There was low evidence supporting greater treatment effects for interspinous-device placement compared to decompression for disability and pain outcomes at 12 months. There was low evidence demonstrating little to no difference in treatment effects between groups for walking distance and complication rates. GRADE criteria suggest that low evidence indicates “further research is very likely to have an important impact on the confidence in the estimate of effect and is likely to change the estimate.”
The observations that are seen in this modified network analysis must be taken within the context that no direct comparisons between interspinous spacers and decompressive laminectomy exist. Thus, the network analysis model analyzes best available evidence from the two different treatment studies for an indirect comparison.
This indirect comparison must be carefully analyzed because of the inherent possibilities of different treatment groups (ie, heterogeneity).
The first caveat is the intent-to-treat analysis as opposed to an as-treated analysis. In the Weinstein et al study 1,5
, 43% of the patients who were in the nonsurgical arm crossed over to having surgery at the 2-year mark. In addition, only 67% of the surgical arm had actually undergone surgery. In the article, the authors' state, “ … the intention-to-treat analysis underestimates the true effect of surgery.” They go on to conclude, “In the as-treated analysis … those treated surgically showed significantly greater improvement … ” Further, none of the patients in Zucherman et al 6,7
crossed over. Thus, because of the cross over in the Weinstein et al study 1,5
, the treatment effect may not be as great as that seen in the Zucherman et al study (,) 6,7
Comparing 12- and 24-month mean percentage improvements in Zurich Claudication subscale scores and treatment effects between interspinous device and conservative management in the study by Zuchermanet al .
Comparing 12- and 24-month improvements in the mean Oswestry Disability Index (ODI) scores and treatment effects between decompression and conservative management in the studies by Malmivaara et al and Weinstein et al , respectively.
Another point to consider is the difference in the enrollment of patients. Weinstein et al 1,5
and Malmivaara et al 4
enrolled patients who were diagnosed with lumbar stenosis who failed conservative therapy. Zucherman et al 6,7
, however, enrolled patients who had pain relief while sitting. Thus, the patients enrolled in that study were known to have alleviation of pain with their lumbar spines in flexion. This may have created a selection bias in that this study only enrolled patients who would respond favorably to an interspinous device.
Morbidity was calculated from the complication rates, both perioperatively and postoperatively. Note that both lumbar decompressive surgery groups (Weinstein et al 1,5
and Malmivaara et al 4
) had patients with instrumented fusion (a more morbid procedure than decompression alone). This would have skewed the complication rate higher for the surgical decompression groups, since these were not simple decompressive procedures.
The indirect treatment effect for disability and pain favors the interspinous device compared to decompression. No significant treatment effect differences were observed for postoperative-walking distance improvement or complication rates; however, findings should be considered with caution due to indirect comparisons and short follow-up periods.
Given the limitations of our network analysis, we nonetheless evaluated the best evidence currently available (). Eventually, studies with direct comparisons evaluating the efficacy between interspinous devices and surgical decompression will hopefully give a more precise answer.
EDITORIAL STAFF PERSPECTIVE
There are several noteworthy factors regarding this topic and the systematic review as performed:
- The current strength of evidence favoring interspinous spacers compared to decompression surgery with or without fusion regarding factors of disability and pain is low, meaning that “Further research is very likely to have an important impact on our confidence in the estimate of effect and is likely to change the estimate.”
- There was general surprise voiced that there is no direct comparison study between decompression surgery alone (in properly selected patients) and interspinous spacers without decompression. For various reasons the higher-grade evidence relies on comparisons, such as interspinous spacers versus nonoperative modalities, decompression and fusion versus decompression and surgical decompression versus nonoperative treatment. The absence of the most compelling form of comparison studies—decompression alone versus interspinous spacer for more stable forms of stenosis and decompression and fusion versus decompression and interspinous spacer for more unstable forms of stenosis—was quoted as being ”lamentable” and ”overdue for correction.”
- The absence of clinically relevant direct comparison studies necessitated a ‘network analysis’ with its inherent shortcomings of error through heterogeneity of its study cohorts and lower level of familiarity to a clinician community.
- The short-term follow-up of studies with interspinous spacers (12 months) was noted to likely underreport complications, such as device loosening and need for more complex revision surgery for patients with interspinous devices, which would appear to be more likely to fail with time.
- The impact of patients' complications with interspinous devices as expressed in invasiveness of potential corrective surgery compared to less expensive and simple decompression surgery is presently not understood.
- The potential for financial conflict of interest affecting several authors related to the development and subsequent marketing of certain interspinous devices was pointed out. To date there are no prospective comparison studies from financially disinterested third-party groups.
(Case provided by Jens Chapman)
Two years after L3/4 interspinous process-spacer placement a physically very active and healthy 65-year-old man presented with severe bilateral leg pain brought on by short-distance walking and relieved by bending and squatting (). At the time the procedure had brought substantial symptom relief, however progressive symptom recurrence was noted about 18 months after the index procedure. The patient was found to have an X-Stop device at his L3/4 interspace with heterotopic bone formation surrounding the implant, as well as a subtle grade 1 degenerative spondylolisthesis.
Preoperative x-ray (AP view).
The MRI scan revealed persistent significant central stenosis with facet hypertrophy and lateral recess compromise ().
Magnetic resonance imaging (axial).
Because of failure of nonoperative treatment the patient underwent removal of the device and simple midline-sparing hemilaminotomies with partial facetectomies through a keyhole laminotomy approach (). Abundant heterotopic bone surrounding the implant was carefully removed, while avoiding injury to the ligaments. Since decompressive surgery the patient experienced complete relief of lower extremity and back symptoms and return to activities of daily living including competitive golf 6 months to date.
The intraoperative site revealed extensive foreign body debris (dark spots) and abundant reactive connective tissue build up around the area of the interspinous spacers.
This case illustrates some concerns about interspinous spacers. The role of simple soft-tissue–sparing decompression surgery, while avoiding destabilizing measures, compared to an implant-based nonfusion procedure remains to be established and cannot be concluded based on the current state of the literature. Does a nonfusion device in fact set up patients for more revision surgeries in the intermediate and longer run compared to simple decompression or fusion surgery in appropriately selected patient? Another question worth debating is how long does a satisfactory result in elective spine surgery have to last to be counted as a success? Alternatively, when is a revision procedure performed at an index level a complication? This case certainly illustrates some of the many unanswered questions.
Preoperative x-ray (extension).
Preoperative x-ray (flexion).