Posterior Minimally Invasive Decompressive Techniques for the Lumbar Spine

CHAPTER PREVIEW

CHAPTER SYNOPSIS:

Recent advances in techniques have enabled surgeons to perform lumbar decompression surgery through smaller incisions, with less soft-tissue disruption. There is a learning curve associated with the minimally invasive approach that must be appreciated. Certain conditions should be approached with caution by the inexperienced minimally invasive technique surgeon, including severe obesity and revision surgery in the multiply operated spine. In the hands of an experienced minimally invasive technique surgeon, minimally invasive lumbar decompression provides less intraoperative morbidity, less postoperative pain, shorter hospitalization, faster recovery, and similar long-term clinical outcomes when compared with traditional open approaches.

IMPORTANT POINTS:

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Do not to compromise the goal of adequately decompressing all neural elements when performing minimally invasive surgery (MIS).
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Indications for MIS lumbar decompression are similar to those for an open decompression.
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Obese patients and patients with prior surgery should be approached with caution.
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The access strategy for each region of the spinal canal where decompression is needed should be planned before surgery, based on imaging.
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Patients who undergo MIS lumbar decompression tend to have less postoperative pain, shorter hospital stays, and faster rehabilitation.

CLINICAL/SURGICAL PEARLS:

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An optimally placed surgical incision is essential to MIS and should be performed under fluoroscopic guidance.
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Use of a small Cobb elevator to clear the soft tissue off of the lamina before placing dilators will help minimize problems with soft tissue during the case.
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Familiarity with tubular retraction is essential.
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The operative microscope is the preferred viewing option for tubular retractor-based surgery.
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Effective use of the “wanding” technique allows for two-level and bilateral decompression through a single unilateral incision.

CLINICAL/SURGICAL PITFALLS:

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At the bottom of the tubular retractor, a small cuff of soft tissue often exists that must be excised to allow visualization.
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The tips of the instruments must be visualized at all times and the field kept dry to minimize the risk for a dural laceration.
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Dural tears often occur when working along the edge of the tubular retractor, where soft tissues can impede visualization.
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Repair of large dural tears may be achieved either by direct suture through the tube or by conversion of the operation to a large open approach.
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Alternative incisions on each side of the spine are needed to perform multilevel decompression beyond two levels.

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Lumbar disc herniation and stenosis have long been recognized as causes of significant disability. In those patients who do not respond to conservative management, surgical treatment has been shown to provide significant improvement in pain and function. Traditional open approaches to lumbar discectomy and decompression are familiar to spine surgeons and are some of the most commonly performed spinal surgeries. Despite their widespread use and documented success, open approaches to lumbar decompression have several potential disadvantages. These include extensive muscle retraction, dissection, and denervation, which have been implicated in postoperative muscle atrophy and increased pain. Furthermore, the open approach for lumbar stenosis involves significant disruption of the posterior spinal elements of the involved levels, which can potentially lead to instability.

Recently, an evolution of minimally invasive technology and techniques for a wide range of lumbar spine pathology has occurred. In general, limiting the amount of soft-tissue retraction and dissection is thought to lead to a decrease in postoperative pain and a faster recovery. Advances in microscopy, tissue retractors, and specialized instruments have enabled surgeons to perform lumbar decompression surgery through smaller incisions, with less disruption of the muscle and soft tissue. As with the traditional open approach, the primary goal of the minimally invasive approach is to adequately decompress all of the neural elements involved. It is important not to compromise this goal when performing minimally invasive surgery (MIS). This chapter discusses the minimally invasive approach to lumbar decompression for the treatment of disc herniation and spinal stenosis, with a specific focus on the indications, contraindications, and surgical technique.

