This analysis of a subset of the SPORT DS patients demonstrated only modest associations between baseline radiographic parameters and surgical and non-operative treatment outcomes. In this study, we have focused on three radiographic measurements: the degree of listhesis, disk space narrowing, and hypermobility at the affected motion segment. We did not evaluate other radiographic factors that may predispose patients to DS, such as facet orientation and segmentation abnormalities,39,40
and this study did not allow for measurement of deformity progression over time.
The effect of listhesis grade on clinical outcomes has not been carefully analyzed in DS.4,5,9
Sengupta and Herkowitz recommended instrumented fusion for the uncommon case of DS where the slip exceeds 50%, but they did not make specific recommendations for Grade I and II listhesis.6
In accordance with prior studies, we found the Grade II listhesis group included a higher proportion of women and low disk height patients than did the Grade I group.6,8
We also found the treatment effect of surgery was significantly greater for Grade II than Grade I patients at one year. These data suggest a high grade slip portends a poorer outcome with non-operative treatment.
Disk height was generally not associated with outcomes in either the surgically or non-operatively treated patients. Patients with low disk height were older, had baseline physical function scores indicative of more severe symptoms, and, as noted, were more likely to have Grade II slips. Some investigators have suggested that loss of disk height, although indicative of greater degeneration, could lead to “re-stabilization” and diminution of back pain over time.8,28,41
However, that same process could be associated with worsening stenotic symptoms due to advancing facet degeneration accompanying the disc space narrowing. This study does not shed light on that question.
Of the three radiographic parameters evaluated, hypermobility (referred to by some as “segmental instability”) has received the most attention in the literature. Many investigators have suggested instrumented fusion is most appropriate for hypermobile DS patients despite a lack of rigorous outcome data to support this.6,9-11,13,14
Surgeons participating in the current study appear to have been influenced by this recommendation, insofar as instrumented fusion was more commonly performed in patients with increased angular and translational movements.
Our mobility analyses did yield some unexpected results. First, the hypermobile group had a significantly lower proportion of females compared to the stable group, a finding contrary to the traditional belief that women are more likely to have segmental hypermobility than men. However, our findings are consistent with those of McGregor et al who found that women had a smaller total lumbar flexion-extension range of motion compared to men.42
Second, radiographically hypermobile patients had better non-operative treatment results than stable patients. Comparison of this finding to prior results is limited since no prior DS study has compared surgical and non-operative outcomes stratified by baseline radiographic mobility. In a small study that combined spinal stenosis and DS patients, Yone et al demonstrated that unstable patients undergoing decompression with instrumented fusion had better results than unstable patients undergoing decompression alone.13,14
In the current study, we were unable to compare surgical outcomes among hypermobile patients who underwent decompression alone, decompression with uninstrumented fusion or decompression with instrumented fusion because of the small sample sizes.
This unexpected finding does bring into sharper focus the long-unanswered question, “What constitutes instability in patients with degenerative diseases?” Traditionally, DS has been viewed as the prototype for degenerative instabilities because it was the condition most associated with the four criteria for instability at a motion segment: pain, translational and rotational hypermobility, risk for progressive deformity, and the potential for neurologic insult.21,43,44
Because we have not obtained follow-up radiographs, we cannot be certain if the deformity increased or if progressive listhesis was associated with increased pain. However, we do know that symptoms decreased over two years with non-operative treatment, particularly in those patients with stringently defined radiographic hypermobility. While surgery resulted in better outcomes for both hypermobile and stable patients, hypermobility should not be considered a contra-indication to non-operative treatment. Given that hypermobile patients improved with non-operative treatment, the need for fusion in these patients, particularly those with significant medical comorbidities, should be further explored.
The current study has some limitations. Although all patients enrolled in the study had spinal radiographs, we were able, for logistic and economic reasons, to digitize and analyze images from only about one third of the patients. Comparison of baseline characteristics of the patients with available images to those not included suggested that the two groups were very similar, so we felt that the convenience sample was fairly representative. Another issue was the relatively small proportion of patients that were classified as having Grade II listhesis, hypermobility or low disk height. These relatively small subgroups may have limited our power to detect significant differences.
A statistical limitation related to the overall SPORT study has been the substantial cross-over between treatment arms.23
In our prior publications, we have addressed this concern and the rationale behind the decision to perform an as-treated analysis using multiple regression models to control for baseline differences.23,37
Despite this approach, it is possible the current analysis could be vulnerable to residual confounding by unmeasured variables.
Similar to prior spinal radiographic studies, the criteria for classifying listhesis, disk height, and mobility were somewhat arbitrary, although they were based on literature standards.25,27,30
The greatest continuing controversy is the radiographic definition of instability.21,43,45-49
Some authors have suggested that there is substantial overlap in radiographic findings between “unstable” and “normal” subjects.50,51
For this reason, we chose a stringent radiographic definition of hypermobility. We also performed analyses in which the listhesis, disk height, and mobility groups were defined by the median value for each characteristic and found similar results (data not shown). The more perplexing problem is that anterolisthesis can be found in 10% of females over age 65, yet the majority of patients are asymptomatic.52,53
In addition, radiographic signs of “instability” do not correlate with symptoms.45,54
SPORT utilized strict clinical and radiographic inclusion criteria, and we wish to emphasize that our findings do not apply to patients without both clinical signs and symptoms of spinal stenosis and associated radiographic abnormalities.
Finally, obtaining reliable radiographic measurements can be difficult.55
Given that the intra-class correlation coefficients for the measurements used in this study ranged from 0.89 to 0.93, we feel that the measurement technique and classification system were reasonably reliable.31
Despite its limitations, the current study has identified associations between baseline radiographic findings and outcomes in DS patients which should be useful to clinicians caring for these patients. Patients with Grade II listhesis had a greater treatment effect of surgery compared to Grade I patients at 1 year, suggesting that surgery is more strongly favored in patients with higher grade slips. The other significant and unexpected finding was that hypermobile patients had better non-operative outcomes than did stable patients. This indicates that hypermobility should not be considered a contra-indication to non-operative treatment, and leaves open the questions “Who benefits most from fusion?” and “Which patients require instrumented fusion?”
- The study evaluated whether baseline radiographic findings predicted outcomes in patients with degenerative spondylolisthesis (DS).
- Regardless of listhesis grade, disk height or mobility, patients who had surgery improved more than those treated non-operatively.
- These differences were due, in part, to differences in non-operative outcomes, which were better in patients classified as Grade 1 or Hypermobile.