Lumbar spinal stenosis (LSS) is a condition in which the space around the spinal cord narrows and causes compression of the spinal cord and nerves. This compression may cause symptoms such as pain, numbness, and weakness of the lower extremities that often worsen with standing or walking and are alleviated by sitting or leaning forward. Back pain may or may not be present. LSS is often associated with age-related changes in disc height and arthritis of the facet joints. It is often seen in individuals over the age of 65. For individuals with severe symptoms that have not been alleviated by rest and conservative therapy (e.g., nonsteroidal anti-inflammatory drugs [NSAIDs], epidural steroid injections [ESIs]), surgery to decompress the spinal cord and/or nerves may be considered.
Percutaneous image-guided lumbar decompression (PILD) is a minimally invasive technique using the minimally invasive lumbar decompression (MILD®) system, which consists of a specialized cannula and surgical tools. This novel technique has been proposed as an alternative, less-invasive treatment for LSS. In contrast to the traditional spinal surgery treatment for LSS such as lumbar laminectomy, PILD with the MILD® system uses fluoroscopic guidance to achieve minimally invasive lumbar decompression for the treatment of LSS. After filling the epidural space with contrast medium under fluoroscopic guidance, a specialized cannula and surgical tools provide access for bone and tissue to be sculpted near the spinal canal. However, the devices are not intended to be used near the lateral neural elements and are contraindicated for disc procedures. In addition, PILD is indicated for central stenosis only, without the capability of addressing nerve root compression or disc herniation that may also be present.
PEER-REVIEWED LITERATURE
In an industry-sponsored, prospective study (MiDAS I), Chopko and Caraway (2010) evaluated the safety and effectiveness of PILD using the MILD® system in 78 individuals with neurogenic claudication (i.e., pain or weakness in the legs) due to LSS. Six-week follow-up data were available for 75 of the study participants. Outcome measurements included visual analog scale (VAS) pain scores, the Oswestry Disability Index (ODI), and Zurich Claudication Questionnaire (ZCQ). At 6-week follow-up, the authors reported statistically significant reductions in pain and improvements in physical function and mobility. They concluded that PILD demonstrated safety and effectiveness for improving mobility and reducing pain associated with LSS. The study is limited in its short-term follow-up and lack of appropriate comparative control groups. In a follow-up study, Chopko (2013) reported on 2-year results from this study, although the longer term results are uncertain because of the high loss to follow-up.
In a randomized controlled trial, Brown (2011) evaluated the safety and effectiveness of PILD using the MILD® system compared with ESIs for the treatment of low back pain, including LSS. Thirty-eight individuals were randomly assigned into PILD (n=21) and ESI treatment groups (n=17). Outcome measurements included VAS pain scores, ODI, and ZCQ patient satisfaction at 12-week follow-up. Individuals in the PILD treatment group reported significantly greater pain reduction (P<0.0001) and functional mobility improvement (P<0.01) when compared with individuals in the ESI group. Individuals in the PILD group reported similar improvements in patient satisfaction. However, at 12-week follow-up, individuals treated with PILD had no significant change in mean VAS from their 6-week follow-up. No major procedure-related complications were reported. The authors concluded that PILD using the MILD® system provided better pain reduction and improved functional mobility when compared with ESI. The study is limited in its lack of long-term follow-up, heterogeneous study population, and lack of comparisons with gold standard surgical treatments (e.g., lumbar laminectomy). Additionally, after only 6 weeks of assessment, 82% of study participants crossed over to PILD (n=14). This crossover design may confound the validity of the author's conclusions.
In a systematic review from the International Spine Intervention Society, Kreiner et al. (2014) evaluated the available peer-reviewed literature to determine the safety and effectiveness of PILD using the MILD® system in individuals with symptomatic, degenerative LSS. One randomized controlled trial and 12 cohort studies were included in the review. Primary outcome measurements included VAS pain scores and ODI, with short (i.e., 4–6 weeks), mid-term (3–6 months), and long-term (>1 year) endpoints. Secondary outcomes included patient satisfaction, adverse events, and complications. VAS pain scores had a mean improvement of 41% in the short term, 46% at 3 months, and 49% at 1 year. However, mean VAS remained greater than 3 at all times after treatment. Ten of the studies assessed function using ODI, which improved by a weighted mean of 16.5 at 6 weeks, 16.2 at 12 weeks, 15.4 at 6 months, and 14.0 at 1 year. However, the mean final ODI was greater than 30 in most of the studies, which did not meet the definition of a minimally acceptable outcome according to one of the included study authors. Although no direct procedure-related complications were identified, the possibility of nerve and dural damage during this minimally invasive procedure was noted. The authors concluded that the available body of evidence was of low quality and that longer term data were needed to determine the overall utility of the procedure. Higher quality studies were needed to clarify which individuals may benefit from PILD and the degree to which the individuals may benefit.
Benyamin et al. (2016) published 6-month and 1-year results for 302 participants from a CMS coverage with evidence development protocol study, MiDAS ENCORE (Evidence-based Neurogenic Claudication Outcomes Research). Their trial concluded a nonblinded, randomized study at 26 interventional pain management centers in the United States, in a 1:1 ratio to image-guided minimally invasive lumbar decompression (IG-MILD) or ESIs. This trial included Medicare beneficiaries 65 years of age and older who had neurogenic claudication symptoms for at least 3 months and in whom standard therapies had failed, including physical therapy, home exercise programs, and oral analgesics.
Individuals in the ESI group were allowed up to four ESI treatments and received a mean of two injections over 1 year. The primary outcome was the proportion of responders achieving the minimal improvement difference (MID) of at least 10 points on the ODI score being significantly higher in the IG-MILD group than in the ESI group at both 6 months and 1 year. Adverse events were low and similar between groups. Researchers report it may be difficult to separate out the effect of comorbidities because over 80% of patients had one or more spinal stenosis comorbidities.
The primary study outcomes were numeric rating scale for pain (NRS), ODI, and ZCQ. This unequal dropout rate raises the possibility of bias due to nonblinding of patients and assessors and patient expectations.
In March 2024, a retrospective review was published by Shahzad et al. The authors evaluated the incidence and characteristics of individuals who required surgical decompression within 5 years following minimally invasive procedures to treat LSS. A total of 5278 individuals were included, of which 3222 (61.04%) underwent PILD, 1959 (37.12%) underwent interspinous spacer device (ISD) placement, and 97 (1.84%) had claims for both procedures. Overall, the incidence of subsequent surgical decompression within 5 years was 6.56% (346 of 5278 patients). The conclusion reached after completion of the author's review was that minimally invasive procedures may provide an additional option to treat symptomatic LSS in individuals who are poor surgical candidates or who do not desire open decompression; however, there still exists a subset of individuals who will require subsequent surgical decompression. Factors such as gender and prior surgical decompression increase the likelihood of subsequent surgery, while older age, obesity, and a higher Charlson Comorbidity Index score reduce it. These findings aid in selecting suitable surgical candidates for better outcomes in the elderly population with LSS.
In 2024, Deer et al. provided Level-1 objective, real-world outcome data on individuals who participated in the MOTION Study, a prospective, multicenter, randomized controlled trial comparing the MILD® procedure as a first-line therapy in combination with nonsurgical conventional medical management (CMM) versus CMM alone as the active control. Individuals in the test group received the MILD procedure at baseline. Both the MILD and CMM group and the control group were allowed unrestricted access to conventional real-world therapies. Individual-reported outcomes included the ODI, the ZCQ, and the Numeric Pain Rating Scale. A validated Walking Tolerance Test, the incidence of subsequent lumbar spine interventions, and the occurrence of adverse events were used to measure objective outcomes. Sixty-nine patients in each group were analyzed at 1-year follow-up. One-year results of this Level-1 study demonstrated superiority of MILD and CMM over CMM alone for individuals with LSS who were suffering from neurogenic claudication secondary to hypertrophic ligamentum flavum. Use of the validated Walking Tolerance Test to objectively measure increased ability to walk without severe symptoms provided evidence of statistically significantly better outcomes for MILD and CMM than for CMM alone. With no reported device or procedure-related adverse events, the authors concluded the long-standing safety profile of the MILD Procedure was reinforced. The test results provided support for early interventional therapy for individuals suffering from symptomatic LSS with the MILD procedure.
SUMMARY
For individuals with severe symptoms that have not been alleviated by rest or conservative therapy, lumbar laminectomy is often considered the gold standard surgical treatment for LSS. While PILD represents a novel, minimally invasive surgical approach for the treatment of central LSS, there is a lack of adequately designed peer-reviewed literature or guidance from relevant medical societies to establish its safety and effectiveness. There is a need for additional research with sufficient follow-up, sample size, and relevant comparative control groups to establish the appropriateness of PILD for the treatment of central LSS.
