In patients with image-confirmed degenerative spondylolisthesis and symptoms persisting for at least 12 weeks, the intention-to-treat analysis found no significant advantage for surgery over nonsurgical care, but the analysis was severely limited by treatment crossover. As-treated analyses showed that surgery was superior to nonsurgical treatment in relieving symptoms and improving function. This treatment effect was seen as early as at the 6-week follow-up and persisted over 2 years. The nonsurgical-treatment group showed only moderate improvement over time. The smaller treatment effect for surgery seen in less-educated subjects is intriguing but unexplained and may be a chance finding in a post hoc subgroup.
The randomized and observational cohorts were remarkably similar at baseline. The only significant differences were small ones in level and location of stenosis on baseline imaging. The cohorts also had similar outcomes, with no significant differences between the treatment effects in the as-treated analyses. These similarities support the validity of the combined analysis presented here.
There was little evidence of harm from either treatment. Often patients fear they will get worse without surgery, but the patients receiving nonsurgical treatment, on average, showed moderate improvement in all outcomes. No patients undergoing surgical or nonsurgical treatment had cauda equina syndrome; 89% of surgical patients had no operative complications.
The characteristics of the participants in the SPORT were similar to those in previous studies of degenerative spondylolisthesis and mixed cohorts of patients with stenosis. The mean age of 66 years was similar to that in the cohorts reported by Herkowitz and Kurz8
(63.5 years), Fischgrund et al.7
(67 years), the Maine Lumbar Spine Study (MLSS)3
(66 years), Yukawa et al.26
(63 years), and Malmivaara et al.5
(63 years). At enrollment, 60% of the SPORT participants reported having had symptoms for more than 6 months, as did 60% of the participants in the MLSS. Baseline functional status in the SPORT was similar to that in the MLSS (mean SF-36 physical function scores, 34 and 35, respectively) and in the randomized trial by Malmivaara et al. (mean Oswestry Disability Index scores, 41.5 and 35.0, respectively).
The surgical outcomes in the SPORT were generally similar to those in previous surgical series. Herkowitz and Kurz8
reported absolute improvements of 33% for back pain and 55% for leg pain (6-point scales) at 3 years, similar to the changes of 31% and 41%, respectively (7-point scales), seen in the SPORT at 2 years. Also, the improvement at 1 year in the patients in the SPORT who were undergoing surgery for degenerative spondylolisthesis was similar to the outcomes of surgery in the MLSS mixed-stenosis (those with and those without degenerative spondylolisthesis) cohort. The improvement in the SF-36 bodily pain score was 32 in the SPORT and 43 in the MLSS, and the improvement in the SF-36 physical function score was 29 in the SPORT and 27 in the MLSS.3
The nonsurgical outcomes in the SPORT were similar to those in the study by Malmivaara et al.5
and in the MLSS.3
Malmivaara et al. reported absolute improvements in back pain at 2 years of 18% on an 11-point scale, as compared with 17% on a 7-point scale in the SPORT, and an improvement in leg pain of 16%, as compared with 17% in the SPORT. Similarly, at 1 year the MLSS reported an improvement of 12.0 points in SF-36 bodily pain, as compared with 12.7 in the SPORT. The nonsurgical functional outcomes, however, were better in the SPORT than in these previous studies. SF-36 physical function improved by 9.6 points in the SPORT nonsurgical group, as compared with 1.0 point in the MLSS, and the Oswestry Disability Index improved by −7.5 points in the SPORT, as compared with −4.5 points in the study by Malmivaara et al. The somewhat greater improvement in the SPORT may be related to the nonsurgical treatments received. The SPORT participants had higher rates of epidural steroid injections than did the MLSS participants (44% vs. 18%), similar rates of physical therapy, and much lower use of activity restriction (16% vs. 29%) and transcutaneous electrical nerve stimulation (2% vs. 14%). Differences in nonsurgical outcomes might also be due to differences in the underlying disease process in patients with degenerative spondylolisthesis as compared with a mixed stenosis population in the study by Malmivaara et al. and in the MLSS.
