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Complications of Minimally Invasive Lumbar Decompression (MILD)

David W. Lee MD¹ and Jason Pope MD²

¹ Fullerton Orthopedic Surgery Medical Center, Fullerton, CA, USA
² Evolve Restorative Center, Santa Rosa, CA, USA

Percutaneous Image-Guided Lumbar Decompression (PILD) involves non-invasive techniques to debulk the posterior elements of the spine (lamina and ligamentum flavum) using instrumentation with image-guidance (CT or fluoroscopy). PILD is not intended to debulk lateral foramen or primary bony abnormalities [1]. Ligamentum flavum hypertrophy is thought to contribute to 50–85% of central canal narrowing and often multiple levels of central canal stenosis due to ligamentum flavum hypertrophy are present producing symptomatic LSS with neurogenic claudication [2].

Presently, MILD ^TM (Minimally Invasive Lumbar Decompression, Vertos Medical, Aliso Viejo, CA, USA) is the only image-guided technique which meets the CMS definition of PILD. MILD is performed percutaneously through a port with fluoroscopic guidance to selectively remove lamina and ligamentum flavum resulting in lumbar spinal decompression. Epidurogram flow allows for assessment of decompression. Unlike many of the other treatments for lumbar spinal stenosis, MILD does not involve any implantation of hardware.

Anatomy

The ligamenta flava are paired ligaments, separated between left and right that run between the spinal laminae of adjacent vertebrae. Each ligament passes from the anterior and inferior aspect of the lamina of the cephalad vertebra down to the posterior and superior aspect of the lamina of the caudal vertebra. Although each ligament is considered to be distinct, a ligamentum flavum frequently merges with the ligamentum flavum of the opposite side, as well as with the interspinous ligament that runs posterior. Small gaps exist between the left and right ligamentum flavum, allowing for the passage of veins that unite the posterior internal (epidural) vertebral venous plexus with the posterior external vertebral venous plexus.

The ligamentum flavum traverses the spine beginning with C1–2 superiorly and ending with L5–S1 inferiorly. At the level of occiput-C1, the posterior atlanto-occipital membrane is the ligamenta flava equivalent. The ligamentum flava are anatomically thinner in the cervical region, gradually become thicker in the thoracic region, and are thickest in the lumbar region.

The ligamentum flavum may undergo degeneration with age or after trauma. Under such circumstances, it usually increases in thickness and may calcify or become infiltrated with fat. These changes cause the ligaments to lose their elastic characteristic, which may result in buckling of the thickened ligamentum flavum into the vertebral canal and neural foramen. The buckling further results in narrowing of these regions, which can compromise the neural elements running within them (e.g., spinal cord, cauda equina, or exiting nerve roots).

Indications

Symptoms of neurogenic claudication
Failure to respond to conservative treatments including physiotherapy and epidural injections
Presence of central canal stenosis due to ligamentum flavum hypertrophy (>2.5 mm, measured perpendicular to the lamina) (Figure 59.1) (A majority of patients in the ENCORE randomized controlled trial had co-morbid foraminal stenosis, facet hypertrophy or disc bulge, and were positive predictors of success with percutaneous decompression).

**Figure 59.1** Sagital T2-weighted MRI image demonstrating spinal stensosis and ligamentum flavum hypertrophy.

Contraindications

Neurogenic claudication in the absence of ligamentum flavum hypertrophy
Previous surgery at the same level
Severe neurologic deficits
Spondylolisthesis greater than Grade III
Presence of infection at the site.

Technique

MILD is a minimally invasive lumbar decompression performed percutaneously through a 5.1 mm port. This limits trauma to the surrounding paraspinal tissues. As a result, the procedure is often performed under only moderate sedation and local anesthetic. Within the percutaneous decompression kit, a trochar, stabilizer, depth gauge, bone, and tissue scuplter are provided (Figure 59.2).

**Figure 59.2** Direct percutaneous decompression kit.

The procedure is performed by accessing the ligamentum flavum through the interlaminar space. Imaging guidance either via CT scan or fluoroscopy is required. Prior to the start of the MILD procedure, an epidurogram is used to visualize the level of interest. The epidurogram allows for a pre-procedure visualization of the severity of stenosis and is to be considered if desired, as the MILD procedure can be performed without its use. Constant visualization via epidurogram throughout the procedure allows for improvement in efficacy and avoidance of adverse effects.

A small incision is made one level below the interlaminar space of interest in the midline. Through that incision, the trochar is then placed at approximately a 45° angle to the surface of the skin. Once the trochar is purchased within the soft tissue, the stabilizer and depth gauge are placed around the trochar.

These tools restrict depth and movement within the interlaminar space. Both tissue and bone sculptures are used to incrementally decompress the region by removing both laminae and ligamentum flavum. During decompression, it is imperative to utilize both AP, lateral and contralateral oblique (CLO) views to optimally visualize the location of the sculptor relative to the lamina and ligamentum flavum. Removal of bone or the ligamentum should be performed on the CLO view (Figures 59.3a–c and 59.4a–c).