression of listhesis over time. In reality, the rate of degenerative spondylolisthesis progression may be quite variable with numerous contributing factors. Periods of peak progression have been described in prospective observational pediatric populations as well as older adult studies (10, 15). Conversely, a slowing of progression or stabilization of spondylolisthesis has been described in the very elderly population. This was also observed in our study in which 9/14 with baseline spondylolisthesis did not show advancement of slippage beyond 1.25%. However, our median follow up period was 23.5 months, arguably not long enough to capture those with a very slow rate of progression of listhesis, assuming a somewhat continuous progression over time.