The results of this meta-analysis demonstrated that the Superion spacer provides the similar clinical outcome in extremity pain severity, back-specific functional impairment, reoperation rate, and complication back function. However, Superion showed a better outcome than X-STOP in axial pain severity and ZCQ patient satisfaction score. The similarity mechanism in two types of ISP may account for the pooled outcome. The mechanism of various ISPs in current spine field includes five points [
17‐
19]: (1) enlargement of the spinal canal area, (2) increase of the neural foramina area, (3) unloading of the posterior annulus and intradiscal pressure, (4) distraction of interspinous distance, and (5) strength of the spinous processes.
Since the 1950s, the first ISP was designed and applied by Konwles [
20] in clinical with many defects. For three decades of progress, the first modern ISP, Wallis spacer, was developed by Senegas [
21] in the early 1980s. Moreover, as a new procedure, the ISP becomes an alternative option for the patient and makes a profound influence in treating LSS. Now, designs of ISP vary from stand-alone spacers represented by the X-STOP to “dynamic” spacers represented by the Coflex, respectively. The X-STOP was approved by the US Food and Drug Administration based on laboratory, mechanical, cadaver, and clinical studies in 2005 for treatment of patients aged above 50 suffering from neurogenic intermittent claudication secondary to a confirmed diagnosis of LSS. Additionally, many studies have shown favorable outcomes compared to decompression surgery (with or without fusion) [
22,
23]. According to a recent report, X-STOP was ceased to sale and distribution by its manufacturer (Medtronic, Inc., Minneapolis, MN, USA) in 2015, leaving the Superion as the de facto clinical option for surgeons and their patients. From experiment to clinical practice paradigm for remedy of LSS, the Superion as the second generation of stand-alone ISP was approved by the US Food and Drug Administration for commercial distribution in the USA on May 20, 2015. Both Superion and X-STOP spacers, albeit similarities in the mechanism of action and clinical outcomes, have distinct differences in device design and surgical technique. Superion spacer (size range from 8 to 16 mm) is inserted through a cannula placed between adjacent spinous processes without dissection of the spinal musculature. Compared to the X-STOP spacer with large surgical exposure, the minimally invasive nature of the Superion spacer contributes to smaller blood loss, less operative time, and shorter hospitalization. In addition to its small incision, the Superion procedure avoids generating large scar tissue around symptomatic levels which may reduce the intricacy of future revision, removal, or further decompression surgery [
24]. The data from previous 2-year follow-up studies suggested that both DI and the Superion procedure provide effective and durable symptom relief of claudication symptom. For back-pain severity, the Superion group and DI group obtain an average percentage improvement of 65 and 52%. For the leg pain severity, the average percentage change with the spacer is 70%, and laminectomy is 62% in comparison to [
8]. Although patients with IPS effectively ameliorate mild to moderate symptom of LSS in midterm and with the relatively small trauma of surgery, the cost-effectiveness issue for IPS should be considered. Parker et al. [
25] use Markov model to evaluate three strategies of care for LSS. The study shows that CC has the lowest cost at $10,540 and the lowest quality-adjusted life year increase, while ISP and DI were nearly identical at about $13,950 and also at quality-adjusted life years. They indicated that surgical care, especially in ISP procedure, provides superior value versus sustained CC in treating LSS. However, another study reveals that for the 1-level procedure, a mean cost of $9291 for DI, $7900 for ISP, and $3478 for CC, and for 2-level $13,429 was a cost for ISP, and the other two treatments remain unchanged [
26]. Despite the merits of the ISP were mentioned above, it is noteworthy that not only complications associated with process fracture, dislodgements, and migrations of the spacer, heterotopic ossification [
27], brought barriers to its safety but also the exceeding indication restricted its clinical application.
It is noteworthy that both IPD and decompressive surgery are current treatment strategies for lumbar spinal stenosis. However, which procedure can bring patients better outcomes is still pending. Moojen et al [
28,
29] showed that, at 2 years, the success rate according to the ZCQ for the IPD group [69% (95% CI 57–78%)] did not show a significant difference compared with decompression [60% (95% CI 48–71%)
p = 0.2]. Furthermore, long-term VAS back pain was significantly higher [36 mm on a 100-mm scale (95% CI 24–48)] in the IPD group compared to the decompression group [28 mm (95% CI 23–34)
p = 0.04]. But the IPD group caused higher reoperation rate (29%) than that in the decompression group (8%) in the early post-surgical period. In 2013, Strömqvist et al. [
22] conducted a trial to compare the X-STOP with conventional decompression in patients with neurogenic intermittent claudication. The results showed that the clinical improvement of two procedures were similar at 6, 12, and 24 months follow-up. Similarly, patients suffered from a higher rate of reoperations in the X-STOP group (26%) than decompression surgery (6%) at endpoint. Intriguingly, in two meta-analysis reviews [
30,
31], we found that there is no significant difference between the IPD and decompression surgery for effective indicator such as Visual Analog Scale, Oswestry Disability Index, and Roland Disability Questionnaire and Complications. In addition, for the reoperation rates, both studies draw the same conclusion that the decompression group is lower than the IPD group.
This meta-analysis has a few limitations. According to our research results and inclusion criteria, five RCT studies are included. A few RCTs and incomplete data may reduce the quality of evidence and strength of analysis. Blinding of patients and surgeons are difficult to evaluate surgical effect in a clinical trial. Inadequate blinding is reported to generate 15% overestimation of treatment effect.