Minimally Invasive Tubular Decompression for Foraminal Stenosis

18 Minimally Invasive Tubular Decompression for Foraminal Stenosis

Jung-Woo Hur and Jin-Sung Luke Kim

18.1 Introduction

Radiculopathy is most commonly caused by nerve root canal stenosis, which can be the result of various pathologies of the spine, including spondylosis, spondylolisthesis, osteophytes, and disk herniation. Lumbar spinal stenosis can be subdivided based on the location of the stenotic pathology: central stenosis (referring to medial stenosis affecting especially the cauda equina), lateral recess stenosis, and foraminal stenosis (Fig. 18.1). Foraminal stenosis can be further categorized into intraforaminal and extraforaminal stenosis. Lumbar foraminal/extraforaminal spinal stenosis (LFSS) is a troublesome disease that can be easily overlooked by surgeons and can result in failed back surgery syndrome (FBSS). Decompression of the whole length of the nerve root from the spinal canal to the extraforaminal zone is often challenging due to the difficulty in identifying the exact site of nerve compression, making preservation of the posterior elements difficult.

18.2 Pathophysiology and Clinical Symptoms

LFSS is defined as compression of a nerve at a site between the medial and lateral borders of the pedicle.¹ Various types of degenerative change can cause LFSS, such as narrowing of the intervertebral disk space, degenerative lumbar scoliosis, bulging of the intervertebral disk, vertebral body osteophyte formation, anterior and posterior spondylolisthesis, and hypertrophy of the ligamentum flavum.

Although LFSS is considered a relatively uncommon disease, Kunogi et al¹ reported that 8% of cases of surgical treatment of lumbar degenerative disease involved LFSS. Furthermore, Burton et al² reported that 60% of cases of failed back syndrome were due to a missed diagnosis of LFSS.

The symptoms of LFSS are similar to those of radiculopathy caused by general lumbar spinal canal stenosis. Patients may present with unilateral foraminal compression and clinical symptoms that are characterized by unilateral radicular pain with or without weakness. Back pain is usually minimal. Surgical intervention is recommended for patients whose symptoms persist despite nonoperative management.³

18.3 Treatment Option for LFSS

Current surgical strategies for treating LFSS can be separated into two categories: strategies that require fusion of the lumbar spine, and strategies that do not require fusion. The traditional surgical approach for LFSS has been to perform a wide, bilateral decompressive laminectomy along with resection of the medial portion of the facet joints to decompress the affected neural elements. Although this approach can successfully alleviate nerve compression symptoms, there are drawbacks of the open approach, including amount of soft tissue dissection, blood loss, postoperative pain, and the potential for iatrogenic instability of the spinal segment. These concerns are magnified when treating an elderly, fragile patient. Fusion is usually performed if the removal of spinal tissue presents a risk for spinal instability or in the presence of a significant spinal deformity, such as scoliosis or spondylolisthesis.

Since there is no established imaging technique for the diagnosis of LFSS, it is usually difficult to identify the site of nerve entrapment. Consequently, the entire length of the nerve (nerve root, dorsal root ganglion, and spinal nerve) from the inside of the spinal canal to the outside of the intervertebral foramen (IVF) must be decompressed in most cases. Total facetectomy combined with spinal fusion using spinal instrumentation, which is unnecessary in many cases, is therefore normally performed. Although it is possible to preserve the posterior elements by combining medial facetectomy and lateral fenestration, the nerves running under the preserved pars interarticularis cannot be decompressed with this method.¹ Additionally, the deep location of intraforaminal lesions makes the surgery technically challenging and more invasive.

Fig. 18.1 Illustration of lumbar foraminal anatomy. SF, superior facet; IF, inferior facet; L, lamina; P, pedicle; SP, spinous process; TP, transverse process.

Fig. 18.2 (a,b) Conventional midline open foraminotomy for the treatment of lumbar radiculopathy.

Fig. 18.3 Illustration of full endoscopic interlaminar approach for spinal and foraminal stenosis.

Surgical strategies that avoid fusion include conventional open foraminal decompression, full endoscopic percutaneous interlaminar/transforaminal decompression, the relatively novel flexible microblade shaver decompression technique,4 and less invasive techniques using tubular or similar retractors via a far-lateral intertransverse approach or facet-sparing contralateral approach.

