Minimally invasive lumbar decompression

Introduction

Minimally invasive lumbar decompression (MILD) is an image-guided approach used in the treatment of symptomatic lumbar central spinal stenosis. This procedure is a good option for patients who are not responsive to conservative or injection therapy and either do not want an open surgical decompression or are not good surgical candidates. Lumbar spinal stenosis (LSS) is a degenerative disease of the aging spine. LSS is the most common indication for spinal surgery in patients above the age of 65.

Indication

The MILD procedure is indicated for patients with symptomatic central LSS due to ligamentum flavum hypertrophy. It is not meant for patients with symptoms due to lateral recess spinal stenosis. It is also not indicated if stenosis is due to any cause other than ligamentum flavum hypertrophy (e.g., anterolisthesis or disc protrusion). Indications and contraindications are summarized in Table 6.1 .

TABLE 6.1

Indications and Contraindications for MILD Procedure

Indications	Contraindications
• Ligamentum hypertrophy preferably >4 mm ² • Central spinal stenosis • Symptoms of neurogenic claudication • No sensory or motor defect	• Coagulopathy • Surgery at that level • Severe scoliosis • True allergy to dye • Only back pain

MILD , Minimally invasive lumbar decompression

Patient selection

Patient selection is the most important part of the MILD procedure ( Table 6.2 ). The patient should have symptomatic central LSS from ligamentum flavum hypertrophy ( Fig. 6.1 ).

TABLE 6.2

Patient Selection Factors

Primary indication	Neurogenic claudication in the presence of radiologically proven ligamentum flavum hypertrophy
Neurogenic claudication symptoms	• Pain • Numbness • Weakness • Tingling in low back, buttocks, and legs initiated by standing, walking, or lumbar extension that is relieved by sitting or forward flexion.
Symptom characteristics	Symptoms do not follow a dermatomal pattern (a hallmark of foraminal or lateral recess stenosis), but the two distinct etiologies can coexist The symptoms tend to be symmetrical, usually above the knee; this proximal distribution helps to symptomatically differentiate lumbar stenosis from vascular claudication
Physical examination	Unremarkable; the presence of sensory or motor deficits should prompt surgical evaluation
Imaging	Imaging is required for the definite diagnosis of LSS A systemic review found CT or MRI equally accurate in the diagnosis of LSS MRI is preferred as ligament hypertrophy can be easily measured
Radiological criteria for spinal stenosis	There are several criteria used, but anterioposterior spinal canal diameter of <10 mm or an area of <70 mm ²is generally considered diagnostic
Diagnostic criteria used in various MILD trials	• Neurogenic claudication for at least 3 months • Ligamentum flavum thickness of >2.5 mm • Anterolisthesis of 5 mm or less • Absence of spinal instability

LSS , Lumbar spinal stenosis

Setup

Table 6.3 describes some important aspects of the procedure setup.

TABLE 6.3

Procedure Setup

Anesthesia	Mild to moderate sedation Deep sedation or general anesthesia should be avoided unless performed with nerve monitoring
Positioning	Prone position with flattened or minimized lumbar lordosis No pressure on the belly or chest for ease of breathing for the patient
Antimicrobial actions	Appropriate antibiotic is given before the start of the procedure The target area is scrubbed with 2%–3% chlorhexidine/70% isopropyl alcohol solution, which is allowed to dry for 3 minutes Full surgical drapes are used, and the target skin area is covered with an antimicrobial incise drape (e.g., Ioban)

Procedure

1.

Obtain an anterioposterior fluoroscopic view of the lumbar spine with the spinous process midway between the pedicles.
2.

Three straight lines are drawn:
- •
  
  One in the middle (over the spinous processes)
- •
  
  Two connecting the medial side of the pedicles on each side ( Fig. 6.2 )
  
  Fig. 6.2
  
  Three green lines drawn define the working area. Black circles outline the pedicles.
3.

Ideally, the insertion point is usually one and one-half to two vertebral levels below the target space, in the paramedian plane between the two lines initially drawn on the patient in the previous step.
4.

Provide cutaneous analgesia with a 25-gauge needle, then further anesthetize the cannula tract with a 22-gauge 3.5 inch spinal needle to the lower lamina, including the laminar periosteum.
5.

Obtain epidural space access. The epidural needle should be placed in the most cephalad part of the interlaminar space with the intent to obtain ipsilateral dye spread.
6.

After a skin stab, a 6-gauge working cannula via use of the trocar is inserted to the lower lamina.
7.

Lateral fluoroscopic guidance is used to ensure that it is not placed too deep (which will result in a large dural tear) ( Figs. 6.3 , 6.4 ).
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