INDICATIONS AND CONTRAINDICATIONS

Patients who are symptomatic from lumbar stenosis typically complain of low back, buttock, and thigh pain that is worse when walking or standing in an upright position (with lumbar spinal extension) and improved when sitting down or leaning forward (with lumbar spinal flexion). This picture of “neurogenic claudication” is the classic presentation, although patient presentation may vary considerably and be confounded by comorbidities, including peripheral vascular disease, peripheral neuropathy, and osteoarthritis of the major joints of the lower extremities. Differences in anatomic areas of stenosis may also affect the presenting symptoms. Patients with primarily central stenosis may present with neurogenic claudication, whereas patients with more isolated foraminal stenosis may present with a radiculopathy in the distribution of the involved nerve root. Patients with a lumbar disc herniation in the lower lumbar segments generally present with pain radiating below the knee in the distribution of the compressed traversing nerve root. In addition to radiating pain, the patient may have weakness and numbness that correspond to the affected nerve root. Patients with symptoms caused by a lumbar disc herniation, in contrast with those with lumbar stenosis, commonly note that their leg pain is made worse with flexion of the spine and lessened with spinal extension.

The natural history of lumbar spinal stenosis is one of insidious onset and progression of symptoms with occasional “flare-ups” often brought on by excessive activity or an injury. Numerous authors have studied the natural history of lumbar stenosis, most of them reporting that, over time, the majority of patients with moderate lumbar spinal stenosis remain essentially unchanged, with a small percentage worsening and a small percentage improving. Except in rare cases of progressive neurologic deficit, a nonsurgical approach is recommended initially for those with symptomatic lumbar spinal stenosis. In most patients, some degree of symptom relief is obtained with nonsurgical therapies including anti-inflammatory drugs, physical therapy, aerobic exercise, and epidural steroid injections either alone or in combination. Patients whose symptoms (i.e., neurogenic claudication with varying degrees of radiculopathy) persist at an unmanageable level despite 3 to 6 months of conservative treatment should be considered for surgery.

When planning a surgical approach to lumbar stenosis, or symptomatic disc herniation, preoperative imaging studies are mandatory and should include plain radiographs of the lumbar spine and an advanced imaging study (magnetic resonance imaging [MRI] or computed tomographic myelography). In addition, if instability of the spinal segment is suspected, lateral flexion/extension radiographs are useful. Cases with a sagittal or coronal plane deformity benefit from long cassette standing scoliosis views of the spine. If evidence of significant instability is documented on preoperative imaging studies, a spinal fusion should be planned in addition to the lumbar decompression.

Approximately 80% to 90% of patients with radiculopathy from a lumbar disc herniation improve with nonoperative management and do not require surgical intervention. Nonoperative treatment options include a short period of bed rest (1–2 days), anti-inflammatory drugs, physical therapy, and epidural steroid injections. Indications for surgery include persistent pain despite 6 to 12 weeks of conservative treatment, progressive or severe weakness, or cauda equina syndrome. Before performing surgery on a lumbar disc herniation, it is essential that an MRI confirms the presence of significant extruded intervertebral disc material causing neural compression at the site suggested by the history and physical examination. If MRI does not confirm a disc herniation at the appropriate level and location, surgery should not be performed because it is unlikely to provide any relief to the patient. Likewise, patients with only back pain and evidence of disc herniation by MRI are poor candidates for lumbar decompressive surgery.

Indications for performing a minimally invasive lumbar decompression are identical to those for a traditional open microdiscectomy. There are, however, some additional considerations when planning the surgical procedure. Minimally invasive lumbar decompression can adequately decompress the central, lateral, and foraminal zones of the spinal canal, and can be used to remove disc material in the extraforaminal region. However, the access strategy for each region of the spinal canal where decompression is needed should be specifically planned before commencing the operation. When significant bony foraminal stenosis is present, this is often easier to access by beginning the approach with a tubular retractor system from the contralateral side and drilling across the spinal canal to the contralateral facet joint with the ligamentum flavum intact as described later. This allows direct access to the exiting nerve root in the firmament, which can be widely decompressed. Extraforaminal compression such as a far-lateral disc herniation should be approached from outside of the spinal canal. In such a case, the tubular retractor is docked onto the transverse process of the upper vertebra, and the intertransverse membrane is identified and released to expose the exiting nerve root. Once the root is identified, the disc material below the nerve in the extraforaminal zone can be accessed deep to the nerve root. Some cases are best performed using a bilateral approach, such as cases with significant bilateral posterolateral disc herniations that are easiest to remove from ipsilateral tubular access on each side.

