MILD Procedure

What Is the Evidence?

Minimally invasive lumbar decompression (MILD) is used to treat lumbar stenosis. Lumbar stenosis results when the spinal canal is narrowed by a combination of disc protrusion into the spinal canal, buckling of the ligamentum flavum, and osteophyte formation from the facet joints.1,2 This can lead to nerve root compression and nerve root ischemia, which can result in radicular pain and nerve dysfunction.1,2 The radicular pain can present as pain radiating down the buttocks into the legs, which is commonly referred to as sciatica. Nerve dysfunction can present in loss of sensation and leg weakness. The loss of sensation and leg weakness does not always present with a specific dermatomal distribution because multiple nerves are often with lumbar spinal stenosis.1,2 Lumbar stenosis can also result in neurogenic claudication. This is a condition characterized by leg numbness, pain, or weakness that limits walking tolerance. The symptoms are often improved or relieved by sitting or leaning forward. Patients can often walk farther if they learn forward on a shopping cart. This occurs because extension decreases the cross-sectional area of the spinal canal and causes nerve root ischemia, which results in radicular leg pain and nerve dysfunction.1,2 Lumbar flexion stretches the ligamentum flavum and increases the cross-sectional area of the spinal canal,1,2 which improves the symptoms and allows patients to walk farther.

In a cadaveric study, Ly et al evaluated the spinal capacity of the lumbar spine in flexion versus extension with myelograms. 3 The authors found a larger capacity of dural sac of 3.5 to 6.0 mL (4.85 +/- 0.75 mL) in flexion than in extension, and the differences were highly significant ( p < 0.001). In a study by Kim et al, the authors evaluated patients clinically and radiographically. 4 The authors performed a comparative analysis to evaluate the association between radiologic and clinical factors. Additionally, comparative analyses were performed between the varying types of surgeries. Among various radiologic factors, the baseline ligamentum flavum thickness was the only major contributing factor to the severity of claudication in the multivariate logistic regression analysis. So, there is evidence that the ligamentum flavum is a major contributor to lumbar spinal stenosis and neurogenic claudication.

MILD is performed percutaneously through a 5.1-mm port. Contrast dye is used along with fluoroscopy to identify where the thecal sac is. Using specialized cutters, lamina and ligamentum flavum are removed (Figure 1). The epidurogram contrast flow allows the physician to evaluate how much of a decompression was performed. Preliminary studies have been promising. Staats et al conducted a prospective, multicenter, randomized controlled trial with 26 centers participating. 5 A total of 302 patients were enrolled, with 149 randomized to MILD and 153 to standard pain management treatment with epidural steroid injections. At 6 months, the Oswestry Disability Index (ODI) improvement in the MILD group (62.2%) was significantly higher than that for in epidural steroid group (35.7%) ( p < 0.001). Longer-term follow-up of this study group has also been reported. At 2 years, there were 143 patients treated with MILD versus 131 treated with epidural steroid injections.6 At 2 years, ODI scores improved by 22.7 points, numeric rating scale improved by 3.6 points, and Zurich Claudication Questionnaire symptom severity and physical function domains improved by 1.0 and 0.8 points, respectively. There were no serious device- or procedure-related adverse events, and 1.3% experienced a device- or procedure-related adverse event.

There has been growing literature on the efficacy of the MILD procedure. Jain et al 7 performed a meta-analysis of the MILD procedure and included 2 randomized controlled trials and 11 other controlled clinical studies. The authors concluded that the MILD procedure had similar complication rates as epidural injections but improved efficacy. Hence, preliminary studies on the MILD procedure have been promising and show it may be better than continuing standard conservative treatments such as epidural steroid injections if patients do not have long-term improvement with those treatments. Despite these findings, there have been reports of serious postoperative complications. Tumialán et al 8 described serious complications in response to a study by Mekhail et al. 9 In the study by Mikhail et al, 9 58 patients underwent 170 MILD procedures at 11 sites. The authors reported no major device- or procedure-related complications. One-year data showed significant reduction of pain as measured by the visual analog scale. Improvements in physical functionality, mobility, and disability were significant as measured by the Zurich Claudication Questionnaire, 12-item Short-Form health survey, and ODI. In contrast, Tumialán et al8 reported that over a 1-year period at their center, 8 patients had refractory neurogenic claudication and 2 patients had cerebrospinal fluid (CSF) leaks after the MILD procedure. One of the patients who had a CSF leak not only had a dural tear but also transected nerve roots identified during revision surgery.

In another article, Tenhoeve and Karsy10 described a case of an epidural hematoma after the MILD procedure. In their case, a 76-year-old woman with lumbar stenosis and neurogenic claudication underwent a L2-3 MILD procedure. Upon discharge following the procedure, she had worsening lumbar and left buttock and hip pain. She also had left leg radiculopathy, lower extremity weakness, and progressive bilateral numbness of her lower extremities. Computed tomography and magnetic resonance imaging showed a large dorsal epidural collection that was later identified as an epidural hematoma on surgical exploration. Postoperatively, the patient showed improved strength and sensation in the lower extremities and reduced radicular pain. At 1- and 3-month follow-ups, she had returned to her neurological baseline. Thus, despite the growing literature that shows improved results of the MILD procedure over longterm epidural steroid injections, there is also growing literature regarding complications related to the MILD procedure.

Conclusion

MILD is a minimally invasive procedure used to treat lumbar stenosis with symptomatic neurogenic claudication. There is evidence that the MILD procedure may be more efficacious than epidural steroid injections long-term. However, physicians must be alert for potential complications such as epidural hematomas and CSF leaks among other surgical complications such as nerve root injury and infections. More studies are needed to better evaluate where the MILD procedure will best fit in the treatment of patients with lumbar stenosis.

References

1. Katz JN, Harris MB. Lumbar spinal stenosis. N Engl J Med. 2008;358:818–825.

2. Porter RW. Spinal stenosis and neurogenic claudication. Spine. 1996;21:2016–2052.

3. Dai LY, Xu YK, Zhang WM, Zhou ZH. The effect of flexion-extension motion of the lumbar spine on the capacity of the spinal canal. An experimental study. Spine (Phila Pa 1976). 1989;14(5):523-525.

4. Kim J, Kwon WK, Cho H, et al. Ligamentum flavum hypertrophy significantly contributes to the severity of neurogenic intermittent claudication in patients with lumbar spinal canal stenosis. Medicine (Baltimore). 2022;101(36):e30171.

5. Staats PS, Benyamin RM; MiDAS ENCORE Investigators. MiDAS ENCORE: Randomized controlled clinical trial report of 6-month results. Pain Physician. 2016;19(2):25-38.

6. Staats PS, Chafin TB, Golovac S, et al; MiDAS ENCORE Investigators. Longterm safety and efficacy of minimally invasive lumbar decompression procedure for the treatment of lumbar spinal stenosis with neurogenic claudication: 2-year results of MiDAS ENCORE. Reg Anesth Pain Med. 2018;43(7):789-794.

7. Jain S, Deer T, Sayed D, et al. Minimally invasive lumbar decompression: a review of indications, techniques, efficacy and safety. Pain Manag. 2020;10(5):331-348.

8. Tumialán LM, Marciano FF, Theodore N. Regarding: long-term results of percutaneous lumbar decompression mild for spinal stenosis. Pain Pract. 2012;12:252–253.

9. Mekhail N, Vallejo R, Coleman MH, Benyamin RM. Long-term results of percutaneous lumbar decompression mild(®) for spinal stenosis. Pain Pract. 2012;12(3):184-193.

10. Tenhoeve SA, Karsy M. Lumbar epidural hematoma as a rare complication from minimally invasive lumbar decompression. Cureus. 2023;15(12):e51083.

Contributor:

Yu-Po Lee, MD

From UCI Health in Orange County, California.