Open decompression has been the gold standard for treatment of radiculopathy since the introduction of foraminotomy by Briggs and Krause in 1945.⁵ Contemporary open methods are based on either a midline or a paraspinal approach. Although simple and direct, the midline approach has historically been associated with tissue damage and blood loss (Fig. 18.2).

A full endoscopic interlaminar approach was recently introduced for the treatment of central and foraminal stenosis (Fig. 18.3). Although the endoscopic approach is theoretically the least invasive, the limited mobility of the instruments, the difficulties in repairing any iatrogenic dural injury, and the demanding learning curve are still problems to overcome.

The newest technique in lumbar foraminal decompression is a flexible blade shaver method to enable inside-out widening of the foramen. The major benefit of the technique is the ability to perform decompression of all four nerve roots at any disk level (two exiting and two traversing nerve roots) with only a single incision and a laminotomy in the intervening interlaminar space. As in the minimally invasive contralateral approach, very little bony tissue is removed for access to the pathology site (Fig. 18.4). However, the microblade approach is not suitable for pathologies of the anterior wall of the foramen, nor is it suitable for patients with concomitant central stenosis unless the stenosis is treated first. The inability to directly visualize the exiting nerve root during the procedure is another major limitation. Further studies are required to establish the efficacy and safety of the flexible blade shaver method.

The paraspinal approach was described by Wiltse and Spencer in 1988⁴ and is associated with less tissue damage. A far-lateral approach is effective for treating foraminal stenosis but is not suitable for combined central pathologies and foraminal stenosis at L5–S1, due to bony hindrance (Fig. 18.5).

With a contralateral approach, the contralateral foramen as well as the bilateral lateral recesses and central canal can all be accessed and decompressed with a single incision and with preservation of mechanical stability. The contralateral approach, therefore, can be applied to many forms of stenotic pathologies of the lumbar foramen, including disk herniations, osteophyte formation, or bony hypertrophy and grade I spondylolisthesis. It is also ideal for treating foraminal stenosis at L5–S1, as the position of the ilium may preclude far-lateral approaches to the foramen.

Fig. 18.4 Schematic illustrations of flexible blade shaver method.

18.4 Minimally Invasive Decompression Technique

Microendoscopic diskectomy (MED), developed by Foley and Smith,6 has been widely used for the treatment of lumbar disk herniation. Recently, as a result of advancements in surgical techniques and instruments, spinal microendoscopy has also come to be applied in various other conditions, such as lumbar spinal canal stenosis and LFSS. Central decompressions and microdiskectomies are now routinely performed at many institutions with the use of minimally invasive techniques.

For foraminal/extraforaminal lumbar lesions, such as far-lateral lumbar disk herniation (FLDH), and far-out syndrome in particular, spinal microendoscopy, which can reach deeper into the back muscles less invasively, is surpassing conventional methods and becoming the standard procedure. While previously there was no possibility of avoiding spinal fusion for decompression in many cases of LFSS, with the help of MED, spinal fusion can be avoided. Benefits of a minimally invasive approach include less tissue dissection, the avoidance of fusion, decreased blood loss, decreased postoperative pain, shortened hospital stay, and earlier mobilization.

18.5 Tubular/Transmuscular Approach

Recently, novel minimally invasive techniques have become available for lumbar decompression, utilizing a tubular retractor system to limit paraspinal muscle trauma.⁶ With the use of a tubular retractor system, a surgeon can reach deep into the body and provide relatively free angles, making the existing MED method even lesser invasive. As experience has grown with this surgical approach, surgeons are routinely treating patients with lumbar stenosis using a combination of a tubular retractor system and an operative microscope.

The METRx system (Medtronic Sofamor Danek, Memphis, TN) was the first commercially available tubular retractor system (Fig. 18.6). Before this system was introduced, a speculum or a polyethylene tube had been used as a kind of tubular dilator system. The biggest advantage of using this system is the application of endoscopic techniques to conventional surgery. The system enables both endoscopic images and direct surgical images to be viewed under a microscope. The images can then be used according to the surgeon’s goals. Moreover, because the METRx system splits the muscle instead of cutting it, it is possible to minimize postoperative back pain by reducing muscle damage.