We can directly compare estimates of treatment effect in the SPORT with those in the MLSS and the study by Malmivaara et al. The estimated 1-year treatment effects for surgery in the SPORT were smaller than those in the MLSS (18.8 vs. 30.4 points for SF-36 bodily pain, and 19.4 vs. 25.5 points for SF-36 physical function). However, the MLSS did not adjust treatment effects for baseline differences between the treatment groups, which probably explains these discrepancies. The estimated 1-year treatment effects were similar in the SPORT and in the study by Malmivaara et al. (Oswestry Disability Index, −17.9 vs. −11.3, respectively; leg pain, 23% [7-point scale] vs. 15% [11-point scale], respectively; and back pain, 20% [7-point scale] vs. 21% [11-point scale], respectively.
The 1-year rate of reoperation for recurrent stenosis or spondylolisthesis was 0.6%, less than the rates reported by Malmivaara et al. (2%) and the MLSS (1.2%). The reoperation rate increased to 3% at 2 years. The perioperative mortality rate was 0.6%, which is less than the 1.3% seen in Medicare patients after fusion surgery for spondylolisthesis.27
The 2-year mortality rate was similar in both treatment groups and less than actuarial projections.
A limitation of this study is the marked degree of nonadherence to randomized treatment. The protocol stipulated that patients assigned to surgery have their surgery within 3 to 6 months after enrollment, a period thought to be appropriate in the clinical experience of the investigators. Although patients consented to this protocol, as in all clinical trials this consent could be changed at the request of the patient, and many chose to do so. This reduced the power of the intention-to-treat analysis to demonstrate a treatment effect. Although the as-treated analysis lost the strong protection from confounding conferred by randomization, these analyses were carefully controlled for important covariates and yielded results similar to previous studies.
Another limitation is the heterogeneity of the treatment interventions. The choice of nonsurgical therapies was at the discretion of the treating physician and the patient. However, with limited evidence regarding efficacy for most nonsurgical treatments for degenerative spondylolisthesis, creating a fixed protocol for nonsurgical treatment was neither clinically feasible nor generalizable. The nonsurgical treatments used were consistent with published guidelines.28,29
Similarly, the surgeries performed varied in terms of the presence, method, and extent of spinal fusion accompanying the decompression. We cannot make direct conclusions regarding the comparison between the effect of surgery and any specific nonsurgical treatment, nor do we directly compare the efficacy of nonsurgical treatment with one specific surgical technique.
The magnitudes of the mean changes reported here after surgery for degenerative spondylolisthesis are less than those reported for patients in a SPORT observational cohort undergoing surgery for intervertebral disk herniation. The mean change scores after 2 years were as follows: SF-36 bodily pain, 29.9 for degenerative spondylolisthesis versus 42.6 for intervertebral disk herniation; SF-36 physical function, 26.7 for degenerative spondylolisthesis versus 43.9 for intervertebral disk herniation; Oswestry Disability Index, −24.2 for degenerative spondylolisthesis versus 37.6 for intervertebral disk herniation.9,10
However, the treatment effects for surgery in the degenerative-spondylolisthesis group were larger than those in the study of intervertebral disk herniation (18.4 for bodily pain in the degenerative-spondylolisthesis group vs. 10.2 in the intervertebral-disk-herniation study) because of dramatic improvements in the nonsurgical group with intervertebral disk herniation not seen in the degenerative-spondylolisthesis group.
In these nonrandomized comparisons with careful control of potentially confounding baseline factors, patients with persistent neurogenic claudication from degenerative spondylolisthesis treated surgically showed substantially greater improvement in pain and function, as well as satisfaction, for 2 years. Characteristics of the patients and treatment outcomes were similar in the randomized and observational cohorts.