Obesity does not generally preclude a minimally invasive approach. In fact, minimally invasive approaches can be advantageous in the obese patient because a more traditional open approach in an obese patient can be plagued by the need for massive incisions, deep wounds with associated poor visibility, and limited exposure. Minimally invasive techniques avoid many of these limitations by placing tubular retractors through the excessive soft tissue, directly to the area of interest along the spinal column, allowing linear access to the necessary pathology. However, proper planning is important to ensure that the length of the tubular retractor will be adequate to reach to the spine. Most tubular retractor systems are available in lengths of greater than 90 to 100 mm, which will provide access for most patients. Longer length tubular retractors are available from some instrument companies. The distance from the skin to the spine can often be measured on a preoperative imaging study if the MRI technician is asked to open up the field of view to include the skin over the spine on some of the axial cuts. Working through a long tube is somewhat more difficult than operating through a shorter access portal but, nonetheless, can allow the pathology to be successfully addressed in the hands of an experienced minimally invasive spinal surgeon.

Another area in which an MIS approach may need to be undertaken with caution is when the surgeon needs to revise the decompression on the multiply operated spine. In such a case, the anatomy is often distorted, and dense epidural scarring can be anticipated. Such a case should be attempted only after significant experience has been gained with tubular access spinal decompression.

ANATOMIC CLASSIFICATION OF LUMBAR STENOSIS

The location of a disc herniation or lumbar stenosis can be divided anatomically into the central, lateral, foraminal, and extraforaminal zones. Central stenosis occurs between the facet joints and is typically caused by a combination of congenital canal stenosis, central disc protrusion, and hypertrophy and buckling of the ligamentum flavum. The borders of the lateral recess are the intervertebral disc ventrally, the superior articular facet dorsally, the medial wall of the pedicle laterally, and the medial aspect of the facet joint medially. Stenosis in the lateral recess is usually due to posterolateral disc protrusion, facet joint hypertrophy, facet cysts, and ligamentum flavum thickening. The foraminal zone is bordered by the intervertebral disc ventrally, the pars dorsally, the lateral recess medially, the lateral wall of the pedicle laterally, the inferior wall of the superior pedicle superiorly, and the superior wall of the inferior pedicle inferiorly. Foraminal stenosis can be caused by collapse of the disc space, lateral herniation of the disc, hypertrophy of the superior tip of the superior articular process, or debris from a chronic nonunion of the overlying pars intra-articularis. The area lateral to the intervertebral foramen is considered the extraforaminal zone, which is affected primarily by far-lateral disc herniations and less commonly by osteophytes from the lateral aspect of the facet joint. It is important to understand the different “zones” of potential compression, particularly when performing MIS, because the location of stenosis will alter the surgeon’s approach to the pathology. Current minimally invasive techniques allow the surgeon to target the area of neural compression with little soft-tissue disruption, whereas areas of the spine not causing neural compression are avoided. Careful study of the preoperative imaging studies is essential to providing an anatomically specific plan to a minimally invasive lumbar procedure.

SURGICAL TECHNIQUE

Setup

Minimally invasive lumbar procedures can be performed under either general or spinal anesthesia depending on the preference of the patient. After appropriate induction of anesthesia, the patient is positioned prone on a radiolucent spinal frame. A sterile prep and drape of the site is performed. Next, C-arm fluoroscopy together with an 18-gauge needle positioned at the site of the proposed surgical incision is used to plan the optimal site for the surgical incision ( Fig. 43–1 ). A lateral fluoroscopy image can be used to document the position of the needle and ensure direct access to the